Geology and genesis of the Aznalcóllar massive sulphide deposits, Iberian Pyrite Belt, Spain

The Aznalcóllar mining district is located on the eastern edge of the Iberian Pyrite Belt (IPB) containing complex geologic features that may help to understand the geology and metallogeny of the whole IPB. The district includes several ore deposits with total reserves of up to 130 Mt of massive sulphides. Average grades are approximately 3.6% Zn, 2% Pb, 0.4% Cu and 65ppm Ag. Mined Cu-rich stockwork mineralizations consist of 30Mt with an average grade of 0.6% Cu. Outcropping lithologies in the Aznalcóllar district include detrital and volcanic rocks of the three main stratigraphic units identified in the IPB: Phyllite-Quartzite Group (PQ), Volcano-Sedimentary Complex (VSC) and Culm Group. Two sequences can be distinguished within the VSC. The Southern sequence (SS) is mainly detritic and includes unusual features, such as basaltic pillow-lavas and shallow-water limestone levels, the latter located in its uppermost part. In contrast, the Aznalcóllar-Los Frailes sequence (AFS) contains abundant volcanics, related to the two main felsic volcanic episodies in the IPB. These distinct stratigraphic features each show a different palaegeographic evolution during Upper Devonian and Lower Carboniferous. Massive sulphides occur in association with black shales overlying the first felsic volcanic package (V^sub A1^) Palynomorph data obtained from this black shale horizon indicate a Strunian age for massive sulphides, and consequently an Upper Devonian age for the V^sub A1^ cycle. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment (V^sub A1^) to hydroclastic subvolcanic conditions for the V^sub A2^. This evolution can be related to compartmentalizing and increasing depth of the sedimentary basin, which may also be inferred from changes in the associated sediments, including black shales and massive sulphides. Despite changes in the character of volcanism, the same dacitic to rhyolitic composition is found in both pyroclastic and subvolcanic igneous series. The main igneous process controlling chemical variation of volcanics is fractional crystallization of plagioclase (+accessories). This process took place in shallow, sub-surface reservoirs giving rise to a compositional range of rocks that covers the total variation range of felsic rocks in the IPB. The Hercynian orogeny produced a complex structural evolution with a major, ductile deformation phase (F^sub 1^), and development of folds that evolved to thrusts