A petrological study of the tin-tungsten deposit at Renosterkop, Augrabies, Northern Cape Province

(1)

A PETROLOGICAL STUDY OF THE T I N - TUNGSTEN DEPOSIT A T RENOSTERKOP, AUGRABIES, NORTHERN CAPE PROVINCE

by

ALLAN EMlLE SAAD BSc Honns.

Submitted i n p a r t i a l fulfilment of t h e requirements f o r t h e degree Master of Science

i n t h e Departement of Geology a t t h e

Potchefstroomse Universiteit v i r Christelike Hoer Onderwys

Potchef stroom June

1987

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i

A b s t r a c t

Renosterkop i s a l a r g e l o w g r a d e t i n - t u n g s t e n - z i n c d e p o s i t located 85km

WSW

o f U p i n g t o n i n t h e n o r t h e r n Cape Province, S o u t h A f r i c a . T h e mineralization i s h o s t e d

by

a n u m b e r o f shallow- d i p p i n g , sheeted g r e i s e n bodies t h a t a r e s u r r o u n d e d

by,

a n d p a r t l y i n t e r c a l a t e d

with

a well f o l i a t e d g r a n i t e gneiss c o u n t r y r o c k . T h e g n e i s s i s t a k e n t o b e l o n g t o t h e i n t r u s i v e Riemvasmaak g n e i s s o f t h e Namaqualand Metamorphic Complex.

T h e mineralized h o s t ( r e f e r r e d t o as T B Q ) i s a g r e y , homogeneous, f i n e t o medium g r a i n e d r o c k composed p r e d o m i n a n t l y o f q u a r t z , b i o t i t e a n d topaz w i t h m i n o r amounts o f f l u o r i t e a n d accessory opaque minerals, z i r c o n a n d s e c o n d a r y c h l o r i t e . T h e u n m i n e r a l i z e d g r a n i t e gneiss c o u n t r y

r o c k i s medium- t o coarse- grained, p i n k i s h i n c o l o u r a n d composed

p r i m a r i l y o f microcline, plagioclase, q u a r t z a n d b i o t i t e , w i t h o r w i t h o u t h o r n b l e n d e . Rock t y p e s , t r a n s i t i o n a l i n m i n e r a l o g y b u t w i t h c l e a r l y d i s t i n g u i s h a b l e contacts, a r e p r e s e n t between t h e T B Q a n d t h e g r a n i t e gneiss.

A

p r o m i n a n t chemical a n d mineralogical halo,

20

m t o 50 m wide, envelopes

t h e Renosterkop d e p o s i t . T h e r e i s a g r a d a t i o n a l t r a n s i t i o n f r o m an u n a l t e r e d h o r n b l e n d e b i o t i t e gneiss, t h r o u g h gneiss c o n t a i n i n g g r e e n i s h - b r o w n b i o t i t e t o a n a p p r o x i m a t e l y

2

m w i d e t r a n s i t i o n zone, c h a r a c t e r i z e d by t h e p a r t i a l replacement o f t h e g r e e n i s h - b r o w n b i o t i t e by c h l o r i t e . T h e t r a n s i t i o n zone i n t u r n y i e l d s t o t h e T B Q i n w h i c h r e d d i s h - b r o w n b i o t i t e f o r m s a t t h e expense o f t h e c h l o r i t e , a n d topaz, q u a r t z a n d . f l u o r i t e a r e f o r m e d a t t h e expense o f t h e f e l d s p a r . Major a n d t r a c e element analyses show a s p e c t r u m o f chemical compositions w i t h c o h e r e n t t r e n d s t h a t s u p p o r t a g r a d a t i o n a l t r a n s i t i o n f r o m t h e h o r n b l e n d e - b e a r i n g g r a n i t e gneiss, t h r o u g h t h e t r a n s i t i o n a l r o c k t y p e s t o t h e T B Q . T h e mineralogical a n d chemical c h a r a c t e r i s t i c s o f t h e R e n o s t e r k o p r o c k t y p e s a r e c o n s i s t e n t w i t h an o r i g i n by p r o g r e s s i v e g r e i s e n i z a t i o n o f a " w i t h i n p l a t e "

A -

t y p e g r a n i t o i d h o s t r o c k . A g e n e t i c model i s p r o p o s e d w h i c h i n v o l v e s t h e f o r m a t i o n o f t h e T B Q g r e i s e n d u r i n g i n t e n s e metasomatic a l t e r a t i o n a n d replacement o f t h e g r a n i t e gneiss w i t h i n a zone

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of structural weakness that provided conduits f o r migrating,

F-

rich, metal- bearing solutions, and t h e r e b y inherited t h e foliation and structural features present in t h e original granite gneiss.

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U i t t r e k s e l

Renosterkop i s

'n

g r o o t laegraadse t i n - wolfram- s i n k a f s e t t i n g gelee 80km

WSW v a n a f U p i n g t o n i n d i e n o o r d e l i k e Kaapprovinsie, S u i d - A f r i k a . Die mineralisasie i s b e p e r k t o t 'n aantal v l a k - hellende p l a a t a g t i g e g r e i s e n liggame w a t as gasheer o p t r e e e n w a t g e d e e l t e l i k tussengelaagd i s met,

sowel as o n d e r - e n o o r l 6 w o r d deur, 'n goedgefolieerde

granietgneisnewegesteente

w a t beskou w o r d as deel v a n d i e i n t r u s i e w e Riemvasrnaakgneis v a n d i e Namakwalandse Metarnorfekornpleks.

Die gemineraliseerde gasheer (waarna v e r w y s w o r d as T B Q ) i s 'n g r y s

homogene, f y n t o t m e d i u m k o r r e l r i g e gesteente w a t hoofsaaklik u i t kwarts, b i o t i e t e n topaas bestaan en w a t k l e i n e r hoeveelhede f l u o r i e t en b y k o m s t i g e opaak minerale, s i r k o o n e n s e k o n d 6 r e c h l o r i e t b e v a t . Die newegesteente v a n granietgneiss, w a t n i e gemineraliseerd i s nie, i s 'n

p i e n k medium- t o t g r o f k o r r e l r i g e gesteente w a t hoofsaaklik uit mikroklien,

plagioklaas, k w a r t s en biotiet, met o f s o n d e r horingblende, bestaan.

Gesteentetipes w a t o o r g a n k l i k i s in mineralogie maar d u i d e l i k

o n d e r s k e i b a r e k o n t a k t e het, i s t e e n w o o r d i g t u s s e n d i e

TBQ

e n d i e g r a n i e t g n e i s s . 'n Prominente v e r a n d e r i n g s k r a n s , 20 m t o t 50 m w y d , omsluit d i e R e n o s t e r k o p a f s e t t i n g . 'n G r a d e r i n g s o o r g a n g i s t e e n w o o r d i g w a t wissel v a n a f d i e o n v e r a n d e r d e horingblendebiotietgneis d e u r ' n b i o t i e t g n e i s w a t 'n g r o e n b r u i n b i o t i e t bevat, t o t by 'n o n g e v e e r

2

m- w y e oorgangsone w a t gekenmerk w o r d d e u r d i e gedeeltelike v e r v a n g i n g v a n d i e g r o e n b r u i n .biotiet d e u r c h l o r i e t . O p s y b e u r t gaan d i e oorgangsone oor na d i e T B Q w a a r i n r o o i b r u i n b i o t i e t t e n k o s t e v a n c h l o r i e t vorm, e n topaas, k w a r t s

en f l u o r i e t v o r m t e n k o s t e v a n veldspaat. Hoof- en

spoorelernentontledings w y s op 'n s p e k t r u m v a n chemiese samestellings w a t k o h e r e n t e n e i g i n g s toon en w a t d a a r o p d u i d a t 'n g r a d e r i n g s o o r g a n g t e e n w o o r d i g i s v a n a f d i e h o r i n g b l e n d e - d r a e n d e gneis, d e u r d i e

oorgangsgesteentetipes, t o t by d i e T B Q .

