Universidade do Porto - Faculdade de
Ciencias
Museu e
Laboratório Mineralógico e Geológico
Memória nº 4, p. 979-983, 1995  ISSN
0871-1607

ALTERATION PROCESSES ASSOCIATED WITH ACTIVE HYDROTHERMAL
SITES: THE LUCKY STRIKE AND MENEZ GWEN EXAMPLES (37º20’N AND 37º50’N,
MID-ATLANTIC RIDGE)

I. Costa1, F.J.A.S. Barriga1, Y.
Fouquet2

1Departamento de Geologia da
Faculdade de Ciências da Universidade de Lisboa, Edifício C2, 5º piso, Campo
Grande, 1700 LISBOA, PORTUGAL
2IFREMER - Centre de Brest, BP 70, 29280 Plouzané,
France

Abstract

Recent studies and direct
observations on the Lucky Strike and Menez Gwen hydrothermal fields reveal
important alteration processes. Oxidation of chimney- and debris-sulphide
material begins as soon as hot hydrothermal fluid is no longer in contact with
the sulphides. The long lasting residues are mostly iron oxides. The low
temperature hydrothermal alteration is represented by an interesting material
named "slab". The nature and mode of occurrence of this material suggests that
hydrothermal fluid percolates slowly through it, in an unfocussed manner. The
result is a kind of crust which tends to seal the fluid circulation in the
oceanic crust. If the process continues for long enough, essentially
closed-system, multi-pass convective circulation may be induced, contributing to
the accumulation of unoxidised sulphides underneath.

Introduction

The Lucky Strike and Menez Gwen hydrothermal fields are both
located in the Mid-Atlantic Ridge (MAR), SW of Azores, in two different ridge
segments (fig. 1). The Lucky Strike segment is composed of a N-S valley with
three volcanic cones in its eastern part. The hydrothermal site is located in a
depression between these volcanoes. The water depth is 1700m. The Menez Gwen
segment lacks a defined central rift, which is unusual. There is an axial graben
with a young volcano in the northern part, and the active chimneys can be
observed in its W flank at a depth of 800m.

Both systems were recently discovered, Lucky Strike in 1993
(Langmuir et al., 1993) and Menez Gwen in 1994 (Équipe scientifique DIVA
1, 1994). Lucky Strike is one of the largest submarine hydrothermal fields yet
discovered. It is also long lived, given that it depicts several hydrothermal
episodes. Menez Gwen is a very active site in spite of its youthfulness; the
first chimneys are just starting to grow.

Recent studies that include and direct observations in these
two hydrothermal sites (Fouquet et al., 1994 and Desbruyères et
al., 1994) and continuing laboratory data (Costa et al., 1995 and
Costa, 1995) reveal the occurrence of important alteration processes, much
obvious in Lucky Strike.

Geology

The Lucky Strike hydrothermal field is located in a central
depression among three volcanoes mostly composed of pillow lavas and volcanic
breccia. In the center of this depression there is a lava lake - the first
discovered in a slow spreading ridge. Here it is possible to recognize several
typical structures: lobated and drapped lavas, lava flows and lava pillars. The
hydrothermal sites are situated around this lake, with the most active venting
present on the western side.

Figure 1. Location
of the Lucky Strike and Menez Gwen hydrothermal fields in
the
Mid-Atlantic Ridge. The Azores islands
and the most important tectonic structures of the
area are represented. (adapted from
Fouquet et al., 1994)
The active sites are dominated by
chimney structures, up to 30 meters high. Some chimneys are clearly reactivated
sometimes with development of lateral hydrothermal growth structures, planar to
concave, holding fluid underneath, as inverted small lakes. These structures are
called flange, and Lucky Strike contains some of the better examples seen to
date. Surrounding the hydrothermal system there are two important lithologies:
"massive sulphides" and slab crusts. "Massive sulphides" are composed of
accumulated, partly oxidized chimney fragments, and slab is a breccia material
that surrounds individual chimneys and chimney clusters.

