Geological,Geochemical Characteristics And Metallogenic Regularity Of The Dashui Gold Deposit In Gansu Province

The Dashui gold deposit is an important deposit in the western part of Gansu Province and is mainly hosted in the contact zone between the Triassic/Permian carbonate strata and the veins.The output of the gold ore body is obviously controlled by the faults,the intermediate-felsic veins,and the ancient karst structure.The shape of ore body is complex.Ore minerals are mainly composed of hematite and low-sulfide,whereas the gangue minerals consist mainly of calcite.Magmatic rocks in this region are relatively developed.The exposed rocks are all small-scale,mostly small stocks or veins,and some of the veins were mineralized into ore body.In this research,the geological and geochemical characteristics of the Dashui gold deposit are systematically studied and the following knowledges are obtained:?1?A large number of hematite veins have been observed in the field work.These hematite veins are interspersed by later calcite veins,which shows that hematite was primarily formed early during reducing state,not formed during later oxidation state.While a small amount of magnetite was oxidized into hematite by microscopic observation,indicating that a small amount of hematite was formed by other minerals being oxidized.?2?The calcite veins were divided into three stages according to inclusion studies.Results show that the ore-forming fluids are characterised by low temperature,low salinity,low density,and shallow mineralization depth and belong to the NaCl-H₂O-CO₂ system.The gas phase composition is mainly H₂ O and CO₂.The ore-forming fluids were accompanied by cooling and depressurization during mineralization.?3?The study of carbon and oxygen isotopes shows that large amount of atmospheric precipitation and magmatic fluid play an important role in the ore-forming fluids in Dashui gold deposit.Interactions and reactions between them and the dilution of a large amount of atmospheric precipitation made the fluids characterized by low temperature,low salinity and low density and so on.The sulfurisotope study shows that the source of the deposit comes from magmatic rocks.?4?Dashui gold deposit is one of the typical epithermal deposits,and experienced multiple tectonic and magmatic transformation in the later stage.At the same time,the surface is constantly subjected to weathering and denudation,and then secondary enrichment and mineralization occur.Therefore,it is considered that this gold deposit belongs to the epithermal superimposed deposit.?5?Based on the study of metallogenic model of Dashui gold deposit,the metallogenic regularity of Dashui gold deposit is analyzed from its ore-controlling factors and ore prospecting indicators,and the prospecting direction for the shallow Au ore body is put forward.?6?Studies on the thermoelectricity of pyrite show that the type of thermal conductivity of pyrite is dominated by P-type.Pyrites were formed mainly between160²00 ?,consistent with the homogenization temperature of calcite fluid inclusions.It is estimated that the denudation rate of this large gold deposits is 43%,indicating that the denudation level is not high;judging the depth of the ore body is still a good prospect of prospecting.