Iron-rich and iron-poor prehnites from the Way Linggo epithermal Au-Ag deposit, southwest Sumatra, and the Heber geothermal field, California

Electron microprobe analyses of prehnites from the Way Linggo low-sulphidat ion epithermal Au-Ag deposit of southern Sumatra, show that the mole fracti on of octahedral Fe3+, expressed as Fe3+/(Fe3++Al-VI), ranges from 0.0 to s imilar to0.6, the higher values being among the most iron-rich reported for prehnite in a hydrothermal environment. Prehnites from a diabase sill in t he Heber geothermal field of California have mole fractions of octahedral F e3+ ranging from 0.03 to 0.3. The Way Linggo prehnites formed below 220 deg reesC, some 20-30 degreesC lower than those at the Heber field; the lower c rystallization temperatures perhaps enhanced the opportunity for Fe3+ to su bstitute in octahedral sites. In both occurrences, prehnite predates late-s tage calcite, consistent with the need for waters depositing prehnite to ha ve aCO(2) <0.01 moles. At higher CO2 activities the stability field of calc ite would swamp the range of aCa(2+)/aH(+) values appropriate for crystalli zing prehnite. Consequently, the presence of prehnite in a hydrothermal env ironment primarily indicates that degassing of the hydrothermal fluid in CO 2 occurred prior to deposition.