Magnetite and ilmenite of Westland beaches : analytical studies bearing on their sources and accumulation patterns.

Abstract

The extensive accumulations of black sands along the beaches of the West Coast of the South Island contain ilmenite and magnetite. In the Jackson Bay - Haast area magnetite predominates, but to the north the sands are ilmenite-rich. It has been established that throughout the entire region both the ilmenite and magnetite have been derived from the schists behind the Alpine Fault. Ultramafic sources near, or within the schists (Red Mountain in South Westland; Pounamu ultramafic pods in northern Central Westland) provide a small amount of chromium-rich magnetite, and because of this, the minor element contents of magnetite have been particularly useful in providing evidence on dispersion and beachsand accumulation patterns of magnetite from the hinterland source rocks. Regional variations in the composition of the Alpine Schist are reflected in the detrital heavy mineral assemblages of sediments from the schist-draining rivers. Schists of South Westland differ from those to the north 4n releasing more magnetite and hornblende, but less ilmenite, biotite and garnet. Because equilibrium has been attained between the various minerals within broad regions of the schists, magnetite and ilmenite samples from specific schist zones reflect the natural distribution of first-row transistion elements between the crystal lattices of these two oxide minerals. Samples of detrital magnetite and ilmenite from Tuhua Granites, however, appear to indicate considerable local variations within these rocks. The analytical work includes a critical examination of methods for dissolving magnetite and ilmenite for Atomic absorption. The procedures used minimise interelement interference effects and enabled any small correction to be made.