| Zheyaoshan copper polymetallic deposit in Gansu province is an important part of the Baiyinchang copper polymetallic ore field, which is the most important and representative ore field in North Qilian Mountains massive sulfide deposits metallogenic province. Thetectonic locationof ore field is in the North Qilian--Zoulangnanshan island arc of Qingling-Qilian-Kunlun orogenic belt. Previous researches have done a lot forthe Zheyaoshan ore deposit, but the deposit genesis and metallogenic mechanism are still not clear. Based on the geological feature research, the text studies the feature of ore-forming fluid, element geochemistry, chronology, etc., hopingprovide the scientific deployment basis for the following prospecting.The spot-surveying, profile survey, and Microscope study of Zheyaoshan deposit indicate that the ore body hostes in keratophyric quartz tuff. The main metal minerals are pyrite and chalcopyrite, the secondary are sphalerite, galena, magnetic pyrite and tetrahedrite. And the gangue minerals are mainly quartz, chlorite and sericite, calcite, barite, ankerite, epidote, etc. The ore body’s wall rock alteration arechloritization, sericitization and carbonatization, rutile, pyritization, epidotization, silicide, barite, etc.The ore-forming process can be divided into two stages, one isvolcanic jet hydrothermal mineralization stage, another is supergene stage.With detailed petrography and thermodynamics research of fluid inclusions, the fluid inclusions are divided into fivekind of liquid inclusion, gaseous inclusion, pure gaseous inclusion, CO2-bearing three-phase inclusion, pure CO2 inclusion. The mobile ore-forming fluid shows a medium to high temperature, low salinity feature,indicate that it forms in H2O-NaCl-CO2-CH4 system withrich Cl-, SO42- and poor F-.Considering the δ18OH2O value ranges from -2.06%o to 5.20%o, the ore-formingfluids are likely derived from the mixture of magmatic hydrothermal and seawater hydrothermal.The 40Ar/36Ar value also suggest that seawater hydrothermal plays an important role in themineralizationprocess as well. The ore sulfides δ34S values is 3.2%o to 6.7%o,206Pb/204Pb one is 17.766 to 18.056,207Pb/204Pb one is variable 15.158 to 15.707,208Pb/204Pb oneis 37.465 to 38.175. All of these evidence illustrate that the source of lead element not only comes from mantle but also from upper continental crust.Compared with average chondrite, the trace element shows that it is enrichment of Th, U and Pb, depletion of Ta, Nb and Sr, andhave similar characteristicswith volcanic rocks in the spiderdiagramoftraceelementratio, so it suggests that both of ore-forming material and volcanicscome from similar source region.The resultshows that Zheyaoshan deposit has the typical VMS depositscharacteristics. In the early-mid Ordovician, with the expansion of the ocean basin, the hot liquid which is bearing the metallic elements and coming from the upwelling magma poured out along the rock fracture or fracture system. When the liquid arrived at the bottom of the sea, it mixed with cold water. Then the combination of the liquid, silica, barite and iron oxide accumulated in the seabed, rapidly formed a solid crust bedded which cut off the mixturingprocess of the seawater and the rising liquid. The hot liquid continued migrating upward, at the same time it mixed with the convective water. Finally it led to the formation of the rich chalcopyrite-sphalerite-pyrite massive ore and disseminated ore. |
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