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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.

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