On the basis of about 100 new whole-rock lithogeochemical analyses plus logging of selected drill core at the Feitais deposit in July 1999, together with results from our earlier studies in 1998-99, the following sequence of volcanic and mineralization events has been established. Five main volcanic units were identified using immobile-element-based methods.
1) Eruption of rhyolite A footwall tuffs. These are mainly aphyric to sparsely feldspar- phyric tuffs and lapilli tuffs, with local intervals (5-20 m thick) of massive rhyolite or autobreccia. The rhyolite A sequence is about 100 m thick under the orebody, but >200 m thick along the downdip margin of the orebody.
2) Local deposition of thin (m-scale beds) of rhyolite B volcaniclastic material, probably shed from the downdip margin of the sulfide depositional basin. Where present, these beds are usually intercalated within the lowest several metres of the massive sulfide lens. At this time, local eruptions of rhyolite B massive lavas and tuffs, totalling a few tens of metres thickness, occurred on the downdip margin of the basin.
3) Accumulation of 20-100 m of mainly fine-grained, locally layered, Zn-Pn-bearing pyritic massive sulfides, with zones of more Cu-rich pyritic sulfides in the lower portion of the orebody, and py-cp-qtz stockworks developed within the underlying rhyolite A sequence.
4) Emplacement of rhyolite C hangingwall tuffs. In most places, these tuffs contain abundant feldspar crystals up to 5 mm across. Rhyolite C is 1-30 m thick. It generally lies directly on the massive sulfides, and is overlain by cherts. In places, cm-scale layers of chert of sulfides are intercalated within the rhyolite C interval.
5) Accumulation of 1-20 m of massive cherts. They are commonly very siliceous (>90% SiO2), but can include hematitic, magnetitic, pyritic or manganiferous varieties.
6) Accumulation of at least 50 m of argillites and terrigenous siltstone turbidites, referred to as the Paraiso formation. The several metres of sediments above the chert formation are in places notably enriched in Fe oxides and Mn phases, or in barite. In a few holes, thin layers of felsic tuff also occur within the lower part of the Paraiso formation, marking the last phase of volcanic activity at Feitais. The Paraiso sediments pass gradually upwards through an increasing proportion of greywacke turbidites into the Culm formation (> 250 m).
Synvolcanic uplift of the footwall just downdip of the orebody is suggested by the higher level of the top of rhyolite A in this area. The general thickening of massive sulfides and rhyolite C in this direction indicate that the sulfides accumulated in an asymmetrical graben bounded on one side by a seafloor growth fault. The latter area was also the site of the localized rhyolite B activity. Metre-scale intercalations of rhyolitic material within the main sulfide lens in some downdip drill holes probably represent debris shed from this fault scarp.
A fourth rhyolite type is represented by quartz-feldspar megacrystic tuff, which outcrops over much of the central part of the Aljustrel property, and also in the Feitais-Moinho tunnel and in the apparent stratigraphic hangingwall at Moinho. This rock has the silica content of rhyolite, but has higher Fe, Ti, P, Cr and V contents. It is technically best described as a crystal-rich ferrorhyolite tuff, or QFX rhyolite for short. Despite variations in the size and proportion of crystals, it has a near-uniform primary composition, and is distinct from any of the other rhyolite types. Two deep drill holes into the Feitais footwall encountered a fifth rhyolite type, termed X, which was followed downhole by QFX rhyolite in one case, and by rhyolite C then QFX rhyolite in the other. These latter units are interpreted to belong to a separate tectonic panel which was juxtaposed against the thrust-out overturned limb of the Feitais anticline.
Alteration effects have been assessed using mass change calculations, which were carried out using the single-precursor method for each of the main rock types (rhyolites A, B, C, X, QFX rhyolite, and the fine-grained terrigenous sediments). Footwall rhyolite A is generally near- depleted in Na and Ca. Large additions of Fe and Mg are present over much of the drilled area below the orebody, although most holes generally did not penetrate >50 m into the footwall, excluding the two deep holes. It is of interest that strong alteration also occurs in the lower part of three holes drilled on the downdip flank of the orebody. The areas with large Fe + Mg gains (± Cu additions) represent conduits along which hot and mineralizing fluids travelled. The current data suggest that at least some of the main feeder zones were located in the vicinity of the downdip fault scarp. As rifted seafloor settings commonly comprise a series of horsts and grabens, the possibility remains open that sulfides were also deposited on the downdip side of inferred horst. In the hangingwall, rhyolite C ranges from little to moderately chloritized, while the lower Paraiso formation contains variable but locally large additions of Fe and Mn, or Ba, representing low-temperature exhalative components. The QFX rhyolite is generally weakly altered apart from alkali exhange effects, although its finer-grained marginal facies can be chloritized, which produces the so-called green facies of the QFX rhyolite. Rhyolite B, located on the downdip flank of the Feitais orebody, is relatively unaltered.
At the Moinho deposit, observations and data from the backfill quarry and drilling on section 60 indicate that rhyolite B constitutes the stratigraphic footwall of the massive sulfide deposit, while QFX rhyolite occurs in the hangingwall. In the quarry, a few metres of chert outcrop which are probably the lateral equivalent of the underground orebody; the chert is followed by 5-10 m-thick interval of argillaceous sediments, rhyolite C, and quartz-veined sediments, then the QFX rhyolite. This sequence is interpreted as being largely in the original stratigraphic order, although probably thrust modified in places. Underground on section 60, the rhyolite B footwall is followed downhole by a sequence of rhyolite C tuffs, chert and Paraiso sediments. As the latter sequence represents an inverted package of rocks, a tectonic contact must be present between it and rhyolite B. This tectonic contact, as it comes to surface, probably cuts out the rhyolite C unit, such that rhyolite B is placed in contact with Paraiso sediments.
The QFX rhyolite extends from the eastern margin of the Moinho quarry to the eastern margin of the Sao Joao quarry, where it is succeeded eastwards by Fe-bearing sediments, then a major interval of chert. If this whole sequence is more or less unbroken and if it youngs eastwards, then the chert interval would overlie the QFX rhyolite, and would represent a different level relative to the cherts which are directly above or lateral to the massive sulfide lenses at Feitais and Moinho. However, this assumption needs to be verified by further field work. At Algares, one surface sample from the immediate hangingwall of the sulfide gossan is of QFX rhyolite, while one from the immediate sulfide-veined footwall is of rhyolite X. If these relations are shown with further sampling to represent the general position of the Algares massive sulfides, then the contact between the QFX and X rhyolites would be an important one to explore at depth. In fact, if the QFX rhyolite also stratigraphically overlies the orebody at Moinho, then the lower contact of this rhyolite is in general a favorable one. This contact presumably also lies below the central part of the Aljustrel property, although its position is unknown as little deep drilling has been done in this area. All of the rocks in the Malpique quarry in the south-central part of the property are QFX rhyolites, apart from two probable infolds of upper chert.
The stratigraphic framework that has been established by this study for the Feitais deposit can be applied to exploration drilling in laterally removed areas. The lithogeochemical methods allow ready identification of specific contacts of exploration interest, even in strongly altered areas. With further field work and sampling across the Aljustrel property, it should be possible to establish the original stratigraphic relations amongst the various massive sulfide deposits, and also the later structural modifications.
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