THE GEOLOGY AND GEOCHEMISTRY OF VEIN-TYPE LEAD DEPOSITS OF THE WILLYAMA COMPLEX, N.S.W.

P.J. HAGARTY

A thesis submitted for the Degree of Master of Science (by research) at The University of New South Wales (W.S. & L.B. Robinson College, Broken Hill). UNIVERSITY OF N.S.W.

27937 13. DEC 77 USRARY The following dissertation is the result of two years independent research in the School of Geology at The University of New South Wales under the supervision of Dr. I.R. Plimer. Where the work of others, whether published, unpublished or personally communicated, has been referred to or made use of, the fullest acknowledgement has been given

P.J. Hagarty, 20th June, 1977 ACKNOWLEDGEMENTS

The author wishes to express many thanks to the following people who contributed to the successful completion of this thesis.

To Dr. I. Plimer who helped greatly with the author's expression and presentation of ideas.

To Professor L.J. Lawrence who gave constructive criticism of ideas throughout this study.

To Professor P. Ypma and Dr. R.A. Both who showed interest in this study and allowed the author to use the fluid inclusion equipment at Adelaide University.

To Dr. K. Norrish who undertook the entire analytical electron microprobe studies for the author at the C.S.I.R.O., Soils Division, Adelaide.

To the staff of the W.S. & L.B. Robinson College, especially Dr. K. Tuckwell and Mrs. B. Lean.

To my wife, Joan, for drafting of diagrams. ABSTRACT

In the Broken Hill district, two types of base-metal deposits occur. The older strataform Pb-Zn-Ag Broken Hill-type deposits and the younger transgressive quartz-siderite-sulphide - fluorite veins previously described collectively as a single type of deposit termed the Thackaringa-type deposits. These vein-type deposits are divided-into three clearly different groups of deposits on the basis of geological and mineralogical relations.

The first group, the quartz-siderite-sulphide veins (e.g., Thackaringa, Umberumberka, Apollyon Valley mines), have filling temperatures of 180°C and are relatively simple fluids with a complex history of post- depositional activity. The second group are quartz-fluorite-sulphide veins (e.g., Mt. Robe group deposits), and have been generated from a complex hypogene fluid with filling temperatures of about 250°C and have undergone little post-depositional stress. The third group are associated with the main Broken Hill lode (i.e., Consols-type deposits) and contain an abundance of Sb-Ag sulphosalts. Fluid inclusions data and mineral chemistry studies have shown that the Group 1 and Group 3 deposits have a similar genesis.

Sulphur isotope data of Both and Smith (1975) are re-interpreted in terms of the different Thackaringa-type deposits proposed. The group 2 deposits data is shown to have a narrow spread of values with means similar to Broken Hill-type deposits whereas the Group 1 and Group 3 deposits have a large spread but with similar overall means to the Broken Hill main lode.

The genetic model proposed is one of base-metal enriched fluids derived from Broken Hill-type deposits,being collected either by shearing of these deposits producing Group 1 and Group 3 deposits,or by complex mobilisation of a mass of material containing Broken Hill-type deposits? which become anatectic melts producing the Mt. Robe group deposits. It is suggested that both types of deposits were emplaced during the retrogression at the waning stages of the prograde metamorphism, with later Cambrian retrogression decrepitating fluid inclusions, brecciating veins and causing limited remobilisation. TABLE OF CONTENTS

Page No.

1. INTRODUCTION 1

1.1 Regional Setting 1

1.1.1 Timing of the High Grade Metamorphism 3 1.1.2 Structures of the High Grade Metamorphism 4 1.1.3 Maximum Conditions of High Grade Metamorphism 5 1.1.4 Age and‘Structures of the Retrograde Shear Zones 6 1.1.5 Conditions of Retrogression 7

1.2 Previous Work 8 1.3 Scope of Thesis 9 1.4 Problems 11

FIELD OBSERVATIONS 12

2.1 Field Relationships 12

2.1.1 Surface Expressions 12 2.1.2 Nature of Ore Horizons 14 2.1.3 Geological Environment 14 Enclosing Lithology Associated Rock Types Metamorphic Grade

Thackaringa-type Mineralisation 18

2.2.1 Umberumberka Region 18 2.2.2 Apollyon Valley Region 23 2.2.3 Daydream Region 26 2.2.4 Thackaringa Mines Region 28 2.2.5 Maybe11 Area 33 2.2.6 Hidden Secrets Area 36 «• 2.2.7 Consols-type Veins 38 2.2.8 Secondary Hydrothermal Veins 39 2.2.9 Mt. Robe, Black Prince, Golden Crest, Silver King and Mayflower 40 (ii)

Page No. 2.3 Classification of the Thackaringa-type Deposits 43 2.4 Summary 44

MINERALOGY AND MINERAL CHEMISTRY 46

3.1 Vein Mineralogy 46

3.1.1 Oxidation Zone 46

3.1.2 Supergene .Zone 53 3.1.3 Primary Zone 55 Galena

Sphalerite Pyrite

Chalcopyrite

Arsenopyrite Tetrahedrite Pyrrhotite

Marcasite Native Silver and Ruby Silver Minerals 3.1.4 Primary Gangue Mineralogy 84

3.2 Distribution of Minerals 86 3.3 Mineral Paragenesis 88 3.4 Summary 90

4. FLUID INCLUSION STUDIES 92

4.1 Group 1 and Group 3 Deposits 92

4.1.1 Daughter Minerals 94

4.1.2 Decrepitation 94

4.2 Group 2 Deposits 96

4.2.1 Daughter Minerals 96

4.2.2 Liquid C0_ 100 2 ,*

4.3 Freezing Data 100

4.4 Heating Data 104 (iii)

Page No.

4.5 Summary 106

5. DISCUSSION 107

5.1 Age of the Thackaringa-type Deposits 107

5.2 Source of Fluids 109

5.2.1 Granite and Associated Pegmatites 110

5.2.2 Adjacent Metasediments 114

5.2.3 Broken Hill-type Deposits 116

5.2.4 Other Sources 119

5.3 Mechanism of Ore Deposition 121

5.4 Controls of Ore Localisation 123

5.4.1 Chemical Controls 123 5.4.2 Structural Controls 124

5.5 Comparison with other Similar Deposits of the World 124 5.6 Summary 125

6. CONCLUSIONS 126

7. REFERENCES 127

APPENDIX 136 CHAPTER 1

INTRODUCTION INTRODUCTION

1. The Broken Hill Block is a large fault-bounded triangular-shaped Proterozoic metamorphic complex near the western border of N.S.W.

It constitutes a rich metallogenic province hosting a vast range of mineral deposits including the massive lead-zinc sulphide types (Broken Hill-type), the carbonate quartz lead-silver sulphide vein- type deposits (Thackaringa-type), small copper deposits associated with amphibolites, massive pyrite deposits, e.g. Big Hill and mineralisation associated with the prolific number of pegmatite intrusives.

* 1.1 Regional Setting

The Willyama Complex consists predominantly of pelitic, psammitic and quartzofeldspathic metasediments, quartzofeld- spathic gneisses, amphibolites and leucocratic intrusives.

Minor rock types include calc-silicates, leuco-adamellites as plugs and associated dykes (Mundi Mundi granite ), dolerite dykes, banded iron formations and quartz-magnetite rocks and rare quartz-gahnite and sulphide rocks.

This sequence is unconformably overlain by Upper Precambrian Adelaidean rocks.

The Willyama Complex has undergone a number of metamorphic events and a number of generations of deformation.

Binns (1963) recognised four metamorphic events in the Broken Hill area, a high grade Willyama metamorphism (M^) up to the lower granulite facies*, retrograde events (M^, M^, & M^) and the Darling Range event (also retrograde).

Binns (1964) then divided the rocks into three zones, A, B & C of increasing metamorphic grade towards the south as shown in figure 1.

♦Metamorphic facies terminology described in this text follows nomen­

clature described by Winkler (1967). Fig. 1

Locality map showing the metamorphic zones of Binns (1964) and Phillips (1975).

|;, Andalusite zone ___ I

Gamet-K' Felspar zone

Sillimanite Muscovite zone

Granulite zone 2.

.Yanco Glen

ZONE A

ZONE B

Broken Hill

ZONE C

O O O O

Figure 1 3.

Zone A corresponds to the sillimanite-almandine-muscovite subfacies of the almandine amphibolite facies. Zone B corres­ ponds to the sillimanite-almandine-orthoclase subfacies of the almandine amphibolite facies. Zone B corresponds to the silli- manite-almandine-orthoclase subfacies of the almandine amphibo­ lite facies and Zone C, the orthoclase-biotite subfacies of the granulite facies.

Binns' (1964)‘principal classification of the metamorphic Zones A, B & C was based on certain Z- absorption axis colour changes of hornblendes in amphibolites, and the appearance of pyroxene for Zone C as follows: Zone A = blue-green hornblende Zone B = brown-green hornblende Zone C = brown hornblende & orthopyroxene

Other changes such as the appearance of orthoclase in the pelitic gneisses and the disappearance of muscovite from Zone A to Zone B also assisted in the classification.

Work by others since then have shown that Binns metamorphic zones are far too simplistic. Hagarty (1974) showed that low granulite facies metamorphism does exist 24 km north of Broken Hill in Binns Zone A.

Other workers (inter alia Barker, 1972; Rutland and Etheridge, 1975; Katz, 1976; Le Couteur, 1976 pers. comm.) have found Zone C assemblages within Binns' Zones A & B. Binns (1965) has also shown that Zone C assemblages occur in Zones A & B.

1.1.1 Timing of the High Grade Metamorphism Most radiometric dating techniques have been under­ taken in the Willyama Complex and a summary of these results is shown in Table 1.

.* From the most recent work by Shaw (1968)^ it is con­ cluded that a high grade metamorphic period reaching the lower granulite facies occurred at about 1700 4.

Table 1

GEOLOGICAL EVENT AGE DETERMINATION TECHNIQUE (in million years) my 87 88 Original age 1820 ± 60 Sr/ Sr, Shaw (1968) of rocks

High grade 1695-1 21 Rb/Sr, Shaw (1968) regional 1640 1 40 Rb/Sr, Pidgeon (1967) metamorphism 1600 K/Ar, Binns, et al., (1963) 1615 Pb, Ostic et al., (1967)

Emplacement of 1520 ± 40 Rb/Sr, Pidgeon (1967) Mundi Mundi 1350 Pb, Richards (1963) Granite 1307 K/Ar, Binns et al., (1963)

Emplacement of 1560 ± 40 Rb/Sr, Pidgeon (1965) muscovite pegma­ 1540 ± 50 " " (1967) tites

Age of the 1190 ± 35 Pb, Russel et al., (1961) Thackaringa- 1020 1 150 Pb, Russel et al., (1960) type deposits 800 Pb, Reynolds (1971) 510 ± 80 Pb, Kanasewich (1962)

Retrograde 530 ± 70 Rb/Sr, Pidgeon (1967) metamorphism 520 K/Ar, Evernden et al., (1961) 440-550 Pb, Richards, et al., (1963)

Age of a pegmatite 495 Rb/Sr, Pidgeon (1967) which cuts the Thackaringa- Pinacles Shear

million years. This high grade metamorphic event may have been protracted over some period of time (up to (}0 million years).

1.1.2 Structures of the High Grade Metamorphism There are a number of structural models proposed to explain features observed in the regional high .* grade rocks.

Hobbs (1966) described two major styles of deform- 5.

ation, one being associated with the high grade meta- morphism (the development of an axial plane schistosity to Group 1 folds) and another folding this high grade schistosity (Group 2 folds).

Rutland (1973) revealed the existance of not one but two high grade regional schistosities. The first schistosity, Group 0, predates the Group 1 structures found by Hobbs (1966).

Rutland and Etheridge (1975) interpreted four phases or periods of high grade deformation. Two being associ­ ated with the prograde metamorphism and the other two associated with the waning stages of the prograde metamorphism.

Tuckwell (1975) has established in the Euriowie Inlier (Figure 1) that metamorphism and deformation were not coeval, isograds are parallel to bedding and isograds have been folded. There is not enough data from the Broken Hill Block to show that this situation also exists but in isolated areas bedding, isograds and are commonly parallel. It appears that deformation may not necessary coincide with the prograde metamor­ phism and in fact that retrogression may have been initiated during the waning stages of the prograde metamorphism.

1.1.3 Maximum Conditions of High Grade Metamorphism From a study of breakdown relationships of hornblendes in metamorphosed rocks, Binns (1968) suggested a temper­ ature of between 700-800°C was attained during the prograde metamorphism with pressures of 5-10 kbs. Recent studies by Scott, Both & Kissin (1976) using the sphalerite-pyrrhotite geobarometer have indicated pressures of 7.8 kbs for the metamorphism of the Broken Hill orebodies. 6.

Hewins (1975) has found temperatures of between 760-790°C as minimum temperature estimates for granulites in the Broken Hill region. This esti­ mate assumed that the environment at the time of the prograde was dehydrated i.e. P^ Q < P^. This is confirmed by petrological observations (Ramdohr, 1951). Maximum temperature estimates of Hewins o range up to 860 C.

1.1.4 Age and Structures of the Retrograde Shear Zones (RSZ) Generally speaking, Thackaringa-type mineralisation is found wholly within or genetically related to retro­ grade zones. Hence it is important to understand the age and structure of these zones. Knowledge to date concerning these two variables is scanty and most unclear. A minimum isotopic age of 495 m.y. has been determined by Pidgeon (1967) for a pegmatite which transgresses the Thackaringa-Pinacles Shear Zone although psammitic gneisses obviously retrogressed from near the Broken Hill orebody gave an isotopic age of 1605 (Pidgeon, 1967). Most retrograde shear zones do not appear to cut the Adelaidean Sequence north of Broken Hill (dated at 1350-700 m.y. by Binns and Miller, 1963 and Compston and Arriens, 1968). The Corona Fault, however, appears to have been active during the deposition of the Adelaidean rocks. Biotites from shear zones and high grade metamorphic rocks have been isotopically re-equilibrated at 530 m.y. (Binns and Miller, 1963; Richards and Pidgeon, 1963). Wasson (1975) has shown that the Mundi Mundi Fault was active in the Pleistocene.

It therefore appears that retrogression has been active over a long period probably commencing immediately after the prograde metamorphism and finally ending with a major event (Darling Event) at about 500 m.y. It has been suggested that the Retrograde Shear Zones may be divided into two principal sets. The older set is a 7.

conjugate NE-NW set and a later set (possibly not

conjugate), WNW and N (Plimer, 1975).

Hobbs (1966) recognised two retrograde structural

events. It is not clear whether these two events coincide with those found by Rutland and Etheridge

(1975).

1.1.5 Conditions of Retrogression From a study of mineral phase relations, Chenhall

(1973) placed the following general conditions of retrogression.

T = 480-600°C P = 4.8-6 kbs

P - P T

Le Couteur (pers. comm.) believes that this temper­ ature may be greater in certain cases being controlled by the size of the shear zone.

The following graphical representation (Figure 2) summarises the metamorphic history of the Broken Hill

area.

Main metamorphism Metamorphic intensity

Cambrian retrogression

1695 1540 500 Time (m.y.)

Fig. 2 8.

The high grade metamorphism of the Willyama Complex appears to consist of two separate pulses (Rutland & Etheridge, 1975). It attained temperatures of between 700-800°C at sometime in this period and this was followed with two separate deformation episodes at the waning stages of the prograde metamorphism.

The emplacement of muscovite pegmatites and Mundi Mundi Granites occurred at about 1560 m.y.

The isotopic re-equilibration of biotite occurred around 530 m.y. This probably represents the last major movement of these shear zones up to the Recent.

Field and age relationships indicate that the Thackaringa-type mineralisation was emplaced between the last pulse of the prograde metamorphism and the 530 m.y. age.

1.2 Previous Work

The first discovery of the Thackaringa-type mineralisation was at Thackaringa in 1874 by C. Nickel. The PIONEER and GYPSY GIRL are the largest mines in the area and have produced 20,000 and 10,000 tonnes respectively.

The next discovery was at Umberumberka and this supported the early township of Silverton. Then followed the DAYDREAM mine in 1881 and the BOBBY BURNS and others in the Apollyon Valley. Most of the deposits were mined above the water table, some­ times only down 10 metres, below which the silver-rich oxidised zone was commonly replaced by sulphides. There was no evidence of any supergene enrichment zone and so tonnages were small and most of them had very short lives (around 5 years).

.* These early discoveries were overshadowed by the discovery in 1884 by Charles Rasp of the main lode at Broken Hill which has to date yielded 200 million tonnes, averaging approximately 9.

Pb 10%, Zn 11.5% and Ag lOOgms/tonne.

Other small Thackaringa-type mines were discovered after the main Broken Hill lodes, notably TERRIBLE DICK (1885), MAYBELL MINES (1885), DEMOCRAT (1885), MAYFLOWER and BRITISHER (1885), CONSOLS (1885) and MT. ROBE (1887). A total of 92 Thackaringa- type mines have been identified in the Broken Hill district, and these produced 65,065 MT of hand picked ore represent­ ing well over 150,000 MT of run-of-the-mine ore. Appendix A4 collates available production figures as extracted from records by Dickinson (1939).

1.3 Scope of Thesis

This thesis is a petrological and structural study of Thackaringa- type mineralisation and is principally aimed at presenting a genetic model and relating these deposits to other lead deposits in the Willyama Complex.

The six most important areas of Thackaringa-type mineralisation are as follows (Figure 3): 1. Umberumberka Region 2. Apollyon Valley Region 3. Daydream Region 4. Thackaringa Region 5. Maybell Region 6. Hidden Secrets Region

Conventional mapping, petrographic studies, x-ray diffraction and electron probe methods were utilised. Fluid inclusion studies were undertaken and phase studies utilised in order to determine the conditions of ore and gangue deposition.

Mapping and laboratory techniques are described in the appendix. This study was undertaken because very little was understood or known about these deposits. Most of the interest in the Broken Hill area centred around the more productive Fig. 3

Locality map showing the areas studied in this thesis. 10.

Maybell \v ' yranco Glen Hidden Mines \ Secrets Mines

Apollyon

Daydream, "• Mine Umberumberika Mines

Broken Hill

Thackaringa kilometres AAines

Figure 3 11.

Broken Hill-type deposits. The main reason for this lack of knowledge centres around the fact that these deposits have not been mapped in sufficient detail.

1.4 Problems

The lack of outcrop in most areas hindered mapping. The limited amount of sulphides and suitable gangue restricted mineragraphic' studies and fluid inclusion work.

Most samples were collected from the dumps and in some safer mines material was collected in situ.

As most mines only extended to the water table complete suites of primary ore were non-existent except for the Pioneer Mine at Thackaringa and Umberumberka. CHAPTER 2 FIELD OBSERVATIONS 12. 2. Introduction

Kenny (1922) recognised two types of base metal deposits in the Willyama Complex which he called "Thackaringa-type" and "Broken Hill-type". This distinction is based primarily on differences in physical form: Thackaringa-type deposits are discontinuous, sinuous vein networks located in retrograde shear zones, whereas Broken Hill type deposits are stratabound massive orebodies.

The relationship between Broken Hill-type and Thackaringa-type deposits has been briefly discussed in the literature. King and Thompson (1953) and Williams (1955) noted a close field correlation between Broken Hill-type and Thackaringa-type mineralisation with the latter occurring on the fringe of quartz-gahnite bodies of the Broken Hill-type deposits. Studies by Rawaz (1961) showed that there was a lack of knowledge about the Thackaringa-type deposits.

During a short mining history, the Thackaringa-type deposits were known for small tonnages of particularly high grade silver and lead ore. The Broken Hill main lode, conversely, is relatively poorer in silver and lead and richer in zinc ores.

