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The Piteiras emerald mine, Minas Gerais, Brazil: fluid-inclusion and gemmological perspectives

Published online by Cambridge University Press:  05 July 2018

E. P. Lynch ,
A. Costanzo ,
M. Feely ,
N. J. F. Blamey ,
J. Pironon  and
P. Lavin
E. P. Lynch
Affiliation:
Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway, Galway, Ireland Mineral Resources Division, Box 670, 751 28, Geological Survey of Sweden, Uppsala, Sweden
A. Costanzo*
Affiliation:
Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway, Galway, Ireland
M. Feely
Affiliation:
Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway, Galway, Ireland
N. J. F. Blamey
Affiliation:
Department of Earth & Environmental Science, 801 Leroy Place, New Mexico Tech, Socorro, NM 87801, New Mexico USA
J. Pironon
Affiliation:
UMR G2R, CREGU, Université Nancy I, B.P. 23, 54501 Vandoeuvre-lés-Nancy, France
P. Lavin
Affiliation:
Jewellery Valuations Ireland, Pound Street, Ballaghaderreen, Co. Roscommon, Ireland
*
* E-mail: [email protected]
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Abstract

New fluid inclusion analyses using a range of analytical techniques including quadropole mass spectrometric analyses coupled with gemmological investigations were conducted on rough and gemquality faceted emeralds from the Piteiras Mine, Minas Gerais, Brazil. These data complement those of Rondeau et al. (2003) who also presented analyses of the Piteiras emeralds. Emeralds are found typically as euhedral-to-anhedral crystals disseminated in biotite-phlogopite schist and range from 0.5 to 50 mm long. Emerald mineralization is associated closely with alkaline metasomatized pegmatite bodies, quartz boudin and veins, and talc-chlorite bands. Four types of fluid inclusions are recognized in the emeralds. These include aqueous brine and aqueous carbonic varieties containing one or two liquid phases, along with gas bubbles and/or solid crystals (e.g. carbonates). Primary fluid inclusions in emeralds record salinities of ~4–24 eq. wt.% NaCl and minimum trapping temperatures from ~350 to 480ºC. Combined microthermometry, Raman spectroscopy and crush-leach gas analyses indicate that the mineralizing fluid was an aqueous carbonic brine enriched in reduced volatile species such as CH4, N2, H2S and alkanes. With respect to their optical properties (RI ε = 1.573–1.580; RI ω = 1.580–1.588; birefringence = 0.006–0.008) and specific gravity (2.65–2.78), the Piteiras emeralds fall within the expected range for metasomatic, schist-hosted emeralds.

Keywords

EmeraldBrazilgemmologyfluid inclusionsmicrothermometryPiteiras
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 7 , December 2014 , pp. 1571 - 1587
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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