陕西镇安西部棋盘沟钨矿区成矿流体特征研究

Study on Characteristics of Ore-forming Fluids in Qipangou Tungsten Mining Area,Western of Zhen'an,Shaanxi

镇安西部棋盘沟矿区内的钨矿体主要为石英脉型钨矿,产出于矿区内近SN向断裂中,矿石中金属矿物以白钨矿为主,矿体规模大且品位高,但矿区内未见有岩浆岩体出露,因此明确其成矿流体来源是理解矿床成因的关键。为了探究成矿流体特征,分析了主成矿阶段形成的白钨矿与石英颗粒中主要发育的气液两相包裹体,结果显示包裹体均一温度介于291.4～423.7℃之间,热液盐度[w(NaCl)]为5.1%～7.7%,流体的密度范围为0.52～1.08 g/cm3。这表明矿区成矿流体属于中高温、低盐度流体,与典型热液钨矿流体相似,H、O同位素测定结果显示其主要来源为岩浆水,S同位素落入岩浆岩S同位素组成范围内,反映矿区钨矿来源于岩浆热液。与周边矿区似层状钨矿流体包裹体相比,棋盘沟矿区成矿流体均一温度明显较高,说明近SN向构造是成矿热液运移的主要通道,在成矿流体运移过程中,顺着该区NNE向断裂裂隙上升,形成石英脉型白钨矿。随着成矿热液温度持续降低,矿液沿层间裂隙灌入形成似层状型白钨矿。

The western part of Zhen’an is an important tungsten molybdenum polymetallic metallogenic belt in Qinling Mountains,and its metallogenic geological conditions are very favorable. A number of tungstenmolybdenum deposits have been discovered in this area in recent years.At present,the study of tungsten deposits in this area mainly focuses on the geological characteristics,ore-controlling factors,mineral assemblages,resource prediction,metallogenic model and prospecting prospects.However,research about systematic fluid inclusions and isotope geochemistry is still lacking.This paper chooses Qipangou mining area in western Zhen’an as a typical mining area,mainly studies the characteristics of fluid inclusions and discusses the properties of oreforming fluids in the mining area.The main tungsten ore bodies in Qipangou are quartz vein scheelite,which occurs in the near north-south faults of the mining area.The metallic minerals in the ore are mainly scheelite.The ore bodies are large in scale and high in grade,but no magmatic rock bodies are found in the mining area. Therefore,the key to understand the genesis of the deposit is to clarify the source of its mineralizing material. In this study,experimental samples of scheelite-bearing quartz veins in Qipangou mining area were taken,and fluid inclusions,pyrite sulfur isotopes,hydrogen and oxygen isotopes were analyzed.The experimental results show that the gas-liquid two-phase water-soluble inclusions are mainly developed in scheelite and quartz grains which formed in the main metallogenic stage of Qipangou. The homogenization temperature of inclusions is mainly between 291.4~423.7 ℃,the hydrothermal salinity is 5.1%~7.7%[w(NaCl)],and the density of fluid is between 0.52~1.08 g/cm^3. The inclusion temperature data show that the ore-forming fluids in the mining area belong to medium-high temperature and low salinity fluids,which is similar to the physical and chemical conditions of fluid in Shizhuyuan super-large tungsten deposit in Nanling area. Hydrogen and oxygen isotope results show that the main source of fluids is magmatic water,and sulfur isotope test results fall into the range of sulfur isotope composition of magmatic rocks,reflecting that tungsten deposits in the mining area are derived from magmatic hydrothermal solution.Compared with the temperature measurement results of fluid inclusions in the near-east-west layered tungsten deposits in the surrounding mining areas, the homogenization temperature of ore-forming fluids in the Qipangou mining area is obviously higher.This result reflects that the near-north-south structure in the area is the main channel for the migration of ore-forming hydrothermal fluids. The migration process of ore-forming fluids first moves along the near-north-south faults,then along the northsouth faults. Then the ore-forming pour into the interbedded fissures to form the lamellar scheelite,and the temperature decreases with the migration of hydrothermal fluids. The study results of ore-forming fluids shows that the Qipangou scheelite deposit is mainly hydrothermal quartz vein type,controlled by NEE-trending faults. The NEE - trending faults are important structures for tungsten mineralization in this area,and are the main channel for ore-forming hydrothermal migration,which is of great significance for indicating prospecting.