The similarities and differences between Broken Hill-type and Thackaringa-type deposits compiled from the studies of Jaquet (1894), Kenny (1922), Ramdohr (1951), King and Thompson (1953), Hobbs (1966), Lawrence (1968, 1973) and observations of the author are shown on Table 2.

2.1 Field Relationships

2.1.1 Surface Expressions

Thackaringa-type mineralisation is characterised by two types of surface expression, as.a rubble of 1shode stones' consisting of malachite, cerussite and embolite and much more commonly as a siliceous ferruginous limonitic gossan. Shode stones were a primary exploration tool for these deposits during the 1890's, however, no exposures remain. These were also referred to as horn silver (cerargyrite), however, the distinction between cerargyrite and embolite is confused (Jaquet, 1894). The more common quartz-limonite 13. Table 2

Characteristics Thackaringa-type Broken Hill-type Age Post prograde metamorphism Pre-prograde metamorphism ages of 500 m.y. (Pidgeon, i.e., greater than 1700 1967), 800 m.y. (Reynolds, m.y. 1971). Age uncertain (Richards, pers. comm.)

Structural relations Hydrothermal vein-type. Massive stratabound.

Main primary Galena with minor Galena, sphalerite, pyrrhotite ore mineralogy sphalerite, pyrite, chalcopyrite, loellingite. chalcopyrite, arseno- pyrite, tetrahedrite.

Gangue mineralogy Siderite-quartz Mn, Fe, Ca silicates, quartz, Fluorite-quartz felspar, calcite, fluorite, gahnite.

Hosts Muscovite, biotite, Pelitic and psammitic rocks. chlorite, schists; granitic dykes.

Associated rocks Granitic rocks, shear and Amphibolites, felsic rocks, fracture zone rocks. garnet-plagioclase gneiss, major shear zone rocks.

Depths of Greater than 150 m. Main lode to 1600 m. mineralisation

Style Pod-like, coarse grained. Massive elongate bodies.

Metamorphic grade Greenschist- Granulite facies. amphibolite facies. 14.

boxwork gossan results from the breakdown of the

quartz-siderite-sulphide primary vein material.

The surface expression of Broken Hill-type deposits is

commonly more siliceous and hence more prominent in

outcrop. Strongly developed limonite boxworks are uncommon (main lode, Rupee) and to some extent they resemble Thackaringa-type deposits. Quartz-gahnite, sulphide-poor lode horizons and banded iron formations

(BIF's) probably are pre-metamorphic extensions or

equivalents of Broken Hill-type deposits.

2.1.2 Nature of Ore Horizons

The main Broken Hill orebody consists of a number of

different lodes of varying chemistry (Tabled in Appendix). They have variable silver values (30 to 336 gms/tonne), variable lead grades (5 to 20%) and consistent high zinc grades of 15-20%. The main lode ore consists predominantly of Fe-rich sphalerite, galena, pyrrhotite, tetrahedrite and chalcopyrite with less important loellingite, arsenopyrite, cubanite and others (Lawrence, 1968).

Broken Hill-type deposits are noted for their conformity

with lithological layering. Recent studies have shown that

the Broken Hill-type deposits are characterised by an enclosing hydrothermal alteration zone (Plimer, 1975).

Thackaringa-type lodes form narrow well defined vein deposits

with no metamorphic or hydrothermal alteration zones.

2.1.3 Geological Environment

Enclosing Lithology

Lawrence (1967) described the Thackaringa-type deposits as being essentially enveloped by the same host lithology.

This observation is not supported by this study. Hosts

for the Thackaringa-type deposits vary from granitic 15.

dykes to pelitic gneiss (Table 3).

Broken Hill-type deposits are characteristically associated with pelitic gneiss. Other expressions of Broken Hill-type deposits, e.g., quartz-gahnite horizons, are present with amphibolite hosts (Champion and Rupee), and it is possible that these are metamorphically remobilised.

In this study, Broken Hill-type deposits are not regarded as stratiform but stratabound.

Table 3

Rock Type Host Locality Granitic dykes Apollyon Valley (H.W.*), Daydream (H.W.), Hen and Chicken (H.W. and F.W.).

Graphite schist Lady Dorothy (H.W. & F.W.), Umberumberka (H.W. & F.W.), Terrible Dick (H.W. & F.W.).

Garnet-biotite- Thackaringa (H.W. & F.W.), Daydream felspar gneiss (F.W.).

Amphibolite Hercules, Golden Crest, Black Prince

Chlorite-muscovite Koh-i-Nor, Brilliant, Gem, Democrat, schist Mayflower, Umberumberka, Maybell.

Andalusite schist Black Prince, Mt. Robe, Silver King.

Granitic gneiss Britisher, Umberumberka.

* H.W. - hanging wall F.W. - footwall 16.

The overall association of Broken Hill-type mineralisation with one lithological host probably related to the original facies environment of deposition.

A number of Thackaringa-type deposits associated with a granitic hanging wall emphasises the age difference between Broken Hill-type and Thackaringa-type mineralisation.

Associated Rock Types Broken Hill-type deposits are associated with amphibolite and quartzofelspathic gneiss (e.g., Parnell and Potosi gneiss). The schists of major shear zones are spatially associated with major Broken Hill-type deposits (Table 4).

Table 4

Mine Shear Zone Attitude & Proximity Angus Mine Hillston Fault Parallel to lode, 200 m away.

Broken Hill Globe Vauxhall Sub-parallel, cuts lode, Main lode greater than 20 m3variable

Champion Champion-Democrat Sub-parallel, 0-50 m cuts (Hagarty, 1974). lode.

Little Main North-South 800 m, sub-parallel to Broken Hill lode.

Allendale Not named. 80 m, sub-parallel to lode. ♦*

Maybe11 Holder Variable. 17.

Table 4 (continued)

Mine Shear Zone Attitude & Proximity Nine Mile Not named. 80 m, sub-parallel to lode.

Pinnacles Globe Vauxhall 1 km (approx), sub-parallel to lode.

The attitudes of these major shear zones are commonly sub-parallel (-8°) to their respective enclosing gneisses which trend 025° to 040°. The association of Broken Hill type mineralisation with antiformal, synformal pairs is also important (King and Thomson 1953; Rutland and Etheridge, 1975).

Thackaringa-type deposits have no common lithological associations. Some deposits are associated with amphibolite and calc-silicate rocks, e.g., Umberumberka, whereas some are associated with granitics, e.g., Apollyon Valley Mines, Terrible Dick and Daydream.

Thackaringa-type deposits are not necessarily associated with shear zones. Some are completely unrelated to retrograde shear zones, e.g., Thackaringa.

Metamorphic Grade Broken Hill-type deposits are commonly enveloped by rocks of the lower granulite facies of metamorphism, e.g., Champion, Rupee, Broken Hill Main Lode, Little Broken Hill, Angus, Allendale (uppermost amphibolite facies, A.M. Symonds, 1975) and Corruga. In many areas later retrogression has obscured the extent of the high grade metamorphism.

With few exceptions, most Thackaringa-type deposits are surrounded by rocks of the lower to middle amphibolite 18.

facies grade of metamorphism, e.g. , Thackaringa-type deposits to the northwest and west of Broken Hill. The exceptions include Umberumberka and Thackaringa-type deposits associated with the Champion mine.

In the east of the Broken Hill block, the main mineralisation-type is the Broken Hill-type deposits. Progressively towards the west, Thackaringa-type become the' most dominant type of base metal mineralisation. This is due to the progressive phasing out of Broken Hill-type mineralisation towards the west of Broken Hill. This observation is important to the genesis of Thackaringa-type deposits and will be discussed later.

2.2 Thackaringa-type Mineralisation

2.2.1 Umberumberka Region

The lithology is a complex sequence of pelitics (with well-developed quartz-rich bands), interlayered quartzite, calc-silicate, amphibolite and thin metadolerite bands. This sequence of metasediments trending 030°, is intruded by a large quartz-felspar pegmatite with a scree which covers about 50% of the available metasediment exposures. The location of this dome-like pegmatite body is shown in a cross section of the Umberumberka mines area compiled from drill core data (Fig. 4). Small muscovite-rich granitic dykes are present but are minor with respect to other areas studied (e.g., Apollyon Valley and Daydream areas). Retrograde shear zones are common in the area.

A large antiformal F2 fold, with a we11-developed axial plane schistosity trending 030°, plunges shallowly towards the northeast. A banded amphibolite defines the fold shape and the hinge of the fold is covered with alluvium. A plot of the poles to layering supports this structural interpretation (Fig. 5). Fig. 4

Geological cross section of the Umberumberka Mines area compiled from drill core data. The main mineralisation occurs in intense shear to the northeast of the UM3 drill hole site. Figure 4

m •< □ □□ *n

CD 3 0> <7' in □ □□ G I ° a 3“ ? i. £ i? » CQ •*\J 3 fiT 0>

-o / / ■*o

HDD 2=o CD3

S. UM3 S. K

2 N CL o 3 N 0> O 3

□ n o_

n UM1 (£>

Q to Fig. 5

Plot of poles to layering in the Umberumberka areas, Contours represent 2-4-8-12% per 1% area with a maximum of 15%

Fig. 6

Schematic structural representation of the Umberumberka area. 20.

Figure 5

later folding

layering trends 030 SCALE : 1cm - 400m

amphibolite

attitude of high grade metamorphic zone

Umberumberka Mines

Figure 6 21.

Two main retrograde shear zones, the Umberumberka Shear and the Chloride Shear are ore-bearing structures (Dudley, 1894). The intersection of these shear zones is the site of the Umberumberka mine which produced in excess of 15,000 tonnes of high grade ore. These shear zones are interpreted as representing a single system which trends at 030°, a similar attitude to the axis of the F2 fold it transects (at a low angle). Later folding and shearing striking approximately due north, have affected these earlier structures (Fig. 6).

From a detailed lithological study of all rock types in the Umberumberka area, a zone of relic high grade (up to the lower granulite facies) metamorphism trending 025° has been delineated. The appearance of orthopyroxene - clinopyroxene associated with euhedral olive-coloured hornblende in amphibolite is the main criteria for delineating this as a Zone C assemblage according to Binns (1964). The presence of orthoclase in pelitics, higher grade textured calc-silicates with diopside and the absence of retrograde tremolite supports a Zone C assemblage.

The main Umberumberka lode strikes N25°E, plunges to the SW at 75° and pitches towards the SE and is conformable with the shear schistosity (Jaquet, 1894). The lode has been displaced by later movements. There is a close correlation between the vein direction and layering trends in country rocks (Fig. 7).

Jaquet (1894) noted a coincidence of graphitic schists with zones of ore concentration. These schists are not a primary ore control because the main hosts to the Umberumberka lode are chlorite-muscovite schist, amphibolite and granitic gneiss. The graphitic schist occurs in two broad bands (up to 50 m thick) and was observed only in the drill core. The graphitic schists are pyritic and probably were originally a carbonaceous sediment. Fig. 7

Plot of layering trends and vein attitudes for the Umberumberka region. 22.

layering

vein attitudes

Figure 7 23.

The gangue mineralogy is commonly siderite, quartz and rare barite (Green, 1894).

These mineralised veins appear to lack an oxidised zone and have no surface enrichment of silver as do the other Thackaringa-type deposits (Dudley, 1894).

The alignment of the high grade metamorphic zone, the axis of the F2 antiformal fold, the major shearing direction (and hence the ore veins) and lithological layering is an important feature of the Umberumberka area. This parallelism has also been observed in the Champion-Hidden Secrets area (Hagarty, 1974). The major shear in the Umberumberka region is possibly a longitudinal shear developed along the axis of the anticline, i.e. the zone of greatest weakness.

The alignment of metamorphic isograds with original layering traces has been observed in other studies of Broken Hill rocks, e.g. sillimanite isograds, Glen, 1975; and Euriowie Inlier, Tuckwell, 1976.

2.2.2 Apollyon Valley Region

The regional geology in this area consists mainly of chlorite-muscovite - garnet schist, retrogressed granite and quartzites which occur in a major shear zone, 950 m wide. This zone trends at 010°-015° and is called Apollyon Valley shear zone (AVSZ). Higher grade staurolite-biotite-muscovite-garnet - chloritoid assemblages are present towards the eastern margin of the shear zone. Retrogressed muscovite-bearing adamellite is prolific in the shear zone and to the east of the shear zone it represents 80% of surface exposures. Amphibolite and calc-silicate rocks are relatively rare as compared with the Umberumberka region and are of low metamorphic grade (lower amphibolite facies). Detailed lithological descriptions are in the Appendix. 24.

Regional structures in the Apollyon Valley are very large scale features and because a relatively small area was mapped, interpretation was limited. However, from a regional traverse across the Lakes Grave-Apollyon Valley-Daydream shear region, Glen (1975) suggests that a broad anticlinal structure is located approximately over the Apollyon Valley shear zone (AVSZ). This is supported by the change in dip in the layering of 40°-50° to' the west on the western side of the AVSZ to 20°-30° to the east on the eastern side near the Daydream mine. Glen (1975) mapped a sillimanite isograd in this area coplanar with beddings (as in the Umberumberka region).

The Apollyon Valley group of mines includes the Patience, Gemini, Gemini 2, New Year, Apollyon, Lady Dorothy, Tom Thumb, Richard Ruby, Block 1 and Gemini East Block mines. These mines can be traced in the AVSZ as two separate arcuate lines of lode dipping 40''-60'" to the east and which transgresses the dominant shear schistosity (Plan 4). The lodes are narrow and discontinuous and have been intensely oxidised at the surface.

Host lithologies include granitic dykes or schist hanging wall and schist footwall. Graphite schists are present on some dumps in the Apollyon Valley. The association of granite host with Thackaringa-type deposits in this area is well documented (Smith, 1922; Burrell, 1937; Dickinson, 1939). Granite-vein contacts observed by the author are sharp and no mineralisation was found in the granite. This probably shows that the granite has no genetic association with the deposits. The mineralising fluids may have migrated along the shear during one of its active phases, intersected the impermeable granitic dykes and then followed the under­ side of the granitic to the surface (Fig. 8). This may explain the arcuate form of these lodes with respect to the shear zones. No Broken Hill-type mineralisation Fig. 8

Schematic diagram illustrating the path of ascent of ore-bearing fluids of the Apollyon Valley deposits. 25.

hanging wall granitic dyke

Thackaringa-type deposit granitic dyke commonly brecciated

movement of

mineralised permeable

fluids \ shear zones

volatile rich ore fluids

Figure 8 26.

is known in this area. One small exposure of spessartine quartzite occurs near the Gemini. This is possibly an expression of a Broken Hill-type lode. The Apollyon Valley shear zone extends northwards to the Terrible Dick and southwards to the Barrier Chief and Red Lion group of mines. The Apollyon Valley region is one which contrasts strongly with the Umberumberka region and is a completely different geological environment with a predominance of granitic rocks.

2.2.3 Daydream Region

The Daydream mine is situated 2.5 km east of the Apollyon Valley shear zone. In this area the lithologies are similar to the eastern side of the Apollyon Valley consisting predominantly of granitic dykes and pegmatite with rarer metaquartzite and sillimanite-garnet-biotite muscovite gneiss. Amphibolite and calc-silicate rocks are absent. A major shear zone trending 010° is present on the eastern edge of the Daydream mine and is probably genetically related to the Apollyon Valley shear zone. Burrell (1937) interprets these shear zones as a complex set of overthrust faults.

The Daydream mine consists of two curvilinear lode horizons outside the major Daydream shear zone (Fig. 9). Both lodes dip towards the south at 20° and have been extensively mined to 40 m vertical depth. The maximum lode channel width observed is 3.5 m. A common vein network noted on underground inspections (Fig. 10) consists of a well defined hanging wall contact with an ore bearing limonite network near the top of the vein.

Host lithologies vary from metaquartzite and granite to pelitic gneiss. The lode horizons are'-widest in the more competent rocks, e.g., adamellite and quartzite. The Hen and Chickens lode, located 500 m due west of the Daydream mine, has a granite host in the foot and Fig. 9

Isometric projection of the Daydream orebody 27. 28.

hanging walls.

The emplacement of the veins appears to have post-dated the main movement of the Daydream shear and has a radial relationship with the shear boundary indicating a tension-gash fill origin for this deposit (Fig. 11). This is supported by the development of ore in more competent rocks. No Broken Hill-type deposits were observed in this area but quartz-gahnite rubble found on one of the dumps from the Daydream workings assayed 8.5% Zn and 0.3% Pb. This material which is coated with limonite stains probably represents part of the material mined from underground.

Both the Daydream area and the Apollyon Valley mines occur in much lower metamorphic grade (middle amphibolite facies) rocks and represent a different geological environment from the substantially higher metamorphic grade Umberumberka region.

2.2.4 Thackaringa Mines Region

In this area, outcrop is poor and detailed geology is impossible. Alluvium covers large low lying areas in the vicinity of the mines.

The dominant mappable lithology is a massive pegmatite body trending 295° flanked by a brown amphibole-bearing amphibolite of middle amphibolite facies metamorphic grade. In the north of the area investigated, amphibolite, pegmatite and calc-silicate rocks are present in the southern extension of the Thackaringa- Pinnacles shear zone. Quartz veins and masses are common in this area and granitic rocks are rare.

The major pegmatite defines the shape of a' major antiformal structure (Archibald, pers. comm.) which is wedged between the intersection of the Mundi Mundi Fault and the Thackaringa-Pinnacles shear zone (Plan 7). Fig. 10

Cross section of a common vein network observed at the Daydream mine. (r\o+ scaJe) .

Fig. 11

Diagrammatic ore migration process at the Daydream mine (not to scale). 29. kaolinised layer rich in silver small pods of chlorides relic galena well defined

hanging wall

Figure 11 30.

The dextral motion of the Mundi Mundi Fault is shown in the contortion of layering structures near the fault.

In the area investigated, three fracture sets are present, two of which form a vertical conjugate pair which trend at 290° and a later fracture set trending 010°. The last set of fractures is probably genetically related to the Mundi Mundi fault. All three fracture sets contain base metal mineralisation.

Underground observations were made in Davy's workings, 150 m south of the Gypsy Girl. A sketch plan of the workings was prepared (Plan 9). Two veins of a conjugate system trending 295° were mined. One of the veins dips towards the southwest at 40° and another to the NE at 20°. These veins intersect at a depth of 20 m and slight displacement is present. The vein has a maximum width of 1.2 m with an average of 0.4 m. Mineralisation is present as small podlike bodies in the vein (Figs. 12 & 13). The host lithology is sillimanite-garnet-biotite gneiss with quartz-felspar-rich bands and common garnet "eyes". Scarlet felspars are common in the hanging and footwalls.

Jaquet (1894) described the Thackaringa deposits as a series of synclines and anticlines resembling saddle lodes. This is reinterpreted as a series of slightly folded conjugate fracture sets.

Vernon (1969) noted that the "Thackaringa lode may have been formed by the replacement of folded layers". No evidence of replacement was observed in the Davy's workings. The mechanism suggested here is one of injection into open fractures.

Three different types of vein structures have been observed in the Thackaringa lodes (Fig. 14). There are two generations of quartz present. The greater the Fig. 12

Galena, bromargyrite, covellite pod near to the hanging wall 30 metres below the surface at Davy's Workings, (x */3j

Fig. 13

A common sulphide-poor limonite vein near to the present water table, 50 m below the surface at Davy's Workings. ^ 5crv»s = 2.^ 31.

Figure 12

Figure 13 Fig. 14

Common vein structures in the Thackaringa-type Group 1 deposits.

