Metallogenesis of the Tibetan collisional orogen: A review and introduction to the special issue

Mineral deposits associated with continental collision are abundant in many orogenic systems. However, the metallogenesis of collisional orogens is often poorly understood, due to the lack of systematic studies on the genetic links between collisional processes and ore formation in collisional orogenic belts. This paper reviews the key metallogenic settings and resultant collision-related ore deposits in the Tibetan Orogen, created by Indo-Asian collision starting in the early Cenozoic. The resulting synthesis leads us to propose a new conceptual framework for Tibetan metallogenic systems, which may aid in deciphering relationships among ore types in other comparable collisional belts. This framework includes three principal metallogenic epochs in the Tibetan orogen, and metallogenesis in: (1) a main-collisional convergent setting (similar to 65-41 Ma); (2) a late-collisional transform structural setting (similar to 40-26 Ma); and (3) a post-collisional crustal extension setting (similar to 25-0 Ma), each forming more than three distinct types of ore deposits in the Tibetan orogen. The main-collisional metaollognesis took place in a convergent setting, i.e., a collisional zone, characterized by collision-related crustal shortening and thickening, associated syn-peak metamorphism and two distinct magmatic series (Paleocene-Eocene crust-derived IOW-fO(2) granitoids generated by crustal anatexis and Eocene high-fO(2) granitoids formed by MASH processes at the base of the Tibetan crust). Metallogenesis during this period formed Sn-W-rare metal deposits related to the low-fO(2) granitoids, skarn-hosted Cu-Au polymetallic deposits related to high-A granitoids, and orogenic-type Au deposits formed by CO2-dominant metamorphic fluids. Late-collisional metallogenesis occurred mainly in a transform structural setting dominated by Cenozoic strike-slip faulting, shearing, thrust systems, and associated potassic magmatism in eastern Tibet, and formed the most economically-significant metallogenic province in the orogen. Four significant ore-forming systems are recognized in the transform zone: porphyry Cu-Mo-Au systems associated with potassic adakitic melts and controlled by Cenozoic strike-slip faults: orogenic-type Au systems related to large-scale left-slip ductile shearing; REE-bearing systems associated with lithospheric mantle-derived carbonatite-alkalic complexes; and Zn-Pb-Cu-Ag systems related to basinal brines and controlled by Cenozoic thrust structures and subsequent strike-slip faults developed in the Tertiary foreland basin. Post-collisional metallogenesis occurred in a crustal extension setting, characterized by lithospheric mantle thinning or delamination at depth, crustal shortening at a lower structural level and synchronal extension at shallower levels. The resulting ore-forming systems include: (1) porphyry Cu-Mo ore systems related to high-K adakitic stocks derived from the newly-formed thickened mafic lower-crust; (2) vein-type Sb-Au ore systems controlled by the south Tibetan detachment system (STDs) and the metamorphic core complex or thermal dome intruded by lecuogranite intrusions; (3) hydrothermal Pb-Zn-Ag ore systems controlled by the intersections of N-S-striking normal faults with E-W-trending thrust faults; and (4) spring-type Cs-Au ore systems related to geothermal activity driven by partial melting of the upper crust. Associated ore deposits lie mostly within the mid-Miocene Gangdese tectono-magmatic belt, in which the scavenging role of fluids derived from evolved magma systems or dewatering of rift basins, and finally discharging at intersections of the orogen-transverse and -parallel faults are extremely important for formation of the low-temperature hydrothermal deposits. Based on the synthesis of deposits in the Tibetan orogen and comparison with the metallogenesis of other orogenic systems, a more complete classification for these collision-related deposits can be proposed. (C) 2009 Published by Elsevier B.V.