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.

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.

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.

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.

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.