Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems

Mueller, Andreas G.

doi:10.1007/s00126-018-0813-5

Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems

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Published: 02 July 2018

Volume 55, pages 697–730, (2020)
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Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems

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A Correction to this article was published on 19 February 2020

This article has been updated

Abstract

The Golden Mile deposit (1751 t Au to 2017) in the Archean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is located in a km-scale envelope of chlorite-calcite-albite alteration overprinting metamorphosed tholeiitic rocks. The propylitic zone (310–340 °C) is centered on D2 strike-slip faults segmenting D1 folds. At the southeast margin of the deposit, 200 m wide hydrolytic paragonite-ankerite-chloritoid alteration (< 0.1 g/t Au) associated with the D2 Trafalgar Fault forms a thermal anomaly (405–425 °C). The Trafalgar alteration zone merges with barren ankerite-sericite replacement in propylitic Paringa Basalt bound to the contact with Golden Mile Dolerite, which extends > 1.5 km northwest to the Paringa South mine. In the mine, D2 faults control Fimiston ore locally overprinted by Oroya ore, whereas D3 reverse faults offsetting the D2-system control only Oroya ore. Representative ore bodies (lodes) were studied by petrographic, geochemical, and thermometric methods. Fimiston ore (4–20 g/t Au) is refractory, characterized by 10% pyrite ± chalcopyrite ± tennantite, and varies in oxidation state (magnetite stable to hematite stable). Gold and tellurides form inclusions in pyrite. In propylitic GMD, lode alteration is zoned from albite stable in veins and selvages (400 °C) to outer hydrolytic phengite-quartz-ankerite replacement (370 °C). The younger Oroya ore in D3 faults contains free gold and is oxidized (5 g/t Au; pyrite-hematite) or reduced (50–120 g/t Au; pyrite-pyrrhotite) close to carbonaceous black schist. Oroya Stage 1 silica-chlorite-siderite replacement (400–425 °C) and As-rich pyrite-arsenopyrite-sphalerite mineralization is crosscut by Stage 2 silica-ankerite fill (385 °C) and Sb-rich pyrite-tetrahedrite-telluride mineralization associated with vanadian muscovite. Mass-balance calculations indicate that the Oroya fluid carried higher concentrations of silica, Fe, V, Ni, and Te than the older Fimiston fluid.

Structural setting of Fimiston- and Oroya-style pyrite-telluride-gold lodes, Paringa South mine, Golden Mile, Kalgoorlie: 1. Shear zone systems, porphyry dykes and deposit-scale alteration zones

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A Palaeoproterozoic multi-stage hydrothermal alteration system at Nalunaq gold deposit, South Greenland

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Early Fimiston and late Oroya Au–Te ore, Paringa South mine, Golden Mile, Kalgoorlie: 4. Mineralogical and thermodynamic constraints on gold deposition by magmatic fluids at 420–300 °C and 300 MPa

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Introduction

The Golden Mile deposit is located at 30° 47′ south latitude and 121° 29′ east longitude in the Kalgoorlie mining district of the 2.7 Ga Eastern Goldfields Province, Yilgarn Craton, Western Australia (Fig. 1). Production is subdivided into underground (1893–1992, 105.6 Mt at 11.27 g/t Au) and open pit (1984–2017, 291 Mt at 1.93 g/t Au recovered grade). About 80–85% of the total gold occurs as inclusions (0.5–20 μm) in pyrite and 15–20% in free Au-Ag tellurides (Travis et al. 1971), the two types of ore commonly termed Fimiston- and Oroya-style, respectively (e.g., Hagemann and Cassidy 2000). The structural setting, broad alteration zones, and associated porphyry dykes of the giant deposit are described in Mueller (2017) as part of this issue with emphasis on the Paringa South mine.

Krusch (1903) and Lindgren (1906) recognized that the ore bodies (lodes) represent sericite-ankerite replacement zones in fractured shear zones defined by the amount of disseminated pyrite. Simpson (1912) and Stillwell (1929) noted that the two largest ore bodies, the Horseshoe No. 3 and No. 4 lodes had central zones of chert-like silica. On levels of the Golden Horseshoe mine 90–244 m below surface, the siliceous core varied in thickness from 2.2 to 5.5 m and in average grade from 29 to 84 g/t gold (Krusch 1903). Tomich (1952, 1959) subdivided such replacement into early grey flint (3.4–5.2 g/t Au), and into late quartz breccia characterized by free gold, tellurides, and Green Leader vanadian muscovite. Sketches and photographs of individual lodes are in Larcombe (1913) and Clout et al. (1990).

Although the general nature of the lodes has been established, an integrated approach is lacking in previous studies. This one is the first combining structural mapping in the Paringa South underground mine, petrography, follow-up microscopy, and multi-element geochemistry to document ore bodies mined out during the recent open pit operation. Structural relations, silicate-carbonate, sulfide-oxide and sulfide-telluride assemblages, chlorite thermometry, and mass-balance calculations are employed to constrain the oxidation/sulfidation state of the ore and the local fluid evolution in space and time. This second contribution on the Paringa South mine precedes another two (this thematic issue) presenting electron microprobe analyses of gangue and ore minerals and related quantitative P-T-X constraints.

Pre-sulfide muscovite-paragonite-ankerite alteration bound to the contact Paringa Basalt-Golden Mile Dolerite extends from the mine southeast and merges with km-scale paragonite-ankerite-chloritoid alteration centered on the Trafalgar Fault. This zone, neglected since Prider (1947), is described in relation to propylitic alteration in the Golden Mile and discussed with respect to other chloritoid-bearing zones in the Eastern Goldfields Province containing pyrophyllite, kyanite and natro-alunite (Fig. 1).

Geologic setting

The Kalgoorlie district is located in the Eastern Goldfields Province (EGP) of the Yilgarn Craton, a continental-margin orogen characterized by 2.73–2.65 Ga greenstone belts. The EGP is composed of the Kalgoorlie, Kurnalpi, and Burtville Terranes (Fig. 1). The stratigraphic, structural, and magmatic-metamorphic evolution of the southern Kalgoorlie Terrane is reviewed in Mueller et al. (2016, this issue).

The lodes in the Golden Mile are confined to three stratigraphic units: the Paringa pillow basalt, the 600–750-m-thick Golden Mile Dolerite sill, and the Black Flag greywacke. The Golden Mile Dolerite (GMD), emplaced at the basalt-greywacke contact, is dated at 2685 ± 5 Ma (zircon U-Pb; Tripp 2013). The sill consists of ten petrographic units (Travis et al. 1971). The variolitic GMD units 1 and 10 represent the lower and upper chilled margins, units 2 and 3 are pyroxene-rich, and units 4, 5, and 9 are ophitic quartz gabbro. The central GMD units 6 to 8 differ due to abundant Ti-magnetite and ilmenite, and are interpreted to represent a pulse of iron-rich tholeiitic magma. The Paringa Basalt (PB), Golden Mile Dolerite, and Black Flag greywacke are folded into the Kalgoorlie Anticline and Syncline (D1) and metamorphosed to the actinolite-albite-zoisite greenschist facies (e.g., Mueller 2017, this issue).

The Golden Mile deposit is controlled by the Boulder Lefroy-Golden Mile fault system (D2a), which displaces the regional upright folds (D1) 11–12 km in a left-lateral sense (Mueller et al. 2016, this issue). The steeply SW-dipping shear zones centered on the Golden Mile Fault, collectively termed the Fimiston Lodes, are bounded by the Boulder Lefroy Fault in the southwest and by the Trafalgar Fault in the northeast. The D2a Trafalgar Fault offsets the sub-vertical contact GMD-greywacke 660 m in a sinistral sense (Woodall 1965). Granodiorite porphyry dykes were emplaced into the fault system at 2676 ± 7 to 2671 ± 10 Ma after peak metamorphism but prior to hydrothermal chlorite-calcite alteration. Barren sericite-ankerite zones in the Golden Mile Fault and at the GMD-PB contact of the Kalgoorlie Anticline formed during the intrusion of diorite dykes at 2663 ± 11 Ma (Gauthier et al. 2007; Mueller 2017).

To the northwest, the Golden Mile deposit is displaced about 2 km at the north-striking dextral Golden Pike Fault (D4). Fimiston lodes northwest of this fault are characterized by laminated quartz veins and contain telluride shoots dominated by hessite causing low Au/Ag ratios (0.12–0.35) in the ore (Mueller and Muhling 2013). These peripheral lodes are not part of this study. They are offset at three closely spaced D4 faults, which control the quartz vein ore bodies of the Mt Charlotte deposit (151 t Au to 2017) dated at 2655 ± 13 to 2651 ± 9 Ma (xenotime U-Pb; Rasmussen et al. 2009; Mueller et al. 2016).

The Golden Mile lodes described below are ordered according to the time of formation of the host structure: D2a sinistral strike-slip faults (Fimiston system), D2b sinistral-reverse faults (Australia East system), and D3a reverse faults (Oroya system). The D2a and D2b faults formed in metamorphic rocks prior to the emplacement of granodiorite dykes at ca. 2675 Ma. Golden Mile hydrothermal activity was synchronous with D2c sinistral strike-slip reactivation and with D3a reverse faulting. It ceased before the development of barren D3b thrusts (Mueller 2017; this issue).

Methods and terminology

Selected ore bodies (B-Lode, Kelly Lode, Blatchford Lode, OHW Lode) on levels 4, 6, 7, and 11 of the Paringa South shaft were mapped by the author at scales of 1:12 to 1:25. The locations are shown on level plans in Mueller (2017; Figs. 8, 9, and 12), and are given below in mine grid coordinates. The Flat Lode stoped between the North Kalgurli shaft 13 and 14 levels is described based on reconnaissance sampling. In structural terms, the ore bodies represent altered shear zones composed of brittle fault-fill veins, extension veins, and breccia (Sibson 1977; Robert and Poulsen 2001) and of S-C mylonites characterized by recrystallized quartz and carbonate (Lister and Snoke 1984).

All samples were ground flat and examined (15–60× magnification). In 21 hand specimens, the short wavelength infrared (SWIR) absorption spectra (350–2500 nm) of white mica and chlorite were recorded with the TerraSpec™ instrument of Analytical Spectral Devices and interpreted using The Spectral Geologist™ software (Table 1). Gangue and ore minerals in 70 polished and covered thin sections and in 23 polished mounts were identified in transmitted and/or reflected polarized light. The covered thin sections were stained with Alizarin Red S + potassium ferricyanide solution. Stained calcite is red brown, ferroan calcite dark blue, dolomite is not stained, and ferroan dolomite is turquoise (Deer et al. 1992). In addition, cores from drill holes intersecting the Trafalgar Fault and 24 related thin sections (Prider 1947) stored at the Clarke Earth Science Museum of the University of Western Australia were re-examined.

