The Xinqiao Cu–S–Fe–Au deposit in the Tongling mineral district, China: synorogenic remobilization of a stratiform sulfide deposit

Abstract

The Cu–S–Fe–Au deposit at Xinqiao is located in the Tongling mineral district, Anhui Province, People's Republic of China. It is a large-scale polymetallic deposit with drilling indicated reserves of 0.5 million tonnes of Cu averaging 0.71%, 75.5 million tonnes of S averaging 29.3% and 24.9 million tonnes of Fe averaging 46%, as well as 11.2 tonnes of Au averaging at 4.7 g/tonne plus 248.4 tonnes of Ag averaging at 248.4 g/tonne. There are two major mineralization types in the Xinqiao deposit: one is the economically significant stratiform orebody hosted in the Lower Carboniferous Gaolishan siltstone (C 1g) and overlying Middle Carboniferous Huanglong limestone (C 2h), while the other is the less important skarn-type mineralization restricted to the contact between the Yanshanian diorite (110–168 Ma) and the Lower Permian Qixia marble (P 1q). The ore in the Xinqiao deposit is dominated by massive pyrite with minor amount of chalcopyrite, magnetite, pyrrhotite, galena, sphalerite, quartz and dolomite. Four stages of deformation have been recognized to affect the sedimentary rocks in the Xinqiao area. The first two (D 1 and D 2) produced mainly NE–SW and NNE–SSW trending open folds, whereas D 3 is characterized by normal faults striking NNE–SSW and D 4 is dominated by post-mineralization strike-slip faults segmenting the orebody. The structural information obtained by this study not only revealed the transgressive nature of the stratiform orebody, but also identified a strong structural control on the formation of the deposit. It is concluded that the Xinqiao polymetallic deposit was formed through remobilization of a Carboniferous sulfide source bed by hydrothermal solutions related to late Yanshanian plutonism (110 Ma), during which D 3 normal faulting played an important role in localization of the mineralization. Such a two-stage ore-forming model is consistent with the most recent genetic interpretations of the deposit.