Abstract

ABSTRACT

A pilot multi-element stream sediment geochemical survey was carried out in the Thames area, Coromandel Peninsula where epithermal Au–Ag and porphyry Cu–Mo–Au style deposits are hosted in Miocene–Pliocene andesite, dacite and rhyolite. Seventy-two samples were collected in a 220 km2 catchment area. The 500 μm to 150 μm size fraction was analysed for 51 elements, mostly by X-ray fluorescence, although inductively coupled plasma mass spectrometry was used for Be, Co, Ge, Se, Sb, Ag, Hg, U and Cd, a Leco analyser for C and N, and proton-induced X-ray emission for Li, B and F. All values for Au, Ag and I were below their detection limits of 1.0 ppm (mg kg−1), 0.1 ppm and 2.0 ppm, respectively. Arsenic, Cd, Cu, K2O, Pb, Sb, Se and Zn exhibit anomalous concentrations, mainly in the western part of the study area, reflecting the occurrence of epithermal quartz vein and porphyry copper mineralization in hydrothermally altered andesites and dacites. Mercury is anomalous in southern samples, reflecting cinnabar occurrences in the Kauaeranga valley (e.g. Mangakirikiri mercury deposits) and as a late-stage mineral in some of the Thames epithermal veins. Barium and U are anomalous in the eastern samples, probably related to the occurrence of rhyolitic rocks in the eastern part of the sampled area. Comparison with archived mining company stream sediment survey data for Cu, Pb and Zn reveals that most of the anomalies identified in the pilot survey have corresponding anomalies in the mining company data, but the much higher density of sampling in the mining company surveys identified additional anomalies not seen in the pilot survey data.

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