滇东南都龙锡多金属矿床中闪锌矿微量元素组成特征及其地质意义

Trace Element Characteristics of Sphalerite from Dulong Tin-polymetallic Deposit in Southeast Yunnan and Their Geological Significances

都龙锡多金属矿床位于滇东南老君山锡多金属矿集区南部,是我国最大的锡石硫化物矿床之一。除Sn、Zn、Cu外,该矿床中还大量伴生富集了In等元素,其主要载体为闪锌矿,但目前伴生元素的富集机理和物理化学条件还不清楚。本文通过野外调查和详细的矿相学工作,在该矿床中识别出三个不同世代的闪锌矿,并对其开展了电子探针和LA-ICP-MS原位成分分析。分析结果显示,闪锌矿总体上相对富集In、Mn、Co和Fe,贫Ga、Ge和Sn。三个世代闪锌矿中主量、微量元素组成具有明显差异,从第一世代闪锌矿(黑色)→第二世代闪锌矿(棕红色)→第三世代闪锌矿(黄色),In、Fe、Cu和Cd含量明显降低,Zn和Co含量显著升高。闪锌矿的Fe/Zn、Ga/In和Zn/Cd值指示成矿温度为中高温,且第一世代闪锌矿→第二世代闪锌矿→第三世代闪锌矿的成矿温度逐渐降低。In的富集具有“铟爆”效应,即呈爆发式富集于第一世代高铁闪锌矿中。In理想的富集环境可能是较高的温度和流体中较充分的Cd^(2+)、Fe^(2+)和Cu^(+)离子浓度,替代机制可能以In^(3+)+Cu^(+)↔2Zn^(2+)为主。该矿床中闪锌矿微量元素组成特征明显不同于MVT型、VMS型、SEDEX型铅锌矿床,与岩浆热液型矿床特征类似,进一步证明该矿床是与燕山晚期岩浆热液活动密切相关的中高温热液交代成因矿床。

The Dulong tin-polymetallic deposit is located in the southern part of the Laojunshan polymetallic concentration area,SE Yunnan province,and it is one of the largest cassiterite-sulphide deposits in China.Trace elements such as In,Cd and Ag are enriched in sphalerite from the Dulong deposit,however,the enrichment mechanism and physicochemical conditions are still unclear.Based on systematic field investigation and detailed mineralogical study,three generations of sphalerite(SpⅠ,SpⅡand SpⅢ)were identified.The trace element compositions of the sphalerite were analyzed by using of EMPA and LA-ICP-MS.The results show that the sphalerite is characterized by high Co,In,Mn and Fe,low Ga,Ge,and Sn.SpⅠ,SpⅡand SpⅢhave obviously different compositions of major and trace element compositions.From SpⅠ→SpⅡ→SpⅢ,the contents of In,Fe,Cu and Cd reduce significantly,while the contents of Zn and Co increase.The Fe/Zn,Ga/In and Zn/Cd ratios indicate that the sphalerite are mainly formed in medium to high temperature,and from SpⅠto SpⅢ,the mineralization temperature decreases gradually.The Cd content of the sphalerite ranges from 861×10^(-6)to 2268×10^(-6),which is lower than those of other similar deposits,such as the Dachang deposit.Indium is preferentially enriched in the high-iron sphalerite(SpⅠ),shows an“indium explosion”effect.Generally,the In-enriched sphalerite has relatively high cadmium content.The In-bearing sphalerite features the dominant coupled substitution of In^(3+)+Cu^(+)↔2Zn^(2+).Therefore,medium-high temperature and sufficient Cd^(2+),Fe^(2+)and Cu^(+)in the ore-forming fluids are conducive to indium enrichment of sphalerite.Trace element compositions of the sphalerite in the Dulong deposit are obviously different from those of MVT,VMS and SEDEX type lead-zinc deposits,while similar to that of magmatic-hydrothermal deposits.Combined with geological characteristics and trace element compositions of the sphalerites,the Dulong deposit is likely formed through metasomatism of the carbonate rocks in the Xinzhai Fromation by magmatic hydrothermal fluids derived form the Late Yanshanian granite.