Otago Schist Belt Research Articles

Detrital gold morphology and recycling around the early Miocene St Bathans paleovalley, Central Otago, New Zealand

ABSTRACT The St Bathans paleovalley developed as a major southwest-draining fluvial system during early Miocene rejuvenation of the Central Otago landscape. Rounded quartz pebbles and detrital gold were recycled from Eocene quartz pebble conglomerates into the headwaters of this Miocene paleodrainage system on the northern margin of the Otago Schist belt. Gold particle morphology is mostly irregular and rough, locally with relict gold crystal shapes, plus a minor component of well-defined flakes. Incremental rounding and surface deformation at the 10 µm scale has occurred through several recycling events in Central Otago, hindering linkage of Miocene and younger placer gold to ultimate basement sources. More intense deformation of the gold, forming thin flakes, occurred in the lower reaches of the Miocene paleodrainage system as gold was transported >100 km to eastern Southland. Hence, the St Bathans paleovalley has been an important source of gold for historic mining in Central Otago and Southland.

Distinct scheelite REE geochemistry and 87Sr/86Sr isotopes in proximally- and distally-sourced metamorphogenic hydrothermal systems, Otago Schist, New Zealand

The Otago Schist in New Zealand hosts orogenic tungsten (W) mineralization in two types. Proximally-sourced (“proximal”) mineralization in sub- to lower-greenschist facies Mesozoic metasedimentary rocks within the northern flank of the Otago Schist Belt comprise quartz-calcite veins with minor scheelite and few sulfides. Distally-sourced (“distal”) deposits are hosted in major crustal discontinuities within Mesozoic sub-greenschist to upper-greenschist facies metasedimentary rocks and are associated with abundant sulfides ± Au. In-situ trace element (particularly REE, Sr, Mo, Na, As) and in-situ 87Sr/86Sr compositions of scheelite show the scheelite occurring in these different mineralization types to be geochemically distinct. Proximal occurrences are characterised by scheelite with both heterogenous trace element and Sr isotope compositions displaying variations that can be linked to different veins, host rock types and scheelite generations. In contrast, scheelite within distal scheelite deposits tends to have fairly homogenous trace element and Sr isotopic ratios at the deposit to grain scale. The heterogenous compositions of proximal occurrences represent local derivation of diverse components from the sub-millimetre to meter scales and small extents of fluid flow, which resulted in a high sensitivity to sources and local rock compositions and thus limited large-scale equilibration of elemental and isotopic systematics within the scheelite they contain. The larger distal deposits formed by regional leaching by metamorphic fluids, probably at the greenschist-amphibolite facies boundary, followed by homogenisation as they ascended through the crust and leading to deposition of scheelite with rather uniform chemical characteristics. Our data re-enforce the model that tungsten mobility in the Otago Schist involved local-scale mobilisation of W in the shallow crust by breakdown of detrital rutile and scheelite at low metamorphic grade. Burial of these veined scheelite-bearing rocks to temperatures of ∼500 °C enabled remobilisation of W by metamorphic fluids, concurrent with scavenging of Au, As and S, followed by return of the metal-bearing fluids towards mid to upper crustal levels, where they precipitated scheelite.

Mineral prospecting from biogeochemical and geological information using hyperspectral remote sensing - Feasibility and challenges

Biogeochemistry is an effective and fast approach for orientation studies in mineral exploration. Vegetative cover has been used in a range of geological environments to gain insight into the composition of sub-surface soils and bedrock found at depth; however, there is little to no records of biogeochemistry being coupled within a remote sensing and spectroscopy framework in the domain of mineral exploration. The study uses lab- and airborne hyperspectral remote sensing to localize gold mineralisation using the concept of biogeochemistry. This pilot study uses plant species, Pinus radiata in close proximity to a mineralised shear zone. This study used a 2 m spatial resolution airborne hyperspectral data, captured by AisaFENIX sensor and was complimented with lab-based hyperspectral data of the sampled vegetation and soil/rock exposed along the Hyde-Macraes Shear Zone, within the Otago schist belt in the South Island, New Zealand. The hyperspectral data, and the biogeochemistry data was explored using several statistical approaches to determine significant relationships between the underlying soil and rock substrate and biogeochemistry of the sampled vegetation with a focus on pathfinder elements to gold-mineralisation. Three commonly used multivariate predictive regression models, were tested, including partial least squares regression (PLSR), kernel based PLSR (kPLSR) and random forest regression (RFR) modelling. The results suggest that hyperspectral scanning can discriminate samples from a mineralised zone spectrally. Furthermore, the lab-based hyperspectral data outcompetes the airborne hyperspectral data for vegetation to identify areas of potential mineralisation. The model performances vary depending on the part of the plant analysed, and bark samples are found to be promising to explore underlying geological resources in the future. The regression model that show up to R2cv > 0.7 using bark samples decreases to R2cv ~ 0.2 when trained with needle samples. The airborne data is significantly more complex, which is observed in poorer model performances with R2cv ranging from 0.2 to 0.5.

