Structures related to the Antimony line, Murchison schist belt, Kaapvaal craton, South Africa

Tectonophysics,

285

154 (1988) 285-308

Elsevier Science Publishers

B.V., Amsterdam

Structures

- Printed

in The Netherlands

related to the Antimony line, Murchison belt, Kaapvaal craton, South Africa

schist

J.R. VEARNCOMBE ‘, P.E. CHESHIRE *, J.H. DE BEER 3, A.M. KILLICK *, W.S. MALLINSON 4, S. MCCOURT 5 and E.H. STETTLER 6 I Department

of Geology, Rand Afrikaans

2 Johannesburg Consolidated Investment 3 National Physical Laboratoty,

University, P. 0. Box 524, Johannesburg 2000 (South Africa) *

Company Limited, James Park, P.O. Box 976, Randfontein

1760 (South Africa)

Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001 (South Africa)

4 Consolidated Murchison Limited, Private Bag 401, Gravelotte 0895 (South Africa) ’ Department

of Geology, University of Pretoria, Pretoria 0001 (South Africa)

6 Geological Survey, Private Bag Xl 12, Pretoria 0001 (South Africa) (Received

May 14, 1987; revised version

accepted

March

10, 1988)

Abstract Veamcombe,

J.R., Cheshire,

Structures

related

P.R., De Beer, J.E., Killick,

to the Antimony

line, Murchison

A.M., Mallinson,

W.S., McCourt,

schist belt, Kaapvaal

craton,

S. and Stettler,

South Africa.

E.H., 1988.

Tectonophysics,

154:

285-308. Sporadic

epigenetic

belt is structurally fractures.

The

lithological dip-slip,

antimony-gold

controlled

Antimony

layering

granitoids

deformed

which

metamorphic

related

along the ENE-trending to carbonated

line is a discontinuous

within

a broad

reverse (north-side-up)

fold structures

mineralisation

and spatially ductile

movement

the Antimony

post-date

D, and

shear

zone

of brittle-ductile

zone. This

tectonically

sense with downward-facing

line and its wallrocks

are not directly

Antimony

related

deformation early

Economic antimony-gold mineralisation in the Archaean Murchison schist belt, South Africa (Fig. 1) occurs along a linear zone known as the Antimony line (Figs. 2 and 3). Controversy over the origin of the mineralisation revolves around the various interpretations of the Antimony line as a stratiform horizon (Viljoen et al., 1978; Pearton, 1982), a fault zone (Van Eeden et al., 1939) or a

fold structures

into steep structures. to mineralisation,

address: Autralia,

0040-1951/88/$03.50

and shear

is locally

in the hangingwall.

The Antimony

the source

oblique

to

zone has an oblique-to Later (D2)

line is intruded

of which

by

may have been

zone of semi-brittle deformation in a ductile shear zone (Boocock et al., 1984,1988). In this paper we present, for the first time, regional structural evidence on the nature of the Antimony line and conclude that this line is a locally discordant zone of semi-brittle deformation in a broad zone of heterogeneous ductile shear with a reverse, oblique-to dip-slip, movement sense.

Antimony-gold

Western

(D1)

schist

veins in tension

fluids.

Introduction

* Present

line of the Murchison

rock types with quartz-carbonate

Department Nedlands,

of Geology,

The University

of

W.A. 6009, Australia.

0 1988 Elsevier Science Publishers

B.V.

mineralisation

Antimony-gold mineralisation (Boocock, 1984; Pearton and Viljoen, 1986; Boocock et al., 1988)

286

free grains, as fine disseminations and as sub-microscopic inclusions in the sulphides and silicates (Davis

et al.. 1986).

arsenopyrite

and

formations SCHIST

j;:::j

BELT

along

Antimony-gold

Additional

pyrrhotite

gold

associated

the Antimony

occurs with

in iron

line.

mineralisation

in the Murchi-

son schist belt is currently serviced by four shafts by Consolidated Murchison Limited and exposed in numerous mony

line

sponsible

for

concentrate) tion (Davis produced

l Bioemfontein

Eel

Fig. 1. Geological

2). The

along

18% (12,060

of the worlds

the 35 km Anti-

working tonnes

mines

Archaean

schist

Archaean

granitoids

map showing

and

greenstone

and

the location

are re-

of antimony

total antimony

produc-

et al., 1986) and 838 kg of gold was in the twelve months

up to 30th June,

1986 (Consolidated Murchison, 1986). Antimony-gold mineralisation also Shabari old mine (Fig. 3), south of the line, and at Galedonian Camp old mine southeastern arm of the Murc~son belt, the Bawa schist belt (Minnitt, 1975).

schist belt in the Kaapvaal

in the Murchison rock, comprising with subordinate quartz-carbonate

old workings (Fig.

occurs at Antimony in the far known as Numerous

belts

gneisses

of the Murchison

craton.

