Observations on the pyrite deposits of the huelva district, Spain, and their relation to volcanism

The felsitic porphyritic rocks that occur with Lower Carboniferous shale or slate in the Huelva district, Spain, were examined at the Rio Tinto, Tharsis, and La Zarza mines. Most of the bodies of porphyry are not intrusive into the shale, but instead consist of rhyolite flows overlain by variable thicknesses of coarse and fine pyroclastic rhyolite. These lie conformably beneath the shale. The pyroclastic beds are the ore horizon, and the ore bodies are confined to this stratigraphic zone. Various modes of formation have been postulated for the ore bodies of the Huelva district. The apparent limitation of ore to one strati-graphic horizon for more than 100 kilometers seems to favor a modified syngenetic origin with metallic elements derived from volcanic emanations.