Regional oxygen isotope systematics of felsic volcanics;: a potential exploration tool for volcanogenic massive sulphide deposits in the Iberian Pyrite Belt

Regional oxygen isotopic sytematics have been performed mainly on the felsic volcanic footwall rocks of the orebodies but also on purple schist characteristic of the hanging wall series, around two giant VMS deposits in the Spanish Iberian Pyrite Belt, Riotinto and La Zarza. As the terranes of the Iberian Pyrite Belt, these two giant deposits have been affected by the Hercynian tectono-metamorphic events, strongly modifying their geometry. About 60 and 40 samples were collected over a 10 x 4 km(2) area at Riotinto and a 3 x 2 km(2) area at La Zarza, respectively. Whole-rock powders were analysed for oxygen by CO2-laser fluorination. At both sites, a same type of low-delta O-18 anomaly down to +3.6 parts per thousand, well differentiated from the regional background (up to 20 parts per thousand), was identified near the orebodies. The lowest delta O-18 values (+4 to +11 parts per thousand) correspond to the chlorite hydrothermal halo, essentially restricted to the feeder zones of the orebody. Intermediate delta O-18 values (+9 to +15 parts per thousand) correspond to the sericite hydrothermal halo, mostly developed laterally to the orebody until 0.5-1 km. The regional background(+16 to +20 parts per thousand) is represented by spilitised volcanic rocks. A same kind of low anomaly, but with less contrast, was defined in purple schist in the immediate hanging wall of the orebodies. All these results demonstrate that, despite high geometrical modifications of the orebodies related to the Hercynian tectonics, oxygen isotopic anomalies recorded by volcanic host rocks during the emplacement of the mineralising hydrothermal systems are still identified. This strongly suggests that oxygen isotopic systematics could be useful to identify target areas in the Iberian Pyrite Belt, as already demonstrated on other VMS targets in the world. (C) 2001 Elsevier Science B.V. All rights reserved.