Ore-forming processes of the Uragen giant sandstone-hosted Zn-Pb deposit in the Ulugqat Basin, Xinjiang, Northwest China: Constraints from sphalerite micro-texture, elemental and in-situ sulfur isotopic compositions

Abstract

The Uragen giant sandstone-hosted Zn-Pb deposit is located in the Ulugqat basin, Northwest China. The mineralization mainly features disseminated sulfides such as sphalerite, galena, minor pyrite and marcasite in the bleached sandstone and conglomerate, with no discernable crosscutting relationships between different mineral assemblages. It is generally agreed that the deposit is of epigenetic nature and formed from hydrothermal fluids, but the detailed ore-forming processes remain poorly understood. In this study, three generations of sphalerite with different colors, morphologies and micro-textures were identified in hydrothermal mineralization, i.e., Sph1 (coarse-grained, grey, non-fluorescence), Sph2 (coarse-grained, honey-yellow, non-fluorescence), and Sph3 (fine-grained, colorless, yellowish-brown and celeste fluorescence). The micro-textures, major and trace elements, and sulfur isotopes of the different generations of sphalerite were examined with microscopy, scanning electron microscopy, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The early sphalerites (Sph1 and Sph2) show relatively high Fe-Mn-Ga-Ge-As-Tl contents, low Cd-Cu-Ag contents and high δ³⁴S values (+9.9‰ ∼ +74‰), whereas the late sphalerites (Sph3) display relatively low Fe-Mn-Ga-Ge-As-Tl contents, high Cd-Cu-Ag contents and low δ³⁴S values (−12.91‰ ∼ +5.06‰). Based on the GGIMFis geothermometer and the FeS mol% of sphalerite, the average precipitation temperatures are estimated to be 124.1 ℃ for Sph1, 148 ℃ for Sph2, and 107 ℃ for Sph3, and the average log fS₂ values are calculated to be −17.1 for Sph1, −14.9 for Sph2, and −16.2 for Sph3. These differences among three generations of sphalerite may indicate the hydrothermal mineralization of an early stage, including two pulsed substages, and a late stage under different physicochemical conditions. H₂S in the early stage is primarily derived from thermal sulfate reduction (TSR) of residual aqueous sulfate with high and variable δ³⁴S values in the sulfate-closed sandstone reservoir, while it is likely formed by in situ bacterial sulfate reduction (BSR) of dissolved sulfate from the evaporite sequences of the Paleocene Aertashi Formation in the late stage. We therefore proposed that the multistage mineralization and multiple sulfur sources may account for the formation of the Uragen giant sandstone-hosted Zn-Pb deposit, which involved episodic influx of hydrocarbon and hydrothermal fluids driven by basin inversion due to the far-field effects of the India-Eurasia collision. This study highlights the necessity of basin structure and evaporites in future ore prospecting of the Uragen-style sandstone-hosted Zn-Pb mineralization in comparable foreland basin in front of South Tianshan and elsewhere.