Geology and geochemistry of the Jianchaling hydrothermal nickel deposit: T–pH–fO2–fS2 conditions and nickel precipitation mechanism

Abstract

The Jianchaling nickel deposit in the Bikou Terrane (Shaanxi Province, China) occurs along the boundaries between granite porphyry and carbonated ultramafic rocks (carbonated serpentinite, talc–carbonate rocks, and listwaenite). Serpentine– magnetite, serpentine– magnesite– magnetite, and magnesite– talc– quartz– pyrite– violarite– millerite– chalcopyrite assemblage formed in carbonated ultramafic rocks during hydrothermal activities. Ni-bearing sulphides, coexisting with magnesite, postdated magnetite in carbonated ultramafic rocks. Compared with serpentinite, Ni, Co, Cu, Mn, and Pb concentrate in talc–carbonate rocks. The fact that the NiO contents of magnetite decrease with progressive carbonation of serpentinite suggests that Ni from magnetite concentrated in fluid and contributed to the formation of the Jianchaling nickel deposit. Sulphides precipitated from fluid with log fO2 value varying from −34.5 to −31.8 and log fS2 value varying from −10.3 to −9.2. High pH and HS− activities triggered by transformation of serpentine into magnesite–talc–quartz assemblage promoted precipitation of Ni-bearing sulphides, and finally formed the Jianchaling hydrothermal nickel deposit.