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Geology and Mineral Resources of the Amherst Quadrangle, Virginia

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GEOLOGY AMD MINERAL RESOURCES OF THE
AMHERST QUADRANGLE, VIRGINIA
A THESIS
PRESENTED TO THE FACULTY OF
THE GRADUATE SCHOOL OF
CORNELL UNIVERSITY
IN
PARTIAL FULFILLMENT
OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
BY
CHARLES HENKEL MOORE, JR.
JUNE, 1940
ProQuest N um ber: 10834646
All rights reserved
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uest
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AUTOBIOGRAPHY
The Author was born October 85, 1915, in New
Market, Virginia, and received his primary and
secondary education in the New Market Public Schools,
graduating from New Market High School in June, 1958.
He entered the University of Virginia in September, 1938
and received his Bachelor of Science degree in June, 1936,
and his Master of Science degree from that institution
in 195?•
He married Miss Elsie Wilson Davis, of Charlottesville,
Virginia, on February 3, 1939.
He has held the following Academic positions:
Laboratory Assistant, University of Virginia, 1935-3?*
Junior Assistant, Cornell university, 1937-38.
Senior Assistant, Cornell university, 1938-40*
Reference to published articles:
Ideas on the Origin of the Natural Bridge of Virginia
Proc* Va. Acad. Scl. (Abstract) May, 1936*
The Stauroiites of Patrick and Henry Countiea,Virginia
Proc* Va. Acad. Sci. (Abstract) May, 193?.
The Staurolite Areas of Patrick and Henry Counties,
Virginia•
Am. Min. Soc., Vol. 88, No. 8, pp. 990-996.
Some Rock® and Minerals of Amherst County, Virginia
Proc. Geol. Soc. America, Jan. 1938.
Origin of the Nelsonit© Dikes of Amherst County,
Virginia•
Economic Geology (In press)•
GEOLOGY AND MINERAL RESOURCES OF THE AMHERST
QUADRANGLE, VIRGINIA
CONTENTS
Pag©
INTRODUCTION
.......................... *...... 1
Field and Laboratory Work.............. ......1
Acknowledgements
............. .
8
Previous Geologic Work...... ............... . 8
.......... .
8
GEOGRAPHY AND PHYSIOGRAPHY
Location of Area*.
..... . 8
Topography and Drainage...... .............. . 8
Physiography....... ........ ........ .......... 9
.
GEOLOGY AND PETROGRAPHY........................ .. 18
Introductory Statement
*................... 18
Metamorphic Rooks of Sedimentary Origin....... 13
Lynchburg Gneiss
..... .... 13
Definition*..*
............
13
Occurrence•••••••*•*••.•«*.•«•*•*...... 15
Megascopic Character. *........ *........1§
Microscopic Character* *.«•••••••»••••*.. 16
Comments*
.......
17
.... 18
Graphite and Seriolte Schist
Megascopic Character
.... 18
Microscopic Character....... *
•••• 19
Comments*
....... .
19
Origin and Relative Age of the Lynchburg
Gneiss***.......... *.....
80
Acid Igneous and Metamorphosed Igneous Rocks•• 80
Lovingston Gneiss
80
Definition......... ...... .............. 80
Occurrence....... .
80
General Character* »• *.... *.............. 81
Normal Lovingston Gneiss........
88
Megascopic Character*
....... ••*•*• 88
Microscopic Character.................. 88
Comments
..... ................. 23
Hydrothermally Altered Lovingston.......
24
Definition*
......
24
Occurrence
.... *.......... 24
Megascopic Character* ** ................. 24
Microscopic Character................... 25
Comments
.............. 26
Origin and Age of the LovingstonGneiss.••• 26
Bypersthene Granodiorl te.........
27
Definition
**.27
-iiPage
Occurrence.
.......
27
.............. 28
General Character.
Blue Ridge Granodiorite.**...*....... ..... 29
Megascopic Character.... * ............. 89
Microscopic Character.................. 29
Coasnents
*• •.31
Tobacco Row Hypersthene Granodiorite...... 51
Megascopic Character........
51
......
31
Microscopic Character
Comments*......
.....
52
Cherry Kill Leucogranodiorite
33
Megascopic Character*
•• 33
Microscopic Character....... • •........
34
Comments ................
••• 35
Sheared Granodiorite••,#••»•••••••••••••*• 35
Comments............................... 37
Age and Relations of Hypersthene Grano­
3?
diorite* .......
Anorthosite* *
..... *****.....
38
Definition and General Character*•**•*• 38
Occurrence
.....
40
40
Megascopic Character.........
Microscopic Character*••*•••*••••*•*••• 40
Origin and Age of Anorthosite*•••••*••• 41
Pegmatite and Granite Dikes
*.....
45
Pegmatites. *
.......
45
Granite Dikes
.... *......
46
Origin
......
46
Basic Igneous Rooks
.... ........... *.......
47
Hornblende Gneiss,
.........47
47
Megascopic Character. ........ *......
Microscopic Character*
.....
47
Origin and Age
.....
43
Hornblende Metagabbro*
......
49
Megascopic Character.•«•••«••*•#•*•••.•.....
49
Microscopic Character•••••••*.......... •••••• 49
Origin and A g e
........
50
Peridotits and Metapyroxinlte......... *.......
51
Perldotite
...... •**•• 51
.....
51
Megascopic Character
Microscopic Character.*••••••*••*••••«••*•••• 51
Metapyroxenite ••*••*•••......
52
Megascopic Character**•.•*••.•*.•••••*.•••••• 52
Microscopic Character.••••••.......
52
Age***..............
53
Greenstone
.......
55
Occurrence and Distribution. .«••••*•...........53
..............
53
Megascopic Character
Microscopic Character. ••••••••• ............
54
Origin and Age...... *.....*
*•••• 55
~iii
Helaonite D
i
k
e
s
Diabase Dikes.................
.......
Megascopic Character.
Microscopic Character.
......
.
*
Page
56
58
58
58
STRUCTURE.
....................
Igneous Structures
.....
Folds,...................
F
a
u
l
t
s
..........
Age of structures.
.....
Igneous Structures. ........
Felds and Faults..,,.*..*.,,*.*1.........
59
59
62
63
65
65
66
METAMGRPHI3M.
.............
Introductory S
t
a
t
e
m
e
n
Regional Dynamic M e t a m o r p h i
Later Dynamic Me tamorphism* .....
......
Hydrothermal Metamorphism.
67
67
67
63
69
GEOLOGIC HISTORY.
............
....
t
*
s m ,
*.
70
ECONOMIC GEOLOGY...............
72
Introduction*
.........
72
Nelsonites. .....
73
Occurrence. .... .......... ......1.......
74
Descriptions of H e l s o n i t e s , , 75
Southern Mineral Company9s Quarry........
76
Lueian Burley Property...................
78
w , B. Gillespy1s Property* ......
80
Relsonite Body on U. S. Route 60.........
80
Other Localities.........................
81
Origin of Relsonites*
..... . •.*.....
81
Conclusions. •...»......
85
Prospecting for Helaonites..................
85
Production,.................................
86
Copper
...................
67
History
.//., • ......
87
Occurrence.
3
8
Production,
..... ... •,
89
Feldspar*
.......
89
Building Stone and Road M e t a l * •
.......
90
iv
ILLUSTRATI OKS
.gur®
I
Z
3
4
5
6
7
8
8
10
II
18
13
14
16
16
17
18
19
30
31
S3
33
34
85
Facing Pag<
Geological Hap of Amherst Quadrangle ...•*
(In folder in Bach)
Index map showing location of Amherst
Quadrangle ............................ .. • 3
Photo from top of long M o u n t a i n . • 10
Photomicrograph of Lynchburg Gneiss....... 16
Photomicrograph of Normal lovingston...... ZZ
Photo of Aplitic Granite Dikes............ 34
Photomicrograph of Hydrothermally Altered
Lovingston Gneiss......................... 36
Residual soil containing weathered boulder
33
of granodiorite ......
Photomicrograph of Blue Ridge Granodiorite 30
Photomicrograph of Tobacco Row Grano­
diorite
31
Photo of granodiorite bodily injected into
altered lovingston........................ 33
Photomicrograph of leucogranodiorite...... 34
Photomicrograph of leucogranodiorite show­
ing ilmenite and epidote in a matrix of
quartz, albite and sericite.......
34
Photomicrograph of Anorthosite............ 41
Photomicrograph of Aplitic Granite.......• 46
Photomicrograph of Peridotite............. 61
Photomicrograph of Catoctin Metabasalt.... 85
General View and Olose-up of Marginal
Fractures in lovingston. .....
60
Diagrammatic Section (Lovingston-granodiorite relationship
.... 61
Ilmenite-apatlte quarry of the Southern
Mineral Products Co. on Plney River....... 76
Photomicrograph of Hard Ore from quarry of
Southern Mineral Products Company......... 77
Photomicrograph of Nelaonite from the
Lucian Burley Property.•• ...........
79
The Southern Chemical Company’s Plant at
.....
87
Piney River.
Shaft of an abandoned copper mine,
Glades Region.
......
@8
Photomicrograph of Mineralized Peridotite. 89
I N T R O D U C T I O N
PURPOSE AND SOOPE OF REPORT
It is the purpose of this report to discuss the
geology end mineral resources of that portion of Amherst
and Nelson counties covered by the Amherst Quadrangle.
The
rock types of this area are discussed in detail as regards
\
their origin, structural features, and areal and genetic
relationships.
The location, mode of occurrence, and origin
of the economically important minerals, as well as building
stone and road metal, are considered in considerable detail.
The geography and geologic history of the region is discussed,
as the successful exploitation of the economic minerals and
correct conclusions regarding the origin of the rocks re­
quire an understanding of these features.
FIELD AND LABORATORY WORK
The field work was done during the summers of 1933
and 1939.
The laboratory investigations were made at
Cornell University during the academic years of 1938-1940.
The Advance Sheet of the Amherst Quadrangle was used
as a base for the geologic map prepared, and the results
were later transferred to the Amherst Quadrangle.
ACKNOWLEDGEMENTS
-2-
Th© writer wishes to express his indebtedness to
Dr. J. D. Burfoot of G o m e l l University, under whose
supervision this report was prepared.
He wishes also to
thank Dr. Anna Jonas Stose for suggesting the problem and
for valuable criticism during the investigation.
He is
further indebted to Dr. H. Hie© of Cornell University for
assistance with the economic geology, and to Dr. 0. M.
Nevin of Cornell University for assistance with the
structural geology.
He also wishes to express his
appreciation to Dr. A. A. Pegau for criticisms in the field,
to Dr. A. 0. Bevan and the Virginia Geological Survey, who
authorized the work and whose financial assistance made this
report possible, and to Dr. A. L. Anderson and other members
of the staff at Cornell University for suggestions and
criticisms.
Finally, he wishes to express his appreciation
to the citizens of Amherst and Nelson counties, whose
friendly cooperation greatly facilitated the field work.
PREVIOUS GEOLOGIC WORK
1
As early as 1809, William Maclure, following the
I------------------------------------------Maclure, William. Observations of the Geology of the United
States. Am. Philos. Soc. Trans. Vol. 6, pp. 411-428. map
1809
teachings of Werner, classified the rocks of the United States
into Primitive, Transitional, Secondary, and Alluvial types.
3-
The rocks of the Piedmont and Blue Ridge provinces were
classed as Primitive.
Since M a d u r a 1s time many persons have studied the
Piedmont and Blue Ridge provinces, in Virginia.
These
included W. B. Rogers, Arthur Keith, I. L. Campbell, P. B.
Laney, G* W. Stose, and E. B. Knopf.
There will be no
attempt to review in this report the works of these men or
of many others.
Only those investigators who have dealt
directly with Amherst County or whose work directly affects
the conclusions arrived at in this report will be considered
here •
Hypersthene granodiorite was first discribed in the
2
Blue Ridge of Virginia by Weed and Watson. They called the
2
Weed, W. H. and Watson, Thomas - The Virginia Copper Deposits
Boon* Geol., Vol. 1, pp. 309-330, 1906. (See esp. pp.318-319.
rock a hypersthene syenite and described it from various
localities along the east side of the Blue Ridge from Warren
County southwest as far as Stony Man.
There is on© descrip­
tion from Amherst County.
3
In 1907, Watson published a brief discussion of
3
Watson, Thomas L.- The Occurrence of Nickel in Virginia.
Trans. Am. Inst. Mining Engrs. 1907, pp.306-316
YJatson, Thomas L.- Mineral Resources of Virginia. 1907, pp.
31-33 and 500-582.
hypersthene syenite and associated hornblende norite from
-4
a locality in the northern part of Floyd County.
Attention
was called to the close similarity of this rock to that at
Milams Gap, in Page and Madison counties, more than 100
miles to the northeast.
4
In 1916, Watson and Cline
published a report on the
4
Watson, T# L. and Cline, Justus H.; Hypersthene Syenite
and Related Rocks of the Blue Ridge Region, Virginia. Bull.
Geol. Soc. of America, Vol. 27, 1916, pp. 193-234.
hypersthene syenite in Virginia.
In their report they noted
that Iddings suggested the name granodiorite for that rock
in which the llme-soda feldspar exceeds the orthoclase.
However, Watson and Cline tended towards the somewhat pre­
valent opinion at that time that the term granodiorite was
synonymous with quartz monzonite.
In order to distinguish
between the two rocks which are closely associated atreally
in Amherst County and elsewhere in the Blue Ridge and which
these authors regarded as oomagmatic, they called the type
with the greatest proportion of lime-soda feldspar a syenite.
They agreed that the rock was pre-Cambrian but did not
commit themselves as to its relation to the associated rooks.
In the Amherst-Helson county area, they concluded that the
syenite (anorthosite), the quartz monzonite (Lovingston
gneiss) and the hypersthene syenite (hypersthene granodiorite)
were undoubtedly differentiates of a common magma, but that
more study was necessary to determine whether there was one
intrusion with a segregation in place or several intrusions.
5
Jonas on the state map of Virginia published in 1928
5
Geological Map of Virginia, 1928
renamed many of the rocks of the Blue Ridge-Piedmont region.
She called the hypersthene syenite of Watson a
hypersthene
granodiorite, applied the place name Lovingston gneiss to the
old quartz monzonite, and called the syenite of Nelson County
an albitite.
She later accepted the name Anorthosite for
6
the last named rock, after it was suggested by Ross . In
6
Ross, C. S.; Mineralization of the Virginia Titanium Deposits
Am. Mineralogist, Vol. 21, No. 3. March, 1936, pp.266-275.
--------- 9----------------------------------------------1955 Jonas more completely described the hypersthene grano?
■
-----------------------------------Jonas, A. I.J Hypersthene Granodiorite in Virginia, Bull,
of the Geological Soc. of America, Vol.46,1955,pp.47-60
1
diorite.
She concluded that its composition was essentially
the same as the hypersthene syenite of Watson. However,
because of its quartz and plagioclase content, she placed it
in the granodiorite class.
She also noted far more hydro-
thermal metamorphism in the rock than had previously been
recognized.
The most recent generalized work directly affecting the
8
Amherst Quadrangle is the work of Jonas and 3tose, in which
8
Jonas, Anna I. and G. W. Stose: Age relation of the I re-
Cambrian Rocks in the Catootin Mountain-Blue Ridge and
Mount Rogers Anticlinoria in Virginia. Am. Jour, of
Soience Vol. 237, August, 1939, pp.575-593.
they produced evidence that they believed invalidated the
conceptions previously held concerning the age of certain
formations in the Catoctin-Blue Ridge and Mount Rogers
anticlinoria.
