Lithogeochemistry as a tracer of the tectonic setting, lateral integrity and mineralization of a highly metamorphosed Mesoproterozoic volcanic arc sequence on the eastern margin of the Namaqua Province, South Africa

The Areachap Group represents the medium- to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1.24–1.30 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the early stages of the Namaqua-Natal Orogeny.Few regional geochemical studies of the succession have been undertaken and despite recent geochronological work from throughout the outcrop belt there is still uncertainty as to the lateral integrity and correlation of the succession from one area to another. This uncertainty has been aided by minimal outcrop and extensive deformation and dislocation of one area from another. This study uses trace element geochemistry combined with Sm–Nd-isotopic data from the northern and southern ends of the succession to test its lateral integrity. Minor differences, including the abundance of certain lithologies, varying influences of a crustal component and differences in age between different areas are apparent, but the succession, as a whole, displays lateral integrity as a continental island arc.All lithologies are characterised by enrichment in Th, U, Pb and the large ion lithophile elements relative to primitive mantle, along with Nb–Ta depletion relative to La, and strong light rare earth element (LREE) enrichment relative to heavy REE depletion, which, along with positive εNd(t) values of mafic gneisses from both ends of the succession, indicates mantle-derivation within a subduction-related environment with a substantial amount of crustal assimilation. Despite some minor geochemical and isotopic differences in the lithological architecture on a local scale the Areachap Group exhibits coherent geochemical characteristics along its entire strike length.Localised remobilization of elements, particularly in areas characterised by granulite grade metamorphism, as well as in the altered footwall zones of volcanogenic massive sulphide lenses, is present. These alteration trends, along with primary geochemical signatures are remarkably well preserved despite the high grade of metamorphism, as illustrated in clearly defined hydrothermal alteration trends on the alteration box plot, and serves as an efficient tool to delineate areas that have undergone hydrothermal alteration spatially associated with massive sulphide ore bodies. The systematic association of hydrothermal alteration and sulphide ore bodies provides support for the isochemical character of rock compositions during high-grade metamorphism and the lack of extensive post-metamorphic alteration processes.

Research Highlights►The manuscript deals with the lithogeochemistry of the highly metamorphosed and deformed Areachap Group on the western margin of the Archean Kaapvaal Craton at its contact with the Proterozoic Namaqua Province of the Namaqua-Natal Belt. ►The research makes use of immobile trace elements from eight sites from throughout the Areachap Group succession in order to test and determine the lateral integrity of the various stratigraphic units from one end of the outcrop belt to the other. Immobile trace elements are used due to the high-grade metamorphic conditions the rocks were subjected to and the likely mobility of major elements and large ion lithophile elements. In addition, the tectonic setting of the entire succession is verified. Despite the fact that there have been previous geochemical and isotopic studies on the Areachap Group they have lacked a comprehensive whole rock major and trace element and isotopic dataset, or have been localized in extent. It is hoped that this study will add an extensive and comprehensive geochemical dataset for the Areachap Group. This study indicates that the Areachap Group is a single volcanic arc but may show differences in age of formation, degree of crustal assimilation and minor local variations. Correlation of various formations from the northern end of the succession may be made with those from the southern end. ►The major and trace element data is supplemented by Sm–Nd isotopic measurements from sites in both the northern and southern ends of the Areachap Group succession that supplement a previous isotopic study from the southern end of the succession. This isotopic dataset helps further define the origin of these rocks. ►Following on from the lithogeochemical study an investigation is carried out as to whether the alteration box plot of Large et al. (2001) could be used as a valid lithogeochemical exploration tool to detect hydrothermal alteration trends associated with base metal volcanogenic massive sulphide mineralisation in a high-grade metamorphic terrain. This part of the study highlights element mobility but also indicates that, despite the high-grade metamorphism, hydrothermal alteration trends associated with base metal sulphide mineralisation are still preserved, most notably in the footwall to the massive sulphide lenses.