An overview of vis-nir-swir field spectroscopy - Spectral International

________________________________________________________________
ALTERATION TYPES ASSOCIATED WITH SKARNS
RETROGRADE ALTERATON
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.
SKARN MODEL
Figure 55 - Zonation of most skarns reflects the geometry of the pluton
contact and fluid flow. Such skarns are zoned from proximal endoskarn to
proximal exoskarn, dominated by garnet. More distal skarn usually is more
pyroxene-rich and the skarn front, especially in contact with marble, may be
dominated by pyroxenoids or vesuvianite. Meinert, L.D., 1992
43