Menez Gwen is a young site with small anhydrite chimneys (1
meter). Sulphides are rarely observed in chimney structures, but frequent in
slab rocks, also present in this site. Because of its youthfulness signs of
alteration are not detected.

Oxidation

Oxidation is one of the alteration mechanisms detected. It
acts on sulphides from active chimneys and from older chimney debris, as soon as
the hot fluid responsible for the formation of these structures is no longer in
contact with them. The physical environmental conditions change drastically from
a high temperature reducing state, induced by the hydrothermal fluid, to a cold
highly oxidising state, dominated by ambient sea water.

Several alteration minerals result from this process. Iron
oxides predominate, in most cases as amorphous minerals, although its possible
to observe, in thin section, goethite and hematite.

Microscopically is also possible to observe, in association
with chimney sulphides (chalcopyrite and in smaller amounts marcasite and
sphalerite), dark alteration material which remains opaque in transmitted light
and doesn’t show any similarity with iron oxides (very low reflectance, earthy).
Microprobe data revealed that this material is composed of Fe, Si, S and Cl
accompanied by other elements depending on the primary mineralogy, in variable
concentrations which probably result both from the original sulphide and from
the altering fluid (sea water or hydrothermal fluid) (Costa, 1995). It was not
possible to define a specific mineral composition for this alteration material.
In the absence of diffraction data and in view of their optical characteristics
they were considered amorphous oxides.

Other alteration minerals include copper sulphides like
covellite and digenite, and copper-iron sulphides like bornite and idaite. All
of them have a secondary hydrothermal origin from chalcopyrite alteration
(mainly oxidation). Covellite is clearly the most abundant and the only one
observable in hand-specimen. Bornite and digenite are less frequent and
sometimes formed after covellite. Idaite can only be seen in large optical
amplification and results from bornite replacement with the formation of
secondary chalcopyrite. These four minerals form a typical paragenetic
association that can be seen in figure 2.

In massive sulphide samples it is very usual to observe in
the exterior alteration layer two additional minerals: an yellow iron sulphate -
jarosite - and a light green chloride - atacamite.

Figure 2. Altered chalcopyrite
crystal, in a pseudo-exolution arrangement. Idaite (not shown), occurs as
minute grains within bornite. (drawn from
photograph)

Slab material

"Slab" was the term found to identify brecciated
rocks recently discovered during oceanographic cruises, in association with
active hydrothermal sites. It forms heterogeneous crusts, more or less
consolidated, with centimetric or metric thicknesses near active sites. This
type of rock is composed of fragments of two different natures - basaltic
fragments and rare chimney fragments, cemented by hydrothermal
precipitates.

The fragments of basaltic glass are highly
vesicular, and contain plagioclase, olivine and pyroxene phenocrysts. Some are
completely unaltered, with typical MAR basaltic chemical composition, whereas
others are variably altered. In extreme cases complete replacement by pure
silica was observed. Intermediate cases were also observed, where only the
border of the fragment has been modified by hydrothermal alteration, again with
a gain in silica.

The vesicles in the glass fragments are rounded to
oval, and partially filled with hydrothermal precipitates, mostly: a) amorphous
silica (lining the vesicles and also in the interior), b) sulphides, and c) iron
and manganese oxides (fig. 3).

The matrix hydrothermal precipitates are
sulphides, silica, barite, iron and manganese oxides and rare carbonates.

Authigenic sulphides are thus disseminated in the
rock or concentrated in basalt vesicles (fig. 3). Marcasite and sphalerite are
abundant, but chalcopyrite and pyrite are also observed. Pyrite is the less
frequent, appearing in the borders of basalt fragments and in spheroidal
aggregates (pyrite spheres). These are typical low temperature hydrothermal
activity structures. Transported sulphide chimney fragments exhibit alteration
contours with covellite, bornite and digenite.

Figure 3. Altered vesicular
basalt fragment in slab. Here it is possible to observe sphalerite (sph)
crystals and amorphous silica (opal) filling the
vesicles.