Table 1 Wall-rock alteration mineralogy, Trafalgar Fault and ore bodies (lodes) in the Paringa South and North Kalgurli mines, Golden Mile, Kalgoorlie

Full size table

The minerals in summary Tables 1 and 2 are named according to 768 wavelength dispersive analyses of silicates, carbonates and oxides, and 580 analyses of sulfides, tellurides, and gold acquired by the author using the JEOL JXA-8530F electron microprobe at the Centre for Microscopy, Characterization and Analysis, the University of Western Australia. Temperatures based on the Al^(IV)-in-chlorite thermometer of Cathelineau (1988) are reported for each mineralized structure (Table 1). The extension of this thermometer to 420 °C has been tested using the “average PT” approach in Thermocalc 3.26 (Powell and Holland 1994), the thermodynamic data in Holland and Powell (1998), and two chlorite-paragonite assemblages listed in the Electronic Supplementary Material (ESM App. 1 and 1a).

Table 2 Mineral assemblages constraining the oxidation and sulfidation state of refractory Fimiston-style and subsequent non-refractory Oroya-style gold mineralization in ore bodies (lodes) of the Paringa South and North Kalgurli mines, Golden Mile, Kalgoorlie

Full size table

The term “silica” is used for chert-like quartz composed of minute granular crystals, and “chalcedony” for silica marked by pores and fibrous quartz (Deer et al. 2004). The term “sericite” denotes unidentified white mica, “phengite” is applied to muscovite with ^[4]Si > 3.1, and “paragonite” to white mica with a ratio K/(K + Na) < 0.2 (Fleet 2003). The carbonates are subdivided by their Mg/(Mg + Fe²⁺) ratio: dolomite > 0.8, ankerite 0.3 to 0.8, and siderite < 0.3 (Chang et al. 1998). Minerals in contact without evidence of reaction are considered in equilibrium and are hyphenated or joined with plus signs. Equilibrium assemblages constrain the local oxidation state (Frost 1991) and sulfidation state (Einaudi et al. 2003) of the ore bodies in f_O2-T and f_S2-T space, respectively (see footnote in Table 2).

Samples pulverized in agate or corundum mills were analyzed at the University of Western Australia for: (1) mineral composition by X-ray diffraction (XRD) and (2) whole-rock major and trace elements by X-ray fluorescence (XRF) using glass discs and pressed-powder pellets (Mueller 1990). Additional trace element analyses were carried out by fire assay, atomic absorption spectroscopy (AAS), and inductively coupled plasma mass spectrometry (ICP-MS) at Intertek-Genalysis, Perth. The analytical details are listed in ESM Appendix 2. A mill-head sample representing 210,410 t of Fimiston sulfide ore mined in 1985/86 from the Lake View and Perseverance shafts was also analyzed (Mueller 1990).

Paragonite-bearing replacement zones

In the Paringa South mine, barren muscovite-ankerite ± paragonite ± chlorite ± magnetite alteration overprints propylitic Paringa Basalt in a zone 150 m thick along the contact with Golden Mile Dolerite on the SW-limb of the Kalgoorlie Anticline (Figs. 2 and 3 in Mueller 2017, this issue). About 1.5 km southeast of the Paringa South shaft, the contact zone closes in the hinge of the anticline and merges with sub-economic paragonite-ankerite-chloritoid alteration in Golden Mile Dolerite centered on the Trafalgar Fault (Fig. 2a). Chloritoid occurs in a zone more than 240 m wide and 1000 m long (Prider 1947). Drill holes TRF-1 and TRF-2 intersect part of this zone (Fig. 2b). The presence of paragonite and chloritoid in barren alteration contrasts with the abundance of muscovite/phengite or chlorite in the ore bodies (Table 1). The paragonite-bearing replacement zones grade laterally into propylitic chlorite-calcite ± epidote alteration (Fig. 2a). In Golden Mile Dolerite, chlorite and Fe-calcite replace metamorphic actinolite and tschermakite in augite sites, and albite ± calcite ± sericite replace zoisite in plagioclase sites (Mueller 2017, this issue).

Trafalgar Fault: paragonite-ankerite zone

In the outer alteration zone of the Trafalgar Fault, the ophitic igneous texture of GMD Unit 9 is preserved (Fig. 3a). Microcrystalline ankerite, minor ferroan calcite, chlorite (3–10 vol.%), and magnetite form pseudomorphs after prismatic augite (1–4 mm). Lath-shaped albite is replaced by paragonite + quartz or, less commonly, by granular albite and quartz. Interstitial igneous quartz (1–3%) and trellised rutile lamellae after Ti-magnetite (2%) characterize GMD Unit 9 (Fig. 3a). Blue-grey chloritoid (1–5%) is in contact with Fe-chlorite, ankerite, paragonite, and quartz (Fig. 3b) and associated with trace tourmaline. Thermocalc temperature estimates vary from 396 ± 32 to 426 ± 34 °C (ESM App. 1a), and agree within error with chlorite thermometry (± 50 °C; Table 1). The zone contains disseminated early magnetite (0.5–2%), locally in contact with chloritoid (Fig. 3c), and late specular hematite, pyrite, chalcopyrite (Fig. 3d), and rare arsenopyrite. Some of the pyrite is zoned from poikilitic cores to solid rims.

Trafalgar Fault: paragonite-albite zone

The inner alteration zone centered on the Trafalgar Fault consists of paragonite-quartz-ankerite schist representing the ductile shear zone and of massive to weakly foliated albite-quartz-ankerite rock (Fig. 2b). Microcrystalline albite constitutes 10–20 vol.% of the schist and 20–40% of the massive rock. The greater abundance of albite and the scarcity of chlorite (about 3%) distinguish the inner from the outer zone. Chloritoid (5–10%, locally 20%) forms fan- or rosette-shaped aggregates (Fig. 3e). Tourmaline and apatite are accessories (Fig. 3f) but magnetite and hematite are absent. Clinozoisite-epidote (up to 5%) is part of the paragonite-albite assemblage locally (Fig. 3g). Subhedral pyrite (10–250 μm) is abundant in spaced zones (5–15%), where it forms aggregates with accessory chalcopyrite and gersdorffite (Fig. 3h). A bulk sample (TRF-2, 126–152 m) contained 75 ppb gold (ESM App. 2b).

Remnant ophitic texture defined by lath-shaped paragonite-quartz pseudomorphs, enclosed in igneous quartz (1–3 vol.%) and in trellised rutile lamellae (1–5 mm), is preserved in the albite-rich rocks. These are crosscut by calcite-ankerite veinlets and by banded veins up to 2 cm thick. The veins are zoned from chloritoid margins to cores of quartz and sparry ankerite and contain accessory pyrite and rutile.

In the schist, primary textures are obliterated and rutile forms granular streaks parallel to foliation. Seams of fine-grained paragonite wrap microcrystalline quartz-albite-ankerite aggregates. Tourmaline needles and a few chloritoid crystals are fractured and pulled apart. Pressure shadows of paragonite and ankerite are common at chloritoid and pyrite crystals. Some paragonite plates and most chloritoid aggregates are post-kinematic and oriented at a high angle to the foliation (Fig. 3e).

Contact zone in Paringa Basalt

Pillows on the Paringa South shaft 6 level (49,320 m N, 19,905 m E), about 5 m below the GMD contact, consist of the alteration assemblage ankerite-quartz, minor white mica and albite, and rare rutile and chlorite. The XRD {001} peak of mica is split into two maxima, one at 9.90 Å and the other at 9.71 Å indicating muscovite interlayered with paragonite. Pillow basalt hosting the Blatchford Lode above the Paringa South shaft 11 level, about 30 m below the GMD contact, is composed of the same minerals except that the mica is muscovite (d₀₀₁ = 9.97 Å). Granular rutile (1–15 μm) is disseminated (3 vol.%) but trellised rutile after Ti-magnetite is absent. Micro-phenocrysts and rare glomerocrysts of plagioclase (1–5%) are pseudomorphed by granular albite ± quartz and embayed by ankerite and muscovite.

The barren alteration zone also overprints two NW-striking quartz diorite dykes crosscutting Paringa Basalt (Fig. 16 in Mueller 2017). They display progressive replacement of albite by muscovite + quartz ± dolomite from the center to the margin: the whole-rock K₂O/Na₂O ratio increases from 1.64 to 6.67, and inner chlorite after hornblende changes to outer muscovite-ankerite pseudomorphs (Stillwell 1929).

Contact zone in GMD Unit 1

The main crosscut on the Paringa South shaft 7 level exposes Golden Mile Dolerite Unit 1 in contact with altered Paringa Basalt. The unit is characterized by orbicular albite-ankerite aggregates (5 mm) and by trellised rutile lamellae after Ti-magnetite (0.5 mm). Grains of igneous quartz (1 vol.%) and apatite (0.5%) are preserved. The GMD 10 m southwest of the PB contact consists of paragonite (30%), quartz (15%), albite (10%), ankerite (25%), chlorite (15%), and rutile (2%). Paragonite and quartz replace albite in lath-shaped sites. Ankerite and chlorite are pseudomorph after prismatic augite (0.5–1 mm). The paragonite XRD {001} peak at 9.67 Å indicates minor phengite interlayers. Chlorite thermometry (370 ± 50 °C) is consistent with Thermocalc estimates of 357 ± 54 °C to 394 ± 50 °C (ESM App. 1/1a).

Fimiston lodes in the Paringa South mine

Fimiston-style refractory ore accounts for 80–85% of the gold in the deposit (Travis et al. 1971), mostly controlled by D2a shear zones dipping steeply southwest (Fig. 2a). In the Paringa South mine, the B-Lode and Kelly Lode are representative of Fimiston-style ore. A post-porphyry phase of syn-mineralization (D2c) sinistral slip along a line pitching 10–20° SE is recorded on the Kelly Lode (Mueller 2017, this issue).

Paringa B-Lode

The B-Lode was mapped on the Paringa South shaft 6 level (194 m above sea level) in the 602 stope (48,900 m N; 19,605 m E). Quartz-ankerite ± pyrite veins 0.2–3 cm thick control red replacement selvages 5–15 cm wide, which grade outward into green-grey GMD Unit 4 (Fig. 4). The white mica is phengite (Table 1). The red selvages are marked by phengite seams sub-parallel to the veins. Quartz-ankerite ladder veinlets connect the seams, and ladder veins filled with quartz, ankerite and specular hematite connect the main fault-fill veins. The outer GMD contains crackle fractures lined with magnetite and chlorite, and veinlets filled with quartz, albite, ankerite, and magnetite. Both form a network roughly parallel to the lode orientation (Fig. 4).

