Characteristics Of Sulfides In The Southern Atlantic Chihu Hydrothermal Vent Field And Their Ore-forming Significance

The Chihu hydrothermal vent field is the first detachment fault-type hydrothermal field discovered by China on the Southern Mid-Atlantic Ridge.It is located at the intersection of the western slope of the Chihu Seamount and the Central Rift Valley.From a tectonic point of view,it is located in the footwall of a detachment fault and the surrounding rock is mafic rock.In contrast,the surrounding rock of previously discovered detachment fault-type hydrothermal fields is mainly ultramafic rock.Therefore,how does the mineralization of the Chihu area differ from other detachment fault-type hydrothermal fields in this special tectonic background?To explore this issue,this paper selected polymetallic sulfides in the Chihu hydrothermal vent field as research objects.The sulfides were studied using mineralogy,element geochemistry,mineral chemistry,and sulfur isotope analysis to analyze the ore-forming characteristics,material sources of the sulfides in the hydrothermal field,and to preliminarily establish an ore-forming model.In terms of mineralogy,sulfides in the Chihu area can be divided into massive sulfides,brecciated sulfides,and impregnated sulfides.Massive sulfides mainly consist of chalcopyrite,pyrite,sphalerite,and amorphous silica.Brecciated sulfides mainly consist of basalt breccia,amorphous silica,and metallic minerals,which are mainly composed of chalcopyrite,pyrite,and sphalerite.Impregnated sulfides contain basalt glass and carbonate minerals,as well as small amounts of fine-grained metallic sulfides.In terms of element geochemistry,T13 station is mainly enriched in Cu,Fe,S,and Si,while T20station is mainly enriched in Fe,S,and Si.Both stations show low Zn content,indicating the reducing nature of the hydrothermal fluids.The high Si content is similar to that of the Si content in the hydrothermal areas of basic rocks,indicating the influence of basic rocks on the hydrothermal area.By analyzing trace elements in single minerals and using stripping curves,it was found that some trace elements have a good correlation with each other.In terms of sulfur isotopes,there is a significant difference between the two stations.Theδ³⁴S composition range of the sulfides at T13 station is 7.59-10.34‰,while theδ34S composition range of the sulfides at T20 station is-16.87-7.08‰.The sulfur isotope composition of the sulfides at T13station indicates that this area may be influenced not only by basic rocks but also by ultrabasic rocks.The sulfides at T20 station have an exceptionally low range of sulfur isotope composition and a large range of variation.The low anomaly values may be related to surface processes and the activity of reducing sulfur bacteria.Based on the comprehensive analysis,the Chihu hydrothermal area may be controlled not only by basic rocks but also by ultrabasic rocks.Therefore,it is preliminary believed that the Chihu hydrothermal area may be a double-source hydrothermal area controlled by both basic and ultrabasic rocks.Due to its off-axis location and long-term weathering,the blocky sulfides at the top of the hydrothermal vents have been eroded.The differences between the two hydrothermal vents in the hydrothermal area indicate that sulfide production in the same hydrothermal system at different times underwent different later modifications.