Study On The Ore-Controlling Structures In The Cuolang Lead Deposit In Basu County Of Changdu,Tibet

Cuolang lead deposit is located in the Basu county of Changdu,Tibet.Tectonically,it belongs to the northern part of the Sanjiang Tethyan tectonic domain and Leiwuqi-Zuogong continental margin Qiangtang-Changdu continental basin-range belt.The huge NW-SE arc-shape structure is developed with a series of major thrusts,strike-slip faults,multiple anticlines and synclines,as well as Indosinian-Yanshanian-Himalayan magmatic arc belts.NW-SE arc-shape structures consist of the structural framework and control the abundant mineral resources in eastern Tibet and western Sichuan.The structural framework in the Cuolang deposit is composed of F₅,F₆ and F₇thrusts.The early NW-SE-striking thrust is the main structure with steep NE-dipping fault plane.In the fault zone,intense cleavage and fracture zones are occurred.Alteration and mineralization are presented.All above indicate pronounced compression.The later structures are characterized by NE,NNE,and NEE-striking faults,which are small scale and offset the NW-SE-striking fault.In summary,the Cuolang deposit belongs to thrust belt of thrust nappe structure,typical of beneficial mineralized environment.The sequence of structural deformation is listed as NW-SE-striking thrust compressional fracture?bedding shear fracture and cleavage?hydrothermal filling and precipitating mineralization?NW-SE fault changed by NE-SW reformation?destroying ore-formation?.The Cuolang lead deposit was formed during the late Yashanian to early Himalayan,typical of tectono-magmatic hydrothermal filling deposit.The ore bodies are occurred as stratiform-like,lenticular and vein.Isotope and fluid inclusion suggested the mineralizaed fluids were derived from deep crust and shown the feature of crust-mantle mixture.It was dominated by magmatic water and mixed by meteoric water or strata water or underground brine.Fluid inclusions mainly consist of H₂O,CO₂,N₂,O₂.The homogenization temperatures of the fluid inclusions in the hydrothermal quartz and calcite have a range of 210-360 ^oC and 150-230 ^oC,respectively.The galena-sphalerite-stibnite group is indicative of medium-low temperature hydrothermal deposits,as well as dufrenoysite,barite.Therefore,we contend the Cuolang lead deposit is a medium-low temperature hydrothermal deposit.The relationship between structure and mineralization can be summarized in following:the Nujiang and Xiaya faults are ore conducting structure;NW-striking faults?F₅,F₆,F₇?,bedding shear fracrures and intersections of the fractures are ore reserving structures;NE,NNE and NEE dextral strike-slip faults are ore destroying structures.In addition,the ore reserving space was increased by the holes in T₃a feldspar quartz sandstone,regarded as important ore reserving strata.Combined regional tectonic evolution,the structure-mineralization model can be concluded as below:The Nujiang deep fault was formed during Late Jurassic to Early Cretaceous and gave a connection for magmatic-hydrothermal fluids or hot brine in the deep crust and upper mantle.Then the Nujiang metallogenic belt was produced,giving the mineral source for this area.The Yanshanian-Himalayan orogeny generated the Xiaya fault?ore conducting structure?and other ore reserving structures.The mineral materials were extracted and activated by structural thermal and strong heat flow effect.Driven by structural force,deep magmatic-hydrothermal and underground ore-bearing fluids ascent through ore conducting structures,and then formed ore bodies in the crushed fracture,bedding shear fractures and intersections.In summary,the Cuolang lead deposit is the product affected by the deep structures and regional tectonic evolution.