(r>o+- 4-o Sco-le) 32.

zoned comb quartz comb quartz with little

development of siderite

unzoned

/ intergranular

quartz' crystals

equigranular siderite or calcite

massive siderite

and sulphides

quartz veneer with

small crystals protruding

into siderite

Figure 14 33.

volume of carbonates in these vein networks, the greater the sulphide content (Fig. 14c). This is explained by the fact that lead, copper and zinc sulphides occur in the carbonate phase present, whereas iron and arsenic sulphides are more common in the quartz-rich outer margins of the vein.

Mineralisation in the 010° trending fracture set is not well developed. The gangue is quartz and calcite (not siderite) and the veins are relatively undeformed. These are possibly later than the mineralisation in the conjugate fracture set.

In the Thackaringa region, the correlation between vein trends (295°) and layering trends is observed (c.f., Umberumberka). No deposits were observed in this present study with conjugate fractured vein systems similar to the Thackaringa Mines. These conjugate fractured veins produced the largest deposits with the widest ore widths (Pioneer, Gypsy Girl) .

No Broken Hill-type deposits are present in the vicinity of the Thackaringa mines. Only two localities are known in the western part of the Broken Hill Block. These are the Angus and minor quartz-gahnite veins in the Thackaringa Pinnacles shear zone (Le Couteur, pers. comm.).

The geological environment of the Thackaringa deposits is similar to the Umberumberka area but the style of mineralisation resembles the Daydream area.

2.2.5 Maybe11 Area

The Maybell group of mines was briefly studied for two reasons:-

(a) because of the close spatial association of these 34.

deposits with Broken Hill-type deposits, and

(b) because the deposits are the mostly easterly expressions of well developed Thackaringa-type deposits.

These deposits include the Maybell North, Main Maybell, the Kate and Silver Dale (Thackaringa-type); the Rob-Roy and Clifton lodes (Broken Hill-type).

The regional geology of the Maybell area is similar to the Champion area. Quartzofelspathic gneiss, amphibolite and pegmatite are present with less common pelitic gneiss whereas granitic dykes are rare. Broken Hill-type lode rocks, e.g., BIF's, garnet quartzite and quartz-gahnite rocks are present.

No high grade metamorphic zones associated with Broken Hill-type deposits have been delineated, although the whole area has undergone regional retrogression and relic high grade zones were possibly destroyed.

There are three shear systems present in the Maybell area (Fig. 15). Two of these are related to the Thackaringa-type mineralisation. One set trends 030° * and is parallel to S2 lineations and to lithological layering (Plan 10). The main Maybell and Silver Dale deposits occur in this shear set. Another shear zone trends 050° and the North Maybell deposit is within this shear zone. A third set (poorly exposed) trends 330°.

* S2 schistosity is defined by sillimanite and biotite elongation. Fig. 15

Relationship between layering and vein trends in the Maybell area. 35.

layering

vein attitudes

Figure 15 36.

From the study of younging directions and lithology repetition, Bunting (1975) has evolved a series of syncline and anticline pairs in the Maybell area. The coincidence of the Broken Hill-type deposits with antiformal fold axes is verified. In the Maybell area, these deposits outcrop on adjacent sides of the antiformal axis (Plan 10) .

No base mebal sulphides were found on the dumps of the Maybell mines and old records of these mines are non-existent.

2.2.6 Hidden Secrets Group

There are several distinctive lithologies in the Hidden Secrets area which are common in the Maybell area. Sillimanite-bearing pelitic gneiss, psammite, quartzofelspathic gneiss and amphibolite are present. Later pegmatite and related leucoadamellite' intrusives trending 030° have a similar attitude to the adjacent layering trends. A linear zone of granulite grade metamorphism has been delineated (Hagarty, 1974). Three shear sets trending 005°, 045° and 023° are present in this area, the last of which contains all Thackaringa-type mineralisation (Fig. 16) .

From the repetition of Parnell gneiss in the Hidden Secrets area, Rutland (1973) interpreted that a major synformal structure with an axial plane trending 030° is present. A few measurements of S2 were made in this study and attitudes of 030° for S2 are common.

Broken Hill-type and Thackaringa-type deposits are common in the area investigated. The Champion lode, the largest of the Broken Hill-type deposits, extends 800 m along strike and crosscuts lithological layering. The Champion is surrounded by a number of Thackaringa-type deposits (Table 5). The close proximity of Broken Hill-type and Thackaringa-type deposits in this area is Fig. 16

Relationship between layering and vein trends in the Hidden Secrets area. 37.

layering

vein attitudes

Figure 16 38.

common in the Maybell area.

Table 5 Thackaringa-type deposits which are in close proximity to Broken Hill-type deposits.

Thackaringa-type Associated Broken Distance between Deposit Hill-type Deposit Deposits Hidden Secirets Champion 400 m Gem Champion 800 m Brilliant Champion 1 km Koh-i-Nor Champion 1.5 km Maybell Silver Dale 500 m Consols Main lode 1.0 km British shear veins Main lode 0

The Hidden Secrets group of mines comprises several shallow quartz-limonite veins. They occur in a primitive shear zone, trending 030° which cuts across the Champion line of lode.

The following similarities with the Maybell area are apparent:-

(a) close spatial association of Broken Hill-type and Thackaringa-type deposits.

(b) a relationship between the attitudes of mineralised shear zones, lithological layering and S2 lineations, and

(c) despite the lack of BIF's in the Hidden Secrets area these two regional areas have similar geological environments with a similar host lithology.

2.2.7 Consols-type Veins

This group includes the East Consols, ABH Consols and 39.

the silver-rich veins in the British shear zone. All of these deposits are in close proximity to the main lode at Broken Hill (Table 5). The British shear zone silver veins are present in contact with the main Broken Hill ore in Brown's Shaft (Plan 1).

Poor surface exposures in the vicinity of the Consols deposits restricts geological interpretation. Two shear zones are present in the Consols area, one trending 325° and another at 045°. The latter shear zone contains the Consols mineralisation and this direction of 045° is similar to layering trends in that area. The British shear veins, occur in flat variably dipping fractures which transgress lithological layering.

Host lithologies of the Consols veins are amphibolite and pelitic gneiss. The British shear veins are hosted by chlorite schist and main lode orebody contacts.

The ABH Consols and the British shear veins are characterised by a dominance of siderite over quartz gangue and a very complex sulphide and sulphosalt mineralogy (Markham & Lawrence, 1962; Lawrence, 1968). These antimony and arsenic silver sulphosalts are virtually absent in the other Thackaringa-type deposits. The predominance of silver-bearing minerals over lead-bearing minerals in these deposits creates problems in grouping these deposits with the other Thackaringa-type deposits.

2.2.8 Secondary Hydrothermal Veins

Throughout the main lode orebodies at Broken Hill are small veins in small fractures and shears. They are characterised by very coarse grained (5-10 cms) quartz sulphide pods with small patches of garnet sandstone and others consisting of epidote, garnet, quartz and

clinozoisite included. 40.

The veins observed by the author on No. 27 level 9 at No. 3 shaft, North Broken Hill Limited, were developed only 20 m from the Globe Vauxhall shear zone and are possibly related to this major structural feature .

These veins have been noted previously by Lawrence (1968) (termed the metahydrothermal veins) and Maiden (19'72) (termed the secondary hydrothermal veins).

2.2.9 Mt. Robe, Black Prince, Golden Crest, Silver King and Mayflower

This group of mines characteristically lacks a siderite gangue. The Mt. Robe, Mayflower and Britisher deposits are characterised by a quartz-fluorite gangue whereas the Golden Crest, Black Prince and Silver King are characterised by a quartz gangue with rare fluorite. These represent a different group of deposits to the other Thackaringa-type deposits.

The Mt. Robe deposit is the largest deposit in this group and a simplified geological plan of the area is presented (Fig. 17).

A flat-lying, thick sequence of andalusite schist interbedded with a fine grained metadolerite is intruded by a dome-like biotite-muscovite-albite-quartz pegmatite. The andalusite schist sequence dips towards the southwest at approximately 10° and strikes due north. The mineralisation occurs in two narrow fissure zones dipping steeply towards the southwest at 60° and striking at 325°. One of the adits is still accessible and some observations of ore/wallrocks were made. The mineralisation consists principally of galena and cerussite and occurs in veins 1.5 m wide. The quartz and multi-coloured fluorite gangue is complexly intergrown with galena. Wallrock contacts are sharp Fig. 17

Simplified geological plan of the Mt. Robe mines area. 41.

camp

KEY J amphibolite

j pegmatite

] andalusite schist

quartz veins

SCALE % triq station 1 0 0 m m 3 zs.

Figure 17 42.

and all lithologies mentioned are in contact with ore at different places. Brecciation of gangue and deformation and remobilisation of galena observed in other Thackaringa-type deposits is absent in this deposit.

Anderson (1966) found that in the quartz-rich phases of the large dome-shaped pegmatite, fluorite and rarely sulphides-'are present, e.g., chalcopyrite, pyrite, arsenopyrite, galena and sphalerite. These quartz-rich phases represent the volatile parts of the intrusion and, if injected into fractures, would have a very similar form to the Mt. Robe deposits.

Another quartz-fluorite vein-type deposit occurs 1.5km to the west of Mt. Robe i.e. Mt. Eltie.

The Silver King, Golden Crest, Consolation and Black Prince all lie in similar stratigraphic positions to the Mt. Robe deposits. They are present in fractures around the margin of the same large pegmatite dome and have similar host rocks to the Mt. Robe area i.e. andalusite schist and metadolerite. The metadolerite is only slightly affected by metamorphism and the ophitic textures of these clinopyroxene hornblende-rich rocks are well preserved. The much lower grade of metamorphism present in this area may explain the less complex undeformed nature of the veins of these deposits.

The Britisher, Diana and Mayflower are located 15 km ESE of the Mt. Robe deposit and similarly have a quartz- fluorite gangue. The Britisher vein is completely surrounded by a granitic gneiss and although there is no direct spatial association with Broken Hill-type deposits, they are present 2 km to the north (Barnes, pers. comm.).

.* The Mayflower vein occurs in a fissure 1 m wide and a gangue of alternating bands of quartz and fluorite. 43.

In summary, the deposits in this group are characterised by a quartz - fluorite gangue with the absence of siderite. They occur in fissure zones (distinct from shear zones), are relatively undeformed and traverse lithological layering trends.

2.3 Classification of the Thackaringa-type Deposits

The present field'"study of Thackaringa-type deposits undertaken has delineated a number of distinct types of Pb-Ag deposits. These have been classified into three groups on the basis of field relations and gangue mineralogy differences.

Group 1 deposits are simple quartz-siderite sulphide veins which are present to the west and northwest of Broken Hill, e.g., Thackaringa, Umberumberka, Apollyon Valley, Daydream, Terrible Dick, Barrier Chief, Orient, Pickup and Lubra. They have no observed spatial association with Broken Hill-type deposits. Most of the well-known Thackaringa-type deposits belong to this group.

Despite a similar gangue mineralogy, these deposits exhibit great variability in host and structural environment.

Group 2 deposits include the quartz - fluorite sulphide veins e.g. Mt. Robe, Britisher, Diana, Mayflower and other closely related quartz-sulphide vein deposits e.g. Black Prince, Golden Crest, Consolation and Silver King. Siderite is absent from these deposits and these veins occur in fissures or fractures instead of the shear zone association of Group 1.

Group 3 deposits are similar to Group 1 deposits. They contain a quartz-siderite-sulphide mineralogy but have a close spatial association with Broken Hill-type deposits. These include the simple quartz-siderite sulphide veins at Maybell, Hidden Secrets, Koh-i-Nor, Gem, Brilliant, Florida, veins at the Angus mine and at Piesses Nob, and also the complex sulphide veins of the ABH Consols, East Consols and the British shear veins. These deposits occur in shear zones which have 44.

traversed Broken Hill-type mineralisation in most cases less than 1 km from the Thackaringa-type deposits.

These three groups plot into well defined areas (Plan 1). Group 1 deposits are present in the western and north-western segments of the Willyama Complex. Group 2 deposits are limited to the northern area near the Torrowangee group and Group 3 deposits are present in areas of Broken Hill-type mineralisation i;e. the eastern two-thirds of the Willyama Complex.

2.4 Summary

(a) Broken Hill-type mineralisation shows the following characteristics:-

- massive in form (coarse grained) and diverse mineralogy. - stratabound - present in areas of high grade metamorphi'sm (commonly granulite facies terrains) - major shear zones occur near these deposits - found predominantly in the eastern half of the Willyama Complex - a common host lithology in all Broken Hill-type deposits. - enclosed by a hydrothermal alteration zone

(b) Thackaringa-type deposits have the following characteristic features:-

- vein-type form with a simple mineralogy - commonly present in shear zones but also in more simple fractured zones - no common host lithology as with the Broken Hill-type deposits - present in lower grade metamorphic rocks than Broken Hill-type deposits though exceptions do exist e.g. Champion and Umberumberka - no contact or hydrothermal aureoles - shear zones that contain Thackaringa-type mineralisation 45.

have similar trends to local layering and S2 attitudes - these deposits commonly occur in positions near the fold axes of major F2 antiforms - these shear zones have similar trends to axial planes of F2 antiformal structures which they are superimposed on and they are interpreted as longitudinal faults genetically associated with the axial plane zone of weakness of the anticline

(c) Field data and gangue mineralogical differences have shown that there are three different groups of deposits. These were originally classified collectively as Thackaringa-type mineralisation.

(d) Coplanar relationship between metamorphic isograds and layering. CHAPTER 3

MINERALOGY AND MINERAL CHEMISTRY 46.

3. Lawrence (1967) described the primary ore mineralogy of sulphides from the Pioneer Mine at Thackaringa. He showed that the mineralogy of the ore is simple and has many counterparts elsewhere in the world.

Lawrence (1968) and Markham and Lawrence (1962) described the silver-rich veins of the British Shear and the ABH Consols lode.

To substantiate any differences established by the field studies within the Thackaringa-type deposits, a detailed sampling program of dumps and a mineragraphic study was undertaken.

3.1 Vein Mineralogy

The vein mineralogy has been subdivided into the following zones:-

(a) Oxidation zone (b) Supergene zone (c) Primary zone

In the Thackaringa-type deposits (except for Umberumberka), a deep oxidation zone is developed. The supergene zone is poorly developed, and there is leaching and depletion of ore mineralisation rather than enrichment in this zone.

3.1.1 Oxidation Zone

The oxidation zone extends from a depth of 12 m to 50 m although, primary ore minerals have been observed within one metre of the surface. The following secondary minerals have been observed in the oxidation zone.

Pyrolusite (MnC>2) has derived from the breakdown of manganoan siderite under strongly oxidising conditions. It occurs in three forms:-

«•

(a) As well developed fibrous radiating crystal aggregates found in the crevasses of limonite which pseudomorph siderite. 47.

(b) As coatings on the skeletal lattices of cerussite completely masking the mineral beneath.

(c) As dendritic forms trapped in fractures commonly a long distance (approximately 30-40 m) away from mineralisation, e.g., Thackaringa mines. It appears from this that manganese is a very mobile element and possibly could be an exploration tool in the search for these deposits.

Pyrolusite is absent from the Mt. Robe, Black Prince Golden Crest, Consolation and Silver King deposits. It is understood to have resulted from the breakdown of manganoan siderite and hence is absent from the Group 2 deposits.

Limonite-goethite is the most common mineral in the oxidised zone resulting predominantly from the breakdown of siderite. In hand specimens it commonly occurs in the form of rhombohedral intergrowths after siderite. Boxwork limonite after galena is common especially along galena cleavages. Fine silky fibrous radiating rosettes of goethite occur rarely. Hematite is common alteration product of goethite and with pyrolusite form fine rims enclosing the earlier mineral Geothite is rare in the Group 2 deposits.

Hematite is a minor mineral in the oxidation zone and occurs as rims around galena and as aggregates associated with covellite. Three types of galena- hematite-cove Hite associations were observed (Fig. 18)

Cerussite is the common oxidation product of galena in the Thackaringa-type deposits. It is present as reticulated twin aggregates and fine grained masses, associated with bromo-chloroargyrite and copper bearing minerals. Cerussite is commonly coated by pyrolusite. Fig. 18

Hematite-covellite-galena associations in the Thackaringa-type Group 1 deposits .f not fo scale^ 48.

galena

intergrowth of

v > V covellite and

hematite (b) \rx/yy/\ *

galena

covellite

(c)

Figure 18 49.

Anglesite is associated with cerussite and is present as small encrustations which pseudomorphs galena. Anglesite was identified at the Pioneer mine by x-ray diffraction and, with barite from Unberumberka, represents the only sulphate-bearing mineral in the Thackaringa-type deposits.

Bindheimite Pb2Sb2°6 ^°' 0H^ ' is a Yellowish earthy massive miheral identified by x-ray diffraction. It was reported by Smith (1926) from the Daydream mine and the ABH Consols. In the oxidised zone of the Daydream mine, which is rich in antimony ores (Table 6), large aggregates weighing over one tonne were extracted, consisting of "canary ore". Bindheimite is absent from the Broken Hill main lode oxidised zone.

Table 6 Assay of the antimony-rich ores from the Daydream mine (after Smith, 1926).

PbO 43.09 47.03 Sb2S3 CaCO_ 5.0 0.2 h20 5.9 101.25

Copper Minerals Azurite, Cu^ (OH) 2 (CO^)^ is present in most of the Group 1 deposits but is not as common as malachite.

The deposits with a close association with amphibolites, e.g., Black Prince, Golden Crest, Silver King have strongly developed copper mineralisation (up to 30% Cu). In other areas where no amphibolites are present, copper mineralisation occurs to a lesser extent, e.g., Terrible Dick, Hen and Chickens, Daydream, Apollyon Valley. 50.

Other rarer copper minerals include cuprite, chrysocolla and atacamite (observed at the Umberumberka East mine). It is associated with pyrite and minor galena in a quartz-dominated vein. These pyrite-quartz veins have been classified as Thackaringa-type but are different in that they lack strong base metal mineralisation and siderite gangue. They occur sporadically throughout the Broken Hill region ancTare very similar to the Thackaringa-type veins at Maybell.

Silver Minerals Silver halides are present at the Thackaringa mines and at the Daydream workings. Using a method determined by Barclay and Jones (1971), the compositions of the Thackaringa-type silver halides are shown on a ternary diagram (Fig. 19). The silver chlorides from Thackaringa-type deposits are similar in composition to the Broken Hill main lode oxidised zone.' Iodoargyrite reported by Smith (1926) from ABH Consols was not recorded in the Broken Hill main lode (Barclay and Jones, 1971) or the Thackaringa-type deposits.

Chlorargyrite is reported by Smith (1922) from most of the Thackaringa-type deposits but was not recorded in the present study.

One analysis of a silver halide from Smith (1926), Table 7, is obviously chlorargyrite (AgCl), but this analysis only recorded all Ag contents as AgCl, whereas AgBr and Agl were not analysed.

Table 7

AgCl 72.23 MgC03 3.75 Insoluble 9.7 Unident 0.85 FeCO 4.2 100.0% 3 A1 2.3 CaCO 6.4 3 51.

Bromo-chlorargyrite, identified in this study, occurs in three principal forms:-

(a) Developed along cleavage places in oxidised galena (Fig. 20).

(b) Cubic crystals in vughs.

(c) As a' core surrounded by cerussite and galena, and not structurally controlled.

Barite (BaSO^) is recorded by Dudley (1894) from the Umberumberka East and Atlas Mine at Purnamoota. This rare mineral was not observed by the author.

Aragonite (CaCO^) was observed at the Hercules Mine (Thackaringa area) and occurs as coating or crusts on calcite and quartz crystals. Identification was by x-ray diffraction.

