Petrological evaluation of Arapuni Dam and Hinuera Quarry rocks and secondary minerals

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Reyes, A.G.; Faure, K.; Soong, C.W.R. 2006 Petrological evaluation of Arapuni Dam and Hinuera Quarry rocks and secondary minerals. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/11 91 p.

Abstract: The majority of drillhole cores from Arapuni dam are Ongatiti Ignimbrite. Pre-Ongatiti rocks were intersected 3 wells at about 14-50 m elevation. The Ahuroa Ignimbrite was intersected in Test Pit 2 and in wells PQ14 and 16. Okareka Tephra was sampled in the test pits. The Potaka Tephra was sampled at the Hinuera Quarry. The secondary minerals and materials in the ignimbrites and tephras found in Arapuni dam and Hinuera Quarry consist of expanding clays such as smectite and vermiculite with the smectites including nontronite, montmorillonite and beidellite; 7A and 10A halloysite, kaolinite, zeolite, calcite, gypsum, quartz, goethite, bitumen used to seal defects in the rock, Mn-Fe oxyhydroxides, birnessite, romanechite, Ba-sulfate(?) and other amorphous biotic materials rich in Fe, Ba, S and P. Nontronite in Arapuni Dam occurs at about 25 to 100 m elevation in the Ongatiti Ignimbrite and basal part of the Ahuroa Ignimbrite. The Ahuroa nontronite typically contains Zn, which is absent in the Ongatiti Ignimbrite nontronite. There are three origins for the nontronite in Arapuni dam and its environs. The majority of nontronite was deposited, in the past, by nontronite-saturated solutions, generated by the interaction of groundwater with volatiles from rootless fumaroles, in the Ongatiti Ignimbrite and earlier phases of Ahuroa Ignimbrite eruptive episodes. A very small percentrage of nontronite is being produced at present by bacterial action. The precursor of these biotic nontronites appear to be coloured core rims found in wells PZ35, 42, 43 and 44. Thermodynamic modelling of Arapuni waters and Ongatiti Ignimbrite indicate that it is possible for abiotic nontronite to form at present at ambient temperatures. Bacterially-mediated materials are all deposited after the dam construction and consist mainly of Mn- and Fe-rich oxyhydroxides. Deposits with Mn/Fe ratios 0.6 are black and consist of sub-microscopic to microscopic globular masses. Deposits with Mn/Fe >0.6 have higher concentrations of Mn, Ba and Al and lower contents of P, Na and Mg than those with Mn/Fe