Rb–Sr geochronology and geochemistry of pyrite from the Shihu gold deposit, central North China Craton: Implication for the timing and genesis of gold mineralization

Abstract

The Shihu quartz vein‐type gold deposit in the central North China Craton occurs within Neoarchean TTG (tonalite‐trondhjemite‐granodiorite) gneisses and shows spatial relationship with quartz diorite porphyry. The occurrence of the ore body is controlled by the N‐S fracture. Primary mineral assemblage of the ore body is composed of quartz, calcite, and base‐metal sulphides, including pyrite, galena, sphalerite, chalcopyrite, and pyrrhotite. Eight pyrite samples from veins of the main mineralization stage yield an isochron age of 129 ± 3 Ma (MSWD = 1.6), coinciding with the major Early Cretaceous magmatic event, and associated lithospheric thinning recognized in the NCC. The (87Sr/86Sr)i values of the pyrite (0.7102 ± 0.0006) are lower than the continental crust mean value (0.719) and higher than that of the mantle initial ratio (0.707), suggesting mixed crust–mantle source for the metallogenic components. Three types of hydrothermal pyrite from the early to major ore‐forming stages were identified in the quartz vein, including Py(I) as disseminated euhedral grains, Py (II) as dense bulks, and Py (III) as intergrowth with base‐metal sulphides. Electron microprobe analysis of the pyrite grains indicates a trend from S‐deficient to S‐enriched and from Fe‐enriched to Fe‐deficient stages, suggesting an increase in sulphur fugacity and reducing conditions. The arsenic in pyrite shows decrease against increasing Au during Stages II and III, implying that As is not the only trigger of gold precipitation, but electrochemical‐geochemical barrier played a major role in the formation of the ore. The Co and Ni concentrations, as well as Co/Ni ratios in pyrite, are relatively high, which provide an additional clue to the link with the regional magmatic event. Combined with the textural features, we propose a wider potential for gold exploration in this region, linking the mineralization with a major transitional tectonic regime.