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Hydrothermal Alteration and Mineralization in the Neves-Corvo Volcanic-Hosted Massive Sulfide Deposit, Portugal. II. Oxygen, Hydrogen, and Carbon Isotopes

Jorge M.R.S. Relvas and Fernando J.A.S. Barriga

CREMINER, Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Edifício C6, Piso 4, Campo Grande, 1749-016 Lisboa, Portugal

Fred J. Longstaffe

Department of Earth Sciences, University of Western Ontario, London, Canada N6A 5B7

Corresponding author: e-mail, jrelvas{at}fc.ul.pt

The oxygen, hydrogen, and carbon isotope compositions of hydrothermally altered rocks and minerals from massive and stockwork ores of the Neves-Corvo volcanic-hosted massive sulfide deposit have been used to infer its thermal history and the origins of the hydrothermal fluids. Quartz and cassiterite separated from the stringer and massive cassiterite ores of the Corvo orebody ("tin corridor") have ¹⁸O values of 12.5 to 13.6 and –1.4 to –0.4 per mil, respectively. Quartz-cassiterite oxygen isotope fractionation indicates temperatures of 174° to 207°C (avg 191°C) for precipitation of this mineral pair. The calculated oxygen isotope composition of the fluid (¹⁸OH₂O = –0.4 to +0.7; avg 0.1) suggests that cassiterite precipitation resulted from extensive mixing of a tin-bearing fluid with seawater at sea-floor hydrothermal vents.

The whole-rock oxygen isotope composition of the hydrothermally altered, felsic volcanic rocks hosting the sulfide stockwork of the Corvo orebody varies from the central stockwork (chloritic alteration zone I, ¹⁸O = 8.7–9.6), to the peripheral zones (sericitic and paragonitic alteration zones IIa, ¹⁸O = 11.6–13.1, and IIb, ¹⁸O = 11.0–14.1). The ¹⁸O values of quartz also increase from alteration zone I (avg 13.9) to alteration zone IIa (avg 14.4) to alteration zone IIb (avg 15.0). Quartz from the hanging-wall jasper unit yielded higher ¹⁸O values of 15.5 to 17.9 per mil. Chlorite from alteration zone I has ¹⁸O values of 6.1 to 8.4 per mil (avg 7.4). Sericite has ¹⁸O and D values that vary slightly from ¹⁸O = 9.3 to 10.1 per mil (avg 9.9) and D = –59 to –47 per mil (avg –53) in alteration zone I, to ¹⁸O = 9.8 to 11.4 per mil (avg 10.5) and D = –50 per mil in alteration zone IIa, to ¹⁸O = 9.4 to 11.7 per mil (avg 10.6) and D = –61 to –36 per mil (avg –45) in alteration zone IIb. Siderite associated with waning hydrothermal activity has low ¹³C values (–11.1 to –5.8), suggesting the incorporation of oxidized, organic carbon from the footwall sequence and/or the addition of magmatic carbon.

In alteration zone I, quartz-chlorite fractionation indicates temperatures of 249° to 402°C (avg 335°C) and ¹⁸O_H2O values of 6.1 to 10.2 per mil (avg 8.3). Quartz-sericite pairs gave isotopic equilibrium temperatures of 250° to 289°C (avg 269°C) and ¹⁸O_H2O values of 5.8 to 7.0 per mil (avg 6.4) in alteration zone IIa, and 170° to 318°C (avg 223°C) and ¹⁸O_H2O = 1.4 to 8.3 per mil (avg 4.5) in alteration zone IIb. The oxygen isotope data show that in the core of the feeder system (alteration zone I), the temperature and the ¹⁸O_H2O values were very high. The D values calculated for the ore fluids (D = –37 to –11) are also significantly lower than seawater. Outward from the central stockwork (alteration zones IIa and IIb) the ore fluid was depleted in ¹⁸O and the temperatures were lower.

The oxygen, hydrogen, and carbon isotope signatures of the Neves-Corvo ore fluids are best explained by incorporation of magmatic and/or metamorphic fluids into a seawater-dominated hydrothermal system. Magmatic fluids, in particular, were a likely source for the tin and much of the copper in the Neves-Corvo ores.

This article has been cited by other articles:

				J. M.R.S. Relvas, F. J.A.S. Barriga, A. Ferreira, P. C. Noiva, N. Pacheco, and G. Barriga Hydrothermal Alteration and Mineralization in the Neves-Corvo Volcanic-Hosted Massive Sulfide Deposit, Portugal. I. Geology, Mineralogy, and Geochemistry Economic Geology, June 1, 2006; 101(4): 753 - 790. [Abstract] [Full Text] [PDF]