云南江城勐野井钾盐成矿新认识

A New Viewpoint concerning the Formation of the Mengyejing Potash Deposit in Jiangcheng,Yunnan

本文通过深入分析滇西南以往地质调查研究资料,而从中发现存在的疑点,从新特提斯形成局限海的成盐背景和滇西南特定的复杂的地质构造条件出发,通过大量地面地质调查、沉积相、盐构造及水化学研究,推断勐野井地区勐野井组中马尾丝状钾盐矿体可能是沿断层由深部侏罗纪含钾岩系挤压塑流到表层来的。因财制宜,选用低频大地电磁法(EH4)、高精度电磁频谱探测法(MES)轻便适用的物探手段,同时结合以往重力勘查(1/10万),综合分析表明,该区深部有多组盐层存在。为验证上述找钾新思路,部署了2口钾盐中深钻,其钻探成果与该认识相一致。其中第1钻井(MK-1)已完钻,在中侏罗统花开左组中部钻遇厚70 m含钾岩盐层;第2钻井(MK-2)已钻遇花开左组上部含膏岩系。与此同时,还进行了Sr同位素和包体测温以及盐体构造和沉积特征等分析。综上所述,论证了勐野井钾盐成矿机制,提出了深部侏罗系盐底辟浅部贯入的"二层楼成矿模式"。以此为指导,部署了区域性高精度重力测量(1/5万)、MES探测和浅钻工程,拓展了兰坪—思茅成盐带深、浅部找钾前景。

Through an intensive analysis of the data obtained from previous geological survey and study in southwestern Yunnan, the authors found some doubtful points. Starting with the salt-forming setting of the restricted sea formed in Neo-Tethys and the specific and complex geological and structural conditions of southwestern Yunnan and based on considerable ground and hydrochemical studies, the authors infer that the geological surveys and sedimentary facies, salt structure horsetail potash beds in the Mengyejing Formation of Mengyejing area were squeezed up and flowed in the plastic form through faults from the deep Jurassic potassic rocks onto the surface. In this study, light, portable and practical geophysical means such as the low-frequency magnetotelluric method （EH4） and high-precision method for detecting the electromagnetic spectrum （MES） were used; in addition, the previously-used gravity survey （1：100,000） was also employed. An integrated analysis indicates that there exist several salt beds at the depth. For the purpose of verifying the above-mentioned new idea of potash search, two potash intermediately deep boreholes were drilled, and the drilling results are consistent with this idea. Of the two boreholes, one （MK-1） has been completed and encountered a 70 m thick potash-bearing halite bed in the central part of the Middle Jurassic Huakaizuo Formation, and the other （MK-2） has intersected a gypsum-bearing sequence in the upper part of the Huakaizuo Formation. Meanwhile, Sr isotope study, inclusion thermometry and analyses of the structure and sedimentary features of the salt body were carried out. In summary, the authors demonstrate the formation mechanism of the Mengyejing potash deposit and put forward a ＂two-storey potash-forming model＂, i.e., Jurassic salt body lies in the deep part and diapirism in the shallow part. Guided by this model, the authors deployed regional, high-precision gravity survey （1：50,000）, MES spectrum and shallow drilling engineering, thus expanding prospects of potash exploration in the deep and shallow parts of the Lanping-Simao salt-forming belt.