The mineralogy, geochemistry, and genesis of the B and · PDF fileHonours Research Report submitted as part of the B.Sc.(Hons) degree in the School of Earth Sciences, University of

Geology of the B Lode, A

Lode and 1 Lens on CML7,

Broken Hill, N.S.W.

Richard B. Tully

Honours Research Report submitted as part of the B.Sc.(Hons) degree in the

School of Earth Sciences, University of Melbourne

Submitted on October 31st, 2002

Chapter Two – Broken Hill Geology

Chapter Two - Broken Hill Geology

2.1 Regional Geology 2.1.1 Stratigraphy

The stratigraphy of the Broken Hill Block (Figure 2.1) described by Stevens et al. (1980,

1988) and by Willis et al. (1983) is still currently generally supported in the literature

including Parr and Plimer (1993), Laing (1996), Stevens and Burton (1998) and Walters

(1998). Due to an intense, prolonged metamorphic and deformation history at Broken Hill,

the recognition of lithological protoliths is exceedingly difficult. As a consequence, there

have been some studies that have disputed parts of the interpreted stratigraphy (e.g.

Haydon and McConachy, 1987; Wright et al., 1987; Vassallo and Vernon, 2000). In light

of new studies, the interpreted stratigraphy of the Willyama Supergroup in proximity of the

orebodies needs constant re-evaluation.

Figure 2.1: Stratigraphy of

the Willyama Supergroup

described by Willis et al.

(1983), also highlighting the

depositional trends and

occurrence of mineralisation

and exhalative sediments

(after Parr and Plimer, 1993).

5


Regional mapping by the New South Wales Geological Survey was instrumental in

the re-classification of the Willyama Supergroup (Willis et al., 1983) into smaller “rock

type associations” called suites (Stevens et al., 1980). The lowest suite of the Willyama

Supergroup identified is the Clevedale Migmatite, followed by the Thorndale Composite

Gneiss, although recently classified units, the Redan Gneiss, Ednas Gneiss and Mulculca

Formation (Stevens et al., 1988) in the south-east of the Broken Hill Block are thought to

occur stratigraphically beneath or equivalent to the Clevedale Migmatite and Thorndale

Composite Gneiss. Overlying these is the Thackaringa Group, which is followed by the

Broken Hill Group, the Sundown Group and the uppermost sequence, the Paragon Group

(Table 2.1). The principal trend is from the quartzo-feldspathic rich lithostratigraphic

sequences of the Thackaringa Group, through the mineralised and exhalative

lithstratigraphic sequences of the Broken Hill Group, to the metasediment-rich sequences

of the Sundown and Paragon Groups (Parr and Plimer, 1993).

Table 2.1.1a: Lithostratigraphic units of the Willyama Supergroup and their interpreted protoliths.

Stratigraphic

Name

Suite

Unit Lithology Interpreted Protolith

Clevedale Migmatite > 500m

1

Poorly bedded feldspathic gneiss, migmatite

metasediment and minor amphibolite (Stevens

et al., 1980).

Detrital sediments containing volcanic detritus

and ash fall tuffs (Stevens et al., 1988).

Thorndale Composite Gneiss 1000m

2

Thinly to poorly bedded metasedimentary and

quartzo-feldspathic composite gneiss and

amphibolite (Stevens et al., 1980)

Fluvial-deltaic feldspathic sand deposition

(Stevens et al., 1988).

Thackaringa Group 1500m

3

Lady Brassey 3.1 Na-plagioclase-quartz rocks (Stevens et al.,

1988).

Na volcanics or air fall tuffs altered by alkaline

lake or seawater (Stevens et al., 1988).

Alma Gneiss 3.2 Coarse grained granitic rock with K feldspar

megacrysts (Willis et al., 1983)

Rhyodacitic lava-flows or ash-flow tuffs (Stevens

et al., 1988) or granitoid intrusion (Vassallo and

Vernon, 2000).

Alders Tank Formation

3.3-

3.4

Metasedimentary and quartzo-feldspathic

composite gneiss (Stevens et al., 1988).

Deep-water turbidite fans or deltaic and

lacustrine deposits (Stevens et al., 1988).

Cues Formation 3.5

Metasedimentary composite gneiss and

metasediments with leucocratic, garnet-biotite

and basic gneisses (Stevens et al., 1988).

Deep-water turbidite fans or deltaic lacustrine

deposits (Stevens et al., 1988).

Himalaya Formation

3.6-

3.9

Na-plagioclase-quartz rocks (Stevens et al.,

1988).



Rasp Ridge 3.10 Quartzo-feldspathic gneiss (Stevens et al.,

1980).



Vernon, 2000).

6


Broken Hill Group 500m

4

Allendale Metasediments

4.1-

4.3

Feldspar-rich and quartz-rich psammitic

metasediments (Stevens et al., 1988).



Parnell Formation

4.4

Amphibolite/basic granulite, garnetiferous

quartzo-feldspathic gneiss and quartz-

garnet/quartz-gahnite rock (Stevens et al.,

1988).

Bimodal rhyodacite-tholeiitic basalt volcanism

with associated exhalites (Stevens et al., 1988) or

marine transgression sequence with storm surge

layers (Wright et al., 1987).

Freyers Metasediment

4.5-

4.6

Well-bedded metasediments of pelitic to

psammitic composition. Offshore marine turbidite or shallow storm/wave base sequence (Stevens et al., 1988).

Hores Gneiss 4.7

Garnetiferous quartzo-feldspathic gneiss and

intercalated metasediments (Stevens et al.,

1988).

Felsic volcanic or volcaniclastic rock (Willis et

al., 1983; Stevens et al., 1988) or arkose

sandstone (Haydon and McConachy, 1987;

Wright et al., 1987).

Silver King Formation

4.8 Massive amphibolites intercalated with


Mass flow acid volcanics with intercalated shelf

muds and silts (Stevens et al., 1988).

Sundown Group 1000m

5

Thin bedded pelitic to psammopelitic

metasediments and minor calc-silicates


Diagenetic carbonate and turbiditic sedimentation

in a marine environment (Stevens et al., 1988) or

sag phase rift-fill sedimentation (Parr and Plimer,

1993).

Paragon Group >2200m

6-8

Well bedded to laminated fine-grained pelitic

schists and psammitic metasediments with

abundant graphitic metasediment (Stevens et

al., 1980).

Deep marine turbidite sequence or shallow

storm/wave base shelf sequence (Stevens et al.,

1988).

2.1.2 Age and Metamorphic History

The Broken Hill Block has undergone a long and intense metamorphic history (Table 2.2).

As a result, precise dating of the Willyama Supergroup stratigraphic units is problematic,

linked to the polymetamorphic history at Broken Hill, with resetting of zircons common

during metamorphism.

Early studies of the Broken Hill Block broadly confined the age of deposition to

1820±100 Ma (Pidgeon, 1967) after the separation of the crust from the mantle at 2100-

2300 Ma (McCulloch and Hensel, 1984). This has recently been constrained more tightly

by U/Pb zircon dating with the interpreted age of the Alma Gneiss (1691±12 Ma; Nutman

and Ehlers, 1998), Rasp Ridge Gneiss (1700-1690 Ma; Nutman and Ehlers, 1998), Hores

Gneiss (1690±5 Ma; Page and Laing, 1992; 1686±3 Ma, Page et al., 2000) and the Paragon

Group (1676±10 Ma; Ehlers et al., 1996). 1690 Ma has become the accepted (Parr and

Plimer, 1993; Stevens and Burton, 1998; Cartwright, 1999; Vassallo and Vernon, 2000)

broad depositional age for Willyama Supergroup.

7


Table 2.1.2a: Summary of the post-depositional tectonic history of the Broken Hill Block.

Event Metamorphic

Conditions

Evidenced

Deformation Date

D1/M1/F1

Olarian Orogeny

Peak prograde event

T= 750-800oC

P= 5-6 kb

(Phillips, 1980; Stevens, 1986).

Prograde mineral growth

defining S1 foliation and

regional F1 E-W-trending

nappe-like folds.

1600 Ma (Page and Laing,

1992) or 1690-1680 Ma

(Nutman and Ehlers, 1998).

D2/M2/F2

Olarian Orogeny Peak prograde event

T= 750-800oC

P= 5-6 kb (Phillips, 1980;

Stevens, 1986).


defining S2 foliation and the

regional F2 steeply dipping

NE-SW-trending folds.

1600-1590 Ma (Page and

Laing, 1992), 1596+3 Ma

(Page et al., 2000) or 1660-

1640 Ma (Nutman and Ehlers,

1998).

D3/M3/F3 (F4)

Olarian Orogeny

Retrograde event

T= 550-600 oC

P= 5-5.5 kb

(Stevens, 1986) or

Peak prograde event


Retrograde mineral growth


shear zone propagation

accompanied by N-S and NE-

SW trending folds.

1600-1570 Ma

(Page and Laing, 1992;

Nutman and Ehlers, 1998).

D4

Grenville Orogeny

Retrograde event

T=300-400oC

(Stevens, 1999).

Retrograde shear zone

reactivation with regional

uplift.

1250-1100 Ma

(Stevens, 1986).

D5

Delamerian Orogeny Retrograde event

T = 350oC

P = 2 kb

(Pidgeon, 1967; Harrison and

McDougall, 1981).

Emplacement of ultrabasic

dykes and plugs followed by

retrograde shear zone

reactivation and the

emplacement of further

pegmatites.

561+7 Ma

520+4 Ma to 458 Ma

(Harrison and McDougall,

1981).

Five main tectonic events (Table 2.2) have been recognised in literature, however

the geochronology of these has been strongly debated. Initial studies suggested peak

metamorphism at 1660+10 Ma, possibly as two thermal events, based on Rb-Sr and zircon

dating (Pidgeon, 1967; Gulson, 1984). Recent studies have suggested two refined, yet

opposing geochronologies. Page and Laing (1992) suggest two high-grade thermal events

occurring in close temporal association during the Olarian Orogeny at approximately 1600

Ma, followed by a single retrograde event dated at 1600-1570 Ma. This geochronology is

commensurate with previous structural studies of the Broken Hill Block (Laing et al.,

1978). Alternatively, Nutman and Ehlers (1998) suggest three peak high-grade thermal

events, occurring during the Olarian Orogeny at 1690-1680 Ma, 1660-1640 Ma and 1600-

1570 Ma. Both geochronologies based on SHRIMP U-Pb dating of zircons, have gained

partial support and remain a source of contention (Stevens, 1999; Nutman and Ehlers,

1999; Page et al., 2000). Peak prograde metamorphic conditions were of high thermal

grade at 750-800oC, and low to moderate pressure of between 5-6 kb, occurring at an

8


estimated depth of 16-20 km (Stevens, 1986). This resulting in the granulite faces

metamorphism of lower stratigraphic sequences, amphibolite facies metamorphism of the

middle sequences and greenshist facies metamorphism of the upper sequences. The

metamorphic grade of the stratigraphic units reflects this, increasing from the NW to the

SE (Figure 2.2) in the Broken Hill Block (Phillips, 1980).

Figure 2.3: Pressure verses temperature

diagram for the metamorphic evolution of

the Broken Hill Block, showing the prograde

evolution path as a result of increased

pressure and temperature during M1 and

M2, and the retrograde evolution path as a

result of the decrease in temperature that

occurred during M3 (from Hobbs et al.,

1984).

Figure 2.2: Broken Hill Block showing the

decrease in metamorphic grade from the

north-west to the south-east on the basis of

the mineral assemblage (from Slack et al.,

1989).

Retrogression started shortly after peak metamorphism with water released from

anatectic melts during cooling (Laing et al., 1978) being taken up by the crystallisation of

kyanite-staurolite and chloritoid assemblages (Figure 2.3)(Stevens, 1986). Muscovite-

bearing pegmatites and granites were emplaced in the region about 1420+20 Ma (Pidgeon,

1967), followed by a thermal pulse of approximately 300-400oC between 1250-1100 Ma

during the Grenville Orogeny causing reactivation of retrograde shear zones and regional

uplift (Stevens, 1986). Following this was a prolonged period of erosion and some

accompanying sedimentary deposition between 1100 Ma and 600 Ma. Dolerite and

pyroxenite dykes intruded close to 830 Ma (Stevens, 1986). The emplacement of ultrabasic

9


dykes and plugs around 561+7 Ma (Harrison and McDougall, 1981) preceded a low-grade

retrogression event occurring during the Delamerian Orogeny at 520+4 Ma (Pidgeon,

1967; Harrison and McDougall, 1981) causing retrograde shear zones to reactivate along

with the emplacement of further pegmatites. Minor fault movement in shear zones, further

erosion, uplift and regolith weathering has continued in the region to the present day.

2.1.3 Structural History

The Broken Hill Block is structurally complex and as a result, the deformation history is

yet to be conclusively determined. Most studies have documented three or four major

folding events and one of shear zone formation (Table 2.2) (Laing et al., 1978; Hobbs et

al., 1984; Laing, 1996) although further folding events and shearing events have been

suggested. The first folding event, F1, was coeval with the first regional peak prograde

metamorphic event of the Olarian Orogeny, M1, and involved very large scale (>10 km),

reclined to recumbent nappe-like isoclinal east-west trending folds (Laing et al., 1978;

Hobbs et al., 1984). S1 is defined in some amphibolite to granulite facies assemblages by

prograde sillimanite and is commonly parallel with bedding (Laing et al., 1978). Bedding

and younging directions have been recognised in some metasediment outcrops. The second

folding event, F2, occurred during the second regional peak prograde metamorphic event of

the Olarian Orogeny, M2 and involved medium scale 1-10 km steeply-dipping, NW-SE-

trending folds. It is recognised by a S2 schistosity commonly defined by sillimanite and is

visible in an array of Willyama Supergroup assemblages.

During the initial stages of the retrograde metamorphic event M3, the F3 folding

event occurred, creating small-scale N-S-trending folds, almost coaxial with F2. A fourth

folding event, F4, suggested by Hobbs et al. (1984) present in the western and north-

western areas of the Broken Hill Block, produced small-scale NE-SW-trending folds, and

occurred shortly after F3, resulting in dome and basin interference patterns with the F3

structures. D5 occurred during the final stages of the M3 retrograde metamorphic event

involving the formation of numerous steeply dipping retrograde shear zones within the

Broken Hill Block primarily in NE-SW and NW-SE orientations. Reactivation of these D5

retrograde shear zones and the creation of further retrograde shear zones occurred during

the Grenville Orogeny, and the Delamerian Orogeny. All shear zones are of lower

amphibolite to greenschist metamorphic grade and comprise fine-grained micaceous

schists (Hobbs et al., 1984).

10


2.2 Main Lode Orebody Geology 2.2.1 Stratigraphy

There are three stratigraphic suites of the Willyama Supergroup present within the main

lode mine leases (Figure 2.4). These include Suite 3, the Thackaringa Group, Suite 4, the

Broken Hill Group and Suite 5, the Sundown Group (Stevens et al., 1980). Further

lithostratigraphic study by Haydon and McConachy (1987) within the mine leases led to

the subdivision of specific suites into numerically ascending units. The Hores Gneiss (Unit

4.7) is the primary ore-bearing unit in the mine leases. It contains a number of “Potosi

Gneiss” lenses as well as pelitic and psammopelitic metasediments and associated lode

rocks (Haydon and McConachy, 1987). Retrograde shear zones, in particular the Globe

Vauxhall Shear, Western Shear and the Main Eastern Shear zones have caused offset of

stratigraphic units (Laing et al, 1978; Haydon and McConachy, 1987).

Figure 2.4: Mine lease cross-section

showing the stratigraphic suites,

Hores Gneiss and location of the main

lode orebody. Also shown are the

Broken Hill Antiform, Hanging Wall

Synform and Globe Vauxhall Shear

(from Laing et al., 1978).

2.2.2 Structure

The structure of the main lode mine leases has been interpreted by a number of authors

(Andrews, 1922; Gustafson et al., 1950; Ransom, 1968; Hodgson, 1975; Laing et al, 1978;

11


Haydon and McConachy, 1987; Rothery, 2001). It is currently accepted (Plimer, 1984;

Haydon and McConachy, 1987; Stevens, 1995) that the main lode orebody lies on a

downward-facing limb of an F1 fold from stratigraphic vergence and younging data (Laing

et al., 1978). The northern extent of the orebody plunges to the north at approximately 40o,

while the southern extent plunges south at approximately 20-25o (Figure 2.5). The main

structures observed in the central mine leases are F2 in age.

Figure 2.5: Longitudinal profile of the Broken Hill orebody showing the CML7 lease boundary,

Kintore pit and generalised ore lode positions. The zinc lodes are concentrated at the southern

extent of the orebody (from Consolidated Broken Hill Ltd.).

2.2.3 Mineralisation

The main lode mineralisation is stratabound within lenses of the quartzofeldspathic garnet-

rich “Potosi Gneiss”. There are eight separate lodes of ore (Haydon and McConachy,

1987) within the central mine leases that form an en echelon stack (Figure 2.6). The

stratigraphic order of the ore lodes (taking into account that the current stratigraphy has

been inverted) is the C Lode, B Lode, Upper A Lode, Lower A Lode, Upper 1 Lens, Lower

1 Lens, 2 Lens and 3 Lens. Each lode has its own characteristic gangue mineralogy and

base-metal content. The C Lode, B Lode, A Lode (Upper and Lower), and 1 Lens (Upper

and Lower) are known as zinc lodes (Matthias, 1974) due to the relatively high Zn:Pb ratio

compared to 2 Lens and 3 Lens.

12


Figure 2.6: Section 30 cross-section

looking south, showing the folded

nature of the eight ore lenses and the

attenuation of the 2 Lens and 3 Lens

caused by the Globe Vauxhall Shear

(from Haydon and McConachy,

1987).

The B and A Lodes are only developed in the CML7 and southern mine leases

while 1 Lens although thickest in the southern mine leases, continues to the northern leases

where it is only a few centimetres thick. B Lode, A Lode and 1 Lens are typically

contained within envelopes of pink garnet quartzite. Close to sulphides, the abundance of

pink garnet quartzite decreases as it is cross-cut by blue lode quartz. Juxtaposed to sulphide

rocks, the percentage of blue quartz increases in association with green gahnite crystals.

The main ore lodes occur as massive sulphides with minor quartz and garnet. Late stage

coarse galena-chalcopyrite-pyrrhotite bearing quartz veins transgress the main ore and

surrounding lode horizon rocks.

2.3 Ore Deposit Genesis Models The origin of the sulphide lodes at Broken Hill has been a source of conjecture for a long

time and there are numerous postulated genetic models. Even after more than 400

published papers on the Broken Hill orebody, no consensus has yet been reached. The

13


earliest ideas were based on the epigenetic deposition of the orebodies (Andrews, 1922;

Gustafson et al., 1950) via the preferential replacement of sedimentary layers. In contrast,

a syngenetic model was proposed (King and Thomson, 1953) and has been refined by

numerous authors (Stanton, 1976; Laing et al., 1978; Willis et al., 1983; Plimer, 1984;

Stevens et al., 1988; Parr and Plimer, 1993). In this model the orebody is coeval with the

enclosing rocks. It is suggested that the orebody is of submarine exhalative origin within

an intracontinental rift setting. Submarine hydrothermal activity is either driven by an

intrusion or by thinning of the crust through rifting (Figure 2.7). This model has been

supported by recent literature (Willis et al., 1988; Parr and Plimer, 1993; Laing, 1996;

Large et al., 1996; Walters, 1998).

Figure 2.7: Proposed model for the genesis of the Broken Hill orebody through syngenetic submarine

exhalative mechanisms (after Parr and Plimer, 1993).

Wright et al. (1987) suggested that mineralisation was generated by compactive

expulsion of metal-bearing basin brines during sedimentation. They suggest the orebody is

formed in a large scale sedimentary basin, with Pb-Zn-rich brines travelling along

preferential pathways such as psammitic sandstones to a point where they were capped by

overlying impermeable pelitic sediments, forming the orebody during diagenesis of the

compacting sediments (Wright et al., 1987). This model is partially supported by Haydon

and McConachy (1987) and Cook and Ashley (1992) while disputed by Plimer and

Lottermoser (1988) and Willis et al. (1988). Further comment was made by Wright et al.

(1988a, b). Recently a revised epigenetic model has been proposed (Ehlers et al., 1996)

14


with the mineralisation suggested to have occurred coeval with regional metamorphism

after the deposition of the enclosing rocks. This model was based on the result of

contentious U/Pb dating of zircons from the Broken Hill orebody, which were later

disputed by Carr and Sun (1996).

15

Chapter One - Introduction

Chapter One – Introduction 1.1 Background The Broken Hill Pb-Zn-Ag ore deposit is situated within an inlier of deformed and

metamorphosed Palaeoproterozoic rocks known as the Broken Hill Block (Figure 1.1)

within the Curnamona Craton, in western New South Wales and South Australia (Willis et

al., 1983). The Curnamona Craton covers approximately 40,000 km2 and consists of the

Broken Hill, Olary and Eurowie Blocks and the Mt Painter and Mt Babbage Inliers (Teale

and Flint, 1993).