They concluded that the Lynchburg gneiss was
in part contemporaneous in age with the Catootin metabas&lt,
and younger than the Lovingston gneiss.
They concluded
that the major structures of the anticlinoria were formed
in late Paleozoic time and that the largest part of the
metamorphism of the volcanic series took place at that time.
Ho previous detailed work has been done, to the
writer*s knowledge, on the greater portion of the area
comprising this report.
However, many of the reports written
on the surrounding areas include small portions of the
Amherst Quadrangle.
9
Watson and Taber, in their investigation of the titanium
9
:
Watson, T. L. and Taber, Stephen: Geology of the Titanium
and Apatite Deposits. Virginia Geological Survey Bull. No.
Ill-A,1913
and apatite deposits of Nelson County, Virginia, discussed
in some detail the associated rock types.
into Amherst County.
These types extend
They concluded that all the intrusive
rocks of the area, excepting the diabase dikes were of
approximately the same age and differentiates of one magma*
-7-
The nelsonite dikes, with their iliaenite and apatite
oontent, were thought to he in general the results of
magmatie segregation*
Ryan
10
mentioned the occurrence of ilmenite todies in
10
Ryan, G. W* ; The Ilmenite Apatite Deposits of WestCentral Virginia. Boon. Geol. Vol. 28, No. 5, May, 1933
pp. 866-275.
Amherst County. He followed the concepts of Watson and Taber
regarding their origin.
11
More recently, Ross
has concluded that the nelsonite
n
Ross, 0. S.: Titanium Deposits of Hoseland District*
International Geol. Cong. Guidebook 11, Northern Virginia,
1933, pp. 29-36.
Ross, C. S.: Op. cit. pp. 143-146.
and rutile deposits are of hydrothermal origin.
He also
called the rock type in which the titanium minerals prim­
arily occur an anorthosite and stated that it was definitely
younger than the associated rocks.
The southeastern part of Amherst County, in the vicinity
12
of the lames River, has been mapped in det&il by Furcron •
IS
'
Furcron, A. S. ; Fames River Iron and Marble Belt, Virginia.
Virginia Geol, Survey* Bull. No. 39, 1935.
He concluded that the Lynchburg gneiss was the oldest rock
In the region and that it was
intruded first by hornblende
gneiss and hornblende gabbro and later by the Lovingston
quartz monzonite gneiss.
-8-
O E O G R A P H T
A N D
P H Y S I O G R A P H Y
LOCATION Of ARM.
The Amherst Quadrange comprises three fourths of
Amherst County and a small portion of Nelson County.
This
Area lies in the west central portion of Virginia, and
Includes parts of the Piedmont and Blue Ridge provinces
(See fig. 1).
The quadrangle extends from Woodson in the
north to Monroe in the south.
Its ©astern boundary is just
west of the Glades region, and its west boundary Is on the
top of Long Mountain, one of the ranges of the Blue Ridge
(See fig. 2).
The area comprises about 300 of the 450 square
miles of Amherst County and approximately 25 square miles
of the southern portion of Nelson County.
TOPOGRAPHY AND DRAINAGE
Topographically this district Is divided into three
distinct types.
The northwest portion lies on the top
and east flank of the Blue Ridge mountains and is rugged
and mountainous.
The central portion of the area lies in
the foothills of the Blue Ridge and is, In general, rugged,
but contains many valleys with wide, relatively level floors.
The eastern portion of the quadrangle is a flat or gently
rolling plateau, with a few residual peaks rising from this
surface.
,J- H
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The James River is the master stream of this region, Its
general direction of flow Is northeast, although it locally
deviates considerably*
The James itself flows for only
about 1/S mile through the extreme southwest corner of the
Amherst Quadrangle. However its tributaries are the main
drainage features of the area and are responsible for the
dissection of the region.
The Amherst Quadrangle Is drained by two large streams
and many small tributaries.
Piney River, which forms the
boundary line between Amherst and Nelson counties and drains
the northeastern portion of the region, rises In the Blue
Ridge and flows southeast into the James.
Buffalo River,
which drains the central portion of the Quadrangle, also
rises in the Blue Ridge and flows southeast into the James.
A multitude of small streams, most of which flows south
Into the James, dra'ln the southern half of the Quadrangle.
A few of the larger ones flow north into Buffalo River.
PHtSIOGRAPHf
The Amherst Quadrangle lies partially In the Blue
Ridge and partially in the Piedmont physiographic provinces.
Strictly speaking, four-fifths of the area, or that part west
of U. S, route 89, should be called the Blue Ridge province,
since it is characterized by outliers of the Blue Ridge
mountains.
However, the Piedmont province is generally de­
fined as extending westward to the foot of the Blue Ridge
Mountains.
In this region the foot of Long Mountain marks
-10the eastern boundary of the Blue Ridge Mountains proper,
but as far east as tJ. S. route 29 the country is mountainous.
The Blue Ridge Province in this area comprises sharp
ridges cut by deep, narrow valleys, yielding a very rugged
topography (See fig. 3).
trellis pattern.
The streams are small and have a
The elevation varies from 4000 feet to
800 feet with a mean elevation of about 1000 feet.
The Piedmont province east cf U» S. route 29, has a
typical Piedmont surface, being a flat, low lying region
with a very low measure of relief.
The elevation varies but
little from the 750 feet average elevation.
The streams,
which have dendritic patterns, have cut this old peneplain
surface so that they now occupy wide, gently sloping valleys.
West of U. S. Rout© 29, however, the Piedmont province
changes, assuming many characteristics of the Blue Ridge
province.
The topography becomes more rugged, with many
small mountains appearing, which are oriented parallel to
the trend of the nearby Blue Ridge.
The average elevation
is greater than the typical Piedmont peneplain by only
about 100 feet, but the measure of relief is increased to
about 1000 feet.
The streams, however, have dendritic
patterns and occupy wide valleys with sloping sides.
Even
the small tributaries have comparatively wide floodplains.
This latter intermediate region which comprises the
entire central portion of the Quadrangle, has many of the
structural features of the Blue Ridge provinoe, but has a
Figure 3. Photo from top of Long Mountain
showing the foothills of the Blue Ridge
dissected by wide valleys.
-11peneplained surface similar in age to that of tbs flatter,
truer Piedmont surface*
Peneplanation is very widespread over the Piedmont
province*
The erosion surface extends from the Blue Ridge
Mountains east to the Pall Zone where it intersects an older
peneplain surface which dips under the late Tertiary
sediments of the coastal plain.
At present the younger
15
erosion surface is considered to b© of Tertiary age •
IS-------------- :
------- --------- 1
-------------Wright, P. J.: The older Appalachians of the South.
Denison Univ. Bull. Jour* Sci. Labs. Pol.26, pp.145-250,
Dec., 1931
Wright, P. J* ; The newer Appalachians of the South.
Denison Univ. Bull. Jour. Sci. Labs. Vol.29, pp.1-105,
Apr., 1934.
The soil, because of this widespread peneplanation, Is
relatively uniform in color and texture throughout the
Piedmont portion of the Amherst Quadrangle.
It is in
general of sandy composition, has a light brown color, and
usually reflects very little th© character of the under­
lying rock.
The soil covering th© Blue Ridge province, on th© other
hand, is chiefly red, residual clay and loam, and where the
different formations are not exposed, their boundaries can
frequently be located by th© change in soil color and texture.
-18O S O L O Q Y
A N D
P E T R O L O G Y
INTRODUCTORY STATEMENT
The rooks comprising the Amherst Quadrangle are,
with the exception of the Triassie diabase dikes,
pre~Cambrian in age*
There are metasadlmentary igneous,
and metalgneous rooks, all of which show a wide range of
variation*
Some of the formations show intense and
varied metamorphism while others exhibit few metamorphie
effects*
The Lynchburg gneiss and the schists overlying it are
of sedimentary origin; whereas the remainder of the rocks
are igneous or metaigneous•
The Lovingston quartz monzonite
gneiss is intruded and hydrothermally altered by the
hypersthene granodiorite and its derivitives, anorthosite,
pegmatite and nelsonite*
Hornblende gabbro and meta-
pyroxenit© dikes Intrude all the rocks of the area except
*
the varpus phases of hypersthene granodiorite and the
diabase.
Perldotite Intrudes the Lynchburg gneiss.
diabase dikes intrude all the rocks of the area.
metabasalt overlies the hypersthene
The
Catoctin
granodiorite on the
top of Long Mountain at th© extreme northwestern edge of
the Quadrangle*
-13-
MSTAMORPHIC ROCKS OF SEDIMENTARY ORIGIN
Lynchburg Gneiss
14
Definition. - The Lynchburg gneiss is defined by Furcron
H
Furoron, A. S.: James River Iron end Marble Belt.
Geol. Survey Bull. No. 39, 1935, p. SO.
7a.
---------------------------------------------------------- jg-------
as follows: "This term was first applied by Jonas
to a belt
Jonas, A* I.: Geologic reconnaissance In the Piedmont of
Virginia. Geol. Soc. Am. Bull*, Vol. 36, p. 845, 1987.
of biotite gneiss and schist west of the Catootin Mountain
border fault, which extends from near Schuyler In Nelson
County, southwest through Lynchburg Into North Carolina,
where it appears to be at least in part equivalent to the
Carolina gneiss of Keith.
It is intruded by rocks that
form the core of the western Piedmont and Blue Ridge
provinces.
Just north of Lynchburg it occupies a belt of
5 to 6 miles wide which widens to at least 20 miles in
southern Virginia."
The Lynchburg gneiss has been considered by Furcron and
others to be the oldest formation in the Piedmont of Virginia,
equivalent to the argillaceous sediments which are the
basement rocks of the older injection complex* In 1939,
16
however, Jonas and Stose, in a paper on the age relationship3
Te
~ ~
Op. cit. pp. 575-593.
-14
of the pre-Cambrian rooks of the Catootin mountain-Blue
Ridge antielinorium in Virginia, place the Lynchburg as
definitely younger than the bordering Lovingston gneiss.
This conclusion is based primarily on the discovery by
17
Nelson of a basal conglomerate beneath the Lynchburg at
17
Nelson, W. A. Informal Communication. lour. Wash. Acad.
Sci. Vol. 22, pp. 456-457, 1932.
Rookfish River, which he called the Rookfist conglomerate.
This conglomerate was traced by Nelson northwestward to
Charlottesville.
The conglomerate contains pebbles of the
Lovingston gneiss and granite comprising th© older inject­
ion complex.
Therefore the Lynchburg is equivalent to the
younger pre-Cambrian series which includes Catootin meta18
basalt at the top. Furcron considers this conglomerate
18
Furcron, A. S.: James River Iron and Marble Belt. Va.
Geol. Survey Bull. No. 39, 1955, p. 38.
to be lower Cambrian Loudoun.
In the Amherst Quadrangle, the Lovingston gneiss was
found intruding a formation which is shown on the state map
as Lynchburg gneiss. This relation was also noted by
19
20
Furcron. It was suggested by Jonas that this rock was
19
Op. oit. p. 43
-15-
20
Verbal communication.
probably the equivalent of the old biotite schist whioh
the Lovingston intruded, and that the true Lynchburg was
located farther east#
Ho Rockfish conglomerate was recognised as such in the
Amherst Quadrangle#
However, one mile east of the Lovingston
quartz mica gneiss contact, the writer found a^arkosio
quartzite about 100 feet wide and in strike with the con­
glomerate at Rockfish River*
If this rock is the southward
extension of the Rockfish conglomerate, then it marks the
contact between the new Lynchburg on the east, and the old
quartz mica sediment on the west.
However, there is no
marked mineralogies! or structural difference between quartz
mica gneiss west of this conglomeratic band and that east
of it.
Because of the present confusion regarding this formation,
this rock type will in this report, be discussed and mapped
according to the older idea#
Occurrence. - In the Amherst Quadrangle the Lynchburg gneiss
is typically exposed at Its contact with the Lovingston
gneiss one mile southeast of Amherst on U. 8. route 60.
About three fourths of a mile of the southeastern
quadrant of the Amherst Quadrangle is underlain by the rock
mapped as Lynchburg gneiss (See fig. 1).
Peridotite
—16—
intrudes the Lynchburg in the extreme southeast corner of
the Quadrangle.
Sericite schist, with a maximum width
of one mile, overlies the central part of th© quadrant
south to Pafcridge Creek.
Megascopic Character. - In Amherst County the Lynchburg
gneiss Is a fine to medium grained, even granular to
slightly porphyritic biotite gneiss, which becomes sohistose
on weathering.
It varies in color from light to dark, but
is usually a light gray salt and pepper rock.
Sometimes
the developement of sericite gives the rock a shiny appear­
ance.
Minerals readily identified with a hand lens are
granular to glassy quartz, gray to white feldspar, biotite,
muscovite and occasionally sericite and epidote. Hornblende
is developed In the vicinity of pegmatites.
Microscopic Character. - The Lynchburg gneiss Is an even
granular to porphyritic rock whose principal constituents
are quartz, biotite, muscovite, and feldspar (See fig. 4).
Quartz is the most abundant constituent.
I t ’s fractured
boundaries Indicate some crushing subsequent to the regional
metamorphism.
Biotite usually occurs in yellow brown to
greenish-brown laths; however, irregular masses locally are
large and form porphyrablasts.
The biotite laths have
a parallel orientation which gives the rock its gneissic
texture.
Muscovite Is usually subordinate, but locally Is
as abundant as biotite.
The muscovite laths are of random
orientation, frequently cutting across the schistosity.
Figure 4* Photomicrograph of Lynchburg
gneiss, showing general gneissic texture.
Muscovite (a) is seen cutting across the
schistosity. Polarized light X25.
-17Orthoclase, microoline, and alblte are present in all thin
sections but vary locally in proportion and abundance.
The potash feldspars are generally predominant over the
plagioclase.
The feldspars are replaced by muscovite and
quartz and are frequently altered to serioite and epidote.
ZfcLxioon and ilmenit© are present in small amounts. Secondary
minerals are excessively developed in slides taken near
the western boundary of the formation.
They include epidote,
some of the biotite, sphene, leucoxene, chlorite, and apatite.
Epidote is relatively abundant as small grains scattered
indescriminately through the rock.
Secondary biotite is
developed in irregular poikilitic masses, is faintly
pleochroic and shows partial alteration to chlorite.
It
contains inclusions of quartz, feldspar, epidote, zircon
and sphene, and it probably formed by replacement of
preexisting porphyroblasts of hornblende,
A few broken
apatite grains were observed*
Comments. - The Lynchburg quartz mica gneiss of the Amherst
Quadrangle is the oldest rock in that locality.
The relative­
ly large amount of secondary minerals present in the
Lynchburg of the Amherst Ouadrangle seem to be peculiar to
this area, and are related to the igneous activity which so
altered the younger rocks of the area.
Descriptions of
the Lynchburg gneiss from other localities mention the
secondary minerals as uncommon and unimportant.
-18Graphite and Sericite Schists
A belt of serioite and graphite schists, with a
maximum width of one mile, occurs in the Lynchburg Gneiss
in the Amherst Quadrangle.
They extend from the eastern
boundary of the Quadrangle for two and one half miles in
a southwestward direction to Patridge Greek.
Although
these schists resemble closely the schist phases of the
Loudoun formation of lower Cambrian age, which has been
31
mapped by Purcron about two miles to the northeast, they
IT
Op. cit. pp. 37-58. And geological map.
are intruded by
hornblende gabbro, which in other parts of
*
the area is of definite pre-Cambrian age.
S r*"'"
....
-
... — —
—
.
-----------............. . —
...
