ABSTRACT
Garnet is a ubiquitous mineral in Fe-, Mn-, and Ca-rich exhalites associated with the 160 Mt Gamsberg deposit in South Africa. It occurs in distinct units of garnet-quartz rocks (coticules) below, above and lateral from the ore horizon, but is also plentiful in Fe-Mn calc-silicates, Ca-Mn marbles and chemogenic amphibole-sphalerite-pyrrhotite mineralisation. REE patterns of both whole rock and individual garnets indicate that these rock types consist predominantly of chemical precipitates with varying contributions of detrital material. A strong hydrothermal signature is evident from most garnets, resulting in a general enrichment of light REE. Positive Eu anomalies in the ore-bearing lithologies suggest a significant input of hot reduced brines, capable of transporting Zn and Pb, whereas Fe-Mn calc-silicates and impure marbles are characterized by relatively cool Fe-Mn-rich fluids. In contrast, coticules distal to the mineralisation exhibit the largest negative Eu anomalies, suggesting large contributions of aluminous clays to the precursor sediment and relatively more oxidizing basin conditions. A systematic increase in the size of the Eu anomaly as the ore body is approached is characteristic and might act as an exploration guide.