Figure 1.1: Location of the

Broken Hill Block within

the Curnamona Craton, in

western New South Wales

and South Australia. The

Mt Painter and Mt Babbage

Inliers (not shown) are

located approximately 200

km north-west of the Olary

Block (after Cook and

Ashley, 1992).

1


The Broken Hill Block contains metasedimentary and metavolcanic sequences of

pelitic, psammitic, quartzofeldspathic and mafic composition comprising the Willyama

Supergroup (Willis et al., 1983). The main lode orebody is estimated to have contained in

excess of 300 million tonnes of sulphide ore prior to mining with grades exceeding 15%

combined lead and zinc (Haydon and McConachy, 1987), with associated silver and gold.

It is considered a super giant orebody (Figure 1.2) by world standards (Large et al., 2002).

Figure 1.2: Tonnes of metal

verses Zn + Pb grade for giant

and supergiant stratiform

sediment hosted Zn-Pb-Ag

ore deposits comprised of

sedimentary exhalative-type

(SEDEX), Mississippi Valley-

type (MVT), Broken Hill-type

(BHT) and Irish style deposits

(after Large et al., 2002).

Recent discoveries of the Burke Street, Crystal Lane, Tin Street, Potosi, Flying

Doctor and Silver Peak mineralisations (Haydon and McConachy, 1987; Stevens and

Burton, 1998) have renewed waning scientific and exploration interest for base metals

within the Broken Hill Block. Consolidated Broken Hill Ltd. (CBH) is the current lease

holder of Consolidated Mining Lease 7 (CML7) which covers the central section of the

Broken Hill main lode, previously known as the Broken Hill South Mine.

1.2 Research Aims The primary objective of this honours research report is to provide a comprehensive

mineralogical and geochemical analysis of the B Lode, A Lode and 1 Lens zinc lodes on

CML7, and document the effects of regional metamorphism and deformation on the

sulphide rocks. This may allow constraints to be placed upon models for the genesis of the

2


Broken Hill orebody. To achieve the primary objectives, a series of aims have been

determined. These are:

o To document the mineralogy and geochemistry of the B Lode, A Lodes and 1 Lens

from core drilled on the CML7 mine lease,

o To document the mineralogy and geochemistry of an ore lode exposed in the base of

the Kintore pit on the CML7 mine lease,

o To investigate textural relationships of sulphide minerals which may indicate

movement of sulphides along the limbs of fold structures during deformation,

o To observe the distribution and phases that Ag occurs within, which may give an

indication of Ag movement during the long history of metamorphism, and

o To look for evidence showing the sulphide rocks have undergone multiple high-grade

metamorphic events and any other evidence that can place constraints on the proposed

models for the genesis of the orebody.

1.3 Research Methods This honours research report presents observations and the analysis of samples collected

during time spent in the field at Broken Hill between Monday 8th of April and Monday 29th

of April 2002. Fieldwork involved logging and sampling three diamond drill cores totalling

746 metres, and the logging and bulk sampling along three traverses across an exposed ore

lode in the Kintore pit, termed the Kintore Lode in this report. From the samples collected,

the following analyses have been completed:

o Major, trace, rare earth and halide elements have been analysed across B Lode, A

Lode, 1 Lens and Kintore Lode samples at AMDEL Ltd. using ICP-OES, ICP-MS,

gravimetric and titration techniques.

o Polished thin section microscopy of sections at The University of Melbourne under

reflected and transmitted light.

o Electron microprobe analysis of sulphide and gangue minerals from B Lode, A Lode, 1

Lens and Kintore Lode samples at The University of Melbourne.

o Stable isotope analyses of O and C from carbonate filled microfractures within B Lode

at Monash University.

3


1.4 Previous Studies

The Broken Hill Pb-Zn-Ag orebody is the world’s largest base metal orebody. Key studies

include Andrews (1912), Gustafson et al. (1950), Laing et al. (1978); Stevens et al. (1980,

1988), Willis et al. (1983), Haydon and McConachy (1987) and Parr and Plimer (1993).

These studies have focused on the provenance of the orebody in a regional, tectonic or

metallogenic context.

Other studies have focussed on geochemistry, either at a regional scale,

investigating the metavolcanic or metasedimentary units of the Broken Hill Block (Main et

al., 1983; Slack and Stevens, 1994) or at a local scale, primarily investigating the

individual lead-rich ore lodes within various orebodies of the Broken Hill Block (Both,

1973; Johnson and Klingner, 1975; Spry and Wonder, 1989). Previous studies that have

investigated the geochemistry of the zinc-rich B Lode, A Lode and 1 Lens in detail include

Stanton (1972) and Matthias (1974). However, neither of these studies analysed samples

from within the CML7 lease area. Two studies (Plimer, 1980; Both and Stumpfl, 1987)

have investigated the distribution and mobilisation of silver within the Broken Hill

orebody. There is one published report by mine geologists at the time, briefly documenting

the geology of the Broken Hill South Mine (van der Heyden and Edgecombe, 1990) and

one report documenting the structure of the orebody, with reference to the Kintore pit

(Rothery, 2001).

CML7 is completely devoid of detailed geochemical and mineralogical studies of

the zinc lodes. This is due to the fact that it was only recently discovered that the zinc lodes

continue onto CML7 from the Pasminco South Mines area. Underground mining on CML7

ceased over 25 years ago. Plans by the current operators to recommence mining operations

shortly have been a driving force behind recent studies on CML7. These include detailed

geochemical analysis of the C Lode and 2 Lens by Sproal (2001) and the Western

Mineralisation and dolerite dykes by Kitchen (2001).

Despite the volume of studies of the Broken Hill orebody and surrounding geology,

there are numerous conflicting views on many issues, from identification of pre-

metamorphic lithologies and origin of the orebodies, to partial melting and subsequent

mobilisation of the ore lodes. As a result, the paradigm for Broken Hill-type deposits,

based on the knowledge of the Broken Hill orebody, is continually evolving.

4


Chapter Two - Broken Hill Geology

2.1 Regional Geology 2.1.1 Stratigraphy

The stratigraphy of the Broken Hill Block (Figure 2.1) described by Stevens et al. (1980,

1988) and by Willis et al. (1983) is still currently generally supported in the literature

including Parr and Plimer (1993), Laing (1996), Stevens and Burton (1998) and Walters

(1998). Due to an intense, prolonged metamorphic and deformation history at Broken Hill,

the recognition of lithological protoliths is exceedingly difficult. As a consequence, there

have been some studies that have disputed parts of the interpreted stratigraphy (e.g.

Haydon and McConachy, 1987; Wright et al., 1987; Vassallo and Vernon, 2000). In light

of new studies, the interpreted stratigraphy of the Willyama Supergroup in proximity of the

orebodies needs constant re-evaluation.

Figure 2.1: Stratigraphy of

the Willyama Supergroup

described by Willis et al.

(1983), also highlighting the

depositional trends and

occurrence of mineralisation

and exhalative sediments

(after Parr and Plimer, 1993).

5


Regional mapping by the New South Wales Geological Survey was instrumental in

the re-classification of the Willyama Supergroup (Willis et al., 1983) into smaller “rock

type associations” called suites (Stevens et al., 1980). The lowest suite of the Willyama

Supergroup identified is the Clevedale Migmatite, followed by the Thorndale Composite

Gneiss, although recently classified units, the Redan Gneiss, Ednas Gneiss and Mulculca

Formation (Stevens et al., 1988) in the south-east of the Broken Hill Block are thought to

occur stratigraphically beneath or equivalent to the Clevedale Migmatite and Thorndale

Composite Gneiss. Overlying these is the Thackaringa Group, which is followed by the

Broken Hill Group, the Sundown Group and the uppermost sequence, the Paragon Group

(Table 2.1). The principal trend is from the quartzo-feldspathic rich lithostratigraphic

sequences of the Thackaringa Group, through the mineralised and exhalative

lithstratigraphic sequences of the Broken Hill Group, to the metasediment-rich sequences

of the Sundown and Paragon Groups (Parr and Plimer, 1993).

Table 2.1.1a: Lithostratigraphic units of the Willyama Supergroup and their interpreted protoliths.

Stratigraphic

Name

Suite

Unit Lithology Interpreted Protolith

Clevedale Migmatite > 500m

1

Poorly bedded feldspathic gneiss, migmatite

metasediment and minor amphibolite (Stevens

et al., 1980).

Detrital sediments containing volcanic detritus

and ash fall tuffs (Stevens et al., 1988).

Thorndale Composite Gneiss 1000m

2

Thinly to poorly bedded metasedimentary and

quartzo-feldspathic composite gneiss and

amphibolite (Stevens et al., 1980)

Fluvial-deltaic feldspathic sand deposition


Thackaringa Group 1500m

3

Lady Brassey 3.1 Na-plagioclase-quartz rocks (Stevens et al.,

1988).



Alma Gneiss 3.2 Coarse grained granitic rock with K feldspar

megacrysts (Willis et al., 1983)



Vernon, 2000).

Alders Tank Formation

3.3-

3.4

Metasedimentary and quartzo-feldspathic

composite gneiss (Stevens et al., 1988).



Cues Formation 3.5

Metasedimentary composite gneiss and

metasediments with leucocratic, garnet-biotite

and basic gneisses (Stevens et al., 1988).

Deep-water turbidite fans or deltaic lacustrine

deposits (Stevens et al., 1988).

Himalaya Formation

3.6-

3.9

Na-plagioclase-quartz rocks (Stevens et al.,

1988).



Rasp Ridge 3.10 Quartzo-feldspathic gneiss (Stevens et al.,

1980).



Vernon, 2000).

6


Broken Hill Group 500m

4

Allendale Metasediments

4.1-

4.3

Feldspar-rich and quartz-rich psammitic




Parnell Formation

4.4

Amphibolite/basic granulite, garnetiferous

quartzo-feldspathic gneiss and quartz-

garnet/quartz-gahnite rock (Stevens et al.,

1988).

Bimodal rhyodacite-tholeiitic basalt volcanism

with associated exhalites (Stevens et al., 1988) or

marine transgression sequence with storm surge

layers (Wright et al., 1987).

Freyers Metasediment

4.5-

4.6

Well-bedded metasediments of pelitic to

psammitic composition. Offshore marine turbidite or shallow storm/wave base sequence (Stevens et al., 1988).

Hores Gneiss 4.7

Garnetiferous quartzo-feldspathic gneiss and

intercalated metasediments (Stevens et al.,

1988).

Felsic volcanic or volcaniclastic rock (Willis et

al., 1983; Stevens et al., 1988) or arkose

sandstone (Haydon and McConachy, 1987;

Wright et al., 1987).

Silver King Formation

4.8 Massive amphibolites intercalated with


Mass flow acid volcanics with intercalated shelf

muds and silts (Stevens et al., 1988).

Sundown Group 1000m

5

Thin bedded pelitic to psammopelitic

metasediments and minor calc-silicates


Diagenetic carbonate and turbiditic sedimentation

in a marine environment (Stevens et al., 1988) or

sag phase rift-fill sedimentation (Parr and Plimer,

1993).

Paragon Group >2200m

6-8

Well bedded to laminated fine-grained pelitic

schists and psammitic metasediments with

abundant graphitic metasediment (Stevens et

al., 1980).

Deep marine turbidite sequence or shallow

storm/wave base shelf sequence (Stevens et al.,

1988).

2.1.2 Age and Metamorphic History

The Broken Hill Block has undergone a long and intense metamorphic history (Table 2.2).

As a result, precise dating of the Willyama Supergroup stratigraphic units is problematic,

linked to the polymetamorphic history at Broken Hill, with resetting of zircons common

during metamorphism.

Early studies of the Broken Hill Block broadly confined the age of deposition to

1820±100 Ma (Pidgeon, 1967) after the separation of the crust from the mantle at 2100-

2300 Ma (McCulloch and Hensel, 1984). This has recently been constrained more tightly

by U/Pb zircon dating with the interpreted age of the Alma Gneiss (1691±12 Ma; Nutman

and Ehlers, 1998), Rasp Ridge Gneiss (1700-1690 Ma; Nutman and Ehlers, 1998), Hores

Gneiss (1690±5 Ma; Page and Laing, 1992; 1686±3 Ma, Page et al., 2000) and the Paragon

Group (1676±10 Ma; Ehlers et al., 1996). 1690 Ma has become the accepted (Parr and

Plimer, 1993; Stevens and Burton, 1998; Cartwright, 1999; Vassallo and Vernon, 2000)

broad depositional age for Willyama Supergroup.

7


Table 2.1.2a: Summary of the post-depositional tectonic history of the Broken Hill Block.

Event Metamorphic

Conditions

Evidenced

Deformation Date

D1/M1/F1

Olarian Orogeny

Peak prograde event

T= 750-800oC

P= 5-6 kb

(Phillips, 1980; Stevens, 1986).



regional F1 E-W-trending

nappe-like folds.

1600 Ma (Page and Laing,

1992) or 1690-1680 Ma


D2/M2/F2

Olarian Orogeny Peak prograde event

T= 750-800oC

P= 5-6 kb (Phillips, 1980;

Stevens, 1986).


defining S2 foliation and the

regional F2 steeply dipping

NE-SW-trending folds.

1600-1590 Ma (Page and

Laing, 1992), 1596+3 Ma

(Page et al., 2000) or 1660-

1640 Ma (Nutman and Ehlers,

1998).

D3/M3/F3 (F4)

Olarian Orogeny

Retrograde event

T= 550-600 oC

P= 5-5.5 kb

(Stevens, 1986) or

Peak prograde event


Retrograde mineral growth


shear zone propagation

accompanied by N-S and NE-

SW trending folds.

1600-1570 Ma

(Page and Laing, 1992;

Nutman and Ehlers, 1998).

D4

Grenville Orogeny

Retrograde event

T=300-400oC

(Stevens, 1999).

Retrograde shear zone

reactivation with regional

uplift.

1250-1100 Ma

(Stevens, 1986).

D5

Delamerian Orogeny Retrograde event

T = 350oC

P = 2 kb

(Pidgeon, 1967; Harrison and

McDougall, 1981).

Emplacement of ultrabasic

dykes and plugs followed by

retrograde shear zone

reactivation and the

emplacement of further

pegmatites.

561+7 Ma

520+4 Ma to 458 Ma

(Harrison and McDougall,

1981).

Five main tectonic events (Table 2.2) have been recognised in literature, however

the geochronology of these has been strongly debated. Initial studies suggested peak

metamorphism at 1660+10 Ma, possibly as two thermal events, based on Rb-Sr and zircon

dating (Pidgeon, 1967; Gulson, 1984). Recent studies have suggested two refined, yet

opposing geochronologies. Page and Laing (1992) suggest two high-grade thermal events

occurring in close temporal association during the Olarian Orogeny at approximately 1600

Ma, followed by a single retrograde event dated at 1600-1570 Ma. This geochronology is

commensurate with previous structural studies of the Broken Hill Block (Laing et al.,

1978). Alternatively, Nutman and Ehlers (1998) suggest three peak high-grade thermal

events, occurring during the Olarian Orogeny at 1690-1680 Ma, 1660-1640 Ma and 1600-

1570 Ma. Both geochronologies based on SHRIMP U-Pb dating of zircons, have gained

partial support and remain a source of contention (Stevens, 1999; Nutman and Ehlers,

1999; Page et al., 2000). Peak prograde metamorphic conditions were of high thermal

grade at 750-800oC, and low to moderate pressure of between 5-6 kb, occurring at an

8


estimated depth of 16-20 km (Stevens, 1986). This resulting in the granulite faces

metamorphism of lower stratigraphic sequences, amphibolite facies metamorphism of the

middle sequences and greenshist facies metamorphism of the upper sequences. The

metamorphic grade of the stratigraphic units reflects this, increasing from the NW to the

SE (Figure 2.2) in the Broken Hill Block (Phillips, 1980).

Figure 2.3: Pressure verses temperature

diagram for the metamorphic evolution of

the Broken Hill Block, showing the prograde

evolution path as a result of increased

pressure and temperature during M1 and

M2, and the retrograde evolution path as a

result of the decrease in temperature that

occurred during M3 (from Hobbs et al.,

1984).

Figure 2.2: Broken Hill Block showing the

decrease in metamorphic grade from the

north-west to the south-east on the basis of

the mineral assemblage (from Slack et al.,

1989).

Retrogression started shortly after peak metamorphism with water released from

anatectic melts during cooling (Laing et al., 1978) being taken up by the crystallisation of

kyanite-staurolite and chloritoid assemblages (Figure 2.3)(Stevens, 1986). Muscovite-

bearing pegmatites and granites were emplaced in the region about 1420+20 Ma (Pidgeon,

1967), followed by a thermal pulse of approximately 300-400oC between 1250-1100 Ma

during the Grenville Orogeny causing reactivation of retrograde shear zones and regional

uplift (Stevens, 1986). Following this was a prolonged period of erosion and some

accompanying sedimentary deposition between 1100 Ma and 600 Ma. Dolerite and

pyroxenite dykes intruded close to 830 Ma (Stevens, 1986). The emplacement of ultrabasic

9


dykes and plugs around 561+7 Ma (Harrison and McDougall, 1981) preceded a low-grade

retrogression event occurring during the Delamerian Orogeny at 520+4 Ma (Pidgeon,

1967; Harrison and McDougall, 1981) causing retrograde shear zones to reactivate along

with the emplacement of further pegmatites. Minor fault movement in shear zones, further

erosion, uplift and regolith weathering has continued in the region to the present day.

2.1.3 Structural History

The Broken Hill Block is structurally complex and as a result, the deformation history is

yet to be conclusively determined. Most studies have documented three or four major

folding events and one of shear zone formation (Table 2.2) (Laing et al., 1978; Hobbs et

al., 1984; Laing, 1996) although further folding events and shearing events have been

suggested. The first folding event, F1, was coeval with the first regional peak prograde

metamorphic event of the Olarian Orogeny, M1, and involved very large scale (>10 km),

reclined to recumbent nappe-like isoclinal east-west trending folds (Laing et al., 1978;

Hobbs et al., 1984). S1 is defined in some amphibolite to granulite facies assemblages by

prograde sillimanite and is commonly parallel with bedding (Laing et al., 1978). Bedding

and younging directions have been recognised in some metasediment outcrops. The second

folding event, F2, occurred during the second regional peak prograde metamorphic event of

the Olarian Orogeny, M2 and involved medium scale 1-10 km steeply-dipping, NW-SE-

trending folds. It is recognised by a S2 schistosity commonly defined by sillimanite and is

visible in an array of Willyama Supergroup assemblages.

During the initial stages of the retrograde metamorphic event M3, the F3 folding

event occurred, creating small-scale N-S-trending folds, almost coaxial with F2. A fourth

folding event, F4, suggested by Hobbs et al. (1984) present in the western and north-

western areas of the Broken Hill Block, produced small-scale NE-SW-trending folds, and

occurred shortly after F3, resulting in dome and basin interference patterns with the F3

structures. D5 occurred during the final stages of the M3 retrograde metamorphic event

involving the formation of numerous steeply dipping retrograde shear zones within the

Broken Hill Block primarily in NE-SW and NW-SE orientations. Reactivation of these D5

retrograde shear zones and the creation of further retrograde shear zones occurred during

the Grenville Orogeny, and the Delamerian Orogeny. All shear zones are of lower

amphibolite to greenschist metamorphic grade and comprise fine-grained micaceous

schists (Hobbs et al., 1984).

10


2.2 Main Lode Orebody Geology 2.2.1 Stratigraphy

There are three stratigraphic suites of the Willyama Supergroup present within the main

lode mine leases (Figure 2.4). These include Suite 3, the Thackaringa Group, Suite 4, the

Broken Hill Group and Suite 5, the Sundown Group (Stevens et al., 1980). Further

lithostratigraphic study by Haydon and McConachy (1987) within the mine leases led to

the subdivision of specific suites into numerically ascending units. The Hores Gneiss (Unit

4.7) is the primary ore-bearing unit in the mine leases. It contains a number of “Potosi

Gneiss” lenses as well as pelitic and psammopelitic metasediments and associated lode

rocks (Haydon and McConachy, 1987). Retrograde shear zones, in particular the Globe

Vauxhall Shear, Western Shear and the Main Eastern Shear zones have caused offset of

stratigraphic units (Laing et al, 1978; Haydon and McConachy, 1987).

Figure 2.4: Mine lease cross-section

showing the stratigraphic suites,

Hores Gneiss and location of the main

lode orebody. Also shown are the

Broken Hill Antiform, Hanging Wall

Synform and Globe Vauxhall Shear

(from Laing et al., 1978).

