TRACE ELEMENT CHEMISTRY OF DEMANTOID VARIETY ANDRADITE GARNET

Bright green to yellow-green gem-quality andradite garnet (Ca₃Fe₂Si₃O₁₂ – variety demantoid) is highly sought-after for its attractive color and fiery brilliance caused by andradite’s unusually high dispersion and refractive index. Demantoid was first discovered in the Ural Mountains in Russia in the 19^th century but has also been produced from several other countries including Italy, Iran, Pakistan, Madagascar, and Namibia. While demantoid usually occurs as nearly endmember andradite (close to 99 mol%), the ability of the garnet structure to easily incorporate a wide suite of cations with varying ionic radii and charges suggests that the minor to trace element chemistry of demantoid might provide a means to separate gems from different geographic sources. We report here the initial results of an LA-ICP-MS study of a suite of samples from serpentinite-hosted and skarn-hosted deposits. Demantoid is easily separated by Cr content which is high in serpentinite deposits (up to 1 wt%) and very low or even below the detection limit (about 1 ppm) for skarn deposits. Rare earth element concentrations, while often a powerful petrogenetic indicator, show little consistent variation between the two types of deposits. These andradite-rich garnets are usually light REE enriched and heavy REE depleted, although in a few cases the trend is reversed. Ultimately, demantoid trace element chemistry will depend on geologic conditions of formation which are assumed to have varied, if only slightly, between deposits. Such geologic conditions include temperature, pressure, oxygen fugacity, and the overall chemistry of the host formation and mineralizing fluids. This contribution outlines general variations in demantoid trace element chemistry while suggesting the potential for separating demantoid from various geologic and geographic sources.