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

Abstract

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 δ18O 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 (δ18OH2O = −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, δ18O = 8.7–9.6‰), to the peripheral zones (sericitic and paragonitic alteration zones IIa, δ18O = 11.6–13.1‰, and IIb, δ18O = 11.0–14.1‰). The δ18O 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 δ18O values of 15.5 to 17.9 per mil. Chlorite from alteration zone I has δ18O values of 6.1 to 8.4 per mil (avg 7.4‰). Sericite has δ18O and δD values that vary slightly from δ18O = 9.3 to 10.1 per mil (avg 9.9‰) and δD = −59 to −47 per mil (avg −53‰) in alteration zone I, to δ18O = 9.8 to 11.4 per mil (avg 10.5‰) and δD = −50 per mil in alteration zone IIa, to δ18O = 9.4 to 11.7 per mil (avg 10.6‰) and δD = −61 to −36 per mil (avg −45‰) in alteration zone IIb. Siderite associated with waning hydrothermal activity has low δ13C 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 δ18OH2O 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 δ18OH2O 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 δ18OH2O = 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 δ18OH2O values were very high. The δD values calculated for the ore fluids (δD = −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.