MULTIPHASE WATER-RHYOLITE INTERACTION AND ORE FLUID GENERATION AT ALJUSTREL, PORTUGAL

Detailed study of the Aljustrel volcanic rocks, through petrography, m ineral chemistry, whole-rock geochemistry, and previously published st able isotope data, show that the rocks experienced extreme metasomatis m. The metasomatic phenomena presented are explained accurately by a s ea-water convective model. The evolution of the sea-water-derived flui d as it progresses downwards through the rocks is evident, from cold, oxidising, Mg-bearing, to hot, reducing, Mg-free, and enriched in Si, Fe, Cu, Zn. Several lines of evidence (feldspar mineralogy, parageneti c relations between regional and ore-zone alteration, and stable isoto pe data) show that the temperature of the hydrothermal circulation inc reased through time, and that the water-to-rock ratio evolved from ver y high to low values. according to our data, about 30 ppm Cu and some 100 ppm Zn were removed from the Aljustrel Volcanics. This implies, fo r both metals, the same amount of leached rock to produce the 2 Mt Cu and 6 Mt Zn contained in the massive sulphide deposits of the Aljustre l area. With regard to Fe, about 1% was on average removed from the ro cks, producing enough metal to account for all the Fe in the MS deposi ts and overlying chemical sediments. This study documents semi-quantit atively the provenance of the main metals in major massive sulphide de posits via sea water leaching of the immediate host rocks.