Dunn, C.E.; Christie, A.B.; Black, J.A. 2018 Biogeochemical surveys at Ohui and Pine Sinter epithermal Au-Ag prospects, Coromandel, and at Waiotapu Thermal Park, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/20. 84 p.; doi: 10.21420/ G25647.
In 2017, biogeochemical surveys were conducted at the Pine Sinter and Ohui Au-Ag prospects located 15 km north of Whangamata near Oputere in the Hauraki Goldfield, Coromandel Volcanic Zone. Foliage from 138 samples of common tree ferns was collected at 117 sample sites (several species from selected sites). In addition, 18 samples of several common plant species were collected from 6 sites around the geothermal pools at Waiotapu, 27 km SSE of Rotorua, in order to compare and contrast the relative abilities of the different plant species to accumulate a range of elements. At Pine Sinter and Ohui, silver fern (Cyathea dealbata [‘ponga’]) proved to be the most widespread species and was the primary focus for these surveys. For comparison of relative element uptake, additional samples were collected of rough tree fern (‘wheki’, Dicksonia squarrosa) and black tree fern (‘mamaku’, Cyathea medullaris). All 156 samples were reduced to ash and analysed for 52 elements. At Pine Sinter, exploration has yet to find a feeder pipe to outcropping sinter that overlies a large flow-banded rhyolite dome and an altered andesite. Silver ferns have slightly anomalous Ag content in a north-trending zone located above the andesite near its contact with the rhyolite. All Au levels were low and no definitive trends were obvious. Over the eastern part of the rhyolite the ferns show a north-south trend of several elements (Ba, Ca, Sr, and Se) that suggest some carbonatisation of the underlying rocks; REE have some coincident trends. At Ohui, the Phoenix and Staircase Au-Ag-bearing quartz veins are quite well defined by the distribution of Ag and Sb, but not by Au. The Staircase veins also show some weak enrichment of As. Nickel, Co, and to a lesser degree S and Ca, define a north-easterly trend that follows some parallel faults. These elements are not enriched in samples from the vicinity of the Great Mexican fault, but some enrichment of Al, B and Mn suggests alteration. Zinc, Tl, Pb and Cu in the southwest are indicative of base metal mineralisation. From earlier soil surveys, there were patterns of Au, As, Sb and Hg that indicated enrichments near the Great Mexican fault, but these signatures are either not present in the ferns or the weak signatures are laterally displaced. Concealed deposits tend to be reflected in plants growing directly above mineralisation, whereas in such rugged terrain, soils may show mechanical dispersion. At Waiotapu, the data show that kanuka (Kunzea ericoides) has higher concentrations than manuka (Leptospermum scoparium) of the epithermal related elements Ag, Au, Sb and As, especially close to Champagne Pool. As with previous studies (Luck-at-Last mine and Puhipuhi (Goldmine) Hill), the data obtained from this study further demonstrate the potential value and some limitations in the use of silver fern (‘ponga’) as an exploration medium in the search for Au-Ag deposits in New Zealand. (auth)