Abstract
The dissolved load of waters were analysed seasonally across a year (Summer - 2013 to Spring - 2014) using major and trace ions and Sr isotope ratios (87Sr/86Sr) to determine mineral provenance in the Waitaki Catchment, South Island, New Zealand. The Waitaki Catchment extends from the central-eastern section of the Southern Alps, through basin and range topography and finally through low-land farm land to the Pacific Ocean. It is New Zealand’s most significant hydro-electricity generating catchment and storage in the basinal lakes causes a well-mixed reservoir.
The average relative molar abundance of major cations across all waters is Ca2+ (69%) > Na+ (19%) > Mg2+ (8%) > K+ (4%). Calcite weathering in the glaciated catchments becomes supply limited and waters are never saturated with respect to calcite and the low silicate flux allows calcite dissolution to effectively titrate the waters there. The large glaciers in the Mt Cook area dominate the dissolved load composition of waters below them by introducing large volumes of highly reactive, superficially weathered silicate suspended load. Once out of the alpine areas silicate decomposition increases with temperature and silicates are weathered to kaolinite.
As waters exit the major storage lakes in the Mackenzie Basin and enter the Waitaki Valley there is a significant influence from numerous small streams which drain marshlands, springs and productive soils. These have a high amount of silicate derived ions, notably Na+ and Si4+ from the dissolution of albite and Mg2+ from chlorite. Similarly, 87Sr/86Sr compositions move towards that of plagioclase as the proportion of silicate derived Sr increases.
Acetic acid leaches of the Torlesse Terrane metasediments showed that greywackes predominantly contribute to the Ca2+ flux while argillites contribute more K+ and Si4+. As a consequence the argillites are more radiogenic but the greywackes disproportionality influence the Sr isotope ratio of waters due to the disseminated calcites high Sr content.