belt is hosted in quartz-carbonate quartz. dolomite and magnesite fuchsite, talc and chlorite. The rock is competent relative to the

other rock-types and mineralisation generally occurs as vein-filled fractures in boudins within this lithotype (Viljoen, 1979; Boocock et al., 1984). The enveloping schist consisting of carbonate and subordinate talc and white and brown micas and contains less quartz than the quartz-carbonate rock. Chlorite schists (with minor quartz, talc and brown mica), talc schists, cherts and arsenopyrite-pyrite-pyrrhotite iron formations are present. Mineralisation along the Antimony line is dominated by stibnite, with lesser amounts of pyrite, berthierite. scheelite, native antimony and other minerals (fisted in Davis et al., 1986). Stibiconite, a yellow oxidation product of stibnite, occurs in surface outcrops. Gold occurs as coarse

other gold workings occur in gabbros of the Rooiwater complex, quartz porphyroclastic schist of the Rubbervale formation, iron formations. carbonated rocks, mafic rocks and adjacent granitoids. Only one of these old workings (Malati) is currently in production, producing a modest 2 to 3 kg of gold per annum. Mercury, in the form of cinnabar has been exploited in the past in workings on the north side of the Antimony line. Murchison schist belt The

Archaean

Murchison

schist

belt

(Jeppe,

1893; Mellor, 1906; Hall, 1912; Van Eeden et al., 1939; Viljoen et al.. 1978) trends east-northeast (Figs. 1 and 2) and is composed of schists derived from ultramafic, mafic and felsic lavas and subvolcanic rocks, carbonated and sedimentary rocks and in the north, the Rooiwater complex (Vearncombe et al., 1986, 1987). Some authors (Anhaeusser et al., 1969; Muff and Saager, 1979; SACS, 1980; Pearton, 1982) have described the Murchison Range as a greenstone belt. However, the rocks are dominantly schists with metamorphic grades from mid-greenschist to upper amphibolite facies, and although well preserved mafic volcanic rocks are present, they occur amongst other mafic rocks lacking evidence of

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288

volcanic

extrusion.

volcanic tracts tic

without

of

the

the

South

Africa

Zimbabwe

original

term

Eeden et al. 1939)

grades

successions (Anhaeusser,

(Hickman,

For reference

ultramafic-mafic

deformation,

greenstone

Australia

(Belingwe), absent.

pervasive

classic

Barberton, Pilbara,

Thus

pile, low metamorphic

vast

characterissuch 1983)

(Bickle

et al., 1975)

belt

(Hall,

as a more accurate

the are

we prefer 1912;

(3) Rubbervale formation of quartz porphyroelastic schists derived from a subvolcanic quartzfeldspar

as

1981), or Mberengwa

to the Murchison

schist

and

Van

representa-

porphyry

well-bedded southern

line on which (Viljoen

a northern,

proximal

tic north-younging stratigraphy was developed. The Murchison schist belt is divided here into subregional domains, without any necessary stratigraphic significance, based on lithology, metamorphism and structure. Where these domains coincide with previous stratigraphic formations the original (Van Eeden et al., 1939 or SACS, 1980) name has been retained. From north to south the schist belt comprises: (1) Silwana’s amphibolite of highly deformed hornblende schists and amphibolite gneisses of the strike-slip Letaba shear zone (Fripp et al., 1980). (2) Rooiwater meta-igneous complex, a thick on-end layered igneous complex of gabbros, anorthosite, subordinate pyroxenite, magnetites, quartz-gabbro and sodic granite, all metamorphosed to amphibolite facies. Much of the 7.5 km thick Rooiwater complex is undeformed except in discrete transecting ENE-trending retrograde shear zones. This complex is southward facing (Vearncombe et al., 1986, 1987).

servations

Rubbervale

deposits

occur

1981; Maiden,

1984).

deposits

consistent is

have

and, a southern

tuffs of southward formation

the silici-

in a discontinuous

sulphide

concentration

Along

formation

chalcopyrite

in graded

from locally

lavas.

zinc-copper

These massive volcanogenic

The

combe et al., 1986, 1987). We confirm the above younging directions and present here, more evidence of folding not considered when the simplis-

several

et al., 1978; Taylor,

been a greenstone

presented by SACS (1980) and Pearton (1982). However, both were developed without an appreciation for the complexity of deformation in the schist belt and contradict observed changes in younging around tight to isoclinal folds (Graham, 1974; Minnitt, 1975; Boocock et al., 1984) and a southward younging for both the felsic volcanic rocks of the Rubbervale formation (Maiden, 1984; Taylor, 1981) and the Rooiwater complex (Vearn-

and

fied tuffs and lavas are exposed

distal, sphalerite

succession.

tuffs

flank of the Rubbervale

tion of the rock types, although prior to deformation and metamorphism the Murchison may have Two similar stratigraphies, both assuming a homoclinal, north-younging sequence have been

and schists derived

felsic

with obyounging.

pervasively

de-

formed and metamorphosed to greenschist facies. (4) Murchison ultramafic, mafic, carbonated and metasedimentary schists, the largest domain, within which the Antimony line is a central feature, comprising a variety of metamorphosed and, usually, deformed sub-volcanic, volcanic and elastic metasedimentary rock types. The rocks include komatiites, serpentinites, tremolite-actinolite schists, mafic rocks including amphibolites and chloritic

pillow lavas, massive schists. Alteration

products include a biotite schist derived from an ultramafic precursor in contact with intrusive granitoid. Carbonated rock types include quartz-carbonate rock and quartz-chloritee carbonate, chlorite-carbonate and talc-carbonate schists. The carbonated rocks are particularly well-developed along the Antimony line and in the area west-southwest of Gravelotte. Iron formations which are locally well banded are principally oxide facies with only local sulphide development. The quartz-mica schists are metamorphosed quartz-rich elastic sedimentary rocks. Quartzwhite mica schists dominate with secondary quartz-fuchsite quartz-chlorite pebble

schists. Subordinate interbedded schist, quartzite, quartz grits and

conglomerates

occur.

