Geological Characteristics And Genetic Analysis Of Sandstone Uranium Deposits In Wukuerqi-Mongqiguer Area,Southern Margin Of YiLi Basin

Yili Basin is the first sandstone-type uranium deposit discovered in China and the earliest successful in-situ leaching uranium mining area.Since 1990s,a series of large uranium deposits,such as 512,511,513 and 510,have been discovered in the southern margin of the basin.At present,it has become one of the most important sandstone uranium deposit bases in China.Mineralization occurs in the coal-bearing clastic rock formation of the Shuixigou Group of the Middle and Lower Jurassic,most of which occurs in the V-cycle.Over the past 20 years,much work has been done on the characteristics and metallogenic regularities of uranium deposits in this area.Basically,it is considered that the characteristics of sandstone uranium deposits in Central Asia are similar and belong to the local interlayer oxidation zone type.In recent years,the new strata VII₂ uranium deposit in Kuaskal area,southern margin of Yili basin,with a large scale,has been mainly explored and discovered.Is it consistent with the characteristics of uranium deposits found in the previous V-cycle horizons or a new type of uranium deposits?Therefore,in this context,the new strata VII₂uranium deposit in Kuosijiaer area is taken as the research object,and the other strata in Mengqiguerr area are taken into account properly.Based on the analysis of the spatial and temporal distribution characteristics of uranium deposits,petrology,elemental geochemistry,inclusion geochemistry and dynamics,stable isotope geochemistry and other aspects of the interlayer oxidation zone are tested and studied in order to reveal the basic characteristics of sandstone-type uranium mineralization in the southern margin of the basin and to explore its genesis.（1）The basic characteristics of uranium deposits are studied,and it is considered to be a typical type of interlayer oxidation zone mineralization.The mineralization horizon is the upper part of the seventh cycle（VII₂）of Xishanyao Formation,which distributes in the oxidation-reduction transition zone and is controlled by the front line of the oxidation zone.The oxidation zone is a local interlayer oxidation zone with limited scale.Sandstones in the oxidation zone show oxidized yellow or partial red tones,mainly altered to limonite,and their geochemical characteristics are high Th/U and Fe³⁺/Fe²⁺ratios,low organic carbon（orgC）and total sulfur（S）.The transitional zone is characterized by high content of organic carbon（orgC）and total sulfur（∑S）,especially the highest content of U;rich in trace and associated beneficial elements such as Re,Mo,Co,Zn;development of alterations such as carbonation,claying and pyritization,and uranium minerals are mainly pitchblende.The reductive zone is dominated by gray sandstones deposited normally.The organic carbon（orgC）and total sulfur（∑S）content in the reductive zone are higher and the ratio of Fe³⁺/Fe²⁺is lower.These understandings basically confirm that the uranium mineralization of the new strata VII₂ in Kuosijiaerl area is obviously similar to that of the major strata before V-cycle in the southern margin of the basin in terms of the spatial distribution of oxidation zone,zoning of altered minerals and geochemical characteristics.This shows that the genesis and characteristics of uranium mineralization series（509,511,512,etc.）in the southern margin of the basin are the same,but the strata are different.It also shows that the sandstone uranium deposits in the southern margin of the Yili Basin have the characteristics of multi-layered system,so they have great exploration potential.（2）The properties and characteristics of ore-forming fluids are studied.According to the tests of calcite colloids and inclusions in the growth edge of quartz fissures,the fluid action is characterized by normal temperature-low temperature and a few medium-low temperature.Based on the measurement and calculation of medium PH and EH values and the characteristics of altered minerals,it is considered that the fluid properties of oxidation zone are mainly acidic oxidation,transition zone being reduction and weak acid-weak alkaline,and reduction zone being normal reductive fluid action.Therefore,it can be considered that the basic mechanism of mineralization should be a geochemical principle of oxidation-reduction.（3）The advanced in-situ microanalysis technology has been fully applied to stable isotope tracing and dating,deepening the understanding of fluid properties and metallogenic mechanism.In-situ observation of pyrite in micro-area reveals that the occurrence types of uranium-pyrite associations in ore-bearing target strata in the study area are mainly automorphic cubic or colloidal,granular or strawberry-like,pyrite in mineral particle dissolution pits and pyrite in calcite cements.The in situ S isotope test of pyrite shows that the mineralization period of the pyrite is characterized by obvious negative values;the non-mineralization period of the pyrite is characterized by a wide distribution of the S isotope,ranging from-36.16‰⁵1.19‰;and the distribution of the S isotope indicates that the mineralization is related to biological processes and organic fluids.The in-situ SIMS age of pitchblende uranium deposits indicates that the metallogenic age of the area is relatively new,mainly in the middle Miocene（13.8±3.0Ma）,followed by the Pleistocene（1.678±0.35Ma）,which is well coupled with the neotectonic movement in the southern margin of the basin.（4）The basic formation process and genetic characteristics of uranium deposits in this area are comprehensively analyzed and studied.Based on the basic characteristics of uranium deposits,fluid action,tectonic evolution of the southern margin of the basin,mineralization age,H-O isotope of altered product fluids and S isotope tracing of pyrite in situ micro-area,it is considered that the formation of slope zone in the southern margin of the basin and the differential uplift and denudation of uranium sources during the periods of Miocene,Late Pliocene-Early Pleistocene and Middle Pleistocene,the atmospheric precipitation layers carrying uranium sources in the Groundwater enters the target strata of Shuixigou Group in Jurassic,and the interlayer oxidation zone is formed by the action of groundwater fluids containing uranium and oxygen,mainly at room temperature and low temperature.The interlayer oxidation zone type uranium deposit in this area is formed by the action of microorganism,coal cuttings organic matter and some oil and gas reducing agents.Therefore,the basic genetic characteristics of uranium deposits in this area are interlayer oxidation zone type,and the basic metallogenic mechanism is characterized by normal-low temperature groundwater fluids and microorganisms.