The Metallogenic Regularity Of Dashuigou Tellurium Deposit,Shimian,Sichuan Province And The Origination Of Prospecting

As an scarce element in the crust, Tellurium, with its crust concentration (1-10×10-9),has been regarded unable to form independent deposit. Existed as an accompanying element, its industrial resources usually are refined from Cu or Ni sulfide kies. Dashuigou Te deposit,an untraditional deposit, in Shimian,Sichuan province, the only independent Te deposit with Te as the main metallogenetic element,has been received much concern and research in the world since its discovery. But There have been many controversies about the properties of the occurred country rock,the metallogenic materials and fluids.The basic study about About the ore-forming mechanism remains weak, ground geological preparations especially the evolutional regularties in time and space of ore forming hardly have been dealt with。On the basis of fully comprehending formers’materials, gitlogy, combined by petrology, geochemistry and tectonics have been employed to study the occurred wall rocks, the resources of metallogenetic elements and fluid, ore controlling factors and the mechanism of the deposit, in which the evolution of mineralizing system in time and space is taken as the main line, and tectonic and fluid as the principal active factors. The spreading rule of the ore-bodies in space is determined and the genetic and metallogenic model is constructed in this paper, which can be used for references for mine exploitation and surrounding prospecting, as well as for the metallogenic regularity of dispersed elements especially tellurium.Dashuigou schist are speculated as tectonic microlithon on the background of nearly meridian compression in the western margin of the Yangtze platform in later Tristan.The initial rocks are mainly composed of calcic argillaceous sediment and volcanic-sedimentary ash tuff intercalated basic igneous rock from Proterozoic to Permian layers.The ore-forming fluid generated from poly-resources are indicated by isotopes of C、H、O and He-Ar in fluid inclusions of minerals. The values of δ DSMOW between-41.6to-82, and δ18OH2O between0.33to11.93imply both the range of magmatic-hydrothermal and metamorphic water. δ18OSMOW between9.52to12.90(average11.24),δ13CPDB between-5.3to -6.5(verage-5.71) probably indicate the homogenization of fluids from different resources in shear zone. Since the fluid characteristics of Na+/K+>1, Na+/(Ca2++Mg2+), and Cl-as the main negative ion with lower F-, reflecting origin of the sedimentary and ground water, ground water should have taken a main share in ore forming process. He isotopes indicates the feature of more crust source and less mantle source.Ar isotopes manifests that the ore fluids were mainly ground water or metamorphic thermal fluid incorporated with some mantle fluid.. In the early metallogenic stage, high temperature fluid inclusion containing sub-crystal indicated the magmatic-hydrothermal,while in the main metallogenic stage, the co-existing inclusions of both low-to-moderate-temperature with high salinity or lower salinity containing much CO2illustrated that the fluids were mainly metamorphic water and reformed fluids.In summary,the fluids of Dashuigou Te deposit had the complex and commingling characteristics of shear zone fluid, and the ore forming fluids were incorporated fluids mainly composed of metamorphic water and more and more underground water combined by magmatic water becoming less in the later ore-forming process and deep gaseous emanation possibly from mantle,showing a tendency from deep fluid to shallow fluid along with the tectonic level from deep to shallow level.Ore forming elements,trace elements and rare earth in wall rocks, altered rocks and orebodies from structural lateral sections revealed that the main ore forming materials such as Te and Bi closely related with wall rocks an altered rocks and early orebodies,and should be the result of re-activation from them. The magmatism in early mineralized stage such as pyrrhotite stage manifested its existence while crust resources dominated in the main stage of matallogenesis.Comparatively homogeneity isotope of S probably resulted from desulfurization of sulfides in the wall rocks. The schist and the underlying marbles had high content of Te amounting to n×10-5～n×10-4, several hundred or thousand times higher than Clarke value in crust, in which considerable proportion of the Te in the wall rocks was found to be existing in forms of easily activated and moving, located in the leptoclases of mineral crystals in the form of microminiature independent minerals such as tellurobismuthite,or probably in the state of Te complexes adsorbed on the surface of minerals especially the carbonate minerals. Therefore, the wall rocks of Dashuigou Te deposit should be the "source bed".The properties of rare earth evolution in surrounding pyrogenetic rocks testified its weak relationship with the Te deposit. A scenario in which relatively high content of Te and Bi in pyrogenetic rock, but lower in the metamorphic rock in the section of exposed pyrogenetic rock than in the rocks without exposed