Chemical variations in biotites—an exploration tool to distinguish between mineralized and barren rocks in the Nigerian tin bearing province

Abstract

Major elements microprobe analyses are presented for biotites from mineralized and barren biotite granite phases in the Afu Younger Granite Complex of Central Nigeria. Mineralized and barren rocks are characterized by different chemical variations in the biotites. These variations are not readily apparent from thin section examinations. Although all the biotites.are iron-rich, the biotites from mineralized phases show higher Σ Fe/Σ Fe + Mg ratios than those from barren rocks. The mineralized biotites are also characterized by lower Mg and Ti contents relative to biotites from backs. There is also higher A1 in the mineralized biotites than biotites from barren phases. The range of biotite compositional trends observed in mineralized and barren intrusions is attributed to a range of likely oxygen fugacity (fO 2 in the melts and differences in temperature during crystalline. Relatively low fO 2 (possibly decreasing during crystallization) is inferred in those phases associated with mineralization.and relatively high to constant fO 2 in the phases that are not associated with mineralization. Conditions of low fO 2 seem appropriate for shallow level magmatic systems that have evolved high volatile-rich hydrothermal solutions as a final crystallization products. An exploratory strategy is suggested using the above observations. Rapid collection of representative samples from the prospective area is followed by aboratory assessment for mineral assemblages indicative of crystallization under low fO 2 conditions (biotite + magnitite + fluorite) and assessment for the mineral chemical variations described above.