Fluid volatile composition associated with orogenic gold mineralization, Otago Schist, New Zealand: Implications of H2 and C2H6 for fluid evolution and gold source

We provide the first large-scale investigation of gold mineralizing fluids from 19 gold deposits distributed in the Mesozoic Otago Schist belt endowed with approximately 580 tonnes of gold (past production and resources). Fluid inclusions from gold-bearing veins were petrographically studied and analysed by solid-probe mass spectrometry for H2O, CO2, N2, C2H6, He, Ar, CH4, SO2, H2S, and H2. Fluid inclusion assemblages are quite similar for all studied gold deposits, a particular feature for orogenic gold districts. Gold-bearing fluids are dominantly aqueous (H2O > 82.13 mol %) with very low contents of CO2 (<12.93 mol) in comparison with other orogenic gold deposits worldwide, with H2 (up to ~8 mol %) and C2H6 (up to ~0.5 mol %). Fluid composition is exceptionally homogeneous at the belt-scale. It is proposed that the relatively low content of CO2 and high content of H2 is related to chemical reactions in the fluids inducing graphite precipitation in the host rocks and mineralized structures and consumption of CO2 and CH4. The unique homogenous composition of fluids through time and space for the Otago Schist is related to the combination of: 1) the relatively homogeneous sedimentary rock source for fluid generation by metamorphic dehydration; 2) large-scale chemical reactions for consuming carbon-bearing volatiles; and 3) lack of external source (meteoric or magmatic) for fluid mixing. The presence of C2H6 is interpreted to be an indication that gold was sourced from primary gold-bearing pyrite hosted in carbon-rich turbidites, which is also the source for fluid generation and its hydrocarbon components.

Early Jurassic felsic and associated mafic meta-igneous rocks in Otago Schist, Central Otago, New Zealand

ABSTRACTMetamorphosed felsic volcanic rocks occur interlayered with metasedimentary schist in close association with a 7 km long metabasaltic (greenschist) horizon in the Otago Schist belt of Central Otago. The felsic metavolcanic rocks are weakly foliated and locally segregated and consist principally of quartz and albite. Geochemical analyses suggest phonolitic compositions and a general within-plate affinity for the felsic metavolcanics rocks, and a MORB/back arc basalt affinity for the associated metabasalts. Zircons from two felsic metavolcanic samples are stubby, sharply faceted, prismatic grains with no visible inherited cores, and are considered to be of primary magmatic origin. Calculated weighted average 206Pb–238U ages for zircons from the two felsic metavolcanic samples are 190.5 ± 0.6 Ma and 190.1 ± 0.3 Ma. We interpret these ages (late Aratauran–early Ururoan; middle Early Jurassic) to date the deposition of the Torlesse Terrane metasediments at that locality, and also possibly the emplacement of the associated metabasalts.

Evolution of the Taieri River catchment, East Otago, New Zealand

ABSTRACTThis paper synthesises geological and biological data to develop an evolutionary history for the Taieri River that currently follows a circuitous 200 km course as one of the main drainages in Otago. The ancestral Taieri River drained only coastal hills initiated in the Miocene, and much of what is now the upper Taieri catchment flowed into the ancestral Clutha River. Major river reorientation events occurred in the upper half of the catchment because of rise of antiformal fold mountains in the Pleistocene, forming a new divide between the Taieri and Clutha catchments. Coeval incision of a gorge through a volcanic rock barrier connected the upper catchment to the lower Taieri River. The sparse Pleistocene sedimentary record documents these drainage changes via contrasting distribution of distinctive clasts derived from greywacke mountains on the northern edge of the Otago Schist belt. These major capture events are also supported by distributions and genetic divergences of freshwater galaxiid fish species. Erosion during Pleistocene rise of the antiformal mountains caused recycling of placer gold into Clutha tributaries before the Taieri River evolved to its present geometry, thereby limiting the placer gold content of the modern Taieri catchment.

