Abstract
The deep brines enriched in potassium (K), lithium (Li), boron (B) resource elements in sedimentary basins are valuable potential mineral resources. There is a widely accepted consensus that evaporite and brine resources were formed in terminal salt lakes, however, newly discovered sand-gravel brine (SGB) occurred in the piedmont alluvial fan zone on the northern Qinghai-Tibetan Plateau (QTP). Currently, there is a key scientific issue on where does the solute of SGB in the alluvial fan zone come from? The existing understanding is that the solute of high Na+, Cl− originates from halite dissolution by meteoric water, but lack of isotope geochemical linkage between brine and evaporite. The Mahai Basin (MHB) on the northern QTP is a natural experimental site for the study on the above unsolved problems, due to harboring similar brine reservoir, small geographical area, thin evaporite layers and various waters in this basin. This study presents detailed analysis of H-O-Cl-Li isotopic compositions and hydrogeochemical parameters for intercrystalline brine, SGB and halite from drilling cores in the MHB. The results reveal several key findings: (1) the SGB in the piedmont alluvial fan zone of Altyn Tagh and Saishiteng Mountains presents similar geochemical characteristics of low TDS, high Na+, Cl− and relatively high K+ contents (1.12–3.65 g/L in MHB), which established new occurrence model of brine resource in the ‘Fan-Lake’ system. (2) The δD (−56.90 ‰∼−17.60 ‰) and δ18O (−5.70 ‰∼6.00 ‰) values of SGB in the MHB are higher, the isotopic disparity exists in these two mountain-fan-lake systems, but they are meteoric origin and accord with their respective hydrologic circulation process. (3) The CNa/CCl molar ratio (∼0.80), δ37Cl (−0.26 ‰∼+0.25 ‰) and δ7Li values (+33.84 ‰∼+43.82 ‰) of SGB are comparable with those of intercrystalline brine in the MHB, indicating that inheritance recharge of salt lake brine for the formation of SGB in this basin. (4) The metallogenic model of SGB in the MHB is multi-condition constraints (northward migration of depocenter, hydrological lateral recharge of river water along the alluvial fault zone and strata subsidence along with the mountain uplift), which develops the topographic difference between brine aquifers of salt lake and alluvial fan zone resulting into subsurface brines recharge for the alluvial-pluvial fan reservoir.
Published Version
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