The

conglomerate

pebbles are usually quartzite, vein quartz and iron formation. The suggestion (Pearton, 1980; Pearton and Viljoen, 1986) based on K/Cs ratios that the quartz-white mica schists are felsic epiclastic sediments is not substantiated since these rocks are interbedded with unambiguous metasediments including conglomerate and lack any felsic volcanic clasts. The outcrop style of the quartz-mica schists

0 1

1. I

2 1

3 ,

4 km. I

Fig. 3. Geological map of the Antimony line and environs. For detailed discussion see text.

ff EFEREWCE -+-

antiforrn

*

rynform

-

fold

plunge

-

D2 DI.

cfeavage cleavclge

-

mineral

A

bed&q

rlongatiao

-

yauegirg

.K

kyanik

locality

workinp

antimc+y

x”

working

gold

X

old

linealion

direcfieo * gold minr

mine

mine

Oranifeids

Silwanlr

‘5

Rooiwster

Lzl 4’

/

wronged

stratigraphic

compi!~

Rubbervole

tormation

Uttromafic,

mafic

Antimony Schist3

Key

amphibolita

and carbonate&

schists

line end cf~v?ritat

in approximafe sipnitfcaoce

N-S

d

w&r

LQ France

with

no

WSP

297

has been referred

to as “bars”

(Van Eeden

et al.,

1939 and subsequent authors) usually after prominent hills.

and The

regarded

as long, linear and

by the old prospectors

each named, “bars” were

continuous, and acted as barriers to mineralising fluids from the southerly granitoids, concentrating mineralisation

on their south

ping (Fig. 3) of the so-called although continuous quartzite, surrounded chlorite

some are long and

sides. In fact map“bars”

and linear

are mostly

quartz-fuchsite

lensoid, schist

by quartz-white schists.

If this

lensoid

carbonate

and talc schists firming

alteration

contacts

(Boocock

an interrelationship and mineralisation

they are not

carbonate

alteration

usually

mony-gold

and

structure

quartzis the

east or west plunging

Alteration and metamorphism

carbonate

gradational

alteration between

rocks occur

exists,

guide to anti-

since

extensively

event.

carbonate

undoubtedly

is not a direct

mineralisation,

and

in chlorite

et al., 1988) thus con-

a second and syn-tectonic

Whilst

with

has

with the foliation

alteration

result of boudinage, the lenses should occur in linear trains each with consistent younging directions; this they do not. The lenses are tectonically modified and isoclinally folded primary sedimentary channel-like features. (5) La France schists of fuchsitic quartzites in quartz-biotite schists which are locally kyanitestauroliteor kyanite-garnet-bearing. The deformation history here differs from that of the rest of the belt, recumbent fold and linear structures being deformed around upright open to tight folds.

some

discordant

that

reveals

or conglomerate

mica

shaft (see Fig. 5) has carbonate porphyroclasts deformed in the second regional cleavage and

umnineralised in the Murchi-

son schist belt and are not restricted

to the Anti-

mony line.

Metamorphism

The Murchison schist belt has a grossly heterogeneous metamorphic pattern. The Rooiwater complex and Silwana’s amphibolite are metamorphosed to mid-amphibolite facies and quartz-biotite schists of the La France area are locally either kyanite-stauroliteor kyanitegarnet-bearing. We metamorphic grade mony line prior to semblages in the greenschists facies.

have not direct evidence of in the vicinity of the Anticarbonate alteration but asschists are essentially midHowever, two outcrops with

kyanite, Carbonate-bearing rocks are one of the characteristic features of the Antimony line. Hall (1912) suggested they were dolomitic limestones altered by metamorphism but subsequent authors (Willemse, 1935; Mendelssohn, 1938; Van Eeden et al., 1939) have preferred an origin by hydrothermal alteration. Minnitt (1975) and Muff (1976) both have an essentially similar explanation of hydrothermal solutions altering an actively accreting volcanic pile. Pearton (1978, 1979) and Viljoen (1979) point out that the carbonates have high Mg, Cr and Ni and low Ca levels implying an altered ultrabasic rock-type and regard the alteration as in part pretectonic and occurring in two phases. Our observations show that carbonatebearing rocks have an early (Dr) cleavage, in part defined by carbonate minerals, thus confirming either a pre- or syn-D, age to the alteration. A post-D,, pre-D, dyke in a quarry at Gravelotte

originally reported by Van Eeden et al. on Castle koppies (Fig. 3) occur in (1939), favourable quartz-white mica lithologies. Kyanites in and adjacent to the margins of quartz veins lacking deformation fabrics, overgrow the D, cleavage in the quartz-white mica schists. Moderately high pressure conditions thus prevailed after D,.

