Exploration for kimberlites in the southwest Goiás region, Brazil: Mineral chemistry of stream sediment samples

Abstract

Ilmenite and garnet analyses are presented for stream sediment heavy mineral concentrate samples collected during aeromagnetic anomaly follow-up for kimberlites in the southwest Goiás region, Brazil. Based on microprobe analyses of over 200 mineral grains, the samples can be divided into five general categories: (1) those associated with an aeromagnetic anomaly overlying an ultramafic breccia which is considered a possible kimberlite; (2) anomalous ilmenite and garnet analyses associated with a second aeromagnetic anomaly where no specific rock type was identified; (3) high-Mg/low-Cr ilmenite analyses from a highly weathered ultramafic breccia; (4) pyropic garnets (±Cr) from the south of the study area; and (5) background high-Mg/low-Cr ilmenites and various garnet compositions from within the study area. The ubiquitous occurrence of high-Mg/low-Cr ilmenites is considered to be due to numerous unmapped alkaline-ultramafic breccias/intrusions in the southwest Goiás region. Chemical analyses of anomalous high-Mg/high-Cr ilmenites and Cr-pyropic garnets from one weathered breccia suggest a probable kimberlite diatreme with an associated later-stage, shallower-level, olivine melilitite intrusion. A review of the mineral chemical signature of associated Cr-pyrope garnets implies that the kimberlite is unlikely to be an economically viable diamond source.