Recent physical volcanology studies at Albernoa, Neves Corvo and Serra Branca, allow the reconstruction of the volcanic and sedimentary facies architecture, bringing new advances to the comprehension of the style of volcanism and mode of emplacement of the felsic volcanic rocks.
Reconstruction of the volcanic and sedimentary facies architecture was based on detailed logging on traverses along creeks, rivers and roads with continuous outcrop exposures, or using data from drill-core. In both cases, textural examination of polished slabs and thin sections complemented the study. Original primary volcanic textures are well preserved in areas where the regional and hydrothermal alteration is minimal and away from the strongly cleaved intervals near thrust faults that disrupt the stratigraphic succession.
The volcanic facies are directly related with volcanic activity or contain elements derived from volcanic activity. These facies are all part of the VSC and include primary volcanic facies, syn-eruptive facies, syn-eruptive resedimented volcaniclastic facies and post-eruptive resedimented volcanogenic facies (McPhie et al., 1993). The non-volcanic facies enclose the volcanic facies and consist mostly of mudstone, in some places containing thin intervals of chemical sediments, such as red jaspers and chert.
In Albernoa area, rhyodacitic coherent facies are subordinate in volume to their autoclastic equivalents, showing gradational contacts with in situ, clast-rotated and redeposited hyaloclastite. Volcanogenic sedimentary facies characterized by thinly, bedded crystal-rich sandstone and mudstone overlie these primary volcanic facies. Along their contact, there is a sediment-matrix volcanic breccia, which could easily be misinterpreted as peperite. The sedimentary component of this breccia is laminated to thinly bedded parallel to the regional bedding. The proportion of the sedimentary component in the breccia increases upwards and passes gradationally to the overlying sedimentary interval with no change of the bedding orientation. We conclude that the breccia was formed by infiltration of sediment into the clast-supported framework of hyaloclastite at the margins of an intrabasinal felsic lava or dome (Rosa C. et al., 2004a).
The principal volcanic facies identified at the Neves Corvo mine are grouped into two facies associations: pumice-rich facies and coherent and monomictic rhyolite breccia facies (Rosa C. et al., 2004b).
The pumice-rich facies association is composed mostly by two pumice breccia units, characterised by lithic-rich coarse bases grading to massive or diffusely stratified intermediate zones and thinly laminated pumiceous mudstone tops. They are interpreted to be deposits from submarine, pumice-rich gravity flows generated by explosive eruptions. Abundant quartz-phyric pumice clasts are preserved in these units. Proximal facies characterised by thick and coarse intervals occur in the south, whereas thinner and finer grained more distal equivalents occur in the central and northern parts of the area. The pumice breccias probably correspond to syn-eruptive pyroclastic deposits. The source vent(s) has not been located but the dominance of lapilli-size pumice and the relatively good hydraulic sorting could indicate that the vent(s) was submarine.
The succession also includes submarine felsic domes or lavas, consisting of massive and flow-banded coherent interiors grading to jigsaw-fit and clast-rotated external zones. Perlitic clasts with planar and curviplanar margins suggest that quenching and autobrecciation were the dominant fragmentation mechanisms. The gradation to overlying sedimentary facies suggests that the rhyolites were extrusive. This facies association corresponds to a near vent setting. The volcanic succession of Neves Corvo also includes a thin clastic interval of rhyodacite that occurs only locally and stratigraphically above the rhyolite facies association. The clast shapes suggest that quenching was the dominant fracturing mechanism.
Integration of the volcanic facies associations in the Neves Corvo stratigraphy defined by Oliveira et al., (2004), indicates that the pumice-rich facies association is of late Famennian age and the rhyolite facies association is of late Strunian age. The rhyodacite age is undetermined but its stratigraphic position suggests that it may correspond to the early Viséan volcanic event reported by Oliveira et al., (2004).
The volcanic sequence at Serra Branca is characterised by a more complex association of facies. Several deposits of pyroclastic origin with rhyolitic composition occur intercalated with thick rhyodacitic lavas. This association representing alternation of effusive and explosive eruptions is disrupted by several small cryptodomes and partly extrusive cryptodomes of rhyolitic composition.
The study areas in the Portuguese part of the Pyrite Belt reveal thick deposits composed of pyroclasts and intrabasinal lavas or domes affected by intense autobrecciation and quench fragmentation (Figure 8). The dominance of lapilli-size pumice in the pumice rich deposits suggests that the source vent was submarine. The rhyolite and rhyodacite facies are constituted by intrabasinal proximal to vent associations.