Die mineralogiese en chemiese k e n m e r k e v a n d i e Renosterkop

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progressiewe greisenisasie v a n

'n "binne plaatse"

A-

t i p e g r a n i t o i e d . 'n

Genetiese model w o r d voorgestel waarin d i e

TBQ g r e i s e n vorrn g e d u r e n d e

intensiewe

rnetasornatiese

v e r a n d e r i n g

e n

v e r v a n g i n g

v a n

n

granietgneisgasheer i n 'n s t r u k t u r e e l v e r s w a k t e sone w a t voerkanale

beskikbaar gestel h e t

vir

rnigrerende,

F-

r y k e ,

metaal-

draende

vloeistowwe, en d a a r d e u r d i e foliasie sowel as d i e s t r u k t u r e l e v e r s k y n s e l s

v a n d i e o o r s p r o n k l i k e granietgneis g e e r f het..

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T A B L E O F CONTENTS

I

.

INTRODUCTION

. . .

1 I

I

.

REGIONAL GEOLOGY

. . .

3 A

.

Previous Geological I n v e s t i g a t i o n s

. . .

3

. . .

B

.

Regional Geological S e t t i n g 4

. . .

.

Ill

.

T H E RENOSTERKOP T I N TUNGSTEN DEPOSIT 12

. . .

.

A General Geology 12 B

.

S t r u c t u r e

. . .

12 C

.

Sampling a n d Methods of I n v e s t i g a t i o n

. . .

19

D

.

General P e t r o g r a p h i c D e s c r i p t i o n

. . .

25

. . .

1 .

T h e c o u n t r y r o c k 25

. . .

2 .

T h e T B Q h o s t r o c k 27

3 .

T h e t r a n s i t i o n zone

. . .

30

4 .

T h e late stage alteration zones

. . .

32

E

.

Mineral V a r i a t i o n i n Borehole Sections

. . .

33

F

.

Mineralogy a n d Mineral C h e m i s t r y

. . .

36

1 .

Q u a r t z

. . .

36

.

. . .

2

Feldspar 37

3 .

B i o t i t e

. . .

37

4 .

C h l o r i t e

. . .

41 5

.

Amphibole

. . .

42

6 .

Topaz

. . .

44

7 .

F l u o r i t e

. . .

44

8 .

Z i r c o n

. . .

44

9 .

Gahnite

. . .

45

10 .

Accessory non-opaque minerals

. . .

47

11 .

Accessory opaque minerals

. . .

47

(a)

.

General statement

. . .

47

(b)

.

T h e g r a n i t e gneiss

. . .

47 (c)

.

T h e T B Q

. . .

50

G

.

Petrochemistry

. . .

53 I V

.

DISCUSSION A N D INTERPRETATION

. . .

59

A

.

T h e N a t u r e o f t h e G r a n i t e Gneiss

. . .

59

1 .

T r a c e element correlations

. . .

59

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. . .

2 .

D e la Roche classification

. . .

3 .

Qz

.

A b

.

O r v a r i a t i o n diagrams

4 .

T r a c e element r a t i o ' s

. . .

5 .

Aluminium s a t u r a t i o n

. . .

6 .

Comparison w i t h A - t y p e g r a n i t e s in s o u t h e a s t e r n A u s t r a l i a

7 .

Comparison w i t h t h e Bobbejaankop g r a n i t e a t Zaaiplaats T i n

Mine

. . .

8 .

Comparison w i t h a n S- t y p e g r a n i t e

. . .

9 .

Conclusion

. . .

B

.

N a t u r e o f t h e T B Q a n d i t s R e l a t i o n s h i p t o t h e G r a n i t e Gneiss

1 .

General h y p o t h e s i s

. . .

2 .

T h e topaz

-

f e l d s p a r a n t i p a t h y

. . .

3 .

T h e c h l o r i t e - b e a r i n g g r a n i t e g n e i s s

. . .

4 .

T h e chemical connection

. . .

C

.

Reference a n d Comparison t o Similar Rocks i n o t h e r Parts o f

. . .

t h e World

1 .

Mineralization associated

with

t h e Mole G r a n i t e a n d t h e

T o r r i n g t o n wolframite- b e a r i n g q u a r t z

-

topaz r o c k (silexite),

A u s t r a l i a

. . .

(a)

.

Discussion a n d comparison t o R e n o s t e r k o p

. . .

2 .

Q u a r t z - topaz- l o e l l i n g i t e r o c k s n e a r Eldorado, V i c t o r i a

. .

(a)

.

M i n e r a l o g y o f t h e topazites

. . .

(b) .

Paragenesis o f t h e t o p a z i t e s

. . .

(c)

.

Discussion a n d comparison t o R e n o s t e r k o p

. . .

D

.

Genesis o f t h e Renosterkop Tin Deposit

. . .

1

. P r e v i o u s models p r o p o s e d

. . .

(a)

.

O r i g i n by greisenization o f a g r a n i t o i d h o s t r o c k

. . . .

.

(b)

.

Sedimentary o r i g i n

. . .

(c)

.

Sedimentary x e n o l i t h

within

i n t r u s i v e g r a n i t e gneiss

. . .

(d)

.

Volcanic e x h a l a t i v e o r i g i n

. . .

2 .

P r e f e r r e d model

. . .

E

.

Conclusions

. . .

V

.

ACKNOWLEDGEMENTS

. . .

V I

.

REFERENCES

. . .

APPENDIX

I

. . .

Field d e s c r i p t i o n s a n d d e p t h s o f samples collected f r o m borehole c o r e

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v i i

APPENDIX I1

. . .

108

Mineral composition of samples of all boreholes together with brief petrographic descriptions

APPENDIX I l l

. . .

124

Graphical presentation of major and trace element chemistry

APPENDIX I V

. . .

₁₃₀

Trace element correlations

APPENDIX V

. . .

137

Niggli values and Niggli norms

APPENDIX V I

. . .

145

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I.

INTRODUCTION

Renosterkop is a large, low- grade t i n - tungsten- zinc deposit located 85 km

WSW

of Upington i n t h e n o r t h e r n Cape Province, South Africa (Figure 1). The mineralization is present i n a topaz biotite quartz host rock (abbreviated TBQ) t h a t is preserved w i t h i n t h e Riemvasmaak gneiss which comprises p a r t o f t h e Namaqualand Metamorphic Complex.

Figure 1: Locality map of t h e Renosterkop Deposit.

The TBQ occurs as a number o f shallow- dipping, sheeted bodies, containing minor intercalations o f unmineralized granite gneiss, and forming an erosion resistant r i d g e (Figure

2)

measuring 1500 m by 300 m in plan.The combined mineralized TBQ bodies v a r y in thickness from a maximum of

60

m t o a minimum o f 10 cm, with an average thickness of 20 m t o 30 m.

