metasomatic replacement

metasomatic replacement, the process of simultaneous solution and deposition whereby one mineral replaces another. It is an important process in the formation of epigenetic mineral deposits (those formed after the formation of the host rock), in the formation of high- and intermediate-temperature hydrothermal ore deposits, and in supergene sulfide enrichment (enriched by generally downward movement). Metasomatic replacement is the method whereby wood petrifies (silica replaces the wood fibres), one mineral forms a pseudomorph of another, or an ore body takes the place of an equal volume of rock.

Replacement occurs when a mineralizing solution encounters minerals unstable in its presence. The original mineral is dissolved and almost simultaneously exchanged for another. The exchange does not occur molecule for molecule, but volume for volume; hence, fewer molecules of a less dense mineral will replace those of a more dense mineral. Replacement takes place first along major channels in a host rock through which the hydrothermal solutions flow. Smaller openings, even those of capillary size, eventually are altered, the smallest by diffusion at the very front of the exchange where solutions cannot flow.

Early-formed replacement minerals are themselves replaced, and definite mineral successions have been established. The usual sequence among the commoner hypogene (deposited by generally ascending solutions) metallic sulfide minerals is pyrite, enargite, tetrahedrite, sphalerite, chalcopyrite, bornite, galena, and pyrargyrite.

Although replacement can occur at any temperature or pressure, it is most effective at elevated temperatures, at which chemical activity is enhanced. Replacement by cold circulating waters mostly is confined to soluble rocks, such as limestone. These may be replaced by iron oxides, manganese oxides, or calcium phosphates; vast surface deposits of copper and zinc carbonates have also formed where limestones were replaced, and valuable deposits have occurred where supergene sulfide enrichment occurs. With higher temperatures, replacement increases until, at high temperatures, hardly any rock may resist. Solutions at intermediate temperatures form simple sulfides and sulfosalts for the most part, and those at higher temperatures form sulfides and oxides. Replacement deposits are the largest and most valuable of all metallic ore deposits except those of iron.