Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass. Differential cooling permits separation of dissolved metals into distinct zones, creating rich, localised metal ore deposits (gold, copper, molybdenum, lead, tin, zinc and tungsten).
[images - roll-over link for preview (where available); large images (well worth a visit) show only as a corner on preview : hand-specimen diabase porphyry : hand-specimen rhyolite porphyry : hand-specimen andesite porphyry, 2 : porphyry : hand specimen of "ore" from the Coed y Brenin porphyry-copper deposit : Wolf porphyry : red porphyry : red Imperial porphyry Egypt : 4th C porphyry head : xenolith-bearing hybrid porphyry : porphyrite dike cutting undeformed post-tectonic granite : closeup of hybrid porphyry texture, 2, microphenocryst of titaniferous augite rimmed with brown sodic ferrohornblende : outcrop of hybrid porphyry from pipe cluster : gabbro xenolith in hybrid porphyry : Guanella Pass porphyry : Porphyry Basin, 2, 3, 4, 5, 6, g : porphyry Trentino : porphyry quarry : porphyry copper mine : gold-copper porphyry at Quispe : thin-section plagioclase, augite, oxides porphyry, 2, 3, Skaergaard intrusion photomicrographs]