Northwest Otago Research Articles

Basanite cobbles in Pleistocene sediments in Central Otago and their implications for intraplate volcanism and Clutha River paleo-drainage

ABSTRACT The occurrence of volcanic basanite cobbles in Pleistocene terraces at Galloway and in the upper Clutha valley in Central Otago, in an area devoid of known volcanic edifices, has implications for Cenozoic intraplate volcanism and Pleistocene drainage in the region. We present petrographic, mineralogical, whole rock geochemical and Sr-Nd-Pb isotopic data, as well as an 39Ar-40Ar date of 24.4 ± 0.06 Ma, together with mineral analyses of enclosed peridotite xenoliths. We conclude from this dataset that these alkaline basanite cobbles are most likely derived from the Alpine Dike Swarm in northwest Otago, probably in the Lake Wanaka-Hawea-Luggate area, which extends the volcanic field east of its known extent. The pilotaxitic textures and presence of intersertal glass, along with the volume of basanitic material, suggests that the source rocks were likely effusive, which would make this the first such known component of the Alpine Dike Swarm and requires emergent land at that time. The occurrence of the Galloway basanite cobbles implies that Pleistocene Clutha River flowed through or over what is now the Dunstan Range. The young (late Pleistocene) age of all the basanite-bearing gravels means that the basanite cobbles have likely been reworked several times before final deposition.

Geological controls on locally elevated arsenic in the Glenorchy area, Otago, New Zealand

ABSTRACT The Glenorchy area of northwest Otago consists of a low-relief Holocene fluvial delta complex below glaciated mountains of Otago Schist basement, and has locally elevated arsenic (As) from two geogenic pathways: Mesozoic hydrothermal processes and Holocene shallow groundwater transport. Soils on the delta complex are primitive and contain abundant sand and silt derived from unoxidised schist basement, with background As contents generally <30 mg/kg. Some soils in wetland areas on the delta have naturally elevated (geogenic) arsenic contents that are locally >300 mg/kg on the cm2 scale. Arsenic enrichment has occurred in the upper 30 cm where dissolved As and Fe from low-redox chemical conditions in shallow groundwater has become oxidised, thereby lowering their solubility. Both As and Fe are being mobilised from background sulfide minerals in schist-derived debris, and groundwater As concentrations up to 0.8 mg/L have been recorded. Historic mining of scheelite for tungsten on the surrounding mountains has exposed small volumes (typically tens to hundreds of m3) of arsenopyrite-bearing mine wastes with spot As contents from <500 to >10 000 mg/kg. Water discharging from mine adits have pH∼8 and dissolved As up to 0.3 mg/L, but these waters are rapidly diluted downstream to <0.02 mg/L.

Late Jurassic detrital zircons from the Haast Schist and their implications for New Zealand terrane assembly and metamorphism

The youngest detrital zircon age groups from three samples within the Haast Schist in northwest Otago are Late Jurassic (154, 155, 160 Ma), as determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of U–Pb isotopes in individual zircons. It is inferred that this is the maximum age of sedimentation for these samples, which is within the range of ages for Haast Schist metamorphism (145–180 Ma). This maximum sedimentation age is at least 50 Ma younger than the previously inferred depositional ages for the Caples and Rakaia terranes which are the protoliths of Haast Schist. The zircon age populations within the samples are also different from those found within the Rakaia and Caples terranes, implying different sedimentary sources and possibly a different terrane. The detrital zircon populations are comparable to those found within the Waipapa Terrane in the North Island.

Lead isotope constraints on the origin of Cenozoic orogenic gold systems in the Southern Alps and northwestern Otago, South Island, New Zealand

Lead isotopic compositions were determined for sulphides from Pliocene-Pleistocene gold-bearing veins in the Alpine Schist and from Miocene gold-bearing veins and vein breccias from the Shotover-Macetown area in the northwest Otago Schist belt. The lead isotopic signatures are consistent with derivation of Pb in the vein minerals predominantly from metasedimentary rocks that underlie the region. Differences in Pb isotopic signatures between deposits are interpreted to result from lateral and vertical lithological variability within the source rock mass. The host rocks also contain metabasic rocks with N-MORB, E-MORB or within-plate basalt chemistry. However, the observed Pb isotopic signatures in the gold-bearing veins preclude incorporation of significant amounts of Pb from the metabasites. The Pb isotopic signatures of lamprophyre dikes that were intruded into the Otago Schist coeval with Miocene gold mineralisation are distinctly more radiogenic than those of the hydrothermal veins. Thus, although the lamprophyre dikes were emplaced into similar extensional structural sites to the gold-bearing veins, there was no genetic relationship between lamprophyre dikes and gold mineralisation.