Element Mobility in the Oxidation Zone The oxidation zone of the Thackaringa-type deposits involves the relative enrichment of the following elements:-

Principal Mineral Pb ■> Cerussite, Bindheimite Ag Bromo-chlorargyrite, Chlorargyrite Sb Bindheimite Cn -> Malachite, Azurite, Chysocolla Fe -*• Goethite, Hematite, Limonite

The enrichment of silver is probably produced by the breakdown of galena to cerussite in the oxidation zone. Cerussite has a lattice structure which does not accept silver so that the silver released from the breakdown of galena forms silver halide.

The galena is poor in silver (Hagarty, 1974) and it is Fig. *9

Trinagular plot of silver halides from Thackaringa-type (+) and Broken Hill-type deposits (0), after Barclay and Jones, 1971).

Fig • 2o •

Break-down of galena to Bromo-chlorargyrite malachite, cerrussite and covellite. 52.

Iodargyrite \ Agl

Chlorargyrite $romargyrite

AgCl Bromo-chlorargyrite Ag(Br.CI)

Figure 19

covellite replacing -- galena

malachite closely -galena (resorbed core) associated with bromo-chlorargyrite cerussite

-bromo-chlorargyrite

Figure 20 53.

thought that silver-rich tetrahedrite (which is commonly exsolved in galena) is broken down in the oxidation process.

The enrichment of other elements, e.g., Cu and Sb, is probably related to the breakdown of tetrahedrite

The enrichment of Fe in this zone is due to the breakdown bf siderite.

In the oxidised zone the following elements are depleted

Zn - no sphalerite As - no arsenopyrite, arsenical pyrite or any other As bearing minerals

3.1.2 Supergene Zone

Two supergene minerals were observed; covellite and neodigenite-chalcocite series minerals. In the oxidised zone these minerals are abundant.

Covellite CuS is present in all Thackaringa-type deposits studied. Three forms have been observed

(a) Rimming, pseudomorphing and uncommonly, completely replacing galena (Fig. 21). Chalcocite-neodigenite and covellite are involved in the replacement process (Fig. 22).

(b) As a rim enclosing sphalerite and chalcopyrite

(c) Covellite commonly occurs as an intergrowth with neodigenite.

.* Neodigenite was identified by its light blue colour and isotropism and occurs as an intergrowth with covellite and rims around chalcopyrite (Fig. 23). Fig. 21

Complete concentric replacement of galena by supergene covellite with galena cleavages preserved. ( X25)

Fig. 22 54.

Figure 21

Figure 22 55.

Element Mobility in the Supergene Zone The supergene enrichment zone is characterised by the depletion of some of the common base-metal elements, e.g., Pb, Zn, Ag and the enrichment in Cu. This can be explained by a number of possibilities:-

(a) The release of copper from the breakdown of tetrahedrite exsolved from galena in the oxidation zone'.'

(b) The breakdown of chalcopyrite in the oxidation zone.

(c) The breakdown of chalcopyrite exsolved from sphalerite.

(d) Copper from adjacent metasediments, e.g., amphibolite.

These will be discussed in a later section.

3.1.3 Primary Zone

The primary ore mineralogy of Thackaringa-type Group 1 deposits is similar to the descriptions by Lawrence (1967) for the Pioneer Mine. The Group 2 deposits, not described by Lawrence (1967), are different in mineralogy and mineral chemistry from the Group 1 deposits. The primary ore minerals of both groups are listed in order of abundance.

Galena Galena is common in all the Thackaringa-type deposits. In the Group 1 deposits it is present as deformed, relic masses rimmed by pyrite and covellite. Exsolved tetrahedrite and chalcopyrite, pyrite and sphalerite are common inclusions.

In areas of low stress, galena forms octahedral crystals 56.

(Fig. 24). In the Group 1 deposits these are rare and stressed galena "tails of lead" (Bleischweif) are more common.

Galena is one of the last sulphide phases to crystallise and encloses many primary phases including arsenopyrite, pyrite and sphalerite (Fig. 25).

Galena is'concentrically oxidised to cerussite, covellite and neodigenite (Figs. 26 & 27).

Chalcopyrite and galena are closely associated and the former appears to crystallise first. In some sections where galena envelops chalcopyrite, a complex unmixing effect results (Fig. 28) and they appear to have coprecipitated. This effect is also observed in the assemblage galena-native silver.

Galena appears predominantly in the carbonabe phase of the gangue. Rarely it does occur with quartz but this is due to later remobilisation and is present as a skeletal matrix around spaces in the quartz structure.

In contrast, Group 2 galena is undeformed and this is illustrated by two etched examples of galena from Group 1 and 2 deposits (Fig. 29). Exsolution of tetrahedrite in galena (the most important source of silver from the Group 1 deposits) is absent in the Group 2 deposits. No tetrahedrite of any form was observed from these deposits. Covellite is a common replacement product from galena.

Mineral Chemistry Galena from sixteen deposits was analysed from Ag, Bi and Sb by electron microprobe analysis. The Group 1 and Group 2 galena are distinguished in the following ways:-

(a) Ag and Bi content of galena is significantly higher in the Group 2 deposits (Fig. 30). No exsolved Fig. 23

Brecciated chalcopyrite (light grey) replaced by neodigenite (medium grey) . (X23)

Fig. 24

Octahedral galena crystals in small vughs of siderite. Lubra. (X I 57.

Figure 23

.*

Figure 24 Fig. 25

Sphalerite (medium grey) replaced by galena (light grey) and surrounded by a siderite gangue (dark grey). Galena is distorted.

( x 25) .

Fig. 26

Corrosion of galena along cleavages by oxidation, (x \-^o) ■ 58.

Figure 25

Figure 26 Fig. 27

Concentric breakdown of galena (light grey) in the oxidised zone.

Pig. 28

"Unmixing" between galena and chalcopyrite. (x \Ao). 59.

Figure 27

Figure 28 Fig. 29

Examples of etched galena textures from:- -{-to secaJe^.

(a) Group 1 deposits

(b) Group 2 deposits

(etching by H Br with slight repolishing) 60.

(a)

(b)

Figure 29 - etched galenas Fig. 30

Plot of the variation in molecular % Bi2S3 molecular Ag^S% in PbS. 61.

Molecular °/o

X Group 1 Deposits

O Group 2 Deposits - 2-5

Black Prince

Golden Crest

Thackaringa

■■ 0-5

Mt Robe

Molecular % Ag„S

Figure 30 62.

Ag and Bi phases were found and it is suggested that this represents galena-matildite solid solid solution. Galena-matildite solid solution commences at 215 - 15°C (Ramdohr, 1938) with up to 30% Ag, Bi S2 in PbS and 15% Ag Bi S2 in PbS at 250°C. Above 350°C solid solution is complete (Craig, 1967). In the Group 1 galena, Ag and Bi are low.

(b) In the Group 1 deposits there is a close correlation between Ag and Sb in galena. This is not present in the Group 2 results (Fig. 31). This suggests that tetrahedrite is the only Ag-bearing phase and is present as an exsolved submicroscopic phase. This is also supported by the average Ag/Sb ratio for the Group 1 galenas of 0.80. This compares with the Ag/Sb ratio for analysed tetrahedrite from the Group 1 deposits of 0.79. Group 2 galenas have an average Ag/Sb ratio of 19.1 (maximum 54) thus having no relationship to tetrahedrite exsolution.

This further suggests that there is negligible solid solution of Ag2S in the PbS structure described by Ramdohr (1969) . A92S solution in PbS is possible up to 0.1% Ag2S.

Sphalerite In the Group 1 deposits, sphalerite occurs as massive blocks which are commonly brecciated and fracture filled with galena. Rims of chalcopyrite, pyrite and covellite are very common. Chalcopyrite exsolution and inclusions of pyrite and pyrrhotite are common in sphalerite. This exsolution has a number of different forms

(a) Exsolved along cleavage planes (Fig. 32).

(b) Exsolved independently of any structural features (Fig. 32). Fig. 31

Ag and Sb concentrations in galena from the Group 1, 2 and 3 deposits.

Abbreviations D.D. - Daydream A. V. - Apollyon Vatley T. D. - Terrible Dick U. M. - Umberumberka B. S.H. - British Shear 63.

4000 ■

••300

2000

Britisher Golden Black Mt Black Crest Prince Robe Prince a9 GROUP 2 DEPOSITS Sb

*—

Orient Koh-i D.D. A.V. Tom T.D. U.M. Lubra BSH Thackaringa -Nor (Bl.10) Thumb veins (A.V) GROUP 1 & 3 DEPOSITS

Figure 31 64.

(c) Exsolved under slight deformation forming lines of distorted chalcopyrite (Fig. 33).

(d) Exsolved and migrated towards the grain boundary.

(e) In zonal arrangements around inclusions in sphalerite, or in undefineable zones and bands (Fig. 34).

Lawrence (1967) suggested that this zoning is due to differences in composition of later generations of sphalerite. While this may prove to be so, some of the observable zoning is not associated entirely with this process and could result from migration of grains into defined areas or zones (Ramdohr, 1969).

Rarely Type (a) exsolution shows different episodes of deformation with the former exsolution patterns being deformed and later ones undeformed. (Fig. 35).

Sections of sphalerite with no exsolution of chalcopyrite are commonly rimmed by chalcopyrite or pyrite. This may be due to migration of exsolved grains during slow cooling,

Inclusions of sulphides (expecially galena) in sphalerite are surrounded by a nucleation of exsolved chalcopyrite grains. Sphalerite is associated with many mineral phases including chalcopyrite, galena, pyrite, tetrahedrite and arsenopyrite (Fig. 36).

In the Group 2 deposits, sphalerite commonly forms blebs with no exsolution. Inclusions of anhedral galena are common and rarely pyrrhotite inclusions are present.

Sphalerite is only associated with galena and no other mineral phase.

Mineral Chemistry Sphalerite from six Thackaringa-type deposits (four from Fig. 32

Exsolution types (a) and (b) of chalcopyrite in sphalerite closely associated. (x Mcd) .

Fig. 33

Type (c) exsolution of distorted lines of chalcopyrite in sphalerite, (x Mo'). 65.

Figure 32

Figure 33 Fig. 34

Nucleation of exsolved chalcopyrite around an inclusion of galena (light grey) in sphalerite. l-4o^ .

Fig. 35

Two generations of chalcopyrite exsolution in sphalerite. bo . 66.

Figure 34

Figure 35 Fig. 36

Step-like contact between sphalerite (medium speckled grey) and arsenopyrite (white), (x 2rOo). 67.

Figure 36 68.

Group 1, one from Group 2 and the metahydrothermal veins) were analysed for Sb, Cd, Ag, Bi, As, Fe, Mn and Cu by electron microprobe techniques.

Fig. 37 presents the data for FeS and MnS, wt% Cd and As for the Thackaringa-type and Broken Hill-type deposits. The chemical data has shown that:-

(a) There is a close correlation of values for FeS, MnS and Cd (wt%) between Broken Hill-type and Group 2 deposits.

(b) There is an inverse relationship between FeS and MnS and sulphur fractionnation or absolute & 34g (data from Both and Smith, 1975). For example, Terrible Dick sphalerite has the highest absolute

(c) There is a close association between sphalerite from the metahydrothermal veins in the main lode and the main Broken Hill lode, particularly with FeS contents.

(d) Similarities of sphalerite from the Broken Hill type deposits, Group 2 deposits and metahydrothermal veins indicates similar origin.

(e) Copper contents are the result of the degree of exsolved chalcopyrite in the probe area analysed.

Pyrite Two forms of pyrite were observed. Pyrite 1 consists of relic, brecciated bands or aggregates, some with moderate anisotropism (bluish) similar to arsenopyrite (Fig. 38). It is commonly fracture-filled by galena and tetrahedrite. Pyrite 1 occurs as inclusions in sphalerite, galena and chalcopyrite. Pyrite 2 is later than pyrite 1 and occurs as veinlets cutting across chalcopyrite and pyrite 1 (Fig. 39). It is distinguished Fig. 37

Geochemical data for sphalerite. The following deposits are abbreviated:-

T.D. Terrible Dick Um. Umbe rumberka Thack. Thackaringa M.V. Metahydrothermal veins 69.

B.H. T.D. main lode

Um. Thack. Britisher M.V l l j—i---- ♦------f. 4- 2 J . 0 2 4 6 8 10 12 14 16 % FeS

T.D.

Um. B.H. B.H.-type Britisher M.V. main (ode deposits Thack.

—t------f— 0-2 0-4 0-6 0-8 10 1‘2 1-4 %> MnS

B.H.-type deposits Thack. M.V Britisher Um. T.D.

—i— jL_ —4------±-----*----- 1000 2000 3000

Cd (ppm)

M.V. Britisher Thack. Um. T

y i V —I— 100 200 300 As(ppm)

Figure 37 Fig. 38

Brecciated band of pyrite I (light grey) from Umberumberka. (x 2S) .

Fig. 39

Veinlets of pyrite II (light grey) filling in voids between siderite grains. Thackaringa. (x 2.5) . 70.

Figure 38 71.

from pyrite 1 by its form and isotropism. Pyrite 2 commonly encloses galena and sphalerite.

A different form of pyrite to 1 and 2 occurs as euhedral, isotropic grains (enclosed by covellite) in massive chalcopyrite (Fig. 40). It is suggested that this has formed by the reaction:-

Chalcopyrite---> Covellite + Pyrite 2 Cu FeS^ 2 Cu S + FeS2 + Fe ;'-nto oxides

In the Group 2 deposits, pyrite 1 is present as anisotropic idiomorphic grains and, unlike the Group 1 deposits, is rarely brecciated. Galena is a common inclusion.

Pyrite 2 has not been observed in the Group 2 deposits.

Mineral Chemistry Pyrite from several Thackaringa-type deposits was analysed for Ag and As. One possible explanation for the anisotropism of Pyrite 1 is the variations on arsenic content (Ramdohr, 1969).

Pyrite 1 contains a considerable variation of As across single anisotropic grains (Figs. 41 & 42).

Pyrite 1 samples from other mines show similar trends, e.g., Black Prince 41-504 ppm, Britisher 34-246 ppm and Lubra n.d. - 804 ppm. Other deposits, conversely, show little variation in As content across pyrite 1 grains, e.g., Umberumberka 60-70 ppm and Orient n.d.

Chalcopyrite In the Group 1 deposits chalcopyrite commonly contains inclusions of sphalerite, galena, tetrahedrite and pyrite. Lamellar twining is present only in large masses of chalcopyrite.

Chalcopyrite rarely rims sphalerite, but more commonly Fig. 40

Idiomorphic pyrite in covellite (bluish and mauve) surrounded by chalcopyrite. (x '40s)

Fig. 41

Assymetrical variation of As content across a pyrite I grain

(outlined) . Thackaringa mines. CMo-o^) 72.

Figure 40

I

Figure 41 73.

it shows a number of different forms when exsolved in sphalerite. Chalcopyrite readily breaks down to neodigenite and covellite.

The textural relationship between chalcopyrite and galena (Fig. 28) indicates that both of these phases are coeval.

Cubanite, & common exsolution product of chalcopyrite above 250°C was not observed. This was noted as a minor accessory in the ores at the Pioneer by Lawrence (1967).

Chalcopyrite is a minor phase in the Group 2 deposits.

Minor Chemistry Chalcopyrite from five Thackaringa-type deposits was analysed for Sb, Cd, Ag, Bi and As (Fig. 43).

There is a direct correlation between Ag, Sb and As. Bi is high (0.06% Bi) and does not vary with different deposits.

No chalcopyrite was analysed from Group 2 deposits.

Arsenopyrite In the Group 1 deposits, arsenopyrite is a common accessory mineral. It occurs as trails of idiomorphic grains and is very rarely massive. Inclusions of gangue, galena and tetrahedrite are common. Zoning and cruciform twining have been observed in some of the Thackaringa lodes (Fig. 44).

An electron microprobe scan across the zoned grains was carried out to determine if there is any increase in As content towards the grain margin. The traverse showed that there are a number of bands of high As which are correlated with light bands in the zoning structures (Figs. 45 & 46). Fig. 42

Symmetrical variation of As content across a pyrite I grain

(outlined). Terrible Dick. 2.$) ■

Fig. 43

Microprobe analyses of chalcopyrite for Sb, Bi, As and Ag.

Abbreviations A.V. - Appolyon Valley TH - Thackaringa OR - Orient Um. Umberumberka 74.

\s 0*4-

0-2-

As Sb Ag Bi " 400 800 2000

200 400 1000 ■-

AV TH OR AV TH UM 133 1 164 2

Figure 43 Fig. 44

A zoned cruciform twinned arsenopyrite grain (X-nicols) , Thackaringa. ^ V 14b).

Fig. 45

A zoned arsenopyrite grain (X-nicols) showing the trace of the step-scan analysis of all major elements. (See Fig. 46). Thackaringa. (x >4o). 75.

Figure 45 Fig. 46

Plot of the variation of As, Fe and S across a zoned arsenopyrite grain (see Fig. 45). Thackaringa. STEP SCAN ACROSS A ZONED

ARSFNOPYRITE GRAIN

As,fe,S(9

core zoning gram bands

distance(30/x between points) S *■——-® As------

Fe *------* Figure 46 77.

In the Group 2 deposits, arsenopyrite is massive. No twining, zoning or the characteristic form of trails of idiomorphic grains in the Group 2 deposits were observed.

Major element analyses of arsenopyrite were undertaken from five deposits. The As concentration in arsenopyrite within each deposit showed considerable variation." This means that any temperature considerations on the basis of As content in arsenopyrite (Barton, 1969) are unreliable (see Chapter 4).

Tetrahedrite Tetrahedrite has been observed in only the Group 1 deposits. It occurs in three forms

(a) As exsolution along cleavage traces in galena (Fig. 47). Other types of exsolution appear to have no relation to cleavage traces or any structural features.

In some of the Group 1 deposits, e.g., the Pioneer, tetrahedrite is brecciated by later tectonic activity, whereas the surrounding galena is plastically deformed (Fig. 47).

(b) Tetrahedrite is present less commonly as massive areas with inclusions of galena, sphalerite, chalcopyrite and pyrite (Fig. 48). This form of tetrahedrite is rimmed by covellite and is common only at Umberumberka and the Pioneer Mine.

(c) A rare third form has been noted where tetrahedrite occurs as small veinlets (Fig. 49) in the gangue, filling brecciated cracks in pyrite and rimming galena (Fig. 50). This is probably a result of migration of exsolved tetrahedrite (type (a)) out of galena into veinlets and rims. A

No tetrahedrite was observed in Group 2 deposits. Fig. 47

Exsolution of tetrahedrite from galena. The exsolved phase is brecciated^whereas the host is remobilised. Thackaringa.

(x Z5Q) .

Fig. 48

Massive tetrahedrite rimmed by native silver and with inclusions of chalcopyrite and galena. Umberumberka. (x 2.5^. 78.

Figure 48 Fig. 49

Veins of tetrahedrite. Apollyon Valley, (x l4o^).

Fig. 50

Galena rimmed by tetrahedrite. British Shear Veins. (x zs). 79.

Figure 49

Figure 50 80.

Mineral Chemistry Tetrahedrite from four Group 1 deposits was analysed for the following major elements: Sb, As, Ag, Cu, Fe, Zn and S.

The tetrahedrite analysed is characteristically low in As (less than 3% at. wt. As). The Thackaringa Group 1 tetrahedrite is high in silver (up to 14.5% at. wt. AgT. This high Ag content in tetrahedrite 2+ represents up to 50% substitution for the Cu ion in the tetrahedrite lattice.

