扬子陆块北缘马元铅锌矿床成矿物质来源探讨: 来自C、O、H、S、Pb、Sr同位素地球化学的证据

呈层状、似层状产于震旦系灯影组角砾状白云岩层间构造带中的马元铅锌矿床是近年来在扬子陆块北缘铅锌找矿的新突破。文章通过碳、氧、氢、硫、铅和锶同位素地球化学特征研究,探讨了成矿流体和成矿金属来源。研究结果表明:矿石中热液脉石矿物的δ13CPDB为-4.24‰~0.93‰,δ18OSMOW为15.92‰~23.24‰,表明成矿流体中的CO2为震旦系碳酸盐岩的溶解成因。矿石中硫化物的δ34S变化于12.94‰~19.4‰之间;硫酸盐矿物的δ34S为32.2‰~33.48‰,表明还原硫主要来自地层中海相硫酸盐的还原。矿石硫化物的铅同位素组成均一,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb分别为17.62~18.02、15.49~15.63和37.57~38.35,成矿金属可能主要来源于震旦系—志留系。脉石矿物石英流体包裹体的δDFI为-92‰和-113‰,如果取成矿温度200℃,根据δ18O石英值计算的相应流体包裹体的δ18O水为6.03‰~12.73‰,推测成矿流体可能起源于大气降水为主的盆地卤水,或为其他来源的流体与有机质反应形成。成矿流体87Sr/86Sr为0.70967~0.71146,高于赋矿围岩震旦系灯影组白云岩锶同位素比值(0.70890~0.70945),表明成矿流体流经了古生代地层(及基底),并与其中具有高锶同位素比值的碎屑岩、页岩和泥岩等进行了水岩反应及同位素交换。

Sources of metallogenic materials for Mayuan Pb-Zn deposit on northern margin of Yangtze Plate: Evidence from C, O, H, S, Pb and Sr isotope geochemistry

The newly-discovered Mayuan Pb-Zn deposit occurs in bedded and stratoid forms and lies in a structural zone of dolomite breccias in the Sinian Dengying Formation on the northern margin of Yangtze landmass.C,O,H,S,Pb and Sr isotope geochemistry was analyzed in this paper in order to understand comprehensively the source of the ore metals and ore fluids.The results show that δ13CPDB values of the hydrothermal gangue minerals calcite and dolomite are-4.24‰~0.93‰,δ18OSMOW are 15.92‰~23.24‰,indicating that the CO2 in the ore-forming fluid was derived from the dissolution of Sinian carbonates.The δ34SV-CDT values of sulfides are 12.94‰~19.4‰,and the δ34S values of barite are 32.2‰~33.48‰,suggesting that the sulfur should be derived from the in-situ thermochemical reduction of sulfates from the sedimentary strata.The 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb values of the ores are in the range of 17.62~18.02,15.49~15.63 and 37.57~38.35 respectively,and the normal Pb model ages are 418~662 Ma,so the ore metals were likely derived from the Sinian-Silurian strata.The δDFI values of fluid inclusions in quartz display a range from-92‰ to-113‰ and the δ18OH2O values calculated from δ18O quartz values range from 6.03‰ to 12.73‰,if their formation temperature was 200℃.These data indicate that the ore fluid was likely meteoric water or resulted from the reaction of organic matter with fluids from other sources,such as sea water and mixing water.The initial 87Sr/86Sr values of the ore-forming fluid are 0.70967~0.71146,higher than 87Sr/86Sr values(0.70890~0.70945) of the host rock Sinian dolomite,implying that the ore-forming fluid probably migrated through the basement and the Paleozoic formation,and experienced water-rock reaction and isotope exchange between the ore fluid and the shale,clastic rocks and mudstones which had high 87Sr/86Sr values.