University of Tasmania
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Facies Architecture of the Volcanic Sedimentary Complex of the Iberian Pyrite Belt, Portugal and Spain

posted on 2023-05-26, 04:58 authored by Rosa, CJP
The Iberian Pyrite Belt is the richest massive sulfide province in the world. The massive sulfide ore deposits occur in a felsic volcanic and sedimentary succession (VS Complex) of late Famennian (Upper Devonian) to late Visean (Middle Carboniferous) age. Volcanic facies analysis has been carried out on three areas in Portugal, including the Neves Corvo mine, and five sections in Spain. In all sections studied, the depositional setting during accumulation of the VS Complex was submarine and below wave base. The principal felsic volcanic facies are: (1) coherent rhyolite and dacite, associated with monomictic breccia; (2) fiamme-rich breccia (with variable amounts of dense volcanic and sedimentary clasts), fiamme-rich sandstone and fiamme-bearing mudstone; and (3) crystal-rich sandstone and mudstone. Mafic units are minor, dominated by coherent facies and have uncertain mode of emplacement (intrusions or lavas). Fiamme typically have lenticular shape and quartz- or quartz- and feldspar-phyric texture, and are interpreted to be altered and compacted pumice clasts. The volcanic facies are typically interleaved with, and regionally less voluminous than, the non-volcanic facies, which are dominated by mudstone. The felsic volcanic facies are interpreted to be the products of numerous, relatively small intrabasinal volcanic centres that generated abundant lavas, domes and pyroclastic units. Some volcanic centres are dominated by lavas, whereas others have similar proportions of lavas and pyroclastic units. The domes and lavas are more voluminous but less laterally extensive than the pyroclastic units. A sediment-matrix breccia typically occurs at the top contact of the felsic lavas with sedimentary units. This sediment-matrix breccia formed from the infiltration of fine sediment into interclast spaces in previously formed hyaloclastite, and could be misinterpreted as peperite. Felsic intrusions are less voluminous than lavas, and were emplaced as cryptodomes and partly extrusive cryptodomes, late in the evolution of the VS Complex. The architecture of the different study areas reflects differences in the eruption style, emplacement processes and proximity to source. Parts of the succession interpreted to be proximal are dominated by thick lavas/domes and intrusions, and coarse pyroclastic deposits. Medial parts comprise resedimented autoclastic facies derived from the lavas and domes, and relatively thin pyroclastic units. Distal parts comprise relatively thinly bedded crystal-rich sandstone and siliceous mudstone. Regional correlations in the VS Complex are impossible, as none of the volcanic facies are regionally extensive and each of the volcanic centres has a unique internal architecture. At Neves Corvo mine, the massive sulfide ore deposits are close to one of the felsic volcanic centre(s), occurring immediately above the rhyolitic lavas/domes. IV


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