Gem Andradite Garnet Deposits (Demantoid Variety) of Kerman: Geology, Mineralogy and Genesis

In order to reveal the formation mechanism of different garnets from the Kouh Gabbri area (SE Iran), a geochemical, mineralogical and gemological study applied XRD, XRF and ICP-MS on gem quality demantoid garnets. Crystals of demantoid with green to yellowish-green color, crystal clusters and single crystals as large as 10–20 mm (most range from 2 to 10 mm in diameter) have been observed in the study area. The deposit is characterized by magnetite and hematite concentrations with distinctly low chromium contents ( 0.03 Cr2O3) and contain Cu sulfide minerals (e.g. chalcopyrite). Because of the large-scale intergrowths, the Kouh Gabbri samples contain four different compositions. First one, pure andradite (Adr 95) associated with sulfide stage, and second one, Al‐rich andradite, formed at the early stage of amphibole‐epidote‐heamatite stage and third one, a solid solution between grossular and andradite. In the final stage, grossular, formed at Ca‐silicate stage. Trace elements element contents as well as Mg, Mn, Ni, Cr, Co, Zn and La values demonstrated in obtaining accurate origin information for the samples. They revealed that demantoids from the study area have a relatively low lithium content ( 200 ppm) compared to demantoids from other places ( 250 ppm). Demantoid garnets from the study area, do not contain Cr3+, and their color is attributed to the presence of Fe3+ in the octahedral sites