Geochemistry and sedimentology of a facies transition — from microbanded to granular iron-formation — in the early Proterozoic Transvaal Supergroup, South Africa

Abstract A transition from microbanded Kuruman to granular Griquatown iron-formation is described in terms of sedimentological, petrographic, and geochemical characteristics (major, minor, trace and rare earth elements) as well as whole rock carbon and oxygen isotopic compositions. Transitions between microbanded and granular iron-formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron-formations. The Kuruman-Griquatown transition zone differs from other granular iron-formation sequences in being non-oolitic i.e., peloidal and magnetite-rich instead of hematite-rich. It is concluded that the coarsening upward microbanded-granular iron-formation units in the Kuruman-Griquatown sequence represent upward-shallowing storm-dominated deposits; that is, primary microbanded, relatively deep water, fair weather deposits having been intermittently reworked by storm wave action form wavy, sharp-based, graded and poorly sorted, irregular interbeds of granular iron-formation. This is in contrast to typical hematitic oolitic iron-formation units which represent true shoal deposits. Geochemical results strongly support the storm-reworking hypothesis. Microbanded and granular iron-formation beds have similar REE patterns, with negative Ce anomalies and depletion in light REE, reflecting deposits in a marine setting below the zone of shallow surface water. Positive Eu anomalies in the REE patterns suggest admixture of a hydrothermal component of the ocean water. Carbonates in the iron-formation are depleted in 13C indicating involvement of organic matter during diagenetic alteration of the iron-formations. There are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13C than in shallower water. Deposition of the iron-formations was sporadically interrupted by influx of tuffaceous material, now represented by centimeter-thick, shard- and lapilli-bearing stilpnomelane lutite beds. These beds differ chemically from the iron-formations in being enriched in elements such as Al, Sc, Y, Zr, Nb, Hf, Ta, Th, U and the lanthanides, and displaying REE patterns with strong negative Eu and slight positive Ce anomalies. The composition of the parent magma of the volcanics remains uncertain because of probable post-depositional alteration of the volcanic glass; however, it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds; typical of Plinian-type deposits. Chemical variations in the minor, trace and REE compositions of the iron-formations are closely correlated with the amounts of admixed volcanic ash material. Most of the iron-formations appear chemically very “uncontaminated” being highly depleted in all minor, trace and REE, and organic carbon contents. The iron-formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository.