| As one of the most significant and typical ore fields of the massive sulfide deposits metallogenic province in North Qilian Mountain, Baiyinchang copper polymetallic ore deposit, Gansu consists of 5 industrial deposits:Zheyaoshan Cu-Zn large deposit, Huoyanshan Cu-Zn medium deposit, Tongchanggou Zn-Pb-Cu small deposit, Xiaotieshan Pb-Zn-Cu large deposit and Sigequan Pb-Zn-Cu regular deposit, respectively. In this study, using the Zheyaoshan and Xiaotieshan ore deposits as the key investigating object, combining comprehensive researches of geological mineralization, metallogenic structural plane and characteristics of mineral fluid, the author systematically summarized the ore-forming processes of the Baiyinchang copper polymetallic ore deposit, further constructed the possible geological prospecting model.Zheyaoshan and Xiaotieshan ore deposits both occur in the keratophyric quartz tuff, controlled by the related volcanism. The upper plate of the ore body mainly tends towards NEE with bed and lens forms, whereas the footwall mostly presents in veins and mesh-veins structure. The near-ores country rock alterations are chiefly chloritization, sericitization, silicification and barite gasification.According to systematic igneous rock studies of petrography, mineralogy, geochemistry and geochronology in Baiyinchang ore deposit, the result indicated that Baiyinchang ore deposit volcanism took placec.a.468～472 Ma ago and was formed under the Ordovician continental margin arc environment. The source area was the mantle wedge, which was contaminated with the melt formed by partial melting of subducting plate (including oceanic sediments). The three volcanic rocks cycling with in the ore deposit were the product of comagmatic evolution from same tectonic environment. Through the study of spatial and temporal relationship between volcanism and ore bodies, it is clear that the volcanism played a significant role in controlling mineragenesis, and mineralization time was determined as during the quiet phase of late periodof acid volcano eruption.Using studies on iron-bearing siliceous rock, barite, chlorite and other representative fluid minerals, the author found there was a typical acid-alkaline and redox geochemical barrier in the process where ore-forming fluid evolved from the early mineralization stage to the mature mineralization stage, namelybarite + iron siliceous rocks (acid, oxidized) and quartz, sericite and chlorite (neutral, partial reduced), carbonates(Alkaline, oxidized).Combining sulfur, lead isotope result and mineral trace elements data, the mineragenesis of Baiyinchang Deposit was volcanic rock formation-spilite keratophyre sequence. The studies of helium, argon, hydrogen and oxygen isotopes together with inclusions indicated that ore-forming fluid was mixtureof volcanic hydrothermal fluid and seawater introduced by the associated volcanism. Through the fluid inclusion studies, the mobile ore-forming fluid in H2O-NaCl-CO2-CH4 system with medium-high temperature, low salinity, rich Cl- and SO42- and poor F- was featured in the relatively oxidizing and acidic environment, whereas the ore-forming fluid in H2O-NaCl-CO2-CH4 system during mineralization with medium-low temperature was under the moderately reducing and neutral environment.Based on this, the study confirmed the geological metallogenic body and metallogenic tructural plane, discussed the representative fluid minerals, the source of ore-forming materials, physical and chemical properties of metallogenic fluid and its sources, and elaborated the forming process of the ore deposit in order to eventually construct the "Trinity", i.e. a possible geological prospecting model. |
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