Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Lower Carboniferous felsic, explosive, submarine volcanism took place at Aljustrel, creating the ores of one of the principal mining centres of the Iberian Pyrite Belt. Some 250 Mt of massive sulphide ore occur near the top of several hundred metres of pyroclastic volcanics which overly a sediment covered continental basement.;Petrographic, mineral chemical, whole rock geochemical and oxygen isotope investigation of the Aljustrel volcanic rocks remote from mineralization shows that their present quartz-keratophyric (felsic spilitic) compositions result from widespread interaction with sea water, at high water/rock ratios and temperatures ranging 0-300(DEGREES)C. Opaque mineralogy and textures show that iron was oxidized in rocks near the sea floor and leached from deeper rocks, with leaching of transition metals. Jaspers and cherts present above the volcanics may have formed concomitantly, through chemical precipitation of silica, forming an impermeable cover.;Investigation of Feitais ore zone rocks suggests that ore formation took place largely in open space, through almost isothermal mixing of sulphur and metal rich waters. There is clear evidence for silica and sulphide precipitation in a feeder stockwork. Cherts and phyllitic sediments covering the ore, show clear evidence of mineralization and reduction by interaction with the ore forming fluids. Deformation of such rocks indicates that such mineralization predated regional metamorphism. The presence of a cap rock is also suggested by temperature profiles from oxygen isotope systematics. Thus the Feitais orebody was not exhalative sensu stricto, but formed beneath a thin impermeable cap which in fact may have been floated off on rising fluids.;The ore forming process postdated sea water alteration of volcanics and involved a second stage convective system with lower water/rock ratios perhaps with a contribution from metamorphic fluids.;The Aljustrel tectonic-thermal setting is clearly one which involves the ideal set of variables producing giant deposits, orders of magnitude larger than those typical of ophiolites.



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