Re[sbnd]Os and S isotopic constraints on the origins of two mineralization events at the Tangdan sedimentary rock-hosted stratiform Cu deposit, SW China

Abstract

The Tangdan stratiform Cu deposit in the Kangdian region of South China is hosted in sedimentary rocks of the late Paleoproterozoic to early Mesoproterozoic Dongchuan Group. Orebodies mainly occur in dolostone and black shale above a thick sequence of hematitic sandstone and siltstone. Mineralization consists mainly of bedding-parallel stratiform ores, with minor amounts of discordant vein-type ores. Both stratiform and vein-type ores are composed of bornite and chalcopyrite with minor chalcocite and digenite, associated with gangue minerals including ankerite, dolomite, calcite, and quartz.Cu-sulfides from vein-type ores contain ~122–293ppb Re, much higher than those of stratiform ores (~0.6–14ppb Re). They have highly radiogenic Os with ReOs model ages ranging from 1413 to 1457Ma and a weighted mean ReOs model age of 1432±19Ma (2σ, MSWD=0.30), which is within uncertainty in agreement with isochron ages of 1401±30Ma (2σ, MSWD=0.67) and 1397±71Ma (2σ, MSWD=0.22) using 187Re/188Os versus 187Os/188Os and 187Re versus 187Os regression, respectively. In contrast, sulfides from stratiform ores did not yield meaningful model ages due to the extremely low concentrations of Re and radiogenic Os. However, early studies have shown that the stratiform ores were likely formed at ~1700Ma during the diagenesis of the host strata.Sulfides from the vein-type ores have δ34S values ranging from −15‰ to −8‰, significantly lower than those of stratiform ores (−3‰ to +10‰), suggesting that organic matter may have been involved in the formation of the vein-type ores. Contrasting ReOs and S isotopes suggest that the two types of ores at the Tangdan deposit were likely formed from different ore-forming fluids. We propose that stratiform ores were precipitated from basinal fluids during the early evolution of the rift basin, whereas the vein-type ores were formed due to a much younger thermal event during which hydrothermal fluids interacted with overlying black shale to account for the high Re and negative δ34S values. This study demonstrates that ReOs isotopes of sulfides can be used to study the origin of hydrothermal system.