Geological, mineralogical and geochemical aspects of Archean Banded Iron-Formation-hosted gold deposits: Some examples from Southern Africa

Abstract

This paper reports on different styles of gold mineralization observed in Archean gold deposits hosted by Algoma-type Banded Iron-Formations (BIF) in southern Africa. Genetic aspects of various occurrences are discussed in the context of mineralogical as well as geochemical data of BIFs from the greenstone terranes of the Zimbabwe and Kaapvaal cratons. The study revealed that, in spite of their different provenance and age (3.5 to 2.6 Ga), the BIFs are geochemically similar, whereas observed mineralogical differences reflect various degrees of metamorphic overprint. Generally, the BIFs belong to mixed oxide-carbonate-(±sulfide)-facies. REE distribution patterns of the investigated Archean BIF samples exhibit positive Eu-anomalies, which suggest a strongly reducing nature of the solutions which also provided the distinctive element contents now present in the chemical sediments. Irrespective of their formation, gold enrichment in BIF only occurs if the S- and/or As-contents of the BIFs exceed specific threshold values, i.e. gold mineralization is always associated with increased contents of the iron-sulfides pyrite, arsenopyrite and pyrrhotite. The studies indicate that BIF-hosted gold occurrences are not products of a single universal metallogenic process, but may be explained by several different genetic processes such as primary syn-sedimentary formation, diagenetic changes, metamorphic remobilization, and epigenetic hydrothermal emplacement.