The Kirganik alkalic porphyry Cu-Au prospect in Kamchatka, Eastern Russia: A shoshonite-related, silica-undersaturated system in a Late Cretaceous island arc setting

Abstract

The Kirganik prospect is a silica-undersaturated, shoshonite-related, alkalic, porphyry Cu-Au system developed within the Late Cretaceous island arc of Central Kamchatka. It is associated with a potassic volcano-plutonic suite of rocks that range from monzogabbro- and monzodiorite- to monzonite- and syenite-porphyry, followed by a second, compositionally overlapping magmatic cycle of trachytic syenite-porphyry, and possibly other, more differentiated monzonitic phases. The rocks exhibit a subduction-related, island arc rock affinity, with the magma generation involving low degree (~1–3 vol%) partial melting of metasomatized, K-rich lithospheric mantle, followed by amphibole fractionation in a deep, probably lower crustal, magma chamber, with subsequent fractionation of accessory minerals, clinopyroxene and possibly biotite in shallower magma chambers. An early calc-potassic alteration assemblage of K feldspar-pyroxene-apatite-magnetite is closely related to small syenite intrusions. This stage is overprinted by more extensive potassic alteration zones of similarly quartz-free biotite-K feldspar-magnetite, some of which contain abundant Cu-sulfides (mostly bornite and chalcopyrite, with minor chalcocite), minor native gold and PGE minerals. Subsequent sodic-potassic to calcic-sodic-potassic alteration comprising albite-magnetite-calcite-epidote, and propylitic assemblages of chlorite-magnetite-epidote ± pyrite are much more weakly developed. These hydrothermal alteration assemblages form lens-shaped, replacement-style zones, rather than stockworks of well-defined veinlets/stringers, particularly quartz-sulfide veinlets, which are absent. Apatite from calc-potassic and potassic alteration assemblages contains low salinity (5–10 wt% NaCl-eq.) liquid-gaseous fluid inclusions indicating a homogenous, low salinity, supercritical aqueous fluid exsolved directly from a cooling and degassing pluton, with no boiling and unmixing into hypersaline and gaseous phases. Under inferred high temperatures of calc-potassic and potassic alteration (450–550 to 400–420 °C) and significant (2.1–2.2 to 1.5–1.8 kbar) pressure, this suggests a deep (>5 km?), possibly root-level of a larger, shallower, since eroded, porphyry Cu-Au system. The current geological setting and distribution of alteration and mineralization can be explained by a sharp, up to 90°, post-mineral tilting of the initial narrow, subvertical, 'finger-like' intrusions and related mineralized system, followed by deep exhumation. As a consequence, a significant part (>1.5 km) of the deep porphyry system is currently exposed at surface.