260
The distribution of gold and silver in the crystalline
rocks of the Malvern Hills.
B y A. BRAMMALL, D.Sc., A s s o c . I n s t . M . M .
a n d D. L. D o w I E , B . E . (Mining), D.I.C.
G e o l o g i c a l D e p a r t m e n t , I m p e r i a l College of
Science a n d T e c h n o l o g y , L o n d o n .
[Read January 30, 1936.]
I
N r e - e x a m i n i n g t h e c r y s t a l l i n e r o c k s of t h e M a l v e r n Hills, t h e
senior a u t h o r has had occasion to apply q u a n t i t a t i v e spectroscopic
d a t a for t h e r a r e r c o n s t i t u e n t s of h o r n b l e n d e s a n d m i c a s . T h e
r e c o g n i t i o n of s i l v e r in t h e s p e c t r o g r a m s of b i o t i t e l e d t o t e n t a t i v e
f i r e - a s s a y s of g r a n i t i c r o c k s in b u l k - - a p r o c e d u r e w h i c h r e v e a l e d t h e
p r e s e n c e also of gold.
T h e a s s a y r e s u l t s f o r t h i s p r e l i m i n a r y s u i t e of g r a n i t i c t y p e s a r e
g i v e n b e l o w ; v a l u e s less t h a n 5 g r a i n s p e r t o n a r e r e t u r n e d as
' traces ' 9
Rock-type, and reference to
Number of
Gold grains
Silver dwt.
' List of localities ', p. 263.
samples assayed, per ton.
per ton. 1
Granites : grey, pink, and reddish :
LOC. a, b, c, d, e, and j
...
4
...
trace ...
Av. 0.5
5
........
,, 1.3
2
........
,, 3-0
Granites : strongly reddened :
Loc. b . . . . . . . . . . . . . . .
2
...
,. . . . . .
9.4
,, d . . . . . . . . . . . . . . .
1
...
,. . . .
,, 33.6
,, b . . . . . . . . . . . . . . .
1
...
72
.....
42-4
Peqmatiteo :
LOC. d : grey ; muscovite-rioh
,, b: red; quartz-felspar
...
...
1
3
...
trace
.....
.....
...
,,
0-4
8.0
T h e h i g h e r v a l u e s a p p e a r t o b e r e l a t e d t o t h e r e d d e n i n g of t h e
f e l s p a r s , w h i c h is a w i d e s p r e a d ' l a t e ' p n e u m a t o l y t i c e f f e c t ; i t is
c o n s p i c u o u s in t h e m a j o r g r a n i t e m a s s e s a n d r e l a t e d p e g m a t i t e s , b u t
s o m e w h a t local a n d ' a r t e r i a l ' in i t s d i s t r i b u t i o n o v e r r o c k s of p r e x 1 pennyweight (24 grains) per ton (2240 lb.) = 1530 milligrams per metric
ton (1000 kg.).
GOLD
AND SILVER
IN
THE
MALVERN
HILLS
261
granite age. Among the igneous rocks of post-granite age, the intrusive granophyric quartz-diabase (' Ivy Scar' type) shows only feeble
reddening; the newer dolerites are unaffected.
As the contaminated character of the granite-granodiorite kindred
here is one of the more obvious features, the possibility that their
content of gold and silver might have been derived in the first instance
from external sources rendered assay work on other rock-types
obligatory.
The assemblage of Malvernian types sampled for this purpose
comprises the following :
I. Crystalline schists :
(a) Sillimanite-bearing and garnetiferous quartz-, quartz-chlorite-, quartz-chlorite-mica-schists, with varieties containing
felspars, hornblende, zoisite, epidote, apatite, sphene, ilmenite,
magnetite, haematite flakes, and calcite in varying proportions.
(b) Biotite-rich schists of pelitic type, grading into varieties
described above.
(c) Schisted sandstones. Stout ribs in granite are highly
modified along their margins.
(d) Amphibolites, hornblende-schists, and hornblende paragneisses, grading into biotite-rick varieties, with varying
amounts of quartz, felspar, garnet, epidote, &c.