2.2.2 Structure

The structure of the main lode mine leases has been interpreted by a number of authors

(Andrews, 1922; Gustafson et al., 1950; Ransom, 1968; Hodgson, 1975; Laing et al, 1978;

11


Haydon and McConachy, 1987; Rothery, 2001). It is currently accepted (Plimer, 1984;

Haydon and McConachy, 1987; Stevens, 1995) that the main lode orebody lies on a

downward-facing limb of an F1 fold from stratigraphic vergence and younging data (Laing

et al., 1978). The northern extent of the orebody plunges to the north at approximately 40o,

while the southern extent plunges south at approximately 20-25o (Figure 2.5). The main

structures observed in the central mine leases are F2 in age.

Figure 2.5: Longitudinal profile of the Broken Hill orebody showing the CML7 lease boundary,

Kintore pit and generalised ore lode positions. The zinc lodes are concentrated at the southern

extent of the orebody (from Consolidated Broken Hill Ltd.).

2.2.3 Mineralisation

The main lode mineralisation is stratabound within lenses of the quartzofeldspathic garnet-

rich “Potosi Gneiss”. There are eight separate lodes of ore (Haydon and McConachy,

1987) within the central mine leases that form an en echelon stack (Figure 2.6). The

stratigraphic order of the ore lodes (taking into account that the current stratigraphy has

been inverted) is the C Lode, B Lode, Upper A Lode, Lower A Lode, Upper 1 Lens, Lower

1 Lens, 2 Lens and 3 Lens. Each lode has its own characteristic gangue mineralogy and

base-metal content. The C Lode, B Lode, A Lode (Upper and Lower), and 1 Lens (Upper

and Lower) are known as zinc lodes (Matthias, 1974) due to the relatively high Zn:Pb ratio

compared to 2 Lens and 3 Lens.

12


Figure 2.6: Section 30 cross-section

looking south, showing the folded

nature of the eight ore lenses and the

attenuation of the 2 Lens and 3 Lens

caused by the Globe Vauxhall Shear

(from Haydon and McConachy,

1987).

The B and A Lodes are only developed in the CML7 and southern mine leases

while 1 Lens although thickest in the southern mine leases, continues to the northern leases

where it is only a few centimetres thick. B Lode, A Lode and 1 Lens are typically

contained within envelopes of pink garnet quartzite. Close to sulphides, the abundance of

pink garnet quartzite decreases as it is cross-cut by blue lode quartz. Juxtaposed to sulphide

rocks, the percentage of blue quartz increases in association with green gahnite crystals.

The main ore lodes occur as massive sulphides with minor quartz and garnet. Late stage

coarse galena-chalcopyrite-pyrrhotite bearing quartz veins transgress the main ore and

surrounding lode horizon rocks.

2.3 Ore Deposit Genesis Models The origin of the sulphide lodes at Broken Hill has been a source of conjecture for a long

time and there are numerous postulated genetic models. Even after more than 400

published papers on the Broken Hill orebody, no consensus has yet been reached. The

13


earliest ideas were based on the epigenetic deposition of the orebodies (Andrews, 1922;

Gustafson et al., 1950) via the preferential replacement of sedimentary layers. In contrast,

a syngenetic model was proposed (King and Thomson, 1953) and has been refined by

numerous authors (Stanton, 1976; Laing et al., 1978; Willis et al., 1983; Plimer, 1984;

Stevens et al., 1988; Parr and Plimer, 1993). In this model the orebody is coeval with the

enclosing rocks. It is suggested that the orebody is of submarine exhalative origin within

an intracontinental rift setting. Submarine hydrothermal activity is either driven by an

intrusion or by thinning of the crust through rifting (Figure 2.7). This model has been

supported by recent literature (Willis et al., 1988; Parr and Plimer, 1993; Laing, 1996;

Large et al., 1996; Walters, 1998).

Figure 2.7: Proposed model for the genesis of the Broken Hill orebody through syngenetic submarine

exhalative mechanisms (after Parr and Plimer, 1993).

Wright et al. (1987) suggested that mineralisation was generated by compactive

expulsion of metal-bearing basin brines during sedimentation. They suggest the orebody is

formed in a large scale sedimentary basin, with Pb-Zn-rich brines travelling along

preferential pathways such as psammitic sandstones to a point where they were capped by

overlying impermeable pelitic sediments, forming the orebody during diagenesis of the

compacting sediments (Wright et al., 1987). This model is partially supported by Haydon

and McConachy (1987) and Cook and Ashley (1992) while disputed by Plimer and

Lottermoser (1988) and Willis et al. (1988). Further comment was made by Wright et al.

(1988a, b). Recently a revised epigenetic model has been proposed (Ehlers et al., 1996)

14


with the mineralisation suggested to have occurred coeval with regional metamorphism

after the deposition of the enclosing rocks. This model was based on the result of

contentious U/Pb dating of zircons from the Broken Hill orebody, which were later

disputed by Carr and Sun (1996).

15

Chapter Three – CML7 Geology

Chapter Three - CML7 Geology 3.1 Geology Introduction This chapter presents the field geology of drill core and open pit rocks observed while

undertaking fieldwork in Broken Hill.

Figure 3.1: CML7 lease showing drill hole locations and Kintore pit as well as the main line of lode

and Western Mineralisation projected to surface (from Consolidated Broken Hill Ltd).

3.2 Drill Core Geology 3.2.1 Drill core introduction

Three complete diamond drill cores ZLDD5000, ZLDD5001 and ZLDD5002A, totalling

746 metres were lithologically logged. The cores were drilled from the western side of the

main lode, close to the southern extent of the CML7 boundary (Figure 3.1) in an

orientation steeply dipping towards the east in an attempt to intersect the main lode ore

lenses. Logging involved identifying and documenting the mineralogy of the core and any

visible structures, such as bedding, younging, cleavages, faults, fractures, folds and

brecciation. Once the three diamond drill cores had been logged for lithology and broad

stratigraphic features, the zinc lode intersections, B Lode, A Lode and 1 Lens were

relogged in greater detail to focus on the mineralogy and the textures of the lode horizon

16


and sulphide rocks. The use of a 10x hand lens, magnet, tungsten-tipped scratching tool

and hydrochloric acid aided in the logging process.

3.2.2 Drill core lithology

The drill cores were logged using a combination of the nomenclature set out by the New

South Wales Geological Survey for the Broken Hill Block and CBH (Appendix 1).

Observations made during logging were recorded on sedimentological core logs (Figures

3.8 and 3.9 and Appendix 2).

The broad stratigraphy of each diamond drill cores was very similar. The first

lithology encountered varied between pegmatite or psammitic and pelitic metasediments.

These lithologies were all highly oxidised down to about 50 metres depth due to their close

proximity to the surface and long history of weathering. Characteristic of oxidation was the

red earthy colour of psammites, the sericitisation and kaolinisation of pegmatites and the

breakdown of micas within pelites to clays. One unusual lithology observed was a 40cm

breccia of pelite with a fine-grained matrix of dark garnet (Figure 3.2).

Figure 3.2:

Brecciated pelite

with a fine-grained

garnet matrix

(from ZLDD5001-

14.8m).

Once below the oxidation zone, the lithologies varied between intercalated pelitic,

psammopelite and psammite with numerous pegmatite intervals and ore lenses surrounded

by distinctive lode horizon rocks. The pelites exhibited a light green to brown hue on

exposed surfaces as a result of slight post-drilling chloritisation of micas, however a light

grey to light brown mottled appearance is more characteristic of fresh samples. Some

sections of pelites exhibited a spotted appearance resulting from the pseudomorphing of

andalusite by sillimanite crystals averaging 4mm in size but much larger in a number of

cases. This distinctive lithology is termed ‘spotted pelite’ (Figure 3.3). The psammopelites

17


and psammites (Figure 3.4) ranged from light grey to dark grey in colour and rarely had

become red with oxidation since drilling. Graded bedding was present in a number of

sections where a minor pelite bed was contained within a large psammopelitic bed (Figure

3.4) and could be used to identify younging in the metasediments. The mineralogy of

pegmatites varied between quartz and K-feldspar, to include moderate amounts of garnet +

biotite and muscovite. The pegmatites commonly occurred crosscutting S0 and S1, rarely

folding ptygmatically (Figure 3.5) in a S2 or even S3 orientation, while in other sections,

follow the S0/S1 orientation. Minor pure white quartz veins are rare. Sulphides are

commonly associated with blue lode quartz and enclosed within fine-grained pink garnet-

rich psammite known as ‘garnet quartzite’ (Figure 3.6).

Figure 3.3: Spotted

pelite in HQ core

(from ZLDD5001-

29.1m).

Figure 3.4: Reverse

bedding in pelitic

to psammopelitic

metasediments in

NQ core (from

ZLDD5001-178m).

18


Figure 3.5: Ptygmatic

folding of pegmatite

in HQ core (from

ZLDD5000-200m).

Figure 3.6: Garnet

quartzite associated

with blue lode quartz

veins in HQ core

(from ZLDD5000-

163.2m).

From the logs of the three drill cores, the abundance for each main lithological type

was quantified down to an interbedded scale of 10cm (Figure 3.7). The lithologies

comprise of psammopelitic metasediment (28%), pelitic metasediments (19%), psammitic

metasediments (17%), pegmatite (13%), lode horizon (17%) and massive sulphides (4%).

Core loss accounted for 1% of the total drill core length.

Sedimentology of Drill Cores

17

28

19

1317

41

0

5

10

15

20

25

30

35

Psammite

Psammop

elite

Pelite

Pegmati

te

Lode

Hori

zon R

ocks

Massiv

e Sulp

hides

Core Lo

ss

Lithology

Abun

danc

e %

ZLDD5000

ZLDD5001

ZLDD5002A

Drill Core Average(value shown)

Figure 3.7:

Abundance of the

broadly defined

lithological types

observed in the drill

cores.

19


3.2.3 B Lode Geology

The best intersection of B Lode was between 133-140m in ZLDD5001 (Figure 3.8). C

Lode was intersected from 81-84m and 89-92m prior to the B Lode intersection,

supporting the inverted nature of the Broken Hill orebody.

Prior to garnet quartzite at 127m, there was a progression from pelitic

metasediments, through psammopelitic metasediments to psammitic metasediments. B

Lode is surrounded by garnet quartzite, which is a fine-grained <2mm primarily anhedral

pink garnet “sandstone”. Minor 1cm veins of blue lode quartz and pyrrhotite>chalcopyrite

veins occur along S0/S1. Towards massive sulphides, the modal abundance of blue lode

quartz veins in the garnet quartzite increases and sulphides begin to occur disseminated

within the garnet quartzite groundmass. The progression into massive sulphides is marked

by the loss of garnet quartzite, a decrease in the blue lode quartz and an increase in

disseminated sulphides to such a degree that the rock is predominately massive sulphides,

of approximately 50% modal abundance. In B Lode, sphalerite is the most abundant

sulphide (50%), followed by galena (35%), pyrrhotite (10%) and chalcopyrite (5%). The

sulphide grains are coarse and angular, from euhedral to anhedral, increasing in grain size

with depth. Garnet quartzite occurring again at 139m is accompanied with minor

disseminated sulphides in blue lode quartz veins, which cease once a depth of 140m is

reached. At 140.1m, the garnet quartzite grades abruptly to psammopelite and by 141m,

into a thick unit of spotted pelite.

Table 3.2.3a: Abbreviations for mineral names in lithological logs (this chapter) and photomicrographs

(Chapter Five).

Gangue Mineral Abbreviation Sulphide Mineral Abbreviation

Quartz Qtz Sphalerite Sph

Garnet Gar Galena Ga

Gahnite Gah Pyrrhotite Po

Biotite Bio Chalcopyrite Cpy

Chlorite Chl Arsenopyrite Apy

Muscovite Mu Tetrahedrite Tet

Carbonate Carb Gudmundite Gud

Sillimanite Sill Pyrite Py

20


Figure 3.8: Lithological log of B Lode from Hole ZLDD5001: 127-140m.

21


Figure 3.9: Lithological log of A Lode from Hole ZLDD5001: 170-175m and of 1 Lens from

ZLDD5000: 181-185m.

22


3.2.4 A Lode Geology

Down hole from the B Lode intersection, the core comprised a spotted pelite sequence,

interlayered with minor pegmatite layers including the distinctively green plumbian

orthoclase-rich “separation pegmatite” at 159-161m, followed by a transgression to

psammopelitic metasediments with minor layers of lode horizon rocks. Just prior to the

lode horizon, a number of pegmatites, some ptygmatically folded and containing garnet

porphyryblasts occur within a groundmass of psammopelitic metasediments. A Lode was

intersected in ZLDD5001 at the depth of 170-175m (Figure 3.9). Initially A Lode is

identified by the pink garnet quartzite lode horizon, with minor gahnite-rich white quartz

veins. A minor spotted pelite divides this unit from a further gahnite-rich quartz vein. This

is followed by further garnet quartzite, which contains a significant amount of blue lode

quartz and is intensely brecciated. The main A Lode mineralisation follows, consisting

primarily of disseminated sulphides within a blue lode quartz-garnet-gahnite rock. Initially

the sulphides are pyrrhotite>chalcopyrite, which further downhole changes to

sphalerite>chalcopyrite>pyrrhotite and becomes massive in form with 1cm crystals of

sphalerite and pyrrhotite within blue lode quartz. The massive sulphides then become

disseminated with pyrrhotite>chalcopyrite>sphalerite>galena, which was the commonly

observed mineralogy of vein style mineralisation. A decrease in sulphide content

corresponds with an increase in the garnet and gahnite abundance. A Lode grades into

intercalated psammitic and psammopelitic metasediments.

3.2.5 1 Lens Geology

In ZLDD5001, 1 Lens was not intersected. However, in drill hole ZLDD5000, 1 Lens was

intersected at a depth of 183-186m (Figure 3.9). Prior to the intersection of 1 Lens, the

sequence consisted of predominantly psammopelitic and psammitic metasediments. The

sequence was largely free of pegmatite leading into ore horizon, which began after a bed of

foliated spotted pelite interbedded with psammopelite. The lode horizon consisted of fine-

grained pink garnet quartzite with minor quartz-gahnite veins which downhole become

mineralised with chalcopyrite-pyrrhotite>galena-sphalerite. The main mineralisation in this

1 Lens intersection is contained within mineralised white quartz veins that contain

chalcopyrite>pyrrhotite-galena and garnet quartzite which contains galena-

pyrrhotite>chalcopyrite-sphalerite. Galena crystals are large and isolated. Downhole the

mineralisation is pyrrhotite>sphalerite-galena>chalcopyrite in association with gahnite. A

loss of sulphides occurs into a foliated gahnite-garnet quartz feldspar rock which is

23


significantly retrogressed either side of a sericitised pegmatite vein at 188.1m, with high

biotite content, followed by a large 4m bed of garnet quartzite lode horizon which grades

into psammopelitic and psammitic sequences.

3.3 Kintore Lode 3.3.1 Kintore pit introduction

Three traverses were mapped across the Kintore Lode, exposed in a bench wall at the base

of the Kintore pit (Figure 3.10). This ore lode had been classified as A Lode by the

previous operators of the then South Mine leases (van der Heyden and Edgecombe, 1990)

however this classification is far from conclusive. The traverses, measuring 9.3m, 6.4m

and 2.9m were then logged using the same criteria as used for the drill core and a total of

13 bulk samples were collected along the traverses with the use of a sledge hammer and

steel wedge to complement the 6 samples already obtained from CBH. The traverses were

located at various intervals along the strike of the exposed ore lode (Figure 3.11).

Figure 3.10: Cross section of the Kintore pit at 335 north, showing extent of the pit when mining

stopped in 1991. Note that this section is at the south end of the pit and the traverses are located at

the north end of the pit (from Minerals Mining and Metallurgy Ltd.).

24


A

N

B

N

Figure 3.11: A) Photograph of the north end of the Kintore pit looking north-west, with the

approximate position of the ore lodes highlighted. B) Photograph showing the position of the

Kintore Lode, the east verging F3 isoclinal fold and location of traverse one and two. Traverse

three was taken on another outcrop of lode 20m to the south-west along strike from this outcrop.

3.3.2 Kintore Lode Geology

The Kintore Lode has a number of distinct lithological layers, and from these the

dip and strike of the lode was measured. The ore lode strikes 30oNE and dips at 54o

towards 267oW into the bench wall. In the north facing section of the bench wall, an

isoclinal fold of the ore lode is visible (Figure 3.11) and the axial plane of this fold is also

approximately 30oNE with a plunge to the south of approximately 15o. The inferred

vergence of the fold is towards the east due to its ‘z’ geometry, and this correlates with the

position of the larger scale F3 structure known as the Western Antiform (Gustafson et al.,

1950). This suggests that the ore lode has undergone parasitic folding associated with the

F3 folding event.

25


Figure 3.12: Complete log of traverse one across the Kintore Lode.

26


Figure 3.13: Complete log of traverse two and three across the Kintore Lode.

27


The ore lode (Figure 3.12 and 3.13) is bound by metasediments. On the bench wall,

below the ore lode, the metasediments comprise strongly foliated psammopelite, with

minor intercalated sillimanite or garnet-rich layers. The psammopelite is dark in

appearance due to oxidation on the exposed surfaces. Psammopelite grades into more

psammitic metasediments with only minor interbedded psammopelitic, corresponding to a

loss of sillimanite and biotite, with an increase in quartz and feldspar. A number of

pegmatites up to 30 centimetres in thickness are present interbedded within the psammitic

metasediments and have a high modal percentage of green plumbian orthoclase. The dip

and strike of the metasediment beds is variable (dipping up to 80o to W with a strike of

20oN) as a result of deformation and the influence of more competent layers such as the

pegmatite. Proceeding along the traverse up the bench wall, the interbedded nature of the

psammitic and psammopelitic metasediments changes for a purely psammitic composition,

rich in garnet and minor associated gahnite, before changing into the mineralised ore zone.

The mineralisation is initially disseminated within blue lode quartz veins before becoming

massive within quartz-garnet rock. The massive sulphide layers have a slightly varied

orientation, striking at 38oNE with a dip of 48o to W. Further plumbian orthoclase-rich

pegmatites are present within the ore lode, however they are very narrow, with a maximum

thickness of 10cm. The massive sulphides in hand specimen comprise coarse galena,

sphalerite, chalcopyrite and pyrrhotite within a quartz-garnet matrix. Oxidation of the

sulphides has occurred on the exposed surfaces and these are brown in appearance

resulting from sphalerite and pyrrhotite oxididation. A return to psammitic layers occurs

grading into psammopelitic metasediments that contain abundant lenses and veins of blue

lode quartz, garnet quartzite, gahnite-rich layers and minor plumbian orthoclase-bearing

pegmatites. Further along the traverses, the complete loss of garnet, gahnite and blue lode

quartz occurs with the return to psammitic-psammopelitic metasediments and this is

accompanied by an increase in the sillimanite and biotite content.

The logs have been superimposed onto a photograph of the Kintore Lode and

correlation between traverse one and two carried out, showing the approximate lithological

position of the Kintore Lode (Figure 3.14). From the logs of the Kintore Lode traverses,

the abundance for each main lithological type was quantified (Figure 3.15). The lithologies

comprise psammopelitic metasediment (27%), psammitic metasediment (20%), massive

sulphides (29%), lode horizon garnet-quartzite and gahnite rocks (17%), pegmatite (6%)

and pelitic metasediment (1%).

28


Figure 3.14: A) Photograph of the Kintore Lode, with lithological logs of traverse one and two

transposed on. B) Interpreted geology of the bench wall containing the Kintore Lode, based on

correlation of the lithological logs of traverse one and two.

A

29


B

29


Sedimentology of Kintore Lode

20

27

16

17

29

05

101520253035404550

Psammite

Psammop

elite

Pelite

Pegmati

te

Lode

Hori

zon R

ocks

Massiv

e Sulp

hides

Lithology

Abu

ndan

ce %

Kintore T1

Kintore T2

Kintore T4

Open Pit Average(value shown)

Figure 3.15:

Abundance of the

broadly defined

lithological types

observed in the

Kintore Lode

traverses.

3.4 Discussion It is well documented that the Broken Hill Block has undergone multiple metamorphic and

deformation events. Evidence of this was in the metasediments preceding the zinc lodes in

drill core. The occurrence of spotted pelite (Figure 3.3), resulting from sillimanite crystals

pseudomorphing andalusite, is characteristic of high-grade metamorphism from

amphibolite to granulite facies (Phillips, 1980). The occurrence of pegmatites within the

metasediments indicates that partial melting has occurred during the geological history of

the metasediments. Pegmatites followed S0/S1, S2 or S3 foliations in drill core (Figure 3.5),

suggesting that multiple partial melting events occurred, which is consistent with the

metamorphic history of the Broken Hill Block. Graded bedding in drill core with

sillimanite defining the upper section of bedding, is a key tool in identifying younging and

fold attitudes. Using graded bedding (Figure 3.4) as a guide, a number of folds were

observed within the drill cores, and these were consistent with the structural history of the

Broken Hill Block. The dominant sillimanite foliation observed in the Kintore Lode may

have resulted from the F3 deformation event. The presence of intensely brecciated pelite

(Figure 3.2) with garnet dominant in the groundmass is suggestive of high-pressure fluid

flow, possibly during metamorphism resulting in the garnet growth.