Inner phengite-albite zone

The red selvage (Fig. 5a) is composed of heterogranular albite (35–40 vol.%), phengite (20%), ankerite (15–20%), quartz (10%), rutile lamellae after Ti-magnetite (2%; 0.5–3 mm), and fluor-apatite (0.5%). Most textures are secondary (Fig. 5b). Fine-grained hematite (3.97 wt.% TiO₂; 0.51% V₂O₃) and pyrite occur disseminated throughout, and magnetite is rare in a 0.5–5 cm wide zone bordering the veins (Fig. 5a). Pyrite decreases outwards giving way to disseminated magnetite (0.34 wt.% V₂O₃). Although magnetite, hematite, and pyrite are locally in equilibrium (Fig. 5c), defining a high oxidation and an intermediate sulfidation state (Table 2), hematite and pyrite also rim and partly replace magnetite (Fig. 5d). The pyrite contains inclusions (1–5 μm) of hematite, chalcopyrite, coloradoite, hessite, and petzite. The Au/Ag ratio of the ore is low (Table 2).

Outer phengite-ankerite zone

The outer GMD Unit 4 is characterized by disseminated aggregates of pale brown trellised rutile tracing grains (2–7 mm) of Ti-magnetite (Fig. 5e). Other igneous textures are pseudomorphs after prismatic augite or lath-shaped plagioclase (Fig. 5f) and interstitial grains of quartz (2%) enclosing apatite needles. The augite sites (about 35 vol.%) are replaced by ankerite, and by minor chlorite and subhedral magnetite (0.12 wt.% V₂O₃). The plagioclase sites (50%) consist of phengite and minor granular quartz and albite (5–10%). Secondary magnetite (0.1–1 mm) replaces chlorite and ankerite in trellised rutile (Fig. 5g), and specular hematite rims magnetite locally. Sulfide aggregates are composed of arsenical pyrite, chalcopyrite, tennantite (Fig. 5h), and rare cobaltite (Table 2).

The average composition of the altered rock in the phengite-ankerite zone is 5 vol.% chlorite, 45% phengite, 25% ankerite, 10% quartz, 5–10% albite, and 3% rutile. In contrast, chlorite-calcite altered GMD Unit 4 exposed on the Perseverance shaft 23 level consists of 25% chlorite, 2% epidote, 25% calcite, 30% albite, 2% quartz, 3% rutile, and 7% sericite, the latter replacing plagioclase laths. The change in composition and a temperature gradient indicate that the phengite-ankerite zone of the B-Lode (370 °C) overprints outer chlorite-calcite alteration (340 °C; Table 1).

Paringa Kelly Lode

The Kelly Lode was mapped parallel to strike on the Paringa South shaft 4 level (255 m above sea level) in the east wall of the drive at 49,275 m north and 19,707 m east. The section illustrates the termination of the lode to the southeast (Fig. 6). A thick brecciated fault-fill vein marks the footwall boundary, and a parallel array of thinner veins extends into the hanging wall. Channel samples cut across the footwall vein indicate grades of 3.9 to 8.8 g/t gold, higher than those in the hanging wall array at the same northing (Fig. 6). The host rock is chlorite-calcite altered GMD Unit 2.

Fault-fill and ladder veins

The footwall boundary vein is up to 15 cm thick and composed of heterogranular fill, micro-brecciated fill, and breccia (Fig. 7a). Heterogranular fill (0.1–3 mm) consists of strained quartz crystals (10–90 vol.%), dolomite zoned to ankerite rims (5–60%), ferroan calcite (1–20%), albite (1–10%), muscovite (2–5%), pyrite (1–3%), and accessory (0.5%) tourmaline, chlorite, rutile, apatite, and chalcopyrite. In places, quartz-ankerite cement encloses angular vein fragments (Fig. 7b) of quartz ± carbonate or quartz-albite-muscovite. Wall-rock fragments (0.5–10 cm) are replaced by twinned albite (Fig. 7c), zoned dolomite-ankerite (10–30%), quartz (5–30%), muscovite (2–15%), and pyrite (2–7%). Some are rimmed by tourmaline (1–5%). Late breccia at the footwall contact contains fragments of chloritic GMD Unit 2 (Fig. 7a) cemented by dolomite, ankerite, and Fe-calcite. The carbonates form bands of granular or comb-textured crystals alternating with bands of quartz, albite, and muscovite. The fault-fill veins in the hanging wall (Fig. 6) are mostly 2–10 mm thick, similar in mineralogy to the main vein, and linked by quartz-carbonate ladder and crackle veinlets. Ladder veinlets are characterized by muscovite and chlorite plates oriented perpendicular to the walls and, locally, by selvages of disseminated tourmaline.

Vein replacement selvages

Pyrite-rich selvages extend 0.5–10 cm from the veins (Fig. 6) grading first into wall rock mottled by cm-sized chloritic domains and then into chlorite-calcite altered GMD Unit 2. The fine-grained inner zone consists of phengitic muscovite (15–40 vol.%), granular quartz and albite (25–45%), dolomite and ankerite (25–35%), pyrite (10%), trellised rutile (0.2–1 mm), and accessory (0.3–2%) chlorite, tourmaline, and apatite (Tables 1 and 2). Igneous textures are not preserved (Fig. 7d). Vein-parallel muscovite seams are present, and some pyrite crystals have muscovite-quartz pressure shadows but are not rotated. Chlorite in contact with pyrite crystallized at 390–400 °C. Pyrite contains inclusions of chalcopyrite (Fig. 7e) and micron-sized grains of coloradoite, melonite, and petzite. Iron oxides are absent. The whole-rock Fe³⁺/Fe²⁺ ratio is lower than in the B-Lode (Table 2).

In the chlorite-mottled transition zone, the amount of disseminated pyrite decreases gradually to about 1% and the igneous ophitic texture is partly preserved (Fig. 7f). The chlorite is remnant and has the same SWIR absorption and temperature estimate (340 °C) as chlorite in the footwall of the lode (Table 1). Augite sites (0.5–3 mm) consist of ankerite (40–50%), dolomite (20–30%), chlorite (5–25%), muscovite (5–10%), and rutile (0.5%). Felsic sites after lath-shaped plagioclase (0.2–1.5 mm) are composed of muscovite (30%), granular quartz and albite (55%), and calcite (5%). Rutile forms lamellae after Ti-magnetite.

Chlorite-calcite altered GMD Unit 2

The GMD in the footwall of the lode is composed of chlorite (60 vol.%), granular quartz and albite (15%), lath-shaped albite (10%), ferroan calcite (10%), muscovite and ankerite (3%), unstrained rutile trellis after Ti-magnetite (0.1–0.8 mm), and trace pyrite and chalcopyrite (0.1%). Igneous quartz is absent. Augite crystals up to 5 mm long are replaced by chlorite (Fig. 7g) and by remnant metamorphic actinolite needles (0.5%). Albite laths (0.2–0.7 mm) are enclosed in chlorite-calcite aggregates after augite interstitial to the larger crystals. Calcite is zoned to iron-rich rims (Fig. 7h).

Post-mineral fault and veins

A barren fault marked by a narrow quartz-calcite-chlorite S-C mylonite offsets the lode at the southeast end of the section mapped. The fault strikes N85°E, dips 85°S, and is associated with quartz-calcite breccia veins. Drag on the boundary vein of the lode indicates a south side down component of movement (Fig. 6).

Fimiston and Oroya ore in the Blatchford Lode

Oroya-style pyrite-telluride ore, visually characterized by chert-like silica and chlorite, overprints Fimiston refractory ore in the central part of D2a shear zones and in sinistral-reverse D2b faults of the Australia East system. The principal mineralized D2b fault in the Paringa South mine is the Blatchford Lode, which forms narrow (1 m) tabular ore bodies in chlorite-calcite altered Golden Mile Dolerite and wide (6 m) pipe-shaped breccia ore in sericite-ankerite altered Paringa Basalt close to the hinge of the Brownhill Syncline (Fig. 8a), a subsidiary fold of the Kalgoorlie Anticline (Mueller 2017, this issue).

Lenses of sheared and altered Black Flag greywacke (Fig. 8b), thickened in the hinge of the syncline, contain carbonaceous matter (5–10 wt.% carbon). In 1905, methane was encountered in silica-chlorite-ankerite breccia intersected by a drill hole inclined from the − 320 m level of the Iron Duke shaft (Fig. 8a). The gas escaped over 2 days (Montgomery 1906). The occurrence suggests that organic methane trapped in local fault zones played a reducing role during gold mineralization.

The Blatchford breccia ore is characterized by Oroya-style chlorite-ankerite-quartz alteration and pyrite-sphalerite-chalcopyrite mineralization (Fig. 8c). Late Green Leader quartz veins (0.5–3 cm) marked by selvages of disseminated arsenopyrite and mineralized with native gold, petzite, coloradoite, melonite, altaite, and native tellurium (as inclusion in altaite) were mined on the Paringa South shaft 9 and 10 levels. The veins persisted into the Blatchford shear zone above the GMD-PB contact but decreased upwards and gave way to Fimiston-style pyrite ore (Scantlebury 1983).

The D2b Blatchford Lode was mapped in muscovite-ankerite altered Paringa Basalt at 48,968 m north and 19,641 m east in the 1109 drive 10 m above the 11 level of the Paringa South shaft (Fig. 8a). At this location, the lode is linked to the adjacent shear zone by a telluride extension vein indicating a syn-mineralization phase of D2c sinistral strike-slip (Mueller 2017, this issue). Strike-slip alternated with reverse movement during D3a shortening, as indicated by the shear-zone S-fabric and by the buckling of the extension vein (Fig. 9). The host rock pillows are slightly flattened in the plane of a cleavage (N60° W/60–75° SW), which is crosscut by both the Blatchford shear zone and the vein (Fig. 9).

Blatchford shear zone

The Blatchford shear zone consists of (1) ductile S-C mylonite 5–12 cm thick (Fig. 10a) marking the main fault surface; (2) fault-fill veins 0.5–2 cm thick forming a parallel hanging wall array (Fig. 9); (3) dark grey pyrite-rich silica-carbonate-chlorite rock (Fig. 10b); and (4) lenses of foliated fault breccia (Fig. 10c) up to 50 cm thick. Three successive alteration and mineralization stages are distinguished: early muscovite-ankerite-quartz characterized by arsenical pyrite ± chalcopyrite (Fimiston-style), intermediate chalcedony-chlorite-siderite characterized by pyrite-pyrrhotite ± arsenopyrite ± sphalerite (Oroya Stage 1), and late quartz-dolomite-calcite veins containing tellurides (Oroya Stage 2). The chlorite temperature estimates of Oroya Stage 1 mineralization are 402 ± 50 °C (Table 1).

Mylonite and fault-fill veins

The footwall S-C mylonite varies along strike and down dip from fault-parallel schist enclosing quartz-ankerite boudins and pinch-and-swell veins (Fig. 10a) to wider parts where the C-foliation wraps domains with an oblique S-fabric. Where chlorite and sphalerite are absent, such as 20 m northwest of the face mapped, the mylonite consists of muscovite, ankerite, and quartz (25–30 vol.% each), pyrite (10%), tourmaline (3%), and rutile (Tables 1 and 2). The Fimiston-style pyrite contains inclusions of chalcopyrite and native gold, and is zoned from poikilitic cores (0.82 wt.% As) to solid rims (0.13% As; Fig. 10d). Trace barite is present but albite is absent.

