Geological Features Of Diagenesis And Mineralization In The Beiya Gold Polymetallic Deposit, Western Yunnan Province

Beiya gold polymetallic deposits is one of the largest gold deposits in westerndistrict of yunnan province. Studies show that the formation of the ore deposit isclosely associated with alkali rich porphyry. But there are some controversy on themineralization type of the mining area, material sources of alkali rich porphyry,diagenetic chronology and metallogenic chronology. This article focus on thegeological feature of Beiya ore deposit, output and geochemical characteristics ofrock mass and chronology characteristic.The mineralization types are divided into two system, including four types basedon the field relationship and mineral associations. Porphyry and skarn mineralizationsystem include the skarn Au-Fe-Cu ore deposit, the porphyry type Au-Fe (Cu) oredeposit, hydrothermal vein Pb-Au-Fe-Ag-(Cu) ore deposit. Supergenemineralization system includes weathering Fe-Au mineralization. Endogeneticmetallogenic period of which the skarn mineralization is the main mineralized typehas five stages including contact skarn stage, retrograde altered stage, quartz sulphidestage and carbonate stage.The whole rock analysis of quartz-orthophyre and orthophyre in Beiya oredeposit indicates that, the alkali rich porphyry in this region is intermediate-acid toacid rocks, high silicon, alkali rich, poor calcium, magnesium, titanium, weaklyperaluminous shoshonite series. Trace element is rich in Rb, Sr, Ba, K and Pb element,but poor in Ta, Nb, P, Ti, HREE elements. The test samples showed cognate magmaevolution characteristics. The total rare earth of alkali rich porphyry is40.05×10-6~298.30×10-6, and there is a high level of light rare earth element.Our isotopic geochronology U-Pb studies on the alkali-rich porphyries associatedwith the ores reveal a broadly same age group including (35.00±0.17)Ma for thequartz-orthophyre from Wandongshan ore section;(35.06±0.16)Ma for thequartz-orthophyre of the west intrusion sampled from the Hongnitang ore section;(36.72±0.25)Ma for the quartz-orthophyre of east intrusion (the Dashadi intrusion)sampled from the Hongnitang ore section;(35.21±0.19)Ma for orthophyre inBijiashan ore section;(35.18±0.22)Ma for orthophyre in Weiganpo ore section;(35.12±0.13)Ma for orthophyre in Matouwan ore section;(34.62±0.25)Ma fororthophyre in Bailiancun ore section. They are the products at the same time.According to the alkali-rich porphyry dating results and existing data, the magmatism of the mining area and the peripheral area can be divided into60~65Ma,31~37Ma,24~26Ma, and3.65~3.78Ma. Combined with molybdenite ages we know that theregional mineralization has a relationship with the quartz-orthophyre and orthophyreemplacement during34~37Ma. And it is the main mineralization epoch, and thediagenesis and mineralization occurred at the same time, the basic correspondingmetallogenic epoch should be Eocene and Oligocene in paleogene period oftransition.The molybdenite Re-Os dating on the gold ore in Beiya major ore district has aweight average age of37.47±0.51Ma and isochron age of38.12±0.82Ma, and anothersample collected from the Wantongshan ore district has a model age of47.43±0.66Ma.From the data we obtained we can explain the age range of34~37Ma are major stageof polymetallic gold mineralization in the area. The presence of47Ma molybdenummay be the indicator of the other magmatic activitie and related mineralization.Alkali rich porphyry in the region should be derived from the mantle sourceregion affected by subduction oceanic plate metasomatism, combined with theanalysis of geochemical data of rich alkali intrusive mass and the background. ofregional tectonic evolution. According to the range of Re content, rich alkalinemagma may also experienced crustal material contamination. The Beiya goldpolymetallic deposit is a typical porphyry, skarn and hydrothermal type ore depositwhich is closely associated with quartz-orthophyre and orthophyre. Existingmineralization zoning phenomenon is showing the same magma mineralization indifferent forms in different spatial positions.