Mohammad Moradi; Zahra Alaminia; Ebrahim Tale Fazel; Mahin Mansouri Esfahani
Abstract
The Dashkasan gold deposit with 52 Mt of oxidized ore reserves at mean grades of 1.77 g/t Au is hosted by porphyritic dacite/rhyodacite rocks. Tourmaline mineralization occurs as ...
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The Dashkasan gold deposit with 52 Mt of oxidized ore reserves at mean grades of 1.77 g/t Au is hosted by porphyritic dacite/rhyodacite rocks. Tourmaline mineralization occurs as disseminated, vein/veinlet, and breccia textures in three types by petrographic studies: needle (type-1), radial (type-2), and elongated bladed (type-3). The geochemical composition of the three types tourmalines on the various diagrams implies to be an indicator for the nature and evolution of ore-forming fluids. According to the electron microprobe analysis, the tourmalines indicated that the first- and third- types of tourmalines are dravite, and the second-type is dravite to schorl in composition. Third type tourmalines, unlike other types of tourmalines, show poor zoning and have higher values of Na, K, Fe and Mg elements and lower values of Ca. On the ternary diagram Fetot-Ca-Mg plot in the Ca-poor metapelites, metapsammites, and quartz-tourmaline rocks host. Magmatic genesis inferred from the higher Fe/(Fe+Mg) ratio for first and second types, while these ratios in the third- type tourmaline is different and shifted to hydrothermal origin.
Fluids inclusions studies on the quartz veins related to transition between stage-1 and -2 of mineralization shows a salinity values from 15.9 to 16.8 wt.% NaCl equiv. and homogenization temperature values between 185 to 254 °C. Evidences of this research indicated that the occurrence of quartz-tourmaline-pyrite veins in the Dashkasan deposit is due to the existence of a fault system under the physicochemical performance of the ore-mineralized fluid by a sudden depressurization and probably gradual mixing with shallow water.
