Sedimentary characteristics and origin of polyhalite in Lop Nur Salt Lake,Xinjiang
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The modern salt crust geomorphy formed in the process of Lop Nur Lake drying up includes 3 basic types that are Flat Salt Crust,Chapped Salt Crust,and Small Mound Salt Crust respectively.Further the Chapped Salt Crust is divided into Polygon Structured Salt Crust,Compound Polygon Structured Salt Crust,and Honeycombed Structured Salt Crust,and so on.The different types of modern salt crust stands for different special stages of salt crust evolution.Under the synthetically influence of environmental factors the modern salt crust geomorphy has evoluted along the order from Flat Salt Crust to Chapped Salt Crust,to Small Mound Salt Crust,and to Flat Salt Crust eventually.Because of the shorter formation time the mineral composition of different types of modern salt crust in Lop Nur region is similar,i.e.50%～92% of salt crust is Chloride,1.7%～11% is Sulfate,and 0%～2.9% is Carbonate.The chemical difference of them is that there is negative correlation between the content of Chloride and the content of Sulfate and Carbonate.During the process of the evaporation,salt crust formed when different salts crystallized out in the order of their solubility and accumulated on the lakeside.The horizontal distribution of their chemical contents indicates the Lop Nur Lake,once been the center of water storage and salt accumulation in the Tarim Basin,was just a shallow salty lake several meters deep in 1958.During the process of evaporation,from the bank to the center of the lake,first calcite,then sulfate(Gypsum),and finally chloride crystallized out.Therefore,the content of rock salt(NaCl) is the most in salt crust.The lake center dried up last and the chloride in the halogen-containing water was highly concentrated.As a result,the content of rock salt(NaCl) was as high as 89%～92% in the lake center region.
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Results of this study indicates that the Salado Salt is composed primarily of fine to coarse-grained halite with polyhalite, anhydrite, and clay minerals. Other minerals detected in small amounts include gypsum, magnesite, quartz, feldspar, sylvite, carnallite, celestite(question), glauconite, and kainite(question). Petrographic evidence (hopper crystals, and intergrowth of halite with other minerals) indicate that the Salado Salt was deposited in rather shallow water and may have been exposed subaerially at times. There must have been a major change in environmental conditions between the deposition of the Castile Formation and that of the Salado. The evidence also suggests that fluids have been able to move through the Salado Salt along beds and seams of clay and silt and somewhat along fractures. Water loss upon heating to 102 +- 5/sup 0/C ranges from 0.0 to 3.5 percent, considerably lower than those for Lyons, Kansas samples. Most of the dehydration water at 100/sup 0/C comes from clay minerals, while at higher temperatures, polyhalite contributes. The rock units in Salado Salt seem to release much less water when dehydrated than the Hutchinson Salt rocks at Lyons. During preparation of some of the samples, H/sub 2/S and possibly some natural gas were released when the samples were crushed. (DLC)
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Abstract Nitrate deposits are rare worldwide, especially potassium nitrate deposits; furthermore, their genesis remains disputed. There is a rare salt-lake type potassium nitrate deposit in the Dawadi area of Lop Nor at the eastern margin of the Tarim Basin, and the ore bodies show coexisting solid and liquid phases. Additionally, there are large sulphate-type potash deposits in the adjoining Luobei Depression, south of the Dawadi area. To determine why there are two different types of potash deposits in adjacent depressions with similar climates, field geological surveys were conducted and samples collected. It was found that the Tertiary clastic layer at the periphery of the Dawadi deposit was rich in high-salinity brine, with nitrate contents of up to 495–16,719 mg/L, much higher than those in the Luobei Depression, 1–35 mg/L. Additionally, a type of deep hydrothermal (Ca–Cl) brine was found in the fault zones, with nitrate contents of up to 8044 mg/L, dozens of times greater than that of ordinary groundwater. Using comprehensive analysis and research, we concluded that the Dawadi and Luobei depressions belong to different hydrological systems with no connection between them; thus, the two deposits belong to different metallogenic systems. Furthermore, groundwater played an important role in the mineralization of the potassium nitrate deposit, and a deep source may have been an important source of the ore-forming materials. The fault system widely developed in Lop Nor provides favorable channels for deep hydrothermal recharge, and the groundwater and deep hydrothermal brine could provide the source for the nitrate mineralization in the Dawadi Depression through water–rock reactions.
