Journal of Conference Abstracts

15°N, Mid-Atlantic Ridge ­ Logatchev Hydrothermal Field

S. G. Krasnov VNIIOkeangeologiya, 1 Angliyski Pr., 190121 St. Petersburg, Russia

hydroth@g-ocean.spb.su

G. A. Cherkashev VNIIOkeangeologiya, 1 Angliyski Pr., 190121 St. Petersburg, Russia

I. M. Poroshina VNIIOkeangeologiya, 1 Angliyski Pr., 190121 St. Petersburg, Russia

Y. Fouquet Department Geosciences Marines, IFREMER, Centre de Brest, B.P. 70,

29280 Plouzane Cédex, France

D. Prieur CNRS, Station Biologique, Pl. Georges Teissier, 29680 Roscoff, France

A. M. Ashadze PMGRE, Sevmorgeologiya Association, 24 Pobedy St., 189510 Lomonosov, Russia

The Logatchev Hydrothermal Field with extensive massive sulfide deposits was discovered near 14°45'N, 45°W in 1993-1994 during Cruise 7 of the RV Professor Logatchev of Sevmorgeologiya Association, St. Petersburg (Batuyev et al., 1994; Krasnov et al., 1995a,b).

The Mid-Atlantic Ridge (MAR) rift valley between 14°40' and 14°50'N is asymmetric in cross-section (Fig. 1) and narrowed because of the presence of an uplifted block of its floor, adjacent to the eastern slope and expressed by a series of stepwise linear and curved ridges and terraces. The block is complicated by a number of transverse WNW-oriented faults. The ridge-parallel normal faults in the lower part of the uplifted block are directly continued northward by the neovolcanic high of the next rift segment. The Logatchev Field lies within one of the terraces at the depths of 2950 to 3000 m, 1000 m above the valley floor.

Fig. 1: Sketch of the structural position of the hydrothermal field. 1 Outer margins of the rift valley; 2 Margins of the inner floor of the rift valley (the valley slopes are shown as speckled areas); 3 Neovolcanic highs; 4 Caldera of the off-axis volcano; 5 Fault scarps; 6 Crests and crestal surfaces of ridges within the valley slope; 7 Terraces and deeps within the valley slope with (a) thin, (b) moderate and (c) thick sediment cover; 8 Hydrothermal field.

A large isometric volcano with a flat summit surface and a deep (120 m) caldera was discovered on the valley slope downward from the hydrothermal field, at the depth of 3500 m in 1995 during the Nautile submersible dive (MICROSMOKE Expedition). Its summit surface is covered by fresh basalts, including sheet flows, and cut by numerous open fissures both parallel and transverse to the spreading direction. The slope above the volcano including the field itself is composed of gabbros and mostly ultramafics (harzburgites and pyroxenites), often serpentinized. All the magmatics showing a single AFM trend probably belong to the same basic-ultrabasic rock complex.

Several sulfide mounds were mapped within the field, occupying an area of about 800 x 250 m (Fig. 2). The largest mound is 200 x 150 x 20 m in size. Several black and white smokers were observed, though the field on the whole is at the waning stage of hydrothermal activity: chimney fragments are rare among massive sulfide samples obtained by TV-controlled grab. Chalcopyrite, chalcocite, covellite, bornite, pyrite, marcasite, pyrrhotite, sphalerite, isocubanite, atacamite and amorphous silica are the main minerals. Unprecedentally high Cu (up to 54% in bulk samples) and relatively low Zn concentrations (below 12%) are notable.

Fig. 2: Sketch of the hydrothermal field. 1 Mafic and ultramafic rocks and rock talus; 2 Carbonate sediments; 3 Sulfide mounds; 4 Sulfide sediments; 5 High-temperature vents; 6 Low-temperature vents.

Bogdanov et al. (1995) reported Co concentrations higher than those in any other MAR hydrothermal field, and Mozgova et al. (1996) discovered cobalt pentlandite never before observed in oceanic sulfides. Au concentrations in bulk samples are mostly above 3 ppm and reach 36 ppm. Small transverse faults play a significant role in localizing hydrothermal activity of the TAG (Karson and Rona, 1990) and Broken Spur (Murton et al., 1995) fields. Similar to the field under study, massive sulfides of the TAG and 24.5°N MAR are also localized by rift valley marginal faults directly continued along strike by neovolcanic zones of the adjacent rift segments (Krasnov et al., 1995b). The importance of faults in localizing near-surface intrusives which control massive sulfide formation has been recognized in ancient on-land sulfide-bearing areas (Bettison-Varga et al., 1992). The position of the volcano in the marginal part of the valley may be determined by intersection of transverse dislocations with marginal faults, serving as conduits for along-axis magma migration from the neovolcanic zone of the adjacent rift segment.

It seems most probable that the formation of the Logatchev Field in the area of limited volcanism near 15°N is directly associated with the subvolcanic magma chamber. Ancient massive sulfides of the Urals and other regions often form close to central-type volcanoes (Baranov et al., 1988). Thus, in spite of the unique position of the Logatchev Field among intrusive and ultramafic rocks, its formation, similar to that of other MAR fields, is associated with basaltic volcanism.

References

Baranov, E.N., Schteinberg, A.D. & Karpukhina, V.S., Proc. 7th Quadrennial IAGOD Symp. 449-460 (1988).

Batuyev, B.N. et al., BRIDGE Newsletter 5, 6-10 (1994).

Bettison-Varga, L., Varga, R.J. & Shiffman, P., Geology 20, 987-990 (1992).

Bogdanov, Yu.A. et al., Doklady Akademii Nauk. 343, 353-357 (1995) [in Russian].

Karson, J.A. & Rona, P.A., Bull. Geol. Soc. Am. 102, 1635-1645 (1990).

Krasnov, S.G. et al., Geol. Soc. Spec. Publ. 87, 43-64 (1995a).

Krasnov, S.G., Poroshina, I.M. & Cherkashev, G.A., Geol. Soc. Spec. Publ. 87, 17-32 (1995b).

Mozgova, N.N. et al., Canad. Miner. 34, 23-28 (1996).

Murton, B.J., Van Dover, C. & Southward, E., Geol. Soc. Spec. Publ. 87, 33-41 (1995).