Magmatic Controls on Au- and Ag-Rich Intermediate-Sulfidation Epithermal Deposits from Northeast China

Abstract Intermediate-sulfidation epithermal deposits are important not only for their significance in terms of Ag endowment but also for their close link with porphyry deposits. There are two subtypes of intermediate-sulfidation deposits, namely Au-rich (Ag/Au <60) and Ag-rich (Ag/Au >60). The tectonomagmatic factors that control the formation of either one of the two types have not been thoroughly examined. Here we investigated the degree of magmatic fractionation, magma oxygen fugacity, and F-Cl-S concentrations in the magmas, as well as their sources, for the magmatic rocks associated with four intermediate-sulfidation deposits (two Ag-rich: Baiyinchagan, Shuangjianzishan; two Au-rich: Naozhi, Zhengguang; all in northeast China) and found that the ore-forming magmas of our two Au-rich deposits in northeast China belong to the I type, whereas those of our two Ag-rich deposits are of the A type. Ore-forming magmas of our Au-rich intermediate-sulfidation deposits belong to the magnetite series and are less evolved, slightly more juvenile, more oxidized (ΔFMQ = 0.5 to 1.5; FMQ = fayalite-magnetite-quartz), more Cl rich, and more hydrous than magmas associated with the Ag-rich deposits. These features are typical of neutral-compressional thick magmatic arcs. By contrast, the ore-forming magmas of our two Ag-rich deposits belong to the ilmenite series and are more evolved, reduced (ΔFMQ <0.5), moderately juvenile, more F rich, and less hydrous. They likely originated from prolonged differentiation of juvenile crust with heterogeneous and minor addition of sediment melts. These features are typical of a back-arc extensional regime developed within accreted juvenile arcs in northeast China. These findings provide a first-order framework for guiding early-stage porphyry-epithermal exploration of untested prospects.