In general, the C[H.sub.4]-rich hydrothermal fluids may come from three ultimate sources: (1) abiogenic origin derived from the mantle [33, 34]; (2) abiogenic origin derived from postmagmatic
alteration by Fischer-Tropsch type synthesis [35-37]; (3) incorporation of thermally decomposed organic material (thermogenesis) or/and products from microbial processes (bacteriogenesis) [38-41].
The extreme case of this process is episyenitization, a natural postmagmatic
process resulting in episyenite formation (Hecht, 1999; Petersson and Eliasson, 1997).
However, we suggest that the skarn-type mineralogy hosted in this metamorphic unit should be related both magmatic and postmagmatic
stages of the Santa Martha Batholith, and is the result of a very complex skarn-type mineralogy zonation (Castellanos and co-workers, personal communication).
mineral deposits of the northern edge of the Bohemian massif (Erzgebirge-Harz), Central Geological Institute GDR.
The upper part of the lava flow became vesicular and scoriaceous due to lava degassing, and strongly affected by postmagmatic zeolitization evidencing volatile-phase activity after the eruption.
Owing to the activity of postmagmatic hydrothermal fluids, a complete conversion of feldspathoid (nepheline, sodalite?) preceded secondary thomsonite-gonnardite crystallization.
The effects of postmagmatic alteration in samples chosen for bulk rock analyses were assessed using petrographic and textural features where possible.
16 are particularly noteworthy as Ce/Yb and Zr/Nb ratios are parameters least affected by postmagmatic alteration (Gill 1981).
They are produced from either magmatic processes (mantle-derived compounds) or postmagmatic
processes (gas-water-rock reactions).
The carbonate rocks were transformed into marble and, at a postmagmatic stage, mineralization formed in the contact zone with the intrusive rocks.
It is interesting to note that all of these minerals are typical of a retrograded calcic skarn (aposkarn) in a postmagmatic phase.