Abstract
The Rakaia River in the South Island of New Zealand is a unique catchment in which to trace active detrital gold transport, deformation and concentration from near-coeval orogenic sources. The catchment hosts widespread small late Cenozoic hydrothermal alteration zones, some of which are gold-bearing, in actively rising and eroding mountain headwaters. Detrital gold liberated from the sources is distinctively porous with abundant micron-scale silicate inclusions, especially albite. Fine gold (∼200-100 µm) has been transported >100 km downstream, but coarse gold remains highly diluted in proximal fluvial sediments. Fine gold is also diluted by coarse fluvial sediments that form the upper kilometre of the Canterbury sedimentary basin southeast of the mountain front. Minor re-concentration of fine gold occurs in a bedrock gorge at the mountain front, and on steep ocean beaches beyond the river mouth. Despite the long-distance transport, the fine gold particles have been only superficially modified by surface smearing and minor flattening. The gold may have travelled in suspension in the river, buoyed by electrostatically adhering fine micas, in contrast to detrital garnets that were rounded in bed-load. In contrast, detrital gold on the western side of the mountains has been flattened to thin flakes during bed-load transport.