Multistage hydrothermal fluids evolution and precipitation mechanism of the Hongshi Cu deposit in Eastern Tianshan, NW China: Insights from cathodoluminescence, fluid inclusions, trace elements of quartz and in situ S isotopes of sulfides

Abstract

The Hongshi Cu deposit is located in the southeastern Kalatag district in Eastern Tianshan, northwest China. The Cu mineralization is mainly hosted by volcaniclastic rocks of the Daliugou Formation and structurally controlled by fracture zones. Comprehensive analyses of fluid inclusions, in situ sulfur isotopes, and trace elements in quartz were employed to unravel the evolution of ore fluids, identify the sources of ore-forming components, and refine the understanding of ore genesis. The ore formation at Hongshi could be divided into three hydrothermal stages, consisting of quartz-pyrite (stage I), quartz-sulfide (stage II), and quartz-calcite (stage III). Five hydrothermal quartz generations have been identified through cathodoluminescence (CL) imaging, including Q1–1 and Q1–2 from stage I, Q2–1 and Q2–2 from stage II, and Q3 from stage III. Aqueous fluid inclusions from stage I exhibit relatively high temperatures (208–313 °C) and moderate salinities (3.1–9.3 wt% NaCl equiv.). Microthermobarometry results confine the quartz−sulfide veins to temperatures ranging from 151 to 243 °C and salinities of 1.9–7.2 wt% NaCl equiv. Quartz−calcite veins formed from low−temperature and low−salinity fluids, with homogenization temperatures and salinities of 123–168 °C and 0.53–3.23 wt% NaCl equiv., respectively. Based on in situ trace element analysis of the five quartz generations, the Ti concentrations for the ore fluids are 33.20–129.00 ppm (mean = 79.88 ppm), 5.12–27.30 ppm (mean = 16.54 ppm), 1.01–17.20 ppm (mean = 6.96 ppm), 1.48–9.98 ppm (mean = 5.15 ppm), and 14.70–52.70 ppm (mean = 24.19 ppm). The Al concentrations for the ore fluids are 2058–4889 ppm (mean = 3562 ppm), 150–1916 ppm (mean = 1109 ppm), 133–2470 ppm (mean = 926 ppm), 32–174 ppm (mean = 82 ppm), and 686–2296 ppm (mean = 1555 ppm). The trace element data and trends across the five generations of quartz are optimally interpreted as indicative of magmatically derived ore fluids that were subsequently diluted by meteoric water, resulting in mineralization. Fluid mixing is regarded as the primary mechanism responsible for Cu precipitation at Hongshi. Sulfides of pyrite, chalcopyrite, and sphalerite yield δ³⁴S_V-CDT values from −0.21 to +6.21 ‰, with no discernible systematic variations among the various paragenetic stages. The data presented herein support a magmatic origin for the Hongshi deposit. This study clarifies the role of fluid dilution in initiating Cu deposition and offers insights into the genesis of Cu-polymetallic epithermal deposits within the Eastern Tianshan region.