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Genesis and exploration implications of epithermal gold mineralization and porphyry-style alteration at the Endeavour 41 Prospect, Cowal district, New South Wales, Australia

journal contribution
posted on 2023-05-18, 07:34 authored by Zukowski, W, David CookeDavid Cooke, Deyell, CL, McInnes, P, Simpson, K

Endeavour 41 is a deep-level, structurally controlled epithermal gold deposit hosted by Early Ordovician subaqueous volcanosedimentary rocks. Calc-alkalic to shoshonitic, mafic to intermediate sills, dikes, and stocks intruded the volcanosedimentary units in the Middle Ordovician. The most felsic intrusions, together with pyroxene-bearing dikes, are temporally related to gold mineralization. Postmineralization intrusions are exclusively of mafic character.

Endeavour 41 evolved from early, high-temperature porphyry-style veins and alteration to lower-temperature epithermal-style gold mineralization. Early magnetite and garnet-bearing veins (stage 1 and 2, respectively) associated with actinolite, magnetite, and biotite-bearing alteration assemblages have been cut by gold-bearing veins and associated alteration assemblages. There were two main epithermal-style gold mineralizing events: (1) quartz-pyrite ± calcite ± adularia ± chlorite veins (stage 3) and (2) carbonate-base metal sulfide veins (calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite, Ag tellurides, arsenopyrite, apatite, hematite, illite-muscovite, and chlorite [stage 4]). Gold occurs principally as a refractory phase in pyrite. It also occurs as grains of Au-Ag tellurides and as inclusions of free gold in pyrite, sphalerite, and chalcopyrite. Hydrothermal alteration associated with gold-mineralized veins produced early epidote and K-feldspar-epidote–bearing alteration halos and later-stage illite-muscovite-K-feldspar and calcite-rich alteration halos. The highest gold grades are associated with muscovite and illite alteration.

Stable isotope analyses and fluid inclusion data provide evidence of a magmatic-hydrothermal component to the mineralizing fluids. Fluid inclusion data suggest that gold precipitated from boiling saline waters (~9.0 wt % NaCl) at temperatures of about 310°C. Stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars). Stage 4 illite formed at temperatures below ~280°C. Stage 3 calcite has δ13Ccalcite and δ18Ocalcite values that range from −5.2 to −4.6 and from 11.6 to 12.1‰, respectively. Calculated fluids for these mineral values at 300°C (δ13Cfluid = −3‰; δ18Ofluid = 6‰) are consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3. A component of meteoric waters is inferred for stage 4, because δ13Ccarbonate and δ18Ocarbonate values range from −6.9 to −0.5 and from 10.9 to 30.1‰, respectively, corresponding to δ13Cfluid and δ18Ofluid values of −5 and −2‰ at 200° to 250°C.

The δ34Ssulfide values for early vein stages range between −4.9 and −0.5‰. Stage 3 has δ34Ssulfide values ranging from −5.2 to +0.8‰ with the most 34S enriched values deposited away from the mineralized center. Stage 4 sulfides have isotopic compositions from −7.5 to +2.5‰. The negative isotopic values are consistent with oxidized (sulfate-predominant) magmatic-hydrothermal fluids. Sulfur isotopic zonation patterns show that the most negative δ34S values correlate with gold-enriched domains and also with areas that contain high-temperature, porphyry-style alteration facies. The negative sulfur isotope values define zones of upflow for the mineralizing magmatic-hydrothermal fluids.

The paragenetic history of Endeavour 41 records a transition from deep-level to shallow-level magmatic-hydrothermal activity. This transition implies erosion and unroofing of the system synchronous with mineralization. High-temperature assemblages (e.g., actinolite-magnetite, biotite, and K-feldspar-epidote) indicate that epithermal mineralization occurred proximal to a magmatic-hydrothermal center and that there is potential for the discovery of porphyry copper-gold mineralization below the current level of diamond drilling.

Funding

Australian Research Council

AMIRA International Ltd

ARC C of E Industry Partner $ to be allocated

Anglo American Exploration Philippines Inc

AngloGold Ashanti Australia Limited

Australian National University

BHP Billiton Ltd

Barrick (Australia Pacific) PTY Limited

CSIRO Earth Science & Resource Engineering

Mineral Resources Tasmania

Minerals Council of Australia

Newcrest Mining Limited

Newmont Australia Ltd

Oz Minerals Australia Limited

Rio Tinto Exploration

St Barbara Limited

Teck Cominco Limited

University of Melbourne

University of Queensland

Zinifex Australia Ltd

History

Publication title

Economic Geology

Volume

109

Issue

4

Pagination

1079-1115

ISSN

0361-0128

Department/School

School of Natural Sciences

Publisher

Economic Geology Publ Co

Place of publication

5808 South Rapp St, Ste 209, Littleton, USA, Co, 80120-1942

Rights statement

Copyright 2014 Society of Economic Geologists Inc.

Repository Status

  • Restricted

Socio-economic Objectives

Precious (noble) metal ore exploration

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