Chlorite and sphalerite indicate the Oroya Stage 1 overprint (Fig. 9). Veins are more abundant (Fig. 10b, e), separated by mm-thick seams of muscovite-quartz-rutile schist, and composed of granular quartz, ankerite, and minor Fe-calcite and tourmaline (0.5–5%). Chlorite and muscovite line the margins. Lithons of wall rock are partly replaced by Fe-chlorite (40%), muscovite (25%), and granular quartz (20%). The sulfide assemblage comprises pyrite, pyrrhotite, chalcopyrite, sphalerite, and arsenopyrite (Table 2). The fault-fill veins in the hanging wall are similar in mineralogy to those in the mylonite.

Silica-carbonate-chlorite replacement

Oroya Stage 1 silica-carbonate-chlorite rock forms lens-shaped bodies within the mylonite and along its footwall contact (Fig. 10b) and replaces the inter-pillow material in the wall rock between the shear zone and the telluride vein. Additional replacement by silica, classified as chalcedony due to its porous nature, also occurs along and between cleavage planes (Fig. 9). The chalcedony encloses siderite-ankerite aggregates (25–30 vol.%), Fe-chlorite (5–10%; Table 1), muscovite (5%), tourmaline (1–3%), and granular rutile (1%) resulting in a finely mottled texture (Fig. 10f). Crackle fractures are lined with the same minerals. Fine-grained pyrite (10–15%) close to the mylonite gives way to an outer pyrite-pyrrhotite assemblage (7–10%) in contact with chalcopyrite, sphalerite and arsenopyrite. Pyrrhotite and the absence of magnetite indicate low oxidation and sulfidation states (Table 2).

Foliated fault breccia

Fault breccia exposed in the rise to the 1109 sublevel pinched out below the junction of the telluride vein with the Blatchford shear zone. The breccia matrix supports angular to sub-rounded fragments of Paringa Basalt (Fig. 10c). The fragments are partly replaced by ferroan calcite (10–15 vol.%) but retain most of their pre-sulfide mineralogy (30–40% ankerite, 30% muscovite, 15% quartz). Most contain plagioclase microlites (5 vol.%) pseudomorphed by granular albite and quartz. The breccia matrix consists of Oroya Stage 1 chalcedony (30%), siderite (25%), ankerite (15%), Fe-chlorite (10%), muscovite (7%), rutile (2%), and disseminated pyrite (7%). The texture varies from finely banded to massive. Mineral bands surround micro-fragments of Paringa Basalt and form nodular aggregates zoned from siderite rims to chlorite-muscovite cores (Fig. 10g).

Oroya Stage 2 crackle veins cut across both matrix and fragments. The veins have straight walls, lack selvages, and are filled with quartz (20%) and carbonate (70%) zoned from dolomite cores to ankerite rims (Fig. 10h). Minor muscovite and trace (0.5%) chlorite, sphalerite, and tellurides are present. The low-density vein network represents the up-flow link to the extension vein (Fig. 9).

Telluride extension vein

The extension vein (120 g/t Au) branches off the Blatchford shear zone, widens to 10 cm in the face mapped, and is gently buckled down dip (Fig. 9). In July 1987, the vein had been stoped 25 m to the east of the 1109 sublevel. It has sharp walls and is zoned from Oroya Stage 1 chalcedony-sulfide margins to central Stage 2 quartz-carbonate-telluride fill (Fig. 11a). A bulk XRD sample indicates that quartz, calcite, and ferroan dolomite are major, whereas pyrite, siderite, and phengitic muscovite are minor (Table 1). The Fe³⁺/Fe²⁺ ratio, presence of pyrrhotite, and absence of magnetite suggest a low oxidation state (Table 2).

Oroya Stage 2 quartz-carbonate fill

The central light grey part of the vein consists of strained quartz crystals pointing inward (30 vol.%; 1–5 mm). Most are rimmed by recrystallized margins, and some are crossed by healed fractures (Fig. 11b). The comb texture indicates open-space filling. The matrix consists of microcrystalline granular quartz (40–45%), dolomite zoned to ankerite rims (10%), and calcite zoned to ferroan rims (10%). Green muscovite (5%), red chlorite (0.5%), and rutile form stylolite seams and are enriched in vanadium (Tables 1 and 2).

The quartz-rich central part is zoned symmetrically to black-grey outer fill (Fig. 11a) composed of ferroan calcite (50–70 vol.%), minor quartz (20%), dolomite (5–15%), V-muscovite (5%), and Fe-V chlorite, rutile, and apatite (all 0.5%). The calcite is zoned to iron-rich rims and the dolomite to outer ankerite (Fig. 11c). Strained columnar calcite (5 mm) and pointed quartz crystals (1.5 mm) oriented at a high angle to the walls indicate that this part of the vein also formed by open-space filling. Like in the center, the matrix between the columnar crystals is composed of microcrystalline quartz and carbonate.

The sulfide assemblage comprises subhedral pyrite (3%), minor chalcopyrite, sphalerite and tetrahedrite, and rare bornite-covellite-digenite aggregates indicating a late trend to higher sulfur fugacity (Table 2). The sulfides are in contact and in spatial association with native gold and tellurides (Fig. 11d, e) including coloradoite (HgTe), montbrayite ([Au,Sb]₂[Te,Sb]₃), tellurantimony (Sb₂Te₃), antimonian calaverite ([Au,Sb]Te₂), altaite (PbTe), and petzite (Ag₃AuTe₂). Melonite (NiTe₂), pyrrhotite, and arsenopyrite are present close to the chalcedony margin of the vein fill.

Oroya Stage 1 chalcedony fill

The blue-grey zones at both walls of the vein (Fig. 11a) consist of granular and lesser fibrous chalcedony (40–50 vol.%), ankerite (10–30%), siderite (5%), muscovite (20%), chlorite (1–2%), rutile (2%), and accessory tourmaline and apatite (both 0.5%). The chalcedony matrix has open vughs (Fig. 11f) and is finely banded in nodular aggregates of ankerite, siderite-sulfide, and muscovite-chlorite. Seams of granular to needle-shaped rutile, in particular at the wall-rock contact, suggest some replacement during the initial opening of the vein. Pyrite (5–10%) forms subhedral crystals and atoll-shaped or dendritic aggregates (Fig. 11g) in contact with pyrrhotite (1–3%) and arsenopyrite, the assemblage indicating a low sulfidation state (Table 2). Minor chalcopyrite and sphalerite, and trace native gold and melonite are associated.

Outer mineralization in Paringa Basalt

Light blue-grey silica selvages extend from the telluride vein 3–10 cm into the wall rock fading outward and merging with the dark grey silica replacement lining the pillows (Fig. 9). Veinlets crossing the selvage control some of the replacement. They are filled with chalcedony and calcite, and with accessory ankerite, siderite, muscovite, and chlorite. Quartz replaces the plagioclase microlites. Rutile (2–10 μm) is more abundant (3 vol.%) in the selvage than in the extension vein. Sulfides line fractures and pyrite-pyrrhotite aggregates (2%), arsenopyrite (2%), and tourmaline (0.3%) are disseminated.

A channel sample cut across the Paringa Basalt between the vein and the Blatchford shear zone assayed 1.85 g/t gold. A bulk sample of least-altered wall rock contained 0.46 g/t gold, the grade attributed to low intensity silica-chlorite alteration and sulfide mineralization controlled by SW-dipping cleavage planes and by crackle veinlets (Fig. 9). The veinlets are filled with quartz, dolomite-ankerite, ferroan calcite, minor muscovite, and Fe-chlorite. Pyrite-pyrrhotite aggregates are in contact with accessory sphalerite, chalcopyrite, arsenopyrite, and ullmannite (NiSbS).

The Oroya Hanging Wall Lode

The Oroya Hanging Wall (OHW) Lode is located in the main reverse fault controlling the telluride-rich Oroya Shoot (115 g/t Au; Simpson 1912), the type locality of Oroya-style mineralization. Both form major ore bodies above level 7 of the Paringa South shaft (Fig. 8a). The D3a OHW shear zone follows the GMD-PB contact at the SW-limb of the Kalgoorlie Anticline and displaces D2a and D2b faults 50 m SW-side up (Mueller 2017, this issue). The strike varies from N25° W to N35° W, and the dip from 35° to 60° SW. The lode was mapped on the Paringa South shaft 7 level (162 m above sea level) at 49,070 m north and 19,792 m east, where the main crosscut meets the OHW drive.

In the crosscut and in pillars of the adjacent stope, the lode replaces sericite-ankerite altered Paringa pillow basalt (Fig. 12a). It consists of siliceous S-C mylonite and fault breccia formed by Oroya Stage 1 silica-chlorite-siderite ore rich in pyrite, and by Stage 2 silica-ankerite ore containing native gold and tellurides. Chlorite temperature estimates are 415–425 °C for Stage 1, and 385 °C for Stage 2 (Table 1). Fimiston-style mineralization is absent. Opposite the crosscut, the stope extends up to 4 m below the lode due to a network of crackle veinlets, silica-sulfide replacement, and extension veins. The veins strike perpendicular to the lode and are characterized by comb-textured quartz (Fig. 12b). Adjacent light brown basalt is replaced by dark green chlorite and siderite.

Internal structure of the OHW Lode

On the Paringa South shaft 7 level, the OHW Lode (48 g/t Au) is 1.5–2.0 m thick and characterized by Stage 1 chert-like silica, probably chalcedony recrystallized during shear movement. The uppermost part consists of black mylonite containing intrafolial veins (Fig. 12c) derived from carbonaceous greywacke trapped below the GMD sill. The black mylonite grades downward into dark grey siliceous mylonite enclosing lenses of semi-massive pyrite (Fig. 12d), which is in discordant contact with a light grey brecciated zone of Stage 2 silica-ankerite fill cementing fragments of the dark silica (Fig. 12d, e). The central part of the lode consists of fractured and micro-brecciated silica-chlorite-siderite rock (Fig. 12f), and of silica-mottled siderite-ankerite-chlorite rock representing less intense silicification. XRD of a bulk sample indicates that quartz, ankerite and chlorite are abundant, siderite and pyrite are minor, muscovite is rare, and albite below detection. The whole-rock Fe³⁺/Fe²⁺ ratio is low (Table 2).

Oroya mineralization in GMD

In Golden Mile Dolerite, economic Oroya Stage 1 mineralization is confined to the 1–2 cm thick muscovite-chlorite-quartz S-C mylonite lining the hanging wall of the lode (Fig. 13). Albite, calcite, siderite, and deformed rutile trellis (0.4 mm) are minor. Pyrite (10 vol.%) is in contact with chalcopyrite and gersdorffite and encloses grains of muscovite, rutile, pyrrhotite, and melonite (5–15 μm).

