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

Published online by Cambridge University Press:  05 July 2018

R. S. Wheeler ,
P. R. L. Browne  and
K. A. Rodgers
R. S. Wheeler
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand
P. R. L. Browne
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand Geothermal Institute, University of Auckland, Private Bag 92019, Auckland, New Zealand
K. A. Rodgers*
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand Australian Museum, Sydney, Australia
*
*E-mail: [email protected]
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Abstract

Electron microprobe analyses of prehnites from the Way Linggo low-sulphidation epithermal Au-Ag deposit of southern Sumatra, show that the mole fraction of octahedral Fe3+, expressed as Fe3+/(Fe3++AlVI), ranges from 0.0 to ~0.6, the higher values being among the most iron-rich reported for prehnite in a hydrothermal environment. Prehnites from a diabase sill in the Heber geothermal field of California have mole fractions of octahedral Fe3+ ranging from 0.03 to 0.3. The Way Linggo prehnites formed below 220°C, some 20–30°C lower than those at the Heber field; the lower crystallization temperatures perhaps enhanced the opportunity for Fe3+ to substitute in octahedral sites. In both occurrences, prehnite predates late-stage calcite, consistent with the need for waters depositing prehnite to have aCO2 <0.01 moles. At higher CO2 activities the stability field of calcite would swamp the range of aCa2+/aH+ values appropriate for crystallizing prehnite. Consequently, the presence of prehnite in a hydrothermal environment primarily indicates that degassing of the hydrothermal fluid in CO2 occurred prior to deposition.

Keywords

prehniteWay Linggo epithermal depositHeber geothermal fieldSumatraCalifornia
Type
Research Article
Information
Mineralogical Magazine , Volume 65 , Issue 3 , June 2001 , pp. 397 - 406
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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