Metallogenetic significance of element mobility in the Aggeneys-Gamsberg ore district (South Africa)

• Applicant: Dr. Stefan Höhn
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525653989

The Aggeneys-Gamsberg ore district, located c. 700 km north of Cape Town (South Africa), represents one of the world's largest base metal anomalies. It comprises four deposits, including the worldclass Gamsberg deposit. All of those were classified as deposits of the Broken Hill-type, traditionally interpreted as former sedimentary exhalative (SEDEX) deposits that became metamorphosed at amphibolite- to granulite-facies conditions. Yet, a number of mineral assemblages, a pronounced metal zonation across the whole ore district, and the appearance of various geochemical anomalies in the vicinity of the ore can´t be explained by a simplistic genetic model. The Koeris Formation is the only stratigraphic unit within the Bushmanland Terrane, which was deposited after the Okiepian orogeny (1210 to 1180 Ma). Therefore, it contains critical information on post-Okiepian processes within the Aggeneys-Gamsberg basin and its subsequent metamorphic overprint during the Klondikean orogeny (1040 to 1020 Ma). Because of its stratigraphic position only meters above the four giant base metal deposits, investigations on post-Okiepian element mobility in the Koeris Formation can help to decipher the complex metamorphic and metallogenetic history of the ore deposits beneath. Preliminary geochemical studies at Gamsberg indicate that both the amphibolite and the metasedimentary rocks have base metal characteristics, which are similar to the characteristics of the ore itself. This is probably the product of an early to peak-metamorphic interaction between the two units during the Klondikean orogeny. Subsequent mineralogical and geochemical investigations proposed for this study will focus on uninvestigated locations of the Koeris Formation. The comparison between their chemical characteristics will shed new light on district-scale, metamorphic interaction between the different ore types and the Koeris Formation. Previous studies by the applicant indicate that the thick layers of basic volcanic rocks of the Koeris Formation were deposited within the intra-mountainous Aggeneys-Gamsberg basin at a time when the ore horizon was still subject to weathering-induced erosion in more exposed places of the basin. Considering that amphibolite cover above deposits with significant Zn-contents is generally thick but thin or absent above Cu-Pb-dominated deposits raises the question whether these rocks affected the weathering-induced element mobility within the basin and caused a redistribution and concentration especially of Zn. Late-metamorphic alteration has been observed in places but hasn´t been investigated yet. Mineralogical and geochemical analyses on the affected rocks will clarify its role in element mobility in the vicinity of the ore. The insights gained in this study will help in a better characterization of Broken Hill-type deposits and in developing new exploration strategies for medium- to high-grade metamorphic base metal deposits all over the world.

• DFG Programme: Research Grants