The S-C mylonite grades upward into a parallel zone 20–30 cm thick of foliated GMD Unit 1 (20–40% muscovite, 5–20% chlorite, 30–50% granular quartz + albite). Pinch-and-swell ankerite-pyrite veins are rimmed by muscovite, quartz and chlorite. The veins are linked by ladder veinlets of similar composition. Seams of rutile, quartz, and pyrite trace the foliation. Primary textures are not preserved except for interstitial igneous quartz (1%) and trellised rutile lamellae after Ti-magnetite.

The GMD Unit 1 above is cut by spaced cleavage planes sub-parallel to the OHW Lode but retains some of its variolitic texture. Ankerite, ferroan calcite, quartz, and minor muscovite and chlorite fill crackle veinlets. Both carbonates also form part of the wall rock (20–30% in total). Muscovite is the only white mica present (35 vol.%; SWIR absorption at 2194.7 nm), at least up to the narrow D2a lode of the Federal system composed of muscovite, ankerite and pyrite (Fig. 13).

Oroya Stage 1 pyrite ore

Below the GMD contact, disseminated pyrite averages 15 vol.% in the upper siliceous mylonite. Intercalated sulfide lenses contain up to 35% pyrite cemented by ankerite, siderite, quartz, and chlorite. Below the crosscutting telluride zone (Fig. 13), the pyrite content is more variable (7–15%) and post-kinematic cubes and aggregates (3–10 mm) are common. Stage 1 pyrite is intergrown with minor arsenopyrite, sphalerite and chalcopyrite and encloses rare grains of anhedral pyrrhotite indicating a low to intermediate sulfidation state (Table 2). Micron-sized grains of native gold, melonite, and coloradoite are in contact with or enclosed in both pyrite and arsenopyrite (Fig. 14a).

Siliceous mylonite

The mylonite consists predominantly of silica (40–60 vol.%) laminated by surfaces lined with Fe-Mg chlorite (7–10%; Table 1), muscovite (2%), and rutile (0.2–0.5%). The upper part is black due to amorphous organic matter (1–2%) and marked by pinch-and-swell or drag-folded veins composed of granular quartz, ankerite, and sulfides. Ankerite (15%), siderite (2%), tourmaline (1–10%), and the sulfides are also concentrated in seams parallel to foliation (Fig. 14b). The tourmaline changes from blue-grey to yellow-red towards the telluride zone but is the only mineral enriched in V₂O₃ (0.2–0.4 wt.%).

Brecciated silica-chlorite-siderite rock

Silica-chlorite-siderite replacement, separated from the upper mylonite by the zone of late-stage telluride ore, forms the central and footwall parts of the lode (Fig. 13). Dark grey silica enclosing micron-sized pyrite (3–5 vol.%), muscovite (1–5%), tourmaline, and apatite (0.2%) alternates with lenses of micro-breccia (0.1–5 mm fragments). A network of fractures roughly parallel to the lode boundaries crosscuts the grey silica (Figs. 12f and 14c). The fractures are lined with mm-thick seams of Fe-chlorite, pyrite, and accessory siderite, rutile, and arsenopyrite. Most chlorite seams terminate at micro-breccia, which consists of embayed fragments of dark grey silica (Fig. 14d) supported by a matrix of Fe-chlorite (35–50%), siderite (15–50%), quartz (5–15%), and rutile (1%). Chlorite-rutile stylolites cross the matrix locally.

Silica-mottled siderite-ankerite-chlorite rock

Light grey carbonate-rich ore is located between the micro-brecciated siliceous zones (Fig. 13). A mottled texture is imparted by aggregates (0.5 mm) of microcrystalline quartz (15–30 vol.%) located within broader domains of siderite (15–25%), Fe-Mg chlorite (5–15%), and ankerite. Stylolite seams of Fe-Mg chlorite, rutile, and muscovite define a spaced foliation sub-parallel to the lode walls. Chlorites in the silica-mottled rock and in the siliceous mylonite are more Mg-rich (Mg / [Mg + Fe²⁺] = 0.4–0.5) than those in brecciated silica (Mg / [Mg + Fe²⁺] = 0.2–0.3) but all are low in vanadium (0.1–0.2 wt.% V₂O₃; Table 1).

Oroya Stage 2 pyrite-telluride ore

The discordant zone of Stage 2 pyrite-telluride ore is subdivided into an upper banded and a lower brecciated part. The upper bands are 2–10 mm thick and composed of fractured light grey silica (25–45 vol.%) and white sparry ankerite (30–65%), some grains enclosing dolomite cores. The ankerite bands contain sparse fragments (1–5 mm) of Stage 1 silica, characterized by micron-sized pyrite and muscovite, and fragments of pure silica derived from intercalated bands. The ankerite is intergrown with siderite (1–3%), red Fe-V chlorite (0.5–2%), and rare fluor-apatite. The bands are separated by seams of red tourmaline (Fig. 14e) and by stylolites of green V-muscovite, both in contact with pyrite (3–5% in total) and accessory rutile, Fe-chlorite and Fe-V chlorite (Table 1).

The lower part of the telluride zone consists of crackle breccia, jig-saw breccia (Fig. 14f), and matrix-supported breccia. The white-grey silica-ankerite matrix (40 vol.%) described above cements fragments (0.5–2 cm) of dark grey siliceous mylonite and of silica-siderite-chlorite rock (about 60 vol.% in total). The cement contains seams and stylolites of vanadian tourmaline, chlorite, and muscovite. Pyrite (3–5%) and associated chalcopyrite, sphalerite, native gold, montbrayite, antimonian calaverite, tellurantimony, coloradoite, and altaite (Fig. 14g; Table 2) are disseminated and concentrated along stylolite surfaces.

The telluride-bearing breccia grades downward into a network of white veins (5–10 vol.%), which crosscuts the Stage 1 silica-chlorite-siderite ore (Fig. 12f). The veins contain fragments of dark grey silica and are composed of ankerite (70–90%), zoned dolomite-ankerite (1–3%), quartz (10–20%), and accessory muscovite, chlorite, pyrite, sphalerite, and native gold (0.1–1% each). Adjacent aggregates of ankerite or ankerite-muscovite partly replace the earlier chlorite-siderite assemblage (Fig. 14d). Some veins are offset at chlorite-lined surfaces but others cut across.

The vein network is linked to concentrically zoned, nodular veins in the silica-mottled siderite-ankerite ore (Fig. 13). These veins have margins of sparry ankerite or microcrystalline quartz-ankerite and cores of granular or columnar quartz. Locally, they contain small fragments of dark grey silica and aggregates of pyrite (0.5 vol.%). The textures suggest some open-space fill, perhaps in leached cavities. Part of the post-kinematic pyrite disseminated adjacent to veins is attributed to Stage 2, as are covellite-digenite rims on chalcopyrite (Fig. 14h) indicating an increase in sulfur fugacity with time (Table 2).

Oroya Stage 3 extension veins

Extension veins characterized by open vughs link late movement surfaces and crosscut all other mineralized structures (Fig. 13). They consist of coarse-grained (5–10 mm) subhedral to euhedral quartz, ankerite, siderite, pyrite, arsenopyrite, and sphalerite (up to 10 vol.% each). The sphalerite is yellow and iron-poor. Native gold (0.5–2 mm) and fan- to rosette-shaped aggregates of Fe-chlorite are accessory (0.5%). Discontinuous replacement selvages of pyrite border some of the veins.

North Kalgurli Flat Lode

The OHW Lode is reduced but Oroya Stage 1 mineralization elsewhere is oxidized, for example in the Flat Lode stoped between the North Kalgurli shaft 13 and 14 levels. The stope (49,140 m N, 19350 m E) is located close to the GMD-PB contact 105 m below sea level. The Flat Lode is 0.6 to 1.2 m thick and controlled by a D3a reverse fault oriented N60–80° W/20–25° SW, which displaces the D2a Lake View Lode (N50° W/70° SW) hanging wall block northeast (Fig. 15a). The D3a lode (5 g/t Au) is composed of red breccia and replacement ore (Fig. 15b).

Oroya Stage 1 breccia ore

The main part of the ore body is clast-supported red breccia. The fragments (2–50 mm) consist of porous, microcrystalline siderite zoned to ankerite rims. Rutile is accessory (0.5 vol.%) and primary textures are absent (Fig. 15c). The fragments are cemented by a coarser-grained matrix of granular or comb-textured siderite and ankerite zoned to inner Fe-chlorite and quartz (Fig. 15d). The chlorite temperature of the breccia cement is 384 ± 50 °C (Table 1). Most fragments are partly replaced by chlorite and by microcrystalline silica, and both fragments and matrix contain disseminated oxide-sulfide aggregates composed of magnetite (0.21 wt.% V₂O₃), hematite (0.40–0.75% V₂O₃; ≤ 2.9% TiO₂), pyrite, and chalcopyrite. Contact relations indicate a high oxidation and intermediate sulfidation state (Table 2). Hematite replaces magnetite (Fig. 15e), rims magnetite-pyrite grain boundaries, and fills fractures in pyrite (Fig. 15f). The pyrite is zoned from solid arsenic-poor cores to poikilitic rims enriched in arsenic (Fig. 15f). Micron-sized grains of native gold and hematite are enclosed in or attached to pyrite rims.

Where the stope terminates on level 13, the Flat Lode is composed of a 10-cm-thick mylonite and a 50-cm-thick red replacement zone in green chlorite-calcite altered GMD Unit 5 characterized by trellised rutile lamellae after Ti-magnetite (2–3 vol.%). Thin quartz-ankerite ± albite ± pyrite veins trace the mylonite fabric. The replacement zone is fractured but igneous textures are preserved (Fig. 15b). Interstitial quartz (5%) encloses lath-shaped albite (0.5–1 mm) partly replaced by sericite + ankerite ± hematite. Ankerite and magnetite fill augite sites. Pyrite and magnetite (1–3% each) are in contact.

Mass-balance calculations

The geochemical data in Table 3 are used for mass-balance calculations after the method of Grant (1986) to constrain element enrichment and depletion in gold ore relative to outer chlorite-calcite altered Golden Mile Dolerite or sericite-ankerite altered Paringa Basalt. Aluminium, titanium, phosphorus, and zirconium are chosen as least-mobile elements based on Al/Ti, Ti/P, and Ti/Zr ratios (Table 3). The data and mass balances of samples from the paragonite-ankerite-chloritoid zone of the Trafalgar Fault (App. 2b and 3), and the rare earth element data (App. 2a) and chondrite-normalized REE-patterns (App. 4 and 5) of all samples are presented and discussed in the Electronic Supplementary Material.