Nature of the mineralisation

Although minor disseminated stibnite occurs in quartz-carbonate-stibnite rock the dominant style of mineralisation is as quartz-carbonate veins, mostly synchronous with the first deformation in brittle fractures in massive quartz(Dr), carbonate rock (Fig. 4) in boudin necks or along contacts between massive quartz-carbonate and schist. Commonly in the Alpha-Gravelotte ore

298

Fig. 4. Tension

fractures

filled with quartz,

carbonate

and stibnite

from the Free State ore body, Monarch

mine. Field of view about

1 m.

body and locally elsewhere mineralisation is remobilised into veins and fractures generated during or after D,. All styles and ages of antimony mineralisation show the microtextural effects of deformation and metamorphism including kinkbands, annealing textures and polygonal granoblastic textures (Ileri, 1973; Maiden and Boocock, 1984; Boocock et al., 1988). Hence it is not possible to conclude (as did Ileri, 1973), from ore textures, that the ore was emplaced prior to deformation. Boocock et al. (1988) have, using the above and other evidence, concluded that major (0,) concentration of ~n~ralisation

syntectonic in brittle

fractures and boudin necks in massive rock and foliation parallel veins was followed by (mostly Dz ) remobilisation. The economic mineralisation, as we see it, is thus probably epigenetic. Subeconomic grade ~neralisation, manifested by antimony levels up to ten times normal in most of the schists adjacent and south of the Antimony line (Pearton. 1980) may represent a pre-tectonic, possibly syngenetic, concentration. Boocock et al. (1984, 1988) document pressure solution cleavages in the adjacent schists and suggest that stress controlled solution processes may have been a major concentrating mechanism. Carbonate alter-

ation, quartz veining and well-developed pressure solution cleavages all imply carbonate and silica mobility in a H,O-CO, fluid (Boocock et al.. 1988). Structure Deformation

in the study area is heterogeneous

and in several distinct phases. For descriptive purposes we have erected an enumerated deformation sequence based on well exposed structures in a quarry north of Gravelotte shaft (Boocock et al., 1984). We describe these structures first and then compare structures of the surrounding areas, emphasising any differences, with reference to the quarry. Gruvelotte shujt quary This quarry (Fig. 5) has already been described in detail (Boocock et al., 1984) and only a brief summary is presented here. The quarry lies immediately north of the Antimony line and exposes carbonate schists with nodules, quartz-white mica and quartz-chlorite schists with quartzites and deformed vein quartz plus iron formation pebble conglomerates. Graded bedding and truncated

299 7

vouwino direction bedding trace

&

bedding

>

D1 CleavaQe

)”

D2 ChVWJB

2

Dl mineral elongation and shape fabric lineation

w

-3,

a

,’

Dl synche

D2 fold plunge

quartz-mica schists

carbonate

schist with

N

#

c

10m

I

Fig. 5. Geology

of the quarry

north of Gravelotte

shaft. For locality

see Fig. 3 and for details see text.

nodules

300

cross-bedding are recognised in the rocks and at least four changes in younging direction clearly indicate

isoclinal

scale folding,

folding.

with mutual

Three

phases

interference

reveal a D, fold phase of ENE-trending a D, of consistently E-W

oriented

kink

bands.

carbonate

asymmetric

axial planes The

nodules

fabrics

parallel

to the axial plane

are transected

in

schist

and

a flattening

of D, isoclines

elongation.

of

developed

show

and a

These shape fabrics

by a spaced pressure

vage axial planar

folds with

in the carbonate

in the conglomerate vertical

isoclines,

“S”

and a third phase

shape

pebbles prominent

of smallstructures,

solution

to D, fold structures.

clea-

Although

the prominent cleavage is a D, structure, this deformation appears to have only a small influence on the shape fabrics. A bedding-parallel micaceous layering, recognised in areas where the transecting (D2) cleavage is poorly developed may represent a D, cleavage. A cross-cutting dyke shows none of the effects of D, deformation but has a D, cleavage and lacks a linear fabric. This dyke intruded between the two deformation events and provides justification for enumerating the distinct and separate deformations.

D, structures south of the Antimony

Exposure

immediately

south

line

of the Antimony

line, between the working mines is very poor but regionally the lithological banding displays a 15-20” angular discordance to the structural Antimony line. To the east quartz-white mica schists of a unit termed the Louws kop bar (Van Eeden et al., 1939) and to the west mafic, carbonated and quartz-fuchsite schists preserve contrasting deforThe sub-vertical to steep mation structures. north-dipping Louws kop bar is without pervasive D, deformation, which is restricted to thin schistose bedding plane slip zones in micaceous horizons. In these horizons a steep mineral elongation lineation approximately parallels the equivalent lineation on the Antimony line (Fig. 6). The absence of pervasive D, fabrics means that sedimentary structures are well preserved and truncated cross-bedding consistently indicate a north-younging for this unit. At Shabari old gold mine, strati-

Fig. 6. Equal

angle

data.

into that on the Antimony

Divided

the line and data

stereonet

to the south

of mineral subdivided

elongation

lineation

line, to the north

of

into that west and

east of Gravelotte.

