Trace element geochemistry of molybdenite from the Shapinggou super-large porphyry Mo deposit, China

Abstract

The Shapinggou deposit, located in the Dabie Orogenic Belt, eastern China, is one of the largest Mo-only porphyry deposits in the world. Alteration types associated with Shapinggou include silicic, potassic-silicic, phyllic, propylitic and argillic. The phyllic alteration can be subdivided into quartz and sericite (mainly muscovite) phases whereas the argillic alteration is distinguished by poorly crystallized kaolinite (kaolinite PX), muscovite and phengite. The ore-forming event can be divided into quartz-K-feldspar, quartz-molybdenite, quartz-sericite and quartz-fluorite-gypsum stages, with molybdenite deposited in the first three stages but mainly in the second. The molybdenites in Shapinggou represent either the 2H1 and/or the 2H1 + 3R polytype, and can be subdivided into two groups (group one and group two), Group 1 is associated with stage 1 alteration whereas Group 2 is found with stages 2 and 3. There is an increase in the Re values from group one to group two with the Re contents likely controlled by both the crustal metal source and temperature. With decreasing temperature, the contents of Re and the ratios of Mo/Cu in molybdenites in Shapinggou decrease gradually, whereas the contents of Cu and W + Sn, and the ratios of Re/Cu, Co/Ni increase. The Pb + Zn values in molybdenite are higher in the porphyry Mo deposit associated with Pb-Zn deposits than in porphyry Cu-Mo/Mo deposits not associated with Pb-Zn deposits. The ratios of Cl−/F−, SO2/H2S, and H+/K+ in the Shapinggou fluids, increasing over time, suggest that the fluids became more oxidised. The initial ore-forming fluid, was >500 °C, carried ions of Mo, Ca, Na, K, Cl, F, S and caused potassic-silicic alteration with deposition of only small amounts of molybdenite. As the temperature dropped down to ∼400 °C, the amounts of Mo, K, Cu and Cl ions in the fluid decreased leading to silicic-sericitic and propylitic alteration and the deposition of the majority of the molybdenite. As the temperatures decreased to ∼300 °C, the fluid contained ions of Ca and F, and Pb-Zn was deposited distally around the main Mo ore-body.