Mineralogy and provenance of auriferous Waimumu Quartz Gravels, Southland, New Zealand

Waimumu Quartz Gravels, 8 km southwest of Gore, New Zealand, were deposited in Pliocene river channels, and were derived by recycling of older sediments on nearby uplifting mountain ranges. The high proportion of quartz pebbles (>90%) is a result of the decomposition of clay-altered lithic clasts during sedimentary recycling. Most of the quartz was derived from the Otago Schist Belt to the north, including upper greenschist facies metachert clasts with spessartine and piemontite. The quartz gravels at Waimumu Stream mine contain abundant Mn-rich garnet clasts that were derived from Otago Schist. In addition, chromite clasts constitute >10% of the heavy mineral sand fraction, and this may have been derived from the Dun Mountain Ophiolite Belt although the exact source is unknown. Most detrital gold occurs as flakes that have undergone long-distance transport. Rare grains of crystalline gold occur, some of which are in association with prismatic quartz crystals. Gold flakes have minor evidence for authigenic gold overgrowths in association with authigenic marcasite, and gold mobility may have been enhanced by marcasite precipitation and/or oxidation.

Contrasting geochemistry of orogenic gold deposits in Yukon, Canada and Otago, New Zealand

The Yukon-Tanana Terrane (YTT) of western Yukon Territory in NW Canada and Otago Schist belt (OSB) of South Island, New Zealand share similar geological evolutionary histories as convergent orogenic belts. Both belts host orogenic gold deposits of mainly Jurassic to Early Cretaceous age. Jurassic mineralization in the YTT occurred during convergent orogenesis and stacking of previously-metamorphosed (Palaeozoic) greenschist-amphibolite facies metasediments, metavolcanic rocks, and metagranitoids. Early Cretaceous OSB mineralization occurred in the latter stages of terrane accretion of un-metamorphosed turbidites with minor basaltic rocks. Metamorphism of the OSB turbidites mobilised background levels of Au (0.6–1.3 ppb), As (2–20 ppm), Sb (0.1–1 ppm), and W (&lt; 10 ppm), primarily under greenschist to lower amphibolite facies conditions when diagenetic pyrite (Au c . 0.5–2 ppm; As c . 500–10 000 ppm) transformed to pyrrhotite on a regional scale. In contrast, the previously-metamorphosed YTT rocks had generally low background As contents (1–2 ppm) apart from some As-rich quartzites (up to 100 ppm As). Consequently, there was less As available for orogenic mobilisation, and YTT Au deposits generally have lower concentrations of this pathfinder element compared to the OSB. YTT host rocks, especially metagranitoids, have anomalous levels of Mo (10–300 ppm), and many orogenic deposits contain elevated Mo, locally including molybdenite. OSB turbidites have elevated Mo (2–200 ppm), along with elevated Au and As, in diagenetic pyrite, but this Mo became largely dispersed through the metamorphic pile as metamorphic grade increased and pyrite transformed to pyrrhotite. OSB orogenic deposits have only marginally elevated Mo ( c . 1 ppm), no molybdenite, and accessory scheelite in these deposits is distinctly Mo-poor. Only minor mobilisation of base metals occurred in these orogenic belts, and orogenic Au deposits contain sparse base metal sulphides. Orogenic deposits in the YTT and OSB differ in that Au (and other associated elements) in many of the orogenic deposits in the YTT was remobilised from relatively local sources (e.g. pre-existing Cu-Mo-Au porphyry or volcanogenic sulphide mineralization) whereas Au in the OSB was mobilised from larger volumes of homogeneous rock at depth.

Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

The Macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides. The host rock resembles zones of unmineralised micaceous graphitic pyritic schists, derived from argillaceous protoliths, that occur locally in background pelitic Otago Schist metasediments. This study was aimed at determining the relationship between these similar rock types, and whether the relationship had implications for ore formation. Argillites in the protolith turbidites of the Otago Schist metamorphic belt contain minor amounts of detrital organic matter ( 50 million years of evolution of the metamorphic belt, from development of sheared argillite in the Jurassic, to postmetamorphic ductile extension in the Cretaceous. Introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken-sided shears, with some veins having >50% noncarbonate carbon. Graphitic foliation seams in low-grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals. Graphite reflectance is anisotropic and ranges from ~1% to ~8% (green light). Texturally similar porphyroblastic pyrite has grown in micaceous schist (up to 10 wt.% S), metamorphic quartz veins and associated muscovite-rich shears. These pyritic schists are weakly enriched in arsenic (up to 60 ppm). The low-grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the Otago Schist belt for later (more restricted) gold mineralisation. Economic amounts of gold, and associated arsenic, were subsequently introduced to carbonaceous sulphidic schists in the Macraes gold deposit by a separate metamorphic fluid derived from high-grade metaturbidites. Fluid flow at all stages in these processes occurred at metamorphic rates (mm/year), and fluids were broadly in equilibrium with the rocks through which they were passing.

Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

The Blue Lake Fault Zone occurs at a major crustal boundary between Otago Schist and Torlesse greywacke. This zone has been active since the middle Cretaceous (c. 112 Ma), when extensional exhumation of the Otago Schist belt was initiated. The Blue Lake Fault Zone is dominated by a set of normal faults that have caused juxtaposition of rocks of different metamorphic grade, from prehnite-pumpellyite facies turbidites to pervasively recrystallised greenschist facies schists. This metamorphic transition has been thinned from c. 15 km to c. 2 km by normal fault motion on the scale of kilometres. This condensed section is the narrowest such section on the Otago Schist margin. The faults in this condensed section are defined by gouge zones that are hundreds of metres wide and constitute about 30% of the section. Gouge zones were locally reactivated as thin Late Cenozoic reverse faults on the centimetre to metre scale.

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.

Age and Origin of Orogenic Gold Mineralization in the Otago Schist Belt, South Island, New Zealand: Constraints from Lead Isotope and 40Ar/39Ar Dating Studies

The Otago Schist belt of southern New Zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after Mesozoic metamorphism. Previous 40Ar/39Ar dating studies in the Otago Schist belt indicate formation ages of ~106 to 101 Ma for late, postmetamorphic, gold-bearing quartz veins and shear zones. Samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40Ar/39Ar method. Muscovite from a gold-bearing quartz stockwork vein at the Macraes mine, which is a relatively old mineralized system that formed during the latter stages of metamorphism, yields a well-defined plateau age of 135.1 ± 0.7 Ma. Fine-grained muscovite from a quartz shear vein at Macraes gives a rising age spectrum with a maximum age of 135.7 Ma. Five 40Ar/39Ar dates from the scheelite-bearing vein swarm in the Glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 Ma, and that relatively slow cooling and/or minor thermal overprints have disturbed the age spectra of some of the samples. Lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the Otago Schist belt are relatively radiogenic, consistent with an entirely upper crustal source for the Pb (and presumably Au). The Pb data define a less radiogenic and a more radiogenic cluster, with no overlap. Lead isotope compositions from the sulfides from the Macraes group of deposits form part of a less radiogenic cluster, together with Pb isotope compositions for sulfides from scheelite-rich veins in the Glenorchy camp. Lead isotope compositions for sulfides from younger, postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster. The new 40Ar/39Ar dating results and Pb isotope studies, in conjunction with previously published work, demonstrate that most orogenic gold mineralization within the Otago Schist belt formed during two discrete mineralizing events—one at 142 to 135 Ma and a later one at approximately 106 to 101 Ma. We interpret the two pulses of gold mineralization to reflect sequential extraction of ore-forming components from the deeper part of the Otago Schist belt during short-lived thermal events. The younger event was probably associated with subduction of an actively spreading ridge, and similar ridge subduction may have been involved in the older event.

