A tectonic model for porphyry copper-molybdenum-gold metallogenic belts on the eastern margin of the Qinghai-Tibet Plateau

Abstract：Accompanied by hypabyssal emplacement of high-potassic alkali-rich porphyries, “paired” Cenozoic intracontinental porphyry copper-molybdenum-gold belts, controlled by large strike-slip faults, occurred on the eastern margin of the Qinghai-Tibet Plateau formed by India-Asia collision. Of the two metallogenic belts, the west belt is the Jomda-Markam-Xiangyun copper-molybdenum belt and the east belt is the Zhongdian-Yanyuan-Yao＇an porphyry copper-gold-lead-silver belt. The rock types of ore-bearing porphyry include granite porphyry, monzogranite porphyry and monzonite porphyry with small amount of syenite porphyry. They are distinguished from barren porphyry by their higher SiO2 (>63%) and lower Y (<20×10-6) and their adakitic magma affinity. Alkali-rich porphyry is relatively enriched in large-ion lithophile elements (K, Rb and Ba) and depleted in high-field strength elements (Nb, Ta, Ti and P) with a wider range of Nb/Y ratios and shows strong REE fractionation but no pronounced negative Eu anomaly—all these suggest that the magma source region underwent metasomatism and concentration of ancient subducted oceanic slab fluids and was injected by small streams of melts from the asthenosphere. Ore-bearing adakite-like porphyry might originate from the basaltic lower crust. The latter underwent high-pressure (>40 km) low amphibolite-eclogite facies metamorphism and slab fluid metasomatism and occurs as lower-crustal amphibolite xenoliths in alkali-rich porphyry. Barren syenite porphyry might originate from the hydrated phologopite peridotite-enriched mantle. Its trace element and Su-Nd-Pb isotope systematics indicate that their source region was subjected to much stronger slab fluid metasomatism and mixing of asthenospheric material. Available data of deep geophysical exploration suggest that since 50 Ma BP the Yangtze continental slab was subducted westwards and collided with the subducted Indian continental slab, thus inducing upwelling, thermal erosion and underplating of asthenospheric melts and giving rise to partial melting of the crust-mantle transition zone. The magma of adakite-like porphyry is characterized by the presence of relatively rich water, rich sulfur and higher oxygen fugacity (fo2). It was an important metal and sulfur carrier and formed porphyry copper-gold-lead-silver deposits in a shallow-level closed system.