老挝甘蒙省钾镁盐矿床含矿段的矿物学和地球化学特征及成因

Mineralogical and geochemical characteristics and genesis of the potassium-magnesium salt deposit in Khammouan Province,Laos

老挝甘蒙省钾镁盐矿床位于呵叻盆地的东隅,本文以该矿床ZK318钻孔的含矿段———下盐层为研究对象,对其开展了系统的矿物学和地球化学特征研究。结果表明,矿石矿物以石盐、钾石盐和光卤石为主,含少量方硼石和硬石膏。根据矿物与主量元素的分布特征,将下盐层分为5个次层,由下至上顺序为:盐岩层(石盐为主)→钾盐岩层(钾石盐为主)→光卤石岩层(光卤石为主)→薄盐岩层(石盐为主)→钾盐岩层(钾石盐为主)。微量元素Br值均大于200×10-6,表明卤水来源于海水;由盐岩层→钾盐岩层→光卤石岩层,B含量呈现出逐渐升高的趋势,表明B含量与卤水盐度呈线性关系。但顶部钾盐岩层的B含量高达890×10-6,与少量方硼石的存在相吻合,指示成矿卤水除了海水外,还有深部热液的贡献。按照卤化物卤水蒸发浓缩的顺序,光卤石是该卤水浓缩的最后阶段,但在光卤石沉积之后,又晶出一薄层石盐,表明曾经历过一次短暂的淡水补给。上、下两层钾石盐具有基本相同的Br含量,分别为1 790×10-6和1 792×10-6,且均直接沉积在石盐之上,故为原生钾石盐,而非光卤石的次生淋滤产物。认为甘蒙省钾镁盐矿床为海源陆相沉积,海水和深部热液是成钾物质的主要来源,钻孔中出露的两层钾石盐均为原生沉积。

The potassium-magnesium salt deposit in Khammouan Province of Laos is located in eastern Khorat Basin which belongs to upper Cretaceous salt sedimentary basin.The formation of the deposit underwent three sedimentary cycles which formed three salt sections,i.e.,upper,middle and lower sections,and every salt section includes two layers,namely salt layer and clay layer.The ore is located in the lower salt layer of lower salt section.This paper mainly studied the mineralogical and geochemical characteristics of the lower salt layer based on ZK318 well.The results show that halite,sylvite and carnallite are main minerals,with small amounts of boracite and anhydrite.In combination with the characteristics of minerals,major elements and trace elements,the lower salt layer can be divided into five layers in upward order of halite layer→ sylvite layer→ carnallite layer→ thin halite layer→ upper sylvite layer.Bromine（Br） values（higher than 200×10^-6） show that the brine came from sea water,and carnallite occurred at the highest stage of brine concentration,as shown by its highest Br value of 2 572×10^-6 among five layers.Boron content showed a gradual increase from halite layer through sylvite layer to carnallite layer,suggesting that boron content is in linear correlation with salinity of the brine.Boron value of the upper sylvite layer is up to 890×10^-6,which is consistent with the existence of boracite mineral revealed by XRD,indicating that ore brine came from sea water and deep hydrothermal solution.In addition,according to the evaporative order of halide brine,the order of crystallization was halite→sylvite→carnallite,and carnallite seemed to be the last sedimentary period in halide brine.However,in ZK318 well,there appears a thin layer of halite above the carnallite layer,which shows that it underwent a short period of fresh water supply which result in the decrease of brine concentration in the basin,but soon the concentration continued to increase with the replenishment of deep hydrothermal solution,followed by the precipitation of the upper sylvite layer.Thus,the sedimentation of the lower salt layer was over.Therefore,the upper and lower sylvite layers are both primary sedimentary layers because they overlie the halite layer directly,and have the same content of boron,being 1 790×10^-6 and 1 792×10^-6,respectively.In summary,potassium sources of the ore deposit were sea water and deep hydrothermal solution,and two sylvite layers in the well are both primary sediments.