See Basic Igneous Rocks - Hornblende gabbro
These schists are probably of the same age as the gneiss
In which they occur, their difference in character being
caused by an initial difference in the composition of the
sediments as well as other factors discussed under Structure
and Metamorphism.
Megascopic Character. - These rooks are fine grained, high­
ly fissile and crumpled phyllites.
The lighter ones show
a lustrous sheen caused by serioite, some segregation of
the quartz into bands, and a brown stain caused by limonite.
-19-
In places where the Iron content is high they are deep red.
Where the graphite content is high, the rocks are gray to
black in color.
In these latter rocks, there is no
noticable segregation of the quartz into bands.
Microscopic Character. - The lighter colored schist appears
microscopically to be a quartz muscovite phyllite.
All
the minerals, with the exception of some muscovite flakes,
are oriented parallel to the sohistosity.
The quartz shows
some crushing and the fracture surfaces are coated with
limonite.
The rock shows a vague banding caused by the
quartz and muscovite.
In the muscovite bands, all the
flakes of that mineral are oriented parallel to the
sohistosity, as are the included quartz stringers.
However,
in the quartzose bands, some included broad grains of musco­
vite cut across the sohistosity.
Zircon and elongated masses
of limonite and graphite are included mainly in the musco­
vite bands.
The dark colored schists differ ohiefly In that they
contain a greater amount of graphite, which is merely
present as an accessory in the lighter phases.
The grain
size of the individual minerals is smaller, so that it is
difficult to identify them.
Sericite, graphite and quartz,
with no segregation, are the principal minerals.
Slongated
masses of limonite are relatively abundant, and a few iron
stained grains of biotite were observed.
Serioite is more
abundant than graphite.
Comments. - These two schists are intimately associated,
-20*
and degrees of gradation between them can be observed.
In the gradational phases intercalated bands of graphite
and muscovite schist can be seen in the handspecimen.
The
graphite schist is always much finer grained than the
sericite schist*
Origin and Relative Age of the Lynchburg Gneiss* -
The
argillaceous sediments which were transformed by progressive
metamorphism to the Lynchburg gneiss of the Amherst Quad­
rangle were evidently the oldest material of this region*
They were intruded by the Loviugston gneiss, forming the
older injection complex, and later by metagabbro, peridotite,
and diabase.
ACID IGNEOUS AND METAMORPHOSED IGNEOUS ROCKS
Lovingston Gneiss
Definition* - The Lovingston gneiss Is a gneissic quartz
<?■f'
o
monzonite with the gneiss bands stricking No. 38 E. It
82
was named Lovingston by lonas from typical exposures near
2
"
Geological Map of Virginia, 1928.
the town of Lovingston, Nelson County, Virginia.
Occurrence. - The Lovingston gneiss is widely exposed along
the eastern flank of the Blue Ridge in Virginia* It is
usually bounded on the east by metasediments and on the
west by the hypersthene granodiorlte.
The Lovingston gneiss underlies more than half the area
comprising the Amherst Quadrangle. It extends through
the entire length of the Quadrangle in a northeastsouthwest direction, and extends from U. S. Route 29 on
the east to the foot of Long Mountain on the west.
It
has been extensively Intruded by the hypersthene granodiorite
and its derivitives, as well as by many small dikes of dia­
base, metapyroxenite, and hornblende gabbro.
General Character. - The Lovingston gneiss was called a
25
biotite-qtuartz monzonite by Watson and Taber who
is
:
Watson, Thos. L. and Taber, Stephen - Geology of the
Titanium and Apatite Deposits. Va. Geol. Survey Bull.
Ho. 111-A, 1913, p. 59.
described it in detail.
They concluded that although the
rock is of definite igneous origin, its gneissio texture
Is secondary and the result of dynamic regional metamor­
phism.
The writer is, in general, In agreement with this
conclusion, although there are certain structural features
developed around the edges and elsewhere in the body, which
were formed independently of the regional metamorphism.
In addition, the Lovingston gneiss contains narrow bands of
mica schist striking roughly parallel to the general trend
of the gneiss and ranging from a few inches in width up to
200 feet.
These structural features will be discussed in
the chapter on Structure.
The extensive intrusion of the Lovingston gneiss by the
hypersthene granodiorite accompanied by exomorphic phenomena
-23
has produced from the former a rook type which appears
in the field to be Lovingston with granodiorite characteristics
This type of Lovingston gneiss in proximity to the grano­
diorite intrusive, has, through the alteration, lost many
of its original structures, and, therefore, merits a sepa­
rate discussion under the heading of Hydrothermally
Altered Lovingston.
Normal Lovingston Gneiss
Megascopic Character. - The Lovingston gneiss is a distinct­
ly banded, medium grained, black and white granular to
porphyritlc gneiss.
Where the original rock was porphyritic,
the mashing out of the feldspars and their subsequent recrystallization under metamorphism has produced an augen
gneiss.
The black bands are primarily biotite, the por-
phyroblasts white feldspar, and the white bands feldspar
and quartz.
Small slips coated with biotite cut the rock.
Microscopic Character. - Thin sections show the rock to be
medium to coarse grained with a distinct porphyritic
texture (See fig. 5).
The principal minerals are potash
and soda-lime feldspar, biotite, quartz and muscovite with
the accessories zircon, ilmenite, and occasionally apatite.
Minerals determined by their relationship to be secondary
are epidote, olinozoisite, sericite, carbonate and sphene.
The porphyroblasts are in some cases albite and in
others microcline.
The proportion of plagioclase to
orthoclase varies from 50-50 in the relatively even
Figure 5* Photomicrograph of normal Lovingston
gneiss showing biotite, feldspar and fine grained
quartz cut by a veinlet of coarse grained quartz.
Polarized light. X25.
-23granular gneiss to 75-25 in the augen gneiss*
The
smaller feldspar grains oecuring in the white hands are
commonly alhite-oligoclase which in some oases carries
antiperthitie microcline.
Quartz occurs as large broken
grains, as extremely fine grains in separated hands and
as veinlet replacement quartz in the feldspar*
The biotite
occurs chiefly as bands, bent around and frequently replacing
the microcline phenocrysts.
random orientation.
Muscovite occurs as laths with
It frequently cuts and offsets laths
of biotite*
The secondary minerals are usually associated with the
bands of biotite and muscovite, the cleavage of these
minerals no doubt providing an easier channel for the
migrating solutions*
Much of the albite-ollgoolase shows
cloudy areas of epidote, olinozoisite, carbonate and seri­
cite leading to the conclusion that the few calcic oligoclase grains preserved are the remains of the once pre­
dominant feldspar.
Comments* - Most of the potash feldspar in this rock is
microcline, altering in places from orthoclase Indicating
a considerable period during which the rock was strained.
The length of this period of change is also attested by
the many veinlets cutting and replacing the previously
formed minerals*
About the only indication of subsequent
hydrothermal alteration of the normal Lovingston is the
24intimately associated apatite, sphene, and epidote.
The
first two of these minerals are evenly distributed
accessories in the hypersthene granodiorite.
This rock grades
into the hydrothermally altered Lovingston.
Hydrothermally Altered Lovingston
Definition.
The hydrothermally altered Lovingston is normal
Lovingston hydrothermally altered by and, in places,
heavily injected with granodiorite.
Occurrence. - This type of Lovingston occurs between the
small granodiorite intrusion of the Tobacco Row mountain
region and the large granodiorite intrusion of the Blue
Ridge mountains (See fig. 1).
The rock is extremely varied
in appearance and character and is cut by nelsonite and
pegmatite dikes.
Megascopic Character. - The hydrothermally altered Lovingston
is a black and white even granular to porphyritio rock
with the gnelssic texture weak but still discernible.
The
biotite is frequently concentrated in patches instead of
showing distinct banding.
Small dikes of aplitio grano-
dlorit© and pegmatite are extremely numerous, aiding in
the graniti2ation of the gneiss.
In places, the true
granodiorite is bodily injected into the hydrothermally
altered Lovingston (See fig. 6).
Feldspar, quartz, both
glassy white and blue, biotite, chlorite and amphibole
can be identified with a hand lens.
Figure 6. Aplitic granite dikes intruding the
hydrothermally altered Lovingston gneiss. Photo
taken six miles northwest of Amherst, on U. 3.
Route 60.
Microscopic Character, - Thin sections of this rook differ
from those of the less altered, normal Lovingston gneiss,
In the following ways (See fig# 7):
The biotite is
partially or entirely altered to chlorite.
It does not
form bands, but is concentrated in patches or disseminated
through the rock#
Ilmenlte has become very abundant and
clinozoisite, epidote, hornblende, actinolite, and sericite
are the dominant minerals in the rock.
In the vicinity of
the nelson!te dikes a colorless amphlbole near tremolit©
in composition sparingly occurs*
Zircon, sphene, ilmenite,
rutile, graphite, hematite and pyrite, although important
in ail the hydrothermally altered Lovingston, are increas­
ingly important near the nelsonites#
Since graphite cannot
be distinguished from ilmenite in thin sections, a heavy
mineral separation is necessary to determine Its presence#
Oligoelas© is partially or entirely replaced by alblte,
clinozoisite and sericite,
Alblte is pink in the vicinity
of pegmatites, and is identical in composition and appear­
ance with the alblte in these pegmatites#
Perthitic
microcline is present in varying amounts in all the slides
of this rock#
In several localities the perthlte stringers
are altered to clinozoisite and sericite,(paragonite(?)),
while the microcline is unaffected#
Veinlets of hematite,
graphite {?), and ilmenite, as well as veinlets of albite,
traverse the ragged and altered oligoclase grains.
quartz Is definitely of at least two generations.
The
The
original quartz of this Lovingston is non-rutilated, coarse
Figure 7# Photomicrograph of hydrothermally
altered Lovingston gneiss,
A. Fibrous amphibole
B, Epidote
C. Albite
D, Quartz,
Black minerals are ilmenite and lesser amounts
of hematite.
Polarized light, X25
-26grained and filled with sub-microscopic impurities.
The
younger quartz is fine grained, fresh, and rutilated. Crushed,
red and colorless garnets are sporadically abundant.
Comments. - The marked increase in secondary minerals
In this rook, as well as the striking difference in texture
and the other evidences of the proximity of a later
intrusive, serve to distinguish it from the normal Lovings­
ton gneiss.
The new minerals and textures developed are
similar to those observed In other areas, where they are
due to exomorphic effects associated with batholithic
intrusion.
Origin and Age of the Lovingston Gneiss. - The Lovingston
quartz monzonite gneiss was Intruded into quartz mica
schists of sedimentary origin.
Later dynamic regional
metamorphism, extending over a considerable time period,
transformed the rock to the Lovingston gneiss and
metamorphosed the quartz mica schist to the Lynchburg
gneiss.
After a period of erosion the hypersthene grano­
diorite and its derivitives were Intruded into the Lovings­
ton, and by exomorphie effects, formed the hydrothermally
altered Lovingston.
The Lovingston gneiss is the second oldest rock in the
area.
It intrudes the Lynchburg gneiss and is intruded
by the other roGks of the area.
7
-27Hynersthene granodiorite
Definition: - Hypersthene granodiorite is the name applied
24
by Jonas to the group of acid, pyroxene bearing intrusive
a
Geological Map of Virginia, 1928
rocks that comprise the cor© of the Blue Ridge mountains*
25
This rook was first described by Weed and Watson who
IB------------------------------------Weed, W, H. and Watson, Thomas: The Virginia Copper
Deposits, Eoon. Geol. Vol. 1, 1906, pp. 306-530,
called it a hypersthene syenite.
This and subsequent work
has led to the present concept of this rock (See chapter
"Previous Geologic Work)•
Occurrence. - The hypersthene granodiorite is widely exposed
in the Blue Ridge mountains in Virginia.
It is bounded
on the east by the Lovingston gneiss which it has intruded,
and on the west by the overlying lower Cambrian sediments.
It is overlain in many places by the Catoctin meta-basalt.
In the Amherst Quadrangle hypersthene granodiorite is
exposed in the Blue Ridge mountain sector, which comprises
the northwestern part of this Quadrangle.
Smaller bodies
of hypersthene granodiorite are exposed In the Lovingston
gneiss in the central part of Amherst County, five to eight
miles east of the main intrusive.
Tobacco Row mountain,
Jennings mountain, and Morley mountain as well as other smaller
-28
ones are composed wholly or essentially of hypersthene
granodiorite*
They form a northeast-southwest chain
through the hydrothermally altered Lovingston gneiss.
General Character* - The hypersthene granodiorite is, when
fresh, a dark, greenish gray, coarse grained rock.
On the
surface however it is weathered to a brown, spongy, pitted
mass which forms a crust up to six inches thick over the
fresh rock.
In many localities residual soil entirely
covers the rock (See Fig. 8).
Many local textural and mineralogical variations are
to be expected in a batholith as large as the hypersthene
granodiorite.
These have been recognized in other areas
where the intrusive has been studied, and it has been
locally subdivided into the Old Rag granite, Marshall
"7
granite, Mt* Airy granite and many other types.
In the Amherst Quadrangle the hypersthene granodiorite
shows segregations into several types with distinctive
characteristics.
There Is no sharp boundary between these
groups, as specimens showing gradational characteristics were
found. For descriptive purposes the granodiorite will be
divided into four types;
the coarse grained type of the
Blue Ridge sector is designated as Blue Ridge granodiorite;
the medium grained, gray granodiorite of the south central
portion of the quadrangle, typically exposed on Tobacco Row
mountain is called Tobacco Row hypersthene granodiorite; a
light colored feldspathic granodiorite of the north central
portion of the Quadrangle, typically exposed in the
Figure 8. Deep residual soil containing
weathered boulders of granodiorite. Photo
taken on State Highway 151 near intersection
with U. S. Route 60.
29vlcinlty of Cherry Hill is called leuoogranodiorite. In
addition to these types of the hypersthene granodiorite,
caused by segregation, zones of shearing have produced rocks
that vary from mylonites to chlorite phyllonites.
These
are discussed under the heading of sheared granodiorite.
Blue Ridge Granodiorite
Megascopic Oharaoter* - This type of granodiorite is the
largest area of the rock exposed In the Amherst area,
comprising the mountainous area in the northwestern part
of the Quadrangle.
It has a coarse granitic texture, with
occasional phenocrysts of feldspar.
It has a general gray
green color, but mineral fractures and boundaries are
coated with Iron oxide, which lends a reddish brown tint
to the entire rook.
Large crystals of bronze pyroxene,
gray green feldspar, bluish quartz sericite and green
chlorite are visible in the hand specimen, the t o t
two
minerals showing polished and striated faces.
Microscopic Character. - Thin sections show this rock to
have a coarse granitic texture, but with occasional
phenocrysts of feldspar.
It is composed principally of
potash and acid plagioclase feldspars, hypersthene, quartz,
apatite, Ilmenite, sphene, and zircon.
Secondary minerals
are sericite, chlorite, amphibole and some epidote.
Antiperthitic oligoclase is the dominant feldspar in the
rock, with perthitic microcline subordinate.
Calcic
-30-
oligoclase was probably even more abundant originally,
but in many cases it has been entirely replaced by albite,
epidote, sericite, and chlorite (See fig* 9).
Even the
plagloclase comprising the rodlike perthites has been
saussuritized*
Myrmekite, a vermicular intergrowth of
quartz and feldspar, is developed in the plagioelase.
