Hydrothermal alteration and mineralization in the Neves-Corvo volcanic-hosted massive sulfide deposit, Portugal. II. Oxygen, hydrogen, and carbon isotopes

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 delta O-18 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 degrees to 207 degrees C (avg 191 degrees C) for precipitation of this mineral pair. The calculated oxygen isotope composition of the fluid (delta O-18(H2O) = -0.4 to +0.7 parts per thousand; avg 0.1 parts per thousand) 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, delta O-18 = 8.7-9.6 parts per thousand), to the peripheral zones (sericitic and paragonitic alteration zones IIa, delta O-18 = 11.6-13.1 parts per thousand, and IIb, delta O-18 = 11.0-14.1 parts per thousand). The delta O-18 values of quartz also increase from alteration zone I (avg 13.9 parts per thousand) to alteration zone IIa (avg 14.4 parts per thousand) to alteration zone IIb (avg 15.0 parts per thousand). Quartz from the hanging-wall jasper unit yielded higher delta O-18 values of 15.5 to 17.9 per mil. Chlorite from alteration zone I has delta O-18 values of 6.1 to 8.4 per mil (avg 7.4 parts per thousand). Sericite has delta O-18 and delta D values that vary slightly from delta O-18 = 9.3 to 10.1. per mil (avg 9.9 parts per thousand) and delta D = -59 to -47 per mil (avg -53 parts per thousand) in alteration zone I, to delta O-18 = 9.8 to 11.4 per mil (avg 10.5 parts per thousand) and delta D = -50 per mil in alteration zone IIa, to delta O-18 = 9.4 to 11.7 per mil (avg 10.6 parts per thousand) and delta D = -61 to -36 per mil (avg -45 parts per thousand) in alteration zone IIb. Siderite associated with waning hydrothermal activity has low delta C-13 values (-11.1 to -5.8 parts per thousand), 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 degrees to 402 degrees C (avg 335 degrees C) and delta O-18(H2O) values of 6.1 to 10.2 per mil (avg 8.3 parts per thousand). Quartz-sericite pairs gave isotopic equilibrium temperatures of 250 degrees to 289 degrees C (avg 269 degrees C) and delta O-18(H2O) values of 5.8 to 7.0 per mil (avg 6.4 parts per thousand) in alteration zone IIa, and 170 degrees to 318 degrees C (avg 223 degrees C) and delta O-18(H2O) = 1.4 to 8.3 per mil (avg 4.5 parts per thousand) in alteration zone IIb. The oxygen isotope data show that in the core of the feeder system (alteration zone 1), the temperature and the delta O-18(H2O) values were very high. The delta D values calculated for the ore fluids (delta D = -37 to -11 parts per thousand) are also significantly lower than seawater. Outward from the central stockwork (alteration zones IIa and IIb) the ore fluid was depleted in O-18 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.