柴达木盆地西部砂岩型铀矿赋矿地层稀土元素地球化学特征及其地质意义

REE Geochemical Characteristics and Geological Significance of Sandstone⁃type Uranium Hosted Strata in Western Qaidam Basin

针对柴达木盆地西部新近系狮子沟组和上油砂山组共25件泥岩和粉砂岩样品进行了稀土元素和物源地球化学指标(La/Sc、Th/Sc、Th/Co、Th/Cr)分析,探讨了研究区狮子沟组和上油砂山组的沉积物源。测试数据显示,25件泥岩和粉砂岩样品的稀土元素总量介于72.03μg/g~187.01μg/g之间,平均含量为150.14μg/g,与全球平均大陆上地壳含量相当,但略低于北美页岩的平均值。稀土配分模式呈现右倾斜型,显示负Eu异常。Hf⁃La/Th和ΣREE⁃La/Yb判别图解显示,研究区样品物源具有明显的上地壳特征,主要源岩为花岗岩类,无基性物质的混入。盆地周缘富铀花岗岩作为主要物源,奠定了新近系狮子沟组和上油砂山组的高铀背景,为区内砂岩型铀矿的形成提供丰富的铀源基础。本文也从铀源角度对已发现矿床的成因进行了探讨,提示应重视新发现层位的找矿工作。

A total of 25 mudstone and siltstone samples collected from the neogene Shizigou formation and the Shangyoushas⁃han formation in the western Qaidam basin were analyzed.Based on the abundance and distribution patterns of REEs,and in combination with other geochemical indices(La/Sc,Th/Sc,Th/Co,Th/Cr),the sediment sources of the Shizigou formation and the Shangyoushashan formation in the study area were discussed.The results show that theΣREE content of these samples ranges from 72.03μg/g to 187.01μg/g,with an average value of 150.14μg/g,which is close to the global average continental upper crust composition and slightly lower than that of North American Shale.The chondrite⁃normalized patterns of REEs show a right⁃inclined trend with negative Eu anomaly.The discriminant diagrams of Hf versus La/Th andΣREE versus La/Yb both in⁃dicate that the provenance of the samples is mainly from the upper crust,dominantly composed by granitoids,mixed with a small amount of ancient sediments,without the input of basic materials.The uranium⁃rich granites in the periphery of the basin,as the main provenance,not only give rise to the high uranium background of the Neogene Shizigou formation and the Shangyoushashan formation,but also lay a rich uranium source foundation for the formation of sandstone type uranium deposits in the area.From the perspective of uranium source,the genesis of the discovered uranium deposit is also discussed,which suggest more impor⁃tance should be attached to the prospecting work in the newly discovered strata.