The origin of granular chert in the BARB3 drill core from the 3.4 Ga Buck Reef Chert, Onverwacht Group, Barberton Greenstone Belt

M.Sc. (Geology)Abstract: The c. 3.4 (Kröner et al, 1991) Ga Buck Reef Chert of the Onverwacht Group, Barberton Greenstone Belt, was recently drilled as part of the Barberton Drilling Project (Hofmann et al, 2013b). The ca. 900 m long BARB3 drill core consists of a variety of chert lithofacies. These include banded white chert, massive carbonaceous chert, and a granular chert lithofacies, which is most abundant in the stratigraphically lowermost part of the drilled succession. Granular chert also occurs in the top part of the Buck Reef Chert, but is rare to absent in its middle part. The processes of granular chert formation are not well-known. The origin of the carbonaceous matter present in this facies is also poorly understood. However, carbonaceous matter locally occurs in the form of possible microfossils and microbial biofilms. Granular chert was subjected to detailed sedimentological, petrographic, and major element geochemical studies. Granular chert is characterised by the presence of sand-sized carbonaceous grains, sand-sized siliceous grains, and a variety of other clasts, ranging from sand to pebble size, derived from adjacent rocks. Carbonaceous grains include: (1) discrete carbonaceous grains which are interpreted to represent reworked microbial mats, (2) amalgamated carbonaceous grains which are interpreted to represent multiple discrete grains cemented by silica. Siliceous grains include grains composed of pure silica, silica with rutile and siliceous grains with variable amount of diffuse and discrete domains of carbonaceous matter. The presence of rutile in siliceous grains indicate that they represent silicified volcanic and/or volcaniclastic grains. Discontinuous carbonaceous laminae are sometimes associated with carbonaceous grains and may represent remnants of microbial mats. Granular chert either forms stratiform layers or fills stratiform as well as cross-cutting veins. The stratiform layers of granular chert are typically planar and tabular with a thickness ranging between 1 mm and 50 mm and are frequently intercalated with massive white chert, crinkly laminated siderite-bearing chert, breccia and conglomerates. White chert clasts in the breccias are frequently angular, parallel orientation to bedding and show jigsaw fitting providing evidence for in situ brecciation, possibly by hydraulic fracturing. The clasts in the conglomerates are..