The Siberia Fault Zone, northwest Otago, and kinematics of mid‐Cenozoic plate boundary deformation in southern New Zealand

The Siberia Fault Zone (SFZ) in northwest Otago, New Zealand, represents a northern extension of the Moonlight Tectonic Zone. The 40 km long fault strikes northeast subparallel to, and 25 km southeast of, the Alpine Fault and dips steeply northwest. Kinematic data indicate an overall dextral strike‐slip sense of movement, with an estimated 3 ± 2 km of displacement. On the southwest section of the fault, strike‐slip movement is largely transformed into dip‐slip, and actual fault displacement is superimposed on uplift in the core of the Shotover Antiform. This results in a total apparent throw 7000 m. Near Haast Pass, the SFZ steps right c. 3.5 km through a complex zone of northeast‐ and east‐striking faults to connect with the Haast River Fault. Fault rocks from the SFZ are mainly foliated cataclasites. Syn‐kinematic temperature‐depth conditions were in the range 200–250°C and 8–10 km. Timing of fault activity is constrained, by mutually crosscutting relationships with dikes of the Alpine dike swarm, to late Oligocene and early Miocene. The kinematics of the SFZ and other structures in the region are consistent with distributed strain in a regime of dextral transcurrent shear. Previous models for Cenozoic deformation in southwest South Island are refined by combining improved plate tectonic data with reconstructed paleogeometry of Cenozoic structures before regional bending and rotation.

The 'spirit of the hills': Mountaineering in northwest Otago,New Zealand, 1882-1940

A defined geographical region and time period is used to examine the growing popularity of mountaineering in relation to its social and cultural context. The study draws on oral histories, diaries, autobiographies, articles and archival material from mountaineers, and blends them with insights from geography, sociology, art history, literary criticism and cultural history. The findings of the study demonstrate that early mountaineering in New Zealand reflected the legacy of Victorian values in relation to the landscape, in combination with a 'pioneering' spirit and a growing sense of a distinctive colonial character. The First World War marked a watershed, followed by an inter-war 'boom' in mountaineering. This growing enthusiasm can be interpreted as a reaction against the dislocation and devastation of the war and the growing rationalization of modern society. It was also part of the development of a sense of belonging to the landscape, and an exploration of what it meant to be a New Zealander. The study concludes that mountaineering is expressive of a relationship between people and landscapes and that, within this context, its historical development reflects wider social and cultural forces.

Biodiversity in the New Zealand divaricating tree daisies: Olearia sect. nov. (Compositae)

The trees and shrubs in Olearia sect. Divaricaster, sect. nov., are found in North, South, and Stewart Islands of New Zealand. They are distinguished by their small, opposite leaves borne on brachyblasts (short shoots), long shoots which abort apically, solitary or fascicled capitula, flowers with purple style arms, and a very distinctive insect fauna. The ten species and three subspecies, including the new O. fimbriata, O. gardneri and O. virgata subsp. centralis are revised and a key and distribution maps provided. The section includes four threatened species: O. hectorii, O. polita and the two new species. Species density is mapped in grid cells 1∞ latitude by 1∞ longitude. Areas west of the geological Median Tectonic Zone (Nelson, Westland and Fiordland) show relatively low diversity. The highest diversity, six species in one grid cell, is recorded in north-west Otago, around Mt Aspiring and the Humboldt Mountains. This region is also geologically significant as several allochthonous terranes are juxtaposed there. In the central South Island O. laxiflora, O. fimbriata and O. odorata have concentric distributions around what were Tertiary basins and inland seas. Members of the section favour fertile, lowland sites on alluvium and have suVered reduction in population size over the last century due to agricultural development. Several species, such as O. hectorii, the ‘swamp gum’, occur in sites which are periodically flooded. Olearia solandri is a coastal species, often growing as a mangrove associate, but occasionally found inland. The anomalous inland populations are discussed and their location attributed to geological uplift.

