Salvi S., Giuliani Gaston, Andrianjakavah P. R., Moine B., Beziat D., Fallick A. (2017). Fluid inclusion and stable isotope constraints on the formation of the ianapera emerald deposit, southern madagascar. Canadian Mineralogist, 55 (4), p. 619-650. ISSN 0008-4476.
Titre du document
Fluid inclusion and stable isotope constraints on the formation of the ianapera emerald deposit, southern madagascar
Salvi S., Giuliani Gaston, Andrianjakavah P. R., Moine B., Beziat D., Fallick A.
Source
Canadian Mineralogist, 2017,
55 (4), p. 619-650 ISSN 0008-4476
The Ianapera emerald deposit is hosted in the Neoproterozoic Vohibory domain of southern Madagascar, within a tightly folded metamorphic sequence of mafic and ultramafic rocks (M-UMR), gneiss, and marble, a few kilometers north of the major tectonic break known as the Ianapera-Ampanihy Suture. Late-collisional metaluminous to peraluminous felsic intrusions outcrop in the area. Emerald occurs mostly within metasomatic phlogopitite and quartz-tourmaline veins, developed within lenses of M-UMR. Recent work recognized granitic pegmatites as the source of the emerald-forming fluids, leading to the classification of proximal and distal mineralization styles, based on whether or not pegmatites are associated with the phlogopitite, respectively (Andrianjakavah et al. 2009). Considerations of the chemistry of tourmaline and scapolite associated with emerald and beryl, data from fluid and solid inclusions in these minerals, thermodynamic calculations, and mineral thermobarometric data confirm that Be originated from the pegmatites, transported in a moderately saline aqueous fluid that exsolved at about 600 to 680 degrees C and 4-6 kbars. This fluid was enriched in alkali elements, incompatible elements and dissolved sulfur, and produced the proximal mineralization. Migration along fractures caused loss of at least part of the pegmatitic elements and local mixing with a metamorphic CO2-rich fluid, leading to the more distal mineralization at somewhat lower temperatures (520 to 650 degrees C). Oxygen and hydrogen stable isotopes were measured from beryl and emerald; the data corroborate a magmatic-hydrothermal origin for the mineralization and a common source for proximal and distal fluids. The calculated delta O-18(H2O) of 12.5% at 520 degrees C for the distal emerald and the delta D-H2O range measured from the channel of the emerald samples fit within the oxygen and hydrogen isotopic range of values defined for peraluminous granitic magmatism. The occurrence of a distal style of emerald mineralization such as typified by Ianapera suggests that metamorphic origin models for similar occurrences worldwide should be taken with care and possibly reconsidered in terms of source and origin of fluids and metals.
