Geochemistry of magnetite from the Hongniu–Hongshan Cu skarn deposit in Yunnan Province, SW China

Abstract The Hongniu–Hongshan Cu skarn deposit is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. We investigated the petrography and trace-element chemistry of magnetite in the deposit to examine the nature and origin of the ore-forming fluids. Three types of magnetite were identified in porphyritic granite and skarn in the Hongniu–Hongshan deposit. Mt-type1 crystallized from magmas, is typical magmatic magnetite. Mt-type2 formed later than type1, but earlier than the syn-ore stage, and was related to magmatic–hydrothermal activity. Mt-type3 formed during the early syn-ore stage, is hydrothermal skarn magnetite. The magnetites have variable K, Mg, Al, Ti, V, Cr, Mn, Co, Ni, Cu, Pb, Zn, Ga, W, Sn, and Ge contents. The high contents of Ti, V and Cr in the Mt-type1 is largely controlled by its magmatic origin. For the type 2 magnetites, the high Al, Ti, and Ga contents were controlled by hydrothermal fluid composition and temperature, and V, Sn, and Cr were controlled by fO2 values. The enrichment of Mg, Mn, and Zn in the Mt-type3 was largely due to fluid–rock interactions. Co-precipitating mineral phases appear to have exerted less control on magnetite compositions in the deposit. Some of the Mt-type2 had experienced coupled dissolution and reprecipitation (CDR) processes. The infiltration of saline hydrothermal fluids is considered to enhance CDR processes in the skarn deposit. We suggest that the crystallization of magnetite in the porphyritic granite reduced the sulfate content of the magma, which resulted in Cu and Au being complexed more readily into fluids. The precipitation of Mt-type3 might have resulted in a change from an oxidized to a more reduced environment, thereby creating conditions favorable for the precipitation of Cu sulfides.