Geology and geochemistry of the submarine volcanism-related Qiaxia Fe–Cu deposit in the southern Altay, Xinjiang, Central Asian Orogenic Belt

Abstract

The Qiaxia Fe–Cu deposit is located on the southern Altay margin, Xinjiang, China, and is hosted in the submarine volcanic–sedimentary rocks of the Lower Devonian Kangbutiebao Formation. Ore genesis of the Qiaxia deposit plays an important role on understanding the metallogenesis of submarine volcanic-hosted Fe deposits. In the Qiaxia deposit, stratiform and podiform Fe orebodies are conformable with host sequence strike, whereas lenticular Cu mineralization is closely related to skarn. Wall-rock alteration is dominated by silicification with minor skarn (actinolite, chlorite, diopside, and tremolite). Based on field evidence and petrographic observations, the paragenesis of the Qiaxia deposit includes the following three periods: (1) a volcanic–sedimentary period characterized by the formation of barite and Fe orebodies; (2) a magmatic–hydrothermal period with the formation of skarn-type Cu mineralization; and (3) a regional metamorphism period. The volcanic–sedimentary period shows moderate temperature and moderate- to low-salinity fluids with compositions of predominantly K+, Cl−, and SO42− in the liquid phase and H2O, CO2, and N2 in the gaseous phase. By contrast, the regional metamorphism period reveals high- to moderate-temperature and high- to low-salinity fluids and compositions of predominantly Na+, K+, Cl−, and SO42− in the liquid phase and H2O, CO2, N2, CH4, and C2H6 in the gaseous phase. The H–O isotopic compositions suggest that the volcanic–sedimentary period involved both magmatic fluids and seawater, while the fluids of the regional metamorphism period were derived from a mixture of metamorphic water and deeply circulated meteoric water. Sulfur isotope analysis shows that sulfur of volcanic–sedimentary period is mainly formed from thermochemical sulfate reduction of seawater sulfate, while sulfur of the magmatic–hydrothermal period reveals a mantle-derived source. The metamorphic quartz diorite yields a zircon U–Pb age of 398.9 ± 6.2 Ma (MSWD = 0.31). Combining with the available data, we conclude that the Fe orebodies formed from 413 to 399 Ma. The geological and geochemical evidence presented in this paper suggest that the Qiaxia deposit was initially a volcanic–sedimentary Fe deposit, which was overprinted by a later skarn Cu mineralization, and finally underwent greenschist-facies metamorphism.