Tree Ferns and Tea Trees in Biogeochemical Exploration for Epithermal Au and Ag in New Zealand

Biogeochemical orientation surveys were undertaken at low sulphidation epithermal Au-Ag occurrences in the Hauraki Goldfield – Coromandel Volcanic Zone (CVZ) and Taupo Volcanic Zone (TVZ), and at the Waiotapu active geothermal area in the TVZ. 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 the most ubiquitous and were easiest 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 CVZ each exhibited biogeochemical anomalies which successfully highlighted most known quartz veins and provided additional anomalies for further investigation. At Goldmine Hill, Puhipuhi (TVZ), rough tree fern is the most common species. Although this species absorbs lower concentrations of many elements than the silver fern, the spatial distribution of elements is of greater significance than absolute concentrations. Highest Au, Ag, As and Al concentrations occurred in samples from a ridge extending west-north-westward from Goldmine Hill. Antimony and Bi were at anomalous levels in an area peripheral to the precious metal anomalies, indicating potential zonation of elements distal from the Au and Ag.