贵州苗龙金锑矿床方解石微量元素、Sr-Nd同位素地球化学特征及其意义  

Characteristics and Implication of Trace Elements and Sr-Nd Isotope Geochemistry of Calcites from the Miaolong Au-Sb Deposit, Guizhou Province, China

作　　者：孙国涛[1,2] 沈能平[1] 苏文超[1] 俸月星[3] 赵建新[3] 彭建堂[1] 董文斗[1,2] 赵海[1,2] 

机构地区：[1]中国科学院地球化学研究所矿床地球化学国家重点实验室,贵州贵阳550081 [2]中国科学院大学,北京100049 [3]昆士兰大学地球科学系放射性同位素实验室

出　　处：《矿物学报》2016年第3期404-412,共9页Acta Mineralogica Sinica

基　　金：国家重点基础研究发展计划973项目(编号:2014CB440904);国家自然科学基金项目(批准号:41272113;40972072);矿床地球化学国家重点实验室"十二五"项目群(编号:SKLODG-ZY125-01);李四光学者创新研究项目

摘　　要：苗龙金锑矿床位于贵州三都-丹寨金锑汞成矿带,矿体赋存于上寒武统三都组海相碳酸盐岩中。本文对该矿床的成矿期方解石微量元素含量、Sr-Nd同位素组成进行了研究。结果表明,该矿床成矿期方解石稀土元素总量变化范围大(5.98~139μg/g,平均值为43.7μg/g),在球粒陨石标准化稀土元素配分模式图中大多数方解石样品具有轻稀土富集特征((La/Yb)_N=1.84~9.18,(Gd/Yb)_N=2.55~6.14),少量样品表现为中稀土富集特征((La/Yb)_N=0.33~1.39,(Gd/Yb)_N=1.29~2.24)。该矿床成矿期方解石样品不具有Ce异常(δCe=0.97~1.19),但具有Eu异常(δEu=0.61~1.72),指示其形成于相对还原条件下。方解石样品的^(87)Sr/^(86)Sr值为0.7108~0.7144,平均值为0.7119,高于赋矿围岩的^(87)Sr/^(86)Sr值(0.7090),表明成矿流体相对富放射成因锶,其不可能主要由赋矿的三都组灰岩提供,而可能来自富放射成因锶的前寒武纪基底。Sm-Nd同位素研究表明,该矿床成矿期方解石样品可构筑两条Sm-Nd等时线,其对应的等时线年龄分别为273±14 Ma和272±43 Ma,但与地质背景不符,因此该次方解石Sm-Nd同位素定年等时线无地质意义。这种假等时线现象可能是由成矿流体初始^(143)Nd/^(144)Nd不均一造成的。方解石ε_(Nd)(0)(-14.02^-9.48)远小于0,指示成矿流体中的Sm和Nd来源于陆壳。Miaolong Au-Sb deposit is hosted in limestone of Upper Cambrian Sandu Formation, located at the Sandu-Danzhai district, Guizhou Province, China. Trace elements, strontium and neodymium isotopic compositions of ore-stage calcites from the deposit were studied. Results show that contents of rare earth elements in calcites range widely（5.98~139 μg/g）. In chondrite-normalized rare earth patterns, most calcites show LREE enrichment（（La/Yb）_N = 1.84~9.18,（Gd/Yb）_N = 2.55~6.14）, while some are enriched with MREE（（La/Yb）_N=0.33~1.39,（Gd/Yb）_N=1.29~2.24）. The calcites do not display Ce anomalies（δCe = 0.97~1.19）, but show Eu anomalies（δEu = 0.63~1.80）, suggesting that calcites are deposited in reductive condition. The ^（87）Sr/^（86） Sr ratios for calcite vary from 0.7108 to 0.7144, with a mean ratio of 0.7119, which are higher than ^（87）Sr/^（86） Sr ratios（0.7090） of the host rocks. It indicates that radioactive strontium in hydrothermal fluids was likely to be transported from Precambrian basement which has high ^（87）Sr/^（86） Sr ratios rather than mobilized from limestone of Sandu Formation. The study of Sm-Nd isotope system shows that ore-stage calcites form two isochrons, corresponding to isochron ages of 273±14 Ma and 272±43 Ma, respectively. However the isochron ages are not coherent geological setting, suggesting that Sm-Nd system in this set of samples cannot be used to date the formation of Miaolong deposit radiometrically. The pseudoisochrons indicate that the calcites may have formed from fluid with heterogeneous initial ^（143）Nd/^（144） Nd. Negative ε_（Nd）（0）（-14.02^-9.48） compositions of calcites indicate a crustal source for the Sm and Nd found in the fluid.

关 键 词：方解石 微量元素 Sr-Nd同位素组成 苗龙Au-Sb矿床 

分 类 号：P579[天文地球—矿物学] P611[天文地球—地质学]