Heavy mineral rich beach sands of the Waitakere coast, Auckland, New Zealand

Abstract

Heavy mineral rich beach sands of the Tasman Sea coast of the Waitakere Ranges, west of Auckland City, include major minerals plagioclase, augite, titanomagnetite, and quartz, with minor ilmenite, hypersthene, hornblende, biotite, and potassium feldspar, and accessory zircon and apatite. Major sources for the sands are Taupo Volcanic Zone rhyolitic volcanics, delivered via the Waikato River, and Taranaki andesitic volcanics, via the longshore drift. Longshore trends from south to north include a slight fining, a reduction in rock fragments, an increase in heavy mineral content, and a tendency for the feldspar/quartz ratio to decline. Ilmenite is a significant component; it is derived from the Taupo Volcanic Zone (not from the Waitakere Ranges as previous authors have suggested), and it increases in quantity from south to north. However, it does not replace titanomagnetite.

The Taupo Volcanic Zone could have supplied all of the ironsand on the west coast north of the Waikato River mouth. Longshore trends and distribution of ironsand and other minerals on the continental shelf all strongly suggest a net northwards littoral drift of sand, which would be expected from the prevailing south-southwesterly wave approach. Possible contributions of sand from relict continental shelf accumulations, and from a large igneous body which underlies the continental shelf, cannot be evaluated.

Keywords:

• beaches
• sand
• Waitakere Ranges
• heavy minerals
• grain size
• littoral drift
• longshore currents
• Taupo Volcanic Zone
• sediments
• iron-rich composition