东天山小尖山金矿床成矿流体特征及成矿模式探讨

小尖山金矿床产于东天山康古尔韧性剪切带南缘,对该矿床流体特征和矿床成因类型了解较少。矿区普遍发育低绿片岩相变质,矿床由多条走向为100°~120°的陡倾斜蚀变岩型矿体组成,金平均品位3.11×10^-6~24.99×10^-6;成矿过程可划分为3个阶段:(1)黄铁矿-磁铁矿-绿泥石-绢云母-石英阶段;(2)黄铁矿-黄铜矿-自然金-石英-绿泥石阶段;(3)石英-方解石-贫硫化物阶段。本文通过对矿床不同成矿阶段石英脉内发育的流体包裹体进行了岩相学、显微测温与氢氧同位素研究分析,发现矿床主要发育H₂O-CO₂及气液两相流体包裹体,从早至晚成矿过程中流体内CO₂包裹体逐渐减少,气液两相包裹体内气液比逐渐减小。各成矿阶段包裹体显微测温结果表明,从早至晚成矿流体均一温度分别为216.9~396.4℃、183.1~319.2℃与145.1~220.8℃;成矿流体盐度分别为1.40%~10.11%NaCleq、1.91%~11.22%NaCleq与1.63%~6.74%NaCleq,成矿流体属于中低温、中低盐度的NaCl-H₂O-CO₂体系,并经历了从中温、中盐度流体向低温、低盐度流体的演化过程;成矿早阶段流体的δD_V-SMOW值为-22.550‰,δ¹⁸O_水值为9.44‰,指示变质水成因;成矿晚阶段δD_V-SMOW值介于-41.913‰~-34.796‰之间,平均值为-37.413‰,δ¹⁸O_水值介于1.99‰~3.98‰之间,平均值为2.99‰,指示混合水成因,但接近变质水;成矿流体主要为变质水,成矿早阶段至晚阶段具有从变质水向混合水演化的特征。综合分析,小尖山金矿床成因类型为造山型金矿,其成矿模式为早期韧性剪切变形过程中产生的变质流体在运移过程中萃取岩石中成矿物质,形成含金成矿流体,并在糜棱岩面理等裂隙处发生结晶作用,导致金的初步富集;晚期地壳快速抬升,地质体由韧性变形向脆-韧性、脆性变形转变,伴随有花岗岩脉的侵入,变质流体在运移过程中从流经岩石中淋滤萃取金等成矿物质,形成含矿流体,岩浆水、大气降水的混入以及深度、压力的降低使得流体内的成矿物质在裂隙或断层发育的有利地段卸载沉淀,形成金矿体。

The characteristics and evolution of the ore forming fluids in the Xiaojianshan gold deposit, eastern Tianshan Mountains

The Xiaojianshan gold deposit is located at the southern margin of the Kanggur ductile shear zone in the eastern Tianshan Mountains.The fluid characteristics and genetic types of ore deposits are poorly understood.Low greenschist facies metamorphism is widely developed in the mining area.The deposit is composed of many steeply inclined altered rock ore bodies with an average grade of Au 3.11×10^-6~24.99×10^-6,striking between 100°~120°.The metallogenic process can be divided into three stages:(1)pyrite-magnetite-chlorite-sericite-quartz stage,(2)pyrite-chalcopyrite-gold-quartz-chlorite stage,and(3)quartz-calcite-low sulfide stage.Based on the study of the fluid inclusions in quartz veins of different stages by petrography,microthermometry and H-O isotope,it appears that CO₂-bearing fluid inclusions and aqueous two-phase fluid inclusions existed in the Xiaojianshan gold deposit inearly to late stages.CO₂-bearing fluid inclusions decreased gradually and the ratio of gas to liquid for aqueous two-phase fluid inclusions was smaller.From early to late stages,microthermometry results show that the average homogenization temperature of fluid inclusions was 216.9~396.4℃,183.1~319.2℃and 145.1~220.8℃;salinity was 1.40%~10.11%NaCleq,1.91%~11.22%NaCleq,1.63%~6.74%NaCleq.The ore forming fluids were medium-low temperature and low salinity NaCl-H₂O-CO₂type solutions and the evolution was from medium temperature and salinity to low temperature and salinity.H-O isotopic composition indicates thatδD_V-SMOWvalues-22.550‰andδ¹⁸O_watervalues 9.44‰in the early stage of mineralization indicating the genesis of the metamorphic water.In the late stage of mineralization,δD_V-SMOWvalues were-41.913‰~-34.796‰andδ¹⁸O_watervalues were 1.99‰~3.98‰,with an average of-37.413‰and 2.99‰respectively indicating that fluids were mixed water and closed to metamorphic water.These results indicate that the source of ore-forming fluids was mainly metamorphic water,and there may be a mixture of magmatic and atmospheric water in the metallogenic process.The ore-forming fluids show the evolution from metamorphic to mixed water.Comprehensive analysis shows that the genetic type of Xiaojianshan gold deposit is orogenic gold deposit.Its metallogenic model is that the metamorphic fluid generated in the early ductile shear deformation process extracts mineralizing materials from rocks during migration,forms gold-bearing ore-forming fluid,and crystallizes at fissures such as mylonite facies,leading to the initial enrichment of gold.In the late stages,with the rapid uplift of the crust,the geological body changes from ductile deformation to brittle-ductile and brittle deformation,accompanied by the intrusion of granite veins,the metamorphic fluid leaches and extracts gold and other metallogenic materials from the rock in the course of migration,forming ore-bearing fluids.Ore-forming materials in the fluid are unloaded and precipitated in favorable areas where fissures or faults develop to form gold orebodies with the mixing of magmatic water and atmospheric precipitation,as well as the reduction of depth and pressure.