Genesis of the sediment-hosted Sanshierzhan Au deposit, NE China: Evidences from the pyrite geochemistry and s-Hg isotopes

Abstract

The Mohe Basin is a famous placer gold (Au) field in Northeast (NE) China, and has garnered more attention due to the discovery of several sediment-hosted Au deposits. However, the genesis of these deposits remains poorly constrained. This study investigates the pyrite geochemical and S-Hg isotopic data from the sediment-hosted Sanshierzhan Au deposit to provide a critical insight into the metallogenic mechanism of the sediment-hosted Au mineralization in NE China. At Sanshierzhan, pyrite predominantly occurs as disseminated grains in altered sandstones, with minor occurrences in quartz veins. Three pyrite generations (Py I, Py II, and Py III) were identified from four mineralization stages. These include: coarse-grained pyrite (Py I) disseminated in altered sandstones, which shows dissolution textures; fine grained zonal pyrite (Py II) disseminated in polymetallic sulfide quartz vein with dissolution surface in its core (Py II-1) and relatively clean rim (Py II-2); medium grained pyrite (Py III) disseminated in calcite-quartz veins. These textural features, particularly zoned structures and inhomogeneous element distributions, record a dynamic change of fluid composition during the Au precipitation. The δ³⁴S values of the pyrite generations Py I, Py II, and Py III range from 3.07 ‰ to 4.27 ‰, 4.41 ‰ to 6.15 ‰, 6.27 ‰ to 9.96 ‰, respectively. Most of these values exceed the magmatic δ³⁴S values (−5.00 ‰ to + 5.00 ‰), indicating the involvement of the crustal sulfur (S) during the Au mineralization. Furthermore, Hg isotopic data from ore (pyritized and silicified rocks) and altered rock samples at Sanshierzhan show slightly negative to positive δ²⁰²Hg values (−0.28 ‰ to 0.76 ‰) and negative to near zero Δ¹⁹⁹Hg values (−0.11 ‰ to 0.04 ‰). Their positive δ²⁰²Hg values suggest preferential leaching of heavy Hg isotopes from source area. Significantly, the slightly negative Δ¹⁹⁹Hg values are similar with those of the terrestrial reservoirs, further indicating that some Hg as well as Au was likely derived from the upper continental crust. Therefore, our findings demonstrate that the Sanshierzhan mineralization is likely the result of strong water–rock reaction triggered by the magmatic-hydrothermal fluids. This hydrothermal system facilitated the remobilization of elements from the upper continental crust, together with the ore-forming metals carried by the magmatic fluids, causing the enrichment of Au in the hydrothermal system.