gems, turquoise, Iran, Neyshabur, Nishapur

The Baghoo turquoise deposit is hosted by the Toroud-Chahshirin intrusive-volcanic belt (Middle Eocene). Lithological units comprise andesite, rhyolite, granodiorite and dacite. The ore minerals occur as disseminated grains and veinlets in granodioritic to dacitic units. The ore minerals (CuAl₆(PO₄)₄(OH)₈·5H₂O) range from 4 mm to 1.2 cm in thickness. The ore zone is limited by the Anjiloo fault on the north and the Toroud fault on the south. In order to achieve a genetic model for turquoise deposition in the area, this study focuses on structural observations made in the field, microscopy and geochemical analysis of ore minerals and host rock. Different types of fractures were analysed, and the results indicate a good correlation with standard R, R′, P, T and Y type fractures. The turquoise mineralization correlates with the strike of P-group fractures. Microscopic studies of polished and thin sections indicate the presence of hypogene and supergene mineralization (oxidation and enrichment zones) in the area. The paragenetic sequence shows that the hypogene zone includes pyrite, chalcopyrite and galena, the oxidation zone includes iron hydroxides (goethite, hematite and limonite), anglesite, cuprite, marcasite, azurite and malachite, and the enrichment zone contains covellite and chalcocite minerals. In addition, textural evidence shows that turquoise deposition occurred in the waning stages of mineralization. Geochemical study confirms the results of IR, NAA, wet chemistry and SEM analyses and indicates that alunite was gradually changed to turquoise. A genetic model is presented for the formation of Baghoo turquoise.