Kimberlites from Central Angola: A Case Study of Exploration Findings

The Dando-Kwanza concession is situated in the incised highlands of Bié Province in central Angola. An aerial reconnaissance survey conducted in 2005 drew attention to artisanal alluvial mining, in the vicinity of six kimberlite occurrences discovered during colonial times. Exploration commenced in 2007 and the subsequent 3-year work program led to the discovery of 39 new kimberlite occurrences. The concession is underlain by Archean gneisses of the Central Shield Zone of the Angolan Shield, which have been intruded by Paleoproterozoic granitoids at ~1.97 Ga (this study). The basement is overlain by Paleozoic red beds of the West Congolian Inkisi Group which are host to many of the kimberlites. The Lubia cluster kimberlites occur within the northern part of the concession and were emplaced at ~238 Ma, based on zircon U–Pb and mica-in-kelyphite Ar–Ar geochronology. The kimberlites are pipes, some with multiple lobes, with surface areas of up to 16.7 ha. They occur as resedimented volcaniclastic, pyroclastic, and coherent magmatic varieties and have been classified as Group 1 kimberlites. Facies types have been interpreted as crater deposits with narrow hypabyssal feeders and one calcite kimberlite as extrusive lava. Microdiamond sampling of the kimberlites returned low modeled grades and it is postulated that erosion of an enriched and eluviated ferricrete cap on many of the kimberlites may explain the prevalence of alluvial diamonds immediately downstream of the kimberlites. Low diamond grades and the low to moderate interest major element composition of garnets and chromites may be related to perturbation of the SCLM by high-T melt-related processes during Paleoproterozoic times.