The Impact of Climate and Transport on the Compositions and Textures of Heavy Minerals in Modern Rivers of New Zealand

The types, abundances and textures of heavy minerals in the 63-125 μm fraction of modern river sands were investigated in order to assess the controls of climate, tectonic setting, source composition, recycling, transport and diagenesis on the nature of sedimentary systems. Two rivers on the South Island of New Zealand were selected for detailed study; the Haast is a high-flow river located in the high precipitation region near the west coast, whereas the Ahuriri River has lower flow rates and is located in a drier region in the central South Island. In both river systems, the Zircon-Tourmaline-Rutile (ZTR) index increases downstream, illustrating that chemical and mechanical weathering gradually degraded the more unstable mineral species relative to ultrastable zircon. Point counting of the light mineral fractions indicates a downstream increase in quartz compared with feldspar, further suggesting chemical and mechanical weathering processes. Although the Chemical Index of Alteration (CIA) values are not high (average 55 for each river system), the heavy and light mineral data, combined with major element results illustrating an increase in SiO2 with decreasing Al2O3, MgO and Fe2O3 downstream, all indicate a weathering signal. Highly fluctuating patterns in mineralogical and geochemical data suggest bar-to-bar variations consistent with mixing from tributary input. Four heavy mineral types were chosen for textural investigations and include garnet, zircon, titanite and tourmaline. Roundness and etching values for each mineral produced no downstream trends. This discrepancy is attributed to the recycled nature of the source rocks, which are schistose and wacke deposits, as well as input of new source material downstream via tributaries, alluvial fans and erosion of Quaternary and Tertiary deposits. Detailed investigations of sediment sampled from a longitudinal bar in the Haast River show that surface samples have lower roundness and etching values than sediment 10 cm below the surface. This result suggests that mineral alteration takes place within the water-laden bar through diagenetic processes rather than from subaerial chemical weathering