Rhyolitic globular peperites from the Aznalcَllar mining district (Iberian Pyrite Belt, Spain): physical and chemical controls

Globular peperites are exposed along the Aznalcóllar mining district, Iberian Pyrite Belt (IPB), Spain. These rocks originated by mingling of a low-viscosity rhyolitic sill with unlithified, wet sediments. Highly amygdaloidal (up to 80% vol., millimeters to centimeters across) rhyolite globules extend from the upper contact of the sill for several centimeters into fine-grained volcaniclastic host rocks. The sill is very similar to other rhyolites in the IPB in terms of petrography and chemistry. It is aphyric to very sparsely quartz–phyric, with small quartz phenocrysts. Vesicularity has been measured in the sill as vesicle area fraction, determined by point counting over a rectangular grid, and number density per unit area, determined by counting every vesicle within the grid area. Amygdale distribution, measured within the sill, indicates a process by which vesicles migrated vertically from the lower portions of the sill to the roof zone. We suggest that this migration process caused local density changes, large enough to allow the rhyolitic magma to become buoyant enough to move through the overlying sediments, producing globular peperites. Globular structures are preserved when rhyolite globules entered the wet sediment.