An Analysis Of Geological Characteristics And Sedimentary Environment Of Shallow Polyhalite-bearing Potassium Deposit In Nongle Area, Quxian County, Sichuan Province

Polyhalite,as a kind of compound K-bearing sulphate and commonly agreed indicator for potassium salt deposit,plays an important role in investigation of mineral resource distribution on various types of potassium salt.Based on the knowledge of Nongle's shallow polyhalite-bearing potassium deposit,combined with regional geological features,to learn the ore-forming mechanism and occurrence of the widely distributed polyhalite can be instructive on guiding potassium salt deposit exploration.Nongle polyhalite deposit is located at Quxian County,Guang'an City,right on the north of Huayingshan anticline yet close to its core of pitching end.As an evaporite salt deposit,Nongle polyhalite deposit in Quxian County occurs in Triassic System strata,which existed so close to the ground surface(about 200 meters underneath).On the basis of data summarizing and study on previous work in study area,guided by theories of outcrop sedimentology,sedimentary petrology,sedimentary geochemistry,lithofacies and palaeogeography,and mineral deposit,the study is combined with more precised field survey and microscopic analysis.Comprehensive studies focused on surrounded region and Nongle polyhalite deposit in Quxian County,covering stratigraphic units,depositional system,together with lithofacies and palaeogeography and evolution of the stratigraphic units on Triassic Fifth Member of Jialingjiang Formation and First Member of Leikoupo Formation leads to the following conclusions.Outcropping geologic formations in Nongle polyhalite deposit at Quxian County include the Triassic Leikoupo Formation and Xujiahe formation,with overlying Jurassic and Quaternary Formation in some part of the region.The deposit was not cut through by any big fault,and existing polyhalite bodies were primarily shaped by folds,while then by small fractures on the ground surface;small-sized anticlines and synclines arrayed from north to south within the deposit.Polyhalite occurs as thin layers with gypsum which prevented ground water away from the salt,and it is strictly in accordance with gypsum layer in occurrence,shape,and folding.Genesis of polyhalite can be classified into sedimentary type and metasomatic type according to its occurence characteristics.Gypsum salt occurs interbeded with the Triassic marine carbonates,with obviously cyclical sequences;in macroscopical point of view,environment of Sichuan basin in Early Triassic Epoch consisted by Sabhkas of long-lasting dry and hot weather and uplift and decending terrain caused by geotectonic differentiation;according to combinative study on study on lithologic association,biocompatibility,sedimentary structure and geochemical characteristics show that deposition of salt happened in Barrier Island System where tidal flat and lagoon existed,and it is also considered that the Fifth member of the Jialingjiang Formation and the Third Member of Leikoupo Formation were formed in marine shallow water carbonate platform and the restricted platform,that salt deposited mostly in Supratidal Zone and Intertidal Zone.Algae contributed to deposit transformation after genesis of salt later in burial stage.Together with tectonic movement,they affected the migration and accumulation of brines.Studies of sedimentary facies are focused on comparison of lithology facies,borehole logging curves,and sedimentary facies distribution in space and time.Comparison of lithology facies shows obvious Intertidal Zone and Supratidal Zone characteristics of fifth section of Jialingjiang Formation;And Subtidal Zone features appears in third section of Jialingjiang Formation;yet transgressive strata indicated by argillaceous limestone sometimes appear in the area.Analysis of borehole logging curves shows high value in polyhalite and other polyhalite bearing rocks,with no clear layering indicator in resistivity;Polyhalite shows high anomaly results in natural gamma logging curves,which can be easily recognized mainly as Box shape or Peak shape.Related research was conducted on the micro level,metallogenic mechanism shows that polyhalite in Nongle Deposit mainly formed by metasomatism;however,sea water as sources material and supplement of salt,can also cause the dissolution of potassium-magnesium salt and desalting of concentrated brines.It is also recongnized by research that widely distributed “green pisolites” in the area which formed by volcanic glassy tuff has obvious positive relation with polyhalite.