Tungsten Metallogenic System And Prospecting Direction In The Nanwenhe-Saxi Area, Malipo County, Yunnan Province

In order to develop the tungsten metallogenic system and prospecting model in Nanwenhe to Saxi area, southeastern Yunnan province, the major compositions, the characteristics of typical deposits, ore-controlling structures, formation and re-transformation mechanisms, distribution regularities and geological models of this tungsten metallogenic system were studied with the guidance of the theories of metallogenic system, petrology and structural geology. The prospecting model of tungsten deposits in this area was also presented on the basis of above studies. The following is the main achievements:1. Petrologic characteristics and structural geology study of main geologic bodies of the study area indicate that the tungsten metallogenic system in Nanwenhe to Saxi area was developed on the basis of the Precambrian metamorphic basement, which had experienced multi-stage evolution of metallogenic tectonic environment during Caledonian, Indosinian and Yanshanian. The continuously compressional process(170-150 Ma) of the lithosphere of South China caused by plate convergence since Middle Jurassic and the following extension-thinning of the lithosphere(130-80Ma) were significant metallogenic background of tungsten deposits in this area.2. A systematic study on petrology, geochemistry and isotopic geochronology of Mengdong group metamorphic rock, Nanlao gneiss, Laochengpo gneissic granite, Tuantian gneissic granite and Laojunshan composite body suggests that the metamorphic formation related to the tungsten metallogenic system and granites of different periods were essentially formed by cycle transformation of materials from a same ancient crust. The cycle transformation was produced by multi-stage deep melting process that related to the plate activity. On the basis of this, “Re-transformation from the same source” could be the dominant mechanism for the Ore-forming material source of the tungsten deposits.3. Geological characteristics of the typical deposits and isotopic geochronology show that the main genetic type of tungsten metallogenic system in our study area includes the stratiform-like skarn type and the greisen(felsic) vein type, which were formed in approximately 150 Ma and 110 Ma, respectively. This is consistent with the two tectonic-metallogenic backgrounds during Late Mesozoic.4. According to the geological characteristics, fluid inclusions, stable isotopes, metallogenic epochs and ore genesis of the typical tungsten deposits, the metallogenic fluid was mainly derived from magmatic water in depth and partly from meteoric water and sea water. The metallogenic fluid centered on the Laojunshan composite body and migrated to the periphery on a horizontal plane, accompanied by the gradual reducing of temperature and salinity. In addition, the metallogenic fluid was mixed with the meteoric water during vertical migration from the bottom to the top, which also caused variation of the salinity.5. The regional structural analysis in Nanwenhe to Saxi area suggests that the space superimposed relationship of Saxi rock group, Laochengpo gneissic granite, Tuantian gneissic granite, Nanyangtian rock group and Nanlao biotite monzonic gneiss from top to bottom was caused by the strong SE-NW trending multi-level nappe tectonic deformations since Mesozoic. These deformations resulted in generation of the shear fractures in the ore-hosting rock, which formed the pathway of the hydrothermal ore-forming fluid migrating, and caused contact replacement process with the ore-host strata like calcareous marl and formation of stably bedding-extended ore-bearing skarn zone.6. On the basis of integrated study on the tungsten mineralization features in Nanwenhe to Saxi area, the stratiform-like skarn type tungsten deposit was enriched by the superimposed reformation of the vein scheelite formed at approximately 110 Ma, which resulted in transformation of the metallogenic system. Therefore, the change mechanism of metallogenic system in this area was related to hydrothermal filling process in the plumose fractures near the NW and nearly EW trending faults caused by the dextral shear stress field.7. The zircon U-Pb dating and apatite fission-track analysis indicate that the Laojunshan composite body had experienced four tectono-thermal events:(63-74) Ma,(42-50) Ma,(37-28) Ma and(25-17) Ma, the last three stages were related to the uplift of the Laojunshan composite body and correspond to the uplift of the Tibetan Plateau. On the basis of this, it can be concluded that the preservation condition of deposits in Nanwenhe to Saxi area was relatively worse, but preservation condition of deep level deposits gets better in the Dulong and Changtian area which located on the south and north side of the Laojunshan composite body. The regional structural analysis further confirms that the NW trending sinistral strike-slip and normal faults since Cenozoic were the mainly destructive factor of the metallogenic belt, deposit and ore body of the tungsten metallogenic system in this area.8. The structural ore control model, dominated by the Mesozoic nappe structures and the “3-location, 4-layer, 2-zone and 1-body” regional prospecting model in Laojunshan ore concentrated area are built in this thesis. Combined with the ore-forming geological conditions, the W-Sn metallogenic ore prospecting directions in the deep and periphery in Nanwenhe deposit are also raised in this thesis, the prospecting proposal of deep stratiform-like ore body under the gneissic granite in the nearby area of Nanwanhe deposit has already been proven by the deep drilling.