Shotgun deposit: granite porphyry-hosted gold-arsenic mineralization in southwestern Alaska, USA

Abstract.

The Shotgun deposit (30 tonnes gold; inferred resource using 0.5 ppm cutoff) consists of stockwork Au-As mineralization in a Late-Cretaceous (69.7±0.3 Ma, ⁴⁰Ar/³⁹Ar) granite porphyry stock located approximately 150 km north of Dillingham, SW Alaska. Preliminary metallurgical results indicate >90% gold is recoverable by cyanide leaching. With average estimated concentrations of 1.4 ppm Au, 6.1 ppm Ag, 0.5% As, 0.07% Cu, 40 ppm Mo, and 8 ppm Bi, Shotgun is a low-fO₂ porphyry Au-As deposit that shares characteristics with both intrusion-related gold and porphyry copper deposits. The bulk of gold occurs in intense quartz stockwork and breccias. It is texturally identical to Cu-Mo porphyries, but has different mineralization and alteration assemblages. Shotgun contains arsenopyrite>>pyrite>pyrrhotite>chalcopyrite, lacks K alteration and, instead, contains vein and pervasive albite-quartz±sericite±carbonate. As temperature decreased, early high-temperature, low-fS₂ mineralization (arsenopyrite-loellingite±pyrrhotite) evolved to later, lower-temperature, moderately high fS₂ conditions (pyrite-bornite-chalcopyrite). In addition, ores at Shotgun contain primary native gold, native bismuth, Bi-Te sulfides, cubanite, maldonite, scheelite, and sphalerite, as well as supergene covellite, chalcocite, marcasite, and native copper. Arsenopyrite geothermometry yields deposition temperatures of 470 to 630 °C. Fluid inclusion microthermometry and laser Raman spectroscopy indicate that vapor-rich inclusions (H₂O>>CO₂>CH₄) homogenize to vapor at ~360 °C, and saline aqueous inclusions contain 40 to >60 wt% NaCl equiv. (probably NaCl-KCl) and homogenize to liquid at 280 to >600 °C. Coexisting vapor- and liquid-rich fluid inclusions in quartz suggest 'boiling' occurred. Stable-isotope data include δ¹⁸O of 16.4 to 17.1‰ (quartz), δD of –124 to –105‰ (sericite), and δ³⁴S of –5.5 to –5.0‰ (asp, cpy). At 380 °C, fluid compositions of δ¹⁸O of 10.4 to 11.1‰, and δD of –95 to –74‰ are compatible with a magmatic fluid source. Gold correlates fairly well (r>0.60) with Ag, Bi, Mo, and Te. When combined with the evidence for high-temperature fluids, the fragmental morphology of the intrusion, stockwork veining, and exsolution of vapor-rich and aqueous saline-rich fluid inclusions indicate a direct relationship between early mineralization and magmatism. The Shotgun deposit shares similarities with porphyry Cu-Mo-Au deposits (stockwork, breccia) and reduced, nonporphyry intrusion-related Au-As-Bi deposits (sulfide mineralogy, geochemistry, and oxidation state). They appear to be products of the same style of magmatic-hydrothermal systems and differ only in depth and thus volatile-release history. The Shotgun deposit is essentially an intrusion-related Au-As-Bi deposit emplaced at pressures and temperatures typical of a porphyry system.