西秦岭加甘滩金矿成因研究:毒砂Re-Os定年、载金矿物原位微量元素和硫同位素限定  

Genesis of the Jiagantan gold deposit in West Qinling,China:Constraints from Re-Os dating,in-situ trace elements and S isotope of gold bearing minerals

作　　者：陈炳翰 张勇[1] 李康宁 CHEN BingHan;ZHANG Yong;LI KangNing(MNR Key Laboratory of Metallogeny and Mineral Assessment,Institute of Mineral Resources,Chinese Academy of Geological Science,Beijing 100037,China;The Third Geological and Mineral Exploration Institute of Gansu Geological and Mineral Exploration and Development Bureau,Lanzhou 730050,China)

机构地区：[1]中国地质科学院矿产资源研究所,自然资源部成矿作用与资源评价重点实验室,北京100037 [2]甘肃省地质矿产勘查开发局第三地质矿产勘查院,兰州730050

出　　处：《岩石学报》2024年第6期1767-1783,共17页Acta Petrologica Sinica

基　　金：国家重点研发计划(2022YFC2903300);科技基础资源调查专项(2022FY101704);中国地质调查局地调项目(DD20230040、DD20230055)联合资助.

摘　　要：西秦岭加甘滩金矿是夏河-合作地区最大的金矿,但其矿床成因存在争议,其中主要问题在于该矿床成矿时代缺乏研究。通过野外工作和镜下观察,识别出两种毒砂和黄铁矿:毒砂Apy1(较为常见,在矿石中呈浸染状产出)和毒砂Apy2(数量较少,围绕黄铁矿Py1产出),黄铁矿Py1(浸染状黄铁矿)和黄铁矿Py2(黄铁矿脉)。本次工作对毒砂进行Re-Os定年,以期确定加甘滩金矿的成矿时代:通过对Apy1进行了Re-Os定年,获得毒砂的Re-Os等时线年龄243.1±8.7Ma,该年龄与夏河-合作区域岩浆作用(232~249Ma)相对应。利用LA-ICP-MS和LA-MC-ICP-MS对成矿期毒砂和黄铁矿进行了微量元素和硫同位素测定。微量元素测试结果显示加甘滩金矿硫化物内的不可见金以固溶体形式赋存,Apy2的不可见金含量最高可达521×10^(-6),其他毒砂和黄铁矿不可见金含量最高为119×10^(-6)。加甘滩金矿的平均品位为2~3g/t,依据质量平衡计算,如果毒砂和黄铁矿内不可见金发生再活化,不可见金含量需要达到20~30g/t,但实际金含量远低于所需,需要额外的岩浆热液来源。Py1、Py2、Apy1和Apy2的原位硫同位素结果分别为-14.3‰~-7.70‰(平均-10.0‰)、-14.5‰~-9.60‰(平均-12.5‰)、-13.0‰~-6.40‰(平均-8.54‰),-11.2‰~-9.00‰(平均-10.1‰),反映其硫同位素发生了分馏。将加甘滩金矿成矿期硫化物的硫同位素与成岩期和典型岩浆热液型黄铁矿硫同位素进行对此,指示其成矿流体为岩浆热液来源。综上,本文研究指示加甘滩金矿属于岩浆热液成因。The Jiagantan gold deposit in the West Qinling Orogeny is the largest gold deposit in the Xiahe-Hezuo region.However,its genesis remains controversial,mainly regarding to its metamorphic or magmatic origin.One main conflict stem from the lack of records on its mineralization time.Through field and microscopic observations,two types of arsenopyrites and two types of pyrites have been identified:Apy1(common and disseminated in the ore),Apy2(rare and occurred around Py1),Py1(disseminated pyrite)and Py2(pyrite veins in the ore).In this study,the Re-Os test on arsenopyrite of Apy1 is taken to determine the metallogenic time,from which the isochron age of 243.1±8.7Ma was obtained,corresponding to magmatism in the Xiahe-Hezuo region(232~249Ma).LA-ICP-MS and LA-MC-ICP-MS were performed on the ore stages of arsenopyrite and pyrite to determine the trace element contents and sulfur isotope values,respectively.Trace element content shows the invisible gold in the sulfide occurs as solid solution in Jiagantan gold deposit.The invisible gold content in Apy2 was 521×10^(-6)at most,while the gold content in other sulfides was up to 119×10^(-6);the grade of Apy2 is 2~3g/t,indicating that additional gold of magmatic origin would have been required even if reactivation occurred.The in-situ sulfur isotope test showed that theδ34 S values of Py1,Py2,Apy1 and Apy2 ranged from-14.3‰to-7.70‰(-10.0‰on average),-14.5‰to-9.60‰(-12.5‰on average),-13.0‰to-6.40‰(-8.54‰on average),and-11.2‰to-9.00‰(-10.1‰on average),respectively,which shows the fractionation of sulfur isotopes.Comparing the sulfur isotope compositions of syngenetic pyrite and magmatic pyrite,the sulfur isotope values of ore stage sulfides indicate a magmatic hydrothermal origin.The results suggest that the Jiagantan gold deposit should be a magmatic hydrothermal gold deposit.

关 键 词：毒砂 RE-OS定年 微量元素 硫同位素 加甘滩金矿 

分 类 号：P597.2[天文地球—地球化学] P618.51[天文地球—地质学]