Chemical Composition and Strontium Isotope Characteristics of Scheelite from the Doranasai Gold Deposit, NW China: Implications for Ore Genesis

Abstract

Scheelite, as a common accessory mineral found in hydrothermal deposits, is an indicator that allows the study of the ore-forming hydrothermal process and the tracing of fluid sources. The Doranasai gold deposit is a large-sized orogenic gold deposit in the South Altai, and orebodies occur as veins in the Devonian Tuokesalei Formation and Permian albite granite dykes. The ores are quartz veins and altered tectonites (rocks). Here, scheelite can be observed in the early-stage milky quartz veins, the middle-stage smoky quartz-polymetallic sulfide veins, and the altered albite granite dykes. In this study, the scheelites of these three types were carefully investigated in terms of texture, element, and isotope geochemistry to understand their ore-forming processes and fluid sources. The results showed that all types of scheelite were rich in Sr and poor in Mo, indicating that their ore-forming fluids had no genetic relation to magmatic–hydrothermal activities. The scheelites were characterized by the enrichment of medium rare earth element (MREE) and positive Eu anomaly in the chondrite-standardized REE patterns. This indicated the REE differentiation between scheelite and fluid, i.e., REE3+ and Na+ were in the form of valence compensation, preferentially replacing Ca2+ and selectively entering the scheelite lattice. The trace element composition of scheelite showed that the ore-forming fluid system was relatively closed, mesothermal, Na-rich, and reductive. The Sr isotope ratio of the scheelite (0.704819–0.70860, average 0.706372) was higher than that of the ore-bearing albite granite dyke (0.704654–0.704735), indicating that the Tuokesalei Formation is the main source for the fluids forming the Doranasai deposit.