兰坪-思茅盆地勐野井矿区地表钾异常形成的深穿透机理研究

In China, the strategic resource potash is suffering from severe shortages, and the ancient marine solid potash locating is still a problem of long impregnability. Till now, only the Mengyejing Potash Deposit was found in the south of the Lanping-Simao Basin. The basin is a small 70,000 km2 one in southern China, characterized by great development of cretaceous sedimentary evaporites, where strong late-reformation of strata occurred because of tectonic movement. Besides fault belts and saline springs are widely distributed, surface potassium anomaly is found in this basin, and this could be a good clue for potash ore locating. The scientific problem that “whether the surface potassium anomaly is carried up along the fault belts by deep fluids” needs to be answered, and material sources of saline springs and the controlling factors of potassium anomaly need further explanation. In this research, fluid “deep penetration” theories and techniques will be developed and applied in the study of potassium anomaly mechanism in the Mengyejing Ore Deposit Area, by means of element geochemistry, stable isotope geochemistry and noble gas isotope geochemistry. Further more, the role of fluid carriers (water and gas) in the formation of potassium anomaly will be discussed. The aim of this research is to offer theoretical support and credible data for further potash deposit locating in the Lanping-Simao Basin.

钾盐是我国紧缺的战略性资源，古代海相固体钾盐是我国“久攻不破”的难题，目前仅在兰坪-思茅盆地南部发现我国唯一一个固体钾盐矿-勐野井钾盐矿。兰坪-思茅盆地面积七万平方千米，白垩统沉积蒸发岩分布广泛，地层受强烈的构造挤压改造。该盆地断裂发育，盐泉水分布广泛，并在地表发现钾异常，可能是找钾的良好标志。地表钾异常控制因素、盐卤水物质来源等尚需科学解释,“钾是否由流体沿断裂带到地表形成”等科学问题尚需要弄清楚。本项目拟发展深穿透理论和方法，运用元素地球化学、稳定同位素和稀有气体同位素地球化学等分析手段,在勐野井钾盐矿区开展地表钾异常形成机理研究，讨论断裂带-运载流体（水、气）在钾异常形成中的作用，为今后兰坪-思茅盆地钾盐找矿靶区预测提供理论依据和数据支持。

项目通过元素地球化学方法研究了钾等化学元素在地表水体(盐泉、温泉等)、固体沉积物（泉华、测线剖面样品）中的分布特征，对异常点和异常区进行甄别；通过同位素地球化学方法研究了泉水中稳定同位素（氢、氧、锶）、泉水中稀有气体同位素的组成特征，辅助判别钾异常形成的补给源和物质来源；讨论了断裂带和深部补给与钾异常的关系。根据收集资料和采样获得的研究区数百件盐泉水样品数据，绘制了钾及相关离子比值的水化学分布图，由此识别研究区地表钾异常的分布情况；研究发现景谷-宁洱一带及周边，磨铺-宝藏（勐野井）-二官寨一带及周边为地表钾异常区；勐野井矿区附近一处盐泉水钾异常特征明显，为I级异常。泉水沉积物矿物学研究表明，泉水的化学组分与火山岩有密切关系。沿垂直于断裂方向的剖面采集的土壤样品中，钾及相关组分的峰值和谷值的出现有一定的规律性，认为各元素含量异常是由深部流体（水、气）沿断裂上升补给造成。从氢氧同位素组成和矿物学两方面推断，岩浆相关的热液在勐野井矿床的形成中起重要作用，并且它可能是该矿床的主要物质来源之一；Sr同位素组成研究表明，泉水有深部补给特征，Sr的来源与碳酸岩和蒸发岩有关；泉水中稀有气体同位素分析发现，He主要为深部壳源，个别为幔源，由此推断研究区的大断裂切至深部地壳，个别深切到软流圈上部。