Gold Deposit, Inner Mongolia Graduated Force He Geological Characteristics And Causes

A large-sized high-grade porphyry Au deposit has been documented for the first time at Bilihe in the Paleozoic subduction-accretion zone of Inner Mongolia,near the northern margin of North China block(NCB).It contain the lowest known copper to gold ratios of any porphyry-type deposit.Here we describe the volcanic rocks,intrusive rocks,hydrothermal alteration,and mineralization types,characters and sources of the ore fluid of Bilihe, finally give the metallogenic model of Bilihe Au deposits.The Bilihe gold deposit is associated with Late-Permian granodiorite porphyry,around～260Ma to 258Ma,emplaced into andesitic-dacitic volcanic rocks. The rhyolite age is～274Ma, andesite is～270Ma.In contrast,the latest intrusion of Bilihe is a barren granite porphyry,around～253Ma. The main gold mineralization is associated with the granodiorite porphyry. Petrologic geochemical datas of the intrusions and volcanic rocks suggest that granodiorite porphyry is product of subduction-related magmatism, in association with subduction-related calc-alkaline magmas.Sr-Nd-Pb isotop data of granodiorite porphyry indicate it from partial melting in the mantle wedge overlying the subducting plate,which may mixed with crust components.Granite porphyry is association with Continent collision.Two alteration types are developed in intrusion and volcanic rocks in and surrounding Bilihe gold deposit.Earily,tourmalinization characterized by visible tourmalite,pervades the granodiorite porphyry and volcanic rocks from early to late.Late alteration type is characterrized by illite-chlorite-carbonate minerals, Chloritization is pervasive in the interior of the porphyry,but weakly in volcanic rocks. Sericitic alteration (illite-sericite±clay minerals) has overprinted most of the tourmalinization and chloritization.Gold mineralization is mainly coincident with UST quartz at the top of granodiorite porphyry,associated with early tourmalinization.Native gold is present in dominantly disseminated form,occasionally gold cluster in some of UST quartz samples.Minor native gold,pyrite,chalcopyrite and even lesser quantities of tennantite-tetrahedrite,galena, sphalerite also occur in quartz veinlets and volcanic rocks surrounded by chlorite-illite-carbonate minerals alteration.