Abstract
Waimumu Quartz Gravels, 8 km southwest of Gore, New Zealand, were deposited in Pliocene river channels, and were derived by recycling of older sediments on nearby uplifting mountain ranges. The high proportion of quartz pebbles (>90%) is a result of the decomposition of clay-altered lithic clasts during sedimentary recycling. Most of the quartz was derived from the Otago Schist Belt to the north, including upper greenschist facies metachert clasts with spessartine and piemontite. The quartz gravels at Waimumu Stream mine contain abundant Mn-rich garnet clasts that were derived from Otago Schist. In addition, chromite clasts constitute >10% of the heavy mineral sand fraction, and this may have been derived from the Dun Mountain Ophiolite Belt although the exact source is unknown. Most detrital gold occurs as flakes that have undergone long-distance transport. Rare grains of crystalline gold occur, some of which are in association with prismatic quartz crystals. Gold flakes have minor evidence for authigenic gold overgrowths in association with authigenic marcasite, and gold mobility may have been enhanced by marcasite precipitation and/or oxidation.
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