A. Reay

Geochronology and geochemistry of the Dunedin Volcanic Group, eastern Otago, New Zealand

Abstract Fifty‐six previously unpublished K‐Ar ages for the Dunedin Volcanic Group and previously published K‐Ar and 40Ar/39Ar ages demonstrate that activity in the centrally situated Dunedin Volcano (here given formal lithostrati‐graphic status) lasted from 16.0 ± 0.4 to c. 10.1 Ma, and that of the surrounding Waipiata Volcanics lasted from 24.8 ± 0.6 to 8.9 ± 0.9 Ma. Apart from a gap at c. 20 Ma, recorded Waipiata activity climaxed at c. 16–14 Ma when activity of the Dunedin Volcano was beginning; it outlasted that of the Dunedin Volcano by c. 1 m.y. The total volume erupted by the Dunedin Volcano may have exceeded that of the largely monogenetic Waipiata Volcanics by an order of magnitude. New major‐ and trace‐element analyses are given for 87 whole‐rock samples and kaersutite. The whole‐rock data demonstrate the exclusively alkalic nature of the group, the Waipiata Volcanics being more strongly alkalic than most of the mafic members of the central volcano. This fractionated to give a much greater volume of phonolitic differentiates than the Waipiata Volcanics. As for other intraplate Cenozoic volcanism in the New Zealand region, ranging overall from tholeiitic to highly alkalic, major‐ and trace‐element patterns support an origin from a garnet‐bearing ocean island basalt source region with high U/Pb mantle characteristics.

Adakites from Solander Island, New Zealand

Abstract The subduction related, island arc Pleistocene volcano of Solander Island in Foveaux Strait is composed of adakite, a new rock type first described from Adak Island in the Aleutian arc. Adakites are formed by the partial fusion of young oceanic crust under eclogitic facies conditions. Comparison with other adakite localities suggests that the oceanic crust presently subducting at the nearby trench may be <25 Ma old.

Lithology, field relationships, and structure of the Pounamu Ultramafics from the Whitcombe and Hokitika Rivers, Westland, New Zealand

Abstract The Pounamu Ultramafics comprise a series of parallel or en echelon lenses and layers of ultramafic and mafic rocks defining a northeaststriking, 2 km wide zone (the Pounamu Ultramafic Belt — PUB) in garnet zone Haast Schists of North Westland. In their most coherent form, the Pounamu Ultramafics comprise serpentinite, gabbro, polymict igneous breccia, and metabasite. Gabbros are intruded by feldsparphyric dikes and unconformably overlain by breccia. Breccia or metabasite is overlain sequentially by a pelagic sequence including: interlaminated limestone and chert, phosphate lenses, and a massive porphyroblastic biotite schist; and a terrigenous series of thin-bedded, upward younging units of graded quartzofeldspathic sandstone and siltstone, similar to distal turbidite lithologies from the western Torlesse terrane. Similar upward grading quartzofeldspathic metasediments occur beneath the ultramafic unit. The Pounamu Ultramafics are interpreted as an ophiolite segment representing the basement to ...

Mobilisation and attenuation of arsenic around gold mines, east Otago, New Zealand

Abstract Arsenic is released into the environment by decomposition of natural arsenopyrite in gold‐bearing veins in east Otago. Natural release is slow, but is accelerated by mining activity. Waters from old mine adits have pH of 6–7, compared to normal groundwater pH of 7–8, and adit waters have arsenic concentrations up to 4 ppm compared to background levels of c. 0.01 ppm. Tailings disposed about 80–90 years ago into what is now a wetland at Barewood have arsenic concentrations of up to 33 000 ppm, principally in the form of the secondary arsenic mineral scorodite. The maximum dissolved arsenic concentration in the Barewood tailings is 0.75 ppm at pH of c. 5.5. Dissolved arsenic release at that pH may be restricted to levels near 1 ppm by the formation of scorodite with solubility and/or kinetic controls on scorodite redissolution. Modern mining activity in the Macraes area has resulted in localised dissolved arsenic concentrations in excess of 200 ppm at pH near 10. Evaporation of this water produces scorodite precipitates. Dissolved arsenic released from arsenopyrite decomposition is attenuated by fine‐grained minerals, predominantly phyllosilicates, in soils around weathered veins, and in wetlands downstream from mine workings. Lithic soils can hold at least 50–100 ppm As on the fine fraction (c. 1 μ,m), and wetland soils can hold hundreds to thousands of parts per million on their fine fractions. Both soil adsorption and scorodite mediation slow the release of arsenic into the environment, allowing more effective dilution by downstream waters to safe arsenic levels.

