Geochemistry and architecture of the host sequence of the massive sul fi des in the northern Iberian Pyrite Belt

The northernmost Iberian Pyrite Belt hosts several small felsic volcanogenic massive sulfide orebodies (< 15 Mt), except the Aguas Tenidas and Magdalena each with ca. 45 Mt, which are economically important because of their high base and precious metal grades if compared with the bulk of the IPB. They occur aligned in a E-W belt located just south of the major suture that limits the South Portuguese Zone with the Pulo do Lobo Unit. The mineralization is related to a specific felsic volcanic unit that occurs in different tectonically dismembered units, but has a specific geochemical signature and a characteristic hydrothermal alteration. Six principal tectonostratigraphic units can be differentiated: (1) The Lower Felsic Unit is composed of feldspar-quartz-phyric rhyodacite (crypto)-dome complexes, intruded by sills and dykes of similar composition. Pumice and glass-rich breccia beds occur at the top and on the lateral margins of the dome complexes. A major shear zone separates the Lower Felsic Unit from the overlying Volcano-Sedimentary Unit; (2) The Volcano-Sedimentary Unit consists of vesicular basaltic lava and associated mafic epiclastic sandstone and siltstone, intercalated with thin beds of shale. A major tectonic contact marks the upper boundary; (3) The Middle Felsic Unit represents a series of intercalated coherent rhyolite domes with associated volcaniclastic rocks, and polymictic epiclastic sedimentary rocks. They are cut by numerous volcanic dykes and geochemically equivalent sub-volcanic intrusions; (4) The Sedimentary Unit is a highly deformed package of grey siltstone with interlayered shale and fine-grained epiclastic sedimentary rocks; (5) The Upper Felsic Unit is a package of dacitic to rhyolitic dome complexes with similar characteristics to the Lower Felsic Unit. Both the basal and upper contacts of this unit are narrow and high-strain tectonized zones. (6) The uppermost unit, the Andesite Unit, is comprised of andesitic dome complexes rich in hyaloclastite breccias and andesitic volcaniclastic rocks. Massive sulfides are hosted dominantly by the Lower Felsic Unit with only some orebodies related spatially to the Upper Felsic Unit (Lomero Poyatos, San Telmo and El Carpio). Geochemically, the Volcanic Sedimentary Complex is dominated by andesitic to rhyolitic calc-alkaline magmatic rocks with a significant proportion of dacite and andesite. The Lower Felsic and Upper Felsic Units are geochemically identical and probably represent dismembered parts of a single volcanic felsic complex. If that holds true, massive sulfide deposits of the northern IPB are hosted by volcaniclastic rocks located in the hanging wall of a single unit dominated by dacite-rhyolite dome complexes. This volcanic unit is characterized by Zr contents< 200 mu g/g (ppm) which are significantly lower than those of the more evolved Middle Felsic Unit (200-750 mu g/g). Structural analysis and the geochemistry suggest that the volcanic sequence is tectonically imbricated, with the youngest units located at the lowest structural position. The proposed reconstructed stratigraphic sequence comprises an andesitic footwall, overlain by the felsic complexes of the Middle Felsic and Lower-Upper Felsic Units. Thus, the massive sulfide deposits formed toward the top of the whole sequence, and associated with the youngest felsic rocks having a specific geochemistry.