Genesis of the Baping quartz deposit in the western Guangxi:Evidences from fluid inclusions,C-H-O isotopes and rare earth elements
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 CO2 liquid-rich two-phase ones and CO2 vapor-liquid three-phase ones locally.Their components of liquid phases include H2O and CO2,and those of vapor phases include H2O and CO2 and a small amount of N2.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%NaCleqv,indicating the ore-forming fluid belongs to the medium-low temperature,low salinity,low density hydrothermal fluid of the CO2-H2O-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 ore-forming 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 δDSMOW and δ18OH2O 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.Theδ13CPDB values of calcites and dolomite vary from-3.92‰to-3.35‰and 2.41‰to 3.59‰.The δ18OSMOW values of calcites and dolomites vary from 18.84‰to 19.13‰and from 23.25‰to 27.45‰,respectively,suggesting that the CO2 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 CO2.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 SiO2.Then the continuous decrease of temperature and pressure resulted in the continuous decrease of H4SiO4 solubility and the continuous precipitation of a large amount of SiO2 in the evolved hydrothermal fluid.Finally the quartz deposit was formed.
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