The Fluid Regime of Orogenic Gold Deposit Formation in the Yana-Kolyma Belt

Abstract

The microthermometric and bulk analysis data on fluid inclusions in vein quartz of 16 gold–quartz deposits located in the different sectors of the Yana-Kolyma belt are presented. The deposits were found to have formed in the temperature range of 350–220°C by weakly mineralized (<5 wt % NaCl-eq.) carbon dioxide-aqueous fluids that are typical for orogenic ore-forming systems. The composition of the salts is dominated by sodium bicarbonate; the trace elements are K, B, As, Sb, Rb, and Cs. The mole fraction of CO₂ varies from 0.19 to 0.03 depending on the RTX conditions. The mole fraction of methane in carbon dioxide does not exceed 0.1. Intense degassing of the solutions in the range of 320°–270°C could be one of the reasons for ore deposition. Gold-bearing quartz veins and veinlets were formed at depths of 6–3 km at a decrease in pressure from 1.5–1.0 to 0.9–0.8 kbar while the dynamic regime of the orogeny evolved. At the final stage, the opening of fractures led to a change of the regime from lithostatic to hydrostatic with a pressure drop to 0.5–0.1 kbar. At the final stages of mineral formation, the fluid systems have increased salinity and a gas phase comprising essentially nitrogen and methane, which indicates their uptake from another source. Thus, the parameters of fluid inclusions in quartz reflect a long history of evolutionary ore-forming processes that are related to development of orogen.