Stratigraphy, provenance, and diagenesis of the Cretaceous Horse Range Formation, east Otago, New Zealand

The Horse Range Formation is a structurally controlled late Early Cretaceous to early Late Cretaceous nonmarine unit in east Otago, South Island, New Zealand, containing immature lithic debris. Clasts are generally rounded, with only minor subangular material. The formation contains clasts derived from two principal basement sources: schist and greywacke. Schist debris is most abundant at the base of the described section, and this material is dominated (>60%) by quartz from the greenschist facies core of the Otago Schist belt. Conglomerates with >70% greywacke clasts constitute most of the upper part of the Horse Range Formation. These greywacke conglomerates have a matrix of sand derived mainly from schist. A 60 m thick wedge of quartz‐rich, locally carbonaceous sand occurs interlayered with greywacke conglomerates. The Horse Range Formation rests on sub‐greenschist facies semischist, which forms only a small proportion (<10%) of conglomerate clasts, and most sediment underwent considerable transport (tens of kilometres). Paleocurrent indicators suggest that sediment was derived mainly from the northwest, and drainage was probably controlled by the ongoing development of the northwest‐striking Waihemo Fault System, which had normal sense of motion at that time. The formation is developed from the remnants of a braided axial river flowing southwest from active uplift of greywacke ridges along the Waihemo Fault System at least 50 km inland. Some lateral input occurred from the adjacent schist belt that was being exhumed by the extensional tectonics. Post‐depositional diagenesis of the sediments has resulted in widespread replacement of silicate minerals, particularly albite, by calcite that forms up to 40% of the rock locally. The calcite has ä18OSMOW near +24‰ and 813CPDB near ‐2‰, and was partly dissolved and redeposited from the immature basement debris (metamorphic calcite) and partly introduced from overlying Late Cretaceous and Teriary marine sediments by groundwater.

Structural and lithological controls on gold mineralisation at Oturehua on the northeastern margin of the Otago Schist, New Zealand

A set of more than 50 mineralised normal faults cuts lower greenschist facies rocks at the northeastern margin of the Otago Schist belt near Oturehua, central Otago. The faults strike northwest and dip steeply (>60°) northeast, and individual mineralised zones are traceable for up to 700 m along strike. The mineralised faults are closely associated with, and are generally parallel to, a swarm of northwest striking, steeply dipping extensional quartz ± ankerite veins (centimetre scale) that occur over an area of at least 25 km2in the host schist. Mineralised faults contain 10–100 cm thick quartz veins that are traceable as interlinked structures for tens of metres along strike. Where faults cut massive quartz‐feldspar‐rich schist, the veins are planar, contain scattered sulfide minerals and visible gold, and there has been little alteration of essentially undeformed adjacent wall rock (centimetre scale). In contrast, muscovite‐rich laminated schist host rock is extensively deformed and altered adjacent to mineralised faults (metre scale), with addition of quartz (silicification) and ankeritic replacement of metamorphic chlorite by kaolinite and/or sericitic muscovite. Veins adjacent to laminated (micaceous) schist contain abundant breccias with variably altered clasts. Widespread addition of auriferous sulfide minerals, mainly arsenopyrite, has occurred in the laminated schist alteration zones and breccias. The mineralised faults formed in the hinge of a northwest‐trending post‐metamorphic antiform, and the faults may have been localised by zones of weakness associated with kink folding in the hinge of the antiform. Hydrothermal fluids responsible for mineralisation were channelled by the normal faults. These faults developed by coalescence of extensional fractures in the schist rock mass during middle Cretaceous regional extensional unroofing of the Otago Schist belt. The location of these mineralised faults near the margin of the Otago Schist belt suggests a genetic relationship between mineralisation and the regional northwest‐striking normal fault systems that cut and define the schist margin.

Evolution of Cretaceous–Cenozoic quartz pebble conglomerate gold placers during basin formation and inversion, southern New Zealand

