Source and evolution of ore fluids in the Zhenyuan orogenic gold deposit, SE Tibet: Constraints from the S-C-O isotopes

Abstract

Orogenic gold deposits commonly have large variation of stable isotopic values of ore minerals induced by complicated ore fluid evolution, which was considered to provide equivocal evidence for ore sources. In contrast, this study has revealed relatively narrow range of S and C-O isotopes of pyrites and carbonate minerals in the Zhenyuan gold deposit (>100 t, at 3.5 g/t) on Ailaoshan shear zone, southeastern margin of Tibet. The Zhenyuan deposit is a disseminated orogenic gold deposit with mineralization and alteration occurring in various types of wall rocks including carbonaceous slate, meta-quartz sandstone, lamprophyre, quartz porphyry and meta-basalt. The pre-ore pyrites include the diagenetic pyrites in carbonaceous slate and meta-quartz sandstone and the barren hydrothermal pyrites in quartz porphyry. Diagenetic pyrites have much varied δ34S values from −54.58 to 38.31‰, and barren hydrothermal pyrites have elevated positive δ34S values (8.76–43.24‰). The ore-related pyrites, disseminated in all types of host rocks and commonly with repetitive gold-rich zones, are the most important gold-bearing minerals. They contain high concentration of Au along with other trace elements, including Cu, As, Sb, Tl, Pb and Bi. Little difference of δ34S values is observed for the pyrites in different types of host rocks and for the low-Au and high-Au zones in single pyrite grain. All ore-related pyrites display a relatively narrow range of δ34S values (−5.22 to 4.16‰) with a peak at ~0‰. This narrow range implies that the deposition of gold from ore fluids at Zhenyuan deposit was mainly controlled by sulfidation of iron-bearing minerals in the wall rocks. The δ34S values of ore-related pyrites are consistent with the explanation that the ore fluids were from the subcrustal source, instead of the crustal source with much varied sulfide δ34S values. Carbonate minerals from different types of host rocks also have consistent and relatively narrow range of δ13CPDB (−3.77 to 0.68‰) and δ18OSMOW (14.85–20.77‰) values, indicating that the composition of ore fluids were little affected by water-rock reaction at deposit scale. The δ13CPDB and δ18OSMOW values of carbonates minerals are between those of the mantle peridotite and marine carbonates, which can be explained by that the ore fluids have experienced crustal carbonate contamination in the subcrustal level or/and along the migration conduit of the ore fluids.