A hydrothermal origin for the large Xinqiao Cu-S-Fe deposit, Eastern China: Evidence from sulfide geochemistry and sulfur isotopes

The Xinqiao Cu-S-Fe deposit in the Tongling ore district, Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB; Eastern China), is located along the northern margin of the Yangtze Craton. The stratiform- and skarn-type Xinqiao mineralization comprises five stages, namely the early skarn (Stage I, garnet and diopside), late skarn (Stage II, epidote-dominated), iron oxides (Stage III, hematite and magnetite), colloform pyrite (Stage IV) and quartz-sulfides (Stage V). There are three pyrite types at Xinqiao, i.e., colloforrn (Py1; Stage IV), finegrained (Py2, from Py1 recrystallization; Stage V) and coarse-grained (Py3; Stage V) pyrites. Scanning Electron Microscope (SEM) imagery for Pyl reveals that they are cubic microcrystalline pyrite aggregates, and the EDS and XRD data indicate that some Py1 contain minor siderite impurities. Electron Microprobe Analysis (EMPA) and LA-ICP-MS geochemical data demonstrate that the three pyrite types have relatively high Fe/S ratios and distinctly high Mn, Cu and As concentrations. Compared to Py2 and Py3, Py1 has higher Pb, Bi and Ag, but lower Co, Ni, Se, Cd, Te and Au. Ratios of Fe/S (0.837 to 0.906), Se/Te (2.39 to 14.50) and Co/Ni (0.67 to 4.67) of the Xinqiao pyrites resemble typical hydrothermal pyrites. delta S-34(CDT) of Pyl (-0.6%circle to 2.7%circle, average 0.58%circle), Py2 (1.8%circle to 2.5%circle, average 2.1%circle) and Py3 (1.9%circle to 4.4%circle, average 3.5,%circle) are close to those of the Xinqiao skarn-type orebodies (1.3%circle to 4.1%circle), but distinct from those of the Upper Carboniferous Huanglong Formation limestone (-9.5%circle to -15.4%circle), suggesting that the three pyrite types (especially Pyl) were genetically linked to the Yanshanian (Jurassic-Cretaceous) magmatic-hydrothermal events, with Py1 probably reflecting rapid crystallization during fluid mixing. We interpret that the Xinqiao stratiform mineralization may have been associated with the Jitou quartz diorite stock, as may be the case also for the skarn-type mineralization hosted in the contact between the Yanshanian Jitou stock and the Lower Permian Qixia Formation limestone. Overall, the Xinqiao Cu-S-Fe mineralization may have been generated by the Jurassic-Cretaceous tectono-thermal event in Eastern China. (C) 2016 Elsevier B.V. All rights reserved.