graphically just beneath the Louws kop bar recently drilled borehole core has revealed multiple repeated graded bedding in quartz-chlorite schists (the cleavage is transecting and probably equivalent to D2) which again consistently young to the north. The first deformation in this area is thus limited to bedding plane slip and no fold structures have been recognised. In the vicinity of Cravelotte town and south of the Maranda granite the distinctive D, “S” folds and oblique cleavage are absent. The quartz-mica schists (here mostly a quartz-fuchsite schist) are folded in a regional structure with an isoclinal synformal closure in the north and an open antiformal closure with tight parasitic folds to the south. Cleavage in the limbs is sub-vertical and intense, bedding and other primary structures having been obliterated. In fold noses the fabric is intense, constrictional and parallel to plunge at about 45” to the east-northeast (Fig. 6). Although the cleavages are parallel, the linear structural

301

component

plunges

Antimony

line.

and

no direct

hence

Gravelotte

less steeply

There

than those on the

is no continuous correlation

shaft quarry

with

is possible.

this area may have been

deformed

outcrop D, of the

The rocks

of

as a separate

to bedding

and are intense

mony line. An indication cleavage

symbol.

Outcrop

schist

lenses

D, structures north of the Antimony

is poor

and mafic

cleavages

locally

recognisable

in sandstones,

zite and quartz-conglomerate

ENE-trending,

impure

plicable.

precursor

rock types

quartz-mica

although

the chlorite,

schists

it is rarely

cross-bedding, quart-

and

between

have well-developed

possible

to categorise

as D, or D,. However

tions are obliquely and truncated

of D,

in Fig. 3 by the use of the D,

cleavage

bedding

of the distribution

is given

carbonate

Graded

to the Anti-

cleavage

entity. line

adjacent

oriented

and the general rule of

D,, and E-W

The carbonated

incompetent

relative

the

the two deformaoriented,

and chlorite

D,, is apschists

to the quartz-mica

are

schists

to the quartz-mica schists, indicate both north and south younging. At Gravelotte shaft quarry these changes in younging are attributable to D, isoclinal folding within the quartz-mica schist lenses. Elsewhere changes in younging occur between the lenses. These changes may relate to D, isoclinal folding but the absence of fold closures in continuous strata prevents conformation and

and may have accommodated

frustrates North

rotates into a steep north-dipping shear zone. S-C structures (Berth6 et al., 1979) show a north over south, reverse, movement sense (Figs. 7 and 8). Elsewhere in this section a competent quartz-

mapping of D, fold axes. of the Antimony line D,

fabrics

are

restricted to thin bedding parallel slip zones with locally intense slaty cleavage and a subvertical stretching lineation. As the Antimony line is approached

D, fabrics become pervasive,

subparallel

the quartz-mica D, structures

more D, strain than

schists. on the Antimony

line

A NW-SE section through the ENE-trending Antimony line a Jack shaft provides critical structural evidence with parallel

carbonate gradational

(Fig. 7). Here a subvertical quartz veins, in carbonate

cleavage schists,

rock lacks a cleavage except at its margins with the carbonate schist. The

SSE

Fig. 7. Line drawing of the section across the Antimony line at Jack shaft. For locality see Fig. 3. Unornamented portions represent unexposed (grass and debris covered) area.

iron formations and quartz-white mica schists. In the intermediate strain state, isolated D, fold structures are preserved and in the highest strain carbonate structures

schists,

in quartz-chlorite state, in chlorite,

primary

banding

are not preserved.

veins with detached

(probably

preserved

rodded

in isolated

slaty or, in carbonate

and

However, D,)

schists talc and D,

fold

early quartz

fold noses are

structures.

An intense

shists, spaced

solution

to isoclinal

and occurs

clea-

vage is well developed. D, folding several

scales,

is tight

the largest

structure

steep ENE-plunging synform hosting stibnite mineralisation

on

seen being

a

in quartz-carbonate in the Free State

ore body at Monarch mine. Smaller-scale folds on this larger structure generally plunge at shallower angles, suggesting non-cylindricity but no unambiguous sheath folds have been recognised. Mineral elongation and pressure shadows to pyrite euhedra show a general parallelism with the plunge of local fold structures. The competent quartz-carbonate rock is deformed by folding, as discussed above, by

Fig.

8. S-C

relationship

at Jack

shaft

vertical section, looking

quartz-carbonate flat-lying and filled fractures.

section,

viewed

in

east.

is boudinaged and cut by NW-SE oriented upright, quartz Other quartz veins in the carbonate

schist dip steeply north at an oblique angle to the cleavage. Only minor stibiconite occurs at this outcrop but its position on the Antimony line enables the massive quartz-carbonate rock to be equated with those seen underground, there hosting stibnite mineralisation. Underground exposures have been critically examined by us on three of the working mines with detailed mapping of the Free State ore body at Monarch shaft. Three styles of D, deformation are recognised as a function of strain which is in turn a function of competency of the rock types. The least strained rocks have remnant banding, probably bedding, and D, fold structures of banding are preserved in quartz-carbonate rocks, siliceous

boudinage and brittle tension fracturing. Tension fractures are filled with quartz, carbonate, stibnite and other ore minerals. They locally developed as an irregular but intense stockwork. Some early veins on the margins of competent units are clearly seen to have been folded during progressive deformation. Mineral elongation and shape fabric lineations are generally subvertical to steep ENE plunging (Fig. 6) although exceptions to this occur where steep to gentle

