Study On Formation Mechanism Of Polyhalite In Kunteyi Playa Based On Hydrochemical Simulations

Polyhalite is an important K-bearing evaporite mineral and is also used as the main mineral raw material for the production of sulfate potassium.Polyhalite has been extensively exploited to produce potash in many countries.Kunteyi Playa located in the northwestern margin of the Qaidam Basin is a depressed subbasin.There are abundant polyhalites in the playa,but the little research on the mineralization process and genetic mechanism of polyhalite deposits has severely restricted the development and utilization of the polyhalite ore.Hydrogeochemical simulation is an important and effective method to study the formation and evolution of geological fluid.This method has been extensively and efficiently applied in the world.Polyhalite from Kunteyi Playa is the main object in this dissertation.Hydrogeochemical simulation has been carried out using EQL/EVP program to simulate the geochemical evolution of different drainage waters(i.e.the drainage waters include sulfate-type surface water from Lake Sugan and Ca-Cl type oilfeild brine from Lenghu area)inflows towards lake water.These simulated results,combining sedimentology,mineralogy and previous data,have been used to study and discuss the genesis of polyhalite.Hydrogeochemical simulation results based on the simulation by EQL/EVP software show that only little amount of polyhalite was deposited during the simulated evaporation of the lake water from the Lake Sugan.While the polyhalite even can not be deposited from the deep oilfield brine during the process.Therefore the formation of polyhalite can not be explained by evolution of the single water body(lake water or oilfield brine)in study area.The simulation results of mixing of the lake water and the oilfield brines in different proportions show that the phenomenon of low content of polyhalites in salt layers in study area can be explained well,but some layers highly enriched in polyhalite cannot be explained yet.Considering that the K-Mg-enriched brine is one of conditions to form polyhalite,when the lake water has been evaporated to the stage of potassium salt precipitation,a component point with highly K and Mg content in simulated evaporation brine was selected,the point is called “intercepting point”.Then the brine at the intercepting point is proportionately mixed with oilfield brine and is evaporated by simulation.The simulation results showed that evaporation of the mixing brine can explain well the phenomenon of enrichment of polyhalite in local sediment seams.Therefore the different occurrences of polyhalites are closely related to the evolution of brines.If the brines in basin are mixed with the Ca-rich oil field water before the potassium salt precipitation,the little amount of polyhalite can be precipitated;otherwise a large amount of polyhalites will be formed.Additionaly it is found that the formation of polyhalite is probablely significantly related to the oilfield brine activity induced by tectonic activity.Based on the above studies,the polyhalites in the Kunteyi playa is mainly formed by the mixing of concentrated Ca-rich oil field brines from the deep structure in study area and the brines in the basin.In addition,research results also illustrated that the formation and enrichment of polyhalites is the result of the coupling of pulsed tectonic activities and climate changes.