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- --. .--- ... --- -

---2

The

economic

potential

of the

Renosterkop

deposit

is currently

being

evaluated

by Rio Tinto

Exploration

(pty)

Limited,

which company

holds

the

mineral

rights

to the

area.

Rio Tinto

has investigated

the deposit

by detailed

geological,

geochemical,

geophysical

and drilling

methods.

The

purpose

of

this

study

was

to

ascertain

the

petrographic

and

petrochemical

nature

of the

mineralized

TBQ and its host rocks,

and to

use this

information

to outline

a probable

mode of origin

for the TBQ.

Such knowledge

is deemed useful for further

exploration

in the area.

Figure

2:

_Looking

_north

_towards

_Renosterkop,

_the

_prominant

topographic

nature

of the ridge as well as the sheeted

nature

of the TBQ, underlain

by granite

gneiss

country

rock,

are

displayed.

The contact

between

the

TBQ and the granite

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II. REGIONAL GEOLOGY

A. Previous Geological Investigations

T h e earliest geological w o r k i n t h e area was undertaken by Rogers and

Swartz (19001, Rogers (19081, and Rogers and Du T o i t (1909 and 1910).

T h e y described t h e geology of t h e Orange River Valley i n t h e Hopetown

and Prieska districts,

undertook

a

geological s u r v e y of p a r t s o f t h e

districts o f Kuruman, V r y b u r g , Hay, and Gordonia, and reported on t h e

geology o f parts of t h e d i s t r i c t s o f Kenhardt, Prieska, Hay, Britstown,

Carnarvon, and Victoria West respectively.

A f t e r t h e outbreak o f t h e Second World War t h e Geological Survey

embarked on

a

programme o f exploration f o r strategic minerals, and

it

was decided t o map t h e rocks i n t h e Orange R i v e r Valley between

Upington and t h e sea i n o r d e r t o access t h e economic potential of t h e area

(Hugo,1969).

T h e task of unravelling t h e complicated geology and

deciphering t h e complex geological h i s t o r y and s t r u c t u r e of an area

embracing 5200 square kilometers between Kakamas and Onseepkans fell

t o J.W. von Backstrom and D.J.L. Visser.

Work commenced

a t

t h e beginning o f 1941 and was completed d u r i n g 1946r

Von Backstrom mapped t h e areas i n t h e v i c i n i t y of Kakamas (sheet 2820D)

and t h e Bokvasmaak Native Reserve, i.e. t h e southern portion of sheet

.2820A. Visser surveyed t h e area south of t h e Orange River between t h e

Hartbees R i v e r and Onseepkans, i.e. t h e northern p a r t s of Sheets 28200

and 2819D, and t h u s covered t h e present s t u d y area. Von Backstrom later

completed his PhD thesis on t h e area 2820D and t h e major portion of area

28210, and t h i s map was later published on a scale o f 1:125 000 (Von

Backstrom, 1964). T h e w o r k undertaken b y Visser was never published

b u t was used as a base plan

by Hugo (1969) who later undertook a

detailed s t u d y of t h e pegmatites o f t h e area.

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The most recent w o r k c o v e r i n g t h e s t u d y area was undertaken b y Praekelt (1984) who remapped Sheet 2820C on a scale o f 1:100 000 f o r h i s MSc thesis. T h i s mapping included t h e Renosterkop deposit and

s u r r o u n d i n g area a n d has since been p a r t l y revised

by

t h e Geological

Survey and incorporated i n t o a recently compiled geological map o f Sheet 28200 (scale 1:250 000) which w i l l soon be published.

B. Regional Geological S e t t i n g

The region is underlain by rocks which are described

by

SACS (Kent,

1980) as forming p a r t o f t h e Korannaland Sequence o f t h e Namaqualand Metamorphic Complex. T h e l i t h o s t r a t i g r a p h i c designation Namaqualand

Metamorphic Complex includes metasedimentary, metavolcanic and

i n t r u s i v e r o c k u n i t s which are predominantly gneissic i n character. The

Complex underlies a Proterozoic tectonic province which has been variously r e f e r r e d t o as t h e Namaqua Mobile Belt, Orange R i v e r Belt,

Namaqua Province o r Sonama Province (Kent, 1980); it is bounded by t h e

Archean Kaapvaal Province, younger cover rocks and t h e A t l a n t i c

coastline.

The l i t h o s t r a t i g r a p h i c subdivision o f t h e Namaqualand Metamorphic

Complex i s presented by SACS (Kent, 1980) as an ad hoc framework f o r

f u r t h e r improvement as more information i s obtained, and i s g i v e n i n

Table

1.

T h e Korannaland Sequence loosely groups together a number o f rock formations, t h e s t r a t i g r a p h i c relations between which are imperfectly known. These formations are given i n Table 2.

The lithology o f t h e constituent formations, and t h e t y p e areas o f t h e

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Table

1 Lithostratigraphic subdivision

of

the Namaqualand Metamorphic Complex

Koperberg Suite

Spektakel Suite

Keimoes Suite

Hoogoor Suite

Little Namaqualand Suite

Gladkop Suite

Vioolsdrif Suite

Orange River Group

Okiep Group

Bushmanland Group

Korannaland Sequence

Marydale and Kaaien Groups

Syntectonic intrusive rock units,

radiometrically dated 1100 to

1900 Ma (Kent, 1980)

Pretectonic metasedimentary

and metavolcanic rock units,

radiometrically dated 1350 to

2000 Ma (Kent, 1980)

Table

2 Formations

of the Korannaland Sequence (Kent,1980)

Toeslaan Formatio:

Goede Hoop Formation

Rautenbach se Kop Formation

Kenhardt Formation

Bies

je Poort Formation

Kokerberg Format

ion

Eierdoppan Formation

Jannelsepan Formation

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20.00' WOW 22.00'

I

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Table 3

Lithology of the Korannaland Sequence (Kent, 1980)

FORMATION

Goede Hoop

Rautenbach se

K ~ P

Kenhardt

Biesje Poort

Kokerberg

Toeslaan

Eierdoppan

Jannelsepan

LITHOLOGY

Metaquartzite, muscovite q u a r t z i t e and

conglomerate

Quartzo

-

feldspathic gneiss

,

Predominantly a leucocratic b i o t i t e gneiss

Calc

-

s i l i c a t e rocks, streaky leucogneiss, b i o t i t e

gneiss, marble and amphibolite

Quartzo

-

feldspathic gneiss with interlayered

metaquartzite

Garnet

-

s i l l i m a n i t e

-

c o r d i e r i t e

-

b i o t i t e gneiss;

garnet

-

bearing quartzo

-

feldspathic gneiss;

b i o t i t e gneiss with amphibolite

Conglomerate and s c h i s t

Amphibolitic rocks and associated b i o t i t e

s c h i s t s and gneisses; c a l c s i l i c a t e rocks;

garnet

-

sillimanite

-

b i o t i t e gneiss

The granite gneiss units found

in the immediate vicinity of the study area

and underlying the

TBQ

a t Renosterkop a r e regarded by SACS (Kent,

1980) as belonging to t h e syntectonic intrusive

rocks of the Hoogoor

Suite, which a r e intrusive into t h e Kokerberg Formations, and are

broadly defined as undifferentiated leucocratic quartzo

-

feldspathic

gneiss

u n i t s that are usually fine- to medium- grained and reddish- brown

.in outcrop.