ENE-or

WSW-plunging

lineations

occur in the competent quartz-carbonate rocks of the ore zones. Although we have no definitive explanation for this variation, it may reflect rotation, subsequent to lineation development of the fault-bounded low strain domains within the rocks of much higher strain. Angular discordance of the Antimony line to iron formations in the vicinity of Monarch mine The discordance of the Antimony line (a structural feature) and iron formations (a lithologic horizon) is illustrated underground on Monarch mine. Between Athens and Monarch (Fig. 3) the

Fig. 9. Geological

banding

mine,

and discordant

map of Monarch

MONARCH

Antimony

line structures

bodies

0

of quartz-carbonate relative to iron formations.

boudin-like

15 LEVEL

15 level, showing

SHAFT

1OOm

Consolidated

Murchison

schist, 100 m spacing

rock in carbonate-bearing

50

rock

alteration

folding

oblique

(local)

(pervasive)

grid shown for clarity.

carbonate

F2 Second

folding

F 1 First

_

cleavage

_

Second

banding cleavage

First

A _._ 52

Lithological

Ferrugnws

-.-

a_

Quartz-carbonate

=

sch,st

metacherts

Carbonate-bearing

m

sch8st

Chlonfic

t

to lithological

w 3

304

line of stibnite line lie parallel rich iron

ore bodies defining the Antimony to a group of arsenopyrite-pyrite-

formations

(the “burnt”

1912 and ‘“arsenopyritical” al., 1939). However, formations

antimony-bearing

reef of Hall,

reef of Van Eeden

on Monarch

swing to an easterly

mine

The

angular

15-20”

(Fig. 9). Other

along

discordance marker

units

such as the Louws kop quartz-white display

a similar

et

the iron

strike whilst

rocks continue

trend.

dipping and transected by the D, cleavage on which they are downward-facing. D, folds vary

the

an ENE-

being further

from close to tight and are responsible erable,

though unevenly

buckling. ding

and

present

east.

rotated.

relationship.

At Alpha steeply

sense

of the D, structure

As the Antimony line is approached deformation changes, from bedding plane slip, to isoclinal folding, to a pervasive D, cleavage and on the Antimony line itself a heterogeneous deformation of isoclinal folds preserved in pods of competent rock types with a surrounding intense fabric masking all previous features. Mineral lineations and elongate shape fabrics, in appropriate rocks, are well-developed and although showing some scatter, particularly amongst the competent and mineralised quartz-carbonate rocks of the Antimony line, are generally oriented down-dip. Assuming this elongation represents the X-axis of the strain ellipsoid and hence movement direction, combined with the evidence of movement

layering, ore-bearing

position

to D, cannot

be

than their

into which they have been

mine asymmetric

D2 “S” folds plunge

the west and the western

mineralisation

occurs

(half wavelength

possibly

by

of bed-

bedding,

quartz-carbonate

limit of

in the nose of a

about

40 m) “S” fold

et al., 1986). The open folds deform mineral D, cleavage

and

the

rocks.

D, encompasses all ductile structures post-dating the second deformation and is principally of kink-bands. A well developed set oriented at about 030” is common. Another, possibly a conjugate set, is oriented about 160”. Two variably oriented crenulation cleavages are common in the chlorite and talc schists.

or thrust, north-overriding-south sense. In this scenario the D, isoclinal folds, north of the Antimony line, are in the hanging wall of the thrust structure and are probably analogous to nappe

Faulting Murchison

A sub-vertical E-W-trending spaced cleavage in rocks north, south and on the Antimony line is axial planar to asymmetric “S” folds of bedding and D, structures. D, fold plunges vary from about 45’ to the west in the vicinity of Belleview old gold mine to sub-vertical in the working mines along the Antimony line (Fig. 3). The southyounging quartz-mica schists of the Castle koppies to Plessis kop area (Fig. 3) are steeply north

attitude

D3 structures

Faults

D, structures

prior

known as the C line (Abbot to close asymmetric “S”

sense deduced at the Jack Shaft section, the shear zone comprising the Antimony line has a reverse

structures.

structures

towards

large-scale

the regional

for considshortening

they may have been flatter

upright

economic ~ooe~ent

D,

determined

about

mica schist

Although

distributed,

is a common belt, particularly

the mines. Oriented fibrous fault planes suggest both

phenomenon on a small

in the scale in

mineral growths on strike- and dip-slip

movements. Displacements determined by offset lithological features, including dykes, are generally small. Faults bound the ore-body quartz-carbonate rock at Monarch mine where they parallel the existing cleavage. Some of the faults, in the vicinity of ore bodies, are mineralised with stibnite. At Alpha mine the mineralisation is remobilised into en echelon fractures oriented approximately E-W and dipping about 60° to the north. These fractures show little or no displacement and are interpreted as hydraulic fractures probably post-dating the D, fold event.

