Ore genesis of the Sadaigoumen porphyry Mo deposit, North China Craton: Constraints from pyrite trace element and lead isotope analyses

Abstract

The Triassic Sadaigoumen porphyry Mo deposit is located in the northern margin of the North China Craton (NCC). To constrain the formation of this deposit, we analyzed in-situ major, trace, and Pb isotopic compositions of pyrite, as well as Pb isotopes of molybdenite and whole rocks. The formation of the deposit can be divided into five stages. Stages 1 and 2 are characterized by the massive development of magnetite with minor pyrite. Stage 3 is the main Mo mineralization stage, with much chalcopyrite deposited in sub-stage 3.1 veins, and only Mo mineralization developed in sub-stage 3.2. Stage 4 is characterized by the development of quartz–pyrite veins. Only quartz with no sulfides was deposited during stage 5. During all five stages, Mo, Cu, Pb and Bi were deposited with pyrite as mechanical inclusions; however, Zn may exist in the pyrite as isomorphic substitutions or nano scale inclusions. The pyrite from stages 1 and 2 has constant Co/Ni ratios, but the ratios were significantly more varied in the stages 3 and 4 pyrite. The variation was largely due to the mixing of meteoric water with the ore-forming system from stage 3. While magnetite and hematite were absent during sub-stage 3.1, the Ag content and Pb isotopic ratio of the pyrite rapidly increased. Geochemical signatures indicated a shift to a reduced ore-forming environment, such as an increase in the Ag content and the highly radioactive Pb isotope composition of the pyrite. The high Cu and Mo content of Py3.2 and Py4 indicated that the ore-forming elements continued to precipitate in the pyrite crystal lattice after the precipitation of chalcopyrite and molybdenite. The precipitation of Cu in the Sadaigoumen deposit is closely related to the addition of a reducing fluid from the upper crust during the evolution of the ore-forming fluid, which has important reference significance for the delicate process of Triassic porphyry Mo deposit in the northern NCC.