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

• Published in: Acta geochimica 2024, Vol.43 (4), p.802-813
• Main Authors: Guan, Jingwei, Ren, Tao, Wang, Lei, Guan, Shenjin, Wu, Lianrong, Shi, Baosheng
• Format: Article
• Language: English
• Subjects: Amphibolites; Anhydrite; Biotite; Chalcopyrite; Chemical composition; Crystallization; Earth and Environmental Science; Earth Sciences; Electron probe; Electron probe microanalysis; Fluids; Fugacity; Geochemistry; Gypsum; High temperature; Lava; Low temperature; Magma; Magnetite; Massifs; Molybdenite; Original Article; Paleoceanography; Porphyry copper; Pyrite; Pyrrhotite; Sedimentary rocks; Silicates; Subduction; Subduction (geology); Tectonics
• ISSN: 2096-0956; 2365-7499
• DOI: 10.1007/s11631-024-00686-2

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 ( f O 2 ). Primary amphibolite and biotite (type I) crystallized at relatively high temperatures (744–827 °C) and low f O 2 (log f O 2  = − 12.26 to − 11.91) during the magmatic stage. Hydrothermal fluids exsolved from the magma have a relatively lower temperature (621–711 °C) and f O 2 (log f O 2  = − 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 f O 2 results, combined with previous research on magmatic f O 2 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.