In places t h i s gneiss

-

henceforth referred to as granite

gneiss

-

contains nodules

with a variable amount of sillimanite

(Kent,1980), o r

it

may

assume a coarse granitic aspect o r become

megacrystic. Bands of fine- grained white quartzo- feldspathic rock as

well as lenses of calc- silicate rock, quartzite, schist and amphibolite are

common (Kent, 1980). The granite gneiss, also referred to as the pink

gneiss, underlies a large area and i t may not necessarily represent a

single rock

u n i t throughout. Accordingly

it

has been interpreted as

intrusive granites by some researchers, and granitized metasediments by

others. Suggested parent rocks range from arkose (Poldervaart and von

(16)

Backstrom, 1949; Geringer, 1973; Moore, 19771,to r h y o l i t e (Joubert,

1974;

Botha e t al.,

1976) and g r a n i t o i d (Coetzee,

1941; Lipson and

McCarthy, 1977; Colliston, 1979). Most o f these speculations a r e based

either on f i e l d relations o r on geochemistry. However, owing t o t h e

immaturity of clastic sediments such as arkoses and greywackes,

t h e

difference i n chemical composition between such sedimentary and igneous

rocks may not be pronounced (Schultz, 1978).

On t h e Geological Map o f t h e Republics o f South Africa, Transkei,

Bophuthatswana, Venda and Ciskei and t h e Kingdoms o f Lesotho and

Swaziland (1984, scale 1:1000 OOO),

t h e p i n k gneiss u n i t s were mapped

as t h e Eendoorn granite, forming p a r t o f t h e L i t t l e Namaqualand Suite.

The Geological S u r v e y presently classifies t h e p i n k gneiss u n i t s as

forming

p a r t

o f t h e i n t r u s i v e Riemvasmaak gneiss (G. Moen,

personal

communication) and describe

it

as a p i n k - weathering g r a n i t e gneiss with

a granular o r Augen t e x t u r e .

Praekelt (1984) mapped t h e area (Figure 4) i n t h e immediate v i c i n i t y of

t h e present s t u d y area on a scale of 1:100 000 and considered t h e

Renosterkop deposit as a zenolith o f metasediments of Omdraai Formation

within t h e Rooipad granite. He described Renosterkop as a topaz- bearing

quartzite and suggested t h a t t h e topaz formed

by

reaction o f

hydrothermally active f l u i d s w i t h t h e zenolith of Omdraai metasediments

as well as with t h e u n d e r l y i n g Rooipad granites.

The Omdraai Formation is described (Praekelt, 1984) as a fine-

to

medium-

grained yellowish leucocratic quartz- feldspar gneiss interbedded with a

few t h i n layers o f massive quartzite, biotite schists a n d amphibolites.

T h e Formation

i s

surrounded

by

later intrusions of Rooipad and Brabees

granites.

T h e Rooipad g r a n i t e is a well foliated medium t o coarse- grained biotite

granite with minor hornblende and

it

exibits conformable contacts with

t h e Seekoeisteek, Brabees, Eendoorn and Augrabies granites. Xenoliths

o f t h e Omdraai formation are common w i t h i n t h e Rooipad granite (Praekelt,

19841, b u t no mineralogical description

i s

given of these xenoliths.

(17)

GEOLOGICAL LEGEND SEDIMENTARY ROCKS FORMATION LITHOLODV IGNEOUS ROCKS LITHOWOV EENWORN GRANITE

OMORAAI OIMRTZ-FELDSPAR GNEISS WITH OUdRTZITE CATbCIASTIC ZONE WITH LAYERED AND

WR'4S _{IAMINATU) WARTZITE AND AMPHlWUlE}

J

2S,b'L

SAAYwERR AND CDVWWERAlE EARIN0 WARTZITE ORIEIOP- WITIUP LEUODCRATIC AUGEN GNEISS WlTH BIOTITE- CROUP

OIMRRITE

F i g u r e

4:

A

simplified geological map of t h e regional geology a r o u n d Renosterkop (Praekelt,

1984).

(18)

T h e Brabees g r a n i t e (Praekelt, 1984) i s

a

w e l l f o l i a t e d a n d i n places p o r p h y r o b l a s t i c r e d - b r o w n g r a n i t e w h i c h contains i n c l u s i o n s o f q u a r t z i t i c a n d mafic inclusions. No r e l a t i v e age r e l a t i o n s h i p between t h i s g r a n i t e a n d t h e Augrabies, Rooipad, o r Eendoorn g r a n i t e s c o u l d b e ascertained

by f i e l d o b s e r v a t i o n s .

T h e A u g r a b i e s g r a n i t e i s d e s c r i b e d by P r a e k e l t (1984) as medium- t o coarse- g r a i n e d w e a k l y f o l i a t e d b i o t i t e a n d h o r n b l e n d e - b e a r i n g g r a n i t e w h i c h contains zenoliths o f a f i n e - t o medium- g r a i n e d g r a n i t e o f t h e same composition. Close i n s p e c t i o n o f t h e A u g r a b i e s g r a n i t e by t h e w r i t e r h o w e v e r i n d i c a t e d a t least t w o w e l l developed superimposed f o l i a t i o n d i r e c t i o n s developed i n t h i s granite, a n d a l t h o u g h w e a k l y f o l i a t e d r e l a t i v e t o t h e o t h e r gneiss i n t h e area, it i s s t i l l well- f o l i a t e d i n t h e absolute sense. P r a e k e l t (1984) f u r t h e r m o r e f o u n d t h e A u g r a b i e s g r a n i t e o n l y t o b e i n c o n t a c t w i t h t h e Rooipad g r a n i t e , a n d a l l c o n t a c t s w e r e f o u n d t o b e conformable. No age r e l a t i o n s h i p between t h e s e t w o g r a n i t e s c o u l d t h u s b e d e r i v e d f r o m f i e l d observations. T h e r e l a t i v e l y weak f o l i a t i o n t h a t i s d e v e l o p e d in t h e A u g r a b i e s g r a n i t e may h o w e v e r p o i n t t o i t s y o u n g e r age.

P r a e k e l t (1984) d i v i d e d t h e area 2820C i n t o t h r e e s t r u c t u r a l terranes, ie. t h e Upington, M a r c h a n d a n d B l a d g r o n d Terranes, w h i c h h e separated f r o m one a n o t h e r by p r o m i n e n t t h r u s t f a u l t s (see F i g u r e

4).

Each t e r r a n e i s c o n s i d e r e d as c o n t a i n i n g u n i q u e lithological, s t r u c t u r a l a n d metamorphic c h a r a c t e r i s t i c s . T h e R e n o s t e r k o p d e p o s i t f a l l s w i t h i n t h e U p i n g t o n T e r r a n e in w h i c h f o u r phases o f deformation w e r e recognized, a n d w h i c h has been t h r u s t - f a u l t e d in a s o u t h - w e s t e r l y d i r e c t i o n o v e r t h e M a r c h a n d T e r r a n e .

Based o n t h e d e g r e e o f

K-

metasomatism as w e l l as t h e development o f

r o c k f a b r i c , P r a e k e l t p r o p o s e d t h a t t h e Rooipad g r a n i t e i s o l d e r t h a n t h e Brabees g r a n i t e , w h i c h i n t u r n i s o l d e r t h a n t h e A u g r a b i e s g r a n i t e .

