Geochemical Characteristics And Origin Of The Adjacent Rock Crystalline Limestone In The Meng Nuo Lead-Zinc Deposits In Western Yunnan,China

Mengnuo lead-zinc deposit is an important large-scale lead-zinc deposit in Sanjiang metallogenic belt.The mining area is located in the eastern flank of Mengnuo syncline.Most ore bodies occur in the lower section of Upper Renhe-Qiao Formation?S₂¹?of Silurian System.Crystalline limestone,bioclastic limestone,slate limestone,slate calcareous mudstone and so on are mainly exposed in this layer.Among them,crystalline limestone is the main host rock of ore bodies,and lead-zinc ore bodies are bedded and permeable.Mirrors occur in crystalline limestone.Crystalline limestone is not only the host rock,but also an important prospecting indicator.The systematic study of crystalline limestone is not only of great significance to the genesis of the deposit,but also provides a basis for prospecting in the deep side of the mining area.On the basis of systematic field geological work,through the study of petrology,petrogeochemistry and isotope geochemistry of the surrounding rocks in the mining area,the sedimentary type,sedimentary environment,metamorphic deformation and element migration mechanism are discussed,and the genesis of crystalline limestone and its relationship with mineralization are summarized.?1?Crystalline limestone is obviously different from other surrounding rocks in field occurrence and microscopic characteristics.Its thickness is unstable,and there are pinch-out,expansion and contraction along the beds.Crystalline limestone is mainly composed of calcite and a small amount of dolomite,quartz,pyrite and so on.Calcite is mostly semi-automorphic fine-medium crystalline with the size of 0.2⁰.5 mm.It is closely embedded with each other.Some calcite is crushed into mylon?<0.02 mm?or gravel?<0.1 mm?.?2?Crystalline limestone?REE is 60.75×10^-6,?La/Yb?_N is 8.11,?Eu is 0.69 and?Ce is1.06.Other surrounding rocks?REE is 232.02×10^-6,?La/Yb?_N is 11.17,?La/Yb?_N is 0.61,and?Ce anomaly is not obvious.Crystalline limestone has a very similar REE distribution pattern with other surrounding rocks,indicating that they have the same material source and diagenetic mechanism.?3?The geothermal model,U-Th diagram and Al-Fe-Mn triangle diagram of surrounding rock Al₂O₃-SiO₂ indicate that they are not hot water deposits,but normal marine deposits.According to the characteristics of trace and rare earth elements,the sedimentary environment of the surrounding rocks in Mengnuo lead-zinc mining area is identified.The tectonic setting of this area is on the passive continental margin,and the sedimentary environment is an open oxidation shallow sea environment with a wide range of organisms and a relatively arid climate during the deposition period.?4?The formation of crystalline limestone is mainly related to two tectonic thermal events.The first stage of tectonism resulted in the crushing of the original mudstone bioclastic limestone,mainly brittle deformation.The ore-bearing hydrothermal fluid was injected into the crushed fracture,resulting in the recrystallization of calcite and the formation of crystalline limestone with a grain size of 0.2⁰.5mm.The second stage of tectonism superimposed and reformed the crystalline limestone formed in the first stage,resulting in the mylonitization of some crystalline limestone and the formation of mylonitized crystalline limestone.Some coarse-grained calcite debris remained in the mylonitized calcite fragments,which provided strong evidence for the sequence of the two stages of tectonics.?5?The C and O isotopes of hydrothermal calcite and crystalline limestone fall into the dissolution area of marine carbonate rocks in the figure of?¹³C_PDB-?¹⁸O_SMOW,and the hydrothermal calcite injection points are horizontally distributed,indicating that hydrothermal reaction occurs between crystalline limestone and ore-bearing hydrothermal solution,thus providing part of the C and O isotope sources for ore-bearing hydrothermal solution.?6?According to the comparison of REE characteristics of lead-zinc ore,hydrothermal calcite and crystalline limestone,it is found that they have similar REE distribution patterns with crystalline limestone,indicating that crystalline limestone may provide some mineralizing materials.