鄂尔多斯盆地南部彬县地区侏罗系直罗组古地貌及其对铀成矿的影响*

以鄂尔多斯盆地南部彬县地区侏罗系直罗组下段为研究对象,采用层拉平法精细恢复彬县地区直罗组下段各个时期的古地貌特征并划分古地貌单元,探讨古地貌特征对有利沉积相带分布的控制,揭示直罗组下段铀储集层的空间演化特征,明确不同时期地下水流的流向,探讨古地貌特征对层间氧化带的控制作用。辫状河道砂体为有利的铀储集层,铀矿(化)体主要赋存于辫状河砂体中,受沉积古地貌控制,主要发育在下切谷中。古地貌控制了地下古水流的流向,从而控制着氧化带的发育和铀成矿。晚侏罗世,彬县地区北部为古隆起区,容易被淋滤氧化,不利于铀矿的预富集;早白垩世—古新世主成矿阶段,直罗组底部古地貌变为“东南高、北西低”特征,含氧含铀水主要来自盆地东南缘,呈扇形沿着直罗组下段辫状河道砂体渗流,形成层间氧化带。古地貌恢复对铀储集层砂体的分布特征和铀成矿规律的研究具有重要的指导意义。

Palaeogeomorphic restoring of the Jurassic Zhiluo Formation and its effect on uranium mineralization in Binxian area,southern Ordos Basin

Taking the Lower Member of Jurassic Zhiluo Formation of Binxian area in southern Ordos Basin as research objects,the palaeogeomorphology of each period by using the method of layer flattening were restored accurately and the palaeogeomorphological units were divided. The evolution characteristics of the space of the uranium reservoir were revealed and the flow direction of groundwater in different periods were defined. The controlling effects by the characteristics of palaeogeomorphology on interlayer oxidation zone were discussed. Braided channel sandbodies are the favorable reservoir sand in which the uranium ore bodies mainly occur,controlled by the palaeogeomorphology and mainly developed in the incised valley. Palaeogeomorphology controls the ancient underground water flow and the development of interlayer oxidation zone and uranium mineralization. On the preconcentration stage of uranium in the Late Jurassic,the uplift in northern Binxian,easy to be leached and oxidized,is not conducible to the enrichment of uranium. At the main metallogenic stage in the Early Cretaceous-Paleocene,the palaeotopography of southeast is higher than that in northwest,which leads water bearing oxygen and uranium to flow the braided river sandbodies in fan-shape northwestward,so the palaeo-interlamination oxidized zone and uranium mineralization were formed. Restoring palaeogeomorphology has an important guide to study the distribution of the uranium sandbodies and regularity of uranium ore formation.