Genesis of the Paleozoic Aqishan-Yamansu arc-basin system and Fe (-Cu) mineralization in the Eastern Tianshan, NW China

Abstract

The formation of the Eastern Tianshan orogen (northern Xinjiang, NW China) with a number of EW-trending mineralization belts was closely linked to the evolution of the Junggar and South Tianshan oceans, probably part of the Paleo-Asian Ocean. Among these Eastern Tianshan mineralization belts, the Carboniferous Aqishan-Yamansu Fe (-Cu) belt hosts a total reserve of 207 Mt Fe and economic Cu endowments for many deposits. These Fe (-Cu) deposits are hosted in submarine volcanic rocks (Lower Carboniferous Yamansu Formation and Upper Carboniferous Tugutublak Formation) and are featured by the presence of extensive skarn alteration without apparently-causative intrusive rocks, which leads to debates over their genesis. In this paper, we have summarized the ore deposit geology, geochemistry and stable isotopes of the Aqishan-Yamansu Fe (-Cu) deposits and discuss their genesis under the regional tectonic framework. We suggest that the Carboniferous Aqishan-Yamansu belt may have been a forearc basin formed by the south-dipping subduction of the Kangguer Ocean beneath the Central Tianshan massif. The Yamansu Formation volcanic rocks and some syngenetic Fe deposits may have formed during the opening of the forearc basin. The Tugutublak Formation volcanic rocks and most Fe (-Cu) deposits may have formed during the subsequent Late Carboniferous basin inversion. In these Aqishan-Yamansu Fe (-Cu) deposits, the Fe mineralization occurred earlier and was likely formed directly from the magmatic-hydrothermal fluids, whereas the subsequent Cu mineralization was likely caused by the gradual increase of seawater or basinal brine influx into the fluid system (external sulfur). The probable external sulfur input, the absence of clear plutonic link, and the temporal coincidence of peak mineralization and basin inversion for the Carboniferous Aqishan-Yamansu Fe-Cu mineralization are comparable to the Mesozoic IOCG mineralization in the Central Andes.