Tree ferns and tea trees in biogeochemical exploration for epithermal Au and Ag in New Zealand

Abstract

Biogeochemical orientation surveys were undertaken at low sulphidation epithermal Au–Ag occurrences in the Hauraki Goldfield–Coromandel Volcanic Zone and the Taupo Volcanic Zone, and at the Waiotapu active geothermal area in the Taupo Volcanic Zone. Several plant species were sampled, including the foliage of tree ferns and tea trees. The ferns – silver fern (ponga), rough tree fern (wheki) and black tree fern (mamaku) – were ubiquitous and were the easiest species to sample, although tea tree was the dominant genus at Waiotapu. At the Waiotapu geothermal area, significantly higher concentrations of Ag, Au, Sb, As, Cs and Rb were present in samples close to Champagne Pool than elsewhere, confirming its location as the main outflow source of Au, Ag and their pathfinder elements. The fern survey areas at Luck at Last mine, Pine Sinter and Ohui in the Coromandel Volcanic Zone each exhibited biogeochemical anomalies, which successfully highlighted most of the known quartz veins and provided additional anomalies for further investigation. Rough tree fern was the most common species at Goldmine Hill, Puhipuhi (Taupo Volcanic Zone). Although this species absorbs lower concentrations of many elements than the silver fern, the spatial distribution of elements is of greater significance than their absolute concentrations. The highest Au, Ag, As and Al concentrations occurred in samples from a ridge extending WNW from Goldmine Hill. Sb and Bi were at anomalous levels in an area peripheral to the precious metal anomalies, indicating the potential zonation of elements distal from the Au and Ag deposits.Supplementary material: The full datasets on the fern and tea tree chemistry, including quality assurance/quality control and multi-element plots, are available free of charge through the GNS Science website (search for Dunn) at http://shop.gns.cri.nz/publications/science-reports/.