Cation distribution diagrams (Fig. 51) show that the 2+ 2 + cations Ag and Cu are extremely variable whereas 2+ 2+ Zn and Fe are relatively constant.

The two tetrahedrite samples from the Apollyon Valley

2+ 24- show great variation in Cu and Ag but other deposits analysed gave consistent results within each deposit, e.g., Thackaringa and Daydream.

Pyrrhotite Pyrrhotite is a rare mineral in the Thackaringa-type deposits and was observed in only the Pioneer, Umberumberka and Britisher workings.

In the Group 1 deposits, pyrrhotite occurs with pyrite and chalcopyrite as late stage xenoblastic forms (Fig. 52). Inclusions of pyrrhotite are present in sphalerite associated with pyrite and chalcopyrite. This type was observed at all three localities.

One sample of pyrrhotite from Umberumberka was analysed for the author and has the chemical formula FeS (Table 8). This is the low temperature monoclinic form of pyrrhotite. Fig. 51

Cation distribution diagrams for major elements of tetrahedrite in the Group 1 deposits. 81.

As ,Sb

As, Sb

Zn, Fe Fig. 52

Pyrrhotite surrounding pyrite. Thackaringa. (y \ 46) . 82

Figure 52 83.

Table 8 Analysis of monoclinic pyrrhotite by electron microprobe analysis.

Fe 61.53 Ni 0.20 Co 0.02 Cu 0.20 Zn •• bid* Mn bid Cd bid As 0.51 S 37.55 99.99

* means below detection

Assuming the pyrite, pyrrhotite and sphalerite' aggregates are in chemical equilibrium, estimates of sulphur activities can be obtained.

At the Britisher (Group 2 deposits), sphalerite with 12% wt. FeS is present with pyrite and monoclinic pyrrhotite, temperatures of ore formation corresponding to 250°C and aS1- of 10 15 are interpreted (Barnes, 1972).

With the Group 1 deposits, FeS content is very low so that 2 temperatures would be much lower with corresponding aS . . -18 of 10

Marcasite Marcasite is reported by Lawrence (1967) in one section from the Pioneer Mine. It was observed rarely by the author in one section of the Umberumberka Mine as elongate hackly crystals.

Native, Silver and Ruby Silver Minerals Native silver has been observed by the author at 84.

Umberumberka, Daydream, Terrible Dick and the British shear zone veins. It is associated with tetrahedrite and/or galena in the form of rims and veinlets resulting from the breakdown of argentian tetrahedrite (Fig. 48). This is the only native element noted in the Thackaringa-type deposits and is a late stage crystallising phase.

In the British shear zone veins, native silver is very common and is closely associated with silver arsenides and antimonides, galena and tetrahedrite. A list of silver primary minerals present at the British shear zone veins is documented by Lawrence (1968).

The rare ruby silver minerals, e.g., pyrargyrite, stephanite and dyscrasite were not observed by the author at any of the Thackaringa-type deposits. These minerals have been recorded by Jaquet (1894) and Smith (1922) from the Umberumberka, Daydream and ABH Consols lodes. .

It is suggested that the great proportion of silver observed commonly in the oxidation zone of Group 1 deposits is due to the breakdown of argentian tetrahedrite.

In the Group 2 deposits, tetrahedrite is absent. The source of silver in these deposits is matildite solid solution in galena which is released when galena oxidised to cerussite at the surface.

3.1.4 Primary Gangue Mineralogy

Quartz Quartz is ubiquitious in all the Thackaringa-type deposits. The Britisher shear vein is the only exception which has Fe-poor siderite and rare crystals of quartz.

.* In the Group 1 deposits, quartz is present in two forms, as milky to colourless zoned comb crystals (with up to five zones in the one crystal) and as small crystals 85.

embedded in the siderite (Fig. 14). The latter of these two forms is probably the younger and is coeval with the siderite.

In the Group 2 deposits quartz occurs as massive intergranular aggregates with fluorite. This form was not observed in the Group 1 deposits.

Siderite ** Siderite is present only in the Group 1 and Group 3 deposits as massive intergranular grains interlocked with quartz.

Siderite develops well formed trigonal crystals in contact with later crystallising galena or in vughs.

Calcite Calcite is present in one Thackaringa-type deposit (Hercules). This deposit occurs in a large amphibolite which is the possible source of this mineral.

The calcite is course grained and well crystallised, and in places is coated with aragonite. Secondary crystal overgrowths are common. Colours of the calcite vary from colourless to pink. The cores of the crystals are pinkish and commonly fluoresce orange under UV light and this is probably a result of a concentration of Mn.

Graphite Graphite is common in a number of Thackaringa-type Group 1 deposits, (Umberumberka, Apollyon Valley Mines and Terrible Dick).

It occurs as sheaths and scallops closely associated to shear schistosity directions and mineralised veins. .* Analyses of d002 spacings of graphite using a method developed by McKirdy (1975) shows that the graphite has been formed under similar metamorphic grade to the 86.

shear zones and not the lower temperature vein-type deposits. This would correspond to chlorite-chloritoid assemblages observed in shear zones.

Fluorite Fluorite is present as a major gangue mineral in the main Thackaringa-type Group 2 deposits (Mt. Robe and Mayflower) and to a minor extent in the Golden Crest and Black Prince mines.

It occurs as intergranular aggregates with quartz and is variably coloured (e.g., colourless, green and purple). In one deposit (Mayflower) a banded layering is common between quartz and fluorite.

Fluorite has not been observed in Group 1 and Group 3 deposits.

3.2 Distribution of Minerals

Table 9 shows the minerals observed in this study combined with the work of Markham and Lawrence (1962), Lawrence (1967) and Lawrence (1968).

Very few deposits have the full suite of primary minerals described by Lawrence (1967) in the Pioneer. This is probably due to a lack of primary ore on the dumps and also due to the shallow depth mined. Most of the deposits were only mined in the oxidised zone. The Thackaringa mines, Umberumberka, Daydream, Terrible Dick and New Year are the only exceptions.

Minerals which are very common to Group 1 and Group 3 deposits, e.g., tetrahedrite, pyrite, arsenopyrite, chalcopyrite, silver chlorides, native silver, siderite and goethite were either rare or absent in the Group 2 deposits.

.* Conversely, fluorite observed in Group 2 deposits is absent in Group 1 and Group 3 deposits. Shaft Apollyon Umber- Terrible Mt. Britisher British ABH Koh- s P- (B t< fB 3 P* Thackaringa Daydream Valley umberka Dick Robe Mayflower Shear Consols I-Nor Orient I I I i I CD P M 3 P (B *

I

I i I I tn P fB P P- 3 P* ft (B P 3 3 >d

I l i I t P P- rt (B

PC 3 P •

| I

I P

Ui o (B rt 3 P •

-

| I I

O W CB H P- 3 cl (B 3 t) PC P •

J

I I S' P P- rt fD CB M 3 P- P • 3 >P

I I < n o cb P- M M rt rD

1 I cb 23 CB 0 P P- H- rt 3 CB 3 *P yQ P 3 ) K 0 p- rt rt (T> I 11*1111111 O P- » s rt CB

I f o P- rt CB 3

LB

I I I 3* i-< P- rt cb P (B

I H- P> 3 rt CD

-

I cn h ft CB

I

CB p- ft v£) *<3 P

I I p 5 P- rt- CB

I I CB <3 Ui p- H CB h{

I Fu wrta>i-

4

I 111111*1 W 3 MH<3'MP3p 3 P- H rtPJPr+^pCfl) (BOH-^PPOPM 3 CB P M

limited to P cb Mineral Paragenesis

In the Group 1 deposits, a complex mineral paragenetic sequence is present. Following the nomenclature used by Sarkar (1970), the main ore emplacement phase is divided into three stages.

Stage 1 involved a large time span because of the complex zoning structures observed in arsenopyrite and quartz.

Stage 2 possibly had a number of small time breaks and periods of constant temperature to allow the migration of exsolved phases, (e.g., tetrahedrite in galena and chalcopyrite in sphalerite).

Stage 3 is not present in all deposits.

Paragenetic Sequences for Group 1 Deposits

Ore Gangue______Stage 1 Loellingite Quartz (1) Arsenopyrite Pyrite (1) and Pyrrhotite Exsolved Products Stage 2 Sphalerite Chalcopyrite (2) Siderite (major Chalcopyrite (1) Calcite phase) Galena Tetrahedrite Quartz (2) Pyrite (2)

Sub-Stage 2 Tetrahedrite (exsolved and migrated in galena) Chalcopyrite (exsolved and migrated in sphalerite)

Stage 3 Stephanite Pyrargyrite Elemental Silver

In the Group 2,deposits a relatively simple ore stage paragenetic sequence represents a single primary injection 89.

phase followed by a steady cooling rate.

Paragenetic Sequence for Group 2 Deposits

Ore______Gangue____ Stage 1 Arsenopyrite Quartz and Pyrite Fluorite Sphalerite Galena Chalcopyrite

In the Group 3 deposits the following paragenetic sequence suggests a different emplacement process to the Group 2 deposits but is similar to the Group 1 deposits, the only difference being an emphasis on Stage 3 crystallisation and the minor quantity of Stage 1 mineralisation.

Paragenetic Sequence for Group 3 Deposits

Gangue Stage 1 Loellingite Quartz (minor) (minor in Pyrite Group 3 deposits) Exsolved Products Stage 2 Sphalerite Chalcopyrite (2) Siderite Galena Tetrahedrite Chalcopyrite Dyscrasite

(1) Tetrahedrite

Stage 3 Stephanite Acanthite Chalcopyrite (major Pyrargyrite phase) Dyscrasite Polybasite Allargenium Antimonial Silver

Sub-Stage 3 Elemental Silver 90.

3.4 Summary

The Thackaringa-type deposits are divided into three groups of deposits based on mineralogy and paragenesis.

Group 1 and Group 3 deposits are similar in mineralogy. They differ in that Group 3 deposits have a more complex primary ore mineralogy. Group 2 deposits are quite distinct from the other deposits.

Table 10 summarises the more important mineralogical differences !between Group 1, Group 2 and Group 3 deposits.

Table 10

Mineralogy Group 1 Group 2 Group 3 Gangue Quartz and Quartz and Siderite with siderite fluorite minor quartz

Main Primary ■ • Mineralogy Galena Commonly deformed, Undeformed, As for brecciated, matildite solid Group 1 strained, solution recrystallisation textures, exsolved tetrahedrite

Sphalerite Brecciated, Blebby, no As for chalcopyrite exsolution Group 1 exsolution, inclusions pyrite

Chalcopyrite Anhedral masses Rare Commonly as with associated ,• anhedral galena, sphalerite inclusions in tetrahedrite, A galena 91.

Table 10 (continued)

Mineralogy Group 1 Group 2 Group 3 Arsenopyrite Small idiomorphic Massive, inclusion Uncommon grains, bands and free trails with inclusions of galena and tetrahedrite zoning and twining common

Tetrahedrite Blebby exsolution Not present As for products in Group 1 galena

Pyrite Common, two Uncommon, one One type only generations type euhedral brecciated masses

Pyrargyrite Rare Not present Very common, Stephanite massive CHAPTER 4 FLUID INCLUSION STUDIES 92. 4. Introduction

The recent work of Wilkins (1976) is the only detailed fluid inclusion study undertaken on Broken Hill rocks. Wilkins showed that the fluid inclusions trapped in main lode orebody gangue minerals are of a very complex nature. No primary fluid inclusions were observed and the secondary inclusions present, which are believed to be a result of many episodes of fluid migration towards the orebody, have homogenisation temperature ranges of 100°C to 300°C.

The fluid inclusions of the Thackaringa-type deposits are simple and can be divided into two groups.

4.1 Group 1 and Group 3 Deposits

103 doubly polished plates were prepared but 66 of these were unsuitable because of the opacity of the gangue minerals and the minuteness of the fluid inclusions (commonly less than O.lyu. ). Quartz, a commonly zoned phase, is milky in colour and it is difficult for light to penetrate it.- Siderite, calcite and manganiferous calcite contain no suitable inclusions for study.

Primary inclusions observed commonly consisted of two phases (gas + fluid) and rarely three phases (gas + fluid + daughter mineral). Two types of primary inclusions were identified in the zoned comb quartz crystals which are common in Group 1 deposits (Fig. 53). One type defines the crystal and zoning borders and consists solely of solid rhombohedral carbonate inclusions (type A). Another type (termed type B) is present perpendicular to these bands of type A inclusions Type B inclusions are ubiquitous throughout the Group 1 deposits and have variable degrees of fill indicating considerable necking-down. These are classified as pseuod-secondary inclusions and appear to form linear trails. Late stage secondary inclusions (type C) are common. They are very small ( < 5yu.) and rarely necked-down.

Pseudo-primary inclusions (type D) occur but are rare and are limited to the Thackaringa Mines. They have resulted from Fig. 53

(a) Common cross section of zoned comb quartz crystal showing

Type A (solid carbonate inclusion) and Type B (pseudo-primary

necking down structure) growth patterns. bo

(b) Common examples of Type A and B inclusions (Plane light). 93.

Type A

(a)

h------1 (b) 0-1 nr, tr\ .*

Figure 53 94.

decrepitation of type B inclusions and consist of an empty cavity about 100yu. in size with many small disoriented fluid inclusions scattered around them.

4.1.1 Daughter Minerals

Fluid inclusions from the Group 1 deposits characteristically contain few daughter minerals and where present they are simple. Those from Group 3 are more common.

Of the different types of inclusions, daughter minerals are uncommon in type B and D inclusions and rare in type C inclusions. The most common phase observed is carbonate which occurs as rhombohedral crystals though rather hackly 'trapped' crystals are present. Halite crystals are not common and only a few were observed in the pseudo-primary inclusions. Where they do occur they are limited to small areas and are euhedral. Iron oxides also occur but are rare and are present in the type B inclusions. All of the daughter phases observed

are very small ( < lyx).

4.1.2 Decrepitation

Decrepitation of fluid inclusions is uncommon in the Thackaringa-type deposits. It results from thermal activity long after the primary inclusions have formed. They are absent from the Group 2 and Group 3 deposits and most of the Group 1 deposits studied.

The six Thackaringa-type mines investigated in this study contain some decrepitated fluid inclusions. The Patience Mine (Apollyon Valley) and the Umberumberka Mine have some rare decrepitated inclusions but no other mines show these features. (Fig. 54). Fig. 54

A decrepitated fluid inclusion consisting of a fluidless central void with very small randomly scattered fluid inclusions around it. This is the result of a primary inclusion bursting and spraying its fluid around it. 95.

*------, Figure 54 CM rrtM. 96.

4.2 Group 2 Deposits

Fluid inclusions from the Black Prince, Golden Crest, Mt. Robe and Britisher were studied.

These fluid inclusions in Group 2 deposits include primary, secondary and pseudo-secondary inclusions.

The primary inclusions are characterised by equal-sized, faceted, negative-shaped inclusions with one or more daughter minerals and a constant degree of fill (Fig. 55). Decrepitation of these primary inclusions is absent, but rare necking-down in the primary and secondary inclusions was observed. No daughter minerals occur in the secondary and pseudo-secondary inclusions. Features that were observed in Group 1 and Group 3 deposits, e.g., necking-down of primary inclusions, pseudo-primary inclusions, primary solid inclusions and decrepitations, were absent in the Group 2 deposits.

4.2.1 Daughter Minerals

A complex array of daughter minerals were observed and studied in primary fluid inclusions from Group 2 deposits.

Eight daughter mineral phases were identified and many other daughter phases were observed and could not be identified. Up to nine daughter minerals were observed in the one fluid inclusion.

Halite is euhedral and commonly < lyj- in size. Fluid inclusions are present throughout the rock but only sporadic areas of inclusions contain halite.

Quartz is common in primary fluid inclusions trapped in a fluorite host. In one inclusion a perfect doubly terminated hexagonal crystal was observed. Fluorite is common in primary fluid inclusions trapped in a Fig. 55

Primary inclusions of the Group 2 deposits each with a single daughter mineral and constant degree of fill (plane light). Mt. Robe.

Fig. 56

A mechanically trapped anhydrite crystal with a very small amount of fluid surrounding it. Other primary inclusions around this only have small daughter minerals (plane light). Mayflower mine. +

.* Figure 56 98.

quartz host. This feature of quartz and fluorite daughter minerals being trapped in alternate hosts is best observed where the quartz and fluorite hosts are closely associated.

Anhydrite is one of the most common daughter phases observed. It occurs as subhedral to euhedral elongate orthorhombic crystals 5yu in size. In places, anhydrite crystals greater than 15yu are present with little fluid associated with them (Fig. 56). These daughter phases probably grew well before the fluids were trapped. According to normal crystal growth in fluid inclusions, anhydrite can form more than one daughter mineral in the one fluid inclusion.

Gypsum is a rare daughter mineral in the fluid inclusions studied (Fig. 57). With increasing temperature, this mineral shrank to about 1/3 of its original size at around 140°C.

Dawsonite occurs as sheaves or semi-radial aggregates of hair-like crystals. Dawsonite was only observed in small isolated pockets of inclusions in fluorite from the Mayflower deposits. There is a common association between dawsonite and liquid C02 (Fig. 58). In this particular inclusion there are three daughter mineral phases including quartz, an unidentified, high refractive index, isotropic mineral and dawsonite.

Carbonates are rare and occur as globular anhedral grains. Very rare rhombs aided identification.

Iron oxides are rare and are hackly in shape.

All of the daughter minerals identified are never found together in the one inclusion. They are present in patches with some of these areas overlapping producing perhaps two or more daughter minerals occurring together. This is schematically shown in Fig. 59. Fig. 57

A primary fluid inclusion containing gas, liquid C0_, gypsum and other unknown daughter minerals. Mayflower mine.

Fig. 58

A primary fluid inclusion with gas, liquid C0o, dawsonite and quartz daughter minerals. Mayflower mine.

Dawsonite is identified by its form (hairlike X/S), strong anisotropism, 11 1 extinction, green colour and lack of ^Peochroism. 99.

<------H Figure 57 IO/+.

Figure 58 100.

Daughter minerals which have this habit are anhydrite, quartz, gypsum, dawsonite and halite.

Complex daughter mineral associations are common and up to nine daughter minerals have been observed in the one inclusion (Fig. 60).

4.2.2 Liquid CO^

Liquid CC^ is only present in primary inclusions from the Group 2 deposits and are found in very small patches

100jj, in diameter. In most inclusions, the liquid CO^ is present as a thin film which rims the gas bubble (Figs. 57 & 58). In others, large CO^ bubbles actually envelop the gas bubble and any daughter phases present (Fig. 61) .

When the liquid C0o fluid inclusions are subjected to heating the thin film of C09 disappears into the gas bubble at temperatures below the critical temperature of CC>2 (31°C) . This indicates that pressures involved in the formation of these fluid inclusions is fairly low (i.e. less than 0.5 K bars).

4.3 Freezing Data

Freezing experiments were conducted on 85 fluid inclusions from 11 separate Thackaringa-type deposits. In most freezing runs (down to -80°C) there were some inclusions that did not freeze (particularly in the Group 1 deposits). In one run a primary inclusion did not freeze when maintained at a temperature of -180°C for thirty minutes. When the inclusion was allowed to warm up slowly it froze at -100°C.

Table 11 summarises all the freezing data obtained for both Group 1 and Group 2 deposits. In the Group 1 deposits, three different sets of results have been defined (Fig. 62). Fig. 59

Schematic representation of the layout of daughter minerals in different areas of primary inclusions in Group 2 deposits.

