THE AKWATIA DIAMOND FIELD, GHANA, WEST AFRICA: SOURCE ROCKS

The Akwatia diamond field is located within Birimian (Early Proterozoic) metasedimentary rocks and has produced more than a 100 million carats of diamonds. These diamonds were originally assumed to be alluvial owing to the lack of any diamond indicator minerals typically associated with kimberlites. However, diamonds show no evidence of transport and are associated with metamorphosed ultramafic rocks with a composition resembling komatiite or boninite-type volcanic rocks. Akwatia is anomalous in that it is associated with arc sediments and does not occur within an Archean craton. The ultramafic rocks form elongate bodies that outcrop over a 1 km2 area and are contained within or intruded into Birimian metasedimentary rocks that include turbidites, phyllites and greywackes. Small-scale diamond workings that bottom in diamond-bearing rock indicate that ultramafic rocks extend to depths greater than 20 m. High diamond-grades in soils correlate with the ultramafic rocks and diamonds are observed in these rocks. Thus, there is little doubt that Akwatia is a residual deposit. We have identified at least two diamond bearing units, one an actinolite/ tremolite schist in elongate bodies and the other of similar composition exhibiting little or no schistocity and distinctive clastic texture with clasts of phyllite and carbon. The latter unit contains the larger, higher quality diamonds. The clastic units are also elongate, are contained within the actinolite schist, are generally concordant with schistocity of the nearly vertically dipping host rock, and range from 1-10 m in width and 10-30m in length. Field evidence suggests that these komatiitic rocks were coeval with the host turbidites, making Akwatia one of the oldest diamond deposits. Major and trace element analysis of these rocks suggest a suite of rocks similar to the diamond bearing volcaniclastic komatiites of French Guiana and/or the metamorphosed suit of komatiite/ boninite type rocks of the Wawa (Superior Province, Canada) greenstone belts. The structural relationship of the Ghana rocks also resembles the Wawa suite of rocks.