Porphyry

Porphyry

Photographed by Michael P. Klimetz

Fremont

COLORADO

Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix. The larger crystals are called phenocrysts and the fine grained matrix is the groundmass. Its chief characteristic is the striking difference between the size of the groundmass and the phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals, like basalt, or the individual crystals of the groundmass may be easily distinguished with the eye, as in granite. Most types of igneous rocks may display porphyrytic texture. Porphyry deposits are formed when a column of rising magma is cooled in two stages. Initially, rising magma is cooled slowly deep in the crust, giving rise to large crystal grains with a diameter in excess of 2 mm. If it rises at a faster rate the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating the small grains that are usually invisible to the unaided eye around and in between the earlier-formed, larger phenocrysts. The cooling also leads to a separation of dissolved metals into distinct zones. This process is one of the main reasons for the existence of rich, localized metal ore deposits such as those of gold, copper, molybdenum, lead, tin, zinc and tungsten.