Indicative Role of Trace and Rare-Earth Elements of the North West Manganese Ore Deposit (South Africa) in Constraining the Genetic Model of Supergene Orebodies

The North West Province of the Republic of South Africa is famously known for its mineral wealth in chrome and platinum group metals associated with the Bushveld Igneous Complex. In addition to this, small-scale supergene manganese deposits are found to occur on the weathered upper crust of the Neoarchean Malmani dolostones in the Transvaal Supergroup. These deposits occur as manganese nodules, wad and crusts confined to karstic depressions, which resulted from surficial weathering and dissolution of the underlying manganiferous dolostones. This study presents the geochemistry (X-ray fluorescence and inductively coupled plasma mass spectrometry analyzed sample data) of rare-earth (REE) and trace (TE) elements and its indicative role in constraining the genesis of the North West Deposit. The ore mineralogy consists predominantly of high valence state (Mn4+) Mn oxide minerals. These minerals are known to be scavengers of relatively high concentrations of trace (TE) and rare-earth elements (REE) into their tunnel crystal structures during mineral precipitation. The stability and coherent nature of these elements in natural environments have shown in their robustness when it comes to discriminating various sources of manganese ore substances. The Post-Archean Australian Shale (PAAS) normalized REE patterns of the ore samples indicated enrichment in Ba, Y, Cu, Sc, Ni, Cu, Co and other TE such as Ce, La and Zr and depletion of Zn, Nb and U. The REE + Y anomalies from the analyzed ore samples further indicated ore-enrichment in light rare-earth elements and notably pronounced positive Ce and negative Y anomalies indicative of hydrogenetic precipitation as a governing mechanism for the formation of manganese nodules. This was supported by the observation on the high field strength (HFS) element discriminating plots; Ce anomalies and YSN/HoSN, Ce anomalies and Nd concentrations and the Fe + Mn + (Co + Ni + Cu)*10, which effectively discriminate between hydrogenetic, hydrothermal, diagenetic and mixed source ore precipitates. An overlap in the normalized REE-Y patterns for Mn nodules and Malmani dolostones is  suggestive of a close genetic relation.