Numerous auriferous fluvial quartz pebble conglomerates (QPCs) are present within the Late Cretaceous–Recent sedimentary sequence in southern New Zealand. The QPCs formed in low-relief settings before, during, and after regional marine transgression, in alluvial fan and a variety of fluvial and near-shore depositional settings: In particular, during slow thermal subsidence associated with Late Cretaceous–early Cenozoic rifting, and during the early stages of orogenic uplift following mid Cenozoic marine regression. QPC maturity characteristics are complex and vary with sediment transport and recycling history, stratigraphic proximity to the transgressive Waipounamu Erosion Surface, and the amount of first-cycle detritus incorporated during recycling. For pre-marine QPCs, the amount of first cycle detritus varies with tectonic intensity and proximity of the depositional setting to remnant Cretaceous topography. For post-marine QPCs, it varies with tectonic intensity and proximity to Late Cenozoic uplift of basement ranges. QPCs do not form during a single bedrock erosion–sediment deposition cycle: Non-oxidised and/or oxidized groundwater alteration (kaolinisation) of labile minerals in immature sediment and the upper part of underlying basement, and repeated sedimentary recycling, are fundamental processes of QPC formation regardless of the tectonic or sedimentary settings. Altered immature rock disaggregates easily upon erosion, and alteration clays are winnowed to leave quartz-rich residues containing resistant heavy minerals such as zircon and gold. Detrital sulfide survives recycling if deposition and burial in saturated sediments are rapid. QPCs result only if sediment recycling is not accompanied by excessive erosion of fresh basement rock. Uplift of many parts of the Otago Schist belt since late Miocene has raised rocks above the water table, increased erosion rates, and inhibited groundwater alteration and QPC formation. QPC formation is still occurring in Southland, where the water table is high, sediments are saturated and undergoing alteration, and uplift and erosion rates, topography, and fluvial gradients are all low. The QPCs accumulate as residual gravel on the valley floors of low-competence streams that are slowly incising pervasively altered dominantly late Miocene–Pliocene immature conglomerates. QPCs formation essentially represents physical and chemical lagging of precursor strata. Accumulation of detrital gold and other heavy minerals is an inevitable consequence, and most QPCs contain some gold. Three types of significant gold placer have developed in the QPCs. Type 1 placers are essentially eluvial and/or colluvial in origin and form without significant fluvial transport, by residual accumulation in low-competence valleys during low-rate uplift, fluvial incision and QPC formation. Type 2 placers have formed during significant fluvial transport and subsequent fluvial incision, mainly in higher energy proximal and medial reaches of larger pre-marine (Eocene) and post-marine fluvial systems. Type 3 placers formed by wave-base and marine current winnowing in the shallow shelf setting during low-rate regional marine transgression, especially in the Eocene.

Geological controls on refractory ore in an orogenic gold deposit, Macraes mine, New Zealand

The Macraes mine is hosted in an orogenic (mesothermal) gold deposit in metasedimentary rocks of the Otago Schist belt. Much gold occurs within altered schist with minimal silica-addition, and this study focuses on altered schist ore types. The unmineralized host schists are chemically and mineralogically uniform in composition, but include two end-member rock types: feldspathic schist and micaceous schist. Both rock types have undergone hydrothermal alteration along a shallow-dipping foliation-parallel shear zone, but their different rheological properties have affected the style of mineralisation. Micaceous schist has been extensively recrystallized and hydrothermally altered during ductile deformation, to form ores characterized by abundant, disseminated millimetre-scale pyrite cubes (typically 1–2 wt% S) and minor silicification. The earliest pyrite contained Ni and/or As in solid solution and no gold was imaged in these pyrites or later arsenopyrite grains. The ore type is refractory and gold recovery by cyanide leaching is less than 50%, with lowest recovery in rocks that have been less affected by later brittle deformation. In contrast, hydrothermally altered feldspathic schist is characterized by mineralised black microshears and veinlets formed during shear-zone related brittle deformation. Microsheared ore has relatively low sulphur content (<0.7 wt%) and muscovite has been illitised during hydrothermal alteration. Pyrite and arsenopyrite in microshears are fractured and deformed, and contain 1–10 μm blebs of gold. Later pyrite veinlets also contain micron- to submicron-scale inclusions of sphalerite, chalcopyrite, galena, and gold (≤10 microns). Gold in microsheared ore is more readily recoverable than in the refractory ore, although encapsulation of the fine gold grains inhibits cyanidation. Both microsheared ore and disseminated pyritic ore pass laterally into mineralised black shears, which contain hydrothermal graphite and late-stage cataclastic sulphides. This black, sheared ore releases gold readily, but the gold is then adsorbed on to gangue minerals (preg-robbed) and net cyanidation recovery can be less than 50%. Hence, low gold recovery during cyanidation results from (1) poor liberation of gold encapsulated in microcrystalline quartz and unfractured sulphide grains, and (2) preg-robbing of liberated gold during cyanidation. Introduction of pressure-oxidation of ore prior to cynidation has mitigated these issues.

