Dawson, G.L, Barrett, T.J., Alverca, R., Azinhaga, V., Caessa, P., Carmichael, B., Cuttle, J., Soares, S. and Sousa, J., 2000.

The Aljustrel massive sulphide deposits are hosted by Upper Paleozoic volcanic rocks of the Iberian Pyrite Belt in southern Portugal. Within an area of 5 x 2 km centered on the town of Aljustrel are five main deposits (Gaviao, San Joao, Moinho, Algares, Estacao and Feitais). In total, they contain at least 230 MT of massive sulphides, one of the largest such accumulations in the world. Indicated and inferred resources have been recently calculated by Steffen, Robertson and Kirsten Limited for the Feitais and Moinho deposits as part of a feasibility study examining the economic viability of selectively mining the higher-grade zinc and copper zones. The zinc resource for the two deposits totals 30 MT grading 0.28 % Cu, 1.74 % Pb, 5.36 % Zn, 59.40 g/t Ag and 0.74 g/t Au, at a 4.0 % zinc cut-off. The footwall of these two deposits contains a separate copper resource of 9.7 MT grading 2.07 % Cu, 0.32 % Pb, 1.03 % Zn, 15.33 g/t Ag and 0.37 g/t Au, at a 1.5 % copper cut-off. The mineralization is open down-dip and down-plunge at both deposits.

At both deposits, the ores occur as stratiform, northwest-striking lenses hosted within the upper part of the Volcanic-Siliceous Complex (VSC), which is overlain by the Flysch Group (Fig. 1). Several large-scale anticlines and synclines (and thrusts) occur within the VSC. The northern end of the VSC is truncated by the east-northeast-striking Messajana Fault. North of this fault, the VSC has been down-dropped and is only exposed in erosional windows through a Tertiary sedimentary cover. The southern end of the VSC appears to plunge gently to the southeast below conformably overlying sediments of the Flysch group.

The Feitais and Moinho sulfide deposits are underlain by >150 m of felsic tuffs with lesser felsic flows and small subvolcanic plugs, and are overlain by feldspar-bearing felsic tuff followed by cherts and minor volcaniclastic sediments. Footwall rhyolites and lowest hangingwall rhyolites are strongly altered to chlorite-sericite-pyrite±carbonate assemblages. Immobile-element plots have identified three chemically different rhyolite types (X, A and B) in the footwall, and a fourth rhyolite (C) in the hanging wall. Zr/Y relations suggest that rhyolites A and C are of tholeiitic affinity, while rhyolites B and X are transitional. These four rhyolites types have similar REE patterns with (La/Yb)n = 4-6 and moderately high ÖREE contents, features which are fairly common for rhyolites in rift-related settings where the underlying crust is continental. A fifth felsic type, which contains coarse-grained quartz + feldspar (QF rhyolite tuff), occurs over a large central anticlinal area which separates the eastern massive sulfide deposits (Estacao, Feitais) from those in the west (Gaviao, San Joao, Moinho, Algares). The QF rhyolite is distinct from the other rhyolite types in having steeper REE patterns and higher initial Ti and Fe contents. Its stratigraphic position is currently uncertain.

The massive sulphide lenses at Aljustrel are up to a few km in length, 100s of m wide, and up to 100 m thick. Although mainly pyritic, they commonly have a Cu-rich base that grades upward through a lower-grade to barren pyrite zone to a Zn±Pb-rich upper portion. Copper stockworks underlie the thicker sections of most deposits and appear to be spatially associated with growth faults that controlled local basin geometry and deposition of the overlying volcanic and cherty units. Mineralization at Aljustrel is inferred to have formed at and near the sea floor in elongate, restricted and possibly reduced basins adjacent to these faults. The overall tectonic setting is inferred to be a rifted continental margin basin behind a volcanic arc. Similar tectonic settings where Zn-Pb-rich massive sulfides are present include the Bathurst camp of Canada and the Mount Windsor subprovince in Australia (Lower Paleozoic), and the recent Okinawa Trough in the western Pacific.