西藏吉瓦地区渐新统日贡拉组物源分析及找矿意义

摘要

通过详实的野外调查和室内研究，在西藏吉瓦地区新发现了砂岩型铜矿床，赋矿层位为渐新统日贡拉组，矿床类型为层控矿床。为探讨日贡拉组砂岩的物源特征及其构造背景、查明其含矿物质来源，通过碎屑矿物定量分析、元素地球化学方法及重矿物组合分析等一系列物源分析方法对日贡拉组的物质来源进行了研究。结果显示，研究区主要岩性为岩屑砂岩，岩屑主要成分为酸性火山岩，砂岩结构成熟度低，分选磨圆差。碎屑组分分析表明物源集中在火山弧物源区，地球化学特征为硅质含量高、LREE 富集、HREE 相对亏损、显示 Eu 负异常，均表明物源与酸性火山岩密切相关；日贡拉组砂岩的大地构造背景主要为大陆岛弧，砂岩碎屑来自上地壳长英质源区。重矿物组合以反映物源为中酸性岩浆岩成分的赤褐铁矿＋磁铁矿、锆石、电气石、石榴子石为主，沉积环境为气候干旱、水体较浅的富氧环境。锆石形态特征指示物源距母岩区较近，重矿物的相关性分析也指示了物源与火山岩密切相关。研究区的日贡拉组砂岩与早白垩酸性火山岩微量元素及重矿物的对比表明，碎屑物质源区特点从岩石学特征、地球化学特征及重矿物组合特征上均表现出了亲缘关系，物源成分与火山作用紧密相关，很可能主要来自班公湖怒江洋壳南向俯冲与雅鲁藏布江洋壳北向俯冲双重制约条件下产生于火山弧环境中的早白垩世火山岩。日贡拉组发现了砂岩型铜矿，火山岩提供了成矿物质来源，为寻找同类型的矿床开启思路。%Many detailed field surveys and comprehensive researches have been made to find sandstone copper deposit in Jiwa area,Tibet.Oligocene Rigongla formation sandstoneswere the ore-bearing horizon.The deposit type is strata-bound deposit.Series of provenance analysis technique,for instance,quantitative detrital minerals analysis,elementary geochemical methods and heavy minerals combination analysis,are used to explorethe provenance features and tectonicbackground of Rigongla sandstones.The main lithology is lithic sandstone with a poor sorting and psephicity and low maturation of texture.The detrital compositions hintthe main setting background of provenance is continental island arc,the geochemical characteristics are high value of siliceous,enrich in LREE,relatively depleted in HREE,and negative Eu anomalies.They all show that the provenance is closely related to acid volcanic rocks,the main setting background of Rigongla formation sandstone is continental island arc,the lithic sandstones came from felsic part of upper crust,heavy mineral combinations mainlyarehematite-limonite and magnetite,zircon,tourmaline,garnet,which were related to intermediate-acid magmatic rocks. Sandstone was deposited in dry,restricted shallow water and oxidizing environment.The morphological characteristics of zircons indicate that provenance is close to mother rocks.The correlation analysis of heavy minerals also shows that provenance is closely related to volcanic rocks.Comparingto acid volcanic rocks of the lower stratum and early cretaceous volcanic rocks in this area,trace elements and heavy mineral sreveal that the characteristics of detrital material source area were showed genetic relationship from petrology characteristics, geochemical characteristics and heavy mineral assemblages. The provenance of fragments were closely related to volcanic activities,may come from the acid volcanic rocks of early Cretaceous associated with the southward subduction of the Bangong Co-Nujiang oceanic crust and northward subduction of the Yalung Zangbo oceanic crust.Sandstone copper deposits was found in Rigongla Formation,volcanic rock provides the source of ore-forming materials.