摘要:
斯弄多银铅锌(金)矿床位于拉萨地体隆格尔—工布江达弧背断隆带南侧上,是冈底斯北缘Pb-Zn-Ag成矿带中段上发现的首例低硫化型浅成低温热液矿床.矿体赋存在林子宗群火山岩中,受火山活动形成的近南北向次级断裂控制.成矿过程可划分为伊利石-绢云母-玉髓-碧玉±铁锰碳酸盐(I),石英-黄铁矿±方铅矿±闪锌矿(II),方铅矿-闪锌矿-石英-黄铁矿-绢云母±银矿物±碳酸盐(Ⅲ),辉银矿-硫砷铜银矿-方铅矿±深红银矿±自然银±闪锌矿±黄铜矿(IV),以及石英±碳酸盐±绢云母(V)等五个阶段.本次选取了34件Ⅲ阶段和12件V阶段样品进行包裹体岩相学、显微测温、包裹体的气相及离子色谱和Si-H-O同位素研究,探讨了斯弄多矿床的成矿流体性质、来源和成矿沉淀机制.研究结果表明:斯弄多矿床主成矿期石英和闪锌矿中发育纯液相型(PL)、富液相水溶液型(WL)和富气相水溶液型(WG)三种类型的包裹体,后期无矿石英脉中仅发育PL和WL两种类型.其中Ⅲ阶段石英均一温度为185.6~290.7℃,盐度介于1.57%~5.71%,密度为0.76~0.92 g/cm3;闪锌矿均一温度为197.2~280.8℃,盐度介于1.22%~5.71%,密度为0.75~0.99 g/cm3;Ⅴ阶段石英均一温度147.6~218.7℃,盐度介于2.07%~8.94%;密度为0.88~0.98 g/cm3.成矿流体均具有低温、低压、低密度的特点.Ⅲ阶段石英包裹体中气相成分主要以H2O和CO2为主;液相成分中阳离子以Ca2+、K+为主,阴离子以SO2–4、Cl–和F–为主.Ⅱ阶段成矿流体的δ30Si分布范围为–0.2‰~0.1‰,δD值介于–151‰~–177‰,计算获得的δ18O H2O值介于–5.02‰~–0.63‰.Si-H-O同位素结果表明,斯弄多矿床成矿流体主要为大气降水;包裹体中大量CO2和SO2–4、F–等酸性气体和离子成分表明流体中仍保留微弱岩浆水的特征.斯弄多矿床Ⅲ阶段流体为不混溶流体,流体沸腾、气体逃逸、热液隐爆是金属物质快速沉淀富集成矿的主要机制.
Abstract:
The Sinongduo silver-lead-zinc (gold) polymetallic deposit is located on the southern side of Longge'er–Gongbujiangda arc fault belt of the Lhasa terrane. It is the first discovered low sulfidation epithermal type deposit in the middle of the Pb-Zn-Ag metallogenic belt on the northern margin of Gangdise. The orebody occurs in the volcanic rocks of the Linzizong Group, and is controlled by the secondary northwest faults formed by the northwest volcanic activity. The mineralization process can be divided into five stages: illite-sericite-chalcedony-jasper±iron-man carbonates (I); quartz-pyrite±galena±sphalerite (II); galena-sphalerite- quartz-pyrite-sericite±silver minerals±carbonate (III); argentite-peaceite-galena±pyrargyrite±sphalerite±chalcopyrite (IV); and quartz±carbonates±sericite (V). In this paper, systematic studies of fluid inclusions from 34 samples of stage III and 12 samples of stage V and Si-H-O isotopes of inclusions were conducted. Based on the previous research, this paper discussed the nature and source of mineralization and the mechanism of mineralization precipitation. Three types of fluid inclusions were distinguished in various kinds of quartz and sphalerite of the main ore-forming period in this deposit, which primarily include the pure liquid (PL), liquid-rich two-phase (WL) and gas-rich two-phase (WG) inclusions. There are only PL and WL types in the late barren quartz-vein. The homogenization temperatures, salinities and densities of the quartz in stage III vary from 185.6℃ to 290.7℃, 1.57% NaCl eqv to 5.71% NaCl eqv and 0.76 g/cm3 to 0.92 g/cm3, respectively. The homogenization temperatures, salinities and densities of the sphalerite in stage III vary from 197.2℃ to 280.8℃, 1.22% NaCl eqv to 5.71% NaCl eqv and 0.75 g/cm3 to 0.99 g/cm3, respectively. The homogenization temperatures, salinities and densities of the quartz in stage V vary from 147.6℃ to 218.7℃, 2.07% NaCl eqv to 8.94% NaCl eqv and 0.88 g/cm3 to 0.98 g/cm3, respectively. Ore-forming fluids are characterized by low temperature, low pressure and low density. The gas composition of the quartz inclusions of stage III mainly consists of H2O and CO2. The cations in the liquid phase of the quartz inclusions of stage III are mainly Ca2+ and K+, and the anions are mainly SO2– 4, Cl– and F–. Theδ30Si distribution range of the ore-forming fluid is –0.2‰~0.1‰, theδD value is between –151‰ and –177‰, and theδ18OH2O value is between –5.02‰ and –0.63‰. The results of Si-H-O isotope studies show that the ore-forming fluids are mainly atmospheric water. Large quantities of CO2, SO2– 4, F– and other acidic gases and ionic components in the inclusions indicate that the the inclusions still retain the characteristics of weak magmatic water. The ore-forming fluid in stage III is immiscible fluid and the fluid boiling is the main mechanism of enrichment and mineralization of metallic materials in the Sinongduo epithermal deposit.
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