北秦岭南台钼多金属矿床成矿流体和稳定同位素研究

Fluid inclusion and stable isotope study of the Nantai molybdenum polymetallic deposit in North Qinling Mountains

南台钼多金属矿床产于北秦岭构造带宽坪群内,矿体主要呈似层状、透镜体状赋存于花岗斑岩内及其与宽坪群大理岩的接触带。矿化主要呈浸染状、团块状和细脉浸染状。围岩蚀变主要为硅化-钾硅酸盐化、矽卡岩化和碳酸盐岩化。矿床的形成经历了高温蚀变-矽卡岩期和石英-硫化物期,钼多金属矿化主要形成于石英-硫化物期。斑晶石英和辉钼矿-石英脉中主要发育4种流体包裹体:L型富液相包裹体、V型富气相包裹体、C型含CO2包裹体和S型含子晶多相包裹体。早阶段斑晶石英中发育140~200℃、220~280℃、340~400℃3个均一温度区间,主成矿期辉钼矿-石英脉中发育120~180℃、200~240℃和280~380℃3个均一温度区间,晚阶段无矿石英脉中仅发育120~240℃一个低温区间。早阶段斑晶石英中的包裹体盐度显示57.90%~>73.96%、30.06%~38.01%、3.39%~18.55%3个不连续的区间,主成矿期的辉钼矿石英脉和晚期无矿石英脉中的盐度范围分别为0.43%~12.85%、1.91%~10.73%。在成矿早阶段的斑晶石英和主成矿期石英辉钼矿脉的3个温度区间,均出现S型、C型、L型、V型等两种或两种以上包裹体共存且均一温度相近,流体沸腾作用明显,表明流体的多次沸腾是南台钼多金属矿床矿质沉淀的重要机制,这一机制与北秦岭秋树湾铜(钼)矿床的成矿机制相似。硫化物的δ34S值集中于-0.3‰~7.2‰,平均3.1‰,表明硫来自深部岩浆。含硫化物石英的δDV-SMOW值介于-103‰^-76‰之间,δ18OH2O的值为4.01‰~5.55‰,表明主成矿阶段的成矿流体主要为岩浆水,混合有大气降水。

The Nantai Mo polymetallic deposit is hosted in the Kuanping Group in the North Qinling belt,and its stratoid and lenticular orebodies occur in the porphyry and the contact zone between the marble of the Kuanping Group and the porphyry.Mineralization occurs as disseminations,lumps and veinlet-disseminations.Wall rock alteration is well developed and mainly comprises potassium-silicate alteration,skarnization and carbonatizatior.The ore-forming process can be divided into two metallogenic periods：skam and quartz-sulfide,and the quartz-sulfide period represents the main molybdenum mineralization episode.Fluid inclusions in quartz phenocrysts and quartz-molybdenite veins can be divided into four types,i.e.,L-type aqueous fluid inclusions,Vtype aqueous fluid inclusions,C-type three-phase CO2-bearing inclusions and S-type multi-phase inclusions containing daughter minerals.Homogenization temperatures of fluid inclusions at the early stage of quartz phenocrysts can be divided into three intervals,i.e.,340～400℃,220～280℃ and 140～200℃,with salinity being 57.90 ％ ～ ＞ 73.96 ％,30.06 ％ ～ 38.01％,3.39 ％ ～ 18.55 ％,respectively.The temperatures of the main mineralization stage in quartz-molybdenite veins were 280 ～ 380℃,200～240℃ and 120～180℃,with salinities ranging from 0.43 ％ to 12.85 ％ w （NaCleq）.The late stage in non-mineralization quartz veins has only homogenization temperatures of 120 ～ 240℃,with salinities ranging from 1.91％ to 10.73 ％ w （NaCleq）.Coexistence of two or more types of fluid inclusions in the three homogenization temperature intervals of the principal mineralization stage with corresponding salinities homogenized to divergent phases at similar temperatures suggests that boiling had already occurred several times during the ore-forming evolution and was probably the dominated factor for metal precipitation.This phenomenon is comparable to things of the Qiushuwan Mo （Cu） deposit in North Qinling.δ34S values of sulfides associated with mineralization range from-0.3‰ to 7.2‰,3.1‰ on average,indicating that sulfur was derived from deep-seated magma.δDv-sMOW values of sulfide-bear-ing quartz vary between-103‰ and-76‰,with corresponding δ18OH2O values between 4.01‰ and 5.55‰,impling that the ore-forming fluids of the main mineralization stage in the Nantai Mo polymetallic deposit were mainly derived from magmatic fluids,with some addition of the meteoric water.