Significance of chlorite hyperspectral and geochemical characteristics in exploration: A case study of the giant Qulong porphyry Cu-Mo deposit in collisional orogen, Southern Tibet

Abstract

Abstract The genetic mechanism and exploration model for porphyry deposits in subduction zones has been comprehensively and systematically investigated; in contrast, our understanding for porphyry mineralization in collisional orogeny is limited. Qulong porphyry Cu-Mo deposit, a typical porphyry Cu deposit (PCD) developed in the collisional setting, was chosen as the subject of this study. The study is mainly focused on interpretation of short-wave infrared (SWIR) spectroscopy combined with mineral geochemistry of chlorite from Qulong deposit to guide exploration in future. Hydrothermal alteration in Qulong mineralization system can be divided into three zones, which are the inner potassic alteration zone, the outside propylitic alteration zone and phyllic alteration. Phyllic alteration can be intensively superimposed on the early potassic and prophylitic alteration zones. Chlorite is widely distributed in both of the propylitic and phyllic zones. Four types of chlorite were observed in Qulong deposit, including three stages of vein-related chlorite (Chl-I, Chl-II, Chl-III) and one type of disseminated chlorite (Chl-D). Mineral assemblage of Chl-I stage includes chlorite–anhydrite–pyrite. Minerals in Chl-II stage consist of chlorite–epidote–pyrite–chalcopyrite. Mineral assemblage of Chl-III stage is composed of chlorite–sericite–quartz–chalcopyrite, which is later than both Chl-I and Chl-II. Chl-D was developed mainly by alteration of primary biotite phenocryst. Chl-I, Chl-II and Chl-D were formed along with the propylitic alteration; however, Chl-III is from the phyllic alteration. Chl-II and Chl-III are both closely related with Cu mineralization. The major element compositions of chlorite show a regular variation of Mg# with fluid evolution. Transition from shorter wavelength chlorite (Fe feature around 2250 nm) to longer wavelength goes with the decrease of Mg#, except for Chl-D. In summary, chlorite associated with Cu mineralization in Qulong is mainly occurred in veins and has low Mg# ratios and longer wavelength. Such chlorite is the indicator for Cu mineralization and should be the target in future exploration. In addition to chlorite, sericite in Qulong also shows systematic change from early-stage phengite to late-stage muscovite. In combination of mineral geochemistry and thermodynamic modelling, we suggest that ore-forming fluid gradually evolved to be more acidic with higher silica activity over time.