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PORPHYRY Cu-Mo-Au DEPOSITS: SOME NEW INSIGHTS AND ADVANCES

HOU Zeng-qian (Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037,China)  
The porphyry-type Cu-Mo-Au deposit is a significant genetic type of Cu-Mo and Cu-Au deposits,to which close attention has been paid widely. Some new advances in the research of such deposits have been achieved in recent years. These advances are based on some new insights into the geodynamic setting,the magmatic affinity of Cu porphyry,the telescoping of alteration system and the overprinting of epithermal system,and the structural control on porphyry Cu system. Many of porphyry-type deposits occur in island-arc and continental margin arc settings,however,some porphyry-type deposits also occur in collision orogenic settings,which shows that continental collision zones have significant potential for generating porphyry Cu systems. The Cu-bearing porphyries in arc settings are mainly calc-alkaline and potassic calc-alkaline,whereas the ones in collision orogen settings are dominantly potassic calc-alkaline and shoshonitic. The majority of porphyries in both two settings show magmatic affinity with adakite. The arc porphyries were derived from adakitic melt produced by partial melting of oceanic-slab due to flat subduction. The adakitic magmas in collision zones were derived from melting of thickened lower-crust,triggered by break-off of subducted continental slab. In arc settings,the kinks or the tear in the subducted slab result not only in flatting of the slab and asthenospheric upwelling,but also in the coupling deformation of the crust and the formation of transverse-arc faults,which control spatial localization of porphyry Cu systems. In collision orogens,the break-off of the subducted continental slab localizes asthenospheric upwelling,which inputs juvenile components and provides enough heat to cause the melting of thickened lower-crust. Like transverse-arc faults,the transverse-collision zone normal faults,formed by post-collisional crust extension,facilitate rapid ascent of magmas to high-levels in the overlying crust,where vapor saturation,exsolution of volatiles and Cu-Mo-Au deposition may occur. Whether in the oblique-convergence arc settings or in the collision orogenic settings,a series of large-scale strike-slip faults and associated pull-apart basins,usually developed either along magmatic arc or orthogonal to the orientation of main collision zone,are the significant conduit systems to facilitate the high-level emplacement of magmas and favor the decoupling of ore-forming fluids from ascending magmas. In many porphyry metallogenic provinces,the early-formed concentric K silicate (inner) and quartz-sericite alteration (outer) zones associated with mineralized porphyry body were usually overprinted by the later,structurally controlled advanced argillic alteration,showing the telescoping of alteration assemblages. Meantime,the early-formed porphyry-type Cu-Mo orebody was overprinted by epithermal Cu-Au mineralization,which formed either isolated,but spatially coexisted,two deposits,or a high-grade giant deposit. The telescoping and overprinting are widely regarded to be related to the regional uplift and exhumation during the syn- or post- mineralization phase.
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