Western Yunnan Baoshan Hetaoping Pb-zn Deposit Skarn Mineral Characteristics And Genetic Research

Baoshan-Zhenkang area located between the Lancang River fault zone and the Nujiang river fault zone is a very important polymetallic metallogenic belt of Yunnan province rich in copper、lead、zinc、gold、silver and so on.Hetaoping ore deposit is a large lead-zinc-copper polymetallic ore deposit in the metallogenic belt.The ore body is strictly controlled by the fracture nearly SN direction and interbedded fracture zones, and it formed in the hydrothermal alteration limestone and similar skarns which belong to upper Cambrian’s Hetaoping group, presenting as stratiform.This paper is based on the national natural fund project "The analysis of microscopic structure of ore-forming fluid in Hetaoping lead-zinc mining area of western Yunnan "On the basis of the serious analysis of the previous data、the detailed record of surface and underground in Hetaoping lead-zinc mining area and the analysis of the indoor test, I get some new results on the geological characteristics of deposits、the division of metallogeny’s times、the characteristics of wall-rocks’alteration、the characteristics of minerals’combinations、the nature of the fluid inclusions and so on.(1) Through the observational study and contrast of the geological characteristics of deposits, I consider that Hetaoping lead-zinc ore deposit doesn’t belong to the typical skarn-type ore deposits.But the minerals’combinations formed from hydrothermal solution of Hetaoping lead-zinc ore deposit are very similar with the minerals’combinations of the typical skarns, I name this ore deposit as "rocks similar with skarn-type" ore deposits.(2) The mineralization of Hetaoping "rocks similar with skarn-type" lead-zinc ore deposit is divided into hydrothermal ore-forming period and supergene period.The hydrothermal ore-forming period is divided into five stages further:(I)the silicate stage with no water,(Ⅱ) the silicate stage with water,(Ⅲ) the silicate-sulfide stage、(Ⅳ) Quartz-polymetallic sulfide stage and (Ⅴ) Carbonate-sulfide stage.The Ⅲ、Ⅳ、Vstages are the main metallogenic stages of copper、lead and zinc.(3) I research the minerals’combinations of "the rocks similar with skarn" scientifically for the first time.Through the detailed research by microscope and the test of composition by electron probe, I select the garnet, pyroxene、hornblende and epidote-orthite silicate minerals as the objects of study, work out the chemical formulas of crystals for confirming and find the minerals which were not confirmed in the past researching work.They are Calcium-almandine、 grossularite、andradite、Mn-Fe sahlite、Fe-sahlite、sahlite、Mn-hedenbergite、ferroactinolite、 Fe-Ca-Mg amphibole、orthite and Fe-epidote.These minerals’combinations help judge the cause of formation of the deposit.(4) Through the analysis of the garnet、pyroxene and hornblende silicate minerals in composition-causes diagrams, I find that the Hetaoping lead-zinc ore deposit is different from the typical skarn-type polymetallic ore deposits in the indicative meaning on minerals’ composition.This result reveals that the Hetaoping lead-zinc ore deposit is different from the typical skarn-type ore deposits on the cause of formation of the deposit.(5) Through the systematic analysis of petrography and microscopic temperature measurement of fluid inclusions, I find most of the fluid inclusions are negative crystal shape、 oval and irregular shape. It reflects that the mineral crystals of inclusions grew by layer; The inclusions are almost pure liquid and gas-liquid two phase. There’s no magmatic liquated inclusions. It reflects that there’s no crystallization in magma in the process of metallogeny.The heterogeneous inclusions’symbiosis in mineral crystals and the large-scale change of the gas to liquid ratio reflect that the hydrothermal solution seethed in the process of metallogeny and the pressure of ore-forming fluid reduced fast.(6) The results of test shows that the uniform temperature of the inclusions in the minerals of the rocks similar with skarn、quartz and calcite in early stage is190～240℃. The peak value is between320and360℃.The uniform temperature of the minerals in late stage is lower (110～160℃).Sphalerite precipitate mainly in190～210℃.As to salinity, the salinity of the inclusions in the minerals of the rocks similar with skarn in early stage is higher (11～20.52wt%NaCl).The salinity of quartz and calcite is divided into two parts:one is less than5.5wt%NaCl and the other is9.5-19wt%NaCl.As to density, the density is1.005～1.033g/cm in the silicate stage with no water.In the silicate stage with water, the density reduce to0.929～1.024g/cm3.The density is0.826～0.976g/cm3in the Quartz-polymetallic sulfide stage and is0.7～0.8g/cm3in the Carbonate stage.As to pressure, the pressure of the inclusions in the minerals of the rocks similar with skarn is9.5-31.2MPa.The average is about12.6-19MPa. The pressure of the inclusions of Sphalerite is5.8-18MPa, whose average is about12MPa.The uniform temperature、salinity、density and the pressure of the fluid inclusions are reduced from the silicate stage with no water to the carbonate stage on the whole.(7) The nature of the fluid of Hetaoping lead-zinc ore deposit of Baoshan is very unique.I consider this ore deposit is hydrothermal lead-zinc-copper polymetallic ore deposit. The hotliquid’s filling and many chemical reactions made the ore deposit’s forming.