Lherzolite xenolith bearing flows from the east Otago province: Crystal fractionation of upper mantle magmas

Abstract The Waipiata Volcanic Formation comprises a large number of minor eruptive vents peripheral to the Dunedin volcano. Many of the flows and associated pyro‐clastic deposits contain spinel‐lherzolite xenoliths. The lherzolite‐bearing flows have relatively low Mg numbers and are the result of low degrees of partial melting followed by fractional crystallisation in the upper mantle. Early fractionation is controlled by tschermakitic clinopyroxene with minor olivine and titanomagnetite, whereas the more evolved flows also include kaersutite as a significant fractionating phase.

Platinum mineralisation potential of the Longwood Igneous Complex, New Zealand

SummaryThe Longwood Igneous Complex is situated at the southern tip of the South Island of New Zealand. The Complex is 32 km long and up to 12 km wide and is part of a more extensive north-trending belt of late Paleozoic volcanic, volcaniclastic and intrusive rocks.The complex is comprised of an extensive series of layered gabbros (Pahia Layered Series) together with diorites, trondjhemite and granitoids.Exploration by Sigma Resources has located significant platinum-in-drainage anomalies which are derived from a 10 km x 1 km area of largely plagioclase-rich, olivinebearing cumulate rocks within the Pahia Layered series. Grains of platinum and palladium sulphide, arsenide and alloy minerals have been identified in placer deposits derived from this area. A total of 1,500 ounces of platinum was reported to have been recovered as a by-product of historic placer gold mining of these deposits. Platinumgroup element mineralisation in float rocks (up to 3.0 ppm Pt and 3.3 ppm Pd) and significant platinum-in-soil anomalies (peak value 0.47 ppm Pt) have been found within this area.The mineralised troctolite and olivine-gabbro float and adjacent exposures are broadly similar to the anorthosite-troctolite-olivine gabbro zones within the Banded Series of the Stillwater Complex. The mineralised rocks are leucocratic plagioclaseolivine adcumulates with minor clinopyroxene oikocrysts, primary amphibole and sparse (<0.5%) sulphide. Sulphide minerals are pyrrhotite, chalcopyrite, pyrite and secondary copper sulphides. The similarity of this mineralisation style and its host rocks to the JM Reef of the Stillwater Complex and AP and PV mineralisation of the Penikat Intrusion highlights the excellent potential for discovery of a large stratiform platinumgroup element deposit in the Pahia Layered Series.ZusammenfassungDer Longwood Igneous Complex liegt au der Südküste der der Südinsel Neuseelands. Er ist 32 km lang und 12 km breit und ist Teil eines Nord-Süd verlaufenden Gesteinsgürtels paläozoischer vulkanischer, vulkano-klastischer und intrusiver Gesteine. Der Komplex beinhaltet geschichtete Gabbros (Pahia Layered Series) gemeinsam mit Dioriten, Trondjhemiten und Granitoiden.Bedeutende Platinanomalien in Bachsedimenten, die aus einem ca. 10 x 1 km großen Gebiet mit Plagioklas-reichen, Olivin-führenden Kumulat-Gesteinen stammen, wurden von Sigma Resources lokalisiert. Platin- und Palladiumsulfide, -arsenide und -Legierungen wurden in Seifenlager-stätten, in diesem Gebiet nachgewiesen. Insgesamt wurden 1500 Unzen Platin während der historischen Bergbautätigkeit auf Gold in diesen Lagerstätten als Nebenprodukt gewonnen. PGE-Mineralisation (bis 3.0 ppm Pt und 3.3 ppm Pd) in Geschieben und bedeutende Pt-Bodenanomalien (max. 0.47 ppm Pt) wurden in diesem Gebiet entdeckt.Die mineralisierten Troktolit- und Olivin-Gabbro-Gerölle und benachbarte Aufschlüsse zeigen weitgehende Ähnlichkeit mit der Anorthosit-Troktolit-Olivin-GabbroAbfolge der Banded Series des Stillwater Komplexes.Die mineralisierten leukokraten Gesteine führen Plagioklas und Olivin als Kumulusphasen sowie untergeordnet Klinopyroxene, primäre Amphibole und geringe Mengen (< 0.5 %) an Sulfiden. Als Sulfide kommen Magnetkies, Kupferkies, Pyrit und sekundäre Kupfersulfide vor. Die Ähnlichkeit dieser Mineralisation und ihrer Nebengesteine mit dem JM Reef des Stillwater Komplexes und der AP und PV Mineralisation in der Penikat Intrusion läßt die Pahia Layered Series als geeignetes Gebiet für die Entdeckung großer stratiformer PGE-Lagerstätten erscheinen.

