One of the central issues in the debate on the genesis of the Witwatersrand gold
deposits is the distinction between early-diagenetic gold and sulfide textures and those
resulting from hydrothermal activity associated with or following regional metamorphism.
Only unmetamorphosed, and therefore very young, placer deposits can
provide examples of early-diagenetic textures, geochemical signatures and the mineralization
itself. To address the possibility that syngenetic characteristics are erroneously
attributed to metamorphic or hydrothermal processes in ancient placer
deposits, we document an example of gold and sulfide mineralization from recent
quartz pebble conglomerate (QPC) gold placers at Belle-Brook and Parker Road in
southern New Zealand.
Gold and early-diagenetic sulfides at Belle-Brook and Parker Road are morphologically
and compositionally similar to gold and sulfides from Witwatersrand QPC.
Gold in the QPC at the sites investigated is fine-grained (< 2 mm) with abundant textures attributed to transport, diagenesis and authigenic gold remobilization. Authigenic gold occurs as overgrowths on the detrital Au-Ag and Au-Ag-Hg alloys and, less commonly, as discrete Au-Ag alloy particles. Sulfide minerals, mainly early-diagenetic marcasite and, to a lesser extent, early-diagenetic pyrite, detrital pyrite and arsenopyrite are locally abundant. Sedimentary and diagenetic processes associated with, and following the formation of the QPC gold placers at Belle-Brook and Parker Road are readily observable at the surface, and hence, more easily understood than those of the Witwatersrand QPC ore bodies. Despite an oxygen-rich atmosphere, there is abundant evidence for diagenetic formation, sedimentary transport, and preservation of sulfides in the surficial environment at Belle-Brook. Thus, although economically insignificant, study of young QPC gold placers, such as those at Belle-Brook and Parker Road, provide new perspectives in the understanding of Witwatersrand-style mineralization.