be effective in changing rock compositions over tens of centimeters adjacent to melt conduits: gradients in mineral composition adjacent to pyroxenite dikes may preserve evidence of the process. Modal metasomatism may result in formation of amphibole and phlogopite, and the presence of these minerals in peridotite xenoliths has been considered strong evidence of metasomatic processes in the mantle. Formation of minerals less common in peridotite, such as dolomite, calcite, ilmenite, rutile, and armalcolite, is also attributed to melt or fluid metasomatism. Alteration assemblages Investigation of altered rocks in hydrothermal ore deposits has highlighted several ubiquitous types of alteration assemblages which create distinct groups of metasomatic alteration effects, textures and mineral assemblages. Propylitic alteration is caused by iron and sulfur-bearing hydrothermal fluids, and typically results in epidote-chlorite-pyrite alteration, often with hematite and magnetite facies. Albite-epidote alteration is caused by silica-bearing fluids rich in sodium and calcium, and typically results in weak albite-silica-epidote. Potassic alteration, typical of porphyry copper and lode gold deposits, results in production of micaceous, potassic minerals such as biotite in iron-rich rocks, muscovite mica or sericite in felsic rocks, and orthoclase (adularia) alteration, often quite pervasive and producing distinct salmon-pink alteration vein selvages. Quartz-sericite-pyrite alteration, in which these minerals can be deposited both in veins and in a disseminated manner; sericite in particular replaces plagioclase and biotite. This is common in porphyry copper and porphyry molybdenum deposits. Argillic alteration, commonly present in the distal areas of porphyry deposits, is a low-temperature assemblage that converts feldspars and some other minerals into clay minerals such as kaolinite and illite. It can overprint older, higher-temperature alteration assemblages. Rarer types of hydrothermal fluids may include highly carbonic fluids, resulting in advanced carbonation reactions of the host rock typical of calc-silicates, and silica-hematite fluids resulting in production of jasperoids, manto ore deposits and pervasive zones of silicification, typically in dolomite strata. Stressed minerals and country rocks of granitic plutons are replaced by porphyroblasts of orthoclase and quartz, in the Papoose Flat quartz monzonites. References Luth, R. W., Mantle volatiles -- distribution and consequences. In The Mantle and Core (ed. R. W. Carlson) Volume 2 Treatise on Geochemistry (editors H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford, pages 319-361 (2003). See also Metamorphic petrology Geological processes