In
many instances, the plagioelase of the myrmekite has been
replaced by epidote, chlorite and sericite so that the
quartz appears to b© intergrown with these minerals*
Orthoclase and microcline have been broken and the
fractures filled with chlorite, sericite, and limonite*
The hypersthene Is partially or entirely altered to uralit©
and chlorite*
The quartz grains are Irregular in shape
and size and usually show strain, except when replacing the
26
feldspars. There It occurs as rounded "droplety
or
26
This term has been suggested by Dr. E. P. Wheeler from
small grains of quartz embedded in feldspar and apparent­
ly of corrosive origin, which he found in an adamellite
series associated with the anorthosite intrusive near
Nain, Labrador.
corrosion, quartz.
Large fractured grains of apatite,
partially altered to osteolite are associated with ilmenite,
leuooxene and zircon*
Ilmenite, similar to apatite in grain
size and amount, occurs associated with the latter mineral,
sphene, sericite, zircon, and chlorite.
Figure 9. Photomicrograph of Blue Ridge
granodiorite.
A. Large orthoelase crystal showing droplet
corrosion quartz in extreme lower right.
B. Apatite grains.
C. Vermicular quartz in sericite and chlorite,
which has replaced original oligoclase. Matrix
is chiefly sericite and chlorite.
Polarized light. X25
-31Qomments • -
The coarse texture of this rock and the
reddish green color serve to distinguish it from the
other types of granodiorite in the area.
The replacement
of original feldspar and hypersthene by secondary minerals
indicates considerable activity, probably deuteric, since
emplacement and crystallization.
Tobacco Bow Hypersthene Granodiorite
Megascopic Character. - This rock is a dark gray, medium
grained, even granular to porphyritic rock, which weathers
to the characteristic spongy, pitted crust.
Gray green to
pink feldspar, bluish quartz, and lesser amounts of mica,
pyroxene, and ilmenite can be seen with the aid of a
hand lens*
Bands of green epidote cut the rock in places.
In areas of intimate association with the hydrothermally
altered Lovingston, the excessive development of biotite
in the latter is the only mineral which serves
in the
field to distinguish the two rocks.
Microscopic Character . - Slides of this rook (Fig. 10)
show alkalic plagioelase, potash feldspar, hypersthene,
clinopyroxene, quartz, and locally apatite to be essential
minerals.
Ilmenite, rutile, zircon, apatite, and in some
places garnet and pyrlte are accessory.
The phenocrysts,
when present, are essentially orthoelase and microcline.
However, the dominant feldspar is albite, which has altered
from antiperthitic oligoclase*
In some localities none of
Figure 10. Photomicrograph of Tobacco Row
granodiorite.
A. Antiperthitic albite with included
clinozoisite grains.
B. Apatite grains and C - quartz.
Black mineral is ilmenite.
Polarized light. X25
-32-
this original plagioelase remains, in others there has been
but little alteration of the calcic oligoolase.
The difference in mineral composition between this
rook and the Blue Ridge granodiorite is due to a difference
in the types of alteration.
In this type there is a very
marked Increase in the amount of albite.
The oligoclase,
instead of being replaced by chlorite and sericite, is
altered to alblte and epidote, which in turn are replaced
by lobate, corrosion quartz.
The pyroxene content is
relatively high, and shows all stages of alteration, from
rims of uralite around fresh hypersthene* to a complete
replacement of hypersthene by uralite and bastite, which
is in turn altered to chlorite.
There are other evidences
of replacement as indicated by replacement veinlets of
alblte, chlorite, and serioite traversing the groundmass
of antipex’thitic oligoclase and quartz and the perthitic
phenocrysts of microcline.
Myrmekite and a peculiar
vermicular intergrowth of quartz and garnet also indicate
later solution action.
Comments. - There is no evidence in this type of granodiorite
of any post-eonsoli&ation movement between the Individual
grains similar to that seen in the peripheral fracturing of
the grains of the Blue Ridge granodiorite.
Where the rock
has been fractured, the breaks, filled with limonite and
mica, out through the grains indescriminately.
The Tobacco Row granodiorite is very intimately associ-
-35-
ated with the intruded Lovingston gneiss.
At Tobacco How
mountain the distinction between the two rocks is clearly
seen.
However, in the center of the Quadrangle in the
vicinity of Morley Mountain, the rocks form a granodiorite
Lovingston complex (Bee fig, 11),
In mapping this complex,
areas in which the granodiorite comprises relatively large
masses, dominant over the surrounding hydrothermally
altered Lovingston, were mapped as Tobacco Row granodiorite.
In the regions in which mappable areas of this granodiorite
are not exposed on the surface but occurs as small dikellke bodies of granodiorite, derived granite, and pegmatite,
the area was mapped as
hydrothermally altered Lovingston
gneiss (See fig, 1),
Cherry Hill Leucogranodiorite
This rock type occurs as many small segregation in the
granodiorite.
The largest and most clearly defined of these
bodies Is found in the vicinity of Cherry Hill School,
about three miles south of the anorthosite.
Megascopic Character, - This rock Is distinguished in the
field primarily by its light brown to pink color, its
partial granulation, and its light-colored residual soil,
which is composed of incompletely weathered particles derived
from the partially crushed parent rock.
Frequently, limonite,
altered from garnet, has dotted the rock with reddish brown
specks.
It Is medium to coarse grained, even granular to
Figure 11, Granodiorite bodily injected into
altered Lovingston, forming a Lovingstongranodiorite complex. Photo taken on U. S.
Route 60, six miles northwest of Amherst.
-34porphyritic (See fig. 12).
Visible minerals are chalky to
pink feldspars, quartz, frequently colored blue by rutile
needles, and sporadically occurring epidote, mica, limonito,
pyroxene and crushed garnets.
Mioroscopio Character. - Chief minerals of this type of
granodiorit© are feldspar and quartz.
Perthitic mierocline
and orthoolase are dominant, with antiperthitio oligoolase
and albit© subordinate.
Crushed garnets, muscovite,
epidote, clinozoisite, biotite, hornblende, hypersthene,
large and abundant apatite, magnetite, zircon, sericite,
sphene, olinopyroxene and rutile are locally present,
usually in small amounts (See fig. 13).
Garnets, which are abundant locally, show a peculiar
vermicular Intergrowth with quartz.
crushed and altered to limonite.
They are generally
Plagioolase, and to a
lesser extent potash feldspar, are filled with epidote,
muscovite and biotite and surrounded by bands of sericite
and epidote.
Oligoolase and orthoolase, as well as one
variety of quartz, also show crushing.
The other variety
of quartz is in large, well defined grains, always associa­
ted with muscovite, epidote and biotite.
The albite of
the perthitic mierocline is altered to sericite and epidote.
All the feldspars except the albite show corrosion by droplet
quartz.
Ilmenite, apatite, and albite frequently are of
hug© sizes, and include biotite, zircon, epidote, and microclina,
Figure 12* Photomicrograph of leucogranodiorite.
A. Large individual epidote grain.
B. Large ragged quartz grain.
C. Small quartz and albite crystals with
scattered, fine grained epidote.
D. Large albite crystal.
Large black mineral is rutile.
Polarized light. X25
Figure 13. Photomicrograph of leucogranodiorite
showing ilmenite and epidote in a matrix of
quartz, albite and sericite.
Polarized light. X25
-35-
ComentSe - The similarity of mineral composition and alter­
ation with, and the gradation of these Irregular shaped
masses into the granodiorit®, show them to be definitely re­
lated to that rook*
It was probably originally composed
of orthoolase phenocrysts in a matrix of andesine-oligoclase,
quartz, garnet, and pyroxene.
Crushing and hydrothermal
metamorphism have given the rock its present form and
composition.
Sheared Granodiorite
numerous shear zones cutting the granodiorite bodies
have given rise to a fine grained, crushed rock which,
in the zones of greatest differential movements, resembles
very closely chlorite sericite schist or phyllite formed
by progressive metamorphism.
However, gradations can be
traced from the schist to a mylonite, to partially granu­
lated granodiorite, and to normal granodiorite in which
the individual mineral grains show strain.
These sheared areas are seldom over 100 feet wide and
o
strike in a general N. 40 £• direction. The two most
accessible localities to observe these sheared zones are on
0. S. Route 60, eight miles northwest of Amherst near Swan
Hill School, and about one mile east of the top of Long
Mountain on 0. S. Route 60.
Traced from the least affected to the most intensely
altered rocks in the sheared zones, the changes from the
coarse grained granodiorite are as follows:
In the
36partly granulated rook, the handapecimen shows polished
and striated mineral surfaces which are coated with sericite.
In thin section* portions of the orthoolase phenoorysts
have been crushed and drawn out,
The large quartz grains are
strained, and their edges granulated, and the small crushed
gragments are filled with dust-like grains of hematite and
ilmenite.
Chlorite and sericite fill the space between
these fragments*
Albitization has proceeded farther here
and essentially all of the plagioclase feldspar is albite,
the biotite content has increased, and pyroxene is relative­
ly rare.
Stringers of sericite, chlorite, and hematite
replace the broken parts of the feldspar phenoorysts.
In the next stage the rock Is distinctly porphyritio
and has a decided green color*
The feldspar phenoorysts
are replaced by veinlets of ilmenite, sericite, and talc(?)*
These minerals also replace original ilmenite and apatite,
but not the fine grained quartz comprising the matrix.
A
relatively large amount of sphene and leucoxene la associa­
ted with this fine grained, later quartz.
Recrystallization
under the stress of dynamic metamorphism has evidently
yielded the fine grained quartz matrix at this stage of
intensity.
A green myIonite, cut by later pink pegmatitic stringers
results from more Intense conditions.
zation characterizes this stage.
grains are bent or broken.
Widespread s e r i a l i ­
The sericitized albite
The rutilated quartz grains
-37form a mosaic.
The pyroxene is entirely altered to
chlorite, and ilmenite is completely replaced by sphene
and leucoxene.
The pink albite stringers are altered
to sericite and epidote.
In the final, most intense stage, a new rock was
formed from the broken constituents of the granodiorite.
This rock is a banded chlorite sericite phyllonite, with
distinct, cleavage at angles to the banding.
The rock is
composed of extremely fine grained altering bands of
quartz and chlorite.
Sericite, epidote and talc(?) occur
abundantly in the chlorite bands.
Porphyroblasts of
chlorite, altered from original pyroxene, and pyrlte are
occasionally seen.
Comments. - There appears to have been alteration in these
rocks other than that resulting from dynamic aetamorphism.
Hydrothermal solutions, rising along the fault planes,
has aided in the alteration of these rocks.
The origin
of these shear-zones will be discussed under the chapter
on Structure,
Age and Relations of Hypersthene Granodiorite. - The
hypersthene granodiorite Is definitely younger than the
Lovingston gneiss which it intrudes, and Is a member of the
younger injection complex.
It is intruded by only the
diabase dikes of Triassic age.
It shows segregation into
a feldspathic type which is but an intermediate type between
true-granodiorite and the anorthosite.
The small grano-
38-
diorite bodies (Tobacco Row type) are the exposed parts
of the roof of the intrusion, which hers has assumed
a cupola shape.
Anorthosite
Definition and General Character. - The rock name
Anorthosite was applied by Ross
17
to a body of white
'
!
Ross, 0. S.: Mineralization of the Virginia Titanium
Deposits. Am. Min., Vol. SI, Ho. 3, 1936 pp.143-146.
feldspathic rock occurring chiefly in Helson County, Virginia,
This rock had been previously described as a syenite by
Watson and an albitite by Jonas.
-
—
-
--------------------------------
See under Watson and Jonas in previous work.
Although disagreeing on name and paragenesis, the
descriptions of Watson and Ross agree that the anorthosite
In Helson County is a coarse grained to granulated, gray
to white rook composed essentially of plagioclase feldspar,
ranging from andesine to albite, and minor amounts of
potash feldspar and blue quartz.
Rutile, ilmenite, and
apatite are locally abundant, and albite, zoisite, clinozoisite, sericite, and chlorite are the chief secondary
minerals.
-39The rook mass becomes decidedly hornblendic towards the
border*
28
According to Johannsea ,
"an anorthosite is a rock of
28
'
'
"
Johannsen, A#: A Descriptive Petrography of Igneous
Bocks, Vol. 3, P. 196,
the gabhro family essentially free from dark components and
composed essentially of labradorite or bytownite.
Andesinites or oligoclasites are not to be considered
29
anorthosites*w However, Grout would also call these latter
15-------------------------------------------------Grout, F. F.s Petrography and Petrology, McGraw Hill
Book Company, Inc* 1932 p. 50 and p p *256-257*
rocks anorthosites*
If we follow Johannsen, the feldspars of the Nelson
County rook are too acid for the name anorthosite, which
Implies derivation from a gabbro magma.
Since the
leucocratic minerals, including quartz, are essentially
the same ,in the area studied by the writer, as those of
the hypersthene granodiorite, this rock would more
appropriately be called here a leucogranodiorlte.
Nevertheless, the author will follow the pravelent termin­
ology and call the rock anorthosite.
Anorthosite occupies a narrow belt, which, because of
ease and uniformity of weathering, is level and low.
The
grayish white residual soil, which covers the area to a
maximum depth of 60 feet, is colored red in the vicinity
-40of the nelsonite dikes.
Fresh rock is rarely exposed on
the surface.
Occurrence. - The anorthosite of the Amherst Quadrangle ,
which is the southeast portion of the main anorthosite
body, comprises that part of Nelson County included in
the Amherst Quadrangle and extends two miles into Amherst
County.
The body as a whole is roughly elliptical in
shape, extends for approximately 14 miles in a northeastsouthwest direction, has a maximum width of two and one
half miles, and includes a total area of about 25 square
miles.
The southern most one-fourth of the body lies in
the area mapped.
Megascopic Character. - The anorthosite is a white to gray
granulated rock, composed essentially of feldspar and
quartz.
The quart2 is usually colored blue^ and the white
feldspars show the greatest amount of granulation.
Biotite,
pink feldspars, and epidote can occasionally be recognized.
Microscopic Character. - Thin sections of this rook conform
30
essentially with the descriptions given by Ross .
35
|
It
“
Ross, C. S. : Titanium Deposits of Roseland District.
Internat. Geol. Cong. Guidebook 11, Northern Va., 1933,
pp 29-36.
is composed essentially unbroken, ragged antiperthio grains
of calcic oligoolase, replaced by untwinned albite.
This
calcic oligoolase shows both the replacement vein anti-
-41-
perthite and the plate, exsolution antiperthite. Secondary
quartz, in the form of vermicular replacement and droplet,
or corrosion, quartz is extensively replacing the
oligoolase and to some extent the later albite.
Apatite,
ilmenite, zoisite, sericite, muscovite, and sphene are
usually associated with the albite, although in some slides
radiating aeicular zoisite is abundantly replacing all
the feldspar*
Mierocline is locally abundant as phenoorysts
in a fine grained matrix of quartz, mierocline, albite,and
zoisite (See fig. 14).
31
Origin and Age of the Anorthosite*-
IT
Batson and Taber
1
Op* Git* pp. 68-91*
concluded that this rpok could not be a pegmatite, but was
a syenltio differentiate of the common magma which gave
rise to the main rook types of the area, and, as such, was
32
the same age as the surrounding Lovingston gneiss* Eoss
Op* cit. - Titanium Deposits of Roseland District, p. 30.
in his work on anorthosite, suggested a pegmatltie origin
33
formerly* but, in his later paper, he concluded that the
is
1
Op. cit* - Mineralization of Virginia Titanium Deposits,
pp. 143-146.
Figure 14* Photomicrograph of anorthosite.
At A - and in lower right of photo is antiperthitic oligoclase-albite. B - Mierocline,
replaced by chessboard albite at (x). Creplacement quartz. Dark grain is ilmenite
surrounded by epidote and sphene.
