Provenance,Ancient Water Temperature,Degree Of Evaporation Of Paleogene Salt Lake And Evaluation Of Potassium-forming Perspective In The Kuqa Basin,Xinjiang

The global potash deposits mainly concentrated in the Tethys domain during the Cretaceous to the Paleogene.The Tarim Basin belongs to the northern branch of the Tethys during late Cretaceous to early Paleogene,namely domain of the Paratethys Sea.Kuqa Basin is the sub-basin of the Tarim Basin,it located in the northeastern of the Tarim Basin and belongs to a part of the ancient salt lake chain in Central Asia.It is one of the key exploration areas for potash deposits in China,which has long been the focus of geologists.However,so far,because the insufficient investment in potash deposits exploration in the Kuqa Basin,resulting in no significant substantive breakthrough.In a way,this situation caused by there are so many important factors have not been find out which affecting potassium formation in the Kuqa Basin:For example,what is the temperature of Paleogene seawater and brine in the Kuqa Basin?Where is saline material from?How is the degree of evaporation and concentration of ancient brine?These problems have not been studied clearly and becoming an important scientific issue that needs to be identified.Therefore,in this paper,by the methods of field geological survey,petrological and mineralogical characteristics,sedimentary geochemistry and isotope geochemistry have been carried out in this thesis,Saline minerals and potash minerals,geochemistry of drilling rock debris,geochemistry of halite crystals and compositions of halite fluid inclusions have been analyzed,with focuses on restoration of chemical evolution information of ancient brine,quantitative reconstruction of paleotemperature information,identification of material source of Paleogene evaporates,discussion of degree of evaporation and concentration of the ancient brine.On the basis of these studies,we evaluated Paleogene potassium-forming potential in the Kuqa Basin,and proposed favorable potassium-forming stratigraphic horizons.Firstly,a lot of potassium or potassium-bearing minerals were found in evaporate stratas.potassium or potassium-bearing minerals that develop in borehole DZK01 of Middle Eocene upper part include glauberite with potassium,syngenite,polyhalite,and sylvite,potassium or potassium-bearing minerals that develop in borehole KL4 of Middle Eocene upper part include primary and secondary sylvite,syngenite,glaserite and gypsum with sylvite,potassium or potassium-bearing minerals that develop in Middle Eocene Wensu salt mine include syngenite and polyhalite.Combined with sedimentary geochemical characteristics of multiple boreholes,Middle Eocene upper part showed favorable potassium-bearing intervals.Secondly,oxygen isotopic compositions of carbonate rocks recover that early Paleocene seawater temperatures ranged from 12.3 to 38.2?,early Eocene water temperature ranged from 41 2 to 60.5?.The homogenization temperatures of halite primary fluid inclusions from borehole QL1 recover that water temperatures of early Eocene ranged from 32.7 to 57.8?,and this high temperatures interval was associated with the intervals of high potassium ion in the upper part of borehole QL1.Base on the study of the homogenization temperatures of halite primary fluid inclusions from boreholes KL4,DW1,DZKO1 and the Wensu salt mine,Middle Eocene brine temperatures ranged from 23.4 to 41.6?,from 24.2 to 43.1?,from 36.2 to 54.1? and from 34.6 to 37.3?,respectively.And the upper high temperature interval of borehole KL4 corresponds to the upper potassium-bearing interval of it,high temperature interval of Middle Eocene(borehole DZKO1)corresponds to a lot of potassium and magnesium minerals.Additionally,the sulfur isotopic compositions of the gypsum or anhydrite indicates that materials of Paleocene gypsum rocks from the Talak outcrop show marine source,materials of Lower Eocene gypsum rocks from the Xiaokuzibai outcrop show that the influxion sources of marine and terrestrial,and materials of Middle Eocene gypsum rocks from borehole DZKOl show that the characteristics of certain marine source supplyment,Miocene gypsum rocks from Kelasu outcrop show that terrestrial source.The 87Sr/86Sr ratio distribution range of the same sample as sulfur isotope shows that both marine and terrestrial sources.At last,the characteristics of the hydrogen and oxygen isotopic composition of halite primary fluid inclusions and the chlorine isotopic composition of halite indicate that the ancient brine have undergone intensive evaporation and condensation.Although it has experienced several transient freshwater infusions,but the continuous evaporation and concretion of ancient salt lakes fail to affect the precipitation of potassium and magnesium saline minerals in multiple intervals.In conclusion,Paleocene-Eocene salt lake in the Kuqa Basin evolved under conditions of seawater recharge from the Paratethys Sea,high temperature“time windows”and highly evaporative capacity.In addition,potassium-magnesium saline minerals develop in multiple intervals and characteristics coefficients of geochemistry indicate that favorable potassium-forming.It is preliminarily believed that Early Eocene upper part and Middle Eocene upper part are two favorable potassium-forming layers.