Demantoid Garnet from Antetezambato, Northern Madagascar—Internal Characteristics and Their Use in Deciphering Geographic Origin

Abstract

: Demantoid garnets from Antetezambato, northern Madagascar, contain a characteristic inclusion suite typical of a skarn occurrence. Unlike demantoid from the classic deposits in the Ural Mountains, Russia, and from other serpentinite-related occurrences, the stones from Madagascar do not contain ‘horsetail’ inclusions, but instead host groups of small, rounded diopside grains, fluorapatite, dolomite, calcite, quartz, pyrite and wollastonite, as well as rare native bismuth. Partially healed fissures contain two-phase fluid inclusions (with H 2 O and a gas bubble) of varying salinity. Negative crystals, commonly large and empty, sometimes also contain H 2 O. Pronounced growth features and parallel hollow tubes are also present. Demantoid from Namibia likewise has a skarn origin and shows some similar internal features, as well as some that are different, such as the common presence of fluorapatite and small multiphase fluid inclusions, along with the absence of large negative crystals and native bismuth. A review of previously advocated methods for geographic origin determination of demantoid based on chemical fingerprinting alone shows that this technique is applicable only to stones with a nearly pure andradite composition when differentiating Malagasy and Namibian localities. In such cases, elevated traces of Mn are indicative of a Namibian origin.