Genesis of the Xiuwenghala Gold Deposit in the Beishan Orogen, Northwest China: Evidence from Geology, Fluid Inclusion, and H–O–S–Pb Isotopes

Abstract

The Xiuwenghala gold deposit is located in the Beishan Orogen of the southern Central Asian Orogenic Belt. The vein/lenticular gold orebodies are controlled by Northeast-trending faults and are hosted mainly in the brecciated/altered tuff and rhyolite porphyry of the Lower Carboniferous Baishan Formation. Metallic minerals include mainly pyrite and minor chalcopyrite, arsenopyrite, galena, and sphalerite, whilst nonmetallic minerals include quartz, chalcedony, sericite, chlorite, and calcite. Hydrothermal alterations consist of silicic, sericite, chlorite, and carbonate. Alteration/mineralization processes comprise three stages: pre-ore silicic alteration (Stage I), syn-ore quartz-chalcedony-polymetallic sulfide mineralization (Stage II), and post-ore quartz-calcite veining (Stage III). Fluid inclusions (FIs) in quartz and calcite are dominated by L-type with minor V-type and lack any daughter mineral-bearing or CO2-rich/-bearing inclusions. From Stages I to III, the FIs homogenized at 240–260°C, 220–250°C, and 150–190°C, with corresponding salinities of 2.9–10.9, 3.2–11.1, and 2.9–11.9 wt.% NaCl eqv., respectively. The mineralization depth at Xiuwenghala is estimated to be relatively shallow (<1 km). FI results indicate that the ore-forming fluids belong to a low to medium-temperature, low-salinity, and low-density NaCl-H2O system. The (Formula presented.) values decrease from Stage I to III (3.7‰, 1.7–2.4‰, and −1.7 to 0.9‰, respectively), and a similar trend is found for their (Formula presented.) values (−104 to −90‰, −126 to −86‰, and −130 to −106‰, respectively). This indicates that the fluid source gradually evolved from magmatic to meteoric. δ34S values of the hydrothermal pyrites (−3.0 to 0.0‰; avg. −1.1‰) resemble those of typical magmatic/mantle-derived sulfides. Pyrite Pb isotopic compositions (206Pb/204Pb = 18.409–18.767, 207Pb/204Pb = 15.600–15.715, 208Pb/204Pb = 38.173–38.654) are similar to those of the (sub)volcanic ore host, indicating that the origin of ore-forming material was mainly the upper crustal (sub)volcanic rocks. Integrating evidence from geology, FIs, and H–O–S–Pb isotopes, we suggest that Xiuwenghala is best classified as a low-sulfidation epithermal gold deposit.