Analysis Of Upper Triassic Rock Assemblage Sequence And Sedimentary Environment In Lanping Jinding Lead-Zinc Mine Area

Lanping Mesozoic-Cenozoic basin is located in the middle part of the Sanjiang orogenic belt in southwest China.It has a large number of metallic deposits.It is one of the most important base metal ore concentration areas in China.Among them,Jinding super-large lead-zinc deposit is the most well-known deposit.The lead-zinc orebodies of Jinding deposit occur in the carbonate-clastic rock strata of the Sanhedong Formation of the Upper Triassic.The upper part is mainly interlayered set of sandstone and limestone-bearing breccia sandstone,showing a complex rock formation.There are controversies over the age and sedimentary environment of the deposit,which leads to disagreements on the genesis of the deposit.Therefore,a thorough investigation of the sedimentary sequence assemblage,sedimentary facies and sedimentary environment of this suite of rocks was needed to carry not only to address its sedimentary age and paleoenvironment,but also help to further clarify the genetic mechanism of Jinding lead-zinc deposit.In this paper,through detailed field geological survey,section observation and observation,the rock assemblage characteristics of the Upper Triassic Maichuqing Formation and Sanhedong Formation exposed in Jinding mining area are observed and described in detail.Their sedimentary systems sedimentary facies,sub-facies and micro-facies are are identified and then classified with the help of paleocurrent study,sandstone clastic components,heavy mineral analysis and geochemical analysis of sandstone whole-rock,the geotectonic settings,source rock type and sedimentary environment.The main findings and understandings of this study are as follows:1.The upper Triassic Maichuqing Formation in the mining area is mainly composed of gray-black mudstone and light-gray fine sandstone interbedded with a small amount of yellow-brown gray sandstone;the lower part of Sanhedong Formation is gray fine-grained limestone and calcareous mudstone with argillaceous limestone,and the middle part is not exposed,while the upper part is gray-limestone with purple-red breccia limestone,limestone breccia sandstone,fine sandstone and argillary sandstone.2.Maichuqing Formation is delta plain subfacies,which can be subdivided into marsh microfacies,distributary channel microfacies and crevasse fan microfacies;Sanhedong Formation is composed of carbonate platform facies and delta front subfacies,in which carbonate platform facies can be subdivided into carbonate tidal flat subfacies,platform margin slope subfacies and lagoon subfacies,and delta front subfacies can be subdivided into submarine distributary channel microfacies and subfacies.The interchannel Bay microfacies and the submarine natural dike microfacies.3.The analysis of sandstone clastic composition shows that the main clastic grains are quartz and lithic debris,and the majority of lithic debris is volcanic debris,suggesting that its material source may be mainly magmatic rocks of orogenic belt.Heavy minerals are mainly zircon,rutile,tourmaline and titanium-oxide,with a small amount of garnet and monazite,indicating that the provenance may be mainly sedimentary or volcanic rocks,with a small amount of metamorphic rocks.4.The geochemical characteristics of the whole sandstone indicate that the source rocks of the Upper Triassic in this area are mainly sedimentary rocks and neutral volcanic rocks,with a small amount of felsic volcanic rocks.The Late Triassic study area is a passive continental margin environment,and the clastic material comes from the orogenic belt or paleocontinent around the basin.