Hydrothermal activity and magma genesis along a propagating back-arc basin: Valu Fa Ridge (southern Lau Basin)

Valu Fa Ridge is an intraoceanic back-arc spreading center located at the southern prolongation of the Lau basin. Bathymetric observations as well as detailed sampling have been carried out along the spreading axis in order to trace hydrothermal and volcanic activity and to study magma generation processes. The survey shows that widespread lava flows from recent volcanic eruptions covered most of the Vai Lili hydrothermal vent field; only diffuse low-temperature discharge and the formation of thin layers of siliceous precipitates have been observed. Evidence of present-day hydrothermal activity at the Hine Hina site is indicated by a thermal anomaly in the overlying water column. Our studies did not reveal any signs of hydrothermal activity either above the seismically imaged magma chamber at 22°25′S or across the southern rift fault zone (22°51′S). Lavas recovered along the Valu Fa Ridge range from basaltic andesites to rhyolites with SiO2 contents higher than reported from any other intraoceanic back-arc basin. On the basis of the highly variable degrees of crystal fractionation along axis, the development of small disconnected magma bodies is suggested. In addition, the geochemical character of the volcanic rocks shows that the transition zone from oceanic spreading to propagating rifting is located south of the Hine Hina vent field in the vicinity of 22°35′S

Hydrothermal precipitates associated with bimodal volcanism in the Central Bransfield Strait, Antarctica

Polymetallic sulfide-sulfate mineralization enriched in Pb-Ag-As-Sb-Hg occurs in the Bransfield Strait, a late Tertiary-Quaternary marginal basin close to the Antarctic Peninsula. The mineralization is associated with bimodal volcanism and pelagic and volcaniclastic sediment in rifted continental crust. Hydrothermal precipitates have been recovered from two shallow (1,050–1,000 m water depth) submarine volcanoes (Hook Ridge and Three Sisters) in the Central Bransfield Strait. Mineralization at Hook Ridge consists of polymetallic sulfides, massive barite, and pyrite and marcasite crusts in semilithified pelagic and volcaniclastic sediment. Native sulfur commonly infills void space and cements the volcaniclastic sediment. The polymetallic sulfides are dominated by sphalerite with minor galena, enargite, tetrahedrite-tennantite, pyrite, chalcopyrite, and traces of orpiment cemented by barite and opal-A. The presence of enargite at Hook Ridge, the abundance of native sulfur, and the low Fe content of sphalerite indicate a high sulfur activity of the hydrothermal fluids responsible for mineralization. The sulfur isotopic composition of Hook Ridge precipitates documents the complexity of the sulfur sources in this hydrothermal system with variable influence of biological activity and possibly magmatic contributions. Homogenization temperatures and salinities of fluid inclusions in barite and opal-A suggest that boiling may have affected the hydrothermal fluids during their ascent. The discovery of massive barite-silica precipitates at another shallow marine volcano (Three Sisters volcano) attests to the potential for hydrothermal mineralization at other volcanic edifices in the area. The characteristics of the mineralization in the Bransfield Strait with rifting of continental crust, the presence of bimodal volcanism, including highly evolved felsic volcanic rocks, the association with sediments, and the Pb-Ag-As-Sb-Hg enrichment are similar to the setting of massive sulfide deposits in the Okinawa Trough, and distinct from those of sediment-dominated hydrothermal systems such as Escanaba Trough, Middle Valley, and Guaymas Basin. The geological setting of the Bransfield Strait is also broadly similar to that of some of the largest volcanogenic massive sulfide deposits in the ancient record, such as the Iberian Pyrite Belt