The FeO/(FeO+MgO) ratio of tourmaline: A useful indicator of spatial variations in granite-related hydrothermal mineral deposits

Abstract In this paper, we present the results of a study of the FeO/(FeO+MgO) ratios (Fe # ) of tourmalines associated with granite-related Sn and Sn-W hydrothermal mineralisation in South Africa, Namibia and New Zealand. Systematic variations of Fe # are observed from endogranitic deposits to vein systems emplaced at some distance from the granitic source. Tourmalines associated with endogranitic to proximal deposits have Fe # ranging from 1 to 0.8; tourmalines from proximal to intermediate deposits have Fe # of between 0.8 and 0.6, whereas Fe # of less than 0.6 are indicative of tourmalines associated with distal deposits.

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