Fernando J. A. S. Barriga Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Edifício C2, Piso 5, Campo Grande, 1700 Lisboa, Portugal
One of the more striking characteristics of "black smoker" type activity is the large amount of dispersion of potential ore-forming metals, as "black smoke". Also, the chimneys and other hydrothermal edifices are rapidly oxidised, even before hydrothermal activity vanishes (see Costa et. al., 1995 a, b). This contrasts sharply with many ancient massive sulphide deposits, especially large ones, where often there are no traces of sulphide oxidation. Outstanding examples are found, for example, in the Iberian Pyrite Belt of South Portugal and Southwest Spain. For the giant Feitais-Estação orebody of Aljustrel (100 Mt), Barriga and Fyfe (1988) have proposed a model of ore genesis under a blanket of chemical sediments, based on (1) presence of a cap rock with clear signs of having been formed and reworked by mineralising fluids; (2) complete lack of oxidation of the sulphide ores; (3) lack of sedimentary dilution. The cap rock is mostly composed of jaspers, cherts and Fe-Mn sediments, with many similarities to the Lucky Strike spire complexes (Costa et al., ). The lack of efficiency of "black smokers" in the generation of orebodies suggests that the deposits that do form are largely the result of sub-surface precipitation of the sulphides, either through hydrothermal clogging of the original black smokers and spire complexes (see Goodfellow, 1993), or under a blanket of sediments. Recently, Humphris et al. (1995) reported on the results of drilling the TAG mound, concluding that most sulphide precipitation occurs within the mound itself, cementing and replacing previously existing material, hydrothermal precipitates and basaltic rock. Some of the textures are strikingly similar to the extremely abundant (millions of tonnes) granular ore in the Aljustrel mine (Leitão, 1993).
At first it was thought that oceanic metamorphism affected exclusively, or nearly so, the uppermost ophiolite/oceanic crust layers (basalts and sheeted dykes), and that the process was essentially completed during the first few million years of the rocks, near the mid-ocean rifts. Hydrothermal processes were thought to be driven almost exclusively by heat from the axial magma chambers. Several lines of evidence suggest that widespread hydrothermal activity continues to take place within the rocks long after the rift stage. These are as follows. (1) The gabbro layer contains hydrothermal mineralization, especially near, but not restricted to, its top (Vokes et al., 1990), and there are hydrothermally remobilized base and precious metal (gold and PGE) occurrences in the ultramafic levels (Graham et al., 1991; McElduff and Stumpfl, 1991). (2) Oceanic metamorphism persists through the plutonic rocks, including the ultramafics, both in ophiolites and in the present day oceanic crust. (3) The degree of alteration of young oceanic basalts and dykes (up to ~6 Ma) is far less pronounced than in their ophiolitic counterparts, suggesting that ~75% of the alteration is late in age (see Bickle and Teagle, 1992).
Study of the intrusive sequences has produced important conclusions. Thus, alteration in these rocks initiates at temperatures of ~700°C, and produces a full spectrum of mineral assemblages, down to low temperatures. Under ~500°C, ultramafic rocks begin to become serpentinized (see Barriga and Fyfe, 1983; Barriga et al., 1985, 1987). This process is probably largely oceanic, and the oceanic Moho may well correspond to the lower boundary of serpentinization. The much lower permeability of the plutonic sequence explains the higher temperatures, and implies lower water/rock ratios. This, in turn, suggests that the deeper fluid circulation takes place with a geometrical pattern clearly distinct from that of fluid circulation through the volcanic (basalts) and hypabyssal (sheeted dykes) levels. The boundary between the two fluid circulation systems corresponds to the greatest permeability contrast in the oceanic crust, where mixing of two thermally and chemically distinct fluids takes place, with related mineral deposition (Barriga, 1980).
The scale of oceanic hydrothermal activity is such that the often modest size of the known mineral deposits is somewhat surprising. This may signify that major deposits do not generally form, given excessive dispersion, but it may also reflect the minuscule size of the ophiolites exposed on land. Mineral exploration in the oceans, which is today just beginning, may still produce surprises of economic value.
Barriga, F.J.A.S., Graduate Seminar, U. of Western Ontario, 29 pp. (1980).
Barriga, F.J.A.S., Munhá, J., Fyfe, W.S. & Vibetti, N.J., Eos, Trans., AGU 66, 1128 (1985).
Barriga, F.J.A.S., Munhá, J., Fyfe, W.S., Vibetti, N.J. & Kerrich, R., Symposium Troodos 87 Ophiolites and Oceanic Lithosphere, Nicósia, Chipre, p. 134 (1987).
Barriga, F.J.A.S. & Fyfe, W.S., Cont. Mineral. Pet. 84, 146-151 (1983).
Bickle, M.J. & Teagle, D.A.H., Earth Planet. Sci. Lett. 113, 219-237 (1992).
Costa I.A., Barriga, F.J.A.S. & Fouquet, Y., Terra Abstracts nº1, Terra Nova 7, 212 (1995a).
Costa I.A., Barriga, F.J.A.S. & Fouquet, Y., Memórias nº 4, Universidade do Porto - Faculdade de Ciencias, Museu e Laboratório Mineralógico e Geológico, 979-983 (1995b).
Goodfellow, W.D., In: Hutchinson, R.W. & Scott, S. (Eds), Proceedings 24th Underwater Mining Institute, Aspen, Colorado, (1993).
Graham, I.T., Marshall, B. & Franklin, B.J., In: Pagel, M, Leroy, J.L. (Eds), Source, Transport and Deposition of Metals. Balkema, Rotterdam, 619-622 (1991).
Humphris, S.E., Herzig, P.M., Miller, D.J., Alt, J.C., Becker, K., Brown, D., Brügmann, G., Chiba, H., Fouquet, Y., Gemmell, J.B., Guerln, G., Hannington, M.D., Holm, N.G., Honnorez, J.J., Iturrlno, G.J., Knott, R., Ludwig, R., Nakamura, K., Petersen, S., Reysenbach, A.-L., Rona, P.A., Smith, S., Sturz, A.A., Tivey, M.K. & Zhao, X., Nature 377, 713-716 (1995).
Leitão, J.C., APIMINERAL, 1.12-1-1.12-21 (1993).
McElduff, B. & Stumpfl, E.F., Mineralium Deposita 26, 307-318 (1991).
Vokes, F.M., Constantinou, G., Panayiotou, A. & Prestvik, T., In: Malpas, J.G., Moores, E.M., Panayiotou, A. & Xenophontos, C. (Eds), Ophiolites. Oceanic Crustal Analogues. Proceedings of the Symposium Troodos 1987. Geological Survey Department, Cyprus, 627-638 (1990).
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