Abstract

Classic schist-hosted emerald deposits are generally attributed to interaction between invading pegmatitic magma or vapor phases with preexisting metasediments, metavolcanics, and/or ultrabasic rocks. Detailed studies of the Habachtal (Austria) and Leydsdorp (South Africa) emerald deposits indicate that the emerald formation was not due to a single-stage contact metamorphism at the border zone of magmatic intrusive bodies. On the contrary, the emeralds are found in metasomatic zones (blackwall zones) associated with ultramafic bodies formed by syn- to post-tectonic reactions during low-grade regional metamorphism. These reactions occur at the contact of preexisting beryl- and phenakite-bearing pegmatites and albitite pegmatoids with biotite-talc and actinolite schists (Leydsdorp), or at the contact of beryllium-rich garnet-mica schists and biotite-plagioclase-gneisses with serpentinites and talc-schists (Habachtal).Critical evidence for the regional metamorphic origin of emeralds includes the following points:1. Augen textures occur in the metagranitic and metapegmatitic country rocks.2. Polystage growth with simultaneous deformation is indicated by sigmoidal inclusion trails in the strongly zoned emeralds.3. No evidence of contact metamorphism is found in the emerald ores, nor in the immediate country rocks.4. Both the rare earth element fractionation pattern and the Yb/Ca vs. Tb/Ca ratios of the fluorites which coexist with the emeralds indicate the existence of an abundant nonpegmatitic fluid phase during the formation of the emeralds.The following reactions for the formation of emerald from preexisting phenakite are suggested by textural evidence:3 phenakite + 2 biotite = 2MgFe-beryl + 2K (super +) + 4Mg (super +2) 3 phenakite + 3 albite + 2Mg (super +2) = 2MgNa-beryl + Na (super +) + Al (super +3) .The high magnesium and chromium contents--mainly in the rims of the emeralds--reflect their formation in an increasingly Mg- and Cr-rich environment. Microthermometric studies of fluid inclusions in the emeralds of both deposits suggest that they formed from an H 2 O-CO 2 mixed fluid phase, with a salinity of 1 to 9 equiv wt percent NaCl and homogenization temperatures around 300 degrees C.Mass balance and geochemical analyses suggest that the transformation of serpentinites and Be-rich country rocks into blackwall zones liberates significant amounts of Be and enables the formation of emerald in the presence of Cr. Due to unfavorable growth conditions during regional metamorphism, the extent of emerald mineralization never exceeds that of pegmatites.

First Page Preview

First page PDF preview
You do not currently have access to this article.