305

Early oriented

Proterozoic

and

Karoo

dolerite

dykes

about 030 ’ to 050 o locally offset marker

little of the deep structure or geometry of the intrusive granodiorite. The deep structure has been investigated

horizons.

soundings Granodiorites

by deep geoelectrical and a gravity

are published

elsewhere

(Schlumberger)

survey,

details

et al., 1984). Only a brief summary The Antimony a series

of granitoid

Maranda Antimony

granite,

sub-vertical

bodies,

marks

associated

one

developed mica-poor

intrude

cleavage. granodiorite

cleaved

with

of which,

the western

limit

line (Fig. 3). The Maranda

an irregularly ing 020°

line is intimately

the

of the

granite

has

At one locality, dykes

granodiorite.

will be presented 3000

Rooiwater about

Qrn

resistivity

contrasting

complex

10,000

of the results

here. The schists of the Murchi-

son belt have an electrical and

of which

(De Beer, 1982; De Beer

with

between values

and surrounding

Qm. The limited

trend-

complicates

interpretation

Another

nevertheless

results

indicate

width

the

granitoids

of

of the belt

of the resistivity maximum

1000

for

data,

depths

of

granodiorite body occurs at Old Gravelotte gold mine (near Gravelotte antimony mine). Here recently drilled borehole evidence shows that the granodiorite thickens, with depth, from a small

between

surface outcrop to cut out the ore body, in iron formation. Underground exposure and borehole core clearly show that this granodiorite is intrusive

belt, the maximum residual occurring over the gabbro-anorthosite series of the Rooiwater complex. Gravity profiles across the schist belt give an

and post-dates the D, cleavage. Other granodiorites occur in the vicinity of Monarch mine, as

absolute maximum depth of 12 km which is in agreement with the maximum estimate obtained from the geoelectrical data. A prominent geo-

thin veins, usually cutting the D, cleavage. At Malati gold mine an “S” shaped aplitic granodiorite body has an upright N-S-trending cleavage of uncertain origin in its central portion. D, folding may be responsible for the “S” shape. Other granodiorite occurrences are at Neill’s Camp and County Down old mines (Fig. 3). Stibnite mineralisation is reported (JCI Exploration files) from borehole core in Maranda granite near its eastern contact with the Antimony line and berthierite and stibnite occur in quartzrich margins to granodiorites near the AlphaGravelotte ore body. Pyritic zones near granodiorite at the Old

the margins of the Gravelotte and Malati

gold mines are spatially related to both the margins and internal brittle fracture zones with quartz veins showing pyrite, pyrrhotite, arsenopyrite gold mineralisation in the veins and altered

and wall-

rock. Deep structure From surface geological data we know that the Antimony line is a sub-vertical structure intruded by fingers of granodiorite. However, this tells us

8.8 and 12.3 km and that most of the belt

has a depth extent of less than 4.5 km. The gravity data (totalling 2700 stations) show that positive Bouger anomalies are associated with the schist

physical discontinuity occurs in the schist belt along the Antimony line from the Maranda granite, in the west, to the Baderoukwe granite, in the east. A series of profiles (Figs. 2 and 10) show granitoid near surface beneath or in close proximity to the Antimony line. The intrusive granodiorite bodies, seen at surface, are thus interpreted as connected, at depth, to a substantial granitoid which intruded the Antimony line. Structural evidence (already discussed) indicates that granitoid magmatism mostly occurred after D, and before D,. The granodiorite thus intruded along the Antimony line and into a dynamic environment. De Beer et al. (1984), following the unsubstantiated suggestion of sinistral strike-slip movement in the vicinity of the Antimony line by Pearton (1978) and Viljoen et al. (1978), have interpreted their geophysical data showing the thickest part of the Rooiwater complex offset to the west of the thickest part of the schist belt as evidence for sinistral shear. Some of this shear may have occurred along the Antimony line (De Beer et al., 1984) but this is incompatible with the fabrics which suggest oblique- to dip-slip movement and the offset could be explained by many other fac-

consider the process of deformation to have been important in upgrading the proto-ore, possibly through

the

(Boocock

et al.. 1984) within

mechanism

units and precipitation Sb line

fractures

such

the competent

of

pressure

of the metals into tensional

as the quartz-carbonate quartz-carbonate

for this process

fluids

the emplacement

may indicate

that

veins

rock.

there is no need to have exotic bodies

solution

the less competent in

Although introduced

of granodiorite

magmatic

fluids

could

have been introduced. Sulphur

isotope

1986) for stibnite

data have

(Pearton 6j4S values

and

Viljoen,

in the range

1.4% to 4.6% with a mean of 2.6%. Pearton and Viljoen interpret this as consistent with a magmatic exhalative or magmatic hydrothermal (including remobilised magmatic) origin of the sulphur. They regard the narrow spread of data as indicative of an epigenetic origin for the mineralisation and therefore suggest derived from the underlying .pile.

Density contrast clla a

0

aF

*go

“C and “0 isotopes in carbonate minerals (Smith, 1986) suggest that the carbon in the

Granltoid

0

Robbervale

m

formatx,n

ree State

that the sulphur was volcanosedimentary

hor~b~nde

granite

Rooiwater

Novengiffa gabbro-anorthOSite Murchison

metasedimentary,

Murchison

ultramafic

complex

i mafic and carbonated

schists

and mafic schists

Murchison schist belt has at least two sources, the first being Archaean seawater and the second source had a 613C value of about -7% and may

on to the section line. This results in inaccuracies

in the vertical

scale and this part of the figure is diagrammatic.

