Rs. Wheeler et al., Iron-rich and iron-poor prehnites from the Way Linggo epithermal Au-Ag deposit, southwest Sumatra, and the Heber geothermal field, California, MINERAL MAG, 65(3), 2001, pp. 397-406
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.