Genesis And Prospecting Prediction Of The Bijiashan Lead-Zinc Deposit In Gansu Province

The Xicheng Lead-Zinc ore field is located on the eastern margin of the West Qinling tectonic zone,near the edge of the Xicheng basin in the southern region of the Qinling microcontinent.The region has been experienced to multi-stage tectonic movements during the Neoproterozoic,Paleozoic,and Mesozoic.Their metallogenic geological conditions are excellent,forming medium-large Lead-Zinc deposits such as Changba-Lijiagou,Bijiashan,Guojiagou and Dengjiashan Lead-Zinc deposits.In order to explore the source of ore-forming materials,the characteristics of ore-forming fluids and the genesis of Bijiashan Lead-Zinc deposit,this paper studied the geological characteristics,fluid geology and isotope geochemistry of Bijiashan deposit,and combined gravity,aeromagnetic and soil geochemical characteristics to analyze the metallogenic prediction.The deposit is controlled by the Bijiashan anticline and its wing interlayer fracture zones,and the ruptures play a destructive role in the ore body.The occurrence of the ore body is consistent with the stratum,which is layered and lenticular,mostly distributed in the contact zone between phyllite and crystalline limestone,with obvious sedimentary characteristics.In addition to quartz veins and calcite veins,there is no large intrusion in the study area.The metallic minerals include galena,sphalerite,pyrite and a small amount of chalcopyrite.Gangue minerals are mainly quartz and calcite.According to the mineral assemblage,formation time and relationship,the Bijiashan deposit can be divided into sedimentary period,hydrothermal reformation period and supergene reformation period.Among them,the hydrothermal reformation period can be further divided into sulfide stage（Ⅰ）and sulfide-carbonate stage（Ⅱ）.The colloidal structure of pyrite,the disseminated structure of pyrite,galena and sphalerite show obvious sedimentary characteristics,while the network vein structure of pyrite indicates the transformation of late hydrothermal fluid.Wall rock alterations include silicification,carbonation and baritization.The temperature measurement results of fluid inclusions show that the inclusions mainly include gas-liquid inclusions and CO₂-containing inclusions.The ore-forming fluid is a low-density（0.76～0.88g/cm³）fluid formed under low temperature（237.1～275.2°C）,low salinity(2.1～8.3wt.%Na Cl_eqv)and low pressure（62.2～75.2MPa）.The H-O isotopes show the characteristics of lowδD and highδ¹⁸O,suggesting that the ore-forming fluid is a mixture of hydrothermal fluid and Middle Devonian seawater,and the late hydrothermal fluid is also involved in mineralization.The S isotopes show the characteristics of enrichment of heavy sulfur,indicating that theδ³⁴S of the Bijiashan deposit may mainly come from marine sulfate or Middle Devonian seawater in the stratum,accompanied by a small amount of mantle-derived sulfur,which may represent the sign of late hydrothermal fluid.Pb isotopes show that the ore-forming materials of Bijiashan deposit are mainly derived from the upper crust,and the mantle-derived materials also have a certain contribution.The above research results show that the Bijiashan Lead-Zinc deposit was formed in the Middle Devonian and experienced early Mesozoic hydrothermal superposition.The ore-forming material comes from the mixture of upper crust and mantle materials,and the deposit is formed by hydrothermal exhalative sedimentary-transformed Lead-Zinc deposit.Through the analysis of prospecting indicators,it can be seen that the Anjiacha Formation is a favorable stratum for mineralization;The northeast and northwest faults and their surrounding basins are the ore-controlling structures in the study area.The Pb,Zn and Cd single element anomalies with obvious concentration centers are the direct geochemical indicators for Lead-Zinc ore prospecting.The intersection area of positive and negative NE-NW magnetic anomalies superimposed in the study area is an important magnetic prospecting indicator.Combined with geological,aeromagnetic,gravity and soil geochemical characteristics,two gradeⅡprediction area was delineated in the area with good metallogenic geological conditions.