鄂尔多斯盆地北部砂岩型铀矿直罗组物源分析及其铀成矿意义

鄂尔多斯盆地北部直罗组原生灰色砂岩具有高铀背景值的特征,在层间氧化阶段砂岩同沉积期富集的铀元素遭受氧化迁出构成该区铀成矿的重要铀源。本文对鄂尔多斯盆地北部铀矿区直罗组砂岩进行了碎屑锆石U-Pb定年、重矿物和古水流分析,深入分析了该区直罗组的沉积物源,并探讨了富铀砂岩的成因。结果显示:矿区直罗组砂岩碎屑锆石U-Pb年龄主要集中在251~308Ma,322~354Ma,1529~2182Ma,2200~2632Ma四个年龄区间;富Mn钛铁矿、锆石、磷灰石和榍石的重矿物组合指示物源主要为中酸性岩浆岩;通过与源区对比分析认为铀矿区直罗组物源主要来自盆地之北的乌拉山—大青山地区和狼山东部地区的新太古代、古元古代和晚古生代中酸性岩浆岩及新太古代、古元古代变质岩。结合源区岩体铀含量特征分析,发现晚古生代中酸性岩浆岩相对于源区其它岩体强烈富集铀元素,是研究区直罗组高铀背景值砂岩形成发育的主要原因。晚古生代中酸性岩浆岩的形成与古亚洲洋的演化密切相关,其分布特征可以作为中东亚成矿域内盆地铀资源远景预测的重要依据。

Provenance Analysis of the Zhiluo Formation in the Sandstone hosted Uranium Deposits of the Northern Ordos Basin and Implications for Uranium Mineralization

The primary gray sandstone of the Middle Jurassic Zhiluo Formation in the northern Ordos Basin is characterized by high uranium content. Uranium extracted from the syn depositional U rich sandstone through oxidation was the key source for uranium mineralization. In this paper, U Pb detrital zircon geochronology, heavy mineral analysis and paleocurrent analysis were used to document sediments sources of the Middle Jurassic Zhiluo Formation in the northern Ordos Basin and to understand the origin of the U rich sandstone. The results show that U Pb ages of the detrital zircons from the Zhiluo Formation concentrate on four groups: 251~308Ma, 322~354Ma, 1529~2182Ma and 2200~2632Ma. Heavy mineral assemblages of Mn rich ilmenite, zircon, apatite and titanite suggest intermediate felsic magmatic rocks were the main source rocks. Comparative study of potential sources shows that the Zhiluo Formation in the study area was derived mainly from Neoarchean Paleoproterozoic magmatic and metamorphic rocks and late Paleozoic magmatic rocks in the Wulashan Daqingshan area and the eastern Langshan area. Statistical analysis of uranium contents from source rocks shows that the late Paleozoic intermediate felsic magmatic rocks are extremely enriched in uranium than other types of rocks, which caused the formation of the primary U rich sandstone in the northern Ordos Basin. Late Paleozoic magmatism was related to the subduction collision evolution of the Paleo Asian Ocean. The distribution of the late Paleozoic magmatic rocks possibly controlled the distribution of pre ore uranium concentration and formation of the uranium deposits in the Central Asia uraniferous province and can be an important factor for the uranium exploration of other basins in this uraniferous province.