F o r t h e p u r p o s e o f t h e l a t e s t 1:250 000 geological map o f t h e area ( i n

p r e s s ) t h e Geological S u r v e y h a s combined t h e Brabees, Rooipad a n d

Seekoeisteek g r a n i t e s a n d c a l l e d t h e m t h e Riemvasmaak g n e i s s (G. Moen, p e r s o n a l communication).

(19)

It i s e v i d e n t f r o m t h e above information t h a t t h e geology o f t h e area i n

t h e v i c i n i t y o f t h e Renosterkop deposit i s n o t completely understood a n d

t h a t much more detailed w o r k remains t o b e done. For t h e p u r p o s e o f t h i s

study, t h e classification proposed

by

t h e Geological S u r v e y is accepted.

A c c o r d i n g l y t h e g r a n i t e gneiss w h i c h hosts t h e Renosterkop deposit is

t a k e n t o b e p a r t o f t h e Riemvasmaak gneiss.

(20)

Ill.

T H E RENOSTERKOP T I N

-

TUNGSTEN DEPOSIT

A.

General Geology

T h e R e n o s t e r k o p d e p o s i t c o n s i s t s o f l a r g e sheeted bodies o f shallow- d i p p i n g t o p a z b i o t i t e q u a r t z r o c k ( T B Q ) v a r y i n g i n t h i c k n e s s f r o m

centimeters u p t o

60

m

in

places. T h e sheets o f T B Q o v e r l i e a well

f o l i a t e d pink g r a n i t e gneiss, i.e. Riemvasmaak gneiss, w i t h a

c o n s i s t e n t l y f l a t shallow- d i p p i n g b o t t o m contact. Conformable

i n t e r c a l a t i o n s o f g r a n i t e gneiss a r e p r e s e n t between t h e i n d i v i d u a l T B Q sheets. No c o n t a c t i s i d e n t i f i a b l e w i t h i n t h e T B Q w h e r e t w o sheets merge.

T h e T B Q h o s t s low- g r a d e tin, t u n g s t e n a n d z i n c mineralization, whereas t h e g r a n i t e i s n o t mineralized.

A t r a n s i t i o n zone, m e a s u r i n g

2

t o

3

m in thickness, in w h i c h t h e b i o t i t e i s p a r t i a l l y o r t o t a l l y r e p l a c e d by c h l o r i t e , a n d in w h i c h topaz, q u a r t z a n d f l u o r i t e a r e f o r m e d a t t h e expense o f feldspar, i s p r e s e n t between t h e T B Q a n d t h e g r a n i t e gneiss. T h e c o n t a c t between t h i s t r a n s i t i o n zone a n d t h e T B Q i s g e n e r a l l y sharp, b u t i s also seen t o b e g r a d a t i o n a l i n places. L a t e stage a l t e r a t i o n zones a r e common w i t h i n b o t h t h e T B Q a n d t h e g r a n i t e gneiss.

A p l a n o f t h e s u r f a c e geology o f t h e d e p o s i t as w e l l as t h e localities o f boreholes u s e d f o r t h i s s t u d y a r e shown

in

F i g u r e

5.

F i g u r e

6

shows n o r t h - s o u t h p r o f i l e s l o o k i n g east across t h e t h r e e sections used.

B. S t r u c t u r e

A n aerial impression o f t h e R e n o s t e r k o p d e p 0 s i t . i ~ t h a t it f o r m s a shallow n o r t h e r l y dipping t i g h t s o u t h - v e r g e n t s y n f o r m a l f o l d w i t h a g e n t l e

(21)

Figure

5 :

Plan of the surface geology of the Renosterkop deposit.

LEGEND

61

TBQ

GRANITE GNEISS

BOREHOLE NUMBER ?AES/l AND INCLINATION

(22)

SECTION 400m E

SECTION ZERO

SECTION 400m W

LEGEND

-

ln

+

+ W

+

R C W

Figure

6:

Profiles across the Renosterkop deposit.

(23)

eastward p l u n g e and t r a v e r s e d

by

prominent f a u l t s on which no definite

direction o f movement can be detected (Hartnady,

1985).

No f i e l d

evidence could however b e f o u n d t o substantiate t h e presence o f such a

tight

south- v e r g e n t synformal f o l d s t r u c t u r e . It would r a t h e r appear t h a t t h e deposit comprises a composite o f sheetlike bodies o f variable

thickness as i l l u s t r a t e d i n Figures

7

and

8.

On a local and regional scale,

t h e dominating f a b r i c element observed i n t h e g r a n i t e gneiss is a tectonic

foliation (Hartnady,

19851,

and i s f o r practical purposes here r e f e r r e d

t o as

St.

No evidence could b e f o u n d f o r

S1

b e i n g o v e r p r i n t e d over an

earlier tectonic fabric, a n d it apparently represents t h e last major

tectonic deformation t h a t was operative i n t h e t e r r a n e . As a general r u l e t h e sheetlike bodies o f TBQ are orientated r o u g h l y parallel t o t h i s foliation i n t h e g r a n i t e gneiss. Locally however t h e y c u t obliquely across t h e foliation o f t h e g r a n i t e gneiss as i l l u s t r a t e d i n F i g u r e

9.

I n t h e TBQ,

S1

i s defined

by

oriented b i o t i t e and also by a mm- t o cm-

scale phase l a y e r i n g defined p r i m a r i l y by variations i n b i o t i t e abundance.

T h i s foliation i s parallel t o t h e foliation i n t h e s u r r o u n d i n g g r a n i t e gneiss,

which i s defined by oriented b i o t i t e and elongated Augen- l i k e q u a r t z -

feldspar aggregates.

T i g h t isoclinal folding (Figures

10

and

11)

w i t h i n c e r t a i n sheets of TBQ,.

and i n t h e wedges o f g r a n i t i c gneiss between t h e sheets, are

superimposed on

S1.

These s t r u c t u r e s do n o t display axial plane cleavage

o r foliation, and a r e non- penetrative w i t h variable plunges of t h e f o l d axes.

.The t h i r d t y p e o f f o l d i n g seen i n t h e TBQ i s represented by open, non-

cylindrical, g e n t l y o r doubly- p l u n g i n g "whaleback" antiforms and

synforms i n

S1

and may b e caused by disharmonic, viscoelastic b u c k l i n g

o f t h e

S1

f a b r i c along NW t o NNW trends. L a t e r i n t e r f e r e n c e patterns

t r e n d i n g NE t o NNE a r e superimposed on t h i s e v e n t and r e s u l t i n t h e formation o f basin- dome i n t e r f e r e n c e p a t t e r n s ( F i g u r e

12).

T h e major

NE

and NW t r e n d i n g f a u l t zones ( F i g u r e

13)

and joints (Figure

(24)

16 Figure

7:

An approximately

1 m wide sheet of dark-

coloured

TBQ seen

along

strike

within

the granite

gneiss

country

rock.

This

sheet

thickens

in the distance

and eventually

merges

with

other

TBQ

sheets.

In the foreground

the sheet

thins

and

eventually

pinches

out (illustrated

in Figure 9).

-Figure 8:

A section of the northern

face of Renosterkop

illustrating

the

composite sheeted

bodies of TBQ reaching

a thickness

of over

40 m in this locality.

In the foreground

late-

stage warping,

(25)

17 Figure 9:

The cross-

cutting

nature

of the TBQ is illustrated

by the

splitting

veins

to the left and

right

of the person

standing

in the photograph.

