桂西巴平石英矿床成因:来自流体包裹体、C-H-O同位素及稀土元素证据

Genesis of the Baping quartz deposit in the western Guangxi:Evidences from fluid inclusions,C-H-O isotopes and rare earth elements

为探讨碳酸盐岩区石英矿床的成因机理,对桂西巴平石英矿床中的水晶、脉石英、方解石和赋矿围岩(白云岩)进行了分析测试。流体包裹体岩相学及激光拉曼分析表明,水晶和脉石英中的流体包裹体主要为气液两相包裹体,可见富CO_(2)两相及三相包裹体,包裹体的液相成分为H_(2)O和CO_(2),气相成分为H_(2)O、CO_(2)和少量N_(2)。流体包裹体的均一温度为160~312℃,盐度为0.41%~6.16%NaCl_(eqv),属CO_(2)-H_(2)O-NaCl体系的中-低温、低盐度、低密度热液流体。水晶和脉石英的稀土元素配分模式呈轻稀土富集右倾型,具有明显的Eu、Tb、Ho、Yb负异常,推测其母岩浆流体是亏损Eu的,成矿流体与深部的隐伏花岗岩体有关。方解石的稀土元素配分模式呈明显右倾型式,具有轻稀土富集和轻微的Eu正异常,指示其形成于氧化环境,白云岩的Ce负异常指示其形成于低温氧化环境。水晶及脉石英的δD_(SMOW)和δ^(18)OH_(2)O分别为-75.5‰~-54.1‰和5.72‰~7.06‰,指示成矿流体主要为岩浆热液;方解石及白云岩的δ^(13)CPDB分别为-3.92‰~-3.35‰和2.41‰~3.59‰,δ^(18)O_(SMOW)分别为18.84‰~19.13‰和23.25‰~27.45‰,表明方解石的CO_(2)主要来源于围岩的海相碳酸盐岩。成矿过程中深部岩浆热液沿深大断裂侵入,富Si流体沿断裂、裂隙运移过程中与碳酸盐岩围岩发生水岩相互作用,大量的CO_(2)进入到热液中。成矿环境的pH值、压力的降低及流体沸腾作用促使方解石和SiO_(2)开始沉淀,并随着温度和压力持续降低,H_(4)SiO_(4)溶解度不断降低,SiO_(2)大量沉淀,最后形成石英矿床。

In order to explore the genetic mechanism of the quartz deposit in the area of carbonate rocks,quartz crystals,quartz veins,calcites and wallrocks(dolomite)in the Baping quartz deposit,Guangxi,China,have been taken for geochemical analysis in this paper.Results of petrographic observation and laser Raman analysis of fluid inclusions in quartz crystals and veins of the deposit show that fluid inclusions are mainly vapor-liquid two-phase ones,with minor CO_(2) liquid-rich two-phase ones and CO_(2) vapor-liquid three-phase ones locally.Their components of liquid phases include H_(2)O and CO_(2),and those of vapor phases include H_(2)O and CO_(2) and a small amount of N_(2).Meanwhile,microthermometric measurements of fluid inclusions show that homogenization temperatures range from 160℃ to 312℃,with corresponding salinities varying from 0.41% to 6.16% NaCl_(eqv),indicating the ore-forming fluid belongs to the medium-low temperature,low salinity,low density hydrothermal fluid of the CO_(2)-H_(2)O-NaCl system.Rare earth elements(REE)analyses show that the REE distribution patterns of quartz crystals and veins are characterized with slightly enriched LREE,obviously negative anomalies of Eu,Tb,Ho,Yb,indicating that the parent magmatic fluid was Eu-deficient,and that the oreforming fluid was related to the hidden granite body in depth.Calcites have characteristic obviously right-declined REE distribution patterns,with enriched LREE and slight positive Eu anomaly,indicating that calcite was formed in an oxidized environment.The negative Ce anomalies of dolomite indicate that it was formed in a low-temperature oxidized environment.The δD_(SMOW) and δ^(18)OH_(2)O values of quartz crystals and veins of the deposit vary from -75.5‰ to -54.1‰,5.72‰ to 7.06‰,respectively,indicating that the ore-fluid was mainly sourced from the magmatic hydrothermal fluid.The δ^(13)CPDB values of calcites and dolomite vary from -3.92‰ to -3.35‰ and 2.41‰ to 3.59‰.The δ^(18)O_(SMOW) values of calcites and dolomites vary from 18.84‰ to 19.13‰ and from 23.25‰ to 27.45‰,respectively,suggesting that the CO_(2) in calcite was primarily derived from marine carbonates in wallrocks.It is believed that the deep magmatic hydrothermal fluid(Si-rich fluid)was migrated along the deep and large major fractures,and interacted with carbonate of the wallrock through the water-rock interaction in the transporting process of Si rich fluid along fractures and fissures to have formed the ore-forming fluid containing a large amount of CO_(2).The decrease of pH value and pressure of the metallogenic environment and the boiling effect of the ore-forming fluid had prompted the precipitation of calcite and SiO_(2).Then the continuous decrease of temperature and pressure resulted in the continuous decrease of H_(4)SiO_(4) solubility and the continuous precipitation of a large amount of SiO_(2) in the evolved hydrothermal fluid.Finally the quartz deposit was formed.