D, folds and

represent carbon derived from the mantle, probably by magmatic processes. Carbonates within the Antimony line have 613C values from -4 to -7% and 6’sO values slightly heavier than those measured in the surrounding schist belt and are interpreted (Smith, 1986) as indicating a major

tors such as original shape, extent of intrusive granitoid or the eastward increase in strain in the

contribution of carbon from a primary source. Either a magmatic source, from the granodiorite, or a metamorphic source, from the surrounding schists, is thus compatible with various aspects of the data. The field evidence suggest that

Fig. 10. Residual for profile granitoid

gravity

anomalies

AA ‘ of Fig. 2. Density at 2670 kg/m3.

above ground

surface

complex

contrasts

The geological

in the section

cleavage

Rooiwater

and models fitting

omitted

are with respect details

are projected

to

documented along strike

for simplicity.

(Vearncombe

Origin of the mineraking

the data

et al., 1987).

fluids

The almost total alteratidn of komatiites to quartz-carbonate rocks along the Antimony line implies the interaction of large fluid/rock ratios. We consider it likely that the embryonic structural discontinuity that developed into the Antimony line acted as a conduit for mineralising fluids. We

most granodiorite intrusions post-date D, and the mineralisation, and we therefore prefer a metamorphic source. Conclusions (1) Economic epigenetic antimony-gold mineralisation in the Murc~son schist belt is spatially restricted to a narrow deformation zone known as the Antimony line.

307

(2) The Antimony deformation

within

zone with reverse, sense. Previous horizon

semi-brittle ductile

to dip-slip,

suggestions

shear

movement

that it is a stratiform

are equivocal. shear zone and semi-brittle

line tectonism

(4) Small intrusive which post-date

developed history

underlain

early (designated

granodiorite

by

An-

of the schist belt.

the D, fabric,

line which geophysical

regionally

Anhaeusser, 1969.

C.R., Mason, The

shield geology.

of some

bodies, intrude

models

granitoids

some of the Anti-

reveal to be at

shallow

depths. (5) Field evidence suggests that the Antimony line has a reverse or thrust movement sense with north overriding south. Metasedimentary schists in the hanging wall of the thrust were isoclinally folded during D, and are locally downward-facing on the later D, cleavage. (6) The mineralising fluids may be of either metamorphic or magmatic origin but the generally post-D, to pre-D, intrusive nature of the granitoids after the principal mineralising event suggests they are not the direct source.

M.J. and Viljoen, aspects

R.P.,

of Precambrian

Geol. Sot. Am. Bull., 80: 2175-2200.

Berthe, D., Choukroune, mylonite

R., Viljoen,

reappraisal

P. and Jegouzo,

and non-coaxial

P., 1979. Orthogneiss,

deformation

ample of the South Armorican

D,) in the deformation

mony

a broad

oblique-

(3) The ductile timony

line is a central,

zone

of granites:

the ex-

shear zone. J. Struct.

Geol.,

1: 31-42. Bickle, M.J., Martin,

A. and Nisbet,

E.G., 1975. Basaltic

peridotitic

komatiites

conformity

in the Belingwe greenstone

Planet.

and stromatolites

C.N.,

1984. Ore genesis

Murchison

Range,

belt, Rhodesia.

C.N.,

un-

Earth

P.E. and

geology

Monarch

antimony

Transvaal.

Trans.

Monarch

J.R.,

and

D.I.

understanding

193 pp. (unpublished) J.R.,

shaft

Murchison

1984.

quarry

and

greenstone

P.E., Killick, A.M., Maiden,

1988. Antimony-gold

belt,

Groves

(Editors), Gold

Ext.-Univ.

K.J. and

mineralisation

schist belt, Kaapvaal

Precambrian

Dep. and Univ.

thesis,

Geol. Sot. S. Afr., 87: 315-326.

mine, Murchison

S. Ho

line,

M.Sc.

Veamcombe,

of Gravelotte

mine,

C.N., Cheshire,

Veamcombe,

the Antimony

Transvaal.

Johannesburg,

Cheshire,

structural

along

northeastern

Univ. Witwatersrand, Boocock,

Boocock,

and

a basal

Sci. Lett., 27: 155-162.

Boocock,

The

above

Recent Deposits,

W. Aust.,

at

craton.

In:

Advances

in

Vol. 2. Geol.

Spec. Publ.,

12, in

press. Consolidated Davies,

Murchison

D.R.,

Antimony

Limited,

Paterson,

D.B.

in South Africa,

1986. Annual and

Griffiths,

Report,

18 pp.

D.H.C.,

1986.

J.S. Afr. Inst. Min. Metall.,

86:

173-193. De Beer, J.H.,

Acknowledgements

greenstone

1982. A geophysical

study

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Africa.

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12:

A.W.A.,

Joubert,

S.J.

105-112.

This research arose as a result of collaboration between JCI and the National Geosciences Programme of the CSIR. Funding from both organisations is gratefully acknowledged. The authors thank, Chief Geologist, Colin Willson and his staff at Consolidated Murchison Ltd. for their interest and support of this work. Johannesburg Consolidated Investment Co. Ltd and Consolidated Murchison Ltd. are thanked for permission to publish this paper. Our work has benefited from discussions with Maarten de Wit, Ken Maiden, Dirk van Reenen and Chris Roering.

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