Minerals Report - International Seabed Authority

Logachev –1 hydrothermal field
Anomalies of and manganese concentrations in near-bottom waters
indicative of hydrothermal activity have been known for some time above the
MAR rift valley segment directly south of the 15 x -20 x fracture zones
(Klinkhammer et al., 1985; Bougault et al., 1990). In addition, objects similar to
hydrothermal mounds were photographed at 14 0 54' N near the base of the rift
valley’s eastern wall (Eberhart et al., 1998). The existence of light attenuation,
temperature and dissolved Manganese anomalies in the eastern part of the rift
valley about 30 km south from the 15*-20* fracture zones was established
during the 10th and 12 th cruises of the R/V Geolog Fersman in 1991 and 1993.
Between November 1993 and February 1994, the RIFT towed system
carrying temperature and spontaneous electrical potential probes, and
potentiometer sensors was deployed for detailed studies of the same area
during the seventh cruise of the R/V Professor Logachev. The system operates
between 30 and 40 m above the seafloor with an interval of 200-500m between
tracks. The results of the survey revealed two electric potential and sulphide
activity anomalies within a bathymetric step of the eastern wall.
Hydrothermal deposits were discovered during a subsequent
TV/photo survey of the sites of these anomalies near 14 0 45' N. The deposits
are situated on a tectonic step, 7 km to the east of the rift axis at a depth
between 2900 – 3050 metres at the base of an ultra-mafic slope with
dimensions of 600 х 300 m.
The deposits are 3 large mound- and veneer-like (stratiform) deposits
surrounded by the groups of smaller mounds. The mounds are up to 20
metres in height and up to 250 metres in diameter. The apron of metalliferous
sediments with more than 0.1% contained copper was sampled.
Hydrothermal activity (mainly high- and mid-temperature) is localized within
the 3 mounds. Sulphides mineralisation is dominated by copper pyrites, and
gold enriched copper and copper-zinc rich ores (Table 1). Mozgova et al
(1996) discovered cobalt pentlandite never before observed in ocean
sulphides. Widespread are low-temperature crust mineralisation comprising
iron-manganese oxides and hydroxides, atakamite, barite and opal crusts that
completely overlap the deposits surfaces.
INTERNATIONAL SEABED AUTHORITY 179