The right-

hand

vein

cuts

across

the

foliation

while

the

Ieft-

hand

split

runs

parallel

to

the

foliation.(---L-

foliation strike

and dip)

Figure

10:

Looking toward

the west,

this

photograph

shows

shallow-dipping

beds on the northern

limb overlying

and overturning

the

beds

on

the

southern

limb of this

structure.

This

structure

together

with other

field evidence

led to the

in-terpretation

that

thrust

faulting

had possibly

been

(26)

18 Fig.ur~ 11:

Remnant of a tight

isoclinal fold clearly

displaying

the phase

layering

within the fold.

-.Figure

12:

Open non-

cyclindrical

"whaleback"

antiforms

and

sinforms

seen well exposed

on the eastern

extension

of

Renosterkop

(27)

Figure

13:

Looking SW across

a section

of Renosterkop

a NE trending

fault

zone may be seen intersecting

the TBQ ridge

(arrows)

and dissapearing

into the granite

gneiss

country

rock in the

foreground.

Hematitic alteration

is strongly

developed

along

the northern

extensions

of this fault zone.

An important

observation

is that

there

is no evidence

for the existence

of non- penetrative

isoclinal folding

in the granite

gneiss

country

rocks,

as is observed

in the deformed TBQ.

c. Sampling and Methods of Investigation

The

Renosterkop

deposit

was geologically

mapped

by a team of six Rio

Tinto geologists.

A total of 80 ha was covered

by grid

controlled

detailed

geological

mapping

undertaken

on a scale of 1:500. A drilling

programme

totalling

55 diamond drill boreholes

and 12 percussion

drill boreholes

was

undertaken

between

1981

and 1985.

19

(28)

' I

Three

repr~s6ntativa~&ological

sections located 400 m apart w e b selected

f o r diamond

drill

b a r e h l e core sampling across t h e Renosterkap

dew&

(Figure 6). Seven boreholes intersected section 400

E, f i v e intersected

section ZERO, and t h r e e intersected section 400W. Each o f t h e fifteen

boreholes

was

logged i n detail

and each

geological

zone

was

representatively sampled. Except f o r borehole no. AES/3 which was

selectively sampled using assay values as criteria, t h e number of samples

collected depended on t h e thickness of each apparently uniform geological

zone.

Field descriptions and depths of samples collected from borehole

core are g i v e n i n Appendix

I. Figures 14,

15

and 16 indicate borehole geology, sample localities, and

sample numbers. Special attention was given t o t h e sampling o f geobgicaj

contacts a n d a total o f 198 diamond drill borehole core samples was

collected f o r t h e purpose o f t h i s study. A l l samples have been numbered

"X"/"Y"

_,

where

"X" represents t h e borehole number, and "Y"

represents t h e sample number. A n additional 23 samples numbered RM"YW

were taken from a previous investigation undertaken b y De Waal(1985)

and included i n t h i s s t u d y .

Thin,

polished and polished t h i n sections were examined using a

conventional petrographic microscope. T h e microscopic identification of

minerals was v e r i f i e d

by means of x -

r a y

d i f f r a c t i o n techniques and

electron microprobe investigations. The chemical analyses o f rock samples

were c a r r i e d o u t b y Bergstrom and Bakker.

Mineral chemistry was ascertained using a Jeol

733 Superprobe

a t

a beam

.voltage o f 15kV a n d a c u r r e n t o f 0,2 x

lo-'

amp. Table 4 lists t h e

(29)

SECTION ZERO

AES/I A E M font. AES/5 cont.

1

9

10

117,70m

AES/4 AES/4 cont.

LEGEND

+

ALTERED T8Q

+

I,FELDSPATHIC

TBQ

GRANITE GNEISS VERTICAL S C A L E 1 : 5 0 0

AEWI : BOREHOLE No.

Figure

14:

Renosterkop drill sections indicating lithology and sample localities.

(30)

VERTICAL SCALE : 1 : 5 0 0

Figure 15: Renosterkop d r i l l sections indicating lithology and sample localities. L E G E N D A L L U V I U M A L T E R E D TBO F E L D S P A T H I C TBO T B Q GRANITE GNEISS N N A E W BOREHOLE No.

(31)

SECTION 4 0 0 m

W

L E G E N D

VERTICAL SCALE1 1

:

5 0 0

Figure

16:

Renosterkop drill sections indicating lithology and sample localities.

ALLUVIUM

G R A N I T E GNEISS

AES/30: BOREHOLE No N I

W

(32)

Table 4

Table o f standards Wlth r e l a t e d elements used f o r mineral a n a l y s e s ' o n t h e Joel mlcroprobe.

Wi I l e m i t e S p h a l e r i t e C u p r i t e C o b a l t 1 t e z n Zn, S Cu Co, Fe

(33)

25 D. General Petrographic

Description

1. The country rock

The

Riemvasmaak

gneiss

that

forms

the

country

rock

is a pinkish,

medium- to coarse-

grained,

sporadically

porphyroblastic,

pronouncedly

foliated

granite

gneiss

consisting,

per

volume,

.of 40 to 45 % feldspar

(roughly

75 % is microcline

and 25 % is plagioclase),

30 -

40

%

quartz

and

10 -

15 % biotite

(Figures

17 and

18).

Hornblende,

up to 15 %, enters

the

mode of the granite

gnefss

50 m below the lower contact

and 20 m

above the

upper

contact

of the TBQ (Figure

19).

The feldspar

forms

lenticular

aggregates

set in a matrix

of quartz

.and orientated

biotite.

The microcline feldspar

is commonly replaced

by sericite

and kaolinite and

the biotite

by chlorite.

I

z-in

_V

.

Figure

17:

Photomicrograph

of

a

typical

granite

gneiss

displaying

greenish-

brown

biotite

in a matrix of quartz

and feldspar.

Three

small fluorite

crystals

(F) and a single

zircon

(Z) are

present.

The feldspar

is partly

sericitized

(Se).

Plane- polarized

light

(34)

26 Figure

18: The same field as in Figure 17 is shown here under

crossed-nicols. The fluorite displays isotropic characteristics and the zircon'shows green interference colours.

Scale 1 : 0,026

Figure 19: Photomicrograph of the hornblende biotite granite gneiss

displaying biotite (8), hornblende (H), allanite (A), sphene (S), zircon (Z), magnetite (M), sericite (Se), microcline (Mi) and quartz (Q).

Plane- polarized light

(35)

Accessory n h r e W F ' f e u n d are small amounts o f f l u o r i t e 4 s p e m d i t d l y

disseminated), zircon, sphene a n d garnet. Traces o f allanite, apatite,

calcite and opaque minerals are also present. Traces o f topaz are sparsely disseminated i n t h e g r a n i t e gneiss.

2.

T h e TBQ h o s t r o c k

T h e TBQ i s a foliated, grey, homogeneous, f i n e - t o medium- grained

topaz b i o t i t e q u a r t z r o c k which t y p i c a l l y contains, p e r volume, 50 t o

60 %

quartz, 15

-

30 %

biotite,

10 -

20

%

topaz, less t h a n 5

%

f l u o r i t e #rd

t r a c e amounts o f opaque minerals, zircons and secondary c h l o r i t e (Figure

20). Variable amounts o f feldspar, usually microcline, are found i n t h e

T B Q e i t h e r as Augen o r as small bands developed parallel t o t h e foliation. Depending on t h e amount o f f e l d s p a r ' i n t h e rock, various intermediate stages may be seen between a feldspar- f r e e TBQ and a g r a n i t e gneiss.

