Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences

Abstract

, the Chinese research cruise DY115-19 discovered an active hydrothermal field at 49°39′E/37°47′S on the ultraslow spreading Southwest Indian Ridge (SWIR). This was also the first active hydrothermal field found along an ultraslow-spreading ridge. We analyzed mineralogical, textural and geochemical compositions of the sulfide chimneys obtained from the 49°39′E field. Chimney samples show a concentric mineral zone around the fluid channel. The mineral assemblages of the interiors consist mainly of chalcopyrite, with pyrite and sphalerite as minor constitunets. In the intermediate portion, pyrite becomes the dominant mineral, with chalcopyrite and sphalerite as minor constitunets. For the outer wall, the majority of minerals are pyrite and sphalerite, with few chalcopyrite. Towards the outer margin of the chimney wall, the mineral grains become small and irregular in shape gradually, while minerals within interstices are abundant. These features are similar to those chimney edifices found on the East Pacific Rise and Mid-Atlantic Ridge. The average contents of Cu, Fe and Zn in our chimney samples were 2.83 wt%, 45.6 wt% and 3.28 wt%, respectively. The average Au and Ag contents were up to 2.0 ppm and 70.2 ppm respectively, higher than the massive sulfides from most hydrothermal fields along mid-ocean ridge. The rare earth elements geochemistry of the sulfide chimneys show a pattern distinctive from the sulfides recovered from typical hydrothermal fields along sedimentstarved mid-ocean ridge, with the enrichment of light rare earth elements but the weak, mostly negative, Eu anomaly. This is attributed to the distinct mineralization environment or fluid compositions in this area. In 1977 the first hydrothermal black smoker was discovered on the Galapagos Rift by scientists diving in a deep-sea

Geology, sulfide geochemistry and supercritical venting at the Beebe Hydrothermal Vent Field, Cayman Trough

“…The BVF fulfils none of these associations yet is auriferous. However, there is growing evidence that vent sites situated on slow‐spreading mid‐ocean ridges, especially those ultramafic hosted, are particularly gold‐rich [ Münch et al ., 2001; Tao et al ., ; Nayak et al ., ; Fouquet et al ., ; Wang et al ., ]. In this geotectonic setting, we suggest the availability of gold‐rich sulfide blebs in the reaction zone and, vitally, the precipitation of gold in beehive chimney structures has enabled the formation of an auriferous sea floor massive sulfide deposit.…”

Along‐axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50°E) from a wide‐angle seismic experiment

“…Here the distance between the axial valley walls is 10 km. To the east of the NTD, segment 28 is marked by an east-west-trending 27 km long axial volcanic ridge, with the Dragon Flag active hydrothermal field nearby [Tao et al, 2011[Tao et al, , 2012. The axial depth in this segment becomes shallower than 3 km and the ridge valley narrows to 5 km.…”

The formation of gold‐rich seafloor sulfide deposits: Evidence from the Beebe hydrothermal vent field, Cayman Trough

“…Despite this, the data for global VMS deposits show that a larger proportion of mafic‐hosted systems are auriferous compared to either bi‐modal mafic or felsic host‐rock types (Table ), although only 11 of the 69 mafic‐hosted deposits are recognized as being mid‐ocean ridge deposits. On the seafloor, there is growing evidence that vent sites situated on ultraslow spreading and ultramafic crust are particularly gold‐rich [ Munch et al ., ; Tao et al ., ; Nayak et al ., ; Fouquet et al ., ; Wang et al ., ]. The mid‐Cayman spreading center is ultraslow spreading, with one of the lowest melt thicknesses in the world [ Klein and Langmuir , ; White et al ., ] producing an extremely thin mafic crust just a few hundred meters thick [ Stroup and Fox , ; ten Brink et al ., ].…”