Te Aroha Research Articles

The Origin of Mineralization at the Tui Mine, Te Aroha, New Zealand, in the Light of Stable Isotope Studies

Within the old Hauraki goldfield, the Tui Mine is 130 km NNW of the Taupo volcanic zone and thermal water is found close to the mine. Mineralized quartz veins occur in hydrothermally altered, Tertiary andesites which overlie Mesozoic greywackes. The Tui vein paragenesis is (1) pyrite+chalcopyrite, (2) galena+sphalerite, (3) minor pyrite and chalcopyrite, and (4) minor hematite, barite, cinnabar, kaolinite and carbonates. Stable isotope studies indicate temperatures of formation of 400+ or -80 degrees C (5 sulphide pairs, stage (1)), 320+ or -60 degrees C (18 sulphide pairs, stage (2)) and 150+ or -30 degrees C (barite-water, stage (4)). delta S 34 values of the sulphides range from +5 to -3 per mil (CDT) and show a 2 per mil decrease with time. The vein mineralogy suggests a corresponding decrease in pH and increase in oxidation potential of the ore fluid. Thus late barite with delta S 34 = +16 per mil fixes the delta S 34 (sub sigma S) = +16+ or -1 per mil for the system. Hence at 350 degrees C H 2 S and HSO 4 in the ore fluid would have delta S 34 values of +5 and +24% respectively, indicating a probable magmatic plus greywacke source for the sulphur. Te Aroha thermal water shows an oxygen isotope shift of 4 per mil relative to meteoric water (delta O 18 -5.5 per mil SMOW, delta D -30 per mil SMOW). Waters extracted from sphalerite and quartz fluid inclusions and calculations from isotopic studies of quartz and kaolinite indicate ore fluid values of delta O 18 0+ or -0.5 per mil, delta D -17+ or -1 per mil (stage (2)) and delta O 18 -1.0+ or -0.2 per mil, delta D -24+ or -1 per mil (stage (3)). An ultimate meteoric origin for the ore fluids is postulated. Published Pb isotope data (Cooper and Richards) suggest the Pb to be derived from the greywackes. Many comparisons with present day geothermal systems can be made.

Te Aroha earthquake, 9 January 1972

The Te Aroha earthquake of 9 January 1972 (N.Z.S.T.) was of only moderate magnitude (5.1) . It occurred at a shallow depth within a few kilometres of Te Aroha, where an intensity of MM VII was reached. Damage was mainly confined to domestic chimneys and contents of buildings in the Te Aroha region, but the felt area extended as far as Auckland, Mangakino and Whakatane. Numerous after shocks occurred in the following month.

Structural control of base metal mineralisation at the Tui Mine, Te Aroha, New Zealand

Abstract The Tui Mine, located at the southern end of the Coromandel Peninsula, occurs in extensively hydrothermally altered andesitic rocks. Two structurally simple lead-zinc lodes, the Champion and the Ruakaka, strike NE and E-W respectively and diverge with depth. Both lodes occur along dominantly normal faults, and consist of a quartz-cemented wall rock breccia, within which is a younger and narrower brecciated zone cemented with sulphides and quartz. Textural relationships of ore and gangue suggest the following sequence of events: fault movement and brecciation, cementation with quartz and minor pyrite and chalcopyrite, renewed fault movement and brecciation, deposition of galena and sphalerite, deposition of a second generation of pyrite and chalcopyrite, deposition of marcasite and chalcopyrite with minor tetrahedrite, and deposition of kaolinite and traces of cinnabar in some of the remaining cavities. Intermittent brecciation and cementation by quartz took place during sulphide deposition. The g...

Lead isotope measurements on volcanics and associated galenas from the Coromandel - Te Aroha region, New Zealand

Isotopic measurements have been made on lead extracted from samples of andesite, propylitized andesite and rhyolite, and from galenas present in quartz-calcite veins occurring in the Miocene Hauraki Goldfield region, New Zealand. Measurement bias due to isotopic fractionation has been removed by the double-spike technique. All samples show varying degrees of J-type anomaly. Although there is some overlap, the galena samples generally appear more anomalous than the andesites; lead in the veins is isotopically different from lead in adjacent andesite. There is no evidence of contamination by ancient, low radiogenic rock systems, or of isotopic fractionation during vein formation. Although the vein galenas may be slightly enriched in preferentially-extracted radiogenic lead, the results are thought to represent isotopic inhomogeneity in the sources. With one exception the vein galena, rhyolite, propylitized andesite, and hydrothermally altered rocks form a linearly-distributed set on the 208Pb/204Pb - 206Pb/204Pb diagram, which appears to be distinct from the unaltered andesite. A similar but less distinct trend is present on the 207Pb/204Pb - 206Pb/204Pb diagram. It is suggested that the vein galenas and rhyolites, together with the propylitization and hydrothermal fluids, could all have originated from the one bulk source-material, the thick Mesozoic greywacke-argillite sequence, and that the andesites have come from another, necessarily deeper source. Propylitization processes appear to have introduced the hydrothermal-rhyolite-galena-type lead into the altered andesites. A criticism is made of the use of lead isotope results from hydrothermal material as input data for the calculation of single-stage model growth-curves.

The Geology of an area of accelerated erosion at Waerengaokuri, near Gisborne

Abstract Upper Tertiary rocks in the Waerengaokuri area form a faulted anticlinal sequence and are overlain unconformably by flat-lying lacustrine sediments of Early Pleistocene age. Rejuvenation of Te Aroha Stream by piracy in its headwaters has initiated a cycle of accelerated erosion in sheared sediments and diapirically extruded bentonitic mudstones associated with th.c fault zone. Rapid down-cutting in these rocks has left the weak lacustrine sediments unsupported, and they are slumping extensively. Diversion of drainage waters, artificial grading of sections of the stream channel, and the establishment of vegetative cover seem the most promising methods of inhibiting the rate of erosion and improving the stability of the slopes.