Reduced magma generation and its implications for the Pulang giant porphyry Cu-polymetallic deposit in Northwest Yunnan, China

The Pulang giant porphyry Cu-Mo polymetallic deposit is located in the Zhongdian area in the center of the Sanjiang Tethys tectonic domain, which was formed by the westward subduction of the Garze-Litang oceanic slab beneath the Zhongza massif. Chalcopyrite-pyrrhotite-pyrite-molybdenite occurs as disseminations, veins, veinlets, and stockworks distributed in the K-silicate alteration zone in the monzonite porphyry, which is superimposed by propylitization. The chemical compositions of biotite and amphibole analyzed by electron probe microanalysis (EPMA) indicate that the ore-forming magma and exsolved fluids experienced a continuous decrease in the oxygen fugacity (fO2). Primary amphibolite and biotite (type I) crystallized at relatively high temperatures (744–827 °C) and low fO2 (logfO2 = − 12.26 to − 11.91) during the magmatic stage. Hydrothermal fluids exsolved from the magma have a relatively lower temperature (621–711 °C) and fO2 (logfO2 = − 14.36 to − 13.32) than the original magma. In addition, the presence of a high abundance of pyrrhotite and an insufficiency of primary magnetite and sulfate in the ore (i.e., anhydrite and gypsum) indicate that the deposit may be a reduced porphyry deposit. Magma and fluid fO2 results, combined with previous research on magmatic fO2 at the Pulang deposit, indicate that the magma associated with the reduced Pulang ore assemblages was initially generated as a highly oxidized magma that was subsequently reduced by sedimentary rocks of the Tumugou Formation.