With t h e exception o f topaz, which occurs as l a r g e r porphyroblasts, most o f t h e major constituents form a granoblastic aggregate w i t h morphologicdl features t y p i c a l o f grain- surface equilibrium (Figures 20, 21 and 22): T h e s t o u t b i o t i t e flakes ( F i g u r e 22) generally show a d i s t i n c t p r e f e r r e d orientation which defines a foliation t h a t i s r o u g h l y followed b y t h e

elongated g r a i n s o f q u a r t z and topaz ( F i g u r e 21). F l u o r i t e and opaque

minerals generally occur as smaller g r a i n s disseminated heterogeneously

t h r o u g h o u t t h e rock and cassiterite (Figures

23

and 24) i s a p r i m a r y

(36)

28 Figure 20:

Photomicrograph

of the TBQ showing granoblastic

aggregates

of quartz

(Q),

topaz

(T),

red-

brown biotite

(B),

fluorite

(F) and sphalerite

(Sp.)

Plane- polarized

light

Scale 1 : 0,026

.

Q

. .

_{.' ,}

..

1 0.":

_..

'.

Figure 21:

TBQ displaying

elongated,

porphyroblastic

grains

of

topaz

(T),

quartz

(Q),

and orientated

red-

brown

biotite

(B).

The opaque

mineral

is sphalerite.

The

elongation

direction

in the topaz is parallel to the orientation

of the biotite flakes.

Plane- polarized

light

(37)

29 ~_.--

.

,.

.

Figure

22:

Stout,

red-

brown

biotite

flakes

in the TBQ that

are partly

chloritized

in places.

A small fluorite

inclusion

(F) is present

in the

biotite

and in the

sphalerite

(Sp)

adjacent

to

the

biotite.

Plane- polarized

light

Scale 1 : 0,026

-Figure 23:

Photomicrograph

of the TBQ displaying

two cassiterite

grains

(e),

red-

brown

biotite

showing

traces

of chloritization,

quartz,

topaz

(T) and sphalerite.

Plane- polarized

light

Scale 1 : 0,026

(38)

30 Figure

24:

The same field as in Figure

23 shown here

under

crossed-nicols.

The

cassiterite

displays

high-

order

interference

colou rs

.

Scale 1 : 0,026

3. The transition

zone

The

transition

zone is present

wherever

the TBQ is in contact

with the

granite

gneiss.

The transitional

rock

type

basically

is a granite

gneiss

in which the

greenish-

brown

biotite,

typical

of the

granite

gneiss,

is

almost

completely

broken

down to form green

chlorite

containing

many

minute

rutile

crystals

(Figure

25),

and minor K- feldspar

is replacing

the

biotite

aggregates.

It is not clear

if the

latter

two

reactions

are

genetically

linked

or not.

Closer

to the TBQ, a reddish-

brown variety

(39)

31 Figure 25:

Biotite of the transition

zone shown here

to be totally

re-placed by chlorite

and surrounded

by quartz

(Q), microcline

(Mi) and

sericite

(Se).

Minute

black

rutile

crystals

(not

discernable)

are

present

in

the

biotite.

Secondary

K

-feldspar

(SMi) formed at the expense

of the biotite.

Plane- polarized

light

Scale 1 : 0,026

-Figure 26:

Photomicrograph

of red-

brown

biotite,

replacing

the green

chlorite

of the transition

zone,

surrounded

by

microcline,

quartz

and sericite.

A small fluorite

inclusion

is seen in the

biotite.

Plane- polarized

light

Scale 1 : 0,026

(40)

32 This

reddish-

brown

biotite

is characteristically

found

throughout

the

TBQ.

Minor

intercalated

lenses

of

TBQ

are

also

developed

in

the

transition

zone.

Figure

27:

The same field as Figure

26 shown here under

crossed-

nicols

after

rotation

by 45 o. The twinning

in the feldspars

is now

clearly

revealed.

Scale 1 : 0,026

4. The late stage alteration zones

.Late stage alteration

of the granite

gneiss

and TBQ includes

silicification,

sericitization

of feldspars,

chloritization

of biotite and hematization.

The

degree

of alteration

varies

from slight

to intense

and occurs

in

zones

ranging

in thickness

from centimeters

to meters.

Silicification

is found

in both

the granite

gneiss

and the TBQ.

In the

vicinity

of major fault

zones,

the

silicification

is found

associated

with

hematization.

(41)

Sericitization o f feldspar and topaz is commonly f o u n d i n t h e TBQ

associated w i t h a corresponding increase i n f l u o r i t e which is seen t o

pseudomorphously replace t h e biotite i n t h e rock.

Sericitization and

saussuritization of t h e feldspar i n t h e granite gneiss are common and t h e

microcline may be seen t o change colour from clear

pink t o h i g h l y

sericitized yellowish- white crystals.

Plagioclase is i n v a r i a b l y zoned and

saussuritized i n t h e core areas.

Chloritization is commonly f o u n d i n late stage alteration zones i n both t h e

TBQ and t h e granite gneiss, and i s seen t o form a t t h e expense of biotite.

Hematization is common i n t h e v i c i n i t y of f a u l t zones and is associated

w i t h an increase i n quartz a n d a decrease i n t h e biotite content of t h e

rock. T h e hematite is f o u n d as mottled reddish- brown patches i n t h e

altered rock.

E. Mineral Variation i n Borehole Sections

Borehole A E S N has been selected as a typical example of a cross- section

t h r o u g h t h e TBQ host rocks, t h e transition zone, and t h e granite gneiss

c o u n t r y rock. The mineral composition of t h e samples of borehole A E S N

i s given i n Table 5 and t h a t o f a l l o t h e r boreholes together with a b r i e f

petrographic description is g i v e n i n Appendix

I I .

.In borehole AES/4 t h e red- brown biotite v a r i e t y is seen developed

t h r o u g h o u t t h e TBQ. Wherever feldspar- r i c h zones are present i n t h e

TBQ, as well as i n t h e transition zone, t h e red- brown biotite takes on

a deeper red- brown colour a n d becomes intermixed w i t h chlorite (Figure

26).

Away from t h e TBQ and beyond t h e chlorite- bearing transition

zone, t h e biotite f o u n d i n t h e g r a n i t e gneiss has t h e d a r k greenish-

b r o w n appearance and as one moves f u r t h e r away from t h e TBQ,

hornblende also enters t h e mode. T h i s general trend, which i s illustrated

i n F i g u r e

28,

is observed i n a l l t h e borehole intersections w i t h t h e

exception o f borehole AES/5.

Extensive hematitic and chloritic alteration

(42)

T a b l e 5 M i n e r a l composition o f samples o f borehole AES/4

I

MINERAL SAMPLE NUMBER

LEGEND

TBQ rock

T r a n s i t i o n zone G r a n i t e g n e l s s

(43)

LEGEND

-

TBO CHLORITIZED TRANSITION ZONE

i-\?

(44)

which i s present throughout borehole AES/5, is ascribed t o late stage alteration related t o t h e f a u l t seen t o pass close t o AES/5 i n t h e plan of t h e surface geology (Figure

5).

