Baogutu Porphyry Cu-Mo-Au Deposit, West Junggar, Northwest China: Petrology, Alteration, and Mineralization

Abstract

Baogutu is the first porphyry Cu-Mo-Au deposit discovered in West Junggar, Xinjiang, China. It is part of the Central Asian orogenic belt. Baogutu is associated with a Carboniferous intrusive complex that was emplaced into lower Carboniferous volcano-sedimentary sequences of the Baogutu and Xibeikulasi Groups. The intrusive complex is made up of main stage equigranular to porphyritic diorites and quartz diorites, and minor late stage diorite porphyries. Intrusive activity occurred at about 325 Ma based on U-Pb (SHRIMP) analyses of zircons. The main stage diorites host the bulk of the Cu-Mo-Au mineralization at Baogutu. They have been overprinted by three alteration assemblages, including an early potassic (biotite) assemblage that occurs in the center of the deposit. A propylitic assemblage surrounds the potassic zone concentrically. Both of these alteration assemblages have been overprinted locally by phyllic alteration (quartz-sericite-pyrite), which is associated with the highest Cu-Mo grades. Mineralized and hydrothermally cemented breccias have disrupted the main stage diorites. The late stage diorites have undergone moderate potassic alteration and contain weak Cu mineralization. Matrix-rich breccias with very weak mineralization have cut the intrusive complex and disrupted the ore-body. Re-Os dating of molybdenite indicates that mineralization occurred at about 310 Ma.