Silica is
a late stage hydrothermal product becoming part of the cement and filling the
basalt vesicles mostly in its amorphous form - opal. Barite is formed after most
of the sulphides, exhibiting often large crystals in cavities and
fractures.

The slab rocks can be classified as hydrothermal breccias,
although some of its components are not hydrothermal. These rocks are formed
"in situ" as the result of diffuse hydrothermal circulation at low
temperatures (around 200ºC), through the basalt fragments present on discharge
sites. The alteration of the basaltic glass fragments is clearly a result of the
hydrothermal circulation processes responsible for slab formation.

Conclusions

Rapid oxidation of sulphide material, immediately after
fluid circulation stops, indicates that preservation of large massive sulphide
bodies is only possible if they are, in some way, protected from sea water
action. Slab crusts can fulfil this rôle, as covers which isolate the fluid
circulation within the crust from sea water after consolidation. A closed system
is formed, where convective circulation may be induced allowing important
accumulation of non oxidized massive sulphides.

References
Costa, I.A.; Barriga, F.J.A.S.; Fouquet, Y.,
1995. Active hydrothermal sites and ancient massive sulphides: Lucky Strike
versus Iberian Pyrite Belt - TERRA abstracts. Abstract supplement no. 1 to TERRA
nova, 7, p. 212.

Costa, I., 1995. Estudo dos
processos de alteração associados aos sistemas hidrotermais Lucky Strike e Menez
Gwen (37º20’N E 37º50’N, Crista Médio-Atlântica). Tese de Mestrado. (in
preparation)

Desbruyères, D.; Alayse, A.M.;
Antoine, E.; Barbier, G.; Barriga, F.; Biscoito, M.; Briand, P.; Brulport, J.P.;
Comtet, T; Cornec, L.; Crassous, P.; Dando, P.; Fabri, M.C.; Felbeck, H.;
Lallier, F.; Médioni, A.F.; Gonçalves, J.; Ménard, F.; Kerdoncuff, F.; Patching,
J.; Saldanha, L. & Sarradin, P. M., 1994. New information on the ecology of
deep-sea vent communities in the Azores Triple Junction Area: preliminary
results of the DIVA 2 cruise (May 31 - July 4, 1994). InterRidge News, 3(2):
18-19.

Équipe scientifique DIVA 1 (Y
Fouquet; J L Charlou; J P Donval; J Radford-Knoery; H Pellé; H Ondréas; M
Segonzac; I Costa; N Lourenço; M Tivey), 1994. Rapport de campagne DIVA 1 (5 Mai
- 29 Mai 1994). (não publicado)

Fouquet, Y.; Charlou, J. L.; Costa,
I.; Donval, J. P.; Radford-Knoery, J.; Pellé, H.; Ondréas, H.; Lourenço, N.;
Segonzac, M. & Tivey, M., 1994. A detailed study of the Lucky Strike
hydrothermal site and discovery of a new hydrothermal site: Menez Gwen;
preliminary results of the DIVA1 cruise (5-29 May, 1994). InterRidge News, 3(2):
14-17.

Langmuir, C.H.; Fornari, D.;
Colodner, D.; Charlou, J.L.; Costa, I.; Desbruyeres, D.; Desonie, D.; Emerson,
T.; Médioni, A.F.; Fouquet, Y.; Humphris, S.; Saldanha, L.; Sours-Page, R.;
Tatcher, M.; Van Dover, C.; Von Damm, K.; Wiese, K. & Wilson, C. (Lucky
Strike Team), 1993. Geological setting and characteristics of the Lucky Strike
vent field at 37 17'N on the Mid-Atlantic Ridge - EOS, American Geophysical
Union Transactions, 74, p. 99.

Acknowledgement

This research has been undertaken in the framework
of the Marflux/ATJ project. We acknowledge the support from the European
Commission’s Marine Science and Technology Programme (MAST II) under contract
MAS2-CT93-0070.