Lode horizon rocks, namely the garnet quartzite horizon (Figure 3.6), and also

garnet-gahnite-quartz rocks are predominantly on the structural hanging wall, rather than

the structural footwall of the ore horizon. This may indicate that these lithologies form an

alteration halo that is D2 or later in age, resulting after overturning of the ore lodes,

30


followed by the percolation of residual metamorphic fluids contained within the ore lodes

dominantly upwards into the surrounding metasediment rocks. Alternatively, if such rocks

formed pre M1/D1, they may represent a pre-metamorphic alteration assemblage (Plimer,

1979). All mineralisation is contained within lode horizons of garnet quartzite, or garnet-

gahnite rocks, suggesting that lode horizons form an alteration halo related to the

emplacement or metamorphism of the sulphides. Two distinct styles of mineralisation were

observed in drill core, massive sulphide and vein style mineralisation. Massive sulphides

commonly have a composition sphalerite-galena>pyrrhotite>chalcopyrite compared to vein

style mineralisation which is typically pyrrhotite-chalcopyrite>galena-sphalerite. Vein

style mineralisation intersections of 1 Lens and to a lesser extent, the A Lode, consists of

low-grade stringer veins with the more mobile sulphides present such as galena and

pyrrhotite, whereas massive sulphides are higher-grade intersections, with an increased

abundance of the lesser mobile sulphides such as sphalerite. The texture of the massive

sulphide ore is coarse grained and commonly brecciated whereas vein style mineralisation

is more flow textured. Both suggest recrystallisation after initial prograde mineral growth.

31

Chapter Four – Sulphide Rock Geochemistry

Chapter Four - Sulphide Rock Geochemistry

4.1 Introduction During the course of fieldwork, a total of 43 rock samples from sulphide rocks were

collected comprising 24 samples as quarter NQ core samples, and eighth HQ core samples

and 19 bulk samples from the Kintore pit. These samples were analysed for major, trace,

rare earth and halide elements as well as carbon and oxygen isotopes, in order to document

and contrast the broad geochemistry of the individual ore lodes, and to identify trends

which may shed some light on their genetic evolution. The results from this study are

contrasted with various other results in the literature from studies of Broken Hill or other

comparable systems around the world. Where contrasted with data from this study, all 2

Lens and C Lode data has been sourced from Sproal (2001) and all Western Mineralisation

data has been sourced from Kitchen (2001).

4.2 Base Metal Geochemistry It has been well documented in the literature (Gustafson et al., 1950; Johnson and

Klingner, 1975) and observed in various mines at Broken Hill that the individual ore lodes

unique individual metal ratios. This holds true on CML7 (Table 4.2.1a and Appendix 3).

As documented in Chapter Three, there are two distinct styles of mineralisation, vein style

and massive style, each having unique mineralogy and this affects the overall geochemistry

of the individual ore lodes.

CML7 Broken Hill Historic Mining Ore Lode Pb wt% Zn wt% Ag g/t Pb wt% Zn wt% Ag g/t

Kintore Lode 16.4 16.0 169.6 - - - B Lode 5.8 12.1 82.6 4.4 12.5 35.0 A Lode 1.7 0.8 21.2 3.3 7.8 30.0 1 Lens 1.9 0.8 17.2 7.7 16.8 50.0

Western Min. 3.3 4.8 4.0 - - - C Lode 2.1 8.8 20.0 2.0 4.0 20.0 2 Lens 9.4 5.7 60.9 14.0 11.0 105.0 3 Lens 21.2 18.3 576.7 12.6 13.6 234.0

Table 4.2.1a: Average base metal abundance on CML7 and for Broken Hill historically (Broken

Hill historic mining figures from Laing, 1996).

32


Figure 4.1: Ternary plot of Pb-Zn-Ag/10 for

the 43 samples in this study from each of the

four sampled zinc lodes.

Figure 4.2: Ternary plot of the average Pb-Zn-

Ag/10 for all lodes on CML7. (3 Lens data

from Groombridge, 2002/03).

Figure 4.3: Ternary plot of average Pb-Zn-

Ag/10 mined at Broken Hill (after Laing,

1996).

Broad classification of the B Lode, A lode and 1 Lens has been as ‘zinc lodes’

based on the high Zn:Pb ratios in these lodes (Matthias, 1974). This is shown by most B

33


Lode samples with dominant Zn (Figure 4.1) or the Kintore Lode with approximately

equivalent Zn:Pb. Samples from A Lode and 1 Lens show enrichment in Pb and Ag rather

than Zn and is highlighted in the comparison between this studies ore lode average metal

ratios and those of the C Lode, 2 Lens and Western Mineralisation on CML7 (Figure 4.2)

and also with those from Laing (1996)(Figure 4.3) which were based on the historical

mining grades at Broken Hill. The Kintore Lode shows a wide distribution with no

dominant metal cation, however it also plots in a similar trend to A Lode and 1 Lens. Each

ore lode shows that the modal abundance of Pb, Zn, Ag and Cu decreases as the modal

abundance of silica increases (Figure 4.4) as the mineralisation changes from massive

sulphides to blue lode quartz-hosted stringer mineralisation.

SiO2 vs Pb

0

5

10

15

20

25

30

35

0 20 40 60 80 100

SiO2 vs Ag

0

50

100

150

200

250

300

350

0 20 40 60 80 100

SiO2 (wt%)

Ag

(g/t)

Kintore Lode B Lode A Lode 1 Lens

SiO2 (wt%)

Pb (w

t%)

Kintore Lode B Lode A Lode 1 Lens SiO2 vs Zn

0

5

10

15

20

25

30

35

0 20 40 60 80 100

SiO2 vs Cu

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

SiO2 (wt%)

Cu

(wt%

)


SiO2 (wt%)

Zn (w

t%)


Figure 4.4: Harker digrams for dominant metal cations showing negative trends with increasing silica.

4.3 Major Element Geochemistry Major elements (Appendix 4) were analysed by ICP-OES and ICP-MS techniques for the

43 rock samples. Elements analysed include Al, Ca, K, Fe, Mg, Mn, Na, P, Si and Ti

(Table 4.3.1a). Major elements are useful for identifying broad geochemical trends and

inter-element relationships in the rock samples and since all the samples were collected

34


from within ore lodes, major elements can be used in comparison with other previous

studies from metasediments in order to see if these trends are observed in both sets of data

therefore possibly constraining the orebody to a coeval evolution with surrounding

stratigraphic units.

Table 4.3.1a: Major element average abundances in order of modal abundance.

Ore Lode Major

Element Kintore Lode

(n=19) B Lode (n=9)

A Lode (n=9)

1 Lens (n=6)

SiO2 32.17 51.31 65.93 68.23 Fe2O3 10.14 10.10 14.86 14.55 Al2O3 5.06 5.36 8.41 8.65 MnO 3.70 0.54 1.87 1.20 CaO 2.34 2.16 1.11 0.30 P2O5 0.91 1.34 0.08 0.05 MgO 0.29 0.57 1.15 1.19 K2O 0.19 1.21 0.86 0.60 TiO2 0.14 0.20 0.36 0.36 Na2O 0.01 0.11 0.06 0.03

SiO2 vs MnO

0

2

4

6

8

10

0 20 40 60 80 100SiO2 (wt%)

MnO

(wt%

)

Kintore Lode B Lode A Lode 1 Lens 2 Lens

Si02 vs Al2O3

0

5

10

15

0 20 40 60 80 1Si02 (wt%)

Al2 O

3 (w

t%)

00


Figure 4.5: Harker diagram showing major element variation of all samples for MnO.

Figure 4.6: Harker diagram showing major element variation of all samples for Al2O3.

SiO2 vs CaO

0

5

10

15

20

25

0 20 40 60 80 100SiO2 (wt%)

CaO

(wt%

)


SiO2 vs Fe2O3

0

10

20

30

40

50

0 20 40 60 80 100SiO2 (wt%)

Fe2 O

3 (w

t%)

Kintore Lode B Lode A Lode 1 Lens 2 Lens Figure 4.7: Harker diagram showing major element variation of all samples for CaO.

35

Figure 4.8: Harker diagram showing major element variation of all samples for Fe2O3.


SiO2 vs K2O

0

1

2

3

4

5

0 20 40 60 80 100SiO2 (wt%)

K2O

(wt%

)


SiO2 vs Na2O

00.05

0.10.15

0.20.25

0.30.35

0 20 40 60 80 100SiO2 (wt%)

Na2

O (w

t%)

Kintore Lode B Lode A Lode 1 Lens 2 Lens Figure 4.9: Harker diagram showing major element variation of all samples for K2O.

Figure 4.10: Harker diagram showing major element variation of all samples for Na2O.

In modal order of abundance, the ore lodes (Table 4.3.1) are dominantly comprised

of Si, with enrichment in Fe, Al, Mn and Ca. Small amounts of P, Mg, Ti and Na complete

the major element composition of the samples. When plotted as Harker diagrams, there

were three distinct trends. There were vaguely parabolic trends such as MnO (Figure 4.5)

and Al2O3 (Figure 4.6) where the modal abundance of Mn and Al increased with

increasing Si up to an inversion point of approximately 50% silica, where the abundance of

the Mn and Al would decrease with further increasing Si. This trend was present for Al2O3,

MgO, MnO and to a lesser extent TiO2. These elements, with the exception of TiO2, are

major components of garnet, a primary constituent of the ore lode mineralogy as shown in

Chapter Three, with Al2O3 abundant in common garnet end-members and MgO and MnO

dominant in the end-members of pyrope (Mg3Al2Si3O12) and spessartine (Mn3Al2Si3O12)

respectively. Two other major element oxides, CaO (Figure 4.7) and Fe2O3 (Figure 4.8)

form two further end-members of garnet, almandine (Fe3Al2Si3O12) and grossular

(Ca3Al2Si3O12) respectively, and show negative trends in relation to increasing Si content.

The third trend is seen in K and Na (Figure 4.9 and 4.10), where there is a positive

relationship with increasing Si content, related to increasing alkali feldspar with increasing

Si. All other major elements show insignificant variation.

36


Major element abundance relative to metasediments

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

10.00

20.00

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5

Major elements

Rel

ativ

e A

bund

ance

(wt %

)

Kintore Lode Average B Lode Average A Lode Average 1 Lens Average Figure 4.11: Abundance of major elements of this study relative to those of Broken Hill

metasediments from Slack and Stevens (1994).

When compared to distal clastic metasediments from Broken Hill in Slack and

Stevens (1994), the averaged ore lode major element abundances in this study (Figure

4.11) show a pronounced depletion of Si, Al and K, with a strong enrichment of Fe and a

lesser enrichment of Mn and Ca.

Prominent trends within certain major element Harker plots have been inferred to

indicate the source of rock chemistry and therefore have been used to constrain genetic

models for Broken Hill (Slack and Stevens, 1994). They showed that major element

Harker diagrams of Broken Hill metasediments had linear negative trends and plotted

within a well-defined spatial field (Figure 4.12 and 4.13). They suggested this trend is

related to a large clastic component in these metasediments. Of the ore lodes observed in

this study, no Harker plots showed any linear features similar to that of Slack and Stevens

(1994) or correlation with the field of clastic metasediments for the elements Al and Fe

(Figure 4.12 and 4.13).

37


Figure 4.12: Comparison of Al2O3

Harker plot for this study (in

colour) showing no correlation

with those of Slack and Stevens

(1994) that are inferred to indicate

the field of clastic metasediments.

EPCM represents average Early

Proterozoic clastic metasediments

(from Taylor and McLennan,

1985).

Figure 4.13: Comparison of Fe2O3

Harker diagrams of this study (in

colour) showing no correlation

with those of Slack and Stevens

(1994) inferred to indicate the

field of clastic metasediments.

EPCM represents average Early

Proterozoic clastic metasediments

(from Taylor and McLennan,

1985).

4.4 Trace Element Geochemistry Trace elements (Appendix 5) were analysed by ICP-OES and ICP-MS techniques for the

43 rock samples. Elements analysed were Ba, Cr, V, As, Bi, Sb, Th, Y, Sr, W, Rb, Co, Cd,

U, Zr, Ta, Nb and REE (Table 4.4.1a). Trace elements typically comprise up to 1% of the

rock.

The ore lodes, especially the Kintore Lode, exhibit an enrichment in As. It has been

noted by a number of authors that As accumulates at the extremity of a given orebody, and

38


this is the case with the Kintore Lode samples. The Kintore Lode is also enriched with Cd,

Sb and Co while being depleted in Zr, Rb and U. The B Lode exhibits an anomalously high

enrichment of Ba, V, Rb and U and a depletion of Co and Nb. The A Lode and 1 Lens are

enriched in Ba, Zr and Th while 1 Lens was the only ore lode that contained Y. Ta is

absent from all lodes.

Table 4.4.1a: Trace element average abundances in order of modal abundance.

Ore Lode Trace

Element Kintore Lode

(n=19) B Lode (n=9)

A Lode (n=9)

1 Lens (n=6)

As 1744.74 377.22 264.44 393.33 Ba 160.00 360.00 210.00 152.50 V 45.38 600.00 36.25 35.00

Cd 400.38 30.00 26.00 15.00 Sb 201.95 124.00 23.11 84.17 Zr 35.38 100.00 126.25 170.00 Rb 14.31 110.00 49.56 48.88 Cr 68.46 50.00 55.00 40.00 U 4.38 105.00 3.50 3.75

Co 48.46 0.00 22.50 30.00 Sr 44.23 40.00 10.63 5.00 W 24.38 12.00 5.50 4.00 Th 5.74 7.33 17.39 15.17 Bi 5.58 21.67 8.00 2.33 Nb 2.69 0.00 3.75 2.50 Y 0.00 - - 6.50 Ta 0.00 0.00 0.00 0.00

When compared to distal clastic metasediments from Broken Hill from Slack and

Stevens (1994), the averaged ore lode trace element abundances in this study show a

pronounced depletion of Rb, Ba, Y and Zr, with a minor enrichment of U and Co. B lode

showed an anomalously high enrichment of V (Figure 4.14).

39


Trace element abundance relative to metasediments

-600.00

-400.00

-200.00

0.00

200.00

400.00

600.00

Rb Sr Ba Y Zr Nb Ta Th U V Co

Trace Elements

Rel

ativ

e A

bund

ance

(ppm

)

Kintore Lode Average B Lode Average A Lode Average 1 Lens Average Figure 4.14: Abundance of trace elements of this study relative to those of Broken Hill

metasediments from Slack and Stevens (1994) where comparable.

4.5 Rare Earth Element Geochemistry Although classified as trace elements and present in similar quantities, rare earth elements

(REE) of the lanthanide series are commonly treated separately due to their unique,

uniform chemical properties resulting from their electronic configurations (Henderson,

1984). REE initially have been used with great success in igneous petrology, and although

their chemical properties are alike, individual REE can be fractionated by various

petrological and mineralogical processes (Henderson, 1984).

Trends related to the abundance of REE in igneous rocks are able to give an

indication of the partial melting, fractional crystallisation and mixing pathways that have

occurred for the genesis of a given igneous rock (Haskin, 1984). Rare earth elements are

generally considered to be immobile except under extreme hydrothermal conditions or

retrogression and only when there is a tremendous amount of fluid flow and long residency

times (Lottermoser, 1992; Bierlein, 1995). REE trends can be applied to hydrothermal

systems where it has been shown that REE signatures reflect the primary hydrothermal

physico-chemical conditions (Lottermoser, 1992; Klinkhammer et al., 1994; Bierlein,

1995). Hydrothermal fluids from a number of modern systems including the Red Sea

(Courtois and Treuil, 1977 in Lottermoser, 1989) (Figure 4.15), the East Pacific Rise

(Michard et al., 1983; Michard and Albarede, 1986) (Figure 4.16), the Southern Explorer

Ridge (Barrett et al., 1990) and the TAG Mound on the Mid-Atlantic Ridge (Mills and

40


Elderfield, 1995) show similar REE signatures. Comparable signatures have been observed

in Paleoproterozoic Australian hydrothermal systems including the Broken Hill orebody

(Lottermoser, 1989; Slack and Stevens, 1994)(Figure 4.17), the Pinnacles deposit (Parr,

1992a) and Cannington (Bodon, 1996).

Figure 4.15: Hydrothermal fluid REE

signature from Red Sea and Romanche

Fracture Zone (Courtois and Treuil, 1977;

Bonatti et al., 1976 in Lottermoser, 1989).

Figure 4.16: Hydrothermal fluid REE

signature from East Pacific Rise and of

seawater (Michard and Albarede, 1986;

Goldberg et al., 1969 in Lottermoser, 1989).

Figure 4.17: REE signatures from

proximal exhalites and sulphide ore

at Broken Hill and proximal

exhalites from other Broken Hill-

type deposits (Lottermoser, 1989).

41


All of the 43 samples collected in fieldwork were analysed by ICP-MS for REE

(Appendix 6). The abundances were then normalised against the values for Average C1

Chondrites (Boynton, 1984) and against the North American Shale Composite

(NASC)(Piper, 1974). Normalisation of REE is necessary to limit the variations between

odd and even REE and provides a comparative benchmark for REE studies. C1 Chondrites

are thought to represent the primordial solar nebula composition (Boynton, 1984) whereas

the NASC represents the average concentration of REE in sediments based on 40 shales in

America which are suggested to be equivalent to the average abundance of REE in the

upper crust as a result of the homogenisation of original igneous REE abundances (Fleet,

1984). REE tends, when normalised against NASC, tend to show a flatter profile

comparative to C1 Chondrite normalised trends.

CML7 Ore Lode REE

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

Rare Earth Elements

Cho

ndrit

e N

orm

alis

ed A

bund

ance

Kintore Lode (n=19) B Lode (n=9) A Lode (n=9) 1 Lens (n=6)3 Lens (n=6) C Lode (n=8) 2 Lens (n=9) Western Min. (n=14)

Figure 4.18: Chondrite normalised average REE abundance for all ore lodes on CML7.

42


CML7 Ore Lode REE

0.01

0.1

1

10

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

Rare Earth Elements

NA

SC N

orm

alis

ed A

bund

ance

Kintore Lode (n=19) B Lode (n=9) A Lode (n=9) 1 Lens (n=6)3 Lens (n=6) C Lode (n=8) 2 Lens (n=9) Western Min. (n=14)

Figure 4.19: NASC normalised average REE abundance for all ore lodes on CML7.

The B Lode, A Lode and 1 Lens show LREE enrichment with distinctive Eu

anomalies for the chondrite-normalised samples (Figure 4.18) and a flatter profile with

significant Eu anomalies when NASC-normalised (Figure 4.19). The Eu/Eu* anomaly was

calculated from McLennan’s formula (McLennan, 1989) for all samples and ranged from

0.37 to 4.32 indicating both positive (27 samples) and negative (16 samples) anomalies.

The extent of the Eu anomaly, appears linked to the abundance of CaO compared to Eu

(Figure 4.20) as the gradient is steeper for ore lodes that exhibited the higher Eu anomalies.

Eu vs CaO

0

5

10

15

20

25

0 5 10 15

Eu (ppm)

CaO

(wt%

)

Kintore LodeB LodeA Lode1 LensTS&IP C LodeTS 2 LensKK Western MinLinear (A Lode)Linear (Kintore Lode)Linear (B Lode)Linear (TS 2 Lens)Linear (1 Lens)

Figure 4.20: Eu vs CaO for all CML7 ore lodes with positive Eu anomaly ore lodes with

steeply trending gradients and negative anomaly Eu anomalies with flat gradients.

43


The Ce/Ce* anomaly of Toyoda and Masuda (1991) ranged from 0.79 to 1.36

including 19 samples with no anomaly, 19 samples with negative anomalies and 5 samples

with positive anomalies.

4.6 Carbon and Oxygen Isotope Geochemistry The Broken Hill orebody has been deformed by late stage fault-constrained fluids that have

resulted in the formation of new carbonate and other minor minerals within the fault zones

and in fracture planes extending out from the faults. It has been documented that individual

lodes exhibit carbonate-bearing faults and microfractures (Lawrence, 1968; Plimer, 1984).

With stable isotope analytical techniques, carbonate veins can be useful in indicating the

origins of fluids interacting with the orebodies and for the relative dating of various

episodes in the deformation history. Of the previous carbon and oxygen isotope studies at

Broken Hill, some have focussed on the analysis of secondary minerals within the oxidised

zone of the orebody (Böttcher et al., 1993; Melchiorre et al., 1999, 2000) whereas others

have focussed on the analysis of carbon-bearing prograde ore lode minerals (Sproal, 2001)

or of broad isotope variation within the Broken Hill Block (Cartwright, 1999).

