The geochemistry of conglomerates, chert and silica veins from the Fig Tree Group, Barberton Greenstone Belt: implications for Paleoarchean silicification processes

Abstract

Considerable disagreement surrounds the causes of silicification and barite formation in Paleoarchean greenstone belts. Determining the mechanisms responsible for the primary precipitation of these chemical sediments could help identify how these rocks record the evolution of the early oceans, biosphere and geosphere. This thesis focuses on silicified conglomerates, chert and silica veins from three different localities of the ~3.26 Ga Fig Tree Group of the Barberton Greenstone Belt: 1) Eastern Barite Valley 2) the Masenjane Barite Workings and 3) Puddingstone Hill. Chert, jaspilite and ferruginous sediment conglomerate clasts display Rare Earth Element & Yttrium (REE+Y) patterns of heavy REEs enriched over light REEs (HREE>LREE), variably positive Eu anomalies and often super-chondritic Y/Ho ratios. These patterns are consistent with precipitation of these chemical sediments from fluids derived from Archean seawater. Eu anomalies in some of these clasts are more positive than those expected from contemporaneous seawater. This has been used to conclude that low-temperature hydrothermal venting was occurring in some segments of the basin whilst silicification was occurring. Silicified sediments are also represented by some of the conglomerates clasts. These sediments often display REE+Y patterns which deviate from the flat REE+Y patterns expected from sedimentary material. Instead, they also exhibit REE+Y patterns consistent with Archean marine waters. Similar REE+Y patterns are observed in the matrix material of the conglomerates. These results have been used to conclude that sedimentary sequences in the Fig Tree Group were silicified by seawater-derived fluids. REE+Y patterns of some silica veins found in the Eastern Barite Valley and Masenjane Barite Workings also imply formation from a seawater-derived fluid. Mafic signatures observed in these samples also disagree with previous interpretations that these silica veins represent silicified felsic ash. Instead, elevated Cr values indicate relatively deep circulation of a silicifying-fluid through the underlying Onverwacht strata caused these silica veins to form.