Hydrothermal alteration geochemistry of Nugget gold vein system, Shotover valley, northwest Otago, New Zealand

Abstract Ankeritic alteration zones are associated with gold mineralisation in the Nugget vein system, Shotover valley, northwest Otago. These zones consist of sheared and brecciated schist in Miocene normal faults. The zones are also extensively silicified. Mineralisation resulted from passage of low‐salinity, low‐CO2 water at 200–250°C within a few kilometres of the surface. Alteration has involved replacement and veining of hostpelitic schist in the mineralised zone. Most elements have been diluted by quartz and carbonate, with insignificant gains or losses. Ba has been leached as it became more soluble with decreasing fluid temperature. Considerable addition of Sr has occurred, as the fluid had higher Sr/Ca than most Otago mineralising fluids. Enrichment in Sr may be a distinctive feature of the Shotover style of mineralisation. Metallic mineralisation is due to addition of S, As, and Au during alteration. In contrast to many Otago gold vein systems, there has been no scheelite mineralisation. Carbona...

Mafic and ultramafic rocks, and platinum mineralisation potential, in the Longwood Range, Southland, New Zealand

Abstract Intrusive rocks in the Longwood Range represent a component of the Permian Brook Street Terrane. They include diffusely layered, cumulate-textured olivine gabbro, troctolite, and gabbro, and gradations into non-cumulate gabbro and gabbronorite. Volumetrically small ultramafic layers occur (plagioclase wehrlite), and thin veins of felsic rocks ranging from quartz diorite to trondhjemite. Primary olivine, plagioclase, clinopyroxene, and subordinate orthopyroxene and hornblende are commonly altered or metamorphosed to amphiboles, minor spinel, magnetite, chlorite, biotite and clinozoisite, and serpentine in olivine-rich rocks. Accessory primary Ti-bearing magnetite and ilmenite occur, and trace Cr-magnetite is characteristic of olivine-rich rocks. Trace pyrrhotite, chalcopyrite, pentlandite, and pyrite could reflect equilibrated late magmatic, and alteration-derived phases. Key petrochemical characteristics of the rock suite are high Mg, Al, Ca, and Sr contents, and low alkali, LILE, and sulfur contents. Platinum and Pd are locally enriched in drill-hole intercepts, but zones appear unrelated to rock type, magnetic properties, or to S, Cu, Ni, Cr, or Au values. Local platinum group element (PGE) enrichment in altered rocks implies metamorphic and/or hydrothermal redistribution. Pervasive PGE enrichment in Longwood rocks is an indicator of potential ‘fertility’, but evidence is currently lacking for the precipitation of primary stratiform PGE accumulations from a sulfide liquid saturated magma.

Composition and regional significance of Mid Bay and Mason Bay reefs, Foveaux Strait, New Zealand

Abstract Petrographic investigation of rocks from Mason Bay reef indicates that it consists of gabbro and amphibolite similar to, but less evolved than, the Walkers Hill diorite of western Stewart Island. Petrographic examination of samples from Mid Bay reef in Te Waewae Bay, and seismic profiling across it, reveal that this reef is predominantly gabbro with some granite, and is raised up, together with probably Pliocene sediments, along a northeasterly oriented structure. This exposure of basement plutonic rocks at Mid Bay reef indicates total uplift of more than 5000 m in the late Tertiary, effectively separating the Waiau Basin to the north from the Solander Basin to the south.

An occurrence of ardennite in quartz veins in piemontite schist, Western Otago, New Zealand

SummaryArdennite of complex composition: (Mn2+3.488Ca0.509Ba0.002)Σ=4(Mg0.916916 Fe3+0.165 Mn3+0.099Cu0.033Ni0.009Zn0.006 Ti0.008Al4.764)Σ=6(As5+0.823V5+0.022P0.005B0.069Al0.042Si5.039)Σ=6O21.81(OH)6.17 occurs in crack-seal quartz veins in quartz-albite-piemontite-spessartine-phengitehematite-chlorite-rutile-tourmaline ± calcite schist of the Haast Schist Group near Arrow Junction, western Otago, New Zealand. The Mn2+/Mn3+-ratio is sensitive to calculations and to accuracy of analyses. Boron is detected in ardennite for the first time. Other properties includeα = 1.734(3),β = 1.735(3),γ = 1.751(3), 2VZ = 30(2)°;a = 8.721(1),b = 5.816(1),c = 18.545(3) Å,V = 940.7(2) Å3. Associated mineral phases are spessartine, hematite, piemontite containing ≈0.7% SrO and ≈0.06% PbO, and phengite. Later-stage vein minerals comprise chlorite, albite, and manganoan calcite which were deposited under less highly oxidizing conditions. Digenite with minor intergrown covellite occurs in small amount with manganoan calcite and quartz in a cross-cutting late-stage veina chalcopyrite and native copper occur in other late-stage veins. Arsenic and other components of the ardennite and associated minerals are derived from highly oxidized ferromanganese oxide- and hydroxide-bearing siliceous pelagic sediments that formed the protolith for the piemontite schist. The veins formed at a relatively early stage after metamorphism peaked in the chlorite zone of the greenschist facies under conditions that have been estimated at about 4.5 kbar, 390 °C.ZusammenfassungArdennit mit der Zusammensetzung (Mn2+3.488Ca0.509Ba0.002)Σ=4(Mg0.916Fe3+0.165Mn3+0.099Cu0.033Ni0.009Zn0.006 Ti0.008Al4.764)Σ=6(As5+0.823V5+0.022P0.005B0.069Al0.042Si5.039)Σ=6O21.81(OH)6.17 tritt in Crack-seal-Quarzgängen in Quarz-Albit-Piemontit-Spessartin-Phengit-Hämatit-Chlorit-Rutil-Turmalin ± Calcit-Schiefern der Haast Schiefer-Gruppe nahe der Arrow Junction, West-Otago, Neuseeland, auf. Die Proportionen von Mn2+/Mn3+ hängen von der Kalkulation und der Genauigkeit der Analyse ab. Bor wird zum ersten Mal im Ardennit bestimmt. Andere Eigenschaften sind:α = 1.734(3),β = 1.735(3),γ = 1.751(3), 2Vz = 30(2)°; a = 8.721(1), b = 5.816(1), c = 18.545(3) Å, V = 940.7(2) Å3. Assoziierte Mineralphasen sind Spessartin, Hämatit, Piemontit, der ≈ 0.7% SrO und 0.06% PbO enthält und Phengit. Spät gebildete Gangmineralien, wie Chlorit, Albit und Mn-Calcit, sind unter geringer oxidierenden Bedingungen entstanden. Digenit mit etwas Covellin tritt in kleinen Mengen zusammen mit Mn-Calcit und Quartz in einem querschlägigen Gang auf, Chalcopyrit und gediegenes Kupfer kommen in anderen späten Gängen vor. Arsen und andere Komponenten des Ardennites and der assoziierten Minerale können von hochoxidierten, Fe-Mn-Oxid- und Hydroxyd-führenden, Sireichen, pelagischen Sedimenten hergeleitet werden, die das Ausgangsgestein für den Piemontit darstellen. Die Gänge wurden in einem relativ frühen Stadium, nach dem Metamorphosehöhepunkt, innerhalb der Chloritzone der Grünschiefer-Fazies, unter ungefähr 4.5 kbar und 390°C, gebildet.