The Geochemistry Characteristics And Metallogenic Series Of Gangcha Pluton In Tongren, Qinghai Province

This paper focuses on lithogenetic and metallogenic geochemical characteristics of the classic ore-deposits associated with Chagang granites in Qinghai province, western China; and gives subsequently out something new on their genetic model, tectonic settings as well as the problem of metallogenic specialization of granitoid in this area.Findings show that:I: The association type of granites in Gangcha region is observed as gabbro-diorite + diorite +granodiorite + adamellite, whose genetic type was identified as I, S series with significant calc-alkaline or high-K calc-alkaline and metaluminous or weakly paral- uminous feature. LILEs and Pb, rather than Sr, P and Ti, seem abound in these granites according to total rock analysis. In terms of REE and microelement geochemical patterns, we found that these granites must belong to a series of products of comagmatic evolution and post-collision tectonic movement, namely controlled by magmatic underplating-mixing processes in such a geodynamic context whose stress condition transforming from extrusion towards extension, triggered by continental collision and the subsequent crust- thickening as a result of the Indosinian tectono-magmatic event in the Central Orogenic belt..Ⅱ:By SHRIMP zircon U-Pb dating, the average age of Gangcha plutons has been identified as 242±3Ma for the Shuangpenxi granodiorite bodies, 215Ma±3M for the Jiangligou biotite-rich granitic bodies and 238±3Ma for Xiekeng gabbro-rich granites. That is to say, the diagenesis in this area peaked at 242 Ma, 232 Ma and 215 Ma, and should be a result of pulsation of homologous magmas formed at different periods(being the products of the Indosinian tectono-magmatic process in West Qinling Mountain), which made up the so-called Gangcha complex series.III: The geology of Jiangligou, Xiekeng and Shuangpengxi deposits has also been systematically investigated. we found commonness and otherness coexist. Their ore-forming solutions are of mid-to-high-temperature, mid-to-high-salinity and intermediate-density. There seems an intimate metallogenic relationship presenting between Gangcha granites and some deposits surrounding them, either for the(diagenetic- and) metallogenic time(about 243.8~214Ma) being consistent with the Triassic tectono-magmatic process and metallogeny related to it in the whole west Qinling-East Kunlun region. Evidence coming form stable isotope showed that metallogenic material of the deposits in and around Chagang granites may be mainly derived from crust in depth and upper mantle. Besides, it seems of reduced porphyry origin with significant W-mineralized potential but a minor Cu-Mo mineralized scale.IV:The regional deposits around Gangcha granites can be subdivided into one metallogenic series, four sub-series and four styles that are Jiangligou-style, Shuangpengxi-style,Dehetongwa-style and Xiekeng-style. Metallogenic patterns of hypabyssal porphyry- type Co-Mo-Au deposits associated with intermediate-acidic intrusive rocks in this area were established also.V:Several lines of evidence came finally to the conclusion that “high permeability zone”,“apical zone of granitic rocks”, and “geochemical or geophysical anomalous area.” should be appreciably favorable for polymetal mineralization(mainly small to medium-sized Cu-Au deposits), corresponding specifically to four advantaged mineral foreground areas.