南岭加里东期钨锡矿床成矿机制研究:以湘西南落家冲矿床为例

Ore-forming mechanism of Caledonian W-Sn deposit in the Nanling Range:A case study from the Luojiachong deposit,sourthwestern Hunan Province

近来南岭地区加里东期钨锡找矿勘查取得较大突破,但其成因机制研究仍相对缺乏。湘西南苗儿山岩体西北部的落家冲矿床是近年新发现的加里东期钨锡矿床,具有良好的钨锡多金属矿找矿前景,其成矿流体的特征和成矿机制有待查明。在详细的野外地质调查基础上,落家冲矿床的成矿过程可划分为四个阶段:蚀变花岗岩-白钨矿阶段(Ⅰ)、云英岩-白钨矿-锡石阶段(Ⅱ)、石英脉-白钨矿阶段(Ⅲ)和石英脉-硫化物阶段(Ⅳ)。本文选取第Ⅰ阶段的锆石和锡石开展了U-Pb定年和锆石Hf同位素分析,对四个成矿阶段的石英矿物进行了流体包裹体以及H-O同位素的研究。获得的锡石U-Pb反等时线年龄(433.0±11Ma)与蚀变花岗岩锆石U-Pb年龄(430.7±2.3Ma)在误差范围内一致,表明矿区的成岩成矿作用均发生于加里东晚期。流体包裹体研究显示第Ⅰ阶段到第Ⅳ阶段,均一温度分别集中在260-380℃、260-320℃、200-320℃和180-220℃之间,呈逐渐降低的趋势,盐度则表现为由第Ⅰ阶段到第Ⅱ阶段陡降,第Ⅱ阶段到第Ⅳ阶段总体变化不大的特点(平均值分别为16.9%、3.9%、4.8%和3.7%NaCleqv)。成岩成矿定年结果、各成矿阶段的成矿流体特征,结合锆石εHf(t)=-7.6--5.0、tDM2=1565-1711Ma,和石英矿物的δ18OH_(2)O值从第Ⅰ阶段(3.97‰-5.34‰)到第Ⅳ阶段(-4.99‰--5.10‰)逐渐降低的特点,以及区域岩体的地球化学特征,本文认为落家冲钨锡矿床是加里东期起源于元古宙地壳重熔的岩浆在经历了高分异演化作用后,流体的沸腾作用以及温度下降造成成矿流体中钨锡等矿质的沉淀所形成。

Recently,many Caledonian W-Sn deposits have been discovered in the Nanling Range,whereas their ore-forming mechanism are still unclear.The Luojiachong deposit in the northwestern Miao ershan batholith,southwestern Hunan Province,is a newly-discovered Caledonian W-Sn deposit.It is considered to be a promising tungsten-tin reserve,whilst the fluids evolution processes and ore genesis of the deposit still remain enigmatic.Based on detailed geological observation,the mineralization process can be divided into four ore-forming stages,namely altered granite-scheelite stage(Ⅰ),greisen-scheelite-cassiterite stage(Ⅱ),quartz-scheelite stage(Ⅲ)and quartz-sulfide stage(Ⅳ).We present new cassiterite and zircon U-Pb ages and zircon Hf isotopes of stageⅠ,as well as detailed fluid inclusion,H-O isotopic studies of quartz in all stages to constrain the ore genesis of the Luojiachong deposit.The cassiterite Tera-Wasserburg U-Pb age(433.0±11Ma,MSWD=0.75)is consistent with the zircon 206 Pb/238 U weighted mean age(430.7±2.3Ma,MSWD=0.05)of biotite monzogranite,suggesting that the W-Sn mineralization is spatial-temporally associated with the Late Caledonian biotite monzogranite.Fluid inclusion microthermometry shows that the homogenization temperatures of the fluid inclusions decrease from stageⅠto stageⅣ(260~380℃for stageⅠ,260~320℃for stageⅡ,200~320℃for stageⅢand 180~220℃for stageⅣ,respectively),whereas the salinities of the fluid inclusions drastically decrease from stagesⅠtoⅡand within a narrow range from stagesⅡtoⅣ(ave.16.9%,3.9%,4.8%and 3.7%NaCleqv,respectively).The ages of granite emplacement and mineralization,the characterics of ore-forming fluids,combined with zirconεHf(t)values(-7.6~-5.0)and two-stage model ages t DM2(1565~1711 Ma),theδ18 O H 2O value of quartz decreses from stageⅠ(3.97~5.34‰)to stageⅣ(-4.99~-5.10‰),as well as the reported geochemical analysis of Miao ershan batholith,we suggest that the ore-forming material in Luojiachong deposit is derived from the partial melting of Proterozoic crustal which experiences highly fractionation.The fluid boiling and temperature reduction are the effective ore-forming mechanisms for the Luojiachong W-Sn deposit.