Heavy δ26Mg values in carbonate indicate a magmatic-hydrothermal origin of Carlin-type Au deposit

Abstract

The Carlin-type Au deposits in Youjiang Basin, SW China, (referred to as Guizhou Carlin-type Au deposits) are the second largest Carlin-type Au province in the world after Nevada, USA. To date, the source of ore fluids that formed the Guizhou deposits remains controversial, hampering the formulation of a genetic model. Compared to Nevada Carlin-type Au deposits, a significant difference is that the Guizhou Carlin-type Au deposits contain abundant ore-stage dolomite in the ore. Herein, we present carbonate Mg isotopes combined with C-O isotopes from the giant Shuiyindong deposit to provide new insights into the source of ore fluids and to constrain the ore genesis of the Guizhou Carlin-type Au deposits.Petrographic observation shows that from least altered bioclastic limestone to high-grade ore, dolomite increased significantly, suggesting that dolomite formed as part of the Au mineralization process. Chemical analyses reflect a small amount of Mg in the ore fluids was variably added to ore during Au mineralization, and this added Mg was fixed in the dolomite. Although most ore-stage dolomite formed from the sulfidation of Fe in Fe-dolomite, some ore-stage dolomite formed by the combination of host rock calcite with Mg from ore fluids.The δ26Mg values of carbonate, primarily dolomite, range from −3.49‰ to −0.07‰, with a median value of −1.01‰. The δ18O values range from 21.7‰ to 27.6‰, with a median value of 23.2‰. The δ13C values vary from −14.6‰ to 1.2‰, with a median value of −0.7‰. The δ26Mg values exhibit a positive correlation trend with MgO, S, SiO2, and Carlin-suite elements (Au, As, Hg, Sb, Tl, and Cu), and a negative correlation trend with CaO. Comparison of Mg isotopes data from this study with the major Mg reservoirs indicates that the Shuiyindong samples contain heavier Mg isotopes that drifts towards the heavy Mg reservoirs (magmatic and metamorphic rocks) compared with normal carbonate rocks. Two mechanisms, namely heavy Mg addition from ore fluids and isotopic exchange reaction with heavy Mg-rich ore fluids, could have attributed to heavy Mg in carbonates in the Shuiyindong deposit.The Mg isotopes results, combined with other isotopes (e.g., S, Hg, and He-Ar) and geologic evidence, support a magmatic-hydrothermal origin for the ore fluids. The thick Devonian-Triassic sedimentary sequence and weak extension following sedimentation prevented igneous activity from reaching the surface. However, buoyant ore fluids released from the deep-seated intrusions are interpreted to have ascended along basement-penetrating faults and eventually produced the Guizhou Carlin-type Au deposits at shallow levels. This study implies that Mg isotopes are a novel proxy to infer ore fluids source and evolution, and that they can provide significant constraints on the genesis of hydrothermal deposit.