Methane origin and oxygen-fugacity evolution of the Baogutu reduced porphyry Cu deposit in the West Junggar terrain, China

• Published in: Mineralium deposita 2015-12, Vol.50 (8), p.967-986
• Main Authors: Shen, Ping, Pan, HongDi
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbon sources; Contamination; Copper; Crystallization; Dikes; Earth and Environmental Science; Earth Sciences; Geochemistry; Geology; High temperature; Iron sulfides; Magma; Methane; Mineral composition; Mineral Resources; Mineralization; Mineralogy; Organic carbon; Rocks
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-015-0580-5

Most porphyry copper deposits worldwide contain magnetite, hematite, and anhydrite in equilibrium with hypogene copper-iron sulfides (chalcopyrite, bornite) and have fluid inclusions with CO 2  >> CH 4 that are indicative of high f O 2 . In contrast, the Baogutu porphyry Cu deposit in the West Junggar terrain (Xinjiang, China) lacks hematite and anhydrite, contains abundant pyrrhotite and ilmenite in equilibrium with copper-iron sulfides (chalcopyrite), and has fluid inclusions with CH 4  >> CO 2 that are indicative of low f O 2 . The mineralized intrusive phases at Baogutu include the main-stage diorite stock and minor late-stage diorite porphyry dikes. The main-stage stock underwent fractional crystallization and country-rock assimilation-contamination, and consists of dominant diorite and minor gabbro and tonalite porphyry. The country rocks contain organic carbons (0.21–0.79 wt.%). The δ 13 Cv PDB values of the whole rocks (−23.1 to −25.8 ‰) in the wall rocks suggest a sedimentary organic carbon source. The δ 13 Cv PDB values of CH 4 (−28.2 to −36.0 ‰) and CO 2 (−6.8 to −20.0 ‰) in fluid inclusions require an organic source of external carbon and equilibration of their Δ 13 C CO2-CH4 values (8.2–25.0 ‰) at elevated temperatures (294–830 °C) suggesting a significant contribution of thermogenic CH 4 . Mineral composition data on the main-stage intrusions, such as clinopyroxene, hornblende, biotite, magnetite, ilmenite, sphene, apatite, and pyrrhotite, suggest that the primary magma at Baogutu was oxidized and became reduced after emplacement by contamination with country rocks. Mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from log f O 2  > FMQ + 1 in the magma stage, to log f O 2   FMQ in the hydrothermal stage. Though oxidized magma was emplaced initially, assimilation-contamination of carbonaceous country rocks decreased its f O 2 such that exsolved fluids contained abundant CH 4 and deposited a reduced assemblage of minerals.