Polarized light. X25
anorthosite was introduced as a mush of crystals*
The
injection zone surrounding the body, which Watson and
34
Taber had called a gabbro, Ross interpreted as residual
54
~~
Op* Cit* p* 79 and pp* 91-94.
magma forced out of the spaces between grains of the partly
crystallized rock by filter press action during intrusion
and consolidation*
The contact between the anorthositic body and the
surrounding Lovingston gneiss was noted by the other workers
to be a gradational one*
Watsons and Taber's (See previous
paragraph) conclusion that this border area was originally
gabbro, which has been so altered as to resemble a
granodiorite, and Ross' opinion that the border rocks were
solidified from residual magma forced from the anorthosite
in filter pressing, both Indicate that on its edges the body
is not an anorthosite.
This rook contains primary and
secondary feldspars identical to those of the true anorthosite and, in addition, hypersthene, amphibole, chlorite, and
biotite•
About five miles of this contact zone was mapped in
Amherst County, and at ©very locality where residual decay
had not entirely destroyed the rooks, a band of altered rock,
varying in width from about 100 feet to three-fourths of
a mile was found between the anorthosite and the Lovingston*
43-
The lithology and mineral content of this rock are
almost identical to altered granodiorite from other
parts of the area#
The anorthosite body is separated from the small
granodiorite (Cherry Hill leucogranodiorite) area by only
about a mile of highly altered Lovingston gneiss#
This
gneiss is veined by small pegmatites which originated in
the granodiorite#
A study of the small amount of anorthosite exposed in
the Amherst Quadrangle may not warrant an interpretation
of origin for the entire body, but the following facts
appear pertinent: (1) the anorthosite in this area grades
on its edges into granodiorite# (S) True anorthosite was
not found directly in contact with the Lovingston gneiss.
The granodiorite rim mentioned in (1) above always lies
between these two rocks. (3) The original feldspar of the
anorthosite is so similar to that of the granodiorite, in
character, types of antiperthite, perthite, and alteration,
that a qualitative distinction between them cannot be made.
(4) Blebs of droplet, corrosion quartz, common in the
feldspars of both the anorthosite and the granodiorite, is
not known to ocour, to the w r i t e r s knowledge, in rocks
other than granodiorites and those derived from that clan.
(5) The secondary minerals of the anorthosite are similar
to those universally found in the granodiorite. (6) Other
44smaller leuoogranodiorite segregation, apparently
intermediate in ferromagnesian content between typical
granodiorite and typical anorthosite, and possessing
feldspars of type and composition similar to both, occur
in other parts of the area,
the above facts seem to point to magmatie segregation
of the granodiorite as the origin of the anorthosite rather
than to a separate intrusion,
This may have been accomplished
by a floating up of the constituent minerals, in a manner
35
somewhat similar to that suggested by Grout, The feldspars,
.gg
.
Grout, F. F.: Petrography and Petrology, 1st. Ed, p,
857, 1938.
being among the first minerals to crystallize from the
consolidating granodiorite magma, floated to the top of the
magma chamber.
The mass was at this stage further intruded
producing granulation of the feldspar crystals.
Similar
but less intense granulation was observed in the leuoo­
granodiorite and the Blue Ridge granodiorite.
The
derivation of this anorthositio rook from the granodiorite
36
magma has been suggested by Jonas, because of the intimate
m ----------------------------------------------------------------
Jonas, A. I.s Hypersthene Granodiorite in Virginia.
Geol. Soc. America* Vol, 46, 1935, pp 54-95.
association of the two rocks.
The anorthositio body is contemporaneous in age with
-45the hypersthen© granodiorite, although it must represent
a relatively early stage in the solidification of that body,
sinoe it is cut by nelsonite and pegmatite dikes, both of
which were derived from the granodiorite magma.
Pegmatite and Granite Dikes
Pegmatite dikes, ranging in width from a few inches
to ten or more feet, cut all the other rocks of the
area except the diabase, but are especially numerous in
the hydrothermally altered Lovingston.
The abundance of
these dikes has been a large factor in influencing previous
workers to oajLss this Lovingston as an injection complex.
Granite dikes are differentiated from the pegmatites
in the field only by their finer texture and grayish color.
Pegmatite; -
These rocks are coarse grained in texture
and whit© to pink in color and are composed essentially
of feldspar and quartz, with smaller amounts of biotite
and garnet.
Thin sections show the rock to be oomposed
chiefly of perthitic mierocline, orthoolase, secondary
albite, rutilated quartz, crushed garnets and large
apatite grains with lesser amounts of the secondary
minerals, zoisite, non-rutilated quartz, biotite, sericite,
hematite, sphene, chlorite, and graphite(?).
The potash
feldspars and rutilated quartz are being replaced by the
secondary minerals of which albite was first and quartz
last.
Small discontinuous veinlets of biotite, clinozoisite,
-46hematit© and graphite(?) frequently terminate in larger
masses of epidote.
Myrmekit© is developed also.
Granite Dikes: - These rocks are of medium to coarse, even
granular aplitlc texture, and are composed essentially of
gray to pink feldspar, quartz, and garnet with minor amounts
of biotite#
A thin section of this rock showed albite
and fine grained quartz to be the most abundant minerals.
They are replacing mierocline, garnet, and an earlier
quartz, and biotite.
Associated with and frequently
replacing this secondary albite are epidote, clinozoisite,
sphene, graphite, and rounded corrosion quartz.
The
biotite and muscovite laths as well as the albite twins
are bent, and the garnet Is crushed and granulated (See fig.
15).
Origin; - The granite dikes probably were originally
composed of minerals of the same bulk-composition as the
leuoogranodiorite.
fudging from their alteration by the
pegmatitic solutions, it is probable that these aplltic
granite dikes were given off from the parent granodiorite
previous to or contemporaneously with the pegmatite dikes
and were later altered by the same sodic solutions which
altered the pegmatite dikes.
The pegmatite dikes occur in abundance only in the
hydrothermally altered Lovingston, but the granite dikes
were frequently noted to occur in anorthosite, leuoo­
granodiorite, and true granodiorite.
Figure 15. Photomicrograph of aplitic granite
Park material at top of photo is epidote.
A - quartz replacing perthitic mierocline.
B - antiperthitic oligoclase-albite.
Bl&ck mineral is ilmenite.
Polarized light. X70
-47BASXC IGNEOUS HOCKS
General statement* - Basic rocks occur in the Amherst
Quadrangle as dike a, small intrusive bodies and altered
flows, and are divided into four general rock types.
They consist of hornblende gneiss, hornblende metagabbro,
metapyroxenite, and peridotite intrusive bodies and dikes,
greenstone flows, and nelsonite and diabase dikes.
Hornblende Gneiss
Hornblende gneiss was found in the Lovingston gneiss
on U, S. Route 60, one and one-half miles east of Amherst*
The boundaries of the body could not be established
accurately, because of the peculiar structure of the adjacent
Lovingston gneiss.
Megascopic Character, - This is a porphyritic, black and
white rock, composed essentially of unbroken, bladed
hornblende In a groundmass of granulated quartz and feldspar,
A rough gneissic texture is apparent.
Microscopic Character. - Dark green, highly pleochroic
hornblende comprises over one half of the rock.
Potash
and plagioelase feldspar filled with quartz inclusions,
biotite, rutile, apatite, sphene, epidote, zircon and
quartz are the remainder of the minerals present.
Biotite,
corroded quartz and feldspar are replaced by sphene, rutile,
and epidote,
slide*
Veinlets of coarse grained quartz cut the
-4 8 -
In the Lovingston gneiss adjacent to the hornblende
gneiss, hornblende porphyroblasts have been altered
to biotite and chlorite.
The original oligoolase has been
destroyed by sausseritization, with the formation of
albite, epidote, and sericite.
replacing plagioelase.
Mierocline was also observed
Apatite altered to osteollte, epidote,
zircon, biotite, and chlorite are Intimately associated.
Origin and Age. - Hornblende gneiss has been found by
Furcron
to intrude the Lynchburg gneiss, and by the
19
O p . cit• p . 40•
m
writer
to intrude the Wissahiokon schist.
In the Lovingston
.g_
Staurolite Belts of Patrick and Henry Counties, Virginia.
Unpublished part of Master’s Thesis, li. Va, 1937, p. S4.
gneiss and the other formations, it shows evidence of
subjection to the metamorphic effects which altered those
rocks.
It was probably a dike-like intrusive into the
Lynchburg gneiss, which was later altered and incorporated
in the intruding Lovingston quartz monzonite, and would
therefore be the oldest igneous rock in the Amherst Quadrangle.
39
Furcron suggests that the gneiss may represent metamorphosed
39
Op. cit. p. 40
flows or sills in the Lynchburg gneiss.
This would not
-49
explain its dike-like appearance in the Wissahiokon schist
and the Lovingston gneiss.
Hornblende Metagabbro
General Statement: -
The hornblende setagabbro occurs as
small stock-like masses and dikes intruding the Lynchburg
gneiss, and as narrow dikes in the Lovingston gneiss. The
stocks are aligned in a general northeast direction and
frequently terminate in dikes (See fig. 1).
Megaacopio Character. - The hornblende gabbro is generally
a light to dark green, medium grained, even granular rook*
In the stocks the texture varies from coarse porphyritic
in the center to very fine in the border zone.
Green
fibrous hornblende, black biotite, white feldspar, and some
quartz can be identified with the aid of a hand lens.
In
the fine grained border facies only green amphibole can be
seen.
Microscopic Character. - Thin sections of this rook vary
from a coarse grained porphyritic to a fine grained
porphyritic texture.
The principal minerals are amphibole
(Hornblende and uralite), biotite, plagioelase (andesine),
zircon, epidote, rutile, quartz, and apatite.
The horn­
blende Is altered to biotite and chlorite and the feldspars
are sausseritized.
Rutile, quartz, apatite, and epidote,
replace the other minerals and appear to be later.
Magnetite and pyrite are looally relatively abundant.
-5039
Burfoot,
who has studied in detail these basic
39
Burfoot, *T. D . J’r.; The Origin of Tale and Soapstone
Deposits in Virginia. Icon. Geology, Vol. 25, No. 8,
1930, pp.805-26.
rocks, In the soapstone areas of Virginia, concluded that
the sequence of mineralization of the metagabbros is as
follows i
Primary crystallization of a rook composed
essentially of pyroxene, and basic feldspar with some biotite.
Beuterlc solutions first altered the pyroxene to uralite
and sausseritized the feldspar and later altered the uralite
and biotite to chlorite.
During the succeeding hydrothermal
stage, talc, carbonate, magnetite, pyrite, and secondary
quartz replaced the earlier minerals.
This sequence applies to the metagabbros of the
Amherst Quadrangle, with the exception of the hydrothermal
minerals.
Here the composition of the solutions seems to
have been favorable for the formation of sericite and
rutile, rather than talc and considerable carbonates.
Origin and Age. *■ The hornblende metagabbro appears, from
Its composition, to be a more feldspathic differentiate of
the magma which gave rise to peridotite and metapyroxenlte.
Hornblende metagabbro dikes Intrude the Lynchburg and
the Lovingston, frequently cutting across the contact of
the two formations and thus are younger than those rooks.
At no locality in the Blue Ridge province of Virginia, to
-51the writer's knowledge, have metagabbro dikes been found
intruding the hypersthene granodiorite, although dikes
In the adjacent Lovingston gneiss are common#
Feri&otlte and Metapyroxenite
General Statement # - A peridotlte body intrudes the
Lynchburg gneiss In the extreme southeastern corner of
the Amherst Quadrangle.
with the Lynchburg.
A fault forms its western contact
Metapyroxenite dikes, differentiates
of the same magma that gave rise to the peridotites,
sparingly intrude the Lynchburg and the Lovingston.
Peridotlte
Megascopic Character. - The peridotlte is a hard, greenish
black, medium grained, even granular to porphyritic rock.
Visible minerals are olivine, amphibole, mica,and quartz.
Microscopic Character. - The microscope shows the rock to
have a porphyritic texture, with the phenoorysts composed
of olivine in various stages of alteration, amphibole, and
an occasional altered pyroxene (See fig. 16).
The
groundmass is composed in some slides of small crystals
of olivine, colorless amphibole, and chlorite altered
from biotite, In others almost entirely of a mineral
whose properties place It Intermediate between common
chlorite and antigorlte.
There are two types of amphiboles
developed, - ordinary hornblende, and a colorless variety
Figure 16. Photomicrograph of Peridotite.
Phenocrysts are olivine, in a fine grained
matrix of oliving and chlorite. Small
veinlets of carbonate can be seen in upper
right.
Polarized light. X25.
52whose indices of refraction place it midway between
hornblende and tremolite.
Definitely later minerals are
tremolite, carbonate, pyrite, magnetite, and talc*
The
quantity of tale developed Is too small for positive
identification.
Metanyroxenlte
Megascopic Character* - Metapyroxenit© is a coarse grained,
green rock with hornblende, biotite, and chlorite visible
in the handspecimen.
Microscopic Character. - In thin sections, this rock
differs from the peridotlte chiefly in that it has no
olivine, but does have variable amounts of plagioelase
near anddsin© in composition.
Amphibole between hornblende
and tremolite, actinolit© and chlorite, are in excess of
tremolite, epidote, olinozoisite, sphene and quartz, which
are replacing the amphibole® and feldspars*
There Is
magnetite but no apatite in the rock*
40
Burfoot’s Idea of mineral paragenesis for these rocks
13
Op. cit. pp. 810-816.
is as follows:
The crystallization of an original rock
probably composed essentially of olivine, aluminous and
aon-aluminous pyroxene, biotite and perhaps very minor
amounts of basic feldspar.
Deuteric solutions, acting
53on these minerals, first altered the pyroxenes to
aluminous and non-aluminous amphiboles and introduced
more biotite*
Later these solutions altered the
aluminous amphibole and the biotite to chlorite, and then
the olivine and non-aluminous amphibole to serpentine
and magnetite.
During a later hydrothermal stage,
cooling solutions in more local areas deposited tremolite,
and then replacement talc, carbonates, magnetite, and
pyrite.
In the Amherst County peridotlte and pyroxenites,
the deuterle and hydrothermal conditions were much less
intense than In those roclcs described by Burfoot.
As
in the metagabbros, the hydrothermal solutions, when
present, developed, clinozoislte, tremolite, rutile
and quartz, with minor amounts of talc and carbonates.
Age. - These rocks are of essentially the same age as
the hornblende metagabbro.
They are younger than the
Lovingston gneiss and Lynchburg gneiss, but older than
the hypersthene granodiorite.
Near St. Marys Church
in the northeastern part of the Amherst Quadrangle, a
metapyroxenite dike oan be traced in the hydrothermally
altered Lovingston to Its contact with the leuoogranodiorite.
Greenstone
Occurrence and Distribution. - Greenstone overlies the
hypersthene granodiorite at various localities throughout
the Blue Ridge province of Virginia,
In northern
Virginia, the formation is schistose and was named the
41
Catoctin schist by Keith, from Catoctin Mountain,
n -----------------------------------------------------------
Keith, Arthur - Geology of the Catoctin Belt, TJ# S.
Geol. Survey, 14 Ann, Report, pt, 2, pp.293-294,1894,
Maryland,
42
fhe rock is described on the state map
4tZ 'r" .....
as
~ ._
Geologic Map of Virginia, 1928.
consisting essentially of metabasalt lava flows and
volcanios altered to amygdaloldal and schistose epidotechlorite amphlbolite.