Fig. 60

A rounded primary fluid inclusion with nine discernible daughter mineral phases. Anhydrite (rectangular elongate phase) is the only phase identifiable (plane light). Mayflower mine. anhydrite-bearing

gypsum-bearing inclusions

quartz-bearing inclusions

dawsonite-bearing inclusions

Figure 59

Figure 60 Fig. 61

A primary faceted fluid inclusion with a large liquid C02 globule enveloping a relatively small gas bubble and small daughter mineral. Black Prince.

Fig. 62

Frequency distribution of freezing results for primary inclusions in the Group 1 and Group 3 deposits. 102.

Figure 61

Type C ^Fype B Type D

Frequency

-28

Temperature °C

Figure 62 103.

Table 11 Freezing Data for Group 1 and Group 2 Deposits

Deposit Range Mean Thackaringa - 3.4° to -21°C -10.4°C 0 CN 0 ^3* o Daydream -10.4° to 1 -13.2°C Umberumberka - 8.3° to -28°C -14.4°C Consols -13.9° to -18.8°C -16.2°C New Year -15.8° to -18.0°C -16.9°C O i O Tom Thumb -14.8° to -19.8°C M CD Patience -15.8° to -21.5°C -18.1°C Mt. Robe - 4.0° to -23.5°C -12.6°C Black Prince - 9.4° to -24.1°C -15.1°C Golden Crest -10.8° to -24.5°C -10.0°C Mayflower - 7.0° to - 8.6°C - 7.6°C

Secondary inclusions have the lowest salt content ( < 6% total solid), pseudo-secondary inclusions (type B) have 15% total solids, whereas the pseudo-primary inclusions (i.e., those formed by decrepitation) have variable total salts averaging 22% (Fig. 62).

In the Group 2 deposits freezing results are much more complex to interpret. A frequency distribution of freezing results is presented in Fig. 63. The variations in these results are possibly related to the diversity of daughter minerals present. For example, the lower temperature freezing point collection of result around -24°C are fluid inclusions with halite daughter minerals. The most common inclusions containing quartz and anhydrite daughter phases average about -8°C (10-12% total solids). A small set of results around -17°C commonly consists of a complex arrangement of daughter minerals (most of which are unidentifiable).

In all polished thin sections a great variation of'freezing points were obtained. The only variable control in these i'1 i _ ; j . results appears to be the daughter minerals present in the respective inclusions. Heating Data

Seventy-two heating runs were conducted and each temperature reading was checked twice (Fig. 64).

In the Group 1 deposits great difficulty was encountered delineating original primary inclusions that had not been necked-down or decrepitated. This involved locating inclusions that were unrelated to any trails or lines of inclusions (necked-down inclusions) and that were not associated with empty decrepitated cavities. It was difficult to avoid including some necked-down inclusions and at times spuriously low results (e.g. +112°C) were obtained.

The Group 2 heating data shows the differences between the two groups. Although a limited amount of work was completed on this group (15 heating runs), the homogenisation temperature difference between the two groups of deposits is considerable (34° - 67°C).

Pressure Corrections In both the Group 1 and Group 2 primary inclusions the degree of fill is high, i.e., 0.92 and 0.90 respectively. The V to Li V ratio was determined using an areal estimation method. L+G

In the Group 1 inclusions, decrepitation took place and this suggests that pressures must have been very low and, therefore, pressure corrections are negligible.

Work on the presence of liquid C02 suggests that the pressures of Group 2 inclusions in this system is less than 0.5 kb. Using Roedder's (1962b) data, pressure corrections of +20°C maximum is indicated.

The filling temperature difference between Group 1 and Group 2 primary inclusions is 55° - 90°C which is quite substantial considering a standard deviation of -15°C. Fig. 63

Frequency distribution of freezing results for primary inclusions in the Group 2 deposits.

Fig. 64

Homogenisation temperature plot for primary inclusions from Group 1, 2 and 3 deposits. 105.

Frequency

Freezing Temperatures °C

Figure 63

Thackaringa

Consols

Daydream

Umberumberka

Tom Thumb

New Year | 237 C Mt Robe

Britisher

Golden Crest

120 130 140 150 160 170 180 190 200 210 220 230 240 250 Homogenisation Temperatures °C

Figure 64 106.

4.5 Summary

The fluid inclusion study has shown that Group 1 and Group 3 deposits have very similar fluid inclusions. It has also shown that these two similar groups are distinctly different from the Group 2 deposits.

Table 12

Group 1 Group 2 Group 3 Types of Complex number of Simple. Primary As for Group 1. inclusions types reflecting a inclusions. Most complex history of common with formation. secondary.

Daughter Uncommon, Eight complex As for Group 1. mineral carbonate and daughter mineral halite. phases identified indicating a complex primary fluid.

Liquid CO^ Absent. Common. Absent.

Decrepitation Uncommon. Absent. Absent.

Freezing pt. -15.1°C -2.8 -11.3°C -3.0 -16.2°C temperature

Homogenis­ +178.0°C -7.4 +231.0°C 9.0 +181°C ation temp.

Filling +178°C -7.4 +251.0°C -9.0 +181°C temperature CHAPTER 5 107. DISCUSSION

Introduction

The data from Chapters 2 to 4 have shown that the Thackaringa-type deposits comprise two different and clearly defineable groups of deposits, not the single type described previously (King and Thompson, 1953; Both and Smith, 1975).

This study has documented these essential differences within the Thackaringa-type deposits. However, to document the genesis of these deposits, many questions remain unanswered and are perhaps unanswerable. These include:-

(a) The age of the Thackaringa-type deposits. (b) The source of the ore-bearing fluids. (c) The mechanism of ore emplacement, and (d) The ore localisation controls.

5.1 Age of the Thackaringa-type Deposits

The only direct age dating technique previously used on the Thackaringa-type deposits involved Pb isotope studies of galena. Two dates of 500 m.y. and 800 m.y. for galena from Thackaringa-type deposits have been determined (Kanasewich, 1962; Reynolds, 1971). Interpretation of these lead ages involves a two stage model but the ages do not fit a model very well. It is possible that this data and hence the ages are meaningless. The only mechanism to yield meaningful age deductions from a simple two stage interpretation model involves an initial lead of unique isotopic composition admixed in varying proportions with a radiogenic lead of just one identifiable composition (Richards, pers. comm., 1976). The Thackaringa-type veins are very narrow and hence there is much opportunity for mixing of different leads. Galena is a poor medium for age determinations. Deformed recrystallisation textures of galena indicate post-depositional mobility, isotopic re-equilibration and possible lead loss or interchange. 108. The isotope studies of Cooper (1970) on the Consols-type veins represents the closest approximation to the simple two stage model with possible contamination from just the one lead source, i.e., the main Broken Hill orebody lead (Richards, pers. comm., 1976).

Geological evidence of these deposits conflicts with the lead isotope ages (Boots, 1972). Group 1 deposits are characterised by an association with retrograde shear zones. It appears that the Cambrian retrogression is the last major tectonic event in the Broken Hill area (Pidgeon, 1967; Shaw, 1968). If the Group 1 deposits were generated at that time, as the lead ages indicate, then there must have been at least two generations of activity in the retrograde shear zone. An earlier event was associated with ore deposition, and a later event decrepitated fluid inclusions, remobilised and deformed galena and brecciated and displaced ore veins. The structural complexities of the Broken Hill region obscure the possibility of a number of generations of activity associated with the Cambrian retrogression, but it may have occurred. The extent of this retrogression, traditionally the cause of most shearing and retrogression observed in the Broken Hill Block, is strongly questioned. High grade assemblages of upper amphibolite facies present in the Thackaringa-Pinnacles shear zone are probably associated with the main 1700 m.y. metamorphic event (Le Couteur, pers. comm., 1976). Temperatures of over 600°C are much too great for a retrograde event which only re-equilibrated the biotite isotopic system and not muscovite (Pidgeon, 1967). Evidence of the intensity of the Cambrian retrogression is recorded in the Torrowangee Group sediments which are only metamorphosed to the lower greenschist facies (Tuckwell, 1976).

The Cambrian retrogression is considered more likely to be associated with the intrusion of pegmatites, i.e., at Thackaringa, Triple Chance and some of these pegmatites could have formed by tectonic-hydrothermal processes (Gresens, 1967). Slight effects in other areas of the Broken Hill region are shown by minor movement along existing shear zones resulting in brecciation of ore veins and deformation of galena in Thackaringa-type veins (e.g., Apollyon Valley). 109.

Retrogression commencing at the waning stages of high grade metamorphism was a long and complex series of events (Chenhall, 1973) and (Plimer, 1975). The Group 1 deposits are present in shear zones coplanar with S2 (Rutland and Etheridge, 1975). Granites dated at 1540 m.y. transect the schistosity of these complex shear zones at Allendale (Pidgeon, 1967) and this supports their association with the retrogression associated with the high grade metamorphism. It is difficult to imagine a metamorphism of such regional intensity to have no tectonic expressions.

Most retrograde shear zones do not appear to cut the Adelaidian sequence north of Broken Hill (dated at 1350-700 m.y., Binns and Miller, 1963; Compston and Ehring, 1968). The deposition of the Adelaidian rocks was in fault-bound basins and these fault and shear zones were active at the time of deposition (Tuckwell and Cooper, 1971). Undoubtedly, there are fracture zones and shear zones in the Broken Hill area that are associated with the 500 m.y. event. These crosscut primitive shears and lithological layering and trend due north, e.g., Lord's Hill fault and De Bavay shear zones. Both biotite and muscovite are isotopically equilibrated in these shear zones (Pidgeon, 1967).

Geochemical studies on the British Shear Vein (appendix) has j shown that the Thackaringa-type deposits have no hydrothermal alteration zone or contact aureole.

The Thackaringa-type deposits are probably younger than the leuco- adamellite dykes because these dykes at the Champion-Hidden Secrets mines are transgressed by shear zones which contain Thackaringa-type deposits (Hagarty, 1974). No thermal aureole is associated with these dykes and, therefore, it is thought that both Thackaringa-type deposits and leuco-adamellite dykes were emplaced into hot rocks. The differences between the emplacement ages is probably not great.

5.2 Source of Fluids

There are many possible sources of the Thackaringa-type deposits:- 110.

(a) Granitic rocks. (b) Adjacent metasediments. (c) Broken Hill-type deposits. (d) Others, e.g., amphibolite, Potosi gneiss and calc-silicate rocks.

5.2.1 Granite and Associated Pegmatite

Adamellite intrusives are common in foot and hanging wall positions of many Thackaringa-type Group 1 deposits, "e.g., Hen and Chickens, Apollyon Valley mines and Daydream. This close spatial association between adamellite and the Thackaringa-type Group 1 deposits coupled with the possible close age difference suggests a genetic relationship.

Two temperature estimates and one pressure estimate for the leuco-adamellite melt have been determined (Figs. 65, 66, 67).

Temperature estimates from both methods indicate a range of 700°-800°C (Average 750°C - 50°C) with pressures greater than 1500 atm. These conditions are very similar to the main pulse of high grade metamorphism where temperatures in excess of 700°C are present. Scott, Both and Kissin (1976) studied the sphalerite geobarometer and estimate pressures of 7.6 kbs. This method has been questioned in other areas, e.g., Sullivan (Ethier et. al., 1976), West Australian Nickel (Groves et. al., 1975), where pressures are commonly in excess of known pressures.

Hewins (1975) studied pyroxene geothermometers in granulite terrains throughout the world. Minimum temperature estimates from granulites include 760°-790°C which is of similar order to the temperature estimates for the adamellite dykes. Temperature estimates in the Broken Hill region are 843° - 50°C. Fig. 65

Hatched area represents the formation conditions of the adamellite investigated in this study. Aluminium silicate triple point after Holdaway (1971). Muscovite stability curve after Kerrick (1974). Almandine curve as depicted by Sobolev (1972). Granite minimum melting curve after Tuttle and Bowen (1958).

Fig. 66

Pressure estimates for the emplacement of adamellite intrusives in the Broken Hill district (after Burnham, 1967). 111.

Kyanite

Sillimanite

granite min. melt curve

Figure 65

500 bars

5 Kbs

10Kbs

KAISioO

Figure 66 112.

The granitic rocks commonly contain up to 15% of pri­ mary hydrous minerals and hence were viscous magmas. They have probably not travelled a great distance in their ascent. They are interpreted as being anatectic melt derivatives produced during the high grade metamorphism. Evidence supporting this origin include:-

(a) Lack of contact aureole. (b) Have characteristics of S-type granites (Chappell and White, 1975), e.g., K20 5% (typically -—' 8%), (Hagarty, 1974). Na20 3.2% (typically 4-5%) and Na20/K20 0.64. (c) The presence of metasedimentary xenoliths.

Chappell and White (1975) classify this type of granite as S-type - derived from anatexis of metasediments.

The work of Holland (1973) has shown that potential ore-bearing granites must contain water, chlorides (in biotite), base metals and reduced sulphur. Studies of Johns and Burnham (1969) on melting characteristics of granites and pegmatites have shown that very Na-Cl-rich aqueous phases become immiscible with granitic magmas below 675°C (Fig. 68). These residual melts contain over 11% H20 and base metals in a silicate magma have an affinity with the aqueous phase because of their inability to substitute into the silicate lattice. Hence concentration of Pb, Ba,F , Ca in these regions is acceptable on physio-chemical grounds.

Holland (1973) states that anatexis would further liberate sulphur. While this may be so, the sulphur species liberated may never produce an ore deposit. 2 For example, if sulphur exists in the form SO^ , or H2S no ore deposition is possible, whereas other forms and S are capable of producing sulphide deposits. According to Barnes and Kullerud (1961) , these sulphur containing species are stable under certain pH and f09 values. Fig. 67

Temperature estimates for the emplacement of adamellite intrusives based on the 5 kbs equilibrium diagram for the granite system after Luth, Johns and Tuttle (1964).

Fig. 68

Partitioning of very NaCl-rich fluids into the aqueous phase after Johns and Burnham (1969). 113.

------\760

K AlSioO,

Figure 67

T°C 1000 800 700 600 j ' ! 1 , \ ; ; PEGMATITE ----- S MELTING CURVE MELT l ( l + \ Xls \ MELT + Xls \ \ + \ \ V V Saline \ \ \ aqueous \ \ \ phase GRANITE MELTING CURVE ^ ^ ^ ____ ^

5 10 .* % H2O in magma

Figure 68 114.

One problem with an origin based on granitic intrusives is the essential age difference between the two as shown by geological evidence. It is well known that ore deposits have been produced from intrusives long after the time of emplacement of the intrusive.

Despite the potential of these S-type adamellites for producing ore deposits there are a number of problems in linking them with the genesis of the Thackaringa-type deposits. No base metal mineralisation has been observed as segregations or veinlets in the adamellite. In many areas where Thackaringa-type deposits are widespread, adanellites are absent and conversely where adanellites cover large areas, no Thackaringa-type deposits are present, e.g., Brewery Well, and at the Mundi Mundi Ruins.

The Group 2 deposits are completely unrelated to the adanellite dykes. However, a large pegmatite, the Mt. Robe pegmatite, is enclosed by most of the Group 2 deposits. It is suggested that this pegmatite is responsible for the transportation and deposition of the Group 2 deposits based on the following evidence:-

(a) Base metal deposits and fluorite deposits have been observed as segregations and veinlets in the Mt. Robe pegmatite (Anderson, 1966).

(b) Fractures which contain the Group 2 deposits have similar form and attitude to the fractures resulting from intrusion of the pegmatite.

(c) Sulphur isotope and fluid inclusion data for the Group 2 deposits indicates an homogenous source.

i Bratus (1974) studied fluorite formations‘in the Korsun-Novomirgorodsky (pegmatite) pluton. He found 115. two forms of fluorite. One formed at temperatures greater than 260°C from concentrated brine solution (no information) and another at 220°-160°C in dilute brine solution (2-2.3% NaCl).

The brines responsible for the deposition of the Group 2 deposits formed fluorite at 250°C -30°C and had NaCl contents of 10-12% total solids - somewhat higher than the low temperature fluorite studied by Bratus, although they may correspond with the high temperature forms.

Polyanski (1973) studied the Bom-Gorkhon granite intrusive and associated ores.

He describes four stages of the development of this granitic intrusive complex:-

(a) The main granitoid intrusive stage (fluid inclusions with T = 1010°-930<£) . H

(b) Magmatic intrusive main stage leucocratic granites (1050°-950°C).

(c) Late stage intrusive pegmatite (720°-550?).

(d) Hydrothermal ore fluids phase.

It is in Stage 4 that mineralisation takes place. Homogenisation temperatures recorded for quartz sulphide rocks are 260°-170°Cand quartz-fluorite calcitej190°-180°C . These homogenisation temperatures are similar to those found for the Group 2 deposits and temperatures expected for the Mt. Robe pegmatite ( ~ 730°C). Fluorite from pegmatites commonly have formation temperatures 300°-250°C and those formed by other means, e.g., hydrothermal have temperatures of formation less than 200°C. Salinities for fluorites from pegmatites are commonly three molar NaCl (c.f., Group 2 deposits and NaCl). Salinities, composition, x densities and homogenisation temperatures are similar 116.

inclusions from base metal deposits generally derived from pegmatites and those of the Group 2 deposits. The Mt. Robe pegmatite has primary hydrous mineral contents of 12% and the previous discussion based on adamellites would apply in this case. 2+ Fluorites and Pb would partition into the substantial saline aqueous phase below 675°C (Burnham, 1967) .

5.2.2 Adjacent Metasediments

The retrograde shear zones are large tectonic stress release zones. Metasediments which are undergoing a granulite facies grade metamorphic episode experience high confining pressures on pore fluids which are necessarily rich in Na, Si, K and possibly Pb and Ag. These pore fluids if they are near an area of low pressure, either a shear or fracture zone, will preferentially migrate into this zone. This is the principle by which shear zones act as favourable regions for the transportation of pore fluids. These fluids bear volatiles, silica, base metals, H20 and hydrophyllic elements (Wintsch, 1975). Further, the shear zone is a zone of weakness which remains as such to any forces and is susceptible to movement at any time. Attempts to date micas from major shear zones in the Broken Hill region are not successful because elemental isotopes are continually re-equilibrating with slight movement (Etheridge, pers. comm.).

A thick sequence of metasediments, moderately enriched in base metals provides a substantial source of base metals if this mechanism can work on a suitable scale. For example, if a metasediment contains 20 ppm lead content, then to produce an average-sized Thackaringa-type deposit of 30,000 tonnes, a volume of 10,000,000 cubic metres would be required. This is a small volume indeed. Table presents data of average lead and.*zinc values of the common rock types in the Broken Hill mine sequence. (Elliott, pers. comm.). 117.

TABLE

Rock Type Sillimanite, garnet, biotite, gneiss 70 120 20 10

Quartzofelspathic gneiss 80 160 40 10

Amphibolite 80 500 60 50

In all of these rock types Pb would be sufficiently concentrated to produce such a Thackaringa-type deposit. There is a problem, however, with the Zn/Pb ratio which is greater than one whereas the Thackaringa-type deposits have Zn/Pb ratios much less than 0.5. This can be partly resolved by the presence of gahnite (the zinc spinel which is common in a number of metasediments). Gahnite acts differently under pressure and seeks an anhydrous environment (Segnit, 1961). Further, zinc in.the sulphide form has a poor mobility compared with lead and silver and this explains the high proportions of lead and silver observed in the Thackaringa-type deposits.

Base metals derived from metasediments would satisfy sulphur isotope data for the Group 1 deposits where gross contamination of sulphur is common. Conversely, the Group 2 deposits have a narrow well-defined range 34 of 6 S% values and hence are unsuitable for this type of mechanism.