δ18O vs δ13C

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

00 5 10 15 20 25 30 35

δ18O (‰ vs. SMOW)

δ13C

(‰ v

s. P

DB)

2 Lens Fault zone Secondary malachite B Lode microfractures (this study)Rhodochrosite-smithsonite

Figure 4.21: δ18O and δ13C isotope analyses from B Lode microfractures with comparison

to other stable isotope analyses at Broken Hill including 2 Lens (Sproal, 2001), oxidised

zone rhodochrosite-smithsonite (Böttcher et al., 1993) and malachite (Melchiorre et al.,

1999) and fault zones (Gallacher, 1995).

44


The carbon and oxygen isotope analyses from B Lode microfractures in this study

(Figure 4.21) were completed at Monash University using a Finnigan-Matt 252 mass

spectrometer. Carbon isotopes results ranged from -13.7 to -14.1 (% vs. PDB) and oxygen

isotopes range from 27.7 to 31.4 (% vs. SMOW) indicating a biogenic source for the

carbon.

4.7 Halogen Geochemistry Halogens are strongly fractionated by various geological processes and as a result are

useful in determining the evolution of fluids and mixing processes within the crust (Bohlke

and Irwin, 1992). They are commonly the primary complexing mechanism for metal

transport (Large et al., 1996) and therefore are of importance to the genetic evolution of

orebodies.

The halogens Cl, F, Br and I are contained within secondary minerals within the

oxidised zone of the Broken Hill main lode orebody (Lawrence, 1968) which generally

extends down 120m from the surface but is present to 610m in highly fractured zones (van

Moort and Swensson, 1982). Common primary minerals are fluorite (CaF2), fluorapatite

(Ca5(PO4)3F), fluorapophyllite ((KCa4Si8)20(F,OH).8H20) and secondary minerals are

iodargyrite (AgI), chloroargyrite (AgCl) and bromargyrite (AgBr). The 43 samples were

analysed for Cl and F by ICP-OES (Appendix 6). Br and I are unable to be analysed by this

method.

F

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

WesternMin.

C Lode B Lode A Lode KintoreLode

1 Lens 2 Lens 3 Lens

Ore Lode

F (w

t%)

F

Cl

0

50

100

150

200

250

300

350

400

WesternMin.

C Lode B Lode A Lode KintoreLode

1 Lens 2 Lens 3 Lens

Ore Lode

Cl (

ppm

)

Cl

Figure 4.22: F and Cl average abundances for the individual ore lodes on CML7.

The average abundances of F and Cl for the individual ore lodes on CML7 (Figure

4.22) show an increasing abundance of both F and Cl in B Lode. This is followed by

relatively low abundances in A Lode, the Kintore Lode and 1 Lens.

45


4.8 Discussion The zinc lodes, (B Lode, A lode and 1 Lens) all show unique metal ratios. The A Lode and

1 Lens samples are somewhat atypical (Figure 4.2 cf. Figure 4.3) to previously

documented metal ratios from historical mining at Broken Hill (Laing, 1996). This results

from the drill intersections of low-grade vein style mineralisation which is rich in mobile

sulphides including galena (Marshall and Gilligan, 1987) and hence has a high Pb:Zn ratio.

This enrichment in Pb may be a consequence of the remobilisation of sulphides during

deformation as suggested by Maiden (1976) and Plimer (1984, 1987). The B Lode and

Kintore Lodes show more typical Zn:Pb ratios commensurate with previous documentation

of the zinc lodes at Broken Hill (Laing, 1996), yet both are rich in Pb and/or Zn and Ag

compared to historic mining data. The abundance of all metals decreases with increasing

SiO2 (Figure 4.4) with changes from massive sulphides to quartz hosted disseminated

mineralisation.

The major element geochemistry of the ore lodes comprises Si, Fe, Al, Mn, and Ca

(Table 4.3.1) reflecting the abundance of garnet and quartz, the two must abundant gangue

minerals within the ore lodes. Some trends were vaguely parabolic such as MnO (Figure

4.5) and Al2O3 (Figure 4.6) where the modal abundance of Mn and Al increased with

increasing Si, to a point of approximately 50% silica where the abundance of the Mn and

Al decreases with further increasing Si. This trend was observed for Al2O3, MgO, MnO

and to a lesser extent TiO2. This may indicate that pyrope and spessartine garnets are

present in only a low abundance in both low and high Si assemblages and present in much

higher abundances in moderate Si assemblages. TiO2 shares a similar trend as it is

commonly found substituting into garnet, in this case substituting preferentially into

pyrope and spessartine garnet. Two other major element oxides, CaO (Figure 4.7) and

Fe2O3 (Figure 4.8) which form two further end-members of garnet, almandine

(Fe3Al2Si3O12) and grossular (Ca3Al2Si3O12) show negative trends in relation to increasing

Si content. This indicates that these garnets are more abundant in low Si assemblages. Fe is

also contained within sulphide minerals pyrrhotite and chalcopyrite and the decrease of

these minerals as Si increases may be the source of the observed trend for Fe. When

compared to major element abundances of Broken Hill Block metasediments (Figure 4.11),

the ore lodes are enriched in Fe, Mn and Ca, but deficient in Si, Al and K. The enrichment

in Fe, Mn and Ca is likely to be the result of increased garnet growth within the ore lodes

46


while the depletion of Al and K is likely to be the result of decreased biotite and sillimanite

in the ore lodes relative to the pelitic to psammitic metasediments.

Slack and Stevens (1994) suggested that major elements in clastic metasediments

show linear negative trends and plot within a well-defined spatial field. On CML7, the C

Lode has been interpreted as containing a high abundance of clastic sediments by Sproal

(2001) on the basis of this association, while Kitchen (2001) showed that the Western

Mineralisation contained no linear features in the data and therefore contained no clastic

input. The major element geochemistry of the B Lode, A Lode and 1 Lens shows no

correlation with those of clastic metasediments from Slack and Stevens (1994) when

contrasted in Figures 4.12 and 4.13. This provides strong evidence that ore lodes are not

hosted in clastic metasedimentary lithologies as suggested by Wright et al. (1987) for their

syngenetic sedimentary model for the orebody genesis. This also provides evidence that

the orebody did not form by the preferential replacement of sedimentary and volcanic

lithologies by sulphide rocks, as is the case in suggested epigenetic replacement models

(Gustafson et al., 1950).

Trace elements are highly variable between and within the ore lodes (Table 4.4.1),

and commonly are the result of substitutions within the framework of existing minerals,

likely to have occurred during metamorphism. When compared to clastic metasediment

trace element abundances, the zinc lodes show a pronounced depletion of Rb, Ba, Y and Zr

and slight enrichment of Co. The depletion may be due to a lack of detrital zircons and

other accessory minerals such as rutile and monazite, which give anomalously high

abundances within the surrounding metasediments and metavolcanics (Slack and Stevens,

1994). This may provide further evidence that indicates the ore lodes and enclosing lode

horizon rocks are significantly different to the surrounding metamorphosed and altered

metasediments.

The ore lodes exhibit REE signatures similar to a number of comparable ancient

orebody systems around the world, as well as being similar to modern systems. They

exhibit enrichment in light REE and depletion in heavy REE. A negative Ce anomaly was

common in the majority of samples, indicating interaction with seawater and a leaching of

Ce during deposition (Toyada and Masuda, 1991). The ore lodes contain both negative and

positive Eu anomalies, which may be related to the oxidation state of deposition related to

changes in temperature and pH, causing instability of the Eu3+ variety under high

temperature or reducing conditions compared to the Eu2+ variety (Sverjensky, 1984; Taylor

and McLennan, 1985; Michard and Albarede, 1986; Lottermoser, 1989). Therefore ore

47


lodes with positive Eu anomalies (3 Lens, 2 Lens, B Lode, Kintore Lode) may have formed

via the exhalation of hot (>250oC), reducing hydrothermal fluids with no clastic input

whereas the ore lodes with negative Eu anomalies (Western Mineralisation, C Lode, A

Lode, 1 Lens) formed via the exhalation of colder (200-250oC), more oxidising

hydrothermal fluids, possibly with intercalated clastic input. It is also noted that in

accordance with Goldschmidt’s Rules, Eu can substitute for other similar size elements

such as Ca (Figure 4.20). It can be seen that ore lodes with highest amounts of Ca/Eu also

have positive Eu anomalies and this is proportional to the size of the anomaly. Therefore it

is suggested that the amount of Ca-Eu substitution has a significant bearing on the Eu

anomaly. REE in the B Lode, A Lode and 1 Lens are comparable to ancient Broken Hill-

type deposits Cannington (Australia)(Bodon, 1996), Bergslagen (Sweden)(Plimer, 1988)

and Gamsberg (South Africa). These ancient systems have modern analogues with

comparable REE signatures in exhaling hydrothermal fluid, in the form of the Red Sea

(Figure 4.15), East Pacific Rise (Figure 4.16), TAG Mound on the Mid-Atlantic Ridge and

the Southern Explorer Ridge.

The close proximity of the carbon and oxygen isotopes of this study (Figure 4.21)

to those of supergene secondary minerals from Bottcher et al. (1993) and Melchiorre et al.

(1999) suggest a common source, and as mentioned by Gallacher and Plimer (2001) could

be from descending fluids containing oxidised biogenic carbon. Gallacher and Plimer

(2001) suggest that these fluids are the result of ascending fault bound fluids with carbon

sourced from the underlying carbonaceous sequences of the Paragon Group. It is suggested

that the carbonate minerals contained within the B Lode microfractures share a chemical

affinity with secondary minerals in the oxidised zone. However since they occur below the

primary oxidation zone, and no evidence was observed of oxidation in drill core at the

depth of the B Lode, it is suggested that the secondary minerals are the result of ascending

biogenic carbon-bearing fault bound fluids.

The average abundances of F and Cl for the individual ore lodes on CML7 (Figure

4.22) suggest that there is an increasing abundance of both F and Cl in the early stages of

the orebody formation from the Western Mineralisation, C Lode and B Lode. This is

followed by relatively low abundances in A Lode, the Kintore Lode and 1 Lens. 2 Lens has

anomalously high levels of both F and Cl while 3 Lens had relatively low abundances. It is

possible that F and Cl levels are related to the initial start up phases of the hydrothermal

system seen by high levels in the Western Mineralisation, C Lode and 2 Lens. There does

not appear to be a correlation between high-grade ore lodes and the abundances of F and Cl

48


as the high grade Kintore Lode has low abundances. Studies have shown that variations in

F and Cl within apatite from magnetite-apatite ore bodies in Chile can be used to constrain

genetic evolution to hydrothermal fluids or magma mixing with P-rich aqueous fluids

(Treloar and Colley, 1996). Another study (Bohlke and Irwin, 1992) has documented the

implications of halogens in fluid inclusions and used these to contrast the sources of fluid

salinity within various hydrothermal systems. Further work needs to be carried out at

Broken Hill to see if the occurrence of halogens at Broken Hill is related to the ore forming

processes or if they are from a later unrelated source. If halides are conclusively linked to

ore forming processes in Broken Hill-type deposits, they may become an important

exploration target in regolith geochemistry due to their relatively mobility and tendency to

diffuse to the surface.

49

Chapter Five – Sulphide Rock Petrography

Chapter Five - Sulphide Rock Petrography 5.1 Introduction A total of 25 polished and one thin section were produced from the samples taken from

core and pit. These sections were observed under reflected and transmitted light using a

Nikon Labophot2-Pol polarising microscope and petrographic observations noted.

Photomicrographs were taken of most minerals and of any features of interest identified

such as metamorphic and deformation features and remobilisation textures within these

sections.

5.2 Polished Section Microscopy 5.2.1 Introduction

The mineralogy of the individual zinc lodes was generally consistent, however the modal

abundance of the mineralogy varied quite considerably (Tables 5.2a-d). On average,

gangue minerals comprised 50% of the B Lode and Kintore samples, while in the A Lode

and 1 Lens, sulphide minerals accounted for 25% and 30% of the total modal abundance of

the sections. The 25 polished sections were taken from the ore lodes including B Lode (6),

A Lode (10) and 1 Lens (4) core samples and from the Kintore Lode (5) pit samples. The

single unpolished section was taken from a sample of spotted pelite metasediment prior to

the ore lodes. In Tables 5.2a-d, an abundance denoted as <5% means that the mineral was

only rarely observed while prograde and/or retrograde refers to the inferred period of

growth for the given mineral.

Table 5.2a: Mineralogy of B Lode in average modal abundance.

Sulphide Minerals (50%) Gangue Minerals (50%)

Sphalerite 50% Prograde/retrograde Quartz 80% Prograde/retrograde

Galena 25% Prograde/retrograde Garnet 10% Prograde/retrograde

Pyrrhotite 20% Prograde/retrograde Biotite 5% Prograde/retrograde

Chalcopyrite 5% Prograde/retrograde Chlorite 5% Retrograde

Pyrite <5% Retrograde Muscovite <5% Retrograde

Gudmundite <5% Retrograde Carbonates <5% Retrograde

50


Table 5.2b: Mineralogy of A Lode in average modal abundance.



Pyrrhotite 40% Prograde/retrograde Garnet 15% Prograde/retrograde

Chalcopyrite 10% Prograde/retrograde Biotite 15% Prograde/retrograde

Galena <5% Prograde/retrograde Gahnite 10% Prograde

Arsenopyrite <5% Retrograde Chlorite <5% Retrograde

Table 5.2c: Mineralogy of 1 Lens in average modal abundance.


Galena 40% Prograde/retrograde Quartz 65% Prograde/retrograde

Sphalerite 35% Prograde/retrograde Garnet 15% Prograde/retrograde

Pyrrhotite 15% Prograde/retrograde Gahnite 15% Prograde

Chalcopyrite 10% Prograde/retrograde Biotite 5% Prograde/retrograde

Tetrahedrite <5% Retrograde Muscovite <5% Retrograde

Gudmundite <5% Retrograde

Arsenopyrite <5% Retrograde

Table 5.2d: Mineralogy of Kintore Lode in average modal abundance.



Galena 30% Prograde/retrograde Garnet 20% Prograde/retrograde

Pyrrhotite 10% Prograde/retrograde Biotite 5% Prograde/retrograde

Chalcopyrite 10% Prograde/retrograde Chlorite 5% Retrograde

Arsenopyrite 10% Retrograde Muscovite <5% Retrograde

5.2.2 Sulphide Minerals

Sphalerite - ZnS

Sphalerite is the most abundant sulphide mineral in the rocks studied, with modal

abundances ranging from 30% to 50%. This supports the high Zn:Pb ratio within the zinc

lodes. In reflected light, sphalerite is dark grey, while in transmitted light appears an

opaque dark grey colour or when mildly oxidised, a red-brown colour as in the Kintore

Lode. Sphalerite occur as subhedral to anhedral grains of various sizes from 100µm to

51


4mm, although the average size is 1mm. Sphalerite commonly contains exsolution

lamellae of pyrrhotite and chalcopyrite and inclusions of sulphides including galena,

pyrrhotite, chalcopyrite and arsenopyrite (Figure 5.8) and rarely prograde gangue minerals

such as quartz (Figure 5.12), garnet and biotite. It has no distinct cleavage but in some

sections is fractured (Figures 5.5 and 5.15), suggesting it is more competent and less easily

deformed than the other sulphides. It has replaced prograde biotite in some places (Figure

5.19) and, in others, is replaced by biotite (Figure 5.20). It is prominent within massive

sulphides but rare within vein style mineralisation suggesting higher ore grade conditions

are required in order to mobilise it compared to other sulphides.

Galena - PbS

Galena is the second most abundant sulphide mineral within the B Lode and 1 Lens,

comprising between 25-40% of the total sulphides for a given lode, however it was

significantly lower to almost non-existent in the A Lode samples. The cleavage is

commonly curvilinear (Figure 5.2). The cleavage orientation between two adjacent crystals

can be highly variable within massive sulphides indicating subgrain rotation, but is more

uniform in orientation when occurring in narrow vein style mineralisation indicating

crystallisation in a more homogenous stress regime. The crystals vary from anhedral to

subhedral, of a large average size between 1.8mm and 4.5mm and occur as seriate

aggregates with other sulphide minerals. In many places small elongate galena crystals fill

interstices between other grains (Figure 5.12) and fractures within both garnet and gahnite

porphyryblasts and quartz (Figure 5.11), indicating late crystallisation or remobilisation

after deformation which has resulted in the fracturing of the more competent gangue

minerals. Galena has undergone replacement by chlorite in B Lode and replacement to

both biotite and to chlorite in A Lode. In 1 Lens, galena hosts tetrahedrite and associated

breakdown minerals chalcopyrite and gudmundite (Figure 5.25).

Pyrrhotite – Fe(1-x)S

Pyrrhotite is the third most abundant sulphide mineral within the zinc lodes, however it is

significantly lower than sphalerite or galena, comprising between 10% and 20% except in

the A Lode samples where it reached as high as 40%. It is commonly oxidised producing

an iridescent light gold colour (Figure 5.18). It can occur as subhedral crystals but is more

common as large anhedral singular grains approximately 1.8mm in size. Pyrrhotite is

mobilised by crosscutting late stage carbonate veins (Figure 5.18). Pyrrhotite can form

52


veins in its own right that crosscut other sulphide minerals (Figure 5.15). Like galena,

pyrrhotite commonly fills interstices between other crystals and fractures within other

minerals (Figure 5.13) indicating it is a relatively ductile sulphide.

Chalcopyrite - CuFeS2

Chalcopyrite is found in all of the zinc lodes in a consistently low abundance of between

5% and 10%. Chalcopyrite occurs primarily in three different styles. It can occur as

preferentially elongate minute exsolution lamellae within quartz (Figure 5.7) and

inclusions in both gangue and sulphide minerals (Figure 5.9) or as small grains of 100-

500µm at the edge of other sulphide crystals. It can fill interstices between other euhedral

to subhedral crystals in a deformed, remobilised texture commonly in association with

galena (Figure 5.14). Chalcopyrite is rarely contained within very fine folded micro-

fractures within quartz (Figure 5.8) indicating it is almost as ductile as pyrrhotite and

galena. In some places chalcopyrite is related to the breakdown of tetrahedrite in

association with gudmundite (Figure 5.25).

Arsenopyrite - FeAsS

Arsenopyrite is a rare sulphide in the zinc lodes, with minor amounts in 1 Lens and up to

10% in the Kintore Lode. Arsenopyrite increases towards the periphery of the ore lodes at

Broken Hill. In reflected light arsenopyrite is a bright light grey-pink colour, somewhat

similar to galena except without the triangular pits or cleavage. It is commonly euhedral

and has a higher relief than the other sulphide minerals. It varies greatly in size, with most

grains being approximately 100µm with rare crystals up to 1mm. In the Kintore Lode,

arsenopyrite occurs commonly as tiny inclusions within sphalerite in a similar way as

chalcopyrite (Figure 5.9). It also occurs as euhedral grains overgrowing garnet, quartz and

galena (Figure 5.10) or occupying interstices between gangue minerals in a strong

association with chalcopyrite (Figure 5.13).

Pyrite - FeS2

Pyrite is a rare sulphide in the Broken Hill ore lodes. It was identified in late stage

microfractures within the B Lode in association with carbonate vein minerals. It is a dull

yellow colour similar to pyrrhotite but forms small strongly subhedral to euhedral crystals.

It may result from the interaction of pyrrhotite with late stage vein fluids which took up

53


iron in the formation of siderite (FeCO3) at the edges of the veins, leaving excess sulphur

and remaining iron within the veins, forming pyrite.

Tetrahedrite - Cu12Sb4S13

Tetrahedrite is a rare mineral and in terms of the zinc lodes, is found only within the 1

Lens at an abundance of up to 5%. It is a light grey to green colour in reflected light and is

in association with galena and chalcopyrite (Figure 5.25). It is hard to distinguish from the

other grey sulphides due to its small size. At Broken Hill it is argentian tetrahedrite,

freibergite ((Ag,Cu)10(Zn,Fe)2[S|((Sb,As)S3)4]). It forms anhedral to subhedral crystals at

the boundaries of galena and is associated with chalcopyrite and gudmundite. This has

been suggested (Lawrence, 1968) to be a consequence of the retrograde breakdown of

silver-rich tetrahedrite which results in the formation of chalcopyrite, gudmundite as well

as sphalerite, lollingite and silver-deficient tetrahedrite and a number of silver minerals

including pyrargyrite, polybasite, stephenite and argentite. None of these silver minerals

were observed.

