Geology and genesis of the post-collisional porphyry–skarn deposit at Bangpu, Tibet

Abstract

Bangpu deposit in Tibet is a large but poorly studied Mo-rich (~0.089wt.%), and Cu-poor (~0.32wt.%) porphyry deposit that formed in a post-collisional tectonic setting. The deposit is located in the Gangdese porphyry copper belt (GPCB), and formed at the same time (~15.32Ma) as other deposits within the belt (12~18Ma), although it is located further to the north and has a different ore assemblage (Mo–Pb–Zn–Cu) compared to other porphyry deposits (Cu–Mo) in this belt. Two distinct mineralization events have been identified in the Bangpu deposit which are porphyry Mo–(Cu) and skarn Pb–Zn mineralization. Porphyry Mo–(Cu) mineralization in the deposit is generally associated with a mid-Miocene porphyritic monzogranite rock, whereas skarn Pb–Zn mineralization is hosted by lower Permian limestone–clastic sequences. Coprecipitated pyrite and sphalerite from the Bangpu skarn yield a Rb–Sr isochron age of 13.9±0.9Ma. In addition, the account of garnet decreases and the account of both calcite and other carbonate minerals increases with distance from the porphyritic monzogranite, suggesting that the two distinct phases of mineralization in this deposit are part of the same metallogenic event.Four main magmatic units are associated with the Bangpu deposit, namely a Paleogene biotite monzogranite, and Miocene porphyritic monzogranite, diabase, and fine-grained diorite units. These units have zircon U–Pb ages of 62.24±0.32, 14.63±0.25, 14.46±0.38, and 13.24±0.04Ma, respectively. Zircons from porphyritic monzogranite yield εHf(t) values of 2.2–8.7, with an average of 5.4, whereas the associated diabase has a similar εHf(t) value averaging at 4.7. The geochemistry of the Miocene intrusions at Bangpu suggests that they were derived from different sources. The porphyritic monzogranite has relatively higher heavy rare earth element (HREE) concentrations than do other ore-bearing porphyries in the GPCB and plots closer to the amphibolite lithofacies field in Y–Zr/Sm and Y–Sm/Yb diagrams. The Bangpu diabase contains high contents of MgO (>7.92wt.%), FeOt (>8.03wt.%) but low K2O (<0.22wt.%) contents and with little fractionation of the rare earth elements (REEs), yielding shallow slopes on chondrite-normalized variation diagrams. These data indicate that the mineralized porphyritic monzogranite was generated by partial melting of a thickened ancient lower crust with some mantle components, whereas the diabase intrusion was directly derived from melting of upwelling asthenospheric mantle. An ancient lower crustal source for ore-forming porphyritic monzogranite explains why the Bangpu deposit is Mo-rich and Cu-poor rather than the Cu–Mo association in other porphyry deposits in the GPCB because Mo is dominantly from the ancient crust.The Bangpu deposit has alteration zonation, ranging from an inner zone of biotite alteration through silicified and phyllic alteration zones to an outer propylitic alteration zone, similar to typical porphyry deposits. Some distinct differences are also present, for example, K-feldspar alteration at Bangpu is so dispersed that a distinct zone of K-feldspar alteration has not been identified. Hypogene mineralization at Bangpu is characterized by the early-stage precipitation of chalcopyrite during biotite alteration and the late-stage deposition of molybdenite during silicification. Fluid inclusion microthermometry indicates a change in ore-forming fluids from high-temperature (320°C–550°C) and high-salinity (17wt.%–67.2wt.%) fluids to low-temperature (213°C–450°C) and low-salinity (7.3wt.%–11.6wt.%) fluids. The deposit has lower δDV-SMOW (−107.1‰ to −185.8‰) values compared with other porphyry deposits in the GPCB, suggesting that the Bangpu deposit formed in a shallower setting and is associated with a more open system than is the case for other deposits in this belt. Sulfides at Bangpu yield δ34SV-CDT values of −2.3‰ to 0.3‰, indicative of mantle-derived S implying that coeval mantle-derived mafic magma (e.g., diabase) simultaneously supplied S and Cu to the porphyry system at Bangpu. In comparison, the Pb isotopic compositions (206Pb/204Pb=18.79–19.28, 207Pb/204Pb=15.64–15.93, 208Pb/204Pb=39.16–40.45) of sulfides show that other metals (e.g., Mo, Pb, Zn) were likely derived mainly from an ancient crustal source. Therefore, the formation of the Bangpu deposit can be explained by a two-stage model involving (1) the partial melting of an ancient lower crust triggered by invasion of asthenospheric mantle-derived mafic melts that provide heat and metal Cu and (2) the formation of the Bangpu porphyry Mo–Cu system, formed by magmatic differentiation in the overriding crust in a post-collisional setting.