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Lop Nur is a playa located in the east of Tarim basin, Xinjiang, China. A plenty of brine is reserved in the upper evaporates of the upper-middle Pleistocene strata in Luobei depression, Lop Nur. The brines have higher salinity and are rich in potassium, with average 1.40% tenor of KCl. And high percentage of tritium in the upper ground water is measured, with average content of 40.6 TU. Laterally the tritium content variation is: higher in periphery and lower in center; while vertically the tritium content becomes lower with the burial depth increasing. With the use of piston model and completely-mixing model, the age of the upper brine is calculated as 36—80 a. Therefore, brine contained large portion of nuclear-bombing tritium, which verified that since 1952, atmospheric water had precipitated and seeped into the playa of this area. The tritium-bearing modern precipitated water is imbibed into sediments through the aeration zone, in which karst is developed intensively, and tritium mixed with shallow brine uniformly. Owing to the slow evaporation of brine, fractional distillation happened, and “lighter water” with less tritium evaporated. So most of tritium has been rich in relic brine. The characteristics of lateral tritium distribution are believed to be the results of groundwater flow pattern in a salt lake basin.
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The purpose of this paper is to discuss from a hydrochemical and physico-chemical view point the formation of recent potash deposits in a salt lake lying in a Quarternary depression of a certain basin in China. Its surface brine, which varies seasonally and annually in composition, is of MgSO4 subtype and is highly minera-lized with a low metamorphic coefficient (MgSO4/MgCl2 = 0.029). These deposits are distributed at the northern and northeastern banks of the lake, far away from river-water supply. Mineralogical composition of the deposits is quite simple, consisting mainly of halits and carnallites. The formation of natural potash took place as the concentration of surface brine reached a certain point during evaporation. Rate of precipitation, composition and grain size of the carnallite sediment are largely de-pendent upon changes in brine concentration. The presence of these recent potash deposits in present-day basins and continental salt lakes demonstrates that continental salt-bearing basins should likewise be taken as promising areas for ancient potassium salt prospecting.
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The brine must have evaporated to reach the late stage of sylvinite precipitation and must have continued to concentrate the salts for a long time. Then, it is possible to form potash deposit. Hence, this paper, examines the isotopic composition of different sequences of halite, sylvite and carnallite minerals for man made chemical solutions and salt lake brine evapo-concentrated precipitations, ancient potash deposits in an attempt to examine whether chlorine isotopic ratios can be used to study potash deposits and then be utilized to prospect for new potash deposits in the Tarim Basin and western Qaidam Basin. The results show that δ~(37)Cl values of the precipitates or salt deposits decreased systematically during the main phase of halite crystallization. Generally, the sequence of δ~(37) Cl values in salt rock is early precipitated halite (0‰) late precipitated halite (0‰) sylvite (-0.5‰) carnallite (-1.0‰). Hence, δ~(37) Cl in salts can be used as an indicator of evaporation stage. Based on this, we conclude that the concentration of ancient brines formed ancient salt rocks is much more concentrated in Tarim-Shache subbasin than that in Tarim-Kuche subbasin. Particularly, the δ~(37)Cl values of salt rock in Kashi depression of western Shache subbasin is greatly negative (-1.00‰) , which may predicate that the ancient brine has been concentrated to the terminal stage of salt deposit or even later stage of sylvinite deposit. Then it is possible to find sylvite bearing deposits around the western region of the Shache subbasin. But to the western Qaidam Basin, all the 8 Cl values of Tertiary salt rock deposit samples from Youdunzi, Nanyishan, Shizigou and Tulingou depressions show obviously positive, which indicate that they are early halite deposits and the primary brine do not evapo-concentrate to reach the late stage of potash bearing minerals precipitation. Therefore, there maybe have no much more hope to find potash deposit in Tertiary salt rock deposit of western Qaidam Basin. The late stage of highly evapo-concentrated brines possibly moved to the south area.
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