Gold dispersal and placer formation in an active oblique collisional mountain belt, Southern Alps, New Zealand

The Southern Alps of New Zealand constitute part of an orogenic belt formin g on the western margin of the Pacific plate due to collision along the Pac ific-Australian plate boundary. The topography of the mountain range is asy mmetrical with a steep inboard zone adjacent to the plate boundary where ve ry high rainfall causes rapid erosion, and a relatively gentle outboard zon e in the rain shadow east of the topographic divide. Detritus, including go ld, shed from the rising mountains are transported eastward from out outboa rd zone into a foreland basin, and westward from the inboard zone across th e plate boundary to the Australian plate. Rivers in the outboard zone are w idely spaced (>20 km) and structurally controlled oblique to the trend of t he orogen. These rivers are long (>150 km) with relatively low gradients, a nd form braid plains showing net aggradation over the past million years. G old placer formation in outboard rivers draining the central Southern Alps requires concentration factors of ca. 10(5) to 10(6), which has occurred on ly during localized postdepositional uplift and recycling of sediments. Tec tonic 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) an d 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 t ransported across the plate boundary on to the indentor (Australian) plate, in till or river gravels, where it is concentrated in degradational lags i n fluvial sediments and in marine beach placers. Dextral tectonic motion at the plate boundary has transported detrital gold up to about 180 km latera lly from Southern Alps sources during the past 5 m.y.