Geochemistry and mineralogy of listwaenite hosting mercury mineralization and its comparison with the barren types in Tavreh area, west of Khoy city

Alteration of ophiolitic ultramafic rocks, formation of listwaenite and associated mineralization are interesting phenomena of metallogeny of Khoy ophiolite. It seems that the hydrothermal circulations responsible for mercury mineralization were derived from geothermal systems caused by intermediate to acidic magmatism in Neogene-Pleistocene time. Reactivation of brecciated serpentinite with hydrothermal solutions led to the silica-carbonate hydrothermal alteration and formation of listwaenite. The importance of these alteration types is due to associated Hg-Au mineralization. Based on the microscopic mineralogical studies, XRD analysis, geochemical characteristics and field relationships, three types of listwaenite including silica, silica-carbonate and carbonate have been recognized. Tavreh mercury occurrence has formed in relation with silica type listwaenite which also named as birbirite. Alteration zones have structural controls and are restricted to shale (marl)/serpentinite fault type contacts. Mineralogically, silica type listwaenite mainly consists of quartz, chalcedony, opal and secondary iron hydroxides. Magnesite, dolomite, calcite and clay minerals are the minor phases. Toward silica-carbonate and carbonate listwaenites, silica minerals decrease while carbonate minerals increase. Geological field relations and existence of residual chrome spinel in the listwaenite of Tavreh area confirmed the ultrabasic host rock. Based on geochemical studies, average values of SiO2, Fe2O3, MgO and L.O.I as the main components of silica listwaenite are 82.6, 6.99, 1.02 and 3.74 wt %, respectively. Among rare elements, average values of Hg, Pb, As and S have been obtained as 646.2 ppm, 517.7 ppm, 329.1 ppm and 281 ppm, respectively. Mass changes (gains and losses) of major and minor elements during the alteration process in the Tavreh area have considered, using Gresens’ equation in this study. Cinabar-bearing silica listwaenite is enriched in SiO2, Al2O3, K2O and Hg components and is depleted in MgO and MnO. Silica-carbonate type shows enrichment in CaO, Na2O, Al2O3, K2O and TiO2 and deplection in MgO, MnO P2O5, SiO2, Cr2O3 and Fe2O3. Carbonate type shows increase in CaO and Sr and reduction in SiO2, Na2O, MgO, Al2O3, Fe2O3, TiO2 and MnO.