Metallogenic System Model Of The Indosinian Porphyry Copper-Molybdenum Deposits In The North Margin Of The West Qinling, China

Porphyry deposits, one of the world's most important repositories of copper, gold, and molybdenum, could form during both subduction and collisional processes. The western Qinling orogenic belt has been overprinting predominantly by the three global tectonic-metallogenic domains, i.e. the Tethys, Paleo-Asia, and Circum-Pacific. The Indosinian granitoids, widely distributed in the Tongren-Xiahe-Hezuo-Tianshui regions within the north margin of the West Qinling belt, are closely associated with the porphyry and skarn deposits spatio-temporally. Systematical studies on the Indosinian porphyry systems with cases from the Wenquan Mo-Cu and Taiyangshan Cu-Mo deposits reach the following conclusions.The multiple-phase granitoids at Wenquan emplaced at 224.6 Ma to 216.2 Ma, and crystallized at ca.208 Ma. The ore-bearing porphyries, and the late barren porphyritic granite at Taiyangshan emplaced at 226.6 Ma to215.0 Ma, and 200.7 Ma, respectively. They have timing of ore mineral precipitation at ca.210 Ma, suggesting their close relationship between magmatism and mineralization corresponding to the collision between the North China and South China blocks during Late Triassic.The ore-bearing granitic magma was derived from partial melting of the ancient Meso-to Neoproterozoic mid-to lower continental crust which could have formed by remobilization of deep lithosphere modified by prior subduction processes in a collisional tectonic setting, with contribution of the SCLM (subducted continental lithospheric mantle)-derived magma triggered by MASH (e.g., melting, assimilation, storage and homogenization) processes, during which significant metals and sulfur supplied from mafic magma recycled into resultant hybrid magmas.At Wenquan, quartz-k-spar-biotite vein, associated with biotite stable alteration, represents channel-ways for fluid causing alteration of former mafic minerals to biotite, and few sulfides precipitated during this stage. Quartz-chalcopyrite vein,the main stage of Cu mineralization,is related to potassic alteration with plagioclase alteredto K-feldspar. Quartz-molybdenite vein represents the main Mo precipitation stage. Quartz-sericite-pyrite vein,as the last event of mineralization, represents the Au deposition stage.At Taiyangshan, the vein timing sesuences are distinguished as, the early K-spar-biotite-quartz±magnetite±apatite vein related to the potassic alteration, K-spar-chlorite-pyrite-chalcopyrite-quartz vein, and quartz-pyrite-sericite vein associated with the phyllic alteration, and post-mineralizing calcite-quartz vein. Mineral assemblages vary across the quartz-pyrite-sericite vein due to chemical interaction between host rock minerals and elements introduced into the system through the vein. Within the vein, there has been an addition of Fe, S, Cu, and K causing the formation of pyrite, quartz, anhydrite, chalcopyrite, K-spar and muscovite. Within the alteration halo, minerals present in the host rock (plagioclase, biotite, quartz and anhydrite) have been altered to form muscovite, K-spar, anhydrite, quartz, pyrite and chalcopyrite. This also required the addition of Fe, S, Cu, and K.Constraints from whole-rock geochemistry, C-H-O-S-Pb-Fe and Re isotopes indicate that the Wenquan and Taiyangshan deposit are typical porphyry deposits, and the late phyllic alteration formed in the hydrostatic environment overprinted the ore-bearing fluid formed in the lithostatic environment associated with the early potassic alteration.Integrated fluid inclusion petrography and cathodoluminescence microscopy, and trace element analysis of quartz showed that stockwork veins in porphyry deposits are composed of quartz of multiple generations, distinguished by different Ti, Aland K concentration as well as their fluid inclusion assemblages. The Wenquan deposit was formed in a deep environment, whereas the Taiyangshan deposit was formed in a relatively shallow porphyry environment. The emplacement depth and change as well as preserve of this metallogenic system are the key factors for the difference between these deposits.