The greenstone, or metabasalt is exposed for about
a mile on TJ. S. Route 60 at the extreme western edge of
the quadrangle, overlying the Blue Ridge hypersthene
granodiorite.
Megascopic Character. - The metabasalt is a dense, green
amygdaloldal altered lava low, consisting essentially
of epidote, chlorite ana quartz.
purple by iron.
Fractures are stained
Epidote, rimmed by chalcedony or pink
albite fills the amygdules.
Irregular fragments of
Jasper locally occur.
Microscopic Character • - Thin sections of the rock show
-55a distinct ophitie texture*
However, the interstices
between the basic plagloolase laths are filled with chlorite
rather than original pyroxene*
Ragged phenocrysts of
aaphibole are being replaced by chlorite*
Rpidote,
magnetite (or ilmenite), and hematite are replacing the
labradorlte and amphlbole, and epidote and albite occur
as amygdule fillings#
The albite, epidote, magnetite
and hematite were apparently formed by later hydrothermal
alteration of the rook (see fig. 17}*
Origin and Age * - The Catoctin metabasalt was formed
as a basic extrusive flow.
Hydrothermal alteration
changed the rock to a dense greenstone*
43
44
Keith and Purcron marshall evidence to prove that
43
Op. cit. pp. 314-318,1892.
Purcron, A* 3. - Igneous rooks of the Shenandoah
Rational Park Area, Jour, of Geology, Vol. 52, pp.
405-440,1934.
the Catoctin metabasalt is older than the hypersthene
granodiorite and related rocks and has been intruded by
45
46
them* Jonas, and Jonas and Stose, on the other hand,
73
Jonas, A. X. - Hypersthene Granodiorite in Virginia.
Bui, Geol. Soo. America, Vol. 46., 1935, pp.47-59.
46
Jonas, A* I. and Stose, G. W.: Op. cit. pp.580-582.
Figure 17. Photomicrograph of Catoctin
metabasalt, showing an epidote filled
amygule at A. Small, light colored
crystals are labrodorite in a matrix of
iron stained chlorite.
Polarized light. £25
-56
conclude that the metabasalt was poured out on the
granodiorite.
The amount of greenstone in the Amherst Quadrangle
is too small to prove the question of age relations, but
the following suggestions appear significant: 1) The
metabasalt of the Amherst Quadrangle appears to be
similar in composition to the hornblende metag&bbro,
metapyroxenite and peridotite, and hence should belong
to the period of basic intrustion. 2) There has been no
evidence of intense hydrothermal activity in this area
subsequent to the intrusion of the granodiorite and
formation of the hydrothermally altered Lovingston.
3) Epidote and albite are the dominant hydrothermal
minerals in this lovingston, and they were the chief
minerals'formed during the hydrothermal alteration of
the metabasalt.
Helsonite Dikes
47
Nelsonite is the name given by Watson
to a group
?7
Watson, Thos. L. - Mineral Resources of Virginia, 1907,
p. 300.
of high titanium-phosphate bearing rocks of igneous origin.
The name was suggested to cover those bodies occurring
chiefly in Helson County, Virginia, and composed essentia48
lly of ilmenite and apatite. ?/atson and Taber later
-57-
48
Op. cit. p. XOO
expanded the usage of the term "nelsonite" to include
many varietal forms of the roek, depending on its
mineral composition.
These varieties are (1) ilmenite
nelsonite, (B) rutile nelsonite,(3) magnetite nelsonite,
(4) biotite nelsonite, (5) hornblende nelsonite, and
(6) gabbro nelsonite.
In the Amherst quadrangle, the nelsonite occurs
4
as intrusive© In the hydrothermally altered Lovingston,
the anorthosite, and the leuoogranodiorite.
None of
these bodies were found in the true granodiorite.
The mineral content of the nelsonites varies
considerably.
Those dikes exposed on the surface consist
essentially of a skeleton frame work of ilmenite and
apatite, with minor amounts of amphibole, biotite, and
quarts, whereas specimens taken from below the weathering
ssone show, in addition, chlorite, sphene, magnetite,
rutile, sericite and quarts.
There have been several theories of origin advance^
for these rocks, but since the nelsonites are the most
important economic asset of the Amherst Quadrangle,
theories of origin, as well as detailed descriptions of
occurrences will be taken up in the chapter on Economic
Geology.
58Dlabase Dikes
Long narrow diabase dikes are common as intrusives
in the rocks in the Piedmont and Blue Ridge provinces
of Virginia.
These dikes show no genetic relation to
the other Igneous rooks of the region.
They vary widely
in strike, but have a uniformly vertical dip.
Since
they intrude all the other rooks of the area, they are
the youngest rocks of the Amherst Quadrangle, and are
generally considered to be of Triassic age.
The dikes seldom exceed five feet in width, but are
easily traced on the surface, because of the rounded
boulders formed by weathering.
Megascopic Character • - Diabase is a medium grained,
ophitic, dark gray rock.
Dark green pyroxene and
white feldspar uniformly distributed give the rook Its
black and white color.
Microscopic Character. - Since this rook is so well known
in the Piedmont of Virginia, no thin sections were made
of the few small diabase dikes found in this quadrangle.
49
Dikes described from the adjacent Tames River area,
49
—
.....
Pureron, A. S. James River Iron and Marble Belt, Virginia.
Va. 0eol. Survey. Bull. 39, 1935, p. 51.
show little variation from the usual composition of
labradorite, augite, magnetite, and quartz.
Chlorite
and biotite are altered from augite, but the rock as a
whole shows very little alteration.
-
59-
S f B U C I U E S
The structures of this area are of two general
types, those which result from Igneous intrusion and
those caused by external stresses,
IG&IOUS STRUCTURE*
Structures associated with Igneous intrusion have
recently received much attention, for it has been shown
that a detailed study of these features sheds considerable
light on the manner of intrusion,
H, Cloos, 35. Cloos,
Balic, Mayo and others (see bibliography) have placed
much significance on structures found on the edges and
the roofs of intrusives, as well as in the associated
country rock.
The oldest structural feature found in the area
is a band of mylonitized rock, with a maximum width of
one hundred feet, at the contact of the Lovingston
gneiss with the Lynchburg gneiss.
This zone is Lovingston
in which the porphyroblasts have been drawn out and the
quartz granulated until it posses a distinctly schistose
o
texture. The schistosity varies in strike from N. 55
o
E, to H. 55 S., but always is parallel to the LynchburgLovingston contact.
The dip is always nearly vertical,
o
the lowest measured being 70 s. E. The mylonitization
is most intense at the contact, where the rock spalls off
-60in layers, and it gradually decreases in intensity
until it grades into normal Lovingston gneiss.
At one
locality poorly developed gneiss bands were observed
to cut this schistosity at a large angle.
The Lynchburg
gneiss at the contact is not mylonitized.
This zone of mylonite is interpreted as being
caused by the shearing of the cooler, more solid walls
of the intruding quartz monzonite against the cold wall
rock while the center of the intrusive was still hot and
mobile.
Directed, regional metamorphism, which later
converted the Lovingston quartz monzonite and the
Lynchburg metasediments to gneisses, was resolved along
these planes of shearing and merely intensified
preexisting conditions.
After regional metamorphism of the Lovingaton and
the Lynchburg gneisses, the Lovingston gneiss was Intruded
by hypersthene granodiorite and intensely hydrothermally
altered in the vicinity of the intrusion.
Where this
hydrothermally altered Lovingston is in contact with
the granodiorite, there is developed in the former a
series of parallel fractures averaging one inch apart
and parallel to the contact (see fig. 18).
These are
exposed on the surface to variable distances from the
contact, depending on whether the contact is at a steep
or a low angle.
These fractures are superimposed on the
A.
B.
Figure 18. A. Marginal fractures developed
in the hydrothermally altered Lovingston
adjacent to the granodiorite intrusion.
Granodiorite is exposed approximately 200
yards to the right of the outcrop photo­
graphed. Photo taken on county road 615,
one and three fourths miles south of Hicks.
B. Closeup of marginal fractures exposed
on U. 3. Houte 60, in vicinity of Morley
Mountain.
-61
original gneiss bands of the Lovingston and, at the
contact almost obliterate them.
The fractures decrease
in abundance away from the granodiorite-Lovingston
contact, dying out several hundred feet from it.
The
dip of the fractures varies from horizontal to nearly
vertical but is generally at a low angle.
On the
eastern side of the small body of granodiorite (See
fig. 19, Morley Mtn.) the intense hydrothermal
alteration of the Lovingston gradually decreases from the
contact, as the fractures fade.
At High Peak Orchard, where county road 643 is cut
through a gap in Tobacco How Mountain, this phenomenon is
clearly shown.
On the eastern side of the mountain, the
fractures were observed in the hydrothermally altered
o
Lovingston, dipping 40 southeast and parallel to the
contact.
They increase in intensity towards the
granodiorite contact, where they disappear.
The
granodiorite is then exposed for about one quarter of
a mile to the west where the western contact is reached.
Here the hydrothermally altered and fractured Lovingston
0
again is to be seen, with these fractures dipping 30
northwest and parallel to this contact.
Seven miles
northwest, near the contact of the hydrothermally altered
Lovingston and the Blue Ridge granodiorite, the fractures
may be again observed, dipping 15
to the contact.
southeast and parallel
dc
m y
m
;'3 ^ y
<&•
01
o»
H
©
u
3
tiQ
•ri
P*l
-62
These fractures are Interpreted as having been
formed during the emplacement of the granodiorite and
are considered to be marginal structures.
These
structures were formed by a stretching of the
Lovingston gneiss, caused by the forceful emplacement
of the granodiorite.
These fractures do not follow the general trend
of the rocks of the area, but vary in strike with the
contact, always being parallel to it.
Hone of these
fractures were found in the granodiorite*
The only other place in the area where this type of
fracturing occurs is in the hydrothermally altered
Lovingston in proximity to the nelsonite dikes.
Here
the joints are much less distinct, closer together, and
are vertical, parallel with the nelsonite dikes.
They
die out a few feet away from the contact of the nelsonite
and the hydrothermally altered Lovingston.
Extreme
weathering of the dikes and of the surrounding rocks
made it impossible to verify the presence of the fractures
in all eases near nelsonite dikes.
For the general structural relationships of the
granodiorite and the intruded Lovingston, see fig. 19.
FOLDS
The rocks of the western Piedmont and Blue Ridge
provinces of Virginia have been arched into a great
antlclinorium of late Paleozoic age.
The formations
of the Amherst Quadrangle lie on the western limb of
this major fold.
The rocks of this latter area have been severely
compressed, with the exception of the hypersthene
granodiorite and its derivitives, and the Triassic
diabase.
The cleavage of the SGhists and banding of
gneisses dip at a high angle.
Original sedimentary
bedding has been destroyed so that it is not possible
to determine the angle the cleavage boars to the bedding.
The regional structure as shown by cleavage, schistosity,
o
and banding, trends K. 40 £•
FAULTS
Thrust faults. - Throughout the area, the Lovingston
o
gneiss contains bands of mica schists striking N. 35 o
50 1*, roughly parallel to the general trend of the
structure of the region, and dipping to the southeast
o
o
at angles varying from 10 to 60 but usually at the
lower angle.
These bands vary in width from a few
inches up to as much as 200 feet.
The bands are here
interpreted as shear zones produced by thrusting during the
late Paleozoic deformation.
However, it is thought by
some, that these schist bands are not the produce of
shearing alone, but represent undigested masses of the
old quartz-biotite schist which the Lovingston intruded,
-64-
and that the later shearing was localized by these
preexisting zones of weakness*
The hypersthene granodiorite contains zones of
myIonite, which in general are parallel in strike and dip
to the schist bands in the Lovingston.
One of these
zones has a maximum, width of 500 feet and hence is large
enough to be mapped (see fig. 1).
These zones have been
discussed under Geology and Petrography where it was
noted that the rock passes successively from slightly
crushed granodiorite to a quartz-chlorite phyllonite.
Actual displacement could not be determined, but it is
obviously not very great.
These crushed and schistose
zones in the granodiorite are interpreted as shears
formed at the same time and by the same forces that
developed the schist bands in the Lovingston.
It is
believed that a tangential stress strong enough to shear
the granodiorite to a phyllonite and mylonite would be
sufficient to
develop the schist bands in the Lovingston.
High Angle Faults. - The only vertical faults recognized
in the area occur in the eastern edge of the peridotite
at its contact with the Lynchburg gneiss.
One of these
faults was recognized by a narrow, schistone, mineralized
o
zone eutting the massive peridotite in a N. 65 iS. direction.
The schlstosity is vertical, and, since the zone parallels
the contact of the Lynchburg gneiss and the peridotite,
50
it has been interpreted by Furcron as a mineralized contact.
65-
50
Op, cit, pp. 109-110.
However, in several localities massive peridotite was
found between this zone and the Lynchburg gneiss.
It,
therefor©, can not represent the contact between these
two rooks.
At anorther locality, this mineralized zone is
sharply offset for about 1/4 mile.
This break has
tentatively been mapped as a cross fault, but the
evidence is not sufficient to determine it positively
as such,
AGS OF STRUCTURES
Igneous Structures; - The raylonltized border of the
Lovingston gneiss is the oldest structural feature of
the area.
It is contemporaneous in age with the
intrusion of the Lovingston, which comprises the older
injection complex, and is considered to be early
Froterozoic in age.
The marginal fractures, formed by
a stretching of the Lovingston by the intrusion of
hypersthene granodiorite, would be of the same age as
that intrusion.
The hypersthene granodiorite comprises
the younger injection complex and is considered to be
late Froterozoic in age.
-66-
Folds and Faults; - The compression responsible for the
neomineralization of the Lynchburg gneiss and affecting
rooks of comparable age in this area took place before
the intrusion of the granodiorite, probably during
middle Froterozoic time.
Folding and faulting in other parts of Piedmont
and Blue Ridge provinces Involve Paleozoic sediments and
are considered to be late Paleozoic in age, occurring
during the Appalachian deformation.
Although there are
no Paleozoic sediments In the Amherst Quadrangle, the
faults found here are clearly related to those found
elsewhere In the Blue Ridge and Piedmont provinces and
are considered to have been formed during the Appalachian
revolution.
-67
M E T A M O R P H I S M
INTRODUCTORY STATEMENT
Two general types of metamorphism are encountered
In the rocks of the Amherst Quadrangle*
Regional dynamic
metamorphism has altered the rocks of the older injection
complex and produced the Lynchburg gneiss, and later
less intense, dynamic metamorphism has created many
shear zones throughout the Pre-Cambrian formations*
Hydrothermal metamorphism has brought about many
changes in the Lovingston gneiss near the granodiorite
as well as in the Catoctin greenstone, the nelsonites,
and the anorthosite.
REGIONAL XYHAMIC METAMORPHISM
Directed stress, active under conditions of high
temperature and pressure for a considerable period of
time, has aligned the minerals of the Lovingston and
the Lynchburg so that these rocks possess a distinctly
gneisslc texture•
Recrystallization and neomineralization
in these rocks was carried on for some time, since the
Lovingston gneiss shows pheonocrysts of feldspar, altered
early in metamorphism, being replaced by veinlets of
quartz formed during later stages of this same process.
The Lynchburg gneiss, which is of sedimentary origin,
-68Shows a mineral assemblage typical of the amphibolite
facies.
Since there is an extremely close parallelism
between the banding of the Lovingston and the Lynchburg,
it is thought that the metamorphism of these two
formations is contemporaneous, caused by a stress active
in a northwest direction upon the rocks during middle
Froterozoic time, while they were buried to considerable
depth.