Gudmundite - FeSbS

Gudmundite is rare, occurring primarily in 1 Lens, in association with tetrahedrite and

chalcopyrite and also in B Lode without associated tetrahedrite. Under reflected light it is

white in colour, and is observed in very small anhedral to subhedral crystals while the

larger crystals can be euhedral (Figure 5.25). It is one of the non-silver derivatives from the

retrograde breakdown of tetrahedrite, with chalcopyrite being another. The presence of

gudmundite in B Lode suggests that tetrahedrite may have been present initially.

5.2.3 Gangue Minerals

Quartz - Si02

Quartz is the most prevalent gangue mineral in all lodes, ranging from 50% to 85% modal

abundance of the gangue minerals. In transmitted light it commonly has a low to moderate

relief, however it can exhibit many fractures (Figure 5.4) or fluid inclusions and exsolution

lamellae (Figures 5.7 and 5.8). It may also occur completely within sulphide minerals or

garnet and gahnite minerals. It has a broad size range from 500µm to 2mm variable across

all ore lodes. It is euhedral to subhedral shape, but in some places is brecciated with

angular edges within a sulphide matrix (Figure 5.4), particularly within the Kintore Lode

and B Lode. It appears the most euhedral and equigranular when it occurs in association

54


with vein style mineralisation observed in some of the A Lode sections. Late stage

carbonate veins crosscut quartz (Figure 5.22).

Garnet - (Mn Fe, Ca)3Al2(SiO4)3

Garnet (10% and 30% abundance), occurs most abundant within or adjacent to massive

sulphides and within the vein style mineralisation. It ranges in size from 200µm in vein

style sections to a maximum of 900µm in the massive sulphide sections. It is isotropic, has

a high relief mottled appearance and is slightly pink in colour under transmitted light. It

occurs predominately poikioblastic with the individual crystals having a euhedral to

subhedral shape and an association with biotite, sulphides and gahnite (Figure 5.7). It can

contain fluid inclusions or inclusions of quartz suggesting growth over quartz or may be

fractured with fractures often filled by sulphides. It some places, it may occur as thin rims

around gahnite (Figure 5.24).

Gahnite - ZnAl2O4

The zinc spinel gahnite occurs sporadically within the A Lode and 1 Lens, ranging from

10% to 15%, but is absent from the B Lode and Kintore Lode sections. Although similar to

garnet in texture, isotropy, shape and size, it is characterised by its higher relief and green

colour under transmitted light. Similar to garnet, it occurs as poikioloblastic grains and is

commonly highly fractured or has quartz crystals in the centre of the gahnite. In many

places the edges of the gahnite grains have been altered to fine grained muscovite, whereas

on other grains, thin garnet rims are present (Figure 5.24).

Biotite - K(Fe, Mg)3AlSi3O10(F, OH)2

Biotite occurs in low abundance across all of the zinc lodes, from 5% to 15% abundance.

Biotite grains are commonly small, subhedral to anhedral and platey when structurally

bound by more competent minerals such as quartz and sulphides. It is a common mineral

within the pelitic metasediments at Broken Hill (Figure 5.1) and within the ore lodes is

commonly associated with garnet, gahnite and sulphide minerals (Figure 5.7). Biotite is

rarely present completely surrounded by sulphide minerals, occurring as unaltered euhedral

crystals, suggesting prograde mineral growth. In some places it is overgrown by sphalerite

(Figure 5.18). Other examples have been completely or partially retrograded to muscovite

(Figures 5.20 and 5.21).

55


Muscovite - KAl2(AlSi3O10)(OH)2

Muscovite is present in low abundances of <5% within the B Lode and 1 Lens. It is also

present in high abundances within pelite outside of the ore lodes (Figure 5.1). Within the

ore lodes it primarily exists as small platey crystals. It occurs around the edges of biotite

minerals or completely pseudomorphs them as a result of retrogression (Figures 5.20 and

5.21).

Chlorite - (Fe, Mg, Al)6(Si, Al)4O10(OH)8

Chlorite is another rare gangue mineral within the zinc lodes, primarily occurring as a

result of the retrogression of biotite within the A Lode and Kintore Lode. It resembles

biotite in shape and form but is a transparent or green to yellow in colour. It is likely that it

was present in the B and 1 Lens, but has been further retrogressed to muscovite.

Carbonates – (Ca, Fe, Mg)CO3

Late stage carbonate veins crosscut the B Lode and the minerals contain fine angular grains

which crosscut various other minerals. The carbonate mineral species were only

conclusively determined later by electron microprobe analysis as siderite (FeCO3), calcite

(CaCO3) and magnesite (MgCO3). Siderite is one of the most common minerals in

Delamerian remobilised shear zone ore at Broken Hill (Lawrence, 1968). The veins

commonly contain pyrite (Figures 5.5, 5.17 and 5.22) and included grains of quartz and

other sulphides including sphalerite entrained during vein propagation through the rock.

Under transmitted light they appear mottled dark green to black, fine grained with distinct

layers from the boundary of the vein to the centre.

5.3 Paragenesis 5.3.1 Introduction

The Broken Hill main lode orebody has undergone the same intense and sustained

metamorphic and deformation history as the whole Broken Hill Block described in Chapter

Two. The results of the complex geological history have already been seen on a regional

scale, a macroscopic scale in core and pit samples and are present on a microscopic scale.

The mineral assemblages have preserved some deformation features, metamorphic

features, remobilisation features and retrogression features described in Section 5.2.

56


5.3.2 Deformation and Metamorphism Features

Qtz

Ga Sill

400µm 550µm

Figure 5.1: Sillimanite pseudomorphs after

andalusite (ZLDD5001-30.2m).

Figure 5.2: Curvilinear cleavages within

galena (ZLDD5001-216.1m).

Sph

Carb Qtz

Sph

400µm 400µm

Figure 5.3: Folding of carbonate vein through

sphalerite (ZLDD5001-137.9m).

Figure 5.4: Highly fractured quartz grain

within dominantly sphalerite matrix (Kintore

Lode Traverse 2-0.7m).

Qtz Carb

GarSph

550µm Py

Figure 5.5: Pyrite filling fractures in sphalerite

and remobilised within carbonate vein

(ZLDD5001-137.9m).

Figure 5.6: Euhedral garnet in association with

biotite and gahnite, forming a prograde

schistosity (ZLDD5002A-132.3m).

Bio400µm Gah

57


5.3.3 Remobilisation Features

Cpy

Figure 5.9: Inclusions of chalcopyrite and

pyrrhotite within sphalerite (Kintore Lode

Traverse 2-0m).

Figure 5.7: Exsolved chalcopyrite within

quartz crystals and in interstices between

quartz crystals (ZLDD5001-136.9m).

Qtz Cpy

Sph

Cpy

Po

550µm

550µm

Ga Qtz

400µmQtz

Figure 5.8: Exsolved chalcopyrite and folded

microfractures filled by chalcopyrite within

quartz (ZLDD5001-136.9m).

Qtz Ga

Apy

Figure 5.10: Euhedral arsenopyrite

overgrowing quartz and galena (Kintore Lode

Traverse 2-1.8m).

400µm

SphQtz

Ga 550µm 550µm

Figure 5.11: Galena filling fractures in

euhedral quartz grain (ZLDD5001-173.5m).

Figure 5.12: Galena and sphalerite occupying

space between quartz crystals and quartz

crystal fragments (Kintore Lode Traverse 2-

0m).

58


Qtz Ga

Figure XX:

550µm

Figure 5.13: Pyrrhotite occupying space

between quartz crystals with euhedral

arsenopyrite (ZLDD5001-173.5m)

550µmCpy Qtz

Figure 5.14: Curved boundary of chalcopyrite

(ZLDD5000-172.3m).

Apy Po

QtzGa

Cpy Sph

Po Cpy Apy

Sph550µm 550µm

Figure 5.15: Pyrrhotite vein crosscutting

chalcopyrite and sphalerite crystals

(ZLDD5001-214.6m).

Figure 5.16: Arsenopyrite partially rimmed by

chalcopyrite, hosted by sphalerite

(ZLDD5002A-132.3m).

Py Qtz Carb

Po 550µm 550µm

Figure 5.17: Pyrite remobilised within late

stage carbonate vein (ZLDD5001-137.9m).

Figure 5.18: Late stage fluid vein crosscutting

pyrrhotite crystal which is mobilised along the

vein (ZLDD5001-133.4m).

59


5.3.4 Retrogression and Alteration Features

Bio Sph

SphMu

Bio Gar

400µm 400µm

Figure 5.20: Partial replacement of sphalerite

with biotite, with later partial replacement of

biotite by muscovite (ZLDD5001-216.1m).

Figure 5.19: Sphalerite overgrowing biotite

(ZLDD5002A-132.3m).

Mu Bio

Carb

Sph Qtz 400µm 400µm

Figure 5.21: Retrograde muscovite alteration

of biotite after replacement by sphalerite

under crossed-polars (ZLDD5001-136.1).

Figure 5.22: Late stage carbonate vein within

quartz with sequential layers from the edge of

the vein to the centre (ZLDD5001-134m).

60


Figure 5.23: Replacement of biotite by gahnite

(ZLDD5001-214.6m).

Figure 5.24: Garnet rim around gahnite

(ZLDD2A-132.3m).

QtzGah Gah

Bio Gar

400µm 400µm

Qtz

Tet

Gud

Cpy

Apy

Ga

Qtz

400µm

Figure 5.25: Breakdown of tetrahedrite to gudmundite and chalcopyrite with

associated euhedral arsenopyrite, hosted by galena (ZLDD5001-216.1m).

5.4 Discussion It has been shown experimentally that sphalerite is more competent and less easily

deformed than other sulphides (Marshall and Gilligan, 1987) and this is observed in the

zinc lodes. It is inferred that sphalerite crystal growth occurred in two episodes, initially

during peak prograde metamorphism and later during post-peak prograde metamorphism.

Sphalerite grown during the second stage of growth is observed replacing prograde biotite

(Figure 5.19) while both prograde and retrograde sphalerite is observed being replaced by

retrograde biotite (Figure 5.20). After prograde growth, galena has undergone

remobilisation during high-grade metamorphic events, probably during M2/D2 and M3/D3,

61


where secondary galena fills fractures within deformed gangue minerals (Figures 5.11 and

5.12). In places, galena has been altered by chlorite and biotite during retrograde events.

Pyrrhotite and chalcopyrite have been remobilised in places (Figures 5.13, 5.14 and 5.15)

and also occur as inclusions within galena and sphalerite (Figures 5.7-5.9 and 5.16).

Recent studies have attempted to show that multiphase sulphide inclusions within gangue

minerals and low interfacial angles between sulphide minerals suggest the melting of the

Broken Hill orebody (Mavrogenes et al., 2001). Experiments have been conducted that

show the melting temperature for the experimental sulphide assemblages is lower than the

temperature reached during peak prograde metamorpism at Broken Hill. Results of the

experiments show that for the experimental assemblages, polymetallic melts form between

500-600oC, which is well under the temperatures identified relating to M1/M2. The results

show that the melt will initially become enriched in the low-melting point chalcophile

elements Ag, As, Au, Bi, Hg, Sb, Se, Sn, Tl and Te, upon further increased temperatures

will become enriched in Cu and Pb, and in the highest temperature conditions, will become

enriched in Fe, Mn, Zn, Si, H20 and F. These experiments however, used mineral

assemblages significantly different to those at Broken Hill, and therefore are not directly

comparable. Furthermore, there are no ores within the Broken Hill orebody that are

enriched in the low-melting point chalcophile elements. The findings do show that partial

melting is possible during metamorphism of sulphide bearing assemblages including

Broken Hill, but it is yet to be proven on assemblages found within the Broken Hill

orebody. Other studies suggest that polymetallic inclusions result from the crystallisation

of sulphides prior to low surface energy solid-state equilibration (Stanton, 1972).

The lack of any Ag-rich minerals after the breakdown of argentian tetrahedrite to

chalcopyrite and gudmundite (Figure 5.25) (Lawrence, 1968) suggests that there has been a

net loss of Ag from the system, therefore showing the orebdy metamorphism is not

isochemical. It has been suggested from sampling and drilling that the shear zones are

enriched with Ag. Therefore, it is possible there is a net movement of Ag from the B Lode,

A Lode, 1 Lens and Kintore Lode into shear zones during retrograde metamorphism, as has

been suggested by Plimer (1980) for the entire Broken Hill orebody.

Quartz is present in both prograde and retrograde rocks, and along with garnet and

gahnite is associated with the ore lodes. Quartz crystals were widely observed in the core

of altered gahnite and garnet crystals, and this suggests that the growth of prograde quartz

occurred before the growth of prograde gahnite and garnet, which provides support to there

being multiple prograde peak metamorphic events. The presence of garnet rims around

62


some gahnite grains (Figure 5.24) suggests multiple garnet growth phases, possibly

correlating with one period of prograde growth and a second period of growth during a

second prograde event or during a later retrograde event. Parr (1992b) suggested a

retrograde reaction rim between gahnite and retrograde Fe, Mn, Mg, Ca, Na and Si-rich

fluids. This feature was also observed in the C Lode on CML7 (Sproal, 2001). Alteration

of garnet, gahnite and sulphide minerals by biotite, chlorite and muscovite (Figures 5.20,

5.21 and 5.23), is inferred to correlate with the multiple retrograde events that have

occurred after peak metamorphism. Carbonate veins crosscut both gangue and sulphide

minerals (Figures 5.5, 5.17, 5.18 and 5.22) suggesting late stage fluid flow through the

rocks, resulting in the fracturing of the competent minerals and minor remobilisation of the

least competent minerals such as pyrrhotite and galena and formation of pyrite from

residual sulphur in the veins with iron liberated from the retrograde breakdown of

pyrrhotite. The sequentially layered veins suggest sequential precipitation of carbonate

minerals (Plimer, 1984) or a series of quick, episodic fluid flows.

63

Chapter Six – Sulphide Rock Mineral Chemistry

Chapter Six - Sulphide Rock Mineral Chemistry 6.1 Introduction From the 25 polished sections, ten were selected for further study with Electron

Microprobe Analysis (EMPA), using a Cameca SX-50 electron microprobe. Sections from

each zinc lode that show a wide variety of minerals or those that contained minerals with

unusual paragenetic relationships were chosen for the analysis. The sections were digitised

and coated with carbon before undergoing the microprobe analysis. EMPA is able to

determine the chemistry of a given point within a mineral or determine compositional

zoning within a single mineral. Galena and garnet were analysed with five-point traverses

from the centre of the mineral to the edge in order to determine compositional variability.

All elements present in greater than one atomic percent (A%) have been included in the

calculated formula, while all elements present in abundances between 0.10 and 0.99 A%

have been listed as trace elements.

6.2 Sulphide Minerals A total of 330 points were analysed on sulphide minerals, comprising 165 galena, 44

sphalerite, 41 pyrrhotite, 48 chalcopyrite, 10 arsenopyrite, 7 tetrahedrite, 8 gudmundite and

7 pyrite. These were analysed for S, Mn, Fe, Co, Ni, Cu, Zn, As, Ag, Cd, Sb, Pb, Bi and

some for Au. A current of 2.012nA and acceleration potential of 19.91kV were used. The

results were scrutinised for any inconsistent results, which may have arisen due to analysis

of minute inclusions, angled grain boundaries, grain pits or adhesive. The average atomic

abundances of all elements was calculated for each mineral in each ore lode. From these,

the average stoichiometric formula for each mineral was calculated for each ore lode, in

accordance with Deer et al. (1992) using empirical formulae from Strunz and Nickel

(2001).

Galena

165 analyses of galena were carried out as five-point traverses, from the centre of 33

galena grains to the edge of the grain in sections from the B Lode (5), A Lode (10), 1 Lens

(8) and Kintore Lode (10). The calculated average chemical formula for galena did not

vary significantly between the four analysed lodes (Table 6.2a), being close to the

stoichiometric formula of galena, PbS. Each showed a slight overfilling of the Pb site for a

corresponding underfilling of the S anion site. The abundance of Ag ranged from 0.033

64


A% in B Lode and Kintore Lode, 0.038 A% in 1 Lens, to a maximum of 0.092 A% in A

Lode (Figure 6.1). In comparison to other studies on CML7, B Lode, Kintore Lode and 1

Lens showed relatively uniform Ag in galena, while A Lode and 3 Lens have anomalously

high Ag. Sb is present in similar abundances as Ag. Trace amounts of Zn were present in

trace abundances, higher in the B Lode and Kintore Lode and may have resulted from

analysis of minute Zn-bearing inclusions in galena or grain boundary diffraction of the

electron beam.

Table 6.2a: Average EMPA analysis for galena in B Lode, A Lode, 1 Lens and Kintore Lode.

Ore Lode Element

B Lode (n=31) A Lode (n=58) 1 Lens (n=46) Kintore Lode (n=50)A%(S) 48.707 48.488 48.490 48.386

A%(Mn) 0.023 0.032 0.028 0.022 A%(Fe) 0.032 0.039 0.038 0.014 A%(Co) 0.015 0.018 0.015 0.024 A%(Ni) 0.009 0.015 0.008 0.012 A%(Cu) 0.036 0.042 0.039 0.044 A%(Zn) 0.041 0.040 0.127 0.122 A%(As) 0.021 0.017 0.012 0.018 A%(Ag) 0.033 0.085 0.035 0.033 A%(Cd) 0.080 0.077 0.077 0.070 A%(Sb) 0.034 0.065 0.053 0.088 A%(Au) - 0.012 - 0.028 A%(Pb) 50.967 51.076 51.078 51.138 A%(Bi) 0.001 0.001 0.001 0.001

TOTAL A% 100.000 100.000 100.000 100.000 Calculated

Formula Pb1.023S0.975 Pb1.027S0.970 Pb1.027S0.97 Pb1.029S0.968

Average Ag in galena

0.000

0.020

0.040

0.060

0.080

0.100

KintoreLode

(n=50)

B Lode(n=25)

A Lode(n=50)

1 Lens(n=40)

WesternMin.

2 Lens 3 Lens

Ore Lode

Ag (A

%)

Figure 6.1: Average

abundance of Ag in

galena at Broken Hill

(Western Min. data

from Kitchen, 2001; 2

Lens and 3 Lens data

from Both, 1973).

65


Traverses across galena grains show that Pb (Figure 6.2) is relatively uniform

across grains from B Lode, 1 Lens and Kintore Lode. A Lode shows a slight increase in Pb

from the first traverse point to the fourth traverse point. A feature commonly observed in

the B Lode, A Lode and 1 Lens is the distinct decrease in Pb at the very edge of the galena

crystal. A feature commonly observed in all the ore lodes except the Kintore Lode is the

distinct decrease in lead at the very edge of the galena crystal. Ag was highly variable

(Figure 6.3) from one point to the next along individual traverses and in comparison to

other traverses.

Kintore Lode galena traverse Pb abundance

47

48

49

50

51

52

53

1 2 3 4 5

Traverse point

Pb

(A%

)

B Lode galena traverse Pb abundance

49.6

50

50.4

50.8

51.2

51.6

52

1 2 3 4 5

Traverse point

Pb

(A%

)

A Lode galena traverse Pb abundance

47

48

49

50

51

52

53

1 2 3 4 5

Traverse point

Pb (A

%)

1 Lens galena traverses Pb abundance

49.5

50

50.5

51

51.5

52

1 2 3 4 5

Traverse point

Pb (A

%)

Figure 6.2: Compositional variation of Pb across galena grains.

66


Kintore Lode galena traverse Ag abundance

0

0.04

0.08

0.12

0.16

0.2

1 2 3 4 5

Traverse point

Ag

(A%

)

B Lode galena traverse Ag abundance

0

0.05

0.1

0.15

0.2

0.25

1 2 3 4 5

Traverse point

Ag (A

%)

A Lode galena traverse Ag abundance

0

0.04

0.08

0.12

0.16

0.2

1 2 3 4 5

Traverse point

Ag (A

%)

Kintore Lode galena traverse Ag abundance

0

0.04

0.08

0.12

0.16

0.2

1 2 3 4

Traverse point

Ag

(A%

5)

Figure 6.3: Compositional variation of Ag across galena grains.

Sphalerite

44 analyses of sphalerite were carried out on sections from the B Lode (15), A Lode (11), 1

Lens (8) and Kintore Lode (10). Sphalerite has the stoichiometric formula of ZnS, however

the analyses show (Table 6.2b) that sphalerite has compositional differences between the

ore lodes. The calculated formula is variable with a significant amount of Fe substituting

for Zn in all ore lodes.

B Lode shows the highest deviance from the stoichiometric formula for sphalerite

as a result of Fe substitution (Figure 6.4), and produced a calculated formula of (Zn0.798,

Fe0.198)S0.999. The Kintore Lode (0.336 A%) contained the highest abundance of Mn

present as a trace element, with B Lode (0.179 A%) also having Mn present as a trace

element (Figure 6.5). A Lode (0.155 A%), 1 Lens (0.390 A%) and Kintore Lode (0.144

A%) show slight enrichment in Cd, which is present as a trace element (Figure 6.6).

