Chapter 5 Porphyry Copper Deposits in China

Abstract

Abstract Porphyry Cu deposits in China contain a total resource of ~47 million tonnes (Mt) Cu at average grades ranging mostly from 0.2 to 0.7% Cu (most <0.5% Cu), accounting for 42% of China’s Cu reserves. In terms of contained Cu, 14 Cu-rich porphyry deposits are classified as giant (≥2.0 Mt Cu), and 38 are classified as intermediate (≥0.06 Mt Cu). These giant and intermediate deposits are mainly concentrated in seven belts or districts: Gangdese belt, southern Tibet; Yulong and Zhongdian belts, eastern Tibet; Duolong district, central Tibet; Dexing district and Middle-Lower Yangtze River Valley belt, eastern China; and the Central Asian orogenic belt in northern China. Other isolated giant deposits (e.g., Tongkuangyu) occur in the North China craton. These deposits were formed during Paleoproterozoic (~2100 Ma), Ordovician (~480–440 Ma), Carboniferous (~330–310 Ma), Late Triassic to Early Cretaceous (~215–105 Ma), and Eocene to Miocene (~40–14 Ma), with the majority forming during the latter two time periods. Adakite-like (e.g., high Sr/Y ratio) magmas are most favorable for the formation of the porphyry Cu deposits in China, although some deposits in the Central Asian orogenic belt and the Duolong district are associated with normal calc-alkaline intrusions with low Sr/Y ratios. Approximately 50% of the giant and ~35% of the intermediate porphyry Cu deposits in China formed in arc settings. The Xiongcun, Pulang, Duobuza, Bolong, and Naruo deposits in Tibet formed in continental arc settings, and the Central Asian porphyry Cu belt deposits (e.g., Tuwu-Yandong, Duobaoshan, Wushan, Baogutu, and Bainaimiao) formed in island-arc settings. Ore-forming porphyry magmas in arc settings in China probably formed by partial melting of metasomatized mantle wedge. Ascent and emplacement of porphyry magmas in arc settings was controlled by transpressional (e.g., strike-slip fault systems) or compressional deformation (e.g., arc-parallel thrust fault systems). Approximately 40% of the giant and ~65% of the intermediate porphyry Cu deposits in China occur in postcollisional settings. These deposits are mainly concentrated in the Tibetan Plateau, including four giant (e.g., Qulong, Jiama, Zhunuo, and Yulong) and more than 15 intermediate-size deposits. The mineralized intrusions in postcollisional settings were generated by partial melting of subduction-modified mafic lower crust. Ore-forming metals and sulfur were derived from remelting of sulfide phases that were introduced during precollisional arc magmatism, and the water in the Cu-forming porphyry magmas was concentrated during dehydration reactions in the upper parts of the subducting continental plate and/or degassing of mantle-derived H2O-rich ultrapotassic and/or alkaline mafic magmas. Porphyry magma ascent and emplacement were controlled by regional shear zones (e.g., strike-slip fault systems) or extensional fracture arrays (e.g., normal fault systems) in postcollisional settings. Porphyry Cu deposits in China mostly show typical alteration zoning from inner potassic to outer propylitic zones, with variable phyllic and argillic overprints. Potassic alteration can be generally subdivided into inner K-feldspar and outer biotite zones, with K-feldspar–rich alteration mostly earlier than biotite-rich alteration. Phyllic alteration generally comprises early-stage chlorite-sericite and late-stage quartz-sericite alteration, and the chlorite-sericite zone typically occurs beneath the quartz-sericite zone. Lithocaps are absent in most of the porphyry Cu deposits in China, even for those in the youngest (~30–14 Ma) ores in the Gangdese belt. Alteration architecture of the porphyry Cu deposits in China is mainly dependent on the structural setting and degree of telescoping. Telescoping of alteration assemblages in the postcollisional porphyry Cu deposits is more strongly developed than that in island and continental arc porphyry Cu deposits. This is probably because postcollisional porphyry Cu deposits and districts in China either experienced higher rates of synmineralization uplift or suffered more complex structural superposition, compared with those formed in magmatic arcs. Hypogene Cu mineralization in some giant porphyry deposits in China (e.g., Xiongcun, Qulong) is associated with potassic alteration and particularly with late-stage biotite alteration. But hypogene mineralization for more than 50% of giant porphyry Cu deposits, including the Dexing, Yulong, Tuwu-Yandong, Duobaoshan, and Tongkuangyu deposits, is characterized by a Cu sulfide assemblage with phyllic alteration, particularly with chlorite-sericite alteration. The presence of several world-class postcollisional porphyry Cu provinces in China demonstrates that the generation of porphyry Cu deposits does not always require a direct link to oceanic plate subduction.