LATER DYNAMIC METAMORPHISM
The late Paleozoic metamorphism, which produced
the large overthrusts with their accompanying retrogressive
metamorphism in the metasediments of the Piedmont, is
expressed in the hypersthene granodiorite and Lovingston
gneiss by irregularly spaced shear zones*
These shear
zones strike in a northeast-southwest direction, dip at
a low angle to the southeast, and vary in width from a
few inches to 500 feet.
The cataolastic structures
found in these zones indicate that the rocks there
suffered considerable differential movement under
low temperature conditions.
The predominance in these
zones of chlorite and white mica phyllonites substantiates
this conclusion.
The massive nature of these rocks prohibited the
formation of large overthrust zones in this area.
The
stress was instead relieved along the numerous small,
-69
irregularly spaced shears*
m m o m m M K L msttamorfhism
This type of metamorphism has been superimposed
upon the regionally metamorphosed Lovingston gneiss
near its contact with the hypersthene granodiorite.
At these localities the gneissic texture of the
Lovingston has in many Instances been almost obliter­
ated*
The mineral changes brought about by these
exomorphio effects in proximity to the granodiorite
intrusion have been discussed under hydrothermally
altered Lovingston.
The Catoctin metabasalt has been altered to a
greenstone in a like manner*
Nelsonites and anorthosite
have also been hydrothermally altered and this process
is considered in detail under descriptions of these
rocks.
-70CrXOLOGlC
B I S T O R T
The early geologic history of the rocks in the
Piedmont and Blue Ridge provinces is not too clearly
defined in the record*
In a region of such highly
metamorphosed rocks definite ages are difficult to
establish and only comparative age relations can be
assigned.
The oldest known rock in the Blue Ridge antiolinorium of Virginia is, under the older classification,
the Lynchburg gneiss.
Under the newer classification
this rock would be an unnamed quartz mica schist.
The
sediments from which this rock was formed were eroded
from the old continent of Appalachia and deposited in
the Blue Ridge geosyncllne in early Proterozoic, or
perhaps late archeozoic time.
The seas then retreated and a period of folding,
uplift and intrusion followed in middle Proterozoic time.
During this period the Lovingston quartz monzonite
intruded the Lynchburg.
Then followed a period of
regional metamorphism which produced the gneissio texture
of the Lovingston and the Lynchburg.
It is not possible to determine, in this area,
whether the metasediments of the Lynchburg were meta­
morphosed before or simultaneously with the metamorphism
of the Lovingston quartz monzonite.
Prom the parallelism
-71-
of their structures, however, it appears that hoth were
deformed by the same metamorphic processes.
Basic intrusion and flows accompanied or followed
the waning stages of the regional metamorphism, and the
hornblende gabbro, metapyroxenite, peridotite, and
Catoctin metabasalt were emplaced.
In late Proterozoic time the hypersthene grano­
diorite with its various phases was intruded Into the
Lovingston gneiss and Catoctin metabasalt*
Pegmatite
and nelsonite dikes were also emplaced at this time,
originating in the granodiorite.
This intrusive
activity was accompanied by hydrothermal action, which
altered the rock adjacent to the granodiorite.
Late Paleozoic deformation formed, by dynamic
metamorphism, extensive zones of shearing in the grano­
diorite and Lovingston gneiss, and faulted the peridotite.
During the late stages of this metamorphism hydrothermal
solutions, rose along this fault and deposited metallic
minerals.
The latest activity in this region was the Intrusion
of diabase dikes during Triassic time, which intruded all
the other rocks of the area.
-78-
E C O N O M I C
G E O L O G Y
INTRODUCTION
The Amherst Quadrangle contains several varieties
of economically important minerals and rocks*
While
the developement of these resources on an important
scale is comparatively recent, some mining was begun
before the war between the states*
Nelsonite, composed principally of ilmenite and
apatite, is the most important rock being exploited at
the present time*
From the Southern Minerals Company*s
quarry on Piney Hiver in the northern part of Amherst
County, ilmenite and apatite are obtained in quantity unequale
elsewhere in the United States.
A feldspar quarry in the anorthosite in the
northern part of Amherst County has recently been opened
by the Dominions Mineral Company and is at present
producing to capacity.
A company has reoently been formed to reopen the
abandoned copper mines in the Glades region, In south­
eastern Amherst County.
In addition to these mineral resources, normal
Lovingston gneiss and hypersthene granodiorite are
quarried for road metal and building stone.
-73NBLS0NIT1
Nelsonite dikes, with their peculiar association
of ilmenite* apatite, and rutile, have long been of
economic and scientific interest.
Although nelsonites
have been recognized in Kelson County and to a lesser
extent in Amherst County, in the west-central past of
Virginia for a considerable period of time, there has been
no agreement as to their origin.
The rutile-ilmenite deposits were studied first
51
by Watson and Taber, who ascribed a magmatie origin to
51
Watson, Thos* 1. and Taber, Stephenj Geology of
the Titanium and Apatite Deposits of Virginia. Va.
Geol. Survey Bull. No. 111-A,1913.
Ss
the nelsonites.
Eyan
supported the conclusions of
58
Ryan, C. W.: The Ilmenite Apatite Deposits of WestCentral Virginia. Boon. Geol. Vol. xxvlll, No. 3,
May, 1933, pp. 866-875.
Watson and Taber as to origin, but extended the area
in which nelsonites occur southward into Amherst County.
53
More recently, Ross has suggested a hydrothermal origin
53
Ross, G. S.: Titanium Deposits of the Roseland District.
Internet. Geol. Cong. Guidebook 11, Northern Virginia,
1933, pp. 31-56.
-74-
for the bodies.
Occurrence: -
Although the association of apatite with
ilmenite deposits is not uncommon, economically important
bodies similar to those found in the Amherst-Nelson
County area have not, to the writer’s knowledge, been
54
described elsewhere. Watson and Taber noted the
54.........
Op. cit, pp. 341-245.
occurrence in Hoanoke County, Virginia, of several small
bodies composed of ilmenite, apatite, and various silicates,
oocuring in syenite (granodiorite) which they described
as similar to the nelsonites.
Nelsonite dikes are found in the northeastern and
central portions of the Amherst Quadrangle.
They intrude
the anorthosite and leuoogranodiorite, but are far more
abundant in the hydrothermally altered Lovingston gneiss.
In Amherst County, all the dikes found were within the
area designated as hydrothermally altered Lovingston (See
fig. 1).
Nelsonite occurs as dike-like bodies which strike
in a northeast-southwest direction, parallel to the
structure of the region.
They vary in width from less
than on© foot to 100 feet, and have been traced, in one
instance, over one half mile.
Only those dikes oocuring
in the hydrothermally altered Lovingston gneiss of Amherst
-75County are discussed in this report.
In Nelson County,
since there was no preexisting gneissic structure to
control their trend and shape, they are extremely
irregular in outline, but in general preserve their
dike-like character.
DESCRIPTIONS OF NELSONITES
Four dikes, great^er than 20 feet in width, and
for this reason sufficiently large to be worked, and
many smaller ones have been found in Amherst County.
There are, therefore, many more of these dikes in
Amherst County than has heretofore been supposed.
In the anorthosite region of Nelson County, the
dikes weather red and stand out plainly against the
gray soil and are, therefore, easy to locate.
In
Amherst County, however, the hydrothermally altered
Lovingston itself weathers to a red soil very similar to
that formed from the weathering of nelsonite.
The deep
residual soil, heavy undergrowth, and the rugged and
mountainous nature of the topography also make the
outcrops difficult to find.
All the surface exposures of nelsonite are not
weathered to the same degree.
In some exposures, only
the ilmenite, limonite, and small grains of osteolite
remain; whereas In others nearly all the minerals of
the dike are left in a relatively fresh condition.
-76Phenoerysts of ilmenite and apatite, with the matrix
composed of fibrous hornblende, biotite altered in part
to chlorite, sphene, hematite, and quartz are the chief
minerals•
Southern Minerals Company*s Quarry: -
The quarry of
the Southern Minerals Company in the northern portion
of Amherst County on the bank of Piney River where it
is crossed by TJ. S. Route 89, is in the only dike
being worked on a commercial scale at the present time.
o
This dike strikes H. 42 E . , has a maximum width of
100 feet, and has been traced along its strike for one
half mile.
It Is the largest dike found so far in the
area (Bee fig. 20).
This is the only place in the area where speoimens
of unweathered nelsonite can be obtained, since the
workings are about 75 feet in depth and extend well below
the zone of weathering.
A vertical section In this
quarry shows the following features:
Gossan Ore: - The term "gossan" is here applied to
the surface outcrops above the weathered ore from which
all except the most stable constituents have been re­
moved by leaching.
The rock is brown in color and
consists chiefly of broken grains of ilmenite and lesser
amounts of apatite, which is altered to osteolite, and
of limonite.
Stringers of iron strained chlorite altered
from biotite, and serieite occupy minute fractures in the
Figure 20. Ilmenite-apatite quarry of the
Southern Mineral Products Company on Piney
River.
77ilmenite and apatite grains.
The limonite is apparent­
ly filling fissures.
Weathered Ores - The weathered ore is also brown in
color and extends from the bottom of the gossan to a
depth of approximately 50 feet.
It is very soft and is
called by the quarrymen "lateritic ore."
consists
This ore
of ilmenite with lesser amounts of graphite,
apatite, biotite and some quartz,
quartz, replacing the
apatite, is In much smaller proportion than it appears
in the handspecimen.
The ilmenite shows the characteris­
tic rhombohedral parting, which differs little in its
Interfacial angles from a cube.
The mineral, therefore,
breaks into rectangular masses.
Hard Ore; - The hard ore comes from the deepest portion
of the quarry and has suffered very little weathering.
It is green in color, with the metallic minerals and
apatite composing the phenocrysts.
They are, in order of
abundance, Ilmenite, apatite, pyrite and pyrrhotlte, and
graphite, and appear to be dominant over the matrix, which
Is composed of hornblende, actinolite, biotite, uralite,
chlorite, sphene, magnetite, rutile, hematite, and quartz
(see fig. SI).
One generation of ilmenite, apatite and
biotite and perhaps hornblende, apparently are replaced
by amphibole, chlorite and the other metallic minerals.
The irregular outline of the ilmenite are of such a
Figure 21. Photomicrograph of hard ore from
the quarry of the Southern Minerals Product Company
A. Large apatite grains.
B. Matrix composed of uralite, actinolite, chlorite
sericite and some quartz. Black mineral is
ilmenite.
Polarized light. X25
nature that this relationship could not be absolutely
verified, but it appears that one generation of ilmenite,
apatite, biotite and perhaps amphibole are earlier than
the other minerals of the rook, which were probably
introduced by hydrothermal solutions*
Polished sections
of these ores show amphibole, chlorite and pyrite
replacing the apatite and some of the ilmenite.
o
This dike, which dips at an angle of 45, has been
drilled along the dip about 1¥5 feet without running
out of the ore and without an appreciable change in the
amount of ilmenite.
Luoian Burley Property& ~
A nelson!te dike on the Lucian
Burley Property, five miles southwest of Amherst was
observed to carry appreciable amounts of allanite.
This
o
dike strikes N. 40 !♦, can be traced about one quarter
of a mile along the strike, and has a maximum width of
about 65 feet*
This width was determined by trenching,
since deep residual soil covers the contact and since
ilmenite float over an area 100 feet in width might have
weathered from a dike less than one foot wide.
At two localities about 1000 feet apart along the
strike, allonite was found in the residual soil weathered
from the dike, and in the dike itself.
The allanite
occurs in pockets in the dike, and has been noted by
55
Pegau, who stated that the mineral is found in pockets
-79-
55
Pegau, A. A*: Pegmatite Deposits of Virginia. Va. Geol.
Survey Bull. Bo. 33, 1932, p. 95
of granodiorite associated with nelsonite.
He also noted
the prevailing country rock to he a heavily injected
quartz monzonite, which is here called hydrothermally
altered Lovingaton.
Mineral Descriptions -
Specimens taken from an abandoned
pit on this property 50 feet deep, would correspond to
the lower limit of the weathered zone of the Southern
Minerals Company Quarry*
These specimens and pieces of
float show a greater proportion of ilmenite than could
be found in the dike on U. S. Route 29 which is being
exploited (See fig. 22).
Directly in strike with the dike on the Burley
property two miles across Pauls Mountain, the writer
found more allanite lying in large pieces on the surface.
There were also specimens of ilmenite with associated
sphene and apatite.
The body from which this float was
derived could not be found.
At the historic locality of Little Friar Mountain,
from which so many specimens of allanite have been
collected, the mineral was observed to occur directly in
the granodiorite.
The allanite here is associated with
feldspar and has been called a pegmatite.
or rutlie was observed from this locality.
No ilmenite
Figure 22. Photomicrograph of nelsonite from
the Lucian Burely Property, five miles southwest
of Amherst. The matrix composed of uralite,
actinolite, chlorite and some quartz. Black
mineral is ilmenite. Note at "A" the apparent
replacement of ilmenite by the non—metallic
minerals.
80W, B* QiXleapyta Property: -
On Shady Mountain within
on© mil© of Camden Gap and two miles south of Sardis
on county rd* 815, four small dikes were found*
The
dikes were found to be all parallel and to strike in
o
a general N. 40 11 direction* They are very narrow, the
largest width found being 15 feet*
Because of their
small size, the heavy undergrowth, and deep residual
weathering, these dikes could not be traced more than
several hundred feet*
Specimens of these weathered dikes showed ilmenite,
apatite altered to osteollte, graphite, and chlorite out
by veinlets of limonlte*
Nelsonite Pike on U. S. Route 80; - A Nelsonite dike,
composed essentially of ilmenite and magnetite was found
two miles east of Dodd*a Store on U* S* Route 60*
This
rock differs from other nelsonites of the area not
only in composition, but also in shape*
It has an oval
o
rather than tabular shape, strikes N* 50 1*, is about
BOO feet long, and has a maximum width of 75 feet*
Specimens from this dike are black in color and
metallic in luster, and are composed of three parts
ilmenite and two parts magnetite*
Polished surfaces from
this dike show small discontinuous veinlets of aotinolite,
chlorite, and garnet apparently replacing the metallic
minerals.
If this apparent replacement is true, then
at least some of the metals must be of an earlier origin
than the non-metals.
-81Qther Localities: - Small, economically unimportant
nelsonite dikes were noted at other localities in the
hydro thermally altered Lovingston but since they do not
differ essentially from the larger ones, they will not
be discussed here*
For descriptions of the nelsonites
56
in the anorthosite of Kelson County see Watson and Taber*
Op* eit* pp* 100-155
Origin of the Nelsonites: -
The origin for nelsonites
57
maintained by Watson and Taber is given in the following
s?
Op* cit* p. 69*
sentences, quoted from their bulletin.
wThey (the
nelsonites) are thought to be basic segregations somewhat
similar in mode of origin to b© well-known segregations
of titaniferous magnetite in gabbros*
They represent the
latest differentiations from the magma which gave rise
upon cooling to syenite and the other igneous rock types
of this area.**
Watson and Taber apparently did not recognize the
large areas of hypersthene granodiorite in this region.
They assume that all of the rocks of the area were
differentiates of one magma and that all were emplaeed
about the same time*
There is little doubt, judging from
the similarity of their constituent minerals, that the
magams which gave rise to the Lovingston gneiss and to
82-
the hypersthene granodiorite were very similar In
composition, but proofs given elsewhere in this report
Indicate that a long time interval elapsed between their
respective intrusions#
From their mineral descriptions it is possible that
the "gabbros" which Watson and Taber found intruding the
anorthosite and the gradations to gabbro which they
describe around the edges of the anorthositic body are
really granodiorite.
