Alkali-rich Magmatism And Au-polymetal Metallogenic System In Northwestern Yunnan

Northwestern of Yunnan province is important part of "southwestern-three river Metallogenic belt" of China. Au-polymetal deposits with characteristics of multiformity (poly-type) and concentricity (one-body) in this area had consanguineous relationships of space-time and genetic with the Himalayan alkali-rich magmatism. The geological and geochemical basic of tectonic-magma-fluid-metallization was formed by the complex tectonic evolvement of Tethys before Cenozoic. Mulriple tectonic dynamical systems of special regional stress field (geography position) , intense collision orogeny of Qinghai-Tibet plateau, long-range effect of the subduction of Pacific Ocean plate and strong crust-mantle interaction which occurred in the condition of multiply-layered structure with layered stress-concentration made up of the intricacy geodynamical environment of magmatism and metallization in Cenozoic. The unite-controlling of removement of intracontinental depth near EW trend paleo-structures(mainly spread) and other trend structures(compressed-twisted) in lower in crust to alkali-rich magmatism and related gold-polymetal mineralization formed the integrative features of the alkali-rich magmatite and the deposits, which in space distribution showed as independence region, EW trend rock and ore belt in the region and many Au multimetallic concentration area in the belt, and in era as simultaneity or near simultaneity. The magmatites emergenced in northwestern of Yunnan was a series of alkali-rich rocks ( K₂O+Na₂O>8% ) from basic to intermediate-acid and related deposit or mineralization concentration area had the common characteristics of poly types, poly elements, poly factors of ore-controlling and concurrent metallogeneic epoch. Studies of H,O,S,C stable isotopes, Pb,Sm-Nd,Rb-Sr,Si,He-Ar radioactive isotopes and REE geochemistry showed that the alkali-rich magma came of the mixed belt of crust and mantle with the geochemical feature of mantle-rich II type and formed by the fractional melting (and/or alkali-metasomatism) of the source rocks as a result of migrating up and infusing into the crust-mantle belt along deep faults(mainly EW-trending nearlly) of the hydrocarbon alkali fluid derived mantle. The fluid (it might be Au and Cu or other elements-bearing) moved up through melting body or accompanied the intrusions of the alkali-rich magma and obtained gold and other elements from the rocks the fluid passed by metasomatism, extraction and remobilization and then changed into ore-bearing hydrothermal solution. When it moved into some favorable space around the porphyry bodies (veins) in the shallow of crust, a series of gold (and other metal) deposits with different types and sizes formed. A regional gold mutlimetal metallogenicsystem related to the alkali-rich magmatism, which formed by complicated couplied of Mulriple tectonic dynamical systems, the mixed area of crust and mantle and alkali-rich magmatism and mantle ore-elements-rich fluid, was established in this paper in northwestern area of Yunnan province. This metallogenic system included two subsystems, one developed in the Yangtze plate and another in the adjacent Lanping-simao basin. The production of the metallogenic system comprised mineralization series (including sub-series) in regional scale and mineralization network of different types ore-bodies (such as porphyry type, skarn type, quartz vein type, structure altered rock type, (explosion) breccia type, laterite type and so on), wallrock alteration, geophysical-geochemical-remote sensing abnormities,etc. in multimetallic concentration area scale. The changing and conserving of the metallogenic system production was detailedly discussed. According to the features and indications of the productions, together with the validity and adaptability of different exploration methods for deposit, a regional exploration model for deposit had been put forward.