In situ geochemical and S-Pb-C-O isotope constraints on the ore-forming process of the Zhulazhaga gold deposit

Abstract

The Zhulazhaga gold deposit is a super-large (235.82 tons) deposit situated within the Alxa Block, the western segment of the northern margin of the North China Craton. The ore bodies are hosted in Mesoproterozoic low-grade metamorphic rocks, and the main ore bodies occurr as stratiform layers. The gold-bearing minerals include native gold, electrum, maldonite, and bismuthian gold telluride. The primary metallic minerals are mainly pyrrhotite and pyrite, with minor amounts of chalcopyrite and arsenopyrite, which are locally accompanied by fine-grained sphalerite, galena, scheelite, and molybdenite. To constrain the source of the ore-forming materials and the mineralization evolution process, in situ S and Pb isotope analyses were conducted on sulfides, C and O isotope analyses were performed on late-stage carbonate veins, and trace element analyses were carried out on pyrite from the five defined stages (Py1, Py2, Py3, Py4, and Py5). The trace element concentrations and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) time-resolved signal spectra of the pyrite demonstrate that most of the gold and other elements in the pyrite are present in the form of invisible solid solutions, but some occur as nano- or sub-micro inclusions. The Cu/Au ratios are predominantly greater than 1, indicating a reducing fluid environment. Furthermore, the fluctuations in the trace element contents of the pyrite suggest the occurrence of several episodes of new fluid injection. Based on systematic variations in the Co, Ni, Se, and Te concentrations, we infer that there was a general cooling trend and a decrease in the redox state from the early to late mineralization stages. The δ³⁴S_V-CDT values range from 1.21 ‰ to 7.39 ‰, with a median value of 4.96 ‰. Based on the equilibrium isotope fractionation factors between sulfides and H₂S, the calculated δ³⁴S_H2S values of the H₂S in the hydrothermal fluids range from −0.07 ‰ to +6.21 ‰, with an average of +3.91 ‰. The lead isotope ratios predominantly plot near the mantle and orogenic belt evolution lines, while the C and O isotopes of the late-stage calcite veins plot close to the granite and the mantle polyphase system field. These characteristics indicate that the Zhulazhaga gold deposit has a magmatic-hydrothermal origin and that it experienced multi-stage hydrothermal superposition.