CHARACTERIZATION OF BANDED FERRUGINOUS CHERT AND BANDED IRON FORMATION AND THEIR PROCESSES OF SEDIMENTATION IN THE FIG TREE GROUP, BARBERTON GREENSTONE BELT, SOUTH AFRICA

Layers and beds of microcrystalline quartz (sedimentary chert) is a characteristic component of Archean greenstone belts. One common form of this chert is banded chert, which occurs in units from one to many hundreds of meters thick. In banded chert, pure silica bands (up to 10 cm) alternate with bands of other rock types of similar thickness. In the Barberton Greenstone Belt (BGB), South Africa, the most common alternating bands are carbonaceous chert, siderite, and hematite. The resulting rock lithofacies are black and white banded chert (B&W), banded ferruginous chert (BFC), and oxide facies banded iron formation (BIF), respectively. Our study characterizes the lithology and lithologic association of BFC and BIF in the BGB, identifies the environments of deposition, and examines the sedimentation processes.

The lithologic components of BFC and BIF are hematite, siderite, jasper, and pure chert. Petrography and geochemical analyses of our samples show that some bands have more mixed mineralogy than expected. For example, hematite bands contain siderite laminations, and siderite bands have more hematite and mica than siderite. In addition, each band type has a distinct population of rare earth elements (REE). Interestingly, jasper and hematite both have positive Eu anomalies, while siderite does not exhibit a strong Eu anomaly.

The siderite bands exhibit different characteristics from the jasper and hematite bands. The high mica levels, lack of Eu anomaly, and local examples of cross-bedding suggest that the siderite grains were deposited by a combination of currents bringing detrital material and chemical precipitation. These observations are not mirrored in the jasper and hematite bands which have low mica levels, positive Eu anomalies, and planar laminations when present, suggesting exclusively chemical precipitation. We conclude that the BFC was likely deposited at shallower depths than BIF.