Biodiversity in the New Zealand divaricating tree daisies:Oleariasect. nov. (Compositae)

The trees and shrubs inOleariasect.Divaricaster, sect. nov., are found in North, South, and Stewart Islands of New Zealand. They are distinguished by their small, opposite leaves borne on brachyblasts (short shoots), long shoots which abort apically, solitary or fascicled capitula, flowers with purple style arms, and a very distinctive insect fauna. The ten species and three subspecies, including the newO. fimbriata,O. gardneriandO. virgatasubsp.centralisare revised and a key and distribution maps provided. The section includes four threatened speciesO. hectorii,O. politaand the two new species. Species density is mapped in grid cells 1° latitude by 1° longitude. Areas west of the geological Median Tectonic Zone (Nelson, Westland and Fiordland) show relatively low diversity. The highest diversity, six species in one grid cell, is recorded in north-west Otago, around Mt Aspiring and the Humboldt Mountains. This region is also geologically significant as several allochthonous terranes are juxtaposed there. In the central South IslandO. laxiflora,O. fimbriataandO. odoratahave concentric distributions around what were Tertiary basins and inland seas. Members of the section favour fertile, lowland sites on alluvium and have suffered reduction in population size over the last century due to agricultural development. Several species, such asO. hectorii, the «swamp gum», occur in sites which are periodically flooded.Olearia solandriis a coastal species, often growing as a mangrove associate, but occasionally found inland. The anomalous inland populations are discussed and their location attributed to geological uplift.

Composition and zoning of garnet and plagioclase in Haast Schist, northwest Otago, New Zealand: Implications for progressive regional metamorphism

Abstract Two garnet isograds are mapped in quartzofeldspathic Haast Schist, northwest Otago. The first garnet isograd is based on the incoming of Ca‐Fe‐Mn garnet coexisting with albite (An0–1), and is employed to delimit an Otago Schist style garnet‐albite‐biotite zone. The second garnet isograd is based on the incoming of almandine‐rich garnet, occurring as rims on Ca‐Fe‐Mn garnet cores, and on the incoming of oligoclase (An23–26) co‐existing with albite (An0–2). The garnet‐oligoclase zone which this second garnet isograd delimits is equivalent to the Alpine Schist garnet zone at Haast River, and is interpreted as a metamorphic overprint on pre‐existing Otago Schist mineral zones. Compositional zoning in Ca‐Fe‐Mn garnet is dominated by substitution of Ca for Mn, whereas zoning in almandine‐rich garnet from the garnet‐oligoclase zone is dominated by substitution of Fe for Ca. Recognition of these substitution trends is an important aid in determining the petrogenetic affiliation of the minerals. Ca‐Mn substitution and rim compositions for garnet in phyllosilicate‐rich schists from the Aspiring Lithological Association indicate an affinity with Ca‐Fe‐Mn garnet in quartzofeldspathic Otago Schist. However, the comparatively Fe‐rich nature of the cores of garnet in Aspiring schist, and the consequent zoning path, are probably influenced by bulk‐rock composition. Two implications of a metamorphic overprint are that: (1) it supports earlier contentions that Otago and Alpine Schists may have experienced distinctly different P‐T maxima; (2) pending further work on the significance of late, porphyroblastic biotite in these rocks, it should be feasible to map a petrological boundary between Otago Schist and Alpine Schist which compliments the traditional, less precisely defined, distinction between Otago and Alpine Schists based on geographical distribution.

Nephrite and metagabbro in the Haast Schist at Muddy Creek, northwest Otago, New Zealand

Abstract At Muddy Creek, northwest Otago, a pod of gabbro, probably in a matrix of serpentinite, was tectonically emplaced into quartzofeldspathic sediments before the main metamorphism of the Haast Schist. Metasomatic diffusion during pumpellyite actinolite to greenschist facies metamorphism produced talc‐, tremolite‐, chlorite‐, and muscovite‐rich reaction zones at the margin of the pod. Subsequent shearing disrupted the metasomatic sequence and produced nephrite from the tremolite zone precursor. Relict magmatic hornblende and clinopyroxene from the metagabbro have compositions appropriate to a volcanic arc tectonic setting. In contrast, relict pyroxenes from four metabasaltic knockers in the Greenstone Melange have compositions indicating generation of magmas in both ocean‐floor and ocean‐island environments. Ultramafic and mafic rocks at Muddy Creek and in the Greenstone Melange are thought to represent fragments of the ophiolitic basement to the Haast Schist, the tectonic settings of which help cons...

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...

An Early Permian fauna from the Mantle Volcanics Formation, Skippers Range, northwest Otago

An Early Permian fauna from the Mantle Volcanics Formation in the Skippers Range is described and correlated with others in New Zealand. The fauna confirms the Permian age of the formation and includes the first New Zealand record of the Gondwana bivalve family Eurydesmidae. This and other bivalves, gastropods, brachiopods, bryozoans, rugosans, and echinoderms are described. The Mantle Volcanics Formation is composed entirely of proximal vent facies pyroclastic and epiclastic sediments extensively intruded by tholeiitic dikes and sills which probably fed the vent(s).

Shallow-level, late-stage gold mineralisation in Sawyers Creek, Shotover valley, northwest Otago, New Zealand

Abstract A set of mainly west and northwest striking normal faults in Sawyers Creek, Shotover valley, crosscuts post-Oligocene Moonlight Generation structures. These faults have provided access for hydrothermal fluids to shallow crustal levels. Silicification and carbonate alteration of breccias and wall rock has occurred, to form reef zones up to 3 m wide. Gold is found with pyrite and arscnopyrite within the siliceous portions of the reef, especially at fault intersections. The hydrothermal fluid was water with minor dissolved CO2 Mineralisation occurred at about 150-200°C at 2–3 km depth. Wall-rock interaction with the fluid caused replacement of chlorite by ankeritic carbonate. The composition of the ankerite is governed by the composition of the chlorite it replaced. Fluid reaction with chlorite caused the fluid to become progressively more alkaline which eventually resulted in precipitation of gold. Similar fluid activity, in similar structural settings, is found throughout northwest Otago.

Structure of schist in the Mt Aspiring region, northwestern Otago, New Zealand

Abstract Five phases of deformation are recognised in the schists of the Mt Aspiring area, northwest Otago, New Zealand.The first two phases resulted in isocIinal ductile folding and macroscopic nappe formation during greenschist facies metamorphism. The third phase, which occurred during waning metamorphic conditions, resulted in tight folding, with east-west overthrusting of macroSCOPIC nappes.Ouring this third phase, earlyformed fold axes in high-strain zones were rotated towards the stretching or translation direction. The fourth deformation phase is characterised by chevron folds (on all scales) with north-trending axes and associated west-dipping thrust faults.The Moonlight Fault, a major regional feature.is associated with this phase of deformation. The fifth phase of deformation involved mesoscopic conjugate sets of crenulations with east- and southeasttrending axes. Steeply dipping, fold axial surface joints developed in these crenulations are commonly intruded by lamprophyric dikes or associated...

Celmisia spedeniiG. Simpson, an ultramafic endemic, andCelmisia markii, sp. nov., from southern New Zealand

Abstract The taxonomic status of Celmisia spedenii G. Simpson is examined. Plants from the type locality at West Dome in Southland are morphologically and ecologically distinct from plants in Fiordland and north-west Otago and are here recognised as two species, contra Given (1975). Celmisia spedenii G. Simpson occurs only on ultramafic soils in northern Southland, and is currently the sole taxon endemic to ultramafic outcrops in the southern half of the South Island. Celmisia markii is described as a new species. It is confined to areas above treeline in western Fiordland and northwest Otago, between Haast River and Caswell Sound.

Lithologic variations in Otago Schist, Mt Aspiring area, northwest Otago, New Zealand

Abstract Psammitic schist, 2 types of pelitic schist (grey and porphyroblastic), and 4 types of greenschist (metavolcanic rock — light, spotted, foliated, and epidote rich) are recognisable on outcrop scale in textural zone 4 of the Otago Schist, northwest Otago, New Zealand. Thin horizons of metachert, marble, and ultramafic rock are commonly associated with greenschist. Broad units with 1 predominant rock type (greenschist, psammite, grey pelite, or porphyroblastic pelite) are mappable on a regional scale. These units also contain most or all of the above rock types and have poorly defined boundaries. The degree of original Stratigraphic continuity within and between these broad units is unknown. The studied area can be subdivided into 2 lithologic associations, the “Aspiring association” and the eastern “Wanaka association”, separated by a north-trending, poorly defined but lithologically gradational boundary. The Aspiring association is made up predominantly of pelitic rock types with considerable qua...

