Contrasting the settings of seafloor massive sulfide deposits on the South Central Indian Mid-Ocean Ridge: Results of the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) INDEX 2013 cruise

English Abstract:

The INDEX 2013 cruise mapped, in detail, four polymetallic seafloor massive sulfide (SMS) deposits located on the western (Sonne) and eastern flanks (Kairei, Edmond, Gauss) of the active, South Central Indian Mid-Ocean Ridge at water depths ranging from 2,440 to 3,300m; a fifth deposit (Score) was discovered along the latter. Altered sediments, hydrothermal crust, sulfide chimney debris and basalt talus cover the mound-like SMS deposits. Actively venting (up to 418˚C at Edmond) and extinct sulfide chimneys define “fields” on the mound surface; the deposits are typically copper-rich. Delicate, bladed Fe-Mn precipitates occur between pillows peripheral to the SMS deposits.

The active Kairei and Edmond, and inactive Gauss and Score SMS deposits occur on rift-parallel, fault-controlled ridges. The deposits formed on and are partly covered by deposits of coarse, blocky talus comprising blocks of fine-grained to aphanitic, olivine and plagioclase porphyritic basalt, typically mantled by centimetres of fine sediment; massive and pillowed basaltic lava are only exposed in fault scarp windows. There is no evidence of recent volcanism. The location of the SMS deposits along the flanks of structural ridges and their association with talus breccia devoid of glassy pillow components indicate that long-lived, ongoing tectonic unroofing of oceanic lithosphere along steep, ridge-parallel (and perpendicular) faults was accompanied by high-temperature hydrothermal activity, and that the faults acted as fluid conduits. In contrast, the hydrothermally inactive Sonne deposit occurs along the crest of an intact rift-parallel, pillow volcano. Variations in topography and bathymetry define two eruptive centres that attain their maximum height where they overlap immediately below the SMS deposit. Pillow lavas and glassy pillow-fragment talus without sediment cover indicate that the volcano is tectonically (volcanically?) active. The spatial coincidence of volcanic and hydrothermal vents at the Sonne SMS deposit is typical of ancient VMS deposits.