Barrett, T.J., 1999.

The present stratigraphic and alteration study of the host rocks of the Feitais massive sulfide deposit is based on treatment and interpretation of 125 whole-rock lithogeochemical analyses from the work of Barriga (1983), plus 50 new samples collected in 1998 during a new drilling and geological program which is being carried out at Aljustrel by EuroZinc Mining Corp. The two data sets have been treated using immobile element methods, which have proved useful in other VMS terranes for the identification of volcanic units, correlation of stratigraphy, and the quantification of hydrothermal alteration. The host rocks at Feitais include three chemically distinct types of rhyolite, termed A, B and C, which can be readily recognized using plots of immobile element ratios. The stratigraphic footwall to the deposit consists of rhyolite A in virtually all holes sampled. Despite the commonly severe stockwork alteration, rhyolite A can be readily identified chemically. It appears to extend at least 100 m down into the footwall (many of the holes which intersected the orebody did not extend far into the footwall). The orebody is overlain by 10-30 m of feldspar-phyric rhyolite C, then 5-15 m of cherty to jaspery to manganiferous chemical sediments, 25-50 m of fine-grained volcaniclastic sediments and argillites (Paraiso formation), and then a thick sequence (>250 m) of coarse-grained turbidites (Culm flysch). Rhyolite C appears to pass laterally into the cherty unit in the updip direction. Just downdip of the orebody, rhyolite A footwall is overlain, in two holes, by 20-50 m of rhyolite B, then thin cherts. In one of these holes, the lower rhyolite B hosts two thin sulfide occurrences. It is not known if rhyolite C occurs above this, as these two holes have not as yet been completely sampled. Minor rhyolite B is present is at the base of the main orebody in two other holes, where it forms thin intervals of altered mineralized material. It is conceivable that this material represents debris shed into a local basin from a marginal area where rhyolite B was exposed. Relative uplift of the footwall just downdip of the orebody is also suggested by the ‘higher’ level of the top of rhyolite A in this area. Together with the fact that the cherty unit maintains a fairly consistent stratigraphic level in the downdip direction, these relations suggest that the thinning out of the orebody is related to paleo-topographic effects rather than later faulting. A fourth type of felsic volcanic rock, here termed quartz-feldspar porphyritic dacite, occurs in the Feitais tunnel and at Moinho. This rock has the silica content of rhyolite, but has higher Fe, Ti, Cr and V contents. It may be derived from a separate magma chamber.

Alteration effects have been assessed using mass change calculations, which were carried out using the single-precursor method for each main rock group (rhyolites A, B, C, QF dacite, and clastic sediments). Large mass additions of Fe and Mg are present over much of the drilled footwall below the orebody. It is of interest that strong alteration also occurs in the lower part of the two holes drilled on the downdip flank of the orebody. Areas with Fe and Mg gains generally show additions of Cu, and to a lesser extent Zn. Further sampling will be required at depth in the footwall and lateral to the orebody in order to establish the geometry of alteration vectors. The areas of large Fe mass gain represent conduits along which hot and mineralizing fluids travelled, whereas areas with some Fe but large Mg gains represent zones of mixing of these fluids with downwelling cooler seawater (i.e. at the margin of a large convection cell). Alteration of the hangingwall rhyolite C and the Paraiso sediments is of a much different type, with no appreciable additions of Fe and Mg, but moderate (locally large) additions of Ba and Mn. Ba also occurs near the top of the footwall at the updip end of the orebody. The footwall rhyolites are almost totally depleted in Na and Ca, while hangingwall strata are less depleted.

The chemostratigraphic framework established in this study for the Feitais deposit should be of use to step-out and exploration drilling in laterally removed areas. The methodology allows ready identification of specific contacts of exploration interest, even in altered areas. If similar data are obtained for host rocks of other deposits at Aljustrel (e.g. Estacao, Moinho), it should be possible to work out large-scale stratigraphic and structural relations across the whole area.