Assuming that the base metal contents in Table 1 are average for the whole Broken Hill Block metasediments. All shear zones should be ore-bearing. This is a problem which conflicts with such a mechanism. There are examples in certain areas of 2 or 3 generations of shearing with only one producing any ore deposit potential. These particular ore-bearing shear zones appear to be related to the one time period and hence 118.

the one episode of activity.

The association of Thackaringa-type Group 1 veins with a great variation of rock-type and geological environment but with constant mineralogy is a further problem. Erratically distributed lead-bearing horizons are suggested as a more likely source to these deposits.

5.2.3 Broken Hill-type Deposits

Sulphur isotope studies by Both and Smith (1975) gives strong support to genetic ties with the Broken Hill-type 34 deposits (Fig. 69). The mean S S% results for Group 1, Group 2 and the Broken Hill-type deposits are similar. Despite local contamination in certain deposits by pyritic carbonaceous beds, it appears that the majority of the ore-bearing sulphides have derived from a single source.

Evidence to support this is based on geological and geochemical grounds. King and Thompson (1953) first recognised the close spatial relationship between Broken Hill-type and Thackaringa-type deposits. Thackaringa-type deposits plot on the fringe of quartz-gahnite lodes.

The Thackaringa-type Group 3 deposits all lie within 1.5 kms of a known Broken Hill-type deposit. These deposits are commonly surrounded by higher grade rocks than the Group 1 and Group 2 deposits. The observation of Broken Hill-type deposits with high grade rocks is thought to be a possible exploration tool for these deposits and are common in the eastern half of the Broken Hill Block where all of the Group 3 deposits occur. In the western part, few Broken Hill-type deposits outcrop and few high grade rocks are present. Exceptions to this observation are rare. At the Daydream mine middle amphibolite facies rocks outcrop fractured and brecciated pieces of quartz-gahnite observed on the surface nearby indicate Broken Hill-type mineralisation at depth. This supports a possible Fig. 69

Sulphur isotope data of Both and Smith (1975) reinterpreted in terms of the different types of deposits in this study. 119.

0-7 V 1------1------1------1------1------1-----.------1------...... - - Group 1 Deposits + 2 0 -2

0-3 , , t—■—rH-----7------7------1 Other B H - type 0 deposits

-0-6 ,------1 1 1 • • —r*~~~—! Group 2 Deposits 0

0-8 v . ~r=+=----- ' Main Broken 0 Hill lode

$34S

F igure 69 120. hypothesis of fluids deriving from Broken Hill-type mineralisation at depth.

The Geological Survey of N.S.W. have completed a simplified stratigraphic succession of Broken Hill rocks (Fig. 70). In the succession mine sequence occur in a volcanic sedimentary sequence which is overlain by a sedimentary sequence. The orthopyroxene isograd is present in this sequence and is parallel to layering.

At Umberumberka, no Broken Hill-type deposits outcrop but a high grade granulite facies core of an F2 antiform indicates high grade sequences and hence possible Broken Hill-type deposits at depth.

It is probable that with increasing depth, high grade sequences may be found which are similar to those in the ESE sections of the Broken Hill Block.

Field evidence from Chapter 2 showed that the major ore-bearing shear zones have a similar attitude to enclosing metasediments. They occur along the axes of major F2 fold axial planes and are interpreted as longitudinal shears. This would provide a suitable mechanism for movement of fluids from high level stratabound Broken Hill-type deposits (within 1.5 kms from the surface) up major F2 shears during the waning stages of the high grade metamorphism.

Thackaringa-type mineralisation is similar to the Broken Hill-type mineralisation. All of the primary minerals in the Thackaringa-type deposits are common in the Broken Hill-type deposits. The relative ore mineral proportions, however, are different. Lead, silver, copper and antimony are enriched in Thackaringa-type lodes, but zinc is depleted. The relative enrichment of copper may be derived from other sources, e.g., amphibolites. Fig. 70

A simplified stratigraphic succession of Willyama Complex rocks. Compiled by the Geological Survey of N.S.W. 121.

Andalusite and Pel i tes — sediments Felspathic zone

A

Mine Sequence — volcanics — ...... - and sediments Granulite zone

Quartzo-felspathic — Igneous

V

FIGURE 70 122.

On geochemical grounds the relative elemental mobilities of Pb, Ag and Sb would explain the observed enrichment of the elements. The large proportion of carbonate observed in these deposits is common in rejuvenated deposits (Huttenlocker, 1953).

Volatiles CC^, 1^0, F and O2 trapped during the initial stages of ore formation would exert dynamic volumetric hydrostatic pressure during periods of intense stress into stress release zones, i.e., shear zones.

Very few examples exist where Broken Hill-type deposits have been sheared and are in contact as proof of being the one unit. The Hidden Secrets lies 200 m NE of the Champion/ (Broken Hill-type) but there is still an element of doubt as to their genesis. The British Shear veins are in contact with main lode ore.

One example which approximates this situation is one cited by Maiden (1976) of an ore "dropper" at the No. 21 level, NBHC from the No. 1 lens. These are tongues of high grade ore intruded by plastic injection from the margin of the lead lode 120 m into the footwall gneiss. Relative elemental enrichment in the ore dropper (Table 12) shows that all sulphides are enriched, principally Ag, Pb and Sb. Despite the fact that this is only plastic injection (partial remobilisation) and not complete remobilisation the observed enrichment agrees basically with that of the Thackaringa-type deposits.

Mineralogical, geological, geochemical and structural evidence indicates a possible rejuvenation of Broken Hill-type deposits to form Thackaringa-type Group 1 and Group 3 deposits. The Group 2 deposits do not fit this model entirely. These deposits do not occur in fold crests. Their mineral chemistry is different and Sb-bearing phases (tetrahedrite and bindheimitd are absent. Siderite is replaced by fluorite as the main gangue minerals and the phases are not brecciated or remobilised. Conversely, lead, copper and silver are enriched in the Group 2 deposits and sulphur isotope data indicates a 123.

genetic relationship with Broken Hill-type deposits.

Table 12 The relative increase in selective elements in the ore dropper compared with the average lead lode grade from Maiden (1976).

Intrusion Relative Element dropper Pb Lode Enrichment Ag 390 ppm 108 ppm 261% Pb 28% 13.4% 108% Sb 84 ppm 47 ppm 80% Co 84 ppm 57 ppm 47% Zn 15.7% 11.2% 40% Ni 20 ppm 15 ppm 33% Cu 390 ppm 108 ppm 18%

5.2.4 Other Sources

The Thackaringa-type deposits are a widespread group of deposits which outcrop over an area of over 1,000 sq. kms in the Broken Hill Block. Brathwaite (pers. comm.) has identified analogous deposits in the Euriowie Inlier.

It is unlikely that these deposits have been derived directly from a hidden source, e.g., a deep seated pluton. No gravity or magnetic lows have been observed which might indicate a pluton at depth.

There are two possible sources for the Thackaringa-type deposits:- (a) Broken Hill-type deposits, and (b) from adjacent metasediments.

The possibility of ore material being derived from average metasediments is not supporting on the following evidence:- 124.

(a) Conjugate fractures observed at Thackaringa have only a small volume of material for source.

(b) Not all shear zones contain Thackaringa-type mineralisation as would be predicted with consistent average metasedimentary material.

It is, therefore, concluded that the Thackaringa-type deposits are genetically related to the Broken Hill-type deposits based on the following evidence:-

(a) The spatial relationship between Thackaringa-type deposits and Broken Hill-type deposits in the eastern half of the Broken Hill Block. (N.B., a model is necessary to explain deposits to the west of Broken Hill not associated with Broken Hill-type deposits).

(b) Similarities in sulphur isotope values means for Broken Hill main lode, Broken Hill-type deposits, Group 1, Group 2 and Group 3 deposits.

(c) Similarities in ore chemistry if element mobility is considered.

(d) The age difference between Broken Hill-type and Thackaringa-type deposits is an argument in favour of rejuvenation.

(e) Similarities in pervasive post-ore saline fluids in the main Broken Hill orebody with ore bearing fluids in Thackaringa-type deposits (Wilkins, pers. comm., 1976). This is related to dewatering of high grade rocks during the high grade metamorphism.

(f) It has been shown that remobilisation of Broken Hill-type deposits does produce a deposit with elemental concentrations similar to Thackaringa-type deposits using the data of Maiden (1976). 125.

5.3 Mechanism of Ore Deposition

A structural mechanism is the most probable ore deposition mechanism. This is based on the presence of shear or fracture systems in all Thackaringa-type deposits.

At present, the following information has been established:-

(a) That the Thackaringa-type deposits are not associated with the 500 m.y. event but probably with the early post-metamorphic stage after the intrusion of the leucoadamellite dykes.

(b) That the Thackaringa-type deposits have probably evolved from the Broken Hill-type deposits by a rejuvenation process.

(c) That shear and/or fracture zones are the conduits for transport for the base metal fluids of these deposits.

The distinction between the Group 1 and Group 3 deposits, on one side, and the Group 2 deposits on the other is important when mechanisms are considered. The differences in the fluid nature of these deposits implies a different mechanism for the origin of the Group 2 deposits.

Group 1 and Group 3 Deposits

These deposits are characterised by a simple comb quartz vein structure. The quartz crystals are commonly strongly zoned which implies a number of episodes of fluid release. Wilkins (1976) describes the main Broken Hill orebody fluid inclusions as deriving from the successive invasions of fluid towards the orebody during retrogression.

Three different types of secondary fluid inclusions were identified in the present study and these are probably related to the possible invasion of fluid.

Other factors which support this continual invasion of fluids of * different composition include mobility of exsolution in 126.

sphalerite, zoning in quartz and arsenopyrite, different generations of quartz and pyrite, secondary overgrowths of calcite and decrepitation of fluid inclusions.

The fluid inclusion data suggests that the Group 2 deposits are derived from a single complex hypogene source. They have been injected into fractures and have cooled with no well crystallised forms. On the basis of fluid inclusion data, it is suggested that these deposits have a strong affinity with hydrothermal primary fluids. There is strong evidence to suggest that the quartz-fluorite-sulphide Group 2 veins have derived from a major anatectic pegmatite intrusion. These include:-

(a) The location of the Mt. Robe, Black Prince, Silver King, Golden Crest and Consolation deposit on the margin of the massive pegmatite.

(b) These deposits occur in fractures aligned with the margin of the pegmatite.

(c) Quartz-rich volatile phases of this pegmatite are fluorite-bearing (e.g., Mt. Eltie) and sulphide-bearing (Anderson, 1966).

No age data is available for this pegmatite intrusive and work carried out on other pegmatites in the district have not resolved the age problem. Age data of 1540 - 50 m.y. and 495 m.y. (Pidgeon, 1967) show clearly that there are at least two episodes of pegmatite intrusions. The beryl-bearing Lady Beryl pegmatite has been dated at 490 m.y. (B. Stevens, pers. comm.).

Broken Hill-type deposits are rare in the environs of the Group 2 deposits (e.g., Allendale) and an origin based on these deposits, except for a source at depth, is not supported. However, sulphur isotope data confirms a genetic relationship between the two. An origin involving the partial*melting of a large volume of metasediments to form an anatectic pegmatitic ^elt is suggested. The Broken Hill-type deposits are possibly present at depth in this area as in the west of the Broken Hill 127.

Block. High hydrous mineral contents in the Mt. Robe pegmatite indicates that the intrusive has ascended a short distance, i.e., possibly in the order of a few kilometres. A pegmatite of this magnitude (10 kms diameter) derived from the partial melting of metasediments would involve a major episode of activity. Since the 500 m.y. event is thought to be a local episode of tectonism in the Thackaringa area, an age of 1540 m.y. (Pidgeon, 1967) is the more probable age considering the intensity and diuturnity of the high grade metamorphism.

5.4 Controls of Ore Localisation

A number of possible controls of ore localisation have been investigated. These include

(a) A chemical control as shown by indicator minerals, e.g., graphite, chlorite.

(b) A structural control due to strain or stress release areas in shear zones or structural traps.

5.4.1 Chemical Controls

Indicator minerals are common in many Thackaringa-type deposits. Graphite is a common gangue and host mineral at the Umberumberka, Terrible Dick and the Apollyon Valley mines. The importance of this mineral as a reduction agent for hydrothermal fluids is strongly questioned. Many examples of ore in these deposits unaccompanied by graphite and vice versa have been noted indicating only a coincidental relationship between these two (Jaquet, 1894).

Other examples of chloritic hosts are common, but there is no direct association between ore and chlorite.

Due to the diversity of host lithologies in the Thackaringa-type deposits it is not thought that chemical controls are important. 128.

5.4.2 Structural Controls

All Thackaringa-type deposits are located in either a fracture or shear zone. This in itself demonstrates the importance of structural control in the emplacement of ore material. It has been demonstrated earlier that granites have acted as impervious structural traps to ore emplacement in the Apollyon Valley mines. In the Daydream mine, the greatest development of ore and the widest veins are in fractured competent rocks where openings are best developed. This represents another type of structural trap. Deposits appear to pinch out in incompetent rocks.

5.5 Comparison with Other Similar Deposits of the World

There are many examples of quartz-siderite sulphide vein deposits including Mpanda, Tanganyika; Coeur d'Alene and Wood River deposits of Idaho, Slocan, British Columbia. Most of these deposits probably derive from an igneous source but the mineralogy is very similar to that of the Thackaringa-type deposits. For example, the Coeur d'Alene deposits consist of galena, sphalerite, argentiferous tetrahedrite, chalcopyrite, pyrrhotite, arsenopyrite and minor bornite chalcocite, gersdorffite, boulangerite and bournonite. The gangue is quartz, siderite and other carbonates with pyrite (Sorenson, 1951). The only real physical difference between these deposits and the Thackaringa-type deposits is their size, the former having produced many millions of tons of ore. The possible reasons for the small amounts of ore in the Thackaringa-type could be the size of the Broken Hill-type deposit at depth. Many of these deposits previously mentioned were recently regarded as volcanogenic (Anderson, 1966).

The mineralogy of deposits which have no igneous source are equally similar to the Thackaringa-type deposits. These include deposits in central and northern Europe, e.g., The Belledonne district (Ypma, 1963). Schneiderhohn (1962) referred to these as regenerated deposits. Vokes (1971) supports a stress induced mechanism with these deposits associated with metamorphism above upper greenschist facies. He believes fluid movement to be limited, i.e., metres not kilometres, though little information 129.

is available on the matter. This view is is contrast to meteoric fluids. Common mobilisation selectivity favours Pb, Ag, Sb, As and Au. Examples of ore mobilisation are common, e.g., sulphide metapegmatites (Lawrence, 1967) and other peculiar felspar pegmatites (Plimer, pers. comm., 1976).

Temperature and pressure is an important variable in this case and conditions of granulite facies metamorphism (~ 800°C temperature and 7 kbs pressure) could be responsible for base metal mobilisation over some distance.

5.6 Summary

There is considerable evidence to suggest that the Thackaringa- type deposits are regenerated deposits deriving from the Broken Hill-type deposits. The Group 1 deposits have evolved by migration of volatile and metal-rich fluids from depth across geothermal gradients to lower metamorphic grade areas along fluid conduits, e.g., shear zones. The Group 2 deposits have probably evolved by the partial melting of metasediments which contain Broken Hill-type deposits. The high level of intrusion of a felsic anatectic pegmatite is the most probable source for the injection of complex volatile-rich phases into fractures in the overlying metasediments.

The differences between Consols-type deposits and other Thackaringa-type deposits is a difficult problem. The solution to this is based on the differences between the Broken Hill main lode and other Broken Hill-type deposits. The complex Consols-type veins (Group 3) have evolved from the complex main Broken Hill lode and do not occur elsewhere in the Broken Hill region. The other more simple Thackaringa-type Group 1 deposits have derived from the other less complex Broken Hill-type deposits by migration of fluids along F2 shears developed during the fluid release stages of waning metamorphism (Fig. 71). Sulphur isotope data supports this hypothesis. This implies that there are no expressions of the massive main lode ores in the Broken Hill district. All concealed mineralisation at depth -is probably the volumetrically smaller and more simple other Broken Hill-type mineralisation. Fig. 71

Model for the generation of Thackaringa-type Group 1 deposits 130.

fau l

axes

fold

Longitudinal align CONCLUSIONS 131.

CONCLUSIONS

Transgressive quartz-siderite-sulphide-fluorite veins have previously been interpreted as a single type of deposit termed the Thackaringa-type deposits. These are divided into two clearly different groups of deposits on the basis of geological and mineralogical relations, fluid inclusion and mineral chemistry studies. In Group 1 and Group 3 the quartz siderite-sulphide veins occur in F2 longitudinal shears and fractures. They have filling temperatures of 180°C and are relatively simple fluids with primary salinities of 12-15% total solids. Decrepitation, necking down and variations in types of secondary inclusions indicate a complex history of post-depositional activity.

The second group (Group 2) are quartz-fluorite-sulphide veins which occur in simple fractures and have undergone little post-depositional stress. The ore bearing fluids are complex and salinities of primary fluid inclusions range from 8 to 25% total solids. They have been generated from the complex hypogene volatile-rich fluid phase exsolved from an anatectic pegmatite with ore deposition temperatures of 250°C.

Sulphur isotope data of Both and Smith (1975) confirms the direct association of both groups of Thackaringa-type deposits with a rejuvenated Broken Hill-type deposit.

Differences observed are possibly a result of the respective emplacement histories of the types of deposits. It is suggested that both groups of deposits were emplaced during the complex retrogression at the waning stages of the prograde metamorphism with later Cambrian retrogression decrepitating fluid inclusions, brecciating veins and causing limited remobilisation. REFERENCES 132.

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SCHNEIDERHOHN, H., (1962) - Die Erzlagerstatten der metamorphen Abfolge, 371 p. Erzlagerstatten. Jena, Fischer, 1962.

SCOTT, S.P., BOTH R.A., AND KISSIN, S.A., (1976) - Sulphide petrology of the Broken Hill region, N.S.W., Australia. Presented at the 25th I.G.C. Symposium 3B, 1976,

SEGNIT, E.F., (1961) - Petrology of the zinc lode, New Broken Hill Consolidated Limited, Broken Hill, N.S.W. Aust. I.M.M. Proc., 199, pp 87-112.

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.•

WINTSCH, R.P. , (1975) - Solid-fluid equilibria in the system KAlSio0o - 3 o NaAlSi30g - Al2Si05 - Si00 - H90 - HC1. Jour. Pet., 16, pp 57-79. 140.

YPMA, P.J.M., (1963) - Rejuvenation of ore deposits as exemplified by the Belledome metalliferous province. Visdruk, Alphen, The Netherlands, pp 212. APPENDIX

Mapping Techniques Detailed Descriptions of Selected Lithologies Laboratory Techniques and Experimental Studies Chemistry of the Broken Hill Orebody and Production Figures for the Major Thackaringa-type Deposits Mapping Techniques

Detailed field mapping was carried out on 1:12000 black and white aerial photographs (Apollyon Valley, Maybell, Daydream and Thackaringa mines), and 2.4 x enlargements of 1:12000 photographs (Umberumberka). The geological plan of the Daydream mine (Plan 5) was carried out using compass and chain technique for geological control.

Due to the severe lack of outcrop in many areas underground, geological investigations assisted greatly with surface interpretations, e.g., the Daydream mine, mines in the Apollyon Valley, Davy's workings at Thackaringa and the Mt. Robe mine.

Structural mapping was difficult in most areas and was based on layering trends, vein trends and high grade schistosities determined by optical methods.

The field mapping program was time intensive because of the paucity of suitable ore samples from most deposits and many fruitless hours were consumed.

Detailed petrological studies have supported lithological inte rpre tation.

