Pebble Size in Matrix of Palaeoplacer Conglomerate Correlated with Gold Grade

Abstract

This study aimed at finding a complementary field guide to recognize the relative gold ‘grade’ of the palaeoplacer banket conglomerate of the Tarkwaian Group in southwestern Ghana, West Africa. The study was conducted on a drill core from the Ajopa pit near Tarkwa, which, intercepted a palaeoplacer conglomerate from a depth of 5 m to 105 m. The conglomerate forms part of the Tarkwaian gold-bearing unit, deposited from 3132 Ma to 2095 Ma. The pebbles in the rock are mainly made up of oligomictic quartz of milky or vitreous whitish grey and coarse-grained. The size ranges from 8.50 mm to 30.50 mm, rounded, well to moderately sorted, and closely packed. Gold occurs in the matrix of the pebbles, composed of fine to medium-grained quartz, and associated with chlorite, and sericite, with heavy minerals mainly comprised of magnetite and hematite. The Tarkwaian Group sediments were sourced from the Birimian Supergroup which is a hydrothermal gold-bearing terrain of about 2.2 billion years (b.y.) old. The results of the study confirm that gold grade increases with increase in pebble sizes in the conglomerate. The study also found out that there is a relationship between bigger grain sizes in the matrix to gold grades. Also, the medium-grained fragments in the matrix which range in size from 0.60 mm to 4.35 mm show a positive correlation with gold grade (0.01 g/t to 3.67 g/t). These findings apply to higher and lower gold grades.