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

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, ^sup 40^Ar/^sup 39^Ar) 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-fO^sub 2^ 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-fS^sub 2^ mineralization (arsenopyrite-loellingite±pyrrhotite) evolved to later, lower-temperature, moderately high fS^sub 2^ 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 (H^sub 2^O>>CO^sub 2^>CH^sub 4^) 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 [delta]^sup 18^O of 16.4 to 17.1[per thousand] (quartz), [delta]D of -124 to -105[per thousand] (sericite), and [delta]^sup 34^S of -5.5 to -5.0[per thousand] (asp, cpy). At 380 °C, fluid compositions of [delta]^sup 18^O of 10.4 to 11.1[per thousand], and [delta]D of -95 to -74[per thousand] 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 minerali