Abstract

The Southern Alps of New Zealand constitute part of an orogenic belt forming on the western margin of the Pacific plate due to collision along the Pacific-Australian plate boundary. The topography of the mountain range is asymmetrical with a steep inboard zone adjacent to the plate boundary where very high rainfall causes rapid erosion, and a relatively gentle outboard zone in the rain shadow east of the topographic divide. Detritus, including gold, shed from the rising mountains are transported eastward from the outboard zone into a foreland basin, and westward from the inboard zone across the plate boundary to the Australian plate. Rivers in the outboard zone are widely spaced (>20 km) and structurally controlled oblique to the trend of the orogen. These rivers are long (>150 km) with relatively low gradients, and form braid plains showing net aggradation over the past million years. Gold placer formation in outboard rivers draining the central Southern Alps requires concentration factors of ca. 10 5 to 10 6 , which has occurred only during localized postdepositional uplift and recycling of sediments. Tectonic shortening coupled with low outboard erosion rates results in capture of some outboard rivers by inboard rivers, with consequent redirection of gold into the inboard region. Inboard rivers are generally short (20 km) and closely spaced (10 km), and oriented perpendicular to the plate boundary. Aggradation of sediments is temporary and periodic scouring of the valleys to bedrock by floods and glaciers results in ephemeral placer deposits in river bed lags and flood gravels on the Pacific plate. Gold is ultimately transported across the plate boundary on to the indentor (Australian) plate, in till or river gravels, where it is concentrated in degradational lags in fluvial sediments and in marine beach placers. Dextral tectonic motion at the plate boundary has transported detrital gold up to about 180 km laterally from Southern Alps sources during the past 5 m.y.

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