Geochronology, fluid inclusion, and C H O S Pb isotopic study of the Huitongshan skarn Cu deposit in Gansu, northwest China

Abstract

The Huitongshan skarn Cu deposit is located in the southern orogenic belt of Beishan and is one of the most important metallogenic belts in northwestern China. The ore body is hosted within the external contact zone between the K-feldspar granite and Ordovician marble. The zircon U–Pb dating shows that ore-related K-feldspar granite formed at 402 ± 3.0 Ma, which thus constrains that the mineralization of the Huitongshan skarn copper deposits mostly likely occurred in the Early Devonian. Through field investigation and petrographic observation, the deposit formation can be divided into two periods and four stages of mineralization; the skarn period contains the garnet–diopside skarn stage I and magnetite–quartz stage Ⅱ; the quartz–sulfide period contains the quartz–polymetallic sulfide stage III and the quartz–calcite–minor pyrite stage IV. The variations of fluid inclusion (FI) types, homogenization temperatures, and salinities show the fluid evolution process. In stage Ⅰ and stage Ⅱ, the FIs are characterzied by daughter mineral-bearing (S-type) and vapor-rich (V-type) with homogenization temperatures of 352 °C–485 °C. The salinities of V-type and S-type FIs are 2.1–9.8 wt% NaCl equivalent (equiv.) and 41.5–57.0 wt% NaCl equiv., respectively, indicating that these fluids would boil at pressures of 150–500 bar and depths of 0.6–2.0 km. In stage III, the FIs are mainly daughter mineral-bearing (S-type), vapor-rich (V-type), or liquid-rich (l-type) with homogenization temperatures of 260 °C–325℃, 265℃–325℃, 260℃–320℃ and salinities of 35.0–39.4 wt% NaCl equiv., 1.7–3.5 wt% NaCl equiv., and 6.4–11.5 wt% NaCl equiv., respectively; fluid boiling would occur at pressures of 40–100 bar and depths of 0.4–1.0 km. In stage IV, only l-type FIs are observed, with homogenization temperatures and salinities of 180 °C–245℃ and 2.1–7.2 wt% NaCl equiv., respectively. The HOC isotopes of mineralization stages II–IV suggest that the ore-forming fluids in the early stage (stage Ⅱ) mainly came from magmatic hydrothermal fluid, while a large quantity of meteoric water was mixed with those in the later stages (stage III and IV). The S–Pb isotopic compositions indicate that ore materials were derived from mixing between the magmatic source and the Huaniushan Formation.