Table 3 Whole-rock major oxide and trace element analyses of D2a Fimiston Lodes (Kelly, B-Lode, LV-PV bulk ore), the telluride vein of the D2b Blatchford Lode (BLF), and the D3a Oroya Hanging Wall (OHW) Lode hosted by Golden Mile Dolerite (GMD) and Paringa Basalt (PB). Lake View (LV), Perseverance (PV) and Paringa South (PS) shafts, Golden Mile, Kalgoorlie

Full size table

Fimiston-style refractory ore

Fimiston-style ore is represented by the Kelly Lode in the Paringa South mine and by the average ore (210,410 t) mined from the Lake View and Perseverance shafts in the Eastern Lode System (Fig. 2a). The pyrite ore of the Kelly Lode lacks iron oxides and has a low Fe³⁺/Fe²⁺ ratio (0.08; Table 3). The sample is composed of vein replacement selvages in GMD Unit 2. A slight increase in volume caused by veinlets shifts the Ti-P isocon to the right of the equal-mass diagonal line (Fig. 16a). Sulfur, K₂O, and CO₂ are strongly enriched, and silica, Na₂O, CaO, and MnO are moderately enriched, whereas H₂O is depleted (Fig. 16a). These changes reflect the abundance of Fe-Mn dolomite, quartz and albite, and lesser pyrite and phengitic muscovite in the selvages, whereas the loss of H₂O reflects the destruction of chlorite. The trace elements are subdivided into lithophile (Fig. 16b) and chalcophile-siderophile (Fig. 16c). Enrichment is attributed to substitution in muscovite (Ba, Rb, Cs), dolomite (Sr), pyrite (As, Sb, Tl), and chalcopyrite (Cu, Sn). Telluride inclusions in pyrite account for Te, Se, Au, Ag, Hg, and Pb, and scheelite may account for W and Mo. In contrast, lithium and zinc are enriched in chloritic GMD below the lode.

The Fimiston mill-head sample has a high Fe³⁺/Fe²⁺ ratio (0.23) equivalent to that of the B-Lode (Table 3), the latter characterized by the assemblage pyrite-magnetite-hematite. The Ti-P isocon almost coincides with the equal-mass diagonal line (Fig. 16d) indicating negligible volume change. Sulfur, K₂O, MnO, CO₂, and Fe₂O₃ are enriched, whereas SiO₂ and Na₂O are slightly and Al₂O₃, CaO, MgO, and H₂O are significantly depleted. The XRD analysis of the mill-head sample indicates that these changes result from the abundance of phengite, quartz, and manganoan ankerite. Albite, siderite, chlorite, and pyrite are minor phases (5–10 vol.%). Hydrothermal magnetite and hematite (≤ 5%) account for the increase in trivalent iron. The selective replacement of albite by phengite + quartz, observed in the outer zone of the B-Lode (Fig. 4), is the likely cause for the depletion of Na₂O, Al₂O₃, and SiO₂ relative to outer chlorite-calcite altered GMD Unit 9, where albite is stable. The metasomatic changes in lithophile (Fig. 16e) and chalcophile-siderophile trace elements (Fig. 16f) resemble those in the Kelly Lode except that lead is the only base metal enriched. The relative increase of U, Th, and REE is caused by a minor component of GMD Unit 8, which has higher background values than GMD Unit 9 (ESM App. 4).

Oroya-style Blatchford vein

The Blatchford vein above the Paringa South shaft 11 level is hosted by sericite-ankerite altered Paringa Basalt and is zoned from outer chalcedony replacement (30 vol.%) to inner extensional quartz-carbonate fill (70%). The vein sample (1.5 kg) has a low Fe³⁺/Fe²⁺ ratio (0.03), and its Al/Ti ratio is close to that of the host rock (Table 3). The Al-Ti isocon is shifted to the right of the equal-mass diagonal line (Fig. 16g) due to the large volume gain.

Sulfur and the major oxides are strongly enriched except Na₂O, which is depleted (Fig. 16g). The XRD analysis identified quartz, calcite, and ankerite as principal and muscovite, chlorite, siderite, and pyrite as minor phases (5 vol.% each). Albite was not detected. Relative to the wall rock, all lithophile (Fig. 16h) and all chalcophile-siderophile trace elements (Fig. 16i) are enriched, though some at the sub-ppm level (Be, U, Mo, Tl). The enrichment is related to substitution in muscovite and chlorite (Li, Cr, Ga, V), carbonate (Sr), rutile (W, Nb, V), and in apatite, monazite, and xenotime (P₂O₅, REE, Y, Th, U). Total P₂O₅ in the vein is low (0.07 wt.%). Ore minerals causing enrichment include: native gold and tellurides (Au, Ag, Hg, Pb, Te, Se), chalcopyrite (Cu), sphalerite (Zn), arsenopyrite (As, Sb), and pyrite (Ni, Co).

Oroya Hanging Wall Lode

The Oroya Hanging Wall (OHW) Lode, sampled in bulk (2 kg) on the Paringa South shaft 7 level (Fig. 13), is located in a dilated flat section of the reverse shear zone (Mueller 2017, this issue). The Fe³⁺/Fe²⁺ ratio of the ore is low (0.03), and its Ti/P ratio is close to that of sericite-ankerite altered Paringa Basalt (Table 3). A large volume gain is indicated by the shift of the Ti-P isocon (Fig. 16j).

Sulfur and the major oxides are enriched except for Na₂O, which is depleted (Fig. 16j). Most remarkable are the strong enrichment in FeO (ankerite, siderite, pyrite), SiO₂ (chert-like quartz), and sulfur (pyrite) and the distinct enrichment in Al₂O₃ (chlorite) and K₂O (muscovite). The XRD analysis confirms that quartz, ankerite, and chlorite are main constituents, pyrite and siderite are major (about 10 vol.%), and muscovite is minor (5%). Albite is absent.

Most lithophile (Fig. 16k) and chalcophile-siderophile trace elements (Fig. 16l) are enriched, and only Ba, Cs, and Tl show no change or slight depletion. Once again, the elements enriched may be grouped according to substitution in: muscovite and chlorite (Li, Rb, Ga, Cr, V), ankerite (Sr), rutile (Nb, Ta, Sc?), apatite, monazite and xenotime (REE, Y, Th, U), native gold and tellurides (Au, Ag, Hg, Pb, Te, Se), chalcopyrite (Cu), sphalerite (Zn), arsenopyrite (As, Sb), pyrite (Ni, Co), and scheelite (W, Mo). The P₂O₅ content of the ore is low (0.02 wt.%; Table 3).

Residual minerals

Titanium residing in rutile is part of all isocons in combination with phosphorus and aluminium. Relative to these isocons, zirconium and hafnium are immobile in Fimiston replacement ore. In contrast, both are enriched in the OHW Lode and in the Blatchford vein (Fig. 16). The Zr/Hf ratios of wall rock and ore, however, vary within narrow limits (Table 3) suggesting that zircon inherited from the wall rock rather than hydrothermal zircon is the main Zr-Hf mineral.

In the OHW Lode, Al₂O₃ is also enriched consistent with the abundance of chlorite. Relative to Ti, the enrichment factors calculated are Al (2.7×), Ga (6.8×), Zr (6.7×), and Hf (5.9×). If these elements are residual, concentrated in chlorite-muscovite ± rutile ± zircon stylolites, then lode formation will include processes such as extensional fill, silica replacement and wall-rock dissolution, the latter indicated by the nodular “cavity-fill” veins in the central part of the lode (Fig. 13).

Discussion

Wall-rock alteration in the Golden Mile has been linked to regional metamorphism during fold-belt evolution (e.g., Witt 1991; Goldfarb et al. 2005), and classified as “mesozonal” (6–12 km crustal depth; Groves 1993) emphasizing the P-T conditions of the metamorphic terrane. As such classification is less useful at the scale of the ore bodies, the base-cation leaching concept of Meyer and Hemley (1967) is introduced. The km-scale paragonite-chloritoid zone at the southeast margin of the Golden Mile is discussed first and compared to similar zones associated with other gold deposits in the Eastern Goldfields Province. The sequence of wall-rock alteration and mineralization in the Golden Mile is summarized in Fig. 17. The results of the Paringa South case study are discussed in the context of published data from other parts of the deposit.

Classification of wall-rock alteration

Wall-rock alteration is subdivided according to the degree of feldspar destruction by base-cation leaching (hydrolytic alteration), an approach emphasizing fluid-wall rock interaction without assumptions about the fluid source (Meyer and Hemley 1967; Rose and Burt 1979). In this context, the chlorite-calcite ± epidote zone enveloping the Golden Mile deposit represents hydrous “propylitic” alteration because albite is stable and abundant. As ankerite is the main carbonate in all ore bodies, white mica remains as the most distinctive gangue mineral in Fimiston Lodes and the Trafalgar Fault zone. In both, hydrolytic alteration predominates involving muscovite-phengite/paragonite-quartz replacement of albite, apart from albite-stable central structures. Albite is absent in ore replacing sericite-ankerite altered Paringa Basalt suggesting that albite-rich propylitic GMD is required as a buffer to stabilize feldspar. Oroya-style ore is characterized by the deposition of silica and chlorite coupled with iron enrichment, and represents combined silicification and “chloritic alteration” in the sense of Meyer and Hemley (1967).

Trafalgar Fault paragonite-ankerite-chloritoid zone

In the Golden Mile, km-scale propylitic alteration (310–340 °C; Fig. 17) overprints the metamorphic actinolite-albite-zoisite assemblage in the tholeiitic host rocks (Travis et al. 1971; Phillips 1986). Zones of sericite-ankerite and chlorite-ankerite alteration, tens of meters wide and variably mineralized with magnetite ± hematite, replace the chlorite-calcite assemblage in parts of the Golden Mile Dolerite and in Paringa Basalt at the GMD contact. Though anomalous in gold (20–200 ppb), these early ankerite zones are crosscut by the ore bodies and predate main-stage sulfide deposition (Clout et al. 1990). They are broadly synchronous with the emplacement of diorite porphyry dykes at 2663 ± 11 Ma (Fig. 17).

In the Trafalgar Fault at the southeast margin of the Golden Mile, the abundance of paragonite and chloritoid contrasts with the predominance of phengite, muscovite or chlorite in all lodes of the deposit. Yet, the zonation from albite-stable alteration in the D2a Trafalgar Fault to paragonite-quartz in the outer hydrolytic zone resembles the lateral zonation in the Paringa B-Lode. Other similarities to D2a Fimiston Lodes include the lack of iron enrichment, the metasomatic addition of K₂O, CO₂, S, Ba, Sr, Rb, Cs, As, and Sb (ESM App. 3), and the assemblage pyrite-chalcopyrite ± magnetite ± hematite. For these reasons, the alteration zone of the Trafalgar Fault is correlated with Fimiston ore formation (Fig. 17). Striking differences are the low gold grade (48–75 ppb; ESM App. 2b), and the lack of iron oxides in the fault. Anhydrite or gypsum were not detected but may be leached below the saprolite (Fig. 2b).

