Abstract The giant (>20 Moz) Telfer Au-Cu deposit is located in the Paterson Province of Western Australia and is hosted by complexly deformed marine Neoproterozoic metasedimentary siltstones and quartz arenites. The Telfer district also contains magnetite- and ilmenite-series granitoids dated between ca. 645 and 600 Ma and a world-class W skarn deposit associated with the reduced, ~604 Ma O’Callaghans granite. Based on monazite and xenotime U-Pb geochronology, Telfer is estimated to be older than O’Callaghans, forming between 645 and 620 Ma. Au-Cu mineralization at Telfer is hosted in multistage, bedding-parallel quartz-dolomite-pyrite-chalcopyrite reefs and related discordant veins and stockworks of similar composition that were emplaced into two NW-striking doubly plunging anticlines or domes. Mineralization is late orogenic in timing, with hot (≤460°C), saline (<50 wt % NaCl equiv) ore fluids channeled into preexisting domes along a series of shallow, ENE-verging thrust faults and associated fault-propagated fold corridors. A combination of fault-propagated fold corridors acting as fluid conduits below the apex of the Telfer domes and the rheology and chemical contrast between interbedded siltstone and quartz arenite units within the dome are considered key parameters in the formation of the Telfer deposit. Based on the presence of the reduced Au-Cu-W-Bi-Te-Sn-Co-As assemblage, saline and carbonic, high-temperature hydrothermal fluids in Telfer ore, and widespread ilmenite-series granites locally associated with W skarn mineralization, Telfer is considered to be a distal, intrusion-related gold deposit, the high copper content of which may be explained by the predominance of highly saline, magmatic fluids in gangue assemblages cogenetic with ore.

Abstract Middle to upper Proterozoic marine sedimentary successions of the Paterson province host gold-copper mineralization in quartz sulfide reefs at Telfer, the largest single producing gold mine in Australia during 1987. The origin of Telfer is controversial: most previous models have emphasized the very continuous, stratiform-strata-bound nature of the auriferous Middle Vale reef and postulated a syngenetic exhalative origin. However, recent deeper mining and drilling of hypogene ore provides evidence that Telfer is an epigenetic deposit. Reconnaissance data suggest that northwest-trending elongate domes in the Paterson province (including the Main and West domes which host the Telfer gold deposits) formed during a noncoaxial deformation event. Veins associated with mineralization form probable conjugate sets, possibly controlled by later coaxial deformation. Widespread bedding-plane slip and dilation was synchronous with vein formation, resulting in the preferential development of laterally extensive concordant veins and mineralized reefs within less competent siltstone units, either during or after the late stages of doming. The Middle Vale reef hosts most of the gold mineralization, and the highest grade ore is spatially coincident with zones of highest vein density in both its footwall and hanging wall. Economic gold mineralization higher in the stratigraphy is in strata-bound horizons and vein stockworks, hosted by pervasively altered sandstones and siltstones. Field observations suggest that at least part of the Middle Vale reef consists of subconcordant quartz sulfide veins. Textural studies suggest that most of the sulfides within the reef are epigenetic and replace carbonate horizons and calcareous mudstones and siltstones, which locally contain carbonaceous material. Rounded aggregates of fine-grained, well-crystallized pyrite within a thin laminated interval at the top of the reef have anomalous geochemistry and provide the only evidence for probable syngenetic pyrite within the mine sequence. A number of granitoids intruded the middle to upper Proterozoic succession late in its tectono-magmatic history. A Pb-Pb mineral isochron for one of these, the Mount Crofton Granite, gives an age of 690 ± 48 Ma. The Pb isotope compositions of ore-associated sulfides in discordant veins and the Middle Vale reef at Telfer, and skarns at two other prospects, are heterogeneous and dissimilar to that shown by most volcanogenic sulfide ores. The Pb isotope sulfide data are best explained by derivation of most of the Pb from the host rock (or basinal brines), with some contribution from a magmatic source such as the Mount Crofton Granite. Fluid inclusions from quartz in veins and the Middle Vale reef contain very complex saline and CO 2 -rich fluids that homogenize between 225° and 440°C: salinities range between 21 and 54 equiv wt percent NaCl. Daughter minerals include halite, Fe-bearing calcite, sylvite (?), and dawsonite. Fluid inclusion data are indicative of mixing of hot, very high salinity (magmatic) fluids with cooler, lower salinity (basinal or meteoric) waters with liberation of CC 2 during replacement of carbonate host rocks. Telfer is reinterpreted to be an epigenetic vein-hosted replacement deposit in which gold mineralization was controlled by both structure and composition of the host rocks. Hot, saline fluids introduced Cu and Au during late deformation and granitoid emplacement, but the source of the Au is not yet established.