I n t h e T B Q and granite gneiss a pronounced antipathetic relationship between feldspar and topaz exists. Topaz is not significantly present i n t h e g r a n i t e gneiss c o u n t r y rock. T h i s is clearly illustrated i n Table 5. Concomitantly with an increase i n t h e topaz content of t h e rock, a corresponding increase is observed i n t h e sphalerite and cassiterite content. T h i s observation is supported

by

an increase i n t h e density o f t h e rock.

F. Mineralogy a n d Mineral Chemistry

I n t h e following sections t h e constituent minerals will b e described roughly i n an o r d e r of decreasing abundance i n t h e TBQ and granite gneiss.

1.

Quartz

I n t h e g r a n i t e gneiss c o u n t r y r o c k t h e q u a r t z i s present as medium- t o .coarse- grained, anhedral crystals which usually show an undulatory extinction.

A

second generation o f smaller quartz grains showing a weak undulatory extinction also is present.

T h e q u a r t z i n t h e TBQ host r o c k forms small- t o medium- sized grains w i t h i r r e g u l a r outlines, and these too show undulatory extinction. Smaller q u a r t z g r a i n s as well as occasional large porphyroblastic quartz grains are also f o u n d i n t h e TBQ, and these show no undulatory extinction.

(45)

2.

F e l d s p a r

T h e f e l d s p a r o c c u r s as medium- t o coarse- g r a i n e d anhedral c r y s t a l s w h i c h commonly e x h i b i t m y r m e k i t i c i n t e r g r o w t h s . T h e microcline i s i n v a r i a b l y T a r t a n - t w i n n e d a n d t h e c o r e zones o f plagioclase c r y s t a l s a r e

o f t e n saussuritized, i n d i c a t i n g a compositional zonation i n t h e o r i g i n a l

c r y s t a l s .

M i c r o p r o b e analyses o f a v a r i e t y o f f e l d s p a r s i n g r a n i t e gneiss located above, below, a n d i n t e r c a l a t e d w i t h i n t h e T B Q a r e g i v e n i n T a b l e

6.

T h e plagioclase has an A n c o n t e n t v a r y i n g between n i l a n d

15 8 ,

ie. a l b i t e

-

oligoclase.

T h e s t o i c h i o m e t r y o f t h e

K-

f e l d s p a r s a n a l y s e d d i f f e r f r o m t h e t h e o r e t i c a l

in t h a t t h e n u m b e r o f Si atoms p e r

32

o x y g e n atoms i s s l i g h t l y lower, a n d t h e n u m b e r s o f

(K

+ Na) a n d A l p e r

32

o x y g e n atoms a r e s l i g h t l y

h i g h e r t h a n w h a t i s t o b e expected. T h e s e deviations, however compare f a v o u r a b l y w i t h t h e analyses o f c o r r e s p o n d i n g f e l d s p a r s q u o t e d by Deer,

Howie a n d Zussman

(19631,

a n d i s n o t a n a r t e f a c t as it may appear a t

f i r s t s i g h t .

3.

B i o t i t e

T h r e e d i f f e r e n t b i o t i t e v a r i e t i e s a r e f o u n d in t h e s t u d y area. These a r e t h e b i o t i t e i n t h e g r a n i t e gneiss c o u n t r y rock, t h a t i n t h e T B Q h o s t rock, a n d t h a t in t h e t r a n s i t i o n zone between t h e T B Q a n d t h e g r a n i t e gneiss. M i c r o p r o b e analyses o f these t h r e e d i f f e r e n t b i o t i t e s a r e g i v e n in T a b l e

(46)

TABLE 6

Mlcroprobe analyses o f f e l d s p a r s I n g r a n l t e g n e i s s above, below and l n t e r c a l l a t e d I n t h e TBQ. TYPl CAL K-FELDSPAR

AES AES AES AES

1/1 1/3 1/12 3/12 ' CaO 0.01

-

0.04 0,02 Na,O 0.69 0.81 0.99 0.84 K d J 16.14 16.22 15.61 16,35 TOTAL 99.84 99.50 99.18 99.25 PLACIOCIASE VARIATIONS S t r u c t u r a l formulae on t h e b a s i s of 32 ( 0 )

*

FeO r e p r e s e n t s t o t a l Fe

(47)

T a b l e 7

Microprobe a n a l y s e s o f b i o t i t e i n t h e TBQ, t h e t r a n s i t i o n zone, and t h e g r a n i t e gneiss. S t r u c t u r a l farinulae on t h e b a s i s o f 16 c a t i o n s

TOTAL

(

16.00

(

1 6 , O O

(

16.00

1

16.00

*

F e O r e p r e s e n t s t o t a l Fe Greenish-brown b i o t i t e i n t h e g r a n i t e gneiss Red-brown b i o t i t e , i n t h e 7 8 9

The granite gneiss hosts the dark greenish- brown biotite variety which

shows a preferred orientation parallel t o t h e foliation planes

in

the gneiss

and often contains inclusions of sphene and zircons. The zircon

in

the

biotite shows pleochroic haloes d u e t o radioactivity. The microprobe

analyses of these biotites show a chemistry that compares favourably

with

that of biotite from a fine grained granite from Rubideaux, Southern

California batholith (Deer, Howie, and Zussman,

1963).

Red-brown b i o t i t e i n t h e c h l o r i t e - r i c h t r a n s i t i o n zone

The biotite

in

the TBQ host rock

is a reddish- brown variety which is

found as subhedral grains

with

a distinct orientation defining a foliation.

The biotite is sometimes seen mantling opaque mineral grains, and where

(48)

hematitic alteration has taken place,

t h e biotite i s seen t o be

pseudomorphously replaced b y hematite. Whenever significant amounts of

feldspar (over

10

%)

are found i n t h e TBQ, chloritization of t h e biotite

is d i r e c t l y related t o t h e amount of feldspar present i n t h e rock. Although

no t i n was detected i n t h e microprobe analyses of t h e biotites i n t h e TBQ,

a previous investigation carried o u t b y Southwood (1983) revealed t h a t

a small proportion of t h e t i n is present i n t h e b i o t i t e lattice. The

microprobe analyses of t h e biotites i n t h e TBQ are similar t o those of

a

blue- green siderophyllite i n a topaz- bearing greisen vein from

Newcastle, Northern Ireland (Deer, Howie, and Zussman, 1963).

I n t h e transition zone a deep reddish- brown biotite v a r i e t y

i s

found

which is

p a r t l y

o r t o t a l l y replaced

by chlorite. From t h e microprobe

analyses (Table

7)

it

may be seen t h a t t h e chemistry of t h i s biotite is

transitional between t h e biotite i n t h e granite gneiss

a n d

t h a t i n t h e TBQ.

A remarkable feature of each o f t h e three biotite varieties is t h e i r

chemical homogeneity w i t h i n each of t h e i r defined boundaries. The

chemistry of t h e b i o t i t e of t h e granite gneiss which lies above t h e TBQ

is f o r example t h e same as t h a t o f t h e biotite i n t h e gneiss a t various

depths below t h e TBQ. T h e chemistry of t h e biotite i n t h e transition zone

between t h e TBQ host rock and t h e granite gneiss c o u n t r y rock is similar

t o t h a t f o u n d i n a transition zone between t h e TBQ a n d a granite gneiss

lens w i t h i n t h e TBQ (eg.AES 26/10).

Unaltered biotite o f t h e TBQ

i s