The barren sericite-ankerite zone bound to the GMD-PB contact in the Kalgoorlie Anticline plunges 20° SE and merges with the paragonite-ankerite-chloritoid zone of the Trafalgar Fault (Fig. 2a). A lateral zonation in mineral composition within the contact zone is not documented. Muscovite and paragonite are both present at the Paringa South shaft where chloritoid is absent. Temperature estimates averaging 410 °C suggest that the Trafalgar Fault alteration zone constitutes a thermal anomaly within the wider propylitic envelope (310–340 °C; Fig. 17).

In the Charlotte vein stockwork at Kalgoorlie, the auriferous vein selvage changes from muscovite-ankerite-pyrite at surface to albite-ankerite-pyrrhotite 1 km below. Gold grade decreases inversely with temperature (Mueller 2015). This vertical transition from high-temperature Na-rich to distal K-rich alteration may be analogous to the lateral one in the Golden Mile.

Paragonite-chloritoid alteration in the Eastern Goldfields

Elsewhere in the Eastern Goldfields Province, chloritoid and paragonite are common in zones centered on inner quartz-sericite-pyrophyllite ± andalusite-kyanite alteration (Fig. 1). In the Leonora area north of Kalgoorlie, km-scale hydrothermal alteration overprints 2.8 Ga greenschist facies komatiite and basalt east of the 2.75 Ga Raeside granite batholith (see the map in ESM App. 6). Gold deposits are located at the contacts of the batholith and its satellite plutons (Duuring et al. 2004). The zircon U-Pb ages of altered diorite-granodiorite dykes at Tarmoola (2667 ± 8 Ma; Fletcher et al. 2001) and at Tower Hill (2670 ± 5 Ma; Dunphy et al. 2003) set maximum time limits for mineralization.

The Sons of Gwalia deposit (179 t Au to 2017) replaces actinolite-albite meta-basalt overprinted by propylitic chlorite-calcite alteration. The pipe-shaped ore body plunges 40–45° E, and consists of biotite-calcite-pyrrhotite and quartz-rich sericite-dolomite-pyrite schist (8.4 g/t Au). North of this deposit, broad zones of chloritoid-chlorite-sericite-quartz and inner quartz-paragonite-andalusite ± kyanite schist are exposed at Mount Leonora (ESM App. 6). The Al-silicates are intergown with natro-alunite and are rimmed by pyrophyllite, which is in contact with kaolinite in the sulfide zone (Hallberg 1983). Ankerite and pyrite, and accessory chalcopyrite, tennantite, pyrrhotite, gersdorffite, and arsenopyrite are disseminated (Skwarnecki 1990). P-T conditions of > 250 MPa and 380–470 °C are indicated by late kyanite in contact with andalusite, and by the absence of sillimanite and the assemblages staurolite-quartz, almandine-staurolite, and almandine-albite. Retrograde pyrophyllite and kaolinite-quartz suggest that the temperature declined first to 350 °C and then to 250 °C (see P-T diagram in ESM App. 7). Hallberg (1983) interprets the Mount Leonora zone as hydrothermal, a conclusion consistent with zircon U-Pb chronology.

The occurrence of natro-alunite is intriguing, as this mineral is characteristic of “advanced argillic alteration” in the sense of Meyer and Hemley (1967). The andalusite- and kyanite-bearing zone at Mount Leonora may represent the high-PT equivalent of this alteration type. The sulfide assemblage, however, indicates a sulfidation state well below that of high-level Au-Ag-Cu deposits (e.g., Einaudi et al. 2003). At the southeast margin of the Golden Mile, there is no evidence of a high-PT advanced argillic zone. If present, it must be located at depth and northeast of the Trafalgar Fault.

Previous studies of Golden Mile ore

Mineralization styles in the Golden Mile outside the Paringa South mine are summarized in Clout et al. (1990) as follows: (1) outer ankerite-dolomite replacement; (2) inner grey pyrite-rich sericite-ankerite ore, locally associated with (2a) Green Leader marked by V-muscovite and tellurides; and (3) inner siliceous ore (> 80% quartz). Most ore bodies are brecciated and variably mineralized with pyrite, magnetite, hematite, and anhydrite. The “lode alteration” of Gauthier (2006) comprises three successive stages: (1) Fimiston-style ankerite-sericite replacement, 10–20 vol.% pyrite + magnetite, gold, and tellurides as inclusions in pyrite; (2) Green Leader comb-textured and brecciated quartz-ankerite veins with silica-pyrite replacement selvages, vanadian muscovite, free tellurides and gold, and local anhydrite ± hematite; and (3) quartz-ankerite veins with sericite-ankerite-pyrite selvages.

These mineralization schemes are compatible with the results of this study suggesting that the ore bodies described are representative of the broader Golden Mile. Fimiston-style refractory pyrite ore constitutes the Paringa B-Lode and the Kelly Lode, both overprinting propylitic Golden Mile Dolerite, and forms part of the Blatchford Lode replacing sericite-ankerite altered Paringa Basalt. Non-refractory Oroya-style ore is represented by the North Kalgurli Flat Lode and by the Oroya Hanging Wall (OHW) and Blatchford lodes in the Paringa South mine. The latter two are composed of the early pyrite and late telluride stages described from silicified lodes in other parts of the Golden Mile (Tomich 1952).

Fluid inclusion studies summarized in Ho et al. (1990) show that the aqueous fluid trapped in Fimiston- and Oroya-style vein quartz is CO₂-rich (X_CO2 = 0.15), of low to moderate salinity (2–5.5 wt.% NaCl_eq), and of high density (0.75–0.95 g/cm³). Methane was detected in veins where the OHW Lode intersects altered carbonaceous greywacke (local fluid: 70–85 wt.% H₂O, 10–25% CO₂, 1–5% CH₄). Median homogenization temperatures not corrected for pressure vary from 220 to 315 °C and median isochore pressures from 200 to 300 MPa. At lithostatic load, a pressure of 300 MPa converts to a depth of about 10 km (3.6 km per 100 MPa at 2.85 g/cm³ density; Bucher and Frey 2002), a deep crustal level of ore formation.

Fimiston ore in the Paringa South mine

Fimiston refractory ore (4–20 g/t Au) occurs in D2a (B-lode, Kelly Lode) and in D2b shear zones (Blatchford Lode) reactivated during D2c sinistral strike-slip. In lodes hosted by propylitic GMD, albite is stable in brittle fault-fill veins and in the innermost vein selvage. Tourmaline and fluor-apatite are accessories. Phengitic muscovite (< 0.2 wt.% V₂O₃) is common in the selvage but increases in the outer hydrolitic zone where phengite + quartz selectively replace albite (B-Lode). Chlorite temperature estimates are 390–400 °C for the vein selvage and 370 °C for the outer hydrolytic zone. Ankerite is the dominant carbonate. In lodes located within the pre-sulfide Paringa Basalt contact zone, S-C mylonite composed of muscovite-ankerite-quartz ± tourmaline traces the shear zones.

The pyrite is either zoned in arsenic content (B-Lode, Blatchford) or is arsenic poor (Kelly Lode). Accessory chalcopyrite is ubiquitous, sphalerite is absent, and tennantite is associated with arsenical pyrite. Sulfide and sulfide-oxide assemblages define an intermediate sulfidation state. Fimiston ore requires roasting because native gold and tellurides occur as inclusions (mostly 1–5 μm) in pyrite. Hessite, petzite, coloradoite, and melonite were detected in B-Lode and Kelly Lode pyrite. The Au/Ag ratios of ore from the B-Lode (0.43), Kelly Lode (0.72), and the Fimiston mill-head (2.06) suggest that gold inclusions predominate but that Au-Ag tellurides are abundant locally.

The strong enrichment of sulfur in the bulk mill-head sample (210,410 t) illustrates the abundance of pyrite in both the inner albite-stable and outer hydrolytic zone of Fimiston Lodes. According to the mass-balance calculations, the ore fluid introduced significant amounts of Mn, Ca, K, CO₂, and sulfur but did not add iron and magnesium. Large-ion lithophile elements (Ba, Rb, Cs), strontium, and the chalcophile-siderophile elements Au, Ag, Hg, Te, Se, Cu, Pb, As, Sb, Mo, and W were also introduced. Lithium may form an outer geochemical halo (110–213 ppm; Table 3) in propylitic GMD adjacent to lodes.

Oroya ore in the Paringa South mine

Oroya-style pyrite-telluride mineralization overprints Fimiston ore in the D2b Blatchford Lode, and is the only ore in the D3a reverse faults controlling the North Kalgurli Flat Lode and the Paringa OHW Lode. The structural relations indicate a hiatus between the Fimiston and Oroya hydrothermal systems. Oroya mineralization is subdivided into Stage 1 silica-chlorite-siderite-pyrite and Stage 2 silica-ankerite-pyrite-telluride ore. In the Flat Lode (5 g/t Au), Stage 1 ore was deposited at 385 °C and Stage 2 is absent. In the OHW and Blatchford Lodes, fluid temperature is estimated at 400–425 °C during Stage 1 and at 385 °C during Stage 2. Early silica and chalcedony are colored dark by dispersed micron-sized pyrite. Fe-chlorite, muscovite and tourmaline are low in vanadium (≤ 0.25 wt.% V₂O₃). During Stage 2, pure silica, comb-textured quartz in veins and breccia, green vanadian muscovite, red Fe-V chlorite, red tourmaline (all > 0.5 wt.% V₂O₃), and fluor-apatite were deposited. The presence of chalcedony has been interpreted to indicate a shallow crustal level of ore formation (Gauthier et al. 2007), in disagreement with the fluid inclusion pressure estimates (Ho et al. 1990). Rapid cooling during each stage and/or precipitation from fluid super-saturated in silica may be alternative interpretations.

In contrast to Fimiston, Oroya ore is not refractory. In the OHW and Blatchford lodes, free gold is associated with pyrite, arsenopyrite, sphalerite, chalcopyrite, tetrahedrite and melonite, and a late Sb- and Pb-rich telluride assemblage comprising montbrayite, tellurantimony, coloradoite, altaite, calaverite, and petzite. The sulfidation state increased with time from low (pyrite-pyrrhotite-arsenopyrite) to intermediate (pyrite-chalcopyrite-tetrahedrite) to high (covellite-digenite), perhaps coincident with decreasing temperature. The local presence of calaverite and native tellurium (Scantlebury 1983) indicates a high tellurium fugacity, at least at the onset of telluride deposition (see Afifi et al. 1988).

The OHW Lode and Blatchford vein are extensional structures hosted in altered Paringa Basalt. Mass-balance calculations indicate that the ore fluid added significant amounts of silica, Fe, Mg, Mn, Ca, K, CO₂, and sulfur, lithophile Li, Rb, Sr, and Ga, siderophile Cr, Ni, V, and W, and chalcophile Au, Ag, Hg, Te, Cu, Zn, Pb, As, Sb, Co, and Se. Sodium is the only major element depleted in the ore.

