Saline fluids drive Cu mineralization in Precambrian metasediments: Evidence from the Trans-North China Orogen

Abstract

Epigenetic hydrothermal Cu-(Co) deposits are widely distributed in the Zhongtiao Mountains within the Paleoproterozoic Trans-North China Orogen in the North China Craton. Here we investigate the Henglingguan Cu deposit hosted in metasediments to gain insights on the fluid evolution characteristics associated with the ore formation. This Cu deposit is characterized by early disseminated mineralization and late quartz-vein type mineralization. The negative sulfur isotope values (about −10‰) of the vein sulfides suggests a sedimentary rock source for sulfur associated with the hydrothermal Cu mineralization. The coexistence of CO2-rich carbonic inclusions and halite-bearing inclusions within a single fluid inclusion assemblage suggests fluid immiscibility in the H2O-NaCl-CO2 system. Based on the fluid phase analysis, our data show that the parent ore fluid was characterized by X(NaCl) of 10 to 20 mol.% (salinity of around 23 to 41 wt% NaCl) and X(CO2) of 10 to 15 mol.% before fluid immiscibility. The estimated temperature–pressure conditions of 320 to 340 °C and 3.5 to 4.0 kbar based on the carbonic fluid inclusions is suggested to be close to the conditions of fluid immiscibility. In-situ composition analyses of fluid inclusions by LA-ICP-MS show that the ore fluids contained Na, Ca, Fe and Mn, and evolved towards Sr-Ba enrichment. The data are consistent with a metamorphic brine derived from the sedimentary rocks. The brine-rich metamorphic fluids were likely derived from the dehydration and decarbonation of evaporitic carbonate rocks developed on the basin margin or a passive continental margin. The decompression during post-collisional exhumation triggered fluid immiscibility resulting in the quartz vein-type Cu mineralization.