pyrogenetic rock, suggests that the magmatism was not the direct controlling factor although it may provide heat source and some materials which probably migrate in gaseous state by deep emanation from mantle.Multilevel ductile shear zone had controlled the distribution of regional deposits and ore spots with the features of poly-layers, multilevel structure and the banding alternation. Brittle-ductile shear structures especially the brittle structures were the direct controlling and host ones of regional Te-Au deposits.In late Triassic, south to north and east to west orientation compressive tectonic had taken place in succession in the region which brought about giant nappe tectonic and basal sliding zone as well as layers folding, metamorphism and deformation. Dashuigou schiefer was overthrusted and localized,meanwhile the two important structural interfaces,the thrust faults were developed on and under the schist layer, first formation of the Liushapo group. In middle-late Jurassic,the two structures were conversed to detachment surface complemented by the main ore hosting structures-brittle shear fractures and faults. In middle Cretaceous, the structures re-activated and broke up the early ore veins which were filled and superimposed by ore forming fluids. In Cenozoic, strike slip yielded nearly east-west shear faults and disrupted the early ore veins. Metamorphism had taken place accompanying with the regional tectonic evolution, e.g. regional constructive metamorphism in late Triassic,dynamometamorphism and magmatic thermal metamorphism in early to middle Jurassic, brittle shear dynamometamorphism and hydrothermal alternation in late Jurassic to middle Cretaceous.Different levels of structures constituted a three dimensional ore forming space system in which Xiyoufang ductile shear belt was the regional controlling structure, the sliding structure between the schist and the underlying Ordovician marble was the derivative structure, brittle shear fractures and faults were the host structures. The schist of first layer in Liushapo group was the main ore hosting layer because of its developed fractures of penetrability and continuity as well as its relatively opened surrounding. The mantle emanation,magmatic water, metamorphic fluid, ground water and some precipitated water constituted a three dimensional fluid system of shear zone.The "source bed" with high Te and Bi was the material base of Dashuigou Te deposit,tectogenesis was the main ore controlling factor, and the coupling of tectogenesis and fluids was the main mineralization mechanism. Tectonic-dynamic force controlled the levels of deformation and metamorphism, generated the dynamometamorphism from medium to lower temperature and pressure in succession, propelled fluids from different levels to blend and extract metallogenic material in wall rocks in process of migration.The earlier rift evolution brought about high Te background in the western margin of Yangtze platform. After Dashuigou schiefer was located,the dynamometamorphism yielded plentiful metamorphic fluid and activated the metallogenic material in wall rocks which migrated into the layers of high effective porosity and produced metallogenic material preliminary enrichment in source bed.In middle Jurassic to middle Cretaceous, on the back ground of south-north slip, actuated by the model of fault valve and earthquake pumping,different levels of fluids such as metamorphic water, ground water and some magmatic water and mantle emanation mingled and extracted ore forming materials in wall rocks,then ascended to the shallow brittle shear zone. precipitated and formed the ore bodies. The impulsive extrusion and expansion of structures and periodic fluids blending, boiling and concentration were the main mechanism of ore forming and upgrading.The multi-periods and poly-stages of metallogenic evolution,multi-resources of ore-forming fluid, metallogenic materials mainly from wall rocks as well as the obvious shear controlling, proved that Dashuigou Te deposit was a typical meta-sedimentary-hydrothermal reformed deposit related to shear zone.On the basis of metallogenetic model, the regional indicators for prospecting have been proposed.(1) The geochemical anomaly of Bi is the main symbol for independent Te deposit.(2)The schist of Liushapo group and the Ordivician marble with high Te content is the stratigraphic guide.(3)The intersections of lineament structures and ring structures especially where the axial region of anticline dome are the important section for mineralization.(4) The dolomitization is the direct signal with itself often being Te mineralized.(5) The former mining pits of sulfurous iron ore in Dashuigou dome are the important marks for accompanying tellurium.The1448m mining level height of Dashuigou Te deposit mining section should be the focal place for deep exploration. Two sites to the north and south of the Dashuigou Te deposit are the advantageous sections for tellurium mineralization for their similar layers, tectonics, altered rocks.The Xiyoufan ductile zear belt and Yele tectonic microlithon adjacent to Yejidong ductile sear zone are the prospecting area for Te deposit.