(e) Phyllites, usually quartz-banded.
II. Thepyroxenite-hornblenditekindred(with felspathie and micaceous
variants) and related appinites, many of which are demonstrably of
' mixed' origin. In the main they are older than the red granite.
With these may be grouped some intrusive masses of 'appinitic'
diorite, syenite, monzonite, &c., and a widespread ' gabbro-diorite'
comparable to the rock quarried at Hollybush.
III. Granites, granadiorites, &c. The true granites belong to more
than one time-stage in the Malvemian sequence. The older types
are represented by thin intercalates of grey granite, &c., and broader
bands of granite, aplite, felsite, and granitoid paragneiss in the ban ded
crystalline schists. The newer granites are more massive, and typically pink or red ; they also contribute intercalates (locally tourmaliniferous) to the injection-complex.
The red granite-pegmatites belong to the latest phase of graniteintrusion.
262
A.
BRAMMALL
AND
D.
L.
DOWIE
T h e m o d i f i c a t i o n of g r a n i t e s , g r a n o d i o r i t e s ,
attributable
to
migratory
solutions
carrying
ON
&c., t o ' u n a k i t e '
the
constituents
is
of
epidote, with haematite, plagioclase, quartz, and sphene.
I V , Post-granite intrusions i n c l u d e t h e f o l l o w i n g :
A granophyric
quartz-diabase
n o r t h of t h e W y c h e .
type, most frequently encountered
T h e t y p e l o c a l i t y is I v y S c a r .
Dolerites.
A t l e a s t o n e o c c u r r e n c e of a p p i n i t e , w h i c h is i n t r u s i v e i n t o t h e
granophyric quartz-diabase.
V. Epidosites, 1 epi~tote-quartz-, and epidote-quartz-playioelase veinrock. T h e s e a r e w i d e l y d i s t r i b u t e d , a n d t r a v e r s e a l l t y p e s , i n c l u d i n g
t h o s e d e s c r i b e d in I V .
The
supplementary
assay-resulta
are
given
in
the
following
sections :
N u m b e r of Gold grains
samples assayed, per ton.
]~oek-type and locality,
Silver dwt.
per ton.
Crystalline schists :
Average.
Banded quartz-chlorite-sericite-phyllite.
Loc. k
...
Quartz- chlorite - sillimanite - garnet mica-schist.
Loc. f
...
Quar tz-ehlorite-miea~schist.
Loc. a
...
1
...
trace
...
0-4
3
...
,. . . .
0-4
1
...
,. . . .
0-4
2
5
2
2
...
,. .
. . . . . .
. . . . . .
...
,. .
...
2
...
,. . . .
...
1
...
9
...
5-2
1
...
8
...
21-5
...
2
L..
trace
...
2-0
...
1
...
...
1
. . . . . . . .
Banded schist-and.gneias complex :
Granitoid: grey, patchily reddened.
Loc. a
...
,, I
--.
,, b
...
,, m
...
....
,,
g
.
.
.
.
.
.
.
.
0-4
0.5
1.1
2.4
2-5
Granitized schisted sandstones, in
contact with red granite.
I.~c.
b
....
Quartz and epidote :
,,
e
,. . . .
6-8
10-4
Quartz aggregate, from red granite
and red granite-pegmatite.
Loc. a, b, c, d
2
...
trace
...
0-5
Epidosite.
,, j s
.,.
1
...
nil
...
trace
.
.
.
.
g
...
1
...
,. . . .
1-2
1 A. F. Smethurst, Anomalies in the analytical determination of w a t e r in
epidote. Min. Mag., 1935, vol. 24, pp. 173-179.
2 A. F. Smethurst, loc. cit., p. 176, analysis specimen A.
GOLD AND
Rock-type
SILVER
IN THE
MALVERN
HILLS
263
Number of
Gold grains
samples assayed,
per ton.
and locality.
Silver dwt.
per ton.
Amphibolites and hornblende.schists :
Quartz-banded.
Compact, uniform.
Loc. k
Average.
...
6
...
trace
...
