Metallogenic Modeling Of The Beiya Gold Deposit In Western Yunnan And Its Relation To The Cenozoic Alkali-rich Porphyries

It is the first time that we obtain 34～33Ma of the ore-bearing quartz syenite porphyry in Beiya ore area by using SHRIMP U-Pb zircon dating. Combined with the ages reported by former researchers, we suggest that there were three periods of magmatism. The earliest one is quartz albitophyre and lamprophyre in 60～ 65 Ma, then quartz syenite porphyry in 30～34Ma, which has a close relationship with mineralization process, and associated with lamprophyre, and the last is the biotite quartz syenite porphyry at 3.7Ma.Based on geochemistry of major elements, trace elements, and Pb, Sr, Nd isotopes, the Beiya alkali-rich porphyries have the same geochemical characteristics as adakite, but are different from the typical adakite in higher w(Na₂O+K₂O), w(K₂O)/w(Na₂O) ratio, and lower w(MgO), w(Cr) and w(Ni). Low Y, medium Ba and Nb/Y > 1 of the ore-bearing porphyries indicate that the enrichment process related to melts had played an important role. Heat and fluids released by subduchtion and dehydration of Yangzi block and asthenosphere upwelling might cause partial melting of the thickened crust and then formed such alkali-rich magma.Orthoclase phenocrysts with Or-rich rims in the Beiya alkali-rich porphyries suggests that fluid phase occurred during the magma crystallization, which is a symbol of ore-bearing porphyries and could be the evidence that the ore-bearing hydrothermal solution came from the magma.A systemic research on composition, zonation and metallogeny of skarns has been conducted. The Beiya Au deposit has the typical characteristics of skarn-type ores. Zonation was developed in skarns at the contact between porphyries and wall rocks, in which most ore bodies are distributed. Later metasomatic alteration was overlapped on skarns with precipitation of a great deal of magnetite, gold and copper. According to our study, the Beiya deposit is a composite deposit, within which skarn-type Fe-Au deposit is predominant, and associated with porphyry-type Au-Cu, and hydrothermal vein-type Fe-Au-Pb-Zn mineralization.From the inside to outside and from the deep to surface, the alkalic-rich porphyries are clearly zoned in element assemblage from (Au)Cu to (Cu)Au, and to Au-(Cu)-Fe(Pb+Zn), corresponding to temperature decreasing. While the contents of Cu, Fe and Ag (high temperature) gradually decreased along the strike from north to south, Au, Pb+Zn (medium to low temperature) increased..In this thesis, we discussed the timing of mineralization of the primary and supergene deposits. The timing of Beiya Au and polymetallic deposit is considered to be a period of 33～28Ma, and the hydrothermal system of magma had been lasted for about 5Ma. According to geological relationship, the lower section of Lijiang formation and the timing of supergene ore-forming process should be younger than Eocene.A metallogenetic model is proposed for Beiya gold and polymetallic deposit in this thesis. The deposit is located at the junction of the large Jingshajiang-Red River strike-slip fault and W-E buried faults during a transitional period from transpressional to transtensional state in the late collisional time. Ore-forming fluids and metals mainly came from the magma of alkalic-rich porphyries. The ore bodies, controlled by the N-S faults and fracture zones, mainly occurred in the contact zone between the middle Triassic carbonates of Beiya formation and quartz syenite porphyries. The actively chemical and mechanic characteristics of carbonate are in favor of precipitation of metallogenetic materials. Volatiles are very important factors for ore-forming elements to be transported, accumulated and concentrated. There are three epochs of ore-forming processes, namely, epochs of prophyries, skarn and supergene, respectively. Finally, indicators for prospecting of the Beiya deposit has been summarized in aspects of magma rocks, structural geology, alternation of wallrocks, geophysics and geochemistry.