But they considered this gabbro
to be merely small dike-like segregations from the main
quartz-monzonite magma rather than parts of the huge
bathollth which comprises the core of the Blue Ridge
Mountains•
The theory of the origin of the nelsonite bodies
formulated by the writer from a study of those dikes
in Amherst County is as follows:
The constituents
forming these bodies were given off from the granodiorite
magma upon cooling, and this material rose upward through
the earlier crystallized, relatively higher anorthosite
and through those portions of the Lovingston gneiss that
overlay the granodiorite#
Contemporaneous and later
solutions rising from the cooling granodiorite magma
altered the overlying Lovingston and the nelsonite bodies,
producing the hydrothermally altered Lovingston and
altering the bodies so that their original composition of
83ilmenite, apatite, hornblende and biotite was supple­
mented by the minerals amphibole, chlorite, sericite,
graphite, pyrite and perhaps ilmenite and allanite*
The sag in the roof of the granodiorite between
the Blue Ridge mountains proper and Morley Mountain (See
fig* 19) is responsible for the preservation of the
nelsonite bodies*
The evidence In favor of the suggested origin of the
nelsonites, which has already been discussed, may be
summarized as follows:
1*
The occurrence of the pegmatite and nelsonite dikes
only in the intensely altered Lovingston and in the
anorthosite*
5.
The presence of Ilmenite-apatite float over the true
granodiorite, without the associated nelsonite dikes,
indicating that preexisting dikes occurred in the
altered Lovingston above the granodiorite and have,
along with that Lovingston, been destroyed by weather­
ing and erosion, leaving fragments of float*
3., The noted occurrence of allanite not associated with
ilmenite, in the true granodiorite at Little Friar
Mountain, and the occurrence of allanite in a nelsonite
dike that intrudes the Lovingston, but no allanite
found In the Lovingston unaccompanied by nelsonite*
4*
Apatite and Ilmenite universally present as the chief
accessories in the granodiorite*
5*
The nelsonite dikes always associated areally with the
pegmatites, whose mineral relations with the granodiorite
prove almost conclusively that they are related to that
body*
6.
Few nelsonite dikes occurring at any localities other
than Kelson and Amherst counties. At no other place,
so far as the writer can determine, is the granodioriteLovingston relation similar to that in these counties
(See fig. 19). The cupola shape of the intrusion of
granodiorite in this locality left an ideal location
between it and the main mass for the preservation of
the covering of altered Lovingston and the dikes that
-84intruded it,
7. The dikes showing no regional me tamor phi sm, occurring
nearly always parallel to the gneissio structure of
the Lovingston, indicating that their trend was
controlled by this structure. Since this structure
is caused by regional metamorphism, the nelsonite
bodies could not be segregations of the rock in which
they occur, as postulated by Watson and Taber?0
11
1
Op, eit, pp. 151-155
8. The widespread similarity of composition of all the
dikes. It is seen from the suite of specimens taken
from various levels in the dike being worked on at
Plney Eiver that the biotite, hornblende, ilmenite,
and magnetite nelsonites of Watson can all be found
in the same dike and are not separate intrusions, but
merely quantitative and qualitative variation in
replacement by later hydrothermal solutions, exposed
by variable depths of weathering. No gabbro nelsonites
or rutile nelsonites were found in Amherst County.
It is realized that while the reasons given above
indicate very strongly the genetic relationship of the
nelsonites to the granodiorite, they do not entirely
eliminate the possibility of a hydrothermal origin for
the dikes.
Indeed, the facts listed could be made to
support the theory that there were two periods of solutions,
the first depositing the metallic minerals, and apatite,
hornblende, and biotite; the second bringing in the nonmetallic minerals that replaced the metallic ones.
However, the following additional evidence points to an
intrusive origin of the bodies:
The bodies occur in dikes with walls which are always
sharply defined.
2 . They have an even-granular texture throughout which
shows little variation.
-853.
Inclusions of apatite occur in ilmenite and viceversa, indicating simultaneous crystallization*
4*
The suites of typical hydrothermal minerals are
considered later than the ilmenite and apatite and
can h© observed apparently replacing them#
5*
Since solutions have been extremely active in the
region surrounding the dikes; if ilmenite and apatite
were carried in quantity by these solution, the
proportion of these minerals in the intensely altered
Lovingston would be much greater than that In the
normal Lovingston* In the Majority of Instances
this was not observed to be the case*
6*
The existance of marginal fractures was observed in
some instances in the rocks surrounding the dikes*
Conclusions i -
The evidence offered for an intrusive origin
of the nelsonites and against a purely hydrothermal origin
necessarily cannot be conclusive until the dikes ar©
further exploited.
However, the main purpose of this
per is to establish a definite genetic relation
between the nelsonite bodies and the hypersthene
granodiorite*
According to the evidence presented,
the nelsonite dikes are definitely younger than their
host rock, the Lovingston gneiss; they were formed at
about the same time as the emplacement of the granodiorite;
and the only genetic connection between the anorthosite
and the nelsonites is that both are related to the
hypersthene granodiorite.
Prospecting for Helsonites: -
It Is thought for reasons
given above, i.e. thick undergrowth, rugged terrain,
and deep weathering, that there are dikes in Amherst
County of commercial value which have not as yet been
-86dlscovered,
Sine© a dike need be only twenty feet wide
to be of commercial value, this can easily be understood.
It is believed that only a few of the total
number of nelsonite dikes are associated with the
anorthosite,
The occurrence of this anorthosite body
did not control the formation of the dikes and there
are more outside the bounds of the anorthosite than in it,
All the area of lovingston gneiss that is designated
as intensely hydrothenaally altered lovingston and which
Is, therefore, underlain by granodiorite is a possible
source for the nelsonites.
Surface prospecting in this
region, because of the deep residual weathering, is not
too satisfactory.
Prospecting should be done with a
magnetometer in the fall of the year when the vegetation
is dead.
Because of the magnetic quality of the ilmenite,
a weak needle can be used to advantage,
It is thought by the writer that thorough exploration
of this portion of Amherst County will probably reveal
domestic reserves of titanium and phosphate which have
heretofore been little suspected.
Production: - Ilmenite is used chiefly in the production
of titanium pigments and ferro-alloys.
The titanium
pigments, because of their whitening and obliterating power,
are widely used in paint, rubber, linoleum, leather,
plastics, soap, Inks, paper, textiles and ceramics.
The
Virginia Geological Survey is not at liberty to publish
figures of domestic production, but the leading producers
are Amherst County, Virginia, Arkansas, and California.
Of these, the Southern Minerals C o m p a n y Q u a r r y in
Amherst County is probably the largest producer (See
fig* 23).
Apatite Is used as a fertilizer and in foodstuffs.
Figures are not available, but sales of the mineral
from Amherst County seem to be rising.
COPPER
History: - According to local reports, copper was
mined in the extreme southeastern portion of the Amherst
Quadrangle in the region known locally as the Glades
before the Civil War.
However, profits were small and
the workings were soon abandoned.
that between 1875 and 1335 a
It is rumored, however,
Colonel Thomas
shipped $80,000 worth of copper from
shafts
"Old Glades Road,” in the glades region.
Dunlap
sunk inthe
This has
59
been in part substantiated by the research of Furcron,
¥ 5 -----------------------------------------------------------
Op. cit. pg. 108
who found a note in the December, 1880 issue of The
Virginias stating that Dunlap had shipped 30,000 pounds
of copper ore from mines in the Glades.
In 1835 the
mines were again abandoned and not reopened until 1917-18
when they were leased by the
Buffalo Kidge Development
Company.
at this time.
No ore was shipped
In June, 1939 the mines were again leased, and a
Figure 23. The Southern Chemical Company’s
plant at Piney River, Nelson County, near
the Amherst County line. It produces
titanium oxide and calcium mono-phosphate
from nelsonite.
company was formed to clean out the shafts and search
for the large vein which they believe exists there {See
Fig, 24)•
At the present time, production has not begun.
Occurrence! -
The zone of copper mineralization is
o
about 800 feet wide and extends in a N. 45 S. direction,
parallel to the old "Glade Road."
It is found in the
peridotit© at its contact with the Lynchburg gneiss.
The zone Is characterized by a schistose structure striko
ing H. 45 1 and is apparently eaused by a vertical fault.
Due to the nature of the rook, relative movement along
this fault could not be determined.
It has been stated
m
by Furcron
that this mineralized zone occurs In the
35---------- ---------------------------Op. cit. pg. 109.
adjoining Lynchburg gneiss, but detailed work by the
writer has placed the contact 100 feet farther west, in
the peridotite.
Specimens taken from the dumps of abandoned shafts,
show, in order of abundance, fibrous tremolite, chlorite,
malachite, chalcedony, quartz, chalcopyrite, and bornite.
The chalcedony and quartz occur in veinlets replacing the
other minerals.
A microscopic study of these specimens
revealed in many oases original olivine, amphibole,
serpentine, and magnetite which were being replaced by
tremolite, chlorite, carbonate, tal©(?), chalcedony,
quartz and chalcopyrite.
Other sections were composed
Figure 24. Shaft of an abandoned copper mine
in the Glades region.
entirely of tremolite, replaced along fractures by
carbonate and tale, chlorite, and veinlets of liaonite,
quartz, chalcedony, chalcopyrite, bornite and malachite
(see fig. 25).
The concentration of these replacement minerals in
a narrow shear zone indicates that they were formed by
hydrothermal solutions rising along a fault.
The order
of crystallization as indicated by the four slides examined
seem to be as follows:
Formation of tremolite and chlorite
were followed closely by the carbonates and talo, which
in turn were followed by the copper minerals and perhaps
some quartz.
The last formed minerals were chalcedonic
quartz, malachite and llmonite*
Productions - Undoubtedly there have been concentrations
of copper minerals of a sufficient value to' be worked in
this region, as Indicated by the past history.
However,
these deposits were soon worked out and evidently were in
the form of pockets rather than in continuous veins.
There is no evidence at present to warrant the assumption
ofi a large vein, rich enough to justify the expenditure of
large sums for equipment necessary for exploitation.
FELDSPAR
Feldspar Is being recovered from a recently opened
quarry on the southern bank of Piney Rive, one mile west
of U. S. Route 29.
This quarry, operated by the Dominions
Mineral Company, Inc. has been producing less than a year.
Figure 25. Photomicrograph of mineralized
peridotite showing vein of carbonate and
quartz cutting dark suphide.
-90Figures on the amount of feldspar shipped are not
available.
The quarry is located in the Anorthosite
which has already been described in detail.
The amount of quartz and ferro-magnesian impurities
In this rook are so great that, were it not for the
proximity of the branch line Blue Ridge Railroad, the
anorthosite would be too poor in feldspar to compete with
other producers,
quarries worked at other localities in
this comparatively large rock mass away from favorable
transportation facilities would not be profitable.
Building Stone and Road Metal
The normal Lovingston gneiss, and to a lesser extent
the hypersthene granodiorite have been utilized locally
for road metal and building stone.
The granitic texture
of phases of the granodiorite makes it the best rock
in the area for building stone.
The normal Lovingston
gneiss Is easy to quarry and binds well, and hence is the
most widely used rock in the area for road metal.
-91BIBLIOGRAPHV
Balk, Hobart, The Structural Behavior of Igneous Rocks.
Memoir 5, Geol. Soc. Amer., 1936.
Burfoot, J. D. Jr., The origin of Talc and Soapstone
Deposits in Virginia, Scon. Geology, Vol. 25, No. 8,
pp. 805-826, 1930*
Oloss, Brnst, Structural Survey of the Granodiorite south
of Mariposa, California, Amer. Jour. Sci, Vol. 23,
pp. 289-304, 1932.
Close, Hans, Tektonic und Magma, Band 111,1927, Abh. d.
Pr. Geol* I»andesanstalt, Berlin.
Fur cron, A. S., James River Iron and Marble Belt, Va.
Geol. Survey Bull. 39, 1935.
Furcron, A# S., Igneous rocks of the Shenandoah National
Park Area. Jour, of Geology, Vol. 52, pp. 405-440,
1934.
Grout, F. F*, Petrography and Petrology. McGraw Hill Book
Company, Inc. 1932, 475 pages.
Johaansen, A., A Descriptive Petrography of Igneous Rocks,
Vol. 3, 1932, 492 pages.
Jonas, A. X., Geological Map of Virginia, 1928. Published
by Virginia Geological Survey.
Jonas, A. 1., Geological Reconnaissance in the Piedmont
of Virginia. Geol. Society of America, Bull. Vol.
38, pp. 837-846,1927.
Jonas, A. 1., Structure of the Metamorphic belt of the
central Appalachians. Geol. Soo. of America Bull.
Vol. 40, pp. 503-514, 1929.
Jonas, A. I., Hypersthene Granodiorite in Virginia, Geol.
Soc. of America Bull. Vol. 46, pp. 47-60, 1935.
Jonas, A. I. and Stose, G. W., Age Halations of the
FreCambrian Rocks in the Oatoctin Mountain-Blue
Ridge and Mount Rogers Antiolinoria in Virginia.
Am. Jour* of Sci., Vol. 237, August, 1939, pp.
875-593.
Keith, Arthur, Geology of the Oatoctin Belt. TJ. S. Geol.
Survey, 14th Ann. report, pt. 2, pp. 287-395,1894.
taney, F. B., The Geology and Ore Deposits of the
Virgillna District of Virginia and North Carolina.
Va. Geol. Survey Bull. 14, 1917.
Maclure, William, Observations of the Geology of the
United States, Am. Philos. Soc. Trans. Vol. 6,
pp. 411-428, 1809, See Map.
Mayo, Bvans B., Structures Associated with Igneous
Intrusion, Chap. VII, Principles of Structural
Geology by C. M. Nevin, pp. 190-211. John Wiley
and Sons, 1936.
Nelson, W. A., Informal Communication. Jour. Wash. Acad.
Sci. Vol. 22, pp. 456-457, 1932.
Pegau, A. A., Pegmatite Deposits of Virginia. Va. Geol.
Survey Bull. 33, 1932.
-92-
Rosa, C. S., Titanium Deposits of Roseland District,
International Geol, Cong, Guidebook 11, Northern
Virginia, pp, 29-36, 1933.
Boss, C. S., Mineralisation of the Virginia Titanium
Deposits, Am. Min., Vol. 21, No. 3, March, 1936,
pp. 266-275.
Byan, C, W«, The ilmenitQ-apatite Deposits of West-Central
Virginia. Scon, Geol. Vol. 28, Ho. 3, May, 1933,
pp. 266-273.
Watson, T, L., Mineral Resources of Virginia, 618 pages,
1907.
Watson, T. I.., The Occurrence of Nickel in Virginia.
Trans. Am. Inst. Mining ISngrs., pp. 306-316, 1907.
Watson, T. L. and Cline, Justus H., Hypersthene Syenite
and related rocks of the Blue Ridge region,
Virginia. Bull. Geol. Soc. of America, Vol. 27,
1916, pp. 193-834.
Watson, T. L. and Taber, Stephen, Geology of the Titanium
and Apatite Deposits, Va. Geol. Survey Bull. No.
Ill-A, 1913
Weed, W. H. and Watson, Thomas, The Virginia Copper
Deposits. Icon. Geol, Vol. 1, pp. 309-330, 1906.
Wright, F. J . , The older Appalachians of the South
Denison, Univ. Bull. Jour. Sci. Labs. Vol. 26,
pp. 143—250, 1931.
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Denison Univ. Bull. Jour. Sci. Labs. Vol. 29,
pp. 1-105,*1934.
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