1.2
1.5
1
1
2
1
...
...
...
...
,. . . .
29
...
trace
...
,. . . .
4.6
0.7
1.4
2-8
2
Migmatitic : highly modified by silicarich solutions from red granite:
reddish and purplish mig-matites.
Derivative
epidiorites.
Loc. c
...
,,
...
b
Derivative
' appinitic '
diorites,
' Hollybush' type.
Loc. l
...
...
2
...
nil
. . . . . . . .
...
3
...
,. . . .
...
2
1
...
...
,,
...
,. . . .
...
1
...
trace
...
1.2
Loc. c
...
,,
hybridized to coarse red appinitic pegmatite.
Loc. c
...
Derivative granodiorite .....
...
Biotite-hornblendite,
with grey felspars.
Loc. c
...
Appinitic derivatives.
Loc. c
...
2
...
nil
...
1.7
trace
...
,,
b
...
trace
0.7
Pyroxenite-hornblendite kindred :
Biotitite:
,,
pegmatitic.
Loc. c
with grey felspars.
Loe. c
Hornblendite.
,,
0.4
trace
0.7
with grey felspars.
Loc. c
Biotite-hornblende-pyroxenite.
1
...
1
. . . . . . . .
8.3
4
3
. . . . . . . .
...
nil
...
0.4
trace
...
1.4
13-2
Quartz-dlabase ( ' Ivy Scar' type) :
Loc.
Newer dolerites :
i
...
1
...
,,
e
...
3
...
trace
,,
b
...
4
. . . . . . . .
,. . . .
1.5
1.6
Pyrite aggregate f r o m s c h i s t s :
...
1
:..
5
...
1.1
Chalcoslne veinlet :
Loc. c
,,
j
...
1
...
trace
...
39-6
Fault-gouges:
,,
c, d
...
5
...
tr.-4
...
0.4
List of Localities.
C.
d.
e.
lg.
h.
Hay Slad quarries, ½-mile nearly due south of Worcester Beacon.
. . . . . .
{-mile due south of Worcester Beacon.
Tollgate quarries, west side of main road, ~-mile north of Wyche.
. . . . . . . . . .
about {-mile north of Wyehe.
Worcester Beacon.
Enclosure immediately north of the ' Gold Mine ', ¼-mile north of Wyche.
Crag (east side of hill-track), ¼-mile SSW. of Worcester Beacon.
,, ¼ - m i l e E S E . o f W o r c e s t e r B e a c o n .
264
i.
BRAMMALL AND DOWIE ON GOLD AND SILVER IN THE MALVERN HILLS
I v y S c a r crags.
j. North Malvern (' Pyx') quarries.
k. Gullet quarry.
l. Hollybush quarry.
m. Hill-side, about 400 yards due east of Worcester Beacon.
The results are consistent among themselves, and their correlation
leads to the following conclusions :
1. In the more basic igneous rocks, comprising pyroxenites, hornblendites, appinitic diorites, &c., intrusive diorites and dolerites, the
prevailing gold and silver values are low, of the order 1 grain of gold
and 1 dwt. of sil~:erper ton.
2. The same low values prevail in
(a) the grey granites and gneisses,
(b) the granitoid crystalline schists and paragneisses,
(c) amphibolites, hornblende-schists, and their biotitic varieties,
(d) hornblende, basic micas, muscovite, epidote, quartz, and
basic plagioclase.
3. The higher values are associated with the red granites and redgranite pegmatites, and with various types of 'mixed' rocks to which
the red granites have been contributory. They are probably related
to disseminations of haematite attributable to pneumatolytic processes which distinguish the latest phase of granite intrusion.
For the sampling schedule, and the interpretation of the assay
results, the senior author alone is responsible. The actual sampling
and the whole of the assay work were done by Mr. Dowie.
Both authors acknowledge their obligation to Prof. 13. G. H. Boswell,
F.R.S., of the Geological Department, and to Prof. Sir Harold
Carpenter, F.R.S., of the Metallurgical Department, for laboratory
facilities placed at their disposal.