云南思茅盆地钾盐矿床的深源浅储成因模式——来自于Sr同位素的证据

【研究目的】思茅盆地赋存有中国唯一的前第四纪固体钾盐矿床，该矿床的成因一直存在争议。客观地认识矿床成因、合理地构建矿床成因模式，不仅是钾盐矿床学研究中亟需解决的基础科学问题，而且关乎盆地内钾盐资源勘查方向的选择。【研究方法】本项研究以思茅盆地L-2井和MZK-3井的盐岩、盐岩上覆和下伏碎屑岩、盐岩中的碎屑岩为主要研究对象，重点分析其锶同位素地球化学特征。【研究结果】结果表明：（1）L-2井全岩样品⁸⁷Sr/⁸⁶Sr值为0.708220~0.727458，平均值为0.712776；盐岩水不溶物⁸⁷Sr/⁸⁶Sr值为0.711342~0.741999，平均值为0.716574；（2）MZK-3井盐岩上覆碎屑岩层⁸⁷Sr/⁸⁶Sr值为0.713318~0.723147，平均值为0.717255；盐岩下伏碎屑岩层⁸⁷Sr/⁸⁶Sr值为0.712470~0.738988，平均值为0.719307；（3）碎屑岩样品经过87Rb校正恢复至初始沉积状态的⁸⁷Sr/⁸⁶Sr值为0.710880~0.727678，平均值为0.712828；（4）盐岩样品⁸⁷Sr/⁸⁶Sr值全部明显小于大陆地表风化系统的平均值，有个别样品⁸⁷Sr/⁸⁶Sr值大于现代海水。【结论】基于思茅盆地基础地质和钾盐矿床地质已有的研究成果，结合盐岩和碎屑岩锶同位素地球化学特征，可以得出：思茅盆地含钾盐岩与碎屑岩处于不同的锶同位素平衡体系；含钾盐岩的物源主体为海水，成盐过程中陆源淡水的补给可使⁸⁷Sr/⁸⁶Sr值增大；碎屑岩沉积于陆相环境，在与固态盐岩或盐岩水溶液接触之前已处于早成岩阶段A亚期；钾盐的成矿模式为勐野井组沉积期深层源盐通过走滑拉分形成的断层迁移至地表，在由高处向汇水盆地迁移过程中捕获了处于早成岩阶段A亚期的碎屑颗粒，形成了现今的含泥砾盐岩；部分含泥砾盐岩在迁移进入汇水盆地之后，经历了溶解-重结晶的过程，形成了盆地内成分较纯的盐岩；后续沉积的碎屑颗粒形成了防止盐岩溶蚀破坏的保护层；新生代的喜山运动使早期形成的钾盐矿床发生调整改造。

The deep source and shallow mineralization model of potash deposits in the Simao Basin: Evidence from Sr isotope

This paper is the result of the geological survey engineering. Objective]The Simao Basin hosts the only pre-Quaternary solid potash deposit in China, but the genesis of this deposit has been still controversial. An objective understanding of deposit genesis as well as rational construction of a metallogenic model is not only fundamental scientific issues that need to be addressed urgently in the study of potash mineral deposits, but also have a bearing on the choice of the direction for potash resource exploration in the basin. Methods]This study focuses on Sr isotope geochemical characteristics of the samples from salt rocks, overlying and underlying clastic rocks and clastic rocks within the salt rocks in Wells L-2 and MZK-3 of the Simao Basin. Results]The results show that: (1) The bulk-rock ⁸⁷Sr/⁸⁶Sr values from the Well L-2 samples are 0.708220-0.727458, with an average of 0.712776; the ⁸⁷Sr/⁸⁶Sr values of water-insoluble matter within the salt rock are 0.711342-0.741999, with an average of 0.716574; (2) The ⁸⁷Sr/⁸⁶Sr values of the clastic rock overlying and underlying the salt layer in Well MZK-3 range from 0.713318-0.723147 and 0.712470-0.738988, with an average of 0.717255 and 0.719307, respectively; (3) The ⁸⁷Sr/⁸⁶Sr values of the clastic rock corrected by 87Rb tore cover their initial sedimentary state are 0.710880-0.727678, with an average of 0.712828; (4) The ⁸⁷Sr/⁸⁶Sr values of salt rock are all significantly lower than the average value of the continental weathering system, with some individual samples having ⁸⁷Sr/⁸⁶Sr values larger than modern seawater. Conclusions]Based on the existing research results of the basic geology and potash deposit geology in the Simao Basin, combined with the Sr isotope geochemical characteristics of salt rocks and clastic rocks, it can be concluded that: The potash-bearing salt rocks and clastic rocks are in different Sr isotope equilibrium systems; the matter source of potash-bearing salt rock is mainly seawater, and the recharge of terrestrial freshwater into the evaporation basin during the salinization process would increase ⁸⁷Sr/⁸⁶Sr values of the samples. The clastic rocks were deposited in the terrestrial environment and were in the eodiagenesis substage A before contacting with solid salt rock or brine; a more rational potash metallogenic model is that the deep source salt migrated to the surface through faults formed by strike-slip pull-apart process during the deposition period of the Mengyejing Formation, capturing the unconsolidated clastic rocks in the eodiagenesis substage As they migrated from the height to the catchment basin, and forming the present-day clastic-bearing salt rocks. After migrating into the catchment basin, parts of the clastic-bearing salt rocks underwent dissolution and recrystallization, resulting in the formation of salt rock with relatively purer composition. The subsequent deposition of the clastic rocks formed a protective layer against rock salt dissolution. Early-formed potash deposits were influenced by the Cenozoic Himalayan movement, leading to the modification not only in physical structure but also in mineral component.