Physical and chemical evolution of a Pleistocene marginal marine gold paleoplacer deposit, southern New Zealand

Abstract

Pleistocene placer gold accumulation in coastal sediments during fluctuating Pleistocene sea levels and active tectonic uplift in southern New Zealand provides a well-constrained model for processes that have affected paleoplacers formed in similar settings in older rocks, going back to the Archean. Initial gold concentration occurred > 200 ky ago on exposed sandy beaches, where sand-blasting processes deformed fluvially-transported (~200 km) flakes into toroids, dumbells and spheroids. This gold was recycled into younger gravel-bearing sediments < 30 m thick during subsequent uplift (>50 m) with negligible morphological changes during < 10 km of fluvial transport. Most of the gold accumulated in basal fluvial gravels hosted in channels incised into gabbroic basement. Groundwater in the sedimentary sequence was derived from rain with marine aerosols and had the composition of dilute sea water. This low-temperature groundwater interaction has altered mafic clasts and immediately underlying basement to clays dominated by smectites and kaolinite, with some clay cementation. Groundwater alteration occurred under a wide range of redox and pH conditions that were lithologically controlled, principally by permeability contrasts at the centimetre to metre scale. Hence, oxidised waters produced clays impregnated with ferric iron oxyhydroxide, and reduced waters generated authigenic pyrite. Some of this pyrite was subsequently oxidised with localised acidification. Micron scale dissolution of gold via chloride, thiosulphate and bisulphide ligands led to authigenic gold impregnation of clays. Basal gravels contain clasts that have been pervasively clay-altered, partially clay-altered, and only incipiently clay-altered, attesting to repeated fluvial reworking to basement of the channelised sediments, and progressive gold concentration.