Geology, mineralogy and geochemistry of the rhyolite-hosted Maungaparerua clay deposit, Northland

Abstract

A rhyolite dome complex at Maungaparerua, Ar/Ar dated at 3.7 ± 0.04 Ma (Early Pliocene), is bounded on the west by a sequence of hydrothermally altered basalt flows intercalated with several non-marine siltstone and rhyolitic tuff units. This basalt sequence and the rhyolite dome complex are overlain by younger unaltered basalt flows. Within the dome complex, several small pits have been worked in the past for china clays. Recent drilling has outlined a halloysite-rich ‘Southern Area’ extending to a depth of up to 24 m below the present-day erosion surface. Primary sanidine and plagioclase phenocrysts are completely leached in halloysite-rich rhyolite, but are only partially leached at greater depth. Halloysite-rich rhyolite is characterised by relative enrichment in loss on ignition (LOI; 5–9%) and Al2O3 (18–24%) and depletion in K2O (<0.5%), compared with 2.0% LOI, 15.0% Al2O3 and 4.1% K2O in least-altered rhyolite. Oxygen and hydrogen isotope compositions of halloysite samples indicate that it is of supergene rather than hydrothermal origin. This is consistent with weathering type clay profiles in halloysite-rich zones. Although there is earlier hydrothermal alteration in the form of silicified rhyolite 800 m to the west of the Southern Area and kaolinite-pyrite alteration in the adjacent basalt, we conclude that the dominant process in the formation of the halloysite was deep weathering of sanidine rhyolite under water-saturated subtropical conditions.