Dawson, G.L., Barrett, T.J., Caessa, P. and Alverca, R., 2001.

The Feitais deposit is one of six polymetallic, volcanogenic massive sulphide deposits located adjacent to the town of Aljustrel in the Iberian Pyrite Belt of southern Portugal.

The massive sulphide lens is over 1000 m in length, 500 m wide and up to 100 m thick. Although mainly fine-grained pyrite, it commonly has a Cu-rich base that grades upwards through a low-grade pyritic zone to a Zn-Pb-rich upper portion. Sulphide minerals include sphalerite, galena, chalcopyrite, tetrahedrite and minor arsenopyrite. Copper stockworks underlie the thicker sections of the deposit and appear to be spatially associated with growth faults that controlled basin geometry. These faults also influenced the distribution of tuffaceous and cherty (exhalative) units which overlie the sulfides.

A recent feasibility study examined the economic viability of selectively mining the higher-grade zinc and copper zones from the deposit. This study identified a total zinc resource, generally located near the hangingwall, of 18.4 million tonnes grading 0.20 % Cu, 1.81 % Pb, 6.02 % Zn, 66.50 g/t Ag and 0.73 g/t Au (at a 4.5 % zinc cut-off); and a separate copper resource in the footwall of 6.74 million tonnes grading 2.12 % Cu, 0.22 % Pb, 0.83 % Zn, 12.60 g/t Ag and 0.25 g/t Au (at a 1.5 % copper cut-off).

The deposit occurs on the upright limb of the Feitais anticline, within the upper part of the regional Volcanic-Siliceous Complex (VS) of Devono-Mississippian age, and near the contact with the overlying Flysch Group. It is underlain by felsic tuffs, flows and subvolcanic intrusions and is overlain by an upward sequence of feldspar-bearing felsic tuff, chert, greywacke and argillite. The deposit is open down-plunge to the northwest for an additional 250 m. It is truncated by the northeast-striking, steeply dipping Represa Fault. The deposit is offset right laterally about 400 m along this fault, north of which it is called the Estacao deposit. The axial plane of the Fetais anticline strikes northwest and dips moderately to steeply east; this deformational event produced well-developed foliation. A second, weaker phase of deformation created large open folds and weakly developed crenulation cleavage; this folding caused the axis of the Fetais anticline to plunge shallowly to the northwest and southeast.

Footwall and hangingwall felsic volcanic rocks, which total up to 200 m in thickness, are altered to assemblages of chlorite-sericite-pyrite-quartz-carbonate. Alteration is strongest around the footwall stockworks. Where footwall tuffs are less altered, they are commonly monolithic and contain delicate cuspate outlines, which suggests that some of them are hyaloclastites. Immobile-element plots identify two chemically distinct rhyolites (A and B) in the footwall of the deposit, and a third rhyolite (C) in the immediate hangingwall of the deposit. Rhyolite A forms most of the footwall to the Feitais deposit, while rhyolite B occurs mainly along the downdip edge of the massive sulphide lens. The three rhyolite types have similar REE patterns. Zr/Y ratios suggest that rhyolites A and C are of tholeiitic affinity, while rhyolite B is transitional. The chemical features of the rhyolites are consistent with their formation in a rift-related basin underlain by continental crust.

Mineralization at Feitais is inferred to have formed at and near the sea floor in an elongate, restricted and possibly reduced basin bounded by major synvolcanic faults. The overall tectonic setting was probably a rifted continental margin behind a volcanic arc. Zn-Pb-rich massive sulphides occur in similar tectonic settings in the Bathurst camp of Canada, the Mount Windsor subprovince in Australia, and the Okinawa Trough in the western Pacific.