Mineralization Of Polymetallic Sulfides On Mid-Atlantic Ridge At Chihu Hydrothermal Field

The hydrothermal field associated with detachment fault has been discovered for the first time at the South Mid-Atlantic ridge.It is possible to study the metallogenic characteristics and metallogenic mechanism of the hydrothermal sulfide deposit con-trolled by detachment fault in the South Atlantic.In this paper,the Chihu hydrother-mal field of 23.7°S at the Mid-Atlantic ridge is selected as the research object.Con-duct mineralogical and geochemical sudies on the hydrothermal samples collected in this area,and analyze the ore-controlling factors,ore-forming material sources,and ore-forming stages of hydrothermal sulfides,summarize the conceptual model of mineralization.On the one hand,this research can provide basic data for regional mineralization characteristics,mineralization laws and resource potential assessment of polymetallic sulfides in the South Atlantic.On the other hand,it can also provide reference materials for enriching the mineralization theory of ancient VMS deposits of the same type on the land.The Chihu hydrothermal field is located at the junction of the foothills of the Chihu hill west slope of the South Mid-Atlantic ridge and the east wall of the central rift,and is controlled by detachment faults.Analyzing the geological structure of the region is located hydrothermal detachment section exposed position on the seabed.Through mineralogical research,hydrothermal products are divided into siliceous sul-fides and mineralized breccias.The mineral composition of siliceous sulfide is mainly pyrite,chalcopyrite,sphalerite,isocubanite,covellite,and a small amount of talc,sap-onite,and goethite.Mineralized breccia is mainly composed of amorphous silica,rock fragment breccia and sulfide breccia.Mineral composition mainly pyrite,chalcopy-rite,marcasite,sphalerite,bornite,Phase A,idaite,silver pentlandite,saponite,idding-site,palagonite,talc,actinolite,serpentine,magnesite,dolomite,goethite.Geochemi-cal analysis shows that the main elements are enriched in Si O₂,and the content of Cu and Zn is low.Low Zn elements indicate that the hydrothermal fluid is more acidic and the hydrothermal products are in a relatively reducing environment.Mg mainly occurs in magnesite.The low Ca content may be related to the lack of sulfate minerals in the minerals studied in this paper.The high content of Si O₂ indicates that the hy-drothermal point is significantly controlled by the mafic rock,and the conduction cooling effect of the hydrothermal fluid leads to the presence of more Si O₂ in the hy-drothermal product.The content of As,Co,Mn and Se in the trace elements is higher than that of other trace elements.However,compared with other hydrothermal areas,trace elements are generally at a lower level.Among them,Se element has a certain correlation with Cu,indicating that it may be related to the content of chalcopyrite in the sample,and it exists in the form of replacing S element in chalcopyrite.Both Co-Fe and Mo-Fe have good correlation.It is speculated that Co element and Mo element are related to pyrite in sulfide,and they exist in an isomorphic way instead of Fe ele-ment.The lower levels of As,Ag,Sb,and Cd are related to the lower content of Zn in the samples in this article.There is no obvious correlation between Mn and any kind of geochemical group,because there is no native manganese sulfide or manganese sulfate in manganese.Manganese oxides in hydrothermal sulfides usually appear in the form of late precipitation on the outer surface of the chimney and in the altered products.The sulfur isotopeδ³⁴S_V-CDTvalue of the sample is 7.87‰～11.29‰,with an average of 9.59‰.Compared with the hydrothermal area dominated by mafic rock in the Mid-Atlantic ridge,the sulfur isotope level is higher,and it is closer to the sulfide sulfur isotope level in the hydrothermal area where the ultrabasic rock is the base rock.It shows that the hydrothermal area studied in this paper is also controlled by ultrabasic rock,and this higher sulfur isotope level shows that proportion of seawater sulfate involved in the mixing is large,indicating that the hydrothermal fluid in the hydrothermal area has a strong reducibility.There are at least two phases of mineralization in this area.The hydrothermal products formed in the first phase of hydrothermal activity have slumped and dis-placed under the effects of seabed weathering,gravity and tectonic action,forming turbidity current accumulation.The second phase of hydrothermal activity became active again with the reactivation of the detached structure.The hydrothermal fluid was poured into the first phase of turbidity deposition during the ascending process,and siliceous precipitation occurred under the action of seawater mixing or conduc-tion to cool down the turbidity.Cemented diagenesis of flow sedimentary clastics.Controlled by the dual source rock of basic rock and ultrabasic rock,the driving force of the heat source may come from the exothermic heat of the deep intrusive rock mass in the central rift and superimposed on the superbasic rock serpentinization exotherm.