Geochemical Research On The Provenance And Sedimentary Characteristics Of Potash Deposits In The Khorat Plateau

The prospective reservoirs of potash resources in the Khorat Plateau are more than26 billion tons?K₂O?,making this evaporites one of the world's largest potash deposits.The deposit is located in the northeastern Thailand and south-central Laos.It is in the same tectonic belt with the northern Mengyejing potash deposit in Simao Basin,China.The researchers have carried out plentiful research on the sedimentary environment,ore-forming age and mineralogy of the deposit.However,there are still controversies about the origin of the deposit?marine,continental,hydrothermal?and the recharge path of solutes?west,east or northeast?.Meanwhile,the study on spatial deformation during the deposition process of this deposit is insufficient.Therefore,"the origin-recharge path-depositional process"of this deposit constitutes the main contents of the dissertation.In-depth study of the ore-forming rules of this potash deposit is of theoretical and practical significance to enriching the forming theories of potash deposits and prospecting potential potash deposits in China.In terms of the origin of potash deposit,two boreholes of evaporite?anhydrite and chloride salts?and five samples of fissure brine in potash mines were collected in the north Sakhon Nakhon Basin and the south Khorat Basin in the Khorat Plateau and analyzed for the elemental concentrations and multi-stable isotopes(?¹¹B,⁸⁷Sr/⁸⁶Sr,?³⁴S,?³⁷Cl).The main conclusions include that:?1?through the petrographic observation,combined with the geochemical characteristics of elements,the anhydrite and gypsum are primary or quasi-primary minerals;?2?through the"phase transformation and chemical separation"method,boron isotopes of gypsum and anhydrite are determined;combination of elemental analysis,X-ray diffraction analysis,scanning electron microscopy,and comparison of the occurrence forms of boron in different evaporite minerals,proved that boron mainly occur in the crystal lattice of anhydrite by co-precipitation;?3?boron isotopes of the basal anhydrite and the anhydrite?+8.20‰⁺27.34‰,average+16.08‰?from salt layers,and mother brine?+38.20‰⁺57.34‰,average+46.08‰?indicate the marine origin of this potash deposits is comparable with those of the Cretaceous seawater?+37.5‰⁺42.5‰?;this conclusion is corroborated by sulfur?+14.39‰⁺15.94‰,average+15.40‰?and strontium?0.70748⁰.70770?isotopes of the anhydrite/gypsum;chlorine isotopic compositions of halite also indicate that the origin is mainly marine,and there is a small amount of non-marine inflow in the late evolutional stage of brines,which is related to the closure of lake basin;meanwhile,the distribution of rare earth elements in halite and anhydrite and their respective mother liquors?reconstructed based on partition coefficient?is comparable with those of marine evaporites and Cretaceous seawater,which also indicates that the source of the deposit is related to seawater;?4?The elemental concentrations and hydrogen-oxygen-boron isotopes of fissure brine from the potash layer indicate that this brine is mainly derived from surface water infiltrated and dissolved the evaporites,rather than the residue brine.Regarding the migration path,comprehensive comparisons of the mineral assemblages,elemental contents,and B-S-Cl isotopic compositions of basal halite between the Simao and Khorat potash deposits reveal that the seawater evolves from the north to south and then to east of the Khorat Plateau;the Mesozoic tectonic evolution and comparisons of stratigraphic features in the Indosinian Block and surrounding blocks show that the Lhasa block and the Qiangtang block have been collided and sutured during the Early Cretaceous?120Ma?,This process caused the Bangong-Nujiang Ocean retreated westward.While the Shanbang Ocean in the West Burma block and the Sibumasu block still exists and the Tengchong-Baoshan blocks in the western Simao Basin is still in the coastal environment at the Early Cretaceous.Therefore,we proposed that the western part of the Simao Block may be the seawater entrance and the seawater enters Simao Basin from Tengchong,then flows to the Khorat Plateau in the south and finally extends to eastern Laos.Through studying the thickness of salt and potash layers in the Lower Member and spatial distribution characteristics of K and Mg contents from hundreds of boreholes in the two basins,it is found that the spatial distribution of the potash deposits in the Khorat Plateau present“thick potash-thin salt in the north,thick potash-thick salt in the south,thin potash-thick salt in the east,and disproportionate scales of rock salt and potash”,and“thick evaporite-shallow buried,thin evaporite-deep buried,salt-potash deposits occurred in same anticlines”.Based on the controlling factors and sedimentary features of the deposits,and tectonic evolution,we suggested that these spatial characteristics were resulted from the primary basin morphology,the tectonic uplift of the eastern plateau and the differential loading in the basin.In summary,the study proposes that the potash deposit in the Khorat Plateau is mainly formed by the transgression of the Meso-Tethys Ocean during the Middle and Late Cretaceous.The seawater migrated from the north Simao block to the south Khorat Plateau.When the potassium salts precipitated,the brine migrated to the west due to the tectonic uplift of the eastern part of Khorat Plateau.After buried,the evaporites deformed and presented salt anticlines.This study provides certain scientific basis for future development of the potash deposits in the Indo-China Peninsula and exploring of the potential potash deposits in China.