内蒙古巴音戈壁盆地塔木素碱矿Na-碳酸盐成因模式

Genetic Model of Na-Cabonate in Tamusu Trona Deposit,Bayingobi Basin,Inner Mongolia

内蒙古巴音戈壁盆地哈日凹陷下白垩统巴音戈壁组沉积时期为典型的碳酸盐型碱湖,盆地咸化过程中发育大量的Na-碳酸盐类矿物.通过对巴音戈壁组碱矿层中似层状、斑点状和脉状Na-碳酸盐矿物开展了电子探针、X衍射、碳氧同位素和激光拉曼光谱等分析,结果表明碱矿层中Na-碳酸盐矿物主要为天然碱、苏打石、碳钠钙石、碳钠镁石和磷碳镁钠石.除Na-碳酸盐外,岩石中含少量黄铁矿、钠型菱沸石、硅硼钠石和钠长石等热液矿物.碳、氧同位素研究结果表明巴音戈壁盆地下白垩统碱矿层形成于封闭的碱湖环境,碳酸盐矿物形成温度为34~80℃(平均值57℃),受热水喷流沉积作用和蒸发作用双重控制,碱矿层中硅酸盐矿物首先形成,然后形成白云石和方解石,在Ca^(2+)、Mg^(2+)消耗殆尽后,天然碱和苏打石发生沉淀,由于热水带来充足的Na^(+),前期形成白云石和方解石发生交代作用形成碳钠钙石和碳钠镁石.在矿物学和地球化学综合分析基础上,建立了热水喷流和蒸发沉积双重控制下的碱湖Na-碳酸盐岩沉积模式,以期为碱矿勘查提供新的思路.

The Lower Cretaceous Bayingobi Formation in the Hari sag,Bayingobi basin,Inner Mongolia,is a typical carbonatetype alkaline lake during the depositional period,and a large amount of Na-carbonate minerals developed during the salinization of the basin.In this paper,electron probe,X-diffraction,carbon and oxygen isotope and laser Raman spectroscopy were carried out to analyze the layered,speckled and vein-like Na-carbonate minerals in the trona layers of the Bayingobi Formation.The Na-carbonate minerals in the trona layer are mainly composed of trona,nahcolite,shortite,eitelite and bradleyite,with a small amount of hydrothermal minerals such as pyrite,chabazite-Na,searlesite and albite.The results of carbon and oxygen isotope testings show that the Lower Cretaceous trona layers in the Bayingobi basin were formed in a closed alkaline lake environment,the Na-carbonate minerals were formed at a temperature range of 34-80℃(average 57℃).Under control of exhalative deposition and evaporation,silicate minerals were first formed in the trona layers,and then dolomite and calcite were formed,after Ca^(2+)and Mg^(2+)were almost consumed,trona and nacholite were precipitated.With sufficient Na^(+)brought by the hydrotherm,the early formed dolomite and calcite underwent metasomatism to form dawsonite and dawsonite.Based on the comprehensive analysis of mineralogy and geochemistry,the Na-carbonate sedimentary model of alkaline lake is established,which is controlled by hydrothermal exhalation and evaporative deposition,it can provide a new idea for the exploration of trona deposit.