Processes and ore genesis at the Yaochong Mo deposit, Henan Province, China

Abstract

The Yaochong Mo deposit in Henan Province is located in the Dabie orogen, and its orebodies primarily occur as numerous veinlets hosted in the contact zone of the Yaochong stock with the Dabie Complex. Based on vein crosscutting relationships, mineral assemblages, and paragenetic sequence, the mineralization is divided into four stages: a quartz–K-feldspar–magnetite–hematite stage, a quartz–K-feldspar–molybdenite stage, a quartz–polymetallic sulfide mineral stage, and a quartz–carbonate stage. Three types of fluid inclusions have been recognized in the quartz: CO2-bearing fluid inclusions, H2O-rich fluid inclusions, and solid-bearing fluid inclusions. Microthermometric analyses of the fluid inclusions suggest that the fluids evolved gradually during the ore-forming process. In stages 1, 2, 3, and 4, the homogenization temperatures varied in the ranges 264–400°C, 175–487°C, 171–433°C, and 119–172°C, respectively, and the salinities varied in the ranges 4.34–7.70wt% NaClequiv, 2.77–32.49wt% NaClequiv, 2.20–30.70wt% NaClequiv, and 1.06–7.73wt% NaClequiv, respectively. The calculated δ18OH2O values of stages 1, 2, 3, and 4 ranged from 2.2‰ to 3.9‰, 0.9‰ to 1.7‰, −0.8‰ to −0.1‰, and −1.3‰ to 0.6‰, respectively, and the δ18D values varied from −68‰ to −62‰, −73‰ to −64‰, −75‰ to −65‰, and −68‰ to −65‰, respectively. These results show that the ore-forming fluids of the Yaochong molybdenum deposit were mainly derived from magmatic water, which likely included a small component of meteoric water. The δ34S values of pyrite and molybdenite varied from −2.9‰ to −1.8‰, and −2.7‰ to −1.6‰, respectively, and the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the sulfide minerals ranged from 16.193 to 16.818, 15.105 to 15.288, and 36.698 to 37.307, respectively. The sulfur and lead isotope compositions indicated that the ore-forming constituents were derived from a deep igneous source originating in the lower crust. The Yaochong porphyry Mo deposit may have formed in a post-collision extensional setting. The Mo mineralization in the Yaochong deposit occurred at depths of 0.87–1.68km, and phase separation and immiscibility of the ore-forming fluids were critical for the formation of the deposit.