2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 64-14
Presentation Time: 5:10 PM


WISE, Michael A., Dept. of Mineral Sciences, Smithsonian Institution, P.O. Box 37012, Washington, DC 20013-7012, wisem@si.edu

The Hiddenite area of western North Carolina has long been known as North America’s most significant source of gem-quality emerald. Emerald from the Hiddenite area is hosted by quartz-carbonate Alpine-type veins that developed within Precambrian migmatitic schists and gneisses. Overshadowed by the presence of emerald is the occurrence of the Cr-bearing gem variety of spodumene known as hiddenite. The two gem minerals are closely associated throughout the area and typically occur within the same mineral deposit. This rare and unique association of emerald (beryl) and hiddenite (spodumene) is unprecedented and is unknown from emerald deposits and Alpine-type veins worldwide.

The gem-bearing veins of the Hiddenite area formed from hydrothermal fluids under low temperature (< 250°C) and low pressure (~1 kb) conditions. Based on geochemical studies, the heterogeneous migmatitic rocks appear to be the most likely source of Cr for the emerald and hiddenite mineralization. Local outcroppings of the Rocky Face granitic pluton and unexposed granitic pegmatites have been proposed as possible sources of Be for the crystallization of emerald, however recent examination of “pegmatitic” leucosomes within the migmatite terrane have revealed the presence of aquamarine thereby suggesting that pre-migmatitic rocks may have been the most important contributor of Be for the crystallization of emerald. Presently, there are no rocks exposed that could be considered as likely sources of Li for the crystallization of hiddenite.

The exploration for new gem-bearing Alpine-type veins in the Hiddenite area is hindered by the lack of suitable outcropping of migmatite and continues to rely heavily upon the haphazard examination of the overlying saprolite layer. Current and past workings of known emerald and hiddenite occurrences suggest that the gem-bearing vein system may be restricted to a northeast-southwest trending belt of unknown width and length. Within individual workings, differences in the crystal morphology of quartz, rutile and pyrite, coupled with differences in the assemblages of carbonate and sulfide minerals can serve as useful indicators of emerald or hiddenite mineralization.