Journal of Conference Abstracts

Geological Studies at Sites of Presumed Hydrothermal Activity Between 25°30' and 28°50'N, Mid-Atlantic Ridge

G. A. Cherkashev VNIIOkeangeologiya, 1 Angliysky Av., 190121 St. Petersburg, Russia

hydroth@g-ocean.spb.su

I. M. Poroshina VNIIOkeangeologiya, 1 Angliysky Av.,, 190121 St. Petersburg, Russia

Three areas of the Mid-Atlantic Ridge (MAR) rift valley centered at (I) 25°30'N; (II) 27°10'N, and (III) 28°40'N, were studied during Cruise 13 of the RV Geolog Fersman in 1993, aimed at prospecting for new hydrothermal fields. Each study area is about 10x10 nautical miles in area. Along with geological sampling the work included CTD measurements of the water column and water sampling. The study areas were mostly chosen using geomorphologic criteria of hydrothermal activity based upon the analysis of tectonic settings of the known MAR hydrothermal fields (Krasnov et al., 1995).

Areas of intense magmatism combined with increased crustal permeability were considered as potentially active sites according to bathymetric data (Purdy et al., 1990). Along with geomorphologic criteria, the presence of hydrologic anomalies near 25°30'N (Cruise 3 of RV Geolog Fersman ) and 27°50'N (Murton et al., 1993) were considered in the selecting of study areas.

Fig. 1: Sketch of Area II. 1. Margins of the inner floor of the rift valley (the valley slopes are speckled); 2. Bathymetric steps within the rift valley floor; 3. Neovolcanic highs; 4. Fault scarps; 5. Zones of transverse dislocations; 6. Perspective zones. Position of the sediment core is shown by a circle.

Within Area I, the southern part of the neovolcanic zone, composed of lobate lawa flows, is attached to the steep marginal fault along which a crustal block with basic and ultramafic rocks, of various degrees of alteration, was uplifted. Sulfides and hematite were sampled at the intersection of marginal (rift-parallel) and transverse faults. These disseminated sulfides in hydrothermally altered basaltic rocks of the rift valley slope may represent the stockwork zone of a massive sulfide deposit lying up slope. Numerous sulfide grains, in sediments sampled at one of the stations in the marginal part of the rift valley, give evidence of the possible destruction of this deposit. Geochemical data on sediments (vertical profiles) show two ancient events of hydrothermal activity.

Fig. 2: Sketch of Area III. 1. Margins of the inner floor of the rift valley (the valley slopes are speckled); 2. Bathymetric steps within the rift valley floor; 3. Neovolcanic highs; 4. Fault scarps; 5. Zones of transverse dislocations; 6. Perspective zones. Position of the sediment core is shown by a circle. Position of stations with sulfide mineralization are shown by triangles.

Study Area II is located near the 27°10'N non-transform fault (Fig. 1). The axial neovolcanic ridge is absent. The displacement zone is marked by a transverse bathymetric step with minimal depths of the valley floor. Its axial part is cut by a narrow linear depression, corresponding to the present position of the rift. The surface of the bathymetric terrace is studded with numerous volcanic edifices. Anomalies of temperature and dissolved Mn were discovered in near-bottom waters of the northern part of the area, and high concentrations of sulfide minerals were found in sediments. Geochemical anomalies corresponding to three events of hydrothermal activity were discovered in a sediment core (Fig. 1). The most promising zone was contoured in the northern part of the area, located at the intersection of faults of various orientation (Fig. 1).

Area III completely covers one of the shortest segments of the rift valley, bounded by non-transform offsets near 28°42' and 28°52'N (Fig. 2). The valley floor generally shallows in the southern part of the segment, where a transverse bathymetric step conjugates with the zone of non-transform displacement.

The zone of tectonic displacements striking at 025° and developed throughout the region is the area's most peculiar feature. The zone covers a considerable part of the eastern valley slope, the neovolcanic ridge, conjugating to it southward from the 28°52'N displacement zone, as well as the western valley slope of the northward segment. Along with the transverse tectonic dislocations, marking sites of non-transform offsets, this displacement zone may comprise an area of increased permeability, localizing hydrothermal activity. The youngest volcanism probably occurs at the bathymetric step in the southern part of Area III. A large volcanic high margined in the south near a non-transform offset is adjacent to this zone in the southern part of the area. Hydrothermally altered basalts and basalt breccia with veined and disseminated sulfides were sampled here; high concentrations of sulfides were found in sediments at two stations and temperature and manganese anomalies in near-bottom waters were discovered. Similarly to Area I, the presence of massive sulfides may be expected here; veined and disseminated sulfides may form their stockwork zone.

The data obtained from all the areas studied confirm the association of the MAR hydrothermal activity with zones of intense volcanism and increased permeability due to tectonic displacements.

According to the whole set of data, Area II seems the most, and Area I the least, promising for future discoveries of hydrothermal activity and massive sulfide deposits.

References

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

Murton, B.J., et al., BRIDGE Newsletter 4, 3-6 (1993).

Purdy, G.M., Sempéré, J.-C., Shouten H., DuBois, D. & Goldsmith, R., Mar. Geophys. Res. 12, 247-252 (1990).