The Formation Mechanism Of Hydrothermal Vein Type Ore Bodies In Dahongshan Iron-copper Deposit

Dahongshan Iron and Copper Mine bed is located in Jiasha Town,Xinping County,Yuxi City,Yunnan Province.The deposit is composed of a large rich iron ore and a large rich copper ore,and is produced in the pre-Sinian marine metamorphic volcanic sedimentary rocks.The forefathers have carried out a lot of research work on Dahongshan iron-copper deposit from different angles,and achieved fruitful results and understanding,but there is still a great dispute about the genetic mechanism of the deposit at present.Especially in recent years,the hydrothermal ore bodies exposed in the deep part of Dahongshan mining area are mostly controlled by structure,which are in direct contact with surrounding rock.The ore has typical metasomatic structure and structural characteristics of hydrothermal filling However,the systematic research work related to formation mechanism has not been carried out,which restricts the deep exploration direction and prospecting breakthrough.In view of this,on the basis of previous studies and through detailed field geological work,this paper focuses on the comparative study of trace element geochemistry of hydrothermal calcite carbon and oxygen isotopes,magnetite with different occurrence,surrounding rock and gabbro.Through the above work,this thesis mainly obtains the following understanding:?1?In Dahongshan iron-copper deposit,volcanic-sedimentary type and hydrothermal type are developed.The former has fine-grained structure,transitional relationship with metamorphic volcanic rock,no obvious boundary line,limited alteration range of surrounding rock;the latter has medium-coarse-grained structure,often controlled by faults,clear boundary with surrounding strata,large range of sodium-fading alteration of surrounding rock,and often produces Huichanghui green rock at the margin.The characteristics of ore body indicate that the deposit mainly experienced volcanic-sedimentary action and later hydrothermal filling metasomatism process.?2?Rock chemistry studies have shown that mineral-bearing metamorphic volcanic rocks are characterized by high Fe₂O₃Fe O?Ca O?Na₂O,low K₂O?Mg O and are fine blue-gray porphyry from island-arc marine facies eruptions.The widely distributed gabbro in the mining area has little difference in content compared with metamorphic volcanic rocks,and the Mg O,K₂O content is larger than that of sodium-forming lava,and the Na₂O content is lower than that of metamorphic volcanic rocks,which belongs to the basic basalt associated with mantle plume activity.The?¹³C_PDBvalue range of calcite in orebody is-5.78‰?-0.53‰,and the?¹⁸O_SMOWvalue range is 4.55‰?9.77‰,which is lower than that of marble in Manganghe formation?-0.8‰?0.6‰and 18.1‰?19.3‰respectively?.The?³⁴S value of pyrite samples during volcanic jet deposition is-0.2‰?2.7‰;,and the?³⁴S variation is normal.This type of pyrite is consistent with the values of?³⁴S=0±3‰mantle sulfur,indicating that the source of sulfur in Dahongshan deposit during volcanic jet deposition is mantle sulfur.The?³⁴S value of pyrite and porphyrite samples in pyrite hydrothermal period is-0.2‰?8.6‰,with an average of 5.43‰.?4?Studies on trace and rare earth elements show that magnetite in II₁ore bodies is similar in composition to trace and rare earth elements in surrounding rock and Huichanghui green rock mass,both of which have relative losses Sr,relative enrichment LREE,with varying degrees of Eu positive anomalies and weak Ce negative anomalies,indicating that they have a close genetic relationship.The coarse-grained hydrothermal magnetite has the characteristics of low Ti?V?Cr content and high Co?Ni content,and the Ni/Cr value is much larger than 1,which indicates that it has hydrothermal origin and the formation temperature is more than 300?.?5?According to the comprehensive study,the Dahongshan deposit is mainly formed by two phases,in which the fine-grained magnetite and disseminated sulfide with transitional relationship with surrounding rock?metamorphic volcanic rock construction?are formed by submarine volcanic eruption-sedimentary action,while the tectonic-controlled coarse-grained magnetite with diametrically contact with surrounding rock may be the result of the interaction between magmatic hydrothermal solution with surrounding rock homologous to gabbro.