Hydrogeochemical Characteristics And Genesis Of Bauxite Under Concealed Conditions

The contradiction between the supply and demand of domestic bauxite resources has become increasingly prominent,and the mining of bauxite under concealed coal seams has become an inevitable trend.In 2017,a super-large aluminum（clay）mine was discovered in the deep part of Yanlong Mining Area in Henan Province,which provided an important resource reserve for the development of the aluminum industry in Henan Province.Its occurrence horizon is mainly the Benxi Formation of the Middle Carboniferous System.The upper part is in communication with the Taiyuan Formation limestone aquifer,and the lower part is in direct contact with the Ordovician limestone aquifer.In addition,coal mining activities in the area are intense.The hydrogeological conditions are complex,so mine water inrush is a bottleneck problem restricting the mining of bauxite under coal.On the basis of collecting water quality data over the years,collecting water samples in the field,laboratory experiments and data analysis,the thesis takes the main aquifers in the Yanlong mining area-Neogene and Quaternary pore aquifers（phreatic water）,Sandstone aquifer of Permian Shanxi Formation（Sandstone water）,Lower Limestone Aquifer of Permian Taiyuan Formation（Taiyuan limestone water）and the Ordovician limestone aquifer（Ordovician karst water）are the research objects,this article firstly analyzes phreatic aquifer and confined aquifers hydrochemical characteristics at different time scales in the mining area,and then qualitatively explores the formation mechanism of the chemical components of the main aquifer water at different time scales,and finally explains the water-rock interaction of Ordovician water by reverse hydrogeochemical simulation.The main research results of the thesis are as follows:（1）The features of chemical constitution of groundwater in mining area are analyzed by Piper three-line diagram.The outcome reflect that the chemical type of phreatic water is mainly HCO₃-Ca·Mg type.Under the influence of coal mining,the chemical composition of sandstone water and groundwater varies greatly,and the type of water chemistry has no obvious distribution pattern over time.The hydrochemical types of limestone water of Taiyuan Formation are mainly HCO₃-Ca·Mg type and HCO₃·SO₄-Ca·Mg type,and the water quality in some areas has evolved into Cl·HCO₃·SO₄-Na·Ca type due to the mixing of the chemical composition of the overlying sandstone water.The main hydrochemical type of Ordovician karst water is HCO₃-Ca·Mg type.（2）Qualitative analysis using the Gibbs diagram and the ion ratio coefficient found that the hydrochemical effect of groundwater in the mining area is dominated by water-rock effect,followed by evaporation.The water-rock effects of phreatic water and Ordovician karst water include the dissolution and cation exchange of rock salt,carbonate rock,and sulfate rock,on this basis,Taiyuan limestone water increased the dissolution of silicate rock and oxidation of pyrite.By comparing the linear fitting slope of cation,it is concluded that the cation exchange effect of Taiyuan limestone water is the strongest.Through principal component analysis and the calculation of principal component scores,the analysis shows that both the phreatic water and the Ordovician water-rock interactions gradually increase over time,and the cation exchange interactions of the limestone water of Taiyuan Formation and the dissolution strength of carbonate rocks and silicate rocks gradually increase over time.Strengthening the trend.（3）According to the experimental analysis of hydrogen and oxygen isotopes,it is concluded that the phreatic water mainly comes from atmospheric precipitation and is greatly affected by evaporation,while karst water mainly comes from atmospheric precipitation and lateral recharge of karst water in southern mountainous area,which is affected by factors such as isotope drift and karst water retention.According to the results of tritium isotope tracing,it is found that phreatic water is mainly modern water aged 5-10 years.Karst water is mainly sub-modern water（recharged before1952）,mixed water bodies recharged recently,and modern water.（4）Ordovician karst water is an important source of water inrush in the bauxite layer,and its water chemical characteristics are affected by the movement of groundwater.From the karst water supply area,runoff area to the drainage area,the content of conventional ion components in the water gradually increases,and the type of water chemistry changes.（5）Through the reverse hydrogeochemical simulation,the water chemistry evolution law of Ordovician karst water was quantitatively simulated,and three inversion paths were selected.Affected by mining in the mining area,the replenishment path a and b have sufficient water and rock effects,and there is the possibility of hydraulic connection with the upper aquifer.Water inrush accidents should be prevented and avoided at all times during bauxite mining.Path c has the weakest water-rock interaction,and is dominated by non-congruent dissolution.The research results can provide technical support for the comprehensive utilization of water resources,mine water disaster prevention,and groundwater environmental protection during the development and utilization of aluminum under coal in the west of Henan Province.At the same time,it has certain strategic significance for ensuring the stable and healthy development of the aluminum industry.