Oxidized versus reduced ore bodies

Fimiston ore in the B-Lode is characterized by the oxidized assemblage pyrite-magnetite-hematite (Fe³⁺/Fe²⁺ = 0.24), whereas the lateral termination of the Kelly Lode lacks iron oxides (Fe³⁺/Fe²⁺ = 0.08). A surprising result is the oxidized nature of the bulk Fimiston ore (240,410 t) indicated by the high Fe³⁺/Fe²⁺ ratio (0.23) and enrichment of Fe₂O₃ in the mill-head sample (Fig. 16d). This suggests the widespread occurrence of magnetite and hematite in Fimiston Lodes controlled by the main D2a shear-zone system. The high oxidation state inferred is supported by negative δ³⁴S median values in pyrite (− 5.1 to − 5.5‰; n = 46) and positive values (+ 12.0 to + 19.2‰; n = 7) in co-genetic anhydrite (Clout et al. 1990; Golding et al. 1990). Oroya-style ore also varies in oxidation state from high in the Flat Lode (hematite stable) to low in the OHW and Blatchford lodes (Fe³⁺/Fe²⁺ = 0.03; no Fe-oxides). The reduced nature of the pyrrhotite-bearing ore bodies may be related to organic methane released from local black schist. Flat Lode pyrite also has a negative δ³⁴S signature of − 4.7‰ (n = 22; Golding et al. 1990).

The sulfide-sulfate relations have been interpreted to reflect the infiltration of a reduced fluid oxidized by reaction with Ti-magnetite in the Golden Mile Dolerite, which deposited hematite after cooling from about 450 °C to less than 280 °C (Evans et al. 2006). This process requires a low fluid/rock ratio (0.1 to 5), a higher Fe³⁺/Fe²⁺ ratio (0.20) than measured in propylitic GMD Unit 9 (0.13; Bartram and McCall 1971), and inward flow from outer alteration zones into the lode structures to deposit the tens of million tons of pyrite mined (Evans et al. 2006). These constraints are inconsistent with the results of this study because (1) zoned lode alteration progressively replaces previously altered wall rock (outward flow), (2) the lode terminations (Kelly Lode) are more reduced rather than oxidized (lateral flow), and (3) the temperature of the ore fluid was higher than permitted for hematite deposition in the rock-buffered system modeled.

Constraints on fluid sources

The Golden Mile ore bodies result from the superposition of two hydrothermal systems separated in time by D3a reverse faulting. The younger Oroya system deposited abundant tellurides after most of the pyrite, a sequence common in magmatic hydrothermal ore deposits (Afifi et al. 1988). The vast volume of hydrous altered rock suggests a fluid-buffered system requiring significant dissolved SO₂-SO₄ relative to H₂S to account for the oxidized nature of many Golden Mile lodes. These constraints favor a felsic magmatic source, as Archean surface water and metamorphic fluid ascending through thick ferrous greenstones were both reduced (e.g. Cameron and Hattori 1987). Along the Boulder Lefroy fault system controlling the Golden Mile deposit, anhydrite-bearing gold deposits at Kambalda (Bath et al. 2013), Cu-Au epidote-magnetite endoskarns at Hannan South and Mt Shea (Mueller et al. 2012), and monzodiorite-granodiorite intrusive complexes all formed during the period 2663 ± 11 to 2651 ± 9 Ma (Mueller 2007, 2017). Gravity lows and reflection seismic sections outline plutons as the source of dykes and oxidized hydrothermal fluid beneath mineralized faults (Neumayr et al. 2008; Bath et al. 2013; Mueller et al. 2016, this issue). Strontium is enriched in both Fimiston- and Oroya-style ore. The initial ⁸⁷Sr/⁸⁶Sr ratios of monzodiorite-suite porphyries and associated lamprophyres (0.7014–0.7018) match those recorded in tourmaline from the Kelly Lode (0.7014), in scheelite from the OHW Lode (0.7018; Mueller et al. 1991), and in anhydrite from Fimiston Lodes (0.7013–0.7016; Cameron and Hattori 1987). The initial ratios of the mantle-derived tholeiitic host rocks are lower (0.7009–0.7011; Faure 2001).

Conclusions

(1)
Prior to gold mineralization, hydrothermal fluid infiltrating the vast Golden Mile shear-zone system generated hydrous propylitic alteration at 310–340 °C in tholeiitic Golden Mile Dolerite (GMD) and Paringa Basalt (PB), and barren ankerite-sericite-chlorite ± magnetite zones in fractured GMD and at the GMD-PB contact of the Kalgoorlie Anticline.
(2)
The ankerite-muscovite ± paragonite ± chlorite zone at the GMD-PB contact extends from the Paringa South mine 1.5 km southeast, where it merges with the km-scale paragonite-ankerite-chloritoid zone in GMD centered on the Trafalgar Fault. Chloritoid-bearing alteration is zoned from albite stable in the fault to outer hydrolytic paragonite-quartz replacement. Pyrite ± chalcopyrite mineralization is of low grade (< 0.1 g/t Au). Temperature estimates of 405–425 °C indicate that the paragonite-ankerite-chloritoid zone represents a thermal anomaly in propylitic GMD (< 350 °C) at the periphery of the deposit.
(3)
Fimiston-style refractory pyrite ore formed by replacement of altered GMD and PB in D2a and D2b shear zones. In propylitic GMD, narrow albite-stable zones centered on veins give way to outer hydrolytic phengite-quartz alteration. Ore bodies are characterized by the oxidized assemblages pyrite-magnetite and pyrite-hematite ± anhydrite but lack iron oxides at their lateral termination. Most pyrite is arsenical and associated with minor chalcopyrite and tennantite. Sphalerite is absent. Native gold and Au-Ag tellurides form micron-sized inclusions in pyrite. The ore fluid introduced mainly Ca, K, CO₂, S, Ba, Rb, Cs, and Sr but not Fe. Pyrite and gold precipitated at 370–400 °C during the sulfidation of iron in the wall rock.
(4)
Oroya-style pyrite-telluride ore formed by silicification and by chlorite-siderite-ankerite ± tourmaline replacement. It overprints Fimiston ore in D2a and D2b shear zones and is the only ore in D3a reverse faults. Ore bodies vary from oxidized (5 g/t Au; pyrite-hematite) to reduced (50–120 g/t Au; pyrite-pyrrhotite). Trapped methane derived from local black schist acted as a reducing agent. In the Paringa South mine, early arsenic- is distinguished from late antimony-rich mineralization. Stage 1 ore (400–425 °C) composed of pyrite (15 vol.%), minor arsenopyrite, sphalerite, chalcopyrite, and free gold is crosscut by Stage 2 (385 °C) comprising pyrite (5%), minor tetrahedrite, montbrayite, tellurantimony, coloradoite, altaite, and gold associated with V-muscovite, Fe-V chlorite, and V-tourmaline. The ore fluid introduced mainly silica and iron, and lesser Mg, Mn, Ca, K, CO₂, S, Te, Li, Rb, Sr, Ga, Cr, Ni, and V.
(5)
The Fimiston and Oroya hydrothermal systems are separated in time. The Fimiston fluid infiltrated only D2a and D2b shear zones during D2c strike-slip reactivation. The Oroya fluid infiltrated the D2a and D2b shear-zone system, and D3a reverse faults offsetting the latter. Both fluids were oxidized and became reduced during interaction with the iron-rich wall rocks and/or trapped organic methane. Both were CO₂-rich and of low to moderate salinity, consistent with a confining pressure of about 300 MPa, and both introduced Ca, K, Rb, Sr, and sulfur. The fluid of the younger Oroya system differed by larger amounts of dissolved silica, Fe, Cr, Ni and V, and by its evolution to a high tellurium fugacity evident in the deposition of calaverite and native tellurium.

Change history

19 February 2020
The author noticed that two element ratios in Table 3 are incorrect due to a calculation error.

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Acknowledgments

The author acknowledges the receipt of a scholarship during his Ph.D. study at the University of Western Australia. Ray Chang assisted with the XRD and XRF analyses and Malcolm Roberts during the electron microprobe sessions at the Centre for Microscopy, Characterization and Analysis (CMCA), the University of Western Australia. Scott Halley kindly measured the SWIR absorption spectra of representative samples. Greg Hall and Patrick Verbeek, former Gold Resources Pty Ltd of the CSR Paringa Project, encouraged mapping in the Paringa South underground mine before closure in 1987. The manuscript benefitted from improvements suggested by Stephen Kesler and David Craw.

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Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Perth, W.A., 6001, Australia
Andreas G. Mueller

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Mueller, A.G. Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems. Miner Deposita 55, 697–730 (2020). https://doi.org/10.1007/s00126-018-0813-5

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Received: 09 October 2017
Accepted: 20 May 2018
Published: 02 July 2018
Issue Date: April 2020
DOI: https://doi.org/10.1007/s00126-018-0813-5

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Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems

Abstract

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Introduction

Geologic setting

Methods and terminology

Paragonite-bearing replacement zones

Trafalgar Fault: paragonite-ankerite zone

Trafalgar Fault: paragonite-albite zone

Contact zone in Paringa Basalt

Contact zone in GMD Unit 1

Fimiston lodes in the Paringa South mine

Paringa B-Lode

Inner phengite-albite zone

Outer phengite-ankerite zone

Paringa Kelly Lode

Fault-fill and ladder veins

Vein replacement selvages

Chlorite-calcite altered GMD Unit 2

Post-mineral fault and veins

Fimiston and Oroya ore in the Blatchford Lode

Blatchford shear zone

Mylonite and fault-fill veins

Silica-carbonate-chlorite replacement

Foliated fault breccia

Telluride extension vein

Oroya Stage 2 quartz-carbonate fill

Oroya Stage 1 chalcedony fill

Outer mineralization in Paringa Basalt

The Oroya Hanging Wall Lode

Internal structure of the OHW Lode

Oroya mineralization in GMD

Oroya Stage 1 pyrite ore

Siliceous mylonite

Brecciated silica-chlorite-siderite rock

Silica-mottled siderite-ankerite-chlorite rock

Oroya Stage 2 pyrite-telluride ore

Oroya Stage 3 extension veins

North Kalgurli Flat Lode

Oroya Stage 1 breccia ore

Mass-balance calculations

Fimiston-style refractory ore

Oroya-style Blatchford vein

Oroya Hanging Wall Lode

Residual minerals

Discussion

Classification of wall-rock alteration

Trafalgar Fault paragonite-ankerite-chloritoid zone

Paragonite-chloritoid alteration in the Eastern Goldfields

Previous studies of Golden Mile ore

Fimiston ore in the Paringa South mine

Oroya ore in the Paringa South mine

Oxidized versus reduced ore bodies

Constraints on fluid sources

Conclusions

Change history

19 February 2020

References

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