The Genetic Model Of Mississippi Valley-type Deposits In The Boundary Area Of Sichuan, Yunnan And Guizhou Provinces, China

The boundaries area of Sichuan-Yunnan-Guizhou province is located on the southwestern margin of Yangtze craton,which is the most important Zn-Pb metallogenic belt in China.Although many work and study have been done there,many problems still exist,such as the genesis of deposits and the relationship between Pb-Zn deposits and Emeishan basalt.Based on the Dissecting the representative deposits in detail,this thesis focused on the three key problems of the relationship between Pb-Zn deposits and organic matter,ore-forming fluid and the geochronology of deposit. By employing many methods to study mainly on the organic matters and metallogenesis,inclusion fluid,source of metal,geochronology and deposits model,such as extract the Chloroform bitumen "A",Group component of organic matters,Chromatogram-mass spectrum,Petrogeochemistry, Fluid inclusion,Scanning electron microscope(SEM) and radioactive isotopic geochronology.The main conclusions that we got are as follows:1.The Pb-Zn deposits of Chuan-Dian-Qian area have not the direct connection with magmatism, whose host by thick carbonate rocks and are the clear post-sedimentary deposits.They controlled by both lithology(lithofacies) and faults.There are three types of orebodies' shape, namely like bedded controlled by strata,vein controlled by faults and anomaly shape controlled by palaeokarst.2.The analysis of the trace and rare element in the ore and country rock suggests that the main strata of Pb and Zn elements are basement rock,Dengying Formation of Sinian, Carboniferous and Permian.Both of them have the same characteristics of rare elements distribution.These strata supply the metals for the formation of Pb-Zn deposits.The analysis of Pb,S,C,H and O isotopes indicate that the metals of deposits were form upper crust, sulfurs came from the reducing of sulfate in carbonate rocks,and ore fluids came form atmospheric water.3.Through particular studying on the genesis,the organic matters in the ore and country rock of Chipu Pb-Zn deposit have the high maturity and come through the same process of high salinity and strong reduction.The source of organic matters is the shale and carbonate rock from the sea environment,and not from the continent environment.The organic matters can reduce the sulfate to HS^- or H₂S and play the key role in the precipitation of metal sulfides.4.The measures on fluid inclusion obtained the homogenization temperatures of 170～270℃and the salinity of 8.0～16.99 wt%(NaCl)_eq from Pb-Zn deposits in Chuan-Dian-Qian area.These data suggested that the Pb-Zn deposits belong to low temperature and middle salinity deposits. The components of fluid inclusion are NaCl-CaCl₂(MgCl₂) -H₂O system.The precipitation of metal sulfides is from the mixing between hot metalliferous brine and reducing fluid.5.By employing the high precision ultra-low procedural blank Rb-Sr method,the age of 166.3±.7.7 Ma is obtained by dating on single grains(2～5mg) of sphalerite from Daliangzi deposit.The SEM analysis implies that the occurrence of Kaolinite is the key factor for getting the isochrone age or not.6.Based on the constraints of geotectonics setting,geochronology and the coupling of controlling factors and ore-forming fluid,the genetic model is as follows:1) the source beds settled be accompanying by the activities of contemporaneous faults at extensive ingression after the movement of Jinning.2) Locally topography uplift and comprehensive fracture developed during the Caledonian tectonic movement.Formation water and atmospheric water descended along strata and faults at the driving of gravity.The descending fluid extracted the mineral substance and gradually grew into hot metalliferous brine.The early stage Pb-Zn ore precipitated at the open space when the condition of brine changed.3) The orogenesis,which occurred at lndosinian or Yanshanian,brought large-scale quantity of heat and made the ore-forming fluid reactivation and movement again.4) Lateral compression developed the locally normal faults and released the pressure,then formed the dilation spaces for the movement and sulfide deposit.The metalliferous brine flow rapidly along the surface of unconformity or fault surface and settled down at the low pressure and open dilation spaces.7.By compared the Pb-Zn deposits in Chuan-Dian-Qian area with the classic MVT deposits,the Pb-Zn deposits in research district are belong to MVT deposits and are associated to the basin fluid.They are the results of post orogenesis process.