Texture, trace elements, sulfur and He-Ar isotopes in pyrite: Implication for ore-forming processes and fluid source of the Guoluolongwa gold deposit, East Kunlun metallogenic belt

Abstract

The Gouli goldfield is the most important gold producers of the East Kunlun metallogenic belt. Gold mineralization in this region is enigmatic, primarily because of uncertainties as to the mineralization processes, and the nature and sources of ore-forming fluids. Here, we investigate the ore-forming fluids and processes through a comprehensive study of the texture and geochemistry (LA-(MC)-ICPMS trace element analysis, mapping, S isotopes, and He-Ar isotopes) of pyrite from the Guoluolongwa gold deposit, the largest deposit in the Gouli goldfield. Four types of pyrite with distinct textures, trace element contents and sulfur isotopes were identified, and provide evidence for a multi-stage hydrothermal fluid evolution. The sedimentary-diagenetic Py0, representing the pre-ore stage, is characterized by high contents of Co and Ni, and negative δ34S values (−11.6‰ to −4.0‰), which are notably different from the subsequent hydrothermal Py1 to Py3. Hydrothermal Py1, comprising subtypes Py1a and Py1b, was deposited in pyrite-quartz veins during stage I. The Py1a subtype shows characteristic Co-Ni-As oscillatory zoning that replaced Py0 via dissolution-reprecipitation, and hence inherited high concentrations of Co and Ni from Py0. In contrast, the Py1b subtype was devoid of Py0 influence and represented the initial ore-forming fluid with low concentrations of Co and Ni, moderate Cu, Pb, Zn, Sb, and Bi, but high As, Au, and Ag. The next stage, Py2, formed in the quartz-pyrite veins during stage II and showed clear textural and compositional contrasts to Py1. Specifically, Py2 has elevated Cu, Pb, Zn, Ag, and Bi contents and somewhat lower δ34S values (+1.5‰ to +3.2‰) compared to Py1 (+1.6‰ to +5.6‰). In addition, Py2 shows irregular compositional zones, which we attribute to fluid phase separation caused by pressure fluctuation. Abundant Py2 precipitation triggered native gold deposition due to desulfidation of the ore-forming fluid. The next stage, Py3 in polymetallic sulfides-calcite-quartz veins at stage III, implies a new pulse of Cu-Pb-Zn-As-Au-Ag-rich and Bi-poor ore-forming fluid. This latter pyrite is characterized by inclusion-rich cores (Py3c), which contain abundant inclusions of chalcopyrite, galena, sphalerite, and native gold, and inclusion-free rims (Py3r). The cores, Py3c, likely represent abrupt destabilization of metal complexes and rapid precipitation during vigorous phase separation, whereas the euhedral Py3r rims formed under steadier physico-chemical conditions, contributing to lower δ34S values of Py3c (+0.5‰ to +3.0‰) than Py3r (+1.9‰ to +3.6‰). Noble gas isotopes of Py1-Py3 have crust-dominated 3He/4He (0.04–0.4 Ra) and 40Ar/36Ar (623–2081), mixed with minor mantle components. Sulfur isotopes and low Co and Ni contents of hydrothermal pyrite, and He-Ar isotopes of fluid inclusions in pyrite, are consistent with a model in which multiple pulses of ore-forming fluids were exsolved during sequential episodes of primarily crustal-derived felsic magmatism with lesser mantle influences. This model is also supported by previous H-O isotope analyses, as well as widespread granitoid magmatism in the eastern Central East Kunlun Belt.