Geology and Genesis of the Giant Pulang Porphyry Cu-Au District, Yunnan, Southwest China

The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/tonne Au) is in the southern segment of the Yidun arc (Zhongdian arc), part of the Sanjiang Tethyan orogenic belt in southwest China. The district consists of three deposits: South Pulang (~96% of the total ore reserves) and the smaller East and North Pulang deposits. Four intrusive phases host the three Pulang deposits, which are, in order of emplacement, premineralization fine-grained quartz diorite and coarse-grained quartz diorite, intermineralization quartz monzonite, and late-mineralization diorite porphyry. The complex intruded carbonaceous rocks of the Late Triassic Tumugou slates. Zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry dating shows that intrusive activity occurred at about 216 ± 2 Ma. Hydrothermal alteration of the intrusions at Pulang includes five main types: K-silicate, epidote-chlorite, chlorite-illite, quartz-illite, and clay alteration. K-silicate alteration, subdivided into early K-feldspar alteration and late biotite alteration (dominant), mainly affected the central quartz monzonite and adjacent coarse-grained quartz diorite and fine-grained quartz diorite. Epidote-chlorite alteration, the most widespread alteration in the district, extends from the deposit core outward and has overprinted K-silicate alteration at South and North Pulang. Late chlorite-illite, quartz-illite, and clay alteration have overprinted preexisting K-silicate and epidotechlorite alteration assemblages and are locally developed in all four intrusive phases. Copper and gold are positively correlated and are mainly (90%) associated with epidote-chlorite alteration and, to a lesser degree, with K-silicate and chlorite-illite alteration. Hypogene pyrrhotite is intergrown with chalcopyrite and mainly occurs in chlorite-illite– and quartz-illite–altered, coarse-grained quartz diorite at East Pulang. Molybdenite Re-Os dating shows that mineralization in the district occurred at 216.54 ± 0.87 to 216.13 ± 0.86 Ma. The sequence of intrusion emplacement, alteration and veining, and sulfide associations at the three deposits suggests that South and North Pulang are two separate porphyry Cu-Au deposits, whereas East Pulang is probably a distal part of South Pulang. The dominance of primary magnetite over ilmenite and the assemblage titanite + magnetite + quartz in the causative quartz monzonite, and the abundant hydrothermal anhydrite veins associated with early K-silicate and main-mineralization epidote-chlorite alteration indicate the oxidized nature of the felsic intrusion and resultant early hydrothermal fluids. The pyrrhotite related to late chlorite-illite and quartz-illite alteration suggests local reduction due to interaction with the carbonaceous Tumugou slates. The atypical association of epidote-chlorite alteration and Cu mineralization at Pulang either is due to fluids from another porphyry deposit nearby overprinting epidote-chlorite alteration onto preexisting copper mineralization and K-silicate alteration at Pulang or is the result of collapse of epidote-chlorite–stable fluids into the K-silicate-altered core during waning hydrothermal activity.This study was jointly funded by the National Key Research and Development Project of China (2016YFC0600305), the Major State Basic Research Program of the People’s Republic of China (2015CB452602), the National Natural Science Foundation of China (41825005, 41473041, 41273051), and the Ministry of Land and Resources of China (201011011