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 The Lihir gold deposit, Papua New Guinea, is the world’s largest alkalic low-sulfidation epithermal gold deposit in terms of contained gold (50 Moz). The deposit formed over the past million years and records a progression from porphyry- to epithermal-style hydrothermal activity. The early porphyry stage was characterized by biotite-anhydrite-pyrite ± K-feldspar ± magnetite alteration and weak gold ± copper mineralization and produced abundant anhydrite ± carbonate veins and anhydrite ± biotite-cemented breccias. These features collectively characterize the deep-seated anhydrite zone at Lihir. Several hundred thousand years ago, one or more catastrophic mass-wasting events unroofed the porphyry system after porphyry-stage hydrothermal activity ceased. Mass wasting may have been facilitated in part by dissolution of porphyry-stage anhydrite veins. Epithermal mineralization occurred after sector collapse, resulting in phreatic and hydraulic brecciation and veining, widespread adularia-pyrite ± carbonate alteration, and formation of mineralized zones at Lienetz, Minifie, Kapit, Kapit NE, Coastal, and Borefields. A NE- to ENE-striking fault array localized several of these orebodies. The pyrite-rich veins and pyrite-cemented breccias that formed during epithermal-stage hydrothermal activity define the sulfide zone at Lihir. This zone mostly contains refractory gold in pyrite, with minor free gold and precious metal tellurides hosted in late-stage quartz veins. A period of diatreme volcanism disrupted the Luise amphitheater during the latter stages of epithermal mineralization. The diatreme breccia complex truncated several of the epithermal ore zones and was crosscut locally by late-stage epithermal veins. Recent geothermal activity produced a steam-heated clay alteration blanket that has overprinted the refractory sulfide-rich epithermal assemblage near the present-day land surface. Gold was remobilized downward from the steam-heated zone into the sulfide zone during argillic and advanced argillic alteration, producing thin gold-rich rims around pyrite grains. This process produced a high-grade tabular enrichment zone immediately beneath the base of the clay blanket.

Abstract The Cadia district of New South Wales contains four alkalic porphyry Au-Cu deposits (Cadia East, Ridgeway, Cadia Hill, and Cadia Quarry) and two Cu-Au-Fe skarn prospects (Big Cadia and Little Cadia), with a total of ~50 Moz Au and ~9.5 Mt Cu (reserves, resources, and past production). The ore deposits are hosted by volcaniclastic rocks of the Weemalla Formation and Forest Reefs Volcanics, which were deposited in a submarine basin on the flanks of the Macquarie Arc during the Middle to Late Ordovician. Alkalic magmatism occurred during the Benambran orogeny in the Late Ordovician to early Silurian, resulting in the emplacement of monzonite intrusive complexes and the formation of porphyry Au-Cu mineralization. Ridgeway formed synchronous with the first compressive peak of deformation and is characterized by an intrusion-centered quartz-magnetite-bornite-chalcopyrite-Au vein stockwork associated with calc-potassic alteration localized around the apex of the pencil-like Ridgeway intrusive complex. The volcanic-hosted giant Cadia East deposit and the intrusion-hosted Cadia Hill and Cadia Quarry deposits formed during a period of relaxation after the first compressive peak of the Benambran orogeny and are characterized by sheeted quartz-sulfide-carbonate vein arrays associated with subtle potassic, calc-potassic, and propylitic alteration halos.

Abstract Some of the world’s largest and highest grade alkalic porphyry Au-Cu-(Mo) deposits and related epithermal Au deposits occur in the southwest Pacific. Alkalic deposits of this region share many geologic similarities in their environments of formation. Source magmas are highly oxidized and alkali rich, being derived from enriched mantle sources that were previously modified by subduction processes. The more Cu rich systems formed by high K calc-alkalic and alkalic magmatism are typically located along the main magmatic arc. These subduction-related fluids and mantle-sourced mafic magmas evolve in an environment associated with a thickened crust. In contrast, more Au rich systems appear to be associated with rifting of oceanic crust in back-arc settings. Here, primitive mantle-derived magmas evolve in upper crustal magma bodies to form Au- and PGE- rich alkalic porphyry and epithermal deposits. The gold-rich alkalic porphyry and epithermal deposits formed in and along the margin of sedimentary basins that were intruded by alkalic dikes and stocks. In the largest example (Cadia East), deep mineralization is hosted by sheeted quartz-sulfide veins associated with potassic alteration, while near-surface mineralization is disseminated in both permeable clastic units and quartz-sulfide veins. Potassic alteration grades laterally into proximal, hematite-bearing propylitic alteration, and transitions upward from deep K-feldspar to shallow biotite-tourmaline. The shallow biotite alteration domain is overprinted by a complex, late-stage assemblage of pervasive K-feldsparalbite-sericite-pyrite, and structurally focused sericite-pyrite. In the alkalic epithermal environment, near-surface K-feldspar-quartz-carbonate-anhydrite (± sericite) alteration associated with epithermal Au-Ag mineralization occurs in and around dikes, fault intersections, and along extensive low-angle faults. Catrastrophic failure of the overlying volcanic edifice has the potential to cause superposition of alkalic epithermal mineralization onto porphyry deposits. Given their potential to form in a back-arc setting, alkalic porphyry deposits are considered more likely to be preserved in the ancient rock record than their calc-alkalic counterparts, due to burial in the sedimentary basins in which they form. Thus, areas of fragmented intraoceanic arc terranes within orogenic belts should be considered prospective for Au-rich alkalic porphyry deposits like those found in the southwest Pacific, particularly when they occur in regions overlain by postmineralization sedimentary and/or volcanic cover. Alkalic epithermal deposits offer more challenging exploration targets, as they are likely to be exhumed and eroded soon after their formation, unless a tectonic switch causes burial before any significant erosion occurs.