Soil changes associated with development of vegetation on an ultramafic scree, northwest Otago, New Zealand

Abstract Chemical and physical properties of soil beneath four plant communities, ranging from pioneer phase through to low forest, on an ultramafic scree are described. The soils are weakly developed and reflect the chemistry of the parent material in having very high amounts of magnesium and nickel and low concentrations of the other macronutrients. Floristic diversity and vegetation stature increase with improving soil nutrient status and declining heavy metal availability, particularly in organic horizons.

Lithostratigraphy and structure of the Caples terrane of the Humboldt Mountains, New Zealand

Abstract Schistose lithostratigraphic units, correlated with non-schistose rocks of the Caples terrane, can be mapped successfully along the western margin of the Haast Schist terrane in the Humboldt Mountains of north-west Otago. The rocks include chert, volcanogenic sediment and lava of the Harris Saddle Formation; the mineralogically immature Momus Sandstone; the new Cosmos Formation, a thin but persistent horizon of green volcanogenic sandstone; and the overlying pelitic and psammitic rocks of the Upper Peak unit, the Mystery Pelite (new), and the Bold Peak unit. They have been reconstituted under metamorphic conditions of the pumpellyiteactinolite and greenschist facies. Three pe1iods of deformation are recognised. The first, synmetamorphic phase resulted in a maior recumbent fold, with an axial surface parallel to the regional schistosity; the second, post-metamorphic phase is defined by folded schistosity and deformed isotects. Major structures of both fold episodes have been verified by direct obse1vation. Penecontemporaneous faulting was an important adjunct to post-metamorphic folding. In a third deformational episode the Greenstone Mélange, a chaotic rock unit containing a variety of igneous, metamorphic, and sedimentary rocks, was emplaced, possibly diapirically, along a narrow extensional zone in the east. Rocks of the Caples terrane grade northwards and eastwards into the Haast Schist terrane which in turn grades eastwards into Torlesse rocks. Caples and Torlesse rocks are petrographically and lithologically distinct; the Haast Schist thus appears to represent a metamorphosed amalgamation of these two rock suites.

Paleocurrents and composition of lower Bryneira strata (Permian) at Barrington Peak, north-west Otago

Abstract Cross-stratification in the Annear Formation (lower Bryneira Group) in northwestern Otago indicates sediment transport from west (c. 250°) towards east (c. 70°). Petrographic and chemical data indicate that these rocks are of mixed quartzo-feldspathic-volcanic provenance and are decidedly more silicic than overlying and underlying Bryneira volcanogenic terrigenous sediments. Compositional and stratigraphic similarities with Permian sediments of the Torlesse terrane are noted, and a common source area is tentatively suggested.

Alabaster group rocks (Lower Permian) in southern skippers range north-west Otago, New Zealand

Abstract A steeply dipping, fault-bounded sequence of Lower Permian metavolcanic and metasedimentary rocks of the Alabaster Group, with associated intrusives, has been mapped in the southern Skippers Range. The Eglinton Sub-Group, for which three new formations are proposed, consists mainly of porphyritic metabasic lavas, volcanic breccias, and metabasic (and metaandesitic) dykes of the Mantle Volcanics. Metabasalt and finely bedded, locally hornfelsed, sediments of the Hidden Flats Formation are intruded by quartz diorite and granodiorite bodies of the Mackay Intrusives. The Camp Conglomerate overlies the Mackay Intrusives in the south-western part of the present area. Much of the Eglin ton Sub-Group has been metamorphosed to prehnite-pumpellyite facies; some areas are transitional to the pumpellyite-actinolite facies. To the west, and separated from the Eglinton Sub-Group by the Wilmot Fault, is the Skippers Sub-Group, here formally upgraded from formational status. The Skippers Sub-Group comprises thre...