Trace-element characteristics of the Buck Reef Chert (Barberton, S. Africa)

The Buck Reef Chert (BRC) forms the boundary between the Hooggenoeg and Kromberg formations of the Barberton Greenstone Belt (S. Africa). The BRC sequence is characterized by the alternation between black, white, black white, iron-rich and greenish grey cherts. The 900m BARB3 drill core of the Barberton Drilling Project provides an unique opportunity to study parts of the BRC that are weathered up to a depth of 30 meters at the surface. Here we present new trace-element data from 60 micro-drilled samples from the quarters of the BARB3 drill core. The Al2O3 content of the samples varies between 0.00 and 19.01 wt. %. The PAAS normalised REE patterns are variable: (La/Yb)N=0.14 - 39, Ce/Ce*(0.5 – 6.0), Eu/Eu* (1.0-7.0) and Y/Ho (0.0 – 8.5) anomalies. The REE data suggest mixing between seawater and a hydrothermal source. Three end-members characterize the cherts of the BARB3 core based on Cr/Th, Ni/Th and Ba/Th systematics: 1) terrigenous, 2) komatiite and 3) hydrothermal. The terrigenous sediment component is predominantly found in the black cherts and the white cherts contain more of the komatiite end-member. The alternating Black White cherts have the lowest input of terrigenous material and are deposited from hydrothermal fluids.