Approximately 500 m of diamond drill core was logged (from two drill holes at Umberumberka drilled for Silverton Tramway). 2.

A2 Detailed Descriptions of Selected Lithologies

There are a number of important critical thin sections observed during this study. Descriptions of these are presented with anotated sketches. These include:-

Description Locality UMl/75 Hornblende,amphibolite Umberumberka UM14/75 Diopside, tremolite, calc-silicate Umberumberka UM32/75 Orthopyroxene,granulite Umberumberka UM74/75 Metadolerite Umberumberka APV132/75 Adamellite Apollyon Valley APV142/75 Schist Apollyon Valley DD180/75 Adamellite Daydream TH215/75 Retrogressed amphibolite Thackaringa MR312/75 Amphibolite Mount Robe M303/75 Quartz,fluorite gangue Mayflower

Abbreviations for Anotated Sketches

Hb hornblende Plag plagioclase Qtz quartz Diops diopside Trem tremolite Hyp orthopyroxene (hypersthene) Sec Amph secondary amphibole Aug augite Orth orthoclase Muse muscovite Biot biotite Chid chloritoid Chit chlorite Ilm ilmenite Staur staurolite Ser sericite Per perthite Fluor fluorite 3.

UM1/75 - Amphibolite Assemblage quartz (subhedral grains and small veinlets), plagioclase (anhedral) and partly sericitised, hornblende (light brown to olive rarely twining with poikiloblastic inclusions of quartz and plagioclase.

Accessories - ilmenite, apatite. Texture - hackly to partly granuloblastic.

UM14/75 - Calc-silicate Diops Assemblage - diopside (pale green to yellow green), poikiloblastic Trem plates, tremolite (hackly intergranular masses), hornblende yy.v\ (brown green-green), plagioclase (minor), quartz (vein-like masses). Texture - irregular and poikiloblastic.

i------1 2 5 mm 4.

JM32/75 - Orthopyroxene Granulite Assemblage - Quartz and plagioclase (equigranular polygonal), orthopyroxene (light green-pink), poikiloblastic, clinopyroxene(non-pleoctroic), hornblende > (olivine brown). Accessories - Ilmenite. Texture - Granuloblastic. Metamorphic Grade - granulite facies.

•------* 0 - 3 m m

UM74/75 - Metadolerite Assemblage - Augite (porphyroblastic) relic grains, secondary amphibole, (rare relic) sericitised plagioclase laths. Texture - hackly porphyroblastic.

j 2 mm 5.

APV132/75 - Adamellite Assemblage - Orthoclase (simple twinned, un­ twinned, poikilitic plagioclase (zoned and complexly twined). Muscovite and biotite laths. Accessories - Apatite, zircon.

2-5mm

APV142/75 - Retrograde Schist Assemblage - Garnet (inclusion free), and staurolite (rotated), pre- kinematic biotite (straw brown), and chlorite, sericite and post kinematic chloritoid. Accessories - Ilmenite.

2 mm 6.

DD180/75 - Adamellite Assemblage - Poikilitic laths of plagioclase (clustered with inclusions of quartz and sericite), perthitic alkali felspars, albite and aggregates consisting of muscovite and biotite. Accessories - Magnetite, zircon.

Orth

Plag Muse

s V J

2mm

Chit TH215/75 - Disequilibrium Amphibolite

Assemblage - Relic euhedral amphiboles (light brown- brownish green) replaced by biotite and chlorite and lower grade amphiboles (yellowish green bluish green), large plagioclase grains (completely sericitised) and poikiloblastic garnets. Accessories - Ilmenite, sphene.

2 mm 7.

MR312/75 - Low Grade Amphibolite Assemblage - Hornblende (subhedral) with secondary amphibole (overgrowths), quartz and plagioclase (minor and sericitisedJ Accessories - Ilraenite and sphene. Texture - hackly irregular.

i------1 2mm

M303/75 - Quartz Fluorite Gangue Assemblage - quartz and fluorite and galena. Accessories - biotite, magnetite. Texture - flowage.

Fluor

2 *5 m m 8.

A3 Laboratory Techniques and Experimental Studies

Associated with the field mapping program the following laboratory studies have been utilised

1. The preparation and observation analysis of 224 thin sections. 2. The preparation and analysis of 170 polished sections. 3. The preparation of 106 polished thin sections for fluid inclusion studies.

As well as normal optical techniques, the following additional techniques were used:-

1. Modal analysis using the point count technique. One thousand counts were recorded on each thin section. A traverse spacing of h x average grainsize was adopted.

UM21 APV APV APV APV DD DD DD Poolamacca C-D* Thin Section (%) 129 139 132 140 173 180 184 Granite Granite Quartz 33 38 42 46 40 33 33 38 26 27 Pot. Fels. 39 18 19 21 25 37 47.6 44 34 33 Plag. 22 35 25 15 22 23 13 15 22 23 Mus. 5 8 12 16 10 4 3.0 0.9 15 14 Biot. 1 1 1 2 3 3.0 1.6 2 3 Opa. 1 1 0.4 0.3 1

* Data from Hagarty (1974).

2. Estimation of plagioclase compositions using the Schusters method on the albite twinning and (001) cleavage. 9.

Experimental Studies

1. Electron Microprobe Analyses

Six sulphide primary phases were analysed

No. of Analyses Elements Analysed______Galena 40 Sb, Bi, Ag Sphalerite 11 Cd, Sb, Fe, Mn, Cu, As, Bi, Ag Chalcopyrite 6 Sb, Cd, Ag, Bi, As Pyrite 17 Ag, As Arsenopyrite 15 Fe , As , S Tetrahedrite 8 Sb, As , Ag, Cu, S , Fe , Zn 97

Minor elements were met with difficulty but overcome using a number of mineral and metal standards of known low concentration. This study was undertaken at the C.S.I.R.O. Soils Division, Adelaide, on a Cambridge instrument electron microprobe with two crystals and four detectors. All of the instrumental work was done by Dr. K. Norrish.

Results are presented in their crude form and values less than 2 x standard deviation (

Galena Samples

Sb(ppm) Ag(ppm) Bi(ppm where not marked)

Koh-i-Nor 43 66 132 26 n .d. 217 Britisher 38 9 352 31 11 339 Lubra 336 331 186 264 426 238 Daydream 81 54 246 54 63 212 other circle 99 30 54 Golden Crest 79 1324 4672 23 1375 4870 Black Prince 58 2347 9329 36 2640 9552 other sample 158 3133 2.6% 108 3257 1.2% 151 3364 1.4%

British Shear Veins 207 664 206 227 754 128 Orient 161 30 195 195 21 135 Umberumberka 432 202 307 398 153 218 404 201 186 Apollyon Valley (155) 134 84 71 166 66 134 146 60 533 Apollyon Valley (133) 102 n.d. 253 112 36 209 Apollyon Valley (164) 29 2117 5207 45 1819 5128 Mt. Robe 262 2514 6060 249 2883 6599 Terrible Dick 281 104 180 258 142 182 Metahydrothermal 807 209 233 Veins, Main Lode 923 265 205 other circle 773 463 249 Thackaringa 1040 575 264 977 754 499 11

1.2 Chalcopyrite Samples

Sb(ppm) Cd(ppm) Ag(ppm) Bi(ppm) As(ppm) Apollyon Valley (164) n.d. 28 208 547 24 n. d. n.d. n.d. 507 n.d. Thackaringa 24 n.d. 7 594 n.d. 793 94 534 589 141 Umbe rumberka 664 n.d. 1920 668 335 Orient 49 n.d. 50 592 43

1.3 Sphalerites (all in ppm where not marked)

Sb Cd Ag Bi As FeS Fe Mn Cu MnS Metahydrotherma1 n.d. 2024 21 99 136 14.3 9.08% 3251 107 0.515% Veins, Main Lode 14 2033 n.d. 38 149 17.9 H.35% 3175 178 0.50% Thackaringa n.d. 3574 50 102 141 5.3 2.66% n.d. 83 - n.d. 3665 40 118 106 5.9 3.8% 35 120 55 Britisher n.d. 1405 n.d. 62 226 12.3 7.8% 772 143 0.12% n.d. 1391 n.d. 56 260 15.0 9.48% 809 105 0.13%

Mt. Robe n.d. 2607 40 141 265 - - - - -

Orient ------Umbe r umbe rk a n.d. 3436 42 101 251 0.81 0.52% n.d. 134 - n.d. 3267 57 68 235 0.39 0.25% n.d. 58 - Terrible Dick n.d. 2242 36 53 226 0.3 2021 74 94 - n.d. 2416 n.d. 83 271 0.21 1339 n.d. 241 -

1.4 Pyrite Samples (all in ppm)

Ag As Ag As Umberumberka 123 68 Orient n.d. n.d. 62 73 19 n.d. Thackaringa n.d. 132 Lubra 47 814 11 1139 Britisher n.d. 34 3783 45 59 110 Terrible Dick 66 638 * 33 246 191 4105 Black Prince n.d. 503 280 504 83 41 T 34 94 12.

Arsenopyrite Samples (all in %)

Fe As S Total Thackaringa 33.7 41.9 21.2 96.8 (traverse across 33.3 44.7 19.7 97.7 zoned crystal) 33.95 44.5 19.9 100.35 35.5 44.4 19.8 99.7 34.8 42.1 20.6 97.5 35.2 44.3 20.5 100.0 Terrible Dick 36.0 43.1 21.0 100.1 34.6 42.8 20.7 98.1 34.5 44.2 19.7 98.4 Silver King 34.9 43.3 20.2 98.4 34.4 43.8 19.8 98.0 Orient 33.2 45.6 18.8 97.6 35.5 47.3 18.5 101.3 Umberumberka 35.2 43.4 21.3 99.9 35.8 43.9 20.7 100.4

1.6 Tetrahedrite Samples (all in %)

Sb As Ag_ Cu S Fe Zn Total Thackaringa 26.5 0.5 21.9 25.8 22.3 1.5 2.9 101.4 24.0 0.85 22.5 23.9 22.5 2.6 2.85 99.2 Umberumberka 25.6 0.15 23.5 25.2 21.4 1.8 2.55 100.2 25.0 0.18 18.5 29.1 19.8 3.0 2.98 98.53 Daydream 27.3 0.92 0.98 41.92 23.8 2.7 2.0 99.62 25.7 2.45 2.10 41.3 23.9 2.3 2.35 100.1 Apollyon 22.5 3.1 19.4 27.8 22.8 2.1 2.2 99.9 Valley (164) 24.5 3.0 11.47 34.4 23.7 1.8 2.6 101.53 13.

2. X-Ray Studies

2.1 Graphite Measurements

Using the analytical method described by McKirdy (1975), the dooz interplanar spacing was measured using CuK c< radiation, Ni filter, 40 KV and 20 mA,goniometer scan rate 1°29 /min, with a chart speed of 75 mm/degree. The ratio of height to width at half height gave values of 3.375 - .005 (Terrible Dick), 3.38 - .005 (Umberumberka) and 3.385 - .005 (Umberumberka), (Fig. Al).

The results show that the graphite belona to the original retrograde metamorphism not the lower temperature ore-bearing veins.

2.2 Arsenopyrite Studies

Method - A quantity of arsenopyrite from the Pioneer Mine material was crushed using mortar and pestle to -300 B.S.

The sample was mixed with an internal standard (CaF2: a = 5.4626 )

Using CuK o< radiation, Ni filter, 40 KV and 20 mA, goniometer scan rate of ^/min, with a chart speed of 5 cms/degree, the

df3i spacing was scanned.

From four scans the following measurements of 29 were recorded

Aspyrites -

50.230 - .005 ) 56.180 - .005 ) Mean 26 = 56.220- .005 56.220 - .005 ) 56.170 - .005 )

Corrections were made for alignment of the goniometer and scale

variations in the machine. Fig. Al - Graphite doo2 spacing vs. metamorphic grade o CN O •H i + — (1) p

- U h G Cr> 02 ft as

•H ft -P V-l a) o c u o

o ' < —

14

(increasing metamorphic grade) 15.

The mean d.^^ spacing measured is 1.6335 - .001. The experimental studies of Clark (1960) relates the d^2^ spacing variation with temperature(°c). The temperatures corresponding with a spacing of 1.6335 are in the range 350°-475°C.

This is far in excess of expected temperatures of ore deposition based on fluid inclusion studies.

3. Mineral Identification Studies

3.1 Silver Halides

The method described by Barclay and Jones (1971) was adopted to study silver halides from the Pioneer and in Davy's workings.

Iodine was determined using a Phillips X-ray analyser operating a Molybdenum X-ray tube operating at 40 KV and 20 mA. A lithium fluoride crystal was the diffraction crystal and intensities were recorded using a scintillation counter.

The samples were pressed after preliminary crushing into circular pellets in aluminium containers. The results are crude and low count rates

Chlorine and bromine were determined by the powder camera method. Grains of silver halide were crushed and mixed with a gum and benzene mixture and rolled into balls and mounted on a hair in the camera. The d200 spacings measured are as follows:-

Mole % d200 Ag Br + Davy's workings 2.850 .005 70% + 2.822 .005 41% + 2.813 .005 32% + Pioneer mine 2.805 .005 28% 16.

3.2 Identification of Minerals

The following minerals were identified by x-ray diffraction.

(a) Bindheimite - PbnSb_0..(0, OH) ------z 2 6

Intensity Bindheimite Bindheimite (experimental) (this study) 3.03 2.99 2.62 2.59 1.85 1.85 1.58 1.57 1.51 1.50 1.31 1.37 1.17 1.19

(b) Cerussite and anglesite were both identified in this study by x-ray diffraction. Results are not included.

4. Fluid Inclusion Studies

This section is divided into three parts:-

(a) Detailed descriptions of fluid and solid inclusions (see text). (b) Freezing experiments. (c) Heating experiments.

Freezing Experiments

Equipment used included a Leitz Wetzlar-Ortholux with a special high intensity light source and an R. Chaix Meca dual heat/freeze stage with automated digital temperature readout and heat/freeze controls.

The 1 cm diameter, 0.3 mm thick doubly polished plates were ♦* broken up and a small piece mounted on the freezing stage. The selected inclusions (commonly primary inclusions) were centred in the viewfinder. * 17.

High pressure nitrogen gas was re-circulated through the stage and a coiled apparatus which was emersed in liquid nitrogen (-180°C).

The temperature was decreased to -80°C and either

(a) allowed to warm up at a normal rate (i.e., 1° every 30 seconds in the freezing point ranges), or

(b) maintained at -80°C for 30 minutes and followed by (a) above.

At critical temperatures the temperature increase rate was slowed for more accurate temperature determinations. The

accuracy of the digital readout was - .05°C (h limit of the readings).

Because of the difficulty of isolating suitable primary inclusions heating runs were carried out on the same inclusion immediately after the freezing runs.

Heating Experiments

These involve heating the grain until the gas filled bubble in each inclusion disappears. The heating apparatus for these experiments is an electrical heating coil which heats up a brass plate that the grain sits up. The difficulty with estimating homogenisation temperatures is that as this temperature is approached the gas bubble

(a) commences in brownian motion, and (b) decreases in size.

For this reason, a number of heating runs were carried out on the same inclusion. If the true homogenisation temperature had been reached the bubble would not appear immediately with a decrease in temperature.

The heating experiments are also useful in determining daughter crystals. Table A1 shows the melting points of common daughter minerals. 18.

M.P. Halite 801° Sylvite 776° Anhydrite 1456° Gypsum 128° Mg carbonate 350° Ca carbonate 1359° Fe carbonate decomposed

The mineral gypsum was identified when it melted at 135°C. 19.

A4 Chemistry of the Broken Hill Orebody and Production Figures for the Major Thackaringa-type Deposits

A list of the minerals present in the Broken Hill orebody which are found in the Thackaringa-type deposits is shown below

Primary Zone Galena Sphalerite Tetrahedrite Arsenopyrite Pyrite Pyrrhotite Chalcopyrite Cubanite Loellingite Silver Ruby silver minerals (stephanite, pyragyrite, etc)

Secondary Zones Limonite Goethite Pyrolusite

Cerussite Azurite Malachite Aragonite Anglesite Brom-argyrite Gypsum Cuprite Chrysocolla Kaolinite Atacamite Phosgenite Covellite 20.

Gangue Quartz Fluorite Calcite Siderite Mangano-calcite Graphite

Bindheimite found in the oxidised zone of Thackaringa-type deposits does not occur in the Broken Hill main lode. 21.

TABLE A2

SELECTED ORE PRODUCTION AND GRADE FIGURE FOR MOST MAJOR THACKARINGA MINES

Production Mine (tons) Grade Comments

Pioneer 20,000 Ag 40 - 12 ozs Mainly oxidised ore - Pb 65% - 25% cerussite, anglesite, malachite Umberumberka 15,000 Ag 76 - 26 ozs Barites, cerussite, Pb 30% - 10% galena Daydream 10,000 Cerargyrite, bindheimite, phosgenite, malachite, azurite Gypsy Girl 10,000 Ag 50 ozs Divided by granite body - Pb 72% cerussite, trace gold, malachite Lily Mine 1,000 Ag 428 - 520 ozs Pb 56% - 28% Cerargyrite, malachite, azurite Mt. Robe 1,000 (?) Ag 3 - 2 ozs Fluorite quartz gangue, Pb 20% galena, small amount of silver Bonanza 650 10-30" vein of galena, cerussite Apollyon Valley 550 Galena, cerussite, native silver, cerargyrite and embolite Outward Bound 500 Ag 40 ozs WNW dipping to the south. Pb 60% 1 oz Au to ton, width 42" to 18" Mayflower 360 Ag 9 ozs Gold - 16 cwts

Barrier Chief 350 Ag 2300 - 175 ozs Strike N, dip 70°E, silver Pb 12% chlorides, bismuth, galena Lady Brassy 350 Ag 40 ozs Silver rich, galena and Pb 65% - 70% arsenical pyrite Goat Hill 280 N20°W dipping at 60°E, galena, cerussite, malachite, pyrite, calcite Hen & Chickens 250 Ag 150 - 25 ozs Two lodes separated by Pb 9% - 5% granite. AgCl, cerussite, malachite, azurite, galena New Year 250 Ag 263 ozs Native silver, cerussite, Pb 47% phosgenite, ceragyrite Gypsy Boy 240 6" to 18" galena, cerussite ,* Alberta 206 Cerussite, galena, flat dipping lode Hercules 200 Ag 40 - 80 ozs -'V Dips east at 50°, Pb 50% - 70% cerussite, galena Eleven Over Six 100 Same as Alberta 22.

Hidden Secrets 100 Ag 12 ozs Galena, pyrite, lode Pb 11% changed 15' - 20' Lubra 88 Cut a granite dyke at 300', galena, silver chlorides, native silver, malachite Bobby Bums 70 Ag 17 ozs Galena, cerussite in Pb up to 72% hanging wall. Richard Ruby 60 Ag 20 - 30 ozs Calcite, cerussite, angles Pb 40% - 60% ite, galena, dip at 55° Homeward Bound 60 2-3' wide, west dipping vein Comstock 56 Fault striking NE, dip 50°SE, galena up to 3' thick Elsie May 50 2 miles W of NY, galena, silver chlorides, pyrite Maggie's Secret 30 18' vein intersected Wheel of Fortune 30 Ag 40 ozs Silver chloride, 'grey' ore A1 Mine 15 Ag 50 ozs Steely galena, silver Pb 50% chlorides Gemini 10 Ag 150 - 260 ozs Fairly rich in silver Pb 17% - 60% but variable in galena Lady Dorothy 10 Ag 36-8 ozs Pb 62% - 6% Others in 350 Thackaringa Mines