Sphalerite contains low Ag, ranging from 0.009 A% (B Lode) to 0.028 A% (A Lode).

67


Table 6.2b: Average EMPA analysis for sphalerite in B Lode, A Lode, 1 Lens and Kintore Lode. Ore Lode

Element B Lode (n=15) A Lode (n=11) 1 Lens (n=8) Kintore Lode

(n=10) A%(S) 49.990 49.714 49.824 49.892

A%(Mn) 0.179 0.061 0.025 0.336 A%(Fe) 9.800 8.383 8.304 7.660 A%(Co) 0.009 0.027 0.034 0.005 A%(Ni) 0.008 0.005 0.010 0.009 A%(Cu) 0.014 0.013 0.081 0.011 A%(Zn) 39.862 41.555 41.231 41.880 A%(As) 0.024 0.025 0.035 0.036 A%(Ag) 0.009 0.028 0.014 0.007 A%(Cd) 0.086 0.155 0.390 0.114 A%(Sb) 0.004 0.003 0.002 0.002 A%(Au) 0.000 - - 0.000 A%(Pb) 0.003 0.003 0.003 0.002 A%(Bi) 0.013 0.028 0.047 0.045


Formula (Zn0.798, Fe0.198)S0.999 (Zn0.834, Fe0.168)S0.994 (Zn0.834, Fe0.166)S0.996 (Zn0.834, Fe0.153)S0.998

Zn vs Fe in sphalerite

0

2

4

6

8

10

12

38 39 40 41 42 43

Zn (A%)

Fe (A

%)

Kintore B Lode A Lode 1 Lens

Figure 6.4: Inter-element plot of Zn

and Fe in sphalerite, with B Lode

having the highest amount of

substitution of Fe for Zn, followed

by equivalent amounts in A Lode,

Kintore Lode and 1 Lens.

Zn vs Mn in sphalerite

00.05

0.10.15

0.20.25

0.30.35

0.40.45

38 39 40 41 42 43

Zn (A%)

Mn

(A%

)



and Mn in sphalerite, with the

Kintore Lode a showing a greater

enrichment in Mn than B Lode with

low abundances in A Lode and 1

Lens.

68


Zn vs Cd in sphalerite

00.10.20.30.40.50.60.70.80.9

38 39 40 41 42 43

Zn (A%)

Cd

(A%

)


Pyrrhotite

41 analyses of pyrrhotite were carried out on sections from B Lode (16), A Lode (15), 1

Lens (5) and Kintore Lode (5)(Table 6.2c). Pyrrhotite has the stoichiometric formula Fe1-

xS. Across all zinc lodes, pyrrhotite commonly was underfilled in the cation site and

overfilled in the anion site, typical for pyrrhotite (Struntz and Nickel, 2001). Zinc was

present in trace abundances within the B Lode and Kintore Lode. Ag is negliable in

pyrrhotite, ranging from 0.003 A% (1 Lens) to 0.008 (Kintore Lode).

Table 6.2c: Average EMPA analysis for pyrrhotite in B Lode, A Lode, 1 Lens and Kintore Lode.

Ore Lode Element

B Lode (n=16) A Lode (n=15) 1 Lens (n=5) Kintore Lode (n=5)

A%(S) 51.928 52.412 52.120 51.368 A%(Mn) 0.007 0.005 0.001 0.015 A%(Fe) 47.616 47.389 47.600 47.896 A%(Co) 0.005 0.025 0.045 0.008 A%(Ni) 0.009 0.020 0.018 0.004 A%(Cu) 0.011 0.008 0.002 0.020 A%(Zn) 0.288 0.011 0.079 0.550 A%(As) 0.087 0.085 0.096 0.100 A%(Ag) 0.007 0.006 0.003 0.008 A%(Cd) 0.004 0.009 0.026 0.005 A%(Sb) 0.004 0.007 0.003 0.001 A%(Au) - - - 0.000 A%(Pb) 0.003 0.005 0.003 0.000 A%(Bi) 0.022 0.018 0.005 0.024


Formula Fe0.952S1.039 Fe0.948S1.048 Fe0.952S1.042 Fe0.958S1.027


and Cd in sphalerite, with 1 Lens

having a significantly higher

abundance of Cd than the B Lode, A

Lode and Kintore Lode.

69


Pyrrhotite has two structural polymorphs, hexagonal pyrrhotite forms during high-

grade metamorphism, whereas monoclinic pyrrhotite forms by the retrogression of

hexagonal pyrrhotite. The atomic abundance of Fe in pyrrhotite varied from between

47.389 (A Lode) and 47.896 A% (Kintore Lode). This indicates that there is both

hexagonal and monoclinic forms of pyrrhotite present in the ore zone assemblages (Scott et

al., 1977).

Chalcopyrite

48 analyses of chalcopyrite were carried out on sections from the B Lode (15), A Lode

(13), 1 Lens (10) and Kintore Lode (10)(Table 6.2d). Chalcopyrite in the zinc lodes is close

to stoichiometric, with a slight overfilling of Fe in relation to Cu and S. The Kintore Lode

shows a trace abundance of Zn. Ag is present in an anomalously high abundance in A Lode

chalcopyrite (0.070 A%)(Figure 6.6) and in low amounts in the other ore lodes (Figure

6.7), ranging from 0.005 (Kintore Lode) to 0.020 A% (1 Lens), where it substitutes for Cu.

As is present in uniform abundances across the ore lodes, while Sb and Cd are relatively

low.

Table 6.2d: Average EMPA analysis for chalcopyrite in B Lode, A Lode, 1 Lens and Kintore Lode.

Ore Lode Element


A%(S) 49.630 49.554 49.611 49.582 A%(Mn) 0.006 0.024 0.007 0.016 A%(Fe) 25.625 25.410 25.491 25.405 A%(Co) 0.007 0.003 0.009 0.003 A%(Ni) 0.007 0.007 0.004 0.005 A%(Cu) 24.538 24.749 24.738 24.706 A%(Zn) 0.096 0.087 0.044 0.150 A%(As) 0.053 0.052 0.043 0.043 A%(Ag) 0.019 0.070 0.020 0.005 A%(Cd) 0.005 0.010 0.005 0.016 A%(Sb) 0.003 0.003 0.004 0.003 A%(Au) 0.012 - - 0.020 A%(Pb) 0.000 0.004 0.003 0.004 A%(Bi) 0.006 0.027 0.020 0.044


Formula Cu0.985Fe1.0327S1.988 Cu0.995Fe1.022S1.983 Cu0.991Fe1.022S1.986 Cu0.991Fe1.024S1.985

70


Average Ag in chalcopyrite

0.000

0.020

0.040

0.060

0.080

KintoreLode(n=10)

B Lode(n=10)

A Lode(n=18)

1 Lens(n=10)

WesternMin.

C Lode 2 Lens

Ore Lode

Ag

(A%

)

Arsenopyrite

10 analyses of arsenopyrite were carried out on sections from the A Lode (5) and Kintore

Lode (5)(Table 6.2e). In both lodes, arsenopyrite contains excess As, deficient S and

deficient Fe as a result of Co substitution. The Kintore Lode shows a widespread linear

trend of Co relative to Fe, whereas A Lode shows a closely grouped linear Fe:Co trend

(Figure 6.8).

Table 6.2e: Average EMPA analysis for arsenopyrite in A Lode and Kintore Lode.

Ore Lode Element

A Lode (n=5) Kintore lode (n=5)

A%(S) 31.391 29.190 A%(Mn) 0.046 0.017 A%(Fe) 31.889 29.305 A%(Co) 1.016 2.938 A%(Ni) 0.009 0.883 A%(Cu) 0.009 0.005 A%(Zn) 0.016 0.005 A%(As) 35.572 37.612 A%(Ag) 0.006 0.005 A%(Cd) 0.008 0.005 A%(Sb) 0.023 0.032 A%(Au) - 0.000 A%(Pb) 0.016 0.003 A%(Bi) 0.000 0.000

TOTAL A% 100.000 100.000 Calculated

Formula (Fe0.956, Co0.030)As1.068S0.942 (Fe0.879, Co0.088)As1.129S0.876

Figure 6.7: Average

abundance of Ag in

chalcopyrite on CML7

(Western Min. data

from Kitchen, 2001; C

Lode and 2 Lens data

from Sproal, 2001).

71


Fe vs Co in arsenopyrite

0.0000.5001.0001.5002.0002.5003.0003.5004.0004.500

27.000 28.000 29.000 30.000 31.000 32.000 33.000

Fe (A%)

Co

(A%

)

Kintore Lode A Lode

Figure 6.8: Inter-element plot for

Fe and Co in arsenopyrite from

the Kintore Lode and A Lode,

showing the linear trend related to

the substitution of Co for Fe.

Pyrite

A total of seven grains of pyrite were analysed from within microfractures in sections cut

from B Lode (Table 6.2f). The calculated formula was Fe1.036S1.960.

Table 6.2f: Average EMPA analysis for pyrite in B Lode.

Element B Lode (n=7)

A%(S) 65.314 A%(Mn) 0.006 A%(Fe) 34.488 A%(Co) 0.011 A%(Ni) 0.029 A%(Cu) 0.007 A%(Zn) 0.012 A%(As) 0.073 A%(Ag) 0.005 A%(Cd) 0.002 A%(Sb) 0.034 A%(Au) 0.002 A%(Pb) 0.000 A%(Bi) 0.014

TOTAL A% 100.000

Calculated Formula Fe1.036S1.960

Tetrahedrite

A total of seven analyses of tetrahedrite were carried out on sections from the 1 Lens

(Table 6.2g). The calculated formula for tetrahedrite was (Cu8.150Ag1.700Fe1.505Zn0.495)

Sb4.435S12.693.

72


Tetrahedrite is a prograde sulphide, which at Broken Hill is argentian tetrahedrite

(freibergite). Tetrahedrite is a complex mineral with variable substitutions. Tetrahedrite is

the primary Ag-bearing mineral at Broken Hill. The samples from 1 Lens reflect the high

Ag contained in tetrahedrite, with an average of 5.861 A% Ag contained within the seven

analysed samples of tetrahedrite, although is considerably lower than observed in the

Western Mineralisation by Kitchen (2001)(Figure 6.9). Fe and Zn, along with Ag are

present in significant abundances, substituting for Cu. Both the Sb and S sites are

overfilled.

Table 6.2g: Average EMPA analysis for tetrahedrite in 1 Lens.

Element 1 Lens (n=7)

A%(S) 43.768 A%(Mn) 0.008 A%(Fe) 5.190 A%(Co) 0.014 A%(Ni) 0.005 A%(Cu) 28.104 A%(Zn) 1.708 A%(As) 0.929 A%(Ag) 5.861 A%(Cd) 0.026 A%(Sb) 14.365 A%(Pb) 0.006 A%(Bi) 0.016

TOTAL A% 100.000

Calculated Formula (Cu8.150Ag1.700Fe1.505Zn0.495)Sb4.435S12.693

Average Ag in tetrahedrite

0.000

2.000

4.000

6.000

8.000

10.000

12.000

14.000

16.000

1 Lens (n=7) 2 Lens Western Min.

Ore Lode

Ag

(A%

)

Figure 6.9: Average

abundance of Ag in

tetrahedrite on

CML7 (Western Min.

data from Kitchen,

2001; 2 Lens data

from Sproal, 2001).

73


It has been suggested in the literature that tetrahedrite breaks down during

retrograde metamorphism to a silver-rich secondary mineral and a silver-poor secondary

mineral (Lawrence, 1968). The silver-poor minerals are commonly gudmundite,

chalcopyrite, arsenopyrite and lÖllingite. The silver-rich minerals were not observed in this

study consist of pyrargyrite, polybasite, stephenite and argentite.

Gudmundite

Eight grains of gudmundite were analysed, including grains from 1 Lens (5) in association

with tetrahedrite and chalcopyrite and B Lode (3) in association with galena and

chalcopyrite (Table 6.2h). Gudmundite has the empirical formula of FeSbS however, in the

Broken Hill orebody, it has a complex chemistry related to its complex formation history.

The average formula for gudmundite in the B Lode was (Fe0.900Co0.045)(Sb1.038As0.066)S0.932

whereas in the 1 Lens was (Fe0.949)(Sb1.033As0.069)S0.939. Along with the major substitutions

of Co and As, there were a number of trace elements recorded, including Ni, Cu, Zn, Bi

and Ag. The abundance of this wide range of trace elements was highly variable within

each ore lode suggesting no dominant substitution patterns.

Table 6.2h: Average EMPA analysis for gudmundite in B Lode and 1 Lens.

Ore Lode Element

B Lode (n=3) 1 Lens (n=5)

A%(S) 31.074 31.297 A%(Mn) 0.004 0.005 A%(Fe) 30.015 31.632 A%(Co) 1.487 0.015 A%(Ni) 0.193 0.049 A%(Cu) 0.233 0.144 A%(Zn) 0.059 0.062 A%(As) 2.210 2.313 A%(Ag) 0.029 0.015 A%(Cd) 0.001 0.001 A%(Sb) 34.612 34.438 A%(Pb) 0.001 0.011 A%(Bi) 0.082 0.018

TOTAL A% 100.000 100.000

Calculated Formula (Fe0.900Co0.045)(Sb1.038As0.066)S0.932 (Fe0.949)(Sb1.033As0.069)S0.939

74


6.3 Gangue Minerals A total of 281 points on gangue minerals were analysed including 225 garnets, 15 gahnite,

33 biotite, 15 muscovite, 4 chlorite and 4 of carbonate minerals. These were analysed for

Si, Ti, Al, Fe, Cr, V, Mn, Ca, Mg, Zn, Na and K. An emission current of 2.514nA and

acceleration potential of 14.94kV were used. The results were scrutinised for any

inconsistent results that may have arisen due to analysis of inclusions, angled grain

boundaries, grain pits or adhesive. The average atomic abundances of all elements were

calculated for each gangue mineral in each ore lode. From these, the average

stoichiometric formula of each gangue mineral was calculated for each ore lode, in

accordance with Deer et al. (1992) using empirical formulae from Strunz and Nickel

(2001).

Garnet

A total of 225 electron microprobe analyses of garnet were completed (Table 6.3a) and the

end members were calculated (Table 6.3b) and contrasted with garnets from ore lodes on

CML7 (Figure 6.10). This derived from the traversing of 45 individual garnet grains with

five-point traverses.

Table 6.3a: Average EMPA analysis for garnet in B Lode, A Lode, 1 Lens and Kintore Lode.

Ore Lode Element B Lode (n=50) A Lode (n=75) 1 Lens (n=50) Kintore Lode (n=50)

Si 3.007 2.955 2.964 2.988 Ti 0.002 0.002 0.001 0.004

Al/Al IV 0.004 0.045 0.036 0.014 Al VI 1.955 1.951 1.925 1.942 Cr 0.002 0.001 0.001 0.002

Fe3+ 0.020 0.081 0.103 0.051 Fe2+ 1.445 1.833 2.200 0.905

V 0.014 0.011 0.007 0.008 Mn2+ 1.021 0.620 0.354 1.322 Mg 0.213 0.263 0.363 0.155 Zn 0.003 0.003 0.004 0.004 Ni 0.000 0.000 0.000 0.000 K 0.000 0.000 0.001 0.000 Na 0.003 0.003 0.002 0.001 Ca 0.305 0.232 0.040 0.601 Nb 0.000 0.000 0.000 0.000

Sum 7.995 8.000 8.000 8.000

75


Table 6.3b: Garnet end member abundances determined from recalculation of EMPA garnet

compositions.

Garnet B Lode (n=50) A Lode (n=75) 1 Lens (n=50) Kintore Lode (n=50)

Pyrope Mg3Al2(SiO4)3

7.131 8.912 12.281 5.215

Almandine Fe3Al2(SiO4)3

48.409 62.195 74.403 30.315

Spessartine Mn3Al2(SiO4)3

34.225 21.024 11.971 44.323

Andradite Ca3Fe2

3+(SiO4)3

0.986 3.298 1.242 2.541

Uvarovite Ca3Cr2(SiO4)3

0.082 0.051 0.059 0.083

Grossular Ca3Al2(SiO4)3

9.168 4.530 0.059 17.523

TOTAL 100.000 100.010 100.015 100.000

Average total ore lode garnet compositions

01020304050607080

B Lode(n=50)

A Lode(n=75)

KintoreLode(n=50)

1 Lens(n=50)

WesternMin.

C Lode 2 Lens

Ore Lode

Abu

ndan

ce (A

%)

PyropeAlmandineSpessartineAndraditeUvaroviteGrossular

Figure 6.10: Average modal abundance of garnet endmembers in each CML7 ore lode, including B

Lode, A Lode, Kintore Lode and 1 Lens (this study), the Western Mineralisation (Kitchen, 2001)

and C Lode and 2 Lens (Sproal, 2001).

From the atomic abundance of the garnets, individual end members were calculated

on the basis of 12 O. These are shown in Table 6.3.2 and Figure 6.3.1. B Lode and the

Kintore Lode consist of dominant almandine, over spessartine and minor pyrope. A Lode,

1 Lens, Western Mineralisation and C Lode are dominantly almandine with minor

76


spessartine and pyrope. 2 Lens contains anomalously high grossular garnet, with

spessartine and almandine also co-dominant.

Each garnet was analysed with a traverse of five points from the centre of the grain

to the periphery and these were calculated to give the average abundance across a garnet

grain in each ore lode (Figure 6.11). The B Lode shows a strong increase in almandine

garnet on the rim for a corresponding decrease in both spessartine and grossular garnet.

The A Lode and 1 Lens show a slight increase in almandine for a slight decrease in

spessartine and grossular, while the Kintore Lode garnets show a decrease in spessartine,

andradite and pyrope for an increase in almandine and grossular garnet. This shows that for

B Lode, A Lode and 1 Lens, a loss of Mn and Ca and a gain of Fe occurred. The Kintore

Lode traverse data suggests a loss of Mg and Mn and a reworking of Ca and Fe in

andradite to grossular and almandine garnet.

B Lode average traverse point garnet composition

0

10

20

30

40

50

60

1 2 3 4 5

Traverse Point

Abu

ndan

ce (%

) PyropeAlmandineSpessartineAndraditeUvaroviteGrossular

1 Lens average traverse point garnet composition

0

10

20

30

40

50

60

70

80

1 2 3 4 5

Traverse Point

Abu

ndan

ce (%


A Lode average traverse point garnet composition

0

10

20

30

40

50

60

70

1 2 3 4 5

Traverse Point

Abu

ndan

ce (%


Kintore Lode average traverse point garnet composition

05

101520253035404550

1 2 3 4 5

Traverse Point

Abu

ndan

ce (%


Figure 6.11: Average garnet composition from core to rim for each ore lode.

Spinel

A total of 15 spinels were analysed, 10 from A Lode and 5 from 1 Lens (Table 6.3c). The

average stoichiometric formula determined from those analyses is (Zn0.6757 Fe0.2686 Mg0.0585

Mn0.0027V0.0092)(Al1.9836)O4 for A Lode and (Zn0.6944 Fe0.231 Mg0.062)(Al2.0046)O4 for 1 Lens.

77


The zinc spinel gahnite was the dominant variety (Table 6.3d), followed by lower

abundances of hercynite and Mg-spinel. Trace amounts of galaxite were present in both

lodes. A vanadium rich variety was present in trace amounts in the A Lode, possibly

magnesiocoulsonite (MgV2O4), coulsonite (FeV2O4) or vuorelainenite (MnV2O4) (Strunz

and Nickel, 2001). Although gahnite is the dominant spinel in all ore lodes, spinel from the

Western Mineralisation and C Lode on CML7 (Figure 6.12) contain slightly higher

gahnite, compared to the A Lode and 1 Lens, and the Pinnacles Mine, which contain

slightly more hercynite.

Table 6.3c: Average EMPA analysis for gahnite in A Lode and 1 Lens.

A Lode (n=10) 1 Lens (n=5) Ion

Average A% Standard Dev. Average A% Standard Dev. Si 0.000 0.001 0.000 0.000 Ti 0.000 0.001 0.000 0.000

Al/Al IV 0.000 0.000 0.000 0.000 Al VI 1.922 0.007 1.940 0.007 Cr 0.000 0.001 0.001 0.001

Fe3+ 0.128 0.010 0.119 0.007 Fe2+ 0.141 0.042 0.112 0.029

V 0.009 0.010 0.004 0.005 Mn2+ 0.003 0.001 0.003 0.001 Mg 0.059 0.011 0.062 0.003 Zn 0.676 0.040 0.694 0.024 Ni 0.000 0.000 0.000 0.000 K 0.000 0.000 0.000 0.000 Na 0.061 0.005 0.064 0.004 Ca 0.000 0.000 0.000 0.000 Nb 0.000 0.000 0.000 0.000

Sum Cat# 3.000 0.000 3.000 0.000

Table 6.3d: Gahnite end member abundances determined from recalculation of EMPA spinel

compositions.

