Native gold from the Inagli Pt–Au placer deposit (the Aldan Shield, Russia): geochemical characteristics and implications for possible bedrock sources

Abstract

The Inagli alluvial Pt–Au placer deposit in the Republic of Sakha (Yakutia), Russia, is linked to the Inagli massif, one of the several Uralian–Alaskan-type alkaline-ultrabasic complexes in the Aldan Shield. Gold from the placer is heterogeneous in composition and is represented by three types. Type 1 gold is the most abundant and is characterized by simple Au–Ag alloys with 4–34 wt% Ag, low Cu (up to 0.08 wt%) and negligible Hg, Pt, and Pd contents, and silver–tellurium sulfosalts (Ag–Cu–Te–S–As compounds) in the inclusion suite. Silicate inclusions are biotite, K-feldspar, Fe–Mg amphibole, chlorite, plagioclase, Fe–Mg pyroxene, zircon, and titanite. Distinctive features of this gold type are most similar to those derived from low-sulfidation systems linked to iron oxide copper–gold or iron skarn types of mineralization. The bedrock source of type 1 gold could be related with monzonite to syenite intrusions surrounding the Inagli massif. Distinctive features of type 2 gold include a wide discontinuous range of Ag content (1–18 wt%), elevated Cu (up to 0.5 wt%), and occasional Pd (up to 0.3 wt%) levels, non-detectable Pt and Hg contents, and rare inclusions of simple sulfides (digenite, pyrrhotite) and Na amphibole. Type 3 gold is distinguished by a narrow range in Ag content (5–8 wt%), elevated Hg (0.5–1 wt%) contents, negligible Cu, Pt and Pd levels, and Au–Pb compounds + K-feldspar inclusions. Microchemical characteristics of type 2 and type 3 gold are interpreted as suggestive of an alkaline–magmatic-related fluid. Based on the grain morphology and microchemical signatures, potential bedrock sources for both gold types could be related to the numerous alkaline veins and potassic alteration zones within the dunite core. A comparison of the Inagli and the Kondyor placer gold allows to generate distinctive generic signatures for gold from Uralian–Alaskan-type alkaline–ultrabasic complexes in the Aldan Shield.