Study On Formation Conditions And Genesis Mechanism Of Triassic Polyhalite Deposits In Northeast Sichuan Province

Potash is a strategic,scarce resource that relates to national agricultural security,economic construction,and resource security.Polyhalite,as a typical insoluble potassium mineral,is a potential resource of potash.The Triassic strata in the northeastern Sichuan Province is rich in polyhalite resources.There are many different opinions on the formation conditions and the genesis mechanism of the polyhalite deposits.No unified conclusion has yet been reached.The study of the above issues is indicative of the exploration of potash in the northeastern Sichuan Province and even the entire Sichuan Basin,revealing the evolution of marine evaporite in the Sichuan Basin,enriching and improving the study of the Chinese marine phase formation mechanism of potassium,which bears important scientific significance and value.With the development of modern geology,it is necessary to introduce new research methods and ideas such as sequence stratigraphy,sedimentary facies analysis,cyclic stratigraphy,and geophysics into the study of the issues.This paper systematically collected and sorted available documents of salt/potassium,oil/gas exploration,regional geological surveys,geophysical prospecting,drilling,geophysics,and other related research documents in northeastern Sichuan Province.It adopts approaches of field outcrop profile survey measurements,sampling analysis,and drilling log data analysis and other comprehensive methods.The main research results are as follows:1.Through systematically collecting and analyzing predecessors’foundational geological data and engaging in on-site geological survey work,on the basis of a detailed study of geological profiles and drilling profiles in the northeastern Sichuan Province,the data of salt/potassium wells,oil/gas wells were reorganized and compared,thus further determining the strata and structural characteristics of the polyhalite deposits in the northeastern Sichuan Province:The Triassic polyhalite deposits strata in the northeastern Sichuan Province mainly consist of the Leikoupo Formation（T₂l）and the Jialingjiang Formation（T₁j）of the Middle-Lower Triassic,with two major mineralizations period:second sub-paragraph of four sedimentary deposition of Jialing Group(T₁j^4-2),second sub-paragraph of five sedimentary deposition of Jialing Group(T₁j^5-2)^the first sub-phase deposition of the Leikoupo Group first section(T₂l^1-1).There are three major ore-bearing rock series:T₁j^4-2,T₁j^5-2,T₂l¹-1-1 ore-bearing rock series;section types are T₁j^5-2 to T₂l^1-1 type(including T₁j^4-2 type)salt-containing sections.2.It is determined that the sedimentary environment of the upper and middle Triassic in the northeastern Sichuan Province is a shallow-water carbonate-evaporite platform through field observations of typical cross-section,according to facies assemblages,sedimentary structures,interior thin section identification and other studies,combing with well logging and seismic data analysis.The Middle-Lower Triassic Jialingjiang Formation-Leikoupo Formation is mainly a carbonate platform sedimentary system.There are three major sedimentary facies,namely,evaporation mesa facies-restricted mesa facies-open mesa facies.The major stratum T₁j^4-2,T₁j^5-2and T₂l^1-1segments mainly develop restricted platform and evaporation platform environments.With the third-order sequences or system tract as mapping unit,the author systematically compiled sequence-based lithofacies-paleogeography map of Jialingjiang Formation to Leikoupo Formation in the northeastern Sichuan Province.3.Through comprehensive analysis,it is believed that the formation conditions of the Triassic polyhalite deposits in the northeastern Sichuan Province are related to many factors（such as paleoclimate,palaeogeography,paleostructure,basin system,etc.）,the causes of which are complex,and there are both original and metasomatism causes.In some cases,there may be metasomatism of K⁺and Mg²⁺brines and gypsum and anhydrite or other salt minerals,and there may be mixed doping of different water bodies;Changshou-Dianjiang in T₁j^4-2 period and Nanchong-Guang’an-Quxian and Dianjiang areas in the period of T₁j^5-2T₂l^1-1 were proved likely to generate polyhalite deposits.