Journal of Conference Abstracts

Comparison of Sulfide Mineralisation Related to Slow Spreading Ridges with High Magmatic Budget (Lucky Strike and Menez Gwen Sites) and Low Magmatic Budget (15°N Area)

Y. Fouquet IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

fouquet@ifremer.fr

P. Murphy IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

M. K. Tivey Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

K. Henry IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

F. Barriga Universidade de Lisboa, Departamento de Geologia, Campogrande, 1700 Lisboa, Portugal

I. Costa Universidade de Lisboa, Departamento de Geologia, Campogrande, 1700 Lisboa, Portugal

P. Cambon IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

H. Bougault IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

C. Langmuir Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

D. Prieur Observatoire Oceanologique de Roscoff, Place Georges Tessier, 29680, Roscoff, France

P. Rona Institute of Marine and Coastal Studies, Rutgers University, P.O. Box 231, New Brunswick,

NJ 08903-0231, USA

S. Krasnov VNIIOkeangeologia, 1 Angliysky Av., 190121 St Petersburg, Russia

I. Poroshina VNIIOkeangeologia, 1 Angliysky Av., 190121 St Petersburg, Russia

During the FARA program, several diving cruises were devoted to the study and sampling of sulfide mineralisation along the Mid-Atlantic Ridge. Two contrasting areas were studied: The Azores domain, during the ALVIN (1993, Chief scientist, C.H. Langmuir) and DIVA (1994, Chief scientist Y. Fouquet) cruises, and the 15°N area, during the FARANAUT (1992, Chief scientist, H. Bougault) and MICROSMOKE (1996, Chief scientist, D. Prieur) cruises. Due to high and low magmatic budgets, sulfide mineralisation is related respectively to E-MORB in the Azores domain, and to ultramafic rocks in the 15°N area. This contrasts with more typical mineralisation which is related to N-MORB (Fig. 1). Four contrasting types of mineralisation were sampled indicating a strong influence of the source rock composition on the mineralisation, and a control due to water depth. The four areas visited are, from north to south: Menez Gwen, Lucky Strike, 15°N and 14°45'N (Fig. 1).

Fig. 1: Types of mineralisation related to their source rock along the Mid-Atlantic Ridge.

Menez Gwen (37°50"N)

This site was discovered in 1994 during the DIVA1 cruise. The active vent field is located at the topographic high of the ridge segment. The central part of the volcano is a 2 x 6 km and 300 m deep axial graben filled with very fresh lava. The site is locally controlled by a young volcano at the center of a central graben partly filled with a recent lava lake (Fouquet et al., 1995). The site is at a water depth of 847 m, and at this relatively shallow depth, low salinity and metal depletion of the fluid indicate that it was produced during phase separation of the ascending hydrothermal fluid. Related mineralisation is depleted in metals and enriched in Ba and Si and locally Pb. Chimneys consist primarily of anhydrite.

Lucky Strike (37°17'N)

This site was discovered in 1992 during the FAZAR (Langmuir et al., 1993) cruise and preliminary studies were made during 6 ALVIN dives in 1993 (Langmuir et al., 1995). The site is controlled regionally by the topographic high along the neovolcanic ridge. During the DIVA cruise (1994) it was demonstrated that the local control is a caldera system at the center of which is a lava lake (Fouquet et al., 1996). Hydrothermal precipitates and slabs are distributed around the lava lake and cover a surface of about 1 km². The depth (1700 m) is greater than that of Menez Gwen but fluid compositions and high gas content also provide clear evidence of boiling (Charlou et al., 1995; Colodner et al., 1993). The very large amount of silicified slabs made of brecciated sulfides and volcanic fragments indicate widespread diffuse venting which contrasts with the highly focused venting of other MAR sites. This is also seen in the great dispersion of chimneys and the abundance of diffuse flow. Sulfide mineralisation was clearly formed in several episodes, corresponding to different mineral assemblages. One important point is the abundance of barite when compared to other Atlantic N-MORB related sites.

15°N area

During the FARANAUT cruise (1992), submersible investigations were conducted on the Mid-Atlantic Ridge near its intersection with the 15°20'N fracture zone. Data collected during surface cruises by American, Russian and French vessels were used to determine the diving areas. The two diving areas were located at the western and eastern intersections of the ridge with the fracture zone. On both intersections, ultramafic rocks were seen as domes and elongated ridges on the valley floor, and on the rift wall. They consist of dunite and harzburgite and were often associated with gabbro. At the Eastern intersection (15°N) the contact of the ultramafic dome with the neovolcanic zone of the rift valley is very sharp and controlled by a major fault parallel to the ridge system. At many places on the ultramafic outcrops, elongated lines, parallel to the ridge, of white material (enriched in chlorite and aragonite) are interpreted as the superficial manifestation of water circulating along faults and participating in the serpentinization of the ultramafic rocks. Locally in these white areas ultramafic rocks are silicified and impregnated with disseminated sulfides. The most spectacular samples are from the major fault at the contact between the ultramafic rocks and the basaltic neovolcanic ridge. Massive sulfide mineralisation has not been observed in the area and the mineralised samples are believed to represent the deeper portions of a fossil hydrothermal system. Disseminated sulfide mineralisation occurs in a number of styles, and in some cases appears to be primary magmatic in origin, while in others it is related to the intrusion and alteration of early gabbroic dykelets, and to later serpentinisation, talc-carbonate alteration, and silicification. The mineralisation and alteration in these latter three styles is similar to the obduction-related listwaenite mineralisation known on land. The sulfides are rich in nickel, which occurs both as Ni-Fe sulfides and as a trace element, particularly in samples from early/deep settings. Dominant sulfides vary from sample to sample but are generally chalcopyrite, cubanite and isocubanite, pyrrhotite, pentlandite, galena, sphalerite, pyrite, bravoite and millerite. Native gold was observed in one sample.

Logatchev field 14°45'N

This field was discovered during a Russian expedition in 1994 (Karson et al., 1995). The site was visited in December 1995 during the MICROSMOKE diving cruise with Nautile. The hydrothermal field is controlled by the eastern rift valley fault at a depth of 2970 m. In this area no basaltic rocks were seen. The sulfide deposit lies directly on ultramafic rocks with local gabbroic intrusions. The field is 300 m long and 150 m wide and comprises several mounds topped by active or inactive chimneys or by a circular depression of 10-15 m in diameter and a few meters deep. Three of these depressions are extremely active with several tens of black smokers in each. The very small size or absence of chimneys related to these smokers indicates that they are very young. Preliminary mineralogical and chemical results and field observations demonstrate that, when compared to most other oceanic sites, this site is particularly rich in copper at the surface of the deposit. Pyrite or sphalerite dominated samples are not common. This contrasts strongly with the pyritic composition of the surface mound samples in basaltic environments.

In conclusion, sulfide mineralisation at these four sites contrasts strongly with sulfide mineralisation associated with N-MORB. In E-MORB environments mineralisation is enriched in Ba and locally in Pb. In ultramafic environments Cu is dominant on surface deposits while Cu, Ni and Co are enriched deeper in the system.

References

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Colodner, D., et al., Eos, Trans., AGU 99, (1993).

Fouquet, Y., et al., Nature 377, 201 (1995).

Fouquet, Y., et al., J. Geophys. Res. submitted (1996).

Krasnov, S.G., et al. Geological Society Special Publication, Hydrothermal vents and processes 87, 43-64 (1995).

Langmuir, C., et al., Ridge Events 4, 3-5 (1993).

Langmuir, C.H., et al., J. Geophys. Res. (1995).