A Lode (n=10) 1 Lens (n=5) Spinel Variety Mineral Formula Abundance A% Abundance A%

Gahnite (Zn) ZnAl2O4 66.59 70.14 Hercynite (Fe) FeAl2O4 26.47 23.33 Mg-spinel (Mg) MgAl2O4 5.77 6.26

V-spinel (Mg, Fe, Mn)V2O4 0.9 - Galaxite (Mn) MnAl2O4 0.27 0.26

78


Figure 6.12: Average spinel endmember

abundances from A Lode and 1 Lens (this

study), Western Mineralisation (Kitchen,

2001), C Lode (Sproal, 2001) on CML7 and

at the Pinnacles Mine (Parr, 1992b).

Biotite

Table 6.3e: Average EMPA analysis for biotite in B Lode, A Lode, 1 Lens and Kintore Lode.

Ore Lode Element


Si 5.645 5.444 5.489 5.600

Ti 0.180 0.193 0.147 0.203

Al/Al IV 2.355 2.556 2.511 2.400

Al VI 0.408 0.565 0.571 0.288

Fe2+ 1.903 2.785 2.500 2.309

Mn2+ 0.037 0.022 0.013 0.064

Mg 3.202 2.274 2.658 2.991

Zn 0.018 0.014 0.014 0.024

K 1.815 1.758 1.708 1.861

Na 0.043 0.058 0.084 0.014

Ca 0.000 0.002 0.002 0.002

Nb 0.000 0.000 0.000 0.000

OH 4.000 4.000 4.000 4.000

Sum 15.684 15.697 15.711 15.779

79


Table 2.3f: Calculated formula for biotite, determined from the recalculation of EMPA biotite analyses

data.

Ore Lode Formula

B Lode (n=5) (K1.815Na0.043)(Mg3.202Fe2+1.903)(Al0.408Ti0.180)[Si5.645Al2.355O20] (OH,F)4

A Lode (n=15) (K1.708Na0.014)(Mg2.991Fe2+2.785)(Al0.565Ti0.193)[Si5.444Al2.556O20] (OH,F)4

1 Lens (n=10) (K1.708Na0.084)(Mg2.658Fe2+2.500)(Al0.571Ti0.147)[Si5.489Al2.511O20] (OH,F)4

Kintore Lode (n=3) (K1.861Na0.014)(Mg2.991Fe2+

2.309)(Al0.288Ti0.203)[Si5.600Al2.400O20] (OH,F)4

33 analyses of biotite were completed from the B Lode (5), A Lode (15), 1 Lens (10) and

Kintore Lode (3) (Table 6.3e), and the chemical formula of biotite in each lode calculated

(Table 2.3f). Biotite can be classed as peraluminous varieties of the phlogopite-annite

series.

Figure 6.13: Plot of biotite

variety from various ore lodes

on CML7.

80


Muscovite

15 samples of muscovite were analysed from B Lode (5), A Lode (7) and 1 Lens (3)(Table

6.3g) and the chemical formula was calculated for the muscovite in each ore lode (Table

6.3h).

Table 6.3g: Average EMPA analysis for muscovite in B Lode, A Lode and 1 Lens.

Ore Lode

Element B Lode (n=5) A Lode (n=7) 1 Lens (n=3)

Si 6.219 6.164 6.663

Ti 0.022 0.019 0.005

Al/Al IV 0.066 0.122 0.000

Al VI 5.421 5.469 4.633

Cr 0.003 0.002 0.003

Fe2+ 0.128 0.160 0.439

Mn2+ 0.003 0.005 0.003

Mg 0.171 0.154 0.309

Ca 0.003 0.004 0.003

Na 0.156 0.133 0.081

K 1.768 1.700 1.805

Zn 0.013 0.007 0.013

Ni 0.000 0.000 0.000

OH 12.571 12.571 12.571

Sum 13.975 13.937 13.957

Table 6.3h: Calculated formula for muscovite, determined from the recalculation of muscovite EMPA

data.

Ore Lode Formula

B Lode (n=5) (K1.768Na0.156)(Al5.421Fe0.128Mg0.171Zn0.013)[Si6.164Al0.066O20](OH,F)4

A Lode (n=7) (K1.700Na0.156)(Al5.469Fe0.160Mg0.154Zn0.007)[Si6.164Al0.122O20](OH,F)4

1 Lens (n=3) (K1.805Na0.081)(Al4.633Fe0.439Mg0.309Zn0.013)[Si6.663Al0.000O20](OH,F)4

The B Lode and A Lode are relatively enriched in Al and Na, while being depleted

in Fe and Mg, whereas the 1 Lens is comparatively Al poor, but rich in Fe and Mg. Mica

81


from the zinc lodes plots in the aluminous phengite field, classified by Guidotti

(1984)(Figure 6.14) compared to C Lode and Western Mineralisation mica which plot in

the muscovite phase and 2 Lens which plots in between ferrimuscovite, muscovite,

phengite and ferriphengite.

Mu

Ph

Figure 6.14: Composition of

white mica from ore lodes on

CML7. Mu=muscovite,

Ph=phengite (divisions from

Guidotti 1984).

Chlorite

Chlorite was a rare mineral in the zinc lodes and, as a result, was only analysed in the A

Lode (1) and 1 lens (3)(Table 6.3i). The analyses were calculated for 36 ions based on 20

O and 16 (OH)(Table 6.3j). The analyses show that the composition of chlorite between

the A Lode and 1 Lens is markedly different compositionally, with A Lode chlorites being

Fe-rich and Mg-poor, whereas the 1 Lens is Mg-rich and Fe-poor. Chlorite in these ore

lodes plot within the ripidolite phase (Figure 6.15)(Hey, 1954).

82


Table 6.3i: Average EMPA analysis for chlorite in A Lode and 1 Lens.

Ore Lode Element A Lode 1 Lens 1 Lens 1 Lens

Si 5.346 5.185 5.029 5.191 Ti 0.002 0.016 0.013 0.009

Al/Al IV 2.654 2.815 2.971 2.809 Al VI 2.924 2.673 2.799 2.715 Cr 0.012 0 0.002 0.003

Fe2+ 6.516 4.64 4.657 4.59 Mn2+ 0.024 0.024 0.032 0.03 Mg 2.329 4.655 4.533 4.646 Ca 0 0 0.004 0.001 Na 0.007 0.003 0.004 0.009 K 0.007 0.001 0.003 0.002

ZnO 0.044 0.045 0.029 0.039 Ni - - - - OH 16 16 16 16

Sum 19.865 20.057 20.076 20.044

Table 6.3j: Calculated formula for chlorite, determined from the recalculation of chlorite EMPA data.

Ore Lode Formula

A Lode (n=1) (Fe6.516Al2.924Mg2.329)(Si5.346Al2.654)O20(OH)16

1 Lens (n=3) (Fe4.629Al2.729Mg4.6113)(Si5.135Al2.865)O20(OH)16

Chlorite varieties in the A Lode and 1 Lens

00.10.20.30.40.50.60.70.8

5.000 5.100 5.200 5.300 5.400 5.500 5.600

Si (A%)

Fe/F

e+M

g

A Lode (n=1)1 Lens (n=3)

Ripidolite

Figure 6.15: Chlorite varieties in the A Lode and 1 Lens.

83


Carbonates

The late stage propagation of veins through the Broken Hill ore lodes has been

documented in the literature by a number of authors (Lawrence, 1968; Plimer, 1984).

These veins have a wide variety of mineralogy and varied stable isotope values as observed

in Chapter 4, suggesting a number of generations of veining containing different fluid

chemistry. Four carbonate minerals were analysed by EMPA (Table 2.3k). Recalculation

of the EMPA data, shows that the primary constituent of the B Lode veins sampled is

siderite, with minor calcite and magnesite.

Table 6.3k: Average EMPA of carbonate minerals in B Lode.

Ore Lode Element B Lode B Lode B Lode B Lode

Si 0 0 0 0 Fe2+ 0.931 0.869 0.85 0.855 Mn2+ 0.014 0.007 0.005 0.009 Mg 0.03 0.054 0.056 0.061 Ca 0.023 0.07 0.088 0.074 Ba - - - - Sr - - - - Na 0.001 0 0.001 0 K 0 0 0 0 Pb - - - - Zn 0 0.002 0 0 Ni - - - - Co - - - - Cd - - - - Cu - - - -

Sum Cat# 1.001 1.002 1 1

Table 6.3l: Calculated formula for carbonate minerals, determined from the recalculation of carbonate

EMPA data. Carbonate Variety Mineral Formula Abundance %

Siderite Fe(CO3) 87.62

Calcite Ca(CO3) 6.34

Magnesite Mg(CO3) 5.04

Rhodochrosite Mn(CO3) 0.86

Eitelite Na2Mg(CO3)2 0.06

Smithsonite Zn(CO3) 0.05

Buetschildite K2Ca(CO3)2 0.01

TOTAL - 99.99

84


6.4 Discussion

At Broken Hill, Ag is associated with tetrahedrite and galena (Lawrence, 1968; Both,

1973; Plimer, 1980). A number of other rare retrograde Ag-rich phases have been

identified including pyrargyrite, polybasite, stephanite, argentite, native silver, dyscrasite

and allargentum (Lawrence, 1968). Previous studies have shown that tetrahedrite is rare

within the zinc lodes (Both and Stumpfl, 1987), and is supported by this study, where

tetrahedrite is only present in 1 Lens.

This study has shown that on CML7, the low-grade vein style intersections of A

Lode and 1 Lens, although not containing high Ag in bulk rock composition, contains

relatively enriched abundances Ag within galena and chalcopyrite phases compared to

those of the B Lode and Kintore Lode. This suggests that Ag moves preferentially into

galena and chalcopyrite during the metamorphism and remobilisation of sulphides in veins.

The low bulk composition abundance of Ag in the A Lode and 1 Lens may represent an

initial low abundance of Ag in the rock prior to mobilisation or reflects an abundance that

is commensurate with the loss of Ag from the system, possibly into shear zones during

retrogression, as has been documented in previous studies (Plimer, 1980, 1987). The

Kintore Lode contained relatively low Ag content in all sulphides analysed by EMPA, yet

was shown to have an anomalous Ag content (169.6 g/t). This suggests that Ag was

contained within phases not observed and not analysed in this study, and provides an area

for further research. The inconsistency of Ag within galena across the grains, suggests that

Ag can be forced randomly into the galena lattice during deformation and metamorphism.

However it has been shown experimentally that the amount of Ag within galena is

constrained thermodynamically to four mole of Ag in galena at 300oC and two mole of Ag

at 250oC (Amcoff, 1984). Also, the amount of Ag within galena is commonly comparable

to the amount of Sb, suggesting an affinity between the two elements, as has been

suggested by Amcoff (1984) and shown to occur at close to a 1:1 coupled substitution in

the Macarthur River Pb-Zn-Ag deposit (Croxford and Jephcott, 1972).

A commonly observed feature in all the ore lodes except the Kintore Lode is the

distinct decrease in lead at the very edge of the galena crystal. This may be due to diffusion

of Pb during retrograde alteration at the extremities of the grains. Cd in sphalerite is

highest in 1 Lens, suggesting Cd enrichment within vein style mineralisation. The

underfilling of Fe within pyrrhotite and the overfilling of Fe within pyrite, suggests that

85


pyrite formation derives from a post-peak high-grade metamorphic interaction of pyrrhotite

with a sulphur-rich fluid, leaving pyrite in the centre of the veins.

The formation of prograde gahnite may have occurred from the desulphidation

breakdown of sphalerite, the reaction of Al-rich clay sediments with Zn-rich fluid or the

metamorphism of silica with Zn-rich hydrothermal clays and carbonates (Plimer, 1987).

86

Chapter Seven – Discussion

Chapter Seven – Discussion The Curnamona Craton represents the surface expression of a polydeformed

Paleoproterozoic intracontinental rift system. The Broken Hill main lode orebody located

within the Broken Hill Block, was deposited as a series of intense episodic pulses of metal-

bearing hydrothermal fluid during rifting. It is the largest accumulation of Pb-Zn-Ag in the

world. After an intense polymetamorphic and deformation history, interpretation of

palaeolithologies central to genetic models is difficult and as a result, many models for the

orebody genesis have been suggested and contested over the years.

This report has documented the B Lode, A Lode and 1 Lens zinc lodes from drill

core and pit, and analysed samples collected during fieldwork for major, trace, rare earth,

halide, stable isotopes, polished section and electron microprobe analysis. The results have

shown that major elements of the zinc lodes show no affinity with regional clastic

metasediments. This disputes early epigenetic models that suggested the orebody formed

via the replacement of clastic sediments (Gustafson et al., 1950), and the recently

described sedimentary expulsion model (Wright et al., 1987). When combined with rare

earth element data, it suggests that the lode horizon and stratabound sulphide rocks are

completely comprised of chemical sediments formed as a submarine hydrothermal

exhalative system during intracontinental rifting. The observed remobilised and deformed

prograde sulphides in association with prograde gangue minerals, suggests that the

sulphide minerals were present prior to metamorphism supporting the syngenetic model

suggested from major and REE geochemistry.

During deformation, sulphides coarsen in grainsize and may move into fold hinges

with corresponding attenuation on the limbs, forming ‘droppers’ within the fold hinges

(Maiden, 1976) which are typically high in Pb and Ag and brecciated from the entrained

gangue minerals. Ore ‘droppers’ have been observed up to 50 metres in length protruding

from fold hinges (Maiden, 1976). The Kintore Lode exposed in the Kintore pit may be an

example of a small-scale dropper where after initially folding some ore is mobilised into

the wall rock were the gangue minerals are brecciated by the injection of the sulphide ore.

The observed brecciation of quartz and garnet crystals in section and higher Pb and Ag

content of the Kintore Lode supports this.

The Kintore Lode is suggested to be the A Lode. Initially the anomalously high Pb,

Zn and Ag grades contained within the Kintore Lode (Table 4.2.1) suggested an affinity

with 2 Lens, as did the rare earth element signatures. The lack of calcite in section within

87


the Kintore Lode suggested clearly an A Lode origin. The major and trace elements show

that when silica is present in abundance greater than 50 A%, the Kintore Lode and A Lode

closely approximate each other. When silica is less than 50 A% the Kintore Lode may

approximate the 2 Lens or plot independently. It is suggested that as a result of the high

sulphide grade, caused by the movement of sulphides into the fold hinge and possible

‘dropper’ propagation, that the unique geochemistry of A Lode is masked in the Kintore

Lode.

The observation of quartz crystals in the core of gahnite and garnet suggests that

prograde quartz growth was first to occur, followed shortly after, by the prograde growth

of both gahnite and garnet. These observations concur with the suggested two-stage

prograde metamorphic events M1/M2 (Page and Laing, 1992). The observation of garnet

rims around prograde gahnite suggests either prograde garnet growth after gahnite,

suggesting a third prograde event or the growth of garnet during a retrograde event. It has

been suggested (Parr, 1992b) that this is a retrograde reaction caused by the interaction of

gahnite with retrograde fluids. Retrogression has caused the alteration of prograde

sulphides by retrograde biotite, muscovite and chlorite. There is net loss of Ag, possibly

into shear zones (Plimer, 1980) showing that under metamorphism, the system is not

isochemical. Late stage carbonate veins crosscut the ore lodes, resulting in the minor

mobilisation of sulphides, fracturing of gangue minerals and the precipitation of

carbonates, with siderite being the dominant variety in the B Lode veins, and pyrite.

The Broken Hill system is comparable to other ancient systems including the

Pinnacles (Parr, 1992a), Cannington (Bodon, 1996, 1998), Pegmont (Williams et al., 1996)

and Gamsberg (Allen, 1996) where massive sulphides are contained within individual

layers within a amphibolite-granulite metamorphosed and deformed Proterozoic terrain.

The Broken Hill orebody is also comparable with some modern systems, including the Red

Sea (Ross et al., 1969), East Pacific Rise (Michard and Albarede, 1986) and TAG Mound

on the Mid-Atlantic Ridge (Mills and Enderfield, 1995). These modern systems are

exhaling hydrothermal fluid, which shows similar hydrothermal REE signatures to ancient

Broken Hill-type deposits, and have been shown to be precipitating sulphides onto the sea

floor. Investigation of these hydrothermal vents, show that hydrothermal fluids may be

exhaled from both white smokers and black smokers depending on temperature. Within the

Atlantis II Deep basin distal to the hydrothermal exhalation, it has been measured that

there is at least 20m of deposited heavy chemical sediments on the basin floor (Ross et al.,

1969). Deep saline-rich brines have also been identified within the Atlantis II Deep basin,

88


showing both chemical differentiation (Brewer and Spencer, 1969) and temperature

variation (Ross et al., 1969), both of which have been suggested in models on Broken Hill

to be responsible for the variations in the ore lode geochemical composition, which are

commonly a source of contention between genesis models.

89

Chapter Eight – Conclusions

Chapter Eight – Conclusions The genetic evolution of the Broken Hill Block within the Curnamona Craton began with

the separation of the crust from the mantle between 2100-2300 Ma (McCulloch and

Hensel, 1984). The lower units of the Willyama Supergroup were deposited in a shallow

marine environment with ash fall tuffs, volcanic detritus and feldspathic sand deposition

during the early stages of intracontinental extensional rifting (Stevens et al., 1988).

Deepening of the water profile led to deep-water turbidite sequences in the Thackaringa

Group deposited between 1700-1690 Ma, accompanied by bimodal volcanics and

hydrothermal precipitates occurring during rifting. The deposition of the Broken Hill

Group followed at approximately 1690±5 Ma comprised of further bimodal volcanics,

hydrothermal precipitates and clastic sedimentation (Page and Laing, 1992). Upon the

cessation of extensional rifting and hydrothermal activity, the rift was rapidly filled by

deep marine tubiditic and carbonate sequences of the Sundown Group, followed by the

deposition of platform shelf sequences of the Paragon Group at approximately 1676±10

Ma.

Hydrothermally-derived exhalative chemical sedimentation occurred as a number

of intense episodic pulses. This produced minor orebodies within the Thackaringa Group,

such as the Pinnacles deposit in the Cues Formation comprising five distinct zinc and lead

lodes and major stratiform orebodies in the Broken Hill Group including the Western

Mineralisation in the Parnell Formation and main ‘Line of Lode’ in the Hores Gneiss,

which contains the B Lode, A Lode and 1 Lens documented by this study. The zinc lodes

show no correlation with regional Broken Hill clastic metasediments of Slack and Stevens

(1994), yet exhibit a distinctive hydrothermal REE signature. This suggests that these ore

lodes are almost entirely comprised of hydrothermal chemical precipitates being deposited

rapidly during a period of low clastic input. Each zinc lode has its own unique geochemical

and mineralogical compositions indicating variation related to the initial composition of

the chemical precipitates, pre-metamorphic alteration, or from differentiation during later

partial melting and mobilisation.

After deposition, the Willyama Supergroup was subject to multiple regional

deformation and metamorphic events beginning with the peak prograde D1/M1 event at

1600 Ma. This resulted in the overturning of the Broken Hill orebody, involving partial

melting and remobilisation of the ore and growth of associated quartz-gahnite and quartz-

garnet alteration halos. The second peak prograde event D2/M2 followed, resulting in F2

90

Chapter Eight – Conclusions

fold structures and promoted the remobilisation of ore into fold hinges for a corresponding

attenuation of ore along the fold limbs. During the initial stages of the retrogression, the

M3 event occurred between 1600-1570 Ma, which included F3 and F4 folds, produced

further quartz-garnet alteration including garnet rims overgrowing prograde gahnite and

the remobilisation of ore into F3/F4 fold hinges as observed in the A Lode within the

Kintore pit. During the final stages of the M3 retrograde metamorphic event, formation of

numerous steeply dipping retrograde shear zones occurred, enriched in remobilised Ag-

rich ore, liberated by the retrograde breakdown of argentian tetrahedrite.

Pegmatites and granites were emplaced in the region about 1420+20 Ma, followed

by a thermal pulse of approximately 300-400oC between 1250-1100 Ma during the

Grenville Orogeny causing regional uplift, shear zone reactivation, carbonate vein

propagation through the orebodies including B Lode and further movement of Ag into

shear zones. Prolonged erosion and some accompanying sedimentary deposition between

1100 Ma and 600 Ma, was followed by dolerite and pyroxenite dykes intruding close to

830 Ma. Further emplacement of ultrabasic dykes and plugs around 561+7 Ma preceded a

low-grade retrogression event during the Delamerian Orogeny at 520+4 Ma, causing

reactivation of retrograde shear zones along with the emplacement of further pegmatites.

Minor fault movement in shear zones, further erosion, uplift and regolith weathering has

continued in the region to the present day.

91

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