Tree ferns in biogeochemical exploration, Goldmine (Puhipuhi) Hill, North Island, New Zealand

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Dunn, C.E.; Christie, A.B. 2017 Tree ferns in biogeochemical exploration, Goldmine (Puhipuhi) Hill, North Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/21 84 p.; doi: 10.21420/G2TG71

Abstract: At Goldmine Hill (also known as Puhipuhi Hill), 30 km east of Rotorua and 14 km southwest of Kawerau, a biogeochemical orientation survey was conducted over volcanic ash-covered Pleistocene dacitic rocks known to contain epithermal Au mineralisation. The survey focussed on the collection of foliage from the dominant fern - rough tree fern (Dicksonia squarrosa ['wheki'] - n = 62 samples). Supplementary samples from the silver fern (Cyathea dealbata ['ponga'] - n = 8 samples) and black tree fern (Cyathea medullaris ['mamaku'] - n = 7 samples) were collected for comparison. Samples were reduced to ash and, after an aqua regia digestion, analysed for a suite of 64 elements by ICP-ES and ICP-MS. Quality control samples showed that the data quality was very good with a precision of <10% RSD for most elements. Element concentrations were at about the levels expected for many plant species, except for high rare-earth element (REE) concentrations that are more typical for ferns. A comparison of data from the three tree ferns collected show some substantial differences, with wheki having the lowest concentrations for most elements. Nevertheless, the spatial distribution of elements is of greater significance than absolute concentrations and because of its abundance, wheki was the primary sample medium. Highest Au, Ag, As and Al concentrations were in wheki samples from a ridge extending west-north-westward at Goldmine Hill. This correlates well with the elevated Hg concentrations reported previously in soils from this area. Farther downslope were elevated concentrations of Fe, Zn, Cd and Cu. Antimony and Bi were at anomalous levels in an area peripheral to the above. (auth)