The effects of transport and weathering on heavy minerals from the Cascade River New Zealand

The effects of transport and weathering processes on heavy mineral suites have been assessed by examining suites from the Cascade-Martyr river system (Westland, New Zealand) and its associated beach. These rivers drain a predominantly ultramafic source that supplies a large amount of unstable minerals, including forsteritic olivine, enstatite, diopside, actinolite-tremolite and Ti-rich brown amphibole. The weathering processes at outcrop, which take place in a wet temperate climate, have not caused a significant decrease in mineral diversity. The only variations in mineralogy that are detectable in the river samples reflect slight variations in catchment lithologies, suggesting that weathering processes on the floodplain have not affected heavy mineral suites during transit. This indicates that most coarse sediment movement takes place during floods and that coarse sediment does not reside for extensive periods on the floodplain. The suites found in the river sands do not differ appreciably from those in the beach, indicating that longshore drift does not introduce extraneous sediment to the Cascade detritus as it enters the beach. Furthermore, mechanical abrasion processes on the beach have not caused any significant decline in abundance of any of the constituent minerals, even of olivine, one of the most mechanically unstable heavy minerals. Therefore, neither mechanical abrasion nor weathering has had a marked effect on the heavy mineral suites in the Cascade-Martyr river system.