The large Bakyrchik orogenic gold deposit, eastern Kazakhstan: Geology, mineralization, fluid inclusion, and stable isotope characteristics

Abstract

The large (>13 Moz Au, averaging 5–6 g/t Au) Bakyrchik Au deposit in eastern Kazakhstan is part of a district-scale mineralized zone, situated in an accretionary terrane. The deposit is localized within a shear zone in carbonaceous sedimentary rocks, which crosscuts the stratigraphy and comprises subconcordant dikes of high-K calc-alkaline (to shoshonitic?) rocks (lamprophyre to monzogabbro to monzodiorite). The tectonized/cataclased carbonaceous rocks contain numerous banded (bedded?) pyrite clasts and bands and abundant disseminated auriferous sulfides (pyrite and acicular arsenopyrite); the latter overprint pyritic clasts and occur proximal to (although mostly outside of) the concordant quartz veins and associated quartz-cemented hydrothermal breccias. Late granodiorite-plagiogranite dikes postdate auriferous sulfides and associated quartz veins/hydrothermal breccia but are overprinted by the late quartz veins containing free native gold and some Cu-Zn-Pb sulfides. This mineralization, however, is minor at the deposit.The most abundant stylolitic quartz veins accompanied by auriferous sulfide mineralization were formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >315–300 to 265–240 °C and pressures of 0.8 to 1.2 kbar. Boiling was a dominant process during the formation of hydrothermal breccias accompanied by most intense auriferous sulfide (pyrite, arsenopyrite) mineralization at the temperatures of 250–230 °C and pressures of ~1.1 kbar. The late quartz veins were also formed from a homogenous and then boiling aqueous-carbonic fluid at temperatures of >350–315 to 235–225 °C and pressures of 1.1 to 1.3 kbar. The stable isotope data support a variable – from sedimentary to magma-derived - source of sulfur (δ34S = −20 to ~0‰), with a dominantly sedimentary to metamorphic source of carbon (δ13C = −27.4 to −22.5‰).The deposit is polygenetic and exhibits combined synsedimentary-exhalative, metamorphic to intrusion-related, magmatic-hydrothermal signatures, with strong structural control and close association with various dikes. Most of auriferous sulfide mineralization occurred after or nearly coeval to the intrusion of mafic to intermediate dikes, with a strong positive correlation of Au with As, Ssulf, Ag, Sb, and Hg. Homogenized δ34S values of auriferous pyrite and arsenopyrite (from −2.4 to −0.5‰), close to the meteorite standard, indicate dominantly magma-derived sulfur. Late quartz veins with free native gold and minor Cu-Zn-Pb sulfides are more distinctly related to granodiorite-plagiogranite intrusions.