Geochemical constraints on the Gano karst bauxite deposit, eastern Alborz Mountains, northern Iran: Implications for provenance, elemental mobility, and distribution of critical metals

Abstract

The Gano karst bauxite deposit is located in the eastern Alborz Mountains, 90 km NE of Semnan city, northern Iran. The layers and lenticular patches of bauxite are hosted in the Triassic karstified carbonate rocks interbedded with calcareous shale of the Elika Formation, unconformably overlain by siliciclastics of the early Jurassic Shemshak Formation. The bauxite ores have a mineral assemblage dominated by diaspore and hematite, with lesser kaolinite, illite, chlorite, goethite, anatase, dolomite, and quartz. In the Gano area, intrusions range from monzonite to quartz monzonite in composition with an alkaline composition that generated in an extensional regime within a plate tectonic setting during the Devonian age. Geochemical signatures of the Gano bauxite ores suggest a derivation from the monzonite intrusions at the base of the Elika Formation, although a contribution of interbedded calcareous shale of the Elika Formation cannot be excluded. Based on the variation of Ce anomaly across the studied profile and the integration of petrographic evidence and mineralogical and geochemical data, the Gano deposit formed in an oxidized environment under warm-humid climates, significantly linked to rainfall seasonality and groundwater-level fluctuations. Gallium, Nb, and V median values of the Gano bauxite ores are a bit above the maximum value of these critical metals in the northwestern Iranian and Alborz bauxite data set, making diaspore-, anatase-, and hematite-rich layers and lenses valuable for further exploration of these critical metals.