Characteristics of retrograde alteration

The term retrograde alteration refers to an alteration stage in which higher temperature, generally anhydrous, minerals are replaced by lower temperature, generally hydrous, minerals. In some cases this process is largely metamorphic, i.e. the bulk composition stays roughly the same but mineral "A" is replaced by mineral "B". In other cases, retrograde alteration is concurrent with strong metasomatism, such as during mineralization with various sulfide minerals.

Retrograde skarn mineralogy, in the form of epidote, amphibole, chlorite, and other hydrous phases, typically is structurally controlled and overprints the prograde zonation sequence. Thus, there often is a zone of abundant hydrous minerals along fault, stratigraphic, or intrusive contacts. This superposition of later phases can be difficult to discriminate from a spatial zonation sequence due to progressive reaction of a metasomatic fluid. It is important to realize that hydrous minerals do not necessarily represent retrograde alteration (e.g. Dick and Hodgson, 1982). For example, the typically high fluorine activities in tin skarns can stabilize many hydrous minerals at high temperatures during early skarn alteration stages (e.g. Lost River, Alaska, Dobson, 1982). In general, retrograde alteration is more intense and more pervasive in shallower skarn systems. In some shallow, porphyry copper-related skarn systems, extensive retrograde alteration almost completely obliterates the prograde garnet and pyroxene skarn (Einaudi, 1982a,b)

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© 1995 (Larry Meinert)