Contrasting alteration mineralogy at an unconformity beneath auriferous terrestrial sediments, central Otago, New Zealand

A regional unconformity cut into the Otago Schist belt in southern New Zealand is overlain by auriferous terrestrial sediments. Formation waters from the overlying sediments have reacted with the schist basement, resulting in a thick (up to 20 m) zone of intensely kaolinitized basement immediately below the unconformity. Primary rock textures are preserved throughout this alteration zone. Two distinct types of alteration zone have developed. Non-oxidizing alteration resulted in kaolinitization of muscovite, but some albite and chlorite was preserved even in the most altered rocks. Chlorite locally altered to ferrous-iron-bearing smectite—vermiculite during non-oxidizing alteration, and this unusual mineral occurs in basement and overlying sediments. Oxidizing alteration has resulted in degradation of almost all schist minerals, leaving kaolinite and goethite, with relict muscovite and quartz. A distinctive 12 Å interlayered clay mineral occurs in altered schist and overlying sediment at one locality. Oxidizing alteration is responsible for localized gold mobility in basement gold deposits. Non-oxidizing fluids have mobilized detrital gold in auriferous terrestrial sediments. The formational waters were mobilized during late Cenozoic regional deformation of the unconformity and overlying sediments.

The relationship between the Hyde‐Macraes Shear Zone, deformation episodes, and gold mineralisation potential in eastern Otago, New Zealand

Abstract The relationship between the mineralised Hyde‐Macraes Shear Zone (HMSZ), an extensive, low‐angle, semibrittle structure in the Otago Schist Belt within the Haast Schist, New Zealand, and folding events affecting metased‐iments is documented. The shear zone postdates F3 folds, although it is subparallel to S2, a shallow, north‐dipping penetrative cleavage. Evidence of the first shortening phase is only rarely preserved because initial fabrics are transposed. D2 resulted in a strong mineral elongation (L2), a penetrative cleavage, and isoclinal folds. D3 produced generally asymmetrical northwest‐plunging folds in S2 at all scales but of variable intensity. In regions where intense D3 shortening has occurred, L2 is apparently rotated into parallelism with F3 axes. A fourth event resulted in micro‐ and mesofolding about a north‐plunging axis; after this, D5 produced a northeast‐trending micro‐crenulation. The HMSZ nucleated on the lower limb of an inclined macroscopic F3 fold, post‐F3 but pre‐D4 vein...

Fission track analysis reveals character of collisional tectonics in New Zealand

During the late Cenozoic, the Pacific plate has been obliquely converging with the Australia plate in South Island, New Zealand. This has resulted in westward obduction of continental crust of the Pacific plate, and consequently, a major mountain range, the Southern Alps, has formed east of the Alpine fault and within the leading edge of the Pacific plate. We have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the Southern Alps, starting at the Alpine fault, to better understand the uplift age and structure of the continental collision. The results show the following: (1) Uplift was underway at 7 Ma in the south at Haast Pass and by 5 Ma in the north at Arthur's Pass. (2) Profiles of the total amount of late Cenozoic uplift show an exponential increase toward the Alpine fault. Adjacent to the Alpine fault there has been 14.5 to 21.5 km of uplift in the vicinity of Haast Pass, 14.5 to 21 km near Mt Cook, and 12.5 to 15.5 km in the Arthur's Pass transect. Adjacent to the Alpine fault in the Mount Cook region the average uplift rate from ∼5.5 Ma to ∼1 Ma was 2.3 to 2.5 mm/yr; between ∼1 Ma and the present it has averaged 6.2 mm/yr. The strongly asymmetric uplift pattern is responsible for the exposure of a 13–25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the Alpine fault known as the Alpine schists. (3) Late Cenozoic uplift of Torlesse greywackes and Alpine schists was preceeded during the Jurassic‐early Cretaceous by 5–8 km of uplift. The Otago schist belt to the south was uplifted much more during this interval, and in addition, western parts of the Otago schists were uplifted 5–6 km during the late Cretaceous. (4) The fission track data do not support the notion of significant late Cenozoic shear heating about the Alpine fault, as previously suggested from spatial variations in K‐Ar ages of the Otago and Alpine schists.

Contrasting fluids in gold-bearing quartz vein systems formed progressively in a rising metamorphic belt; Otago Schist, New Zealand

Four different quartz vein systems occur within 600 km 2 in east Otago. The vein systems have formed within fault systems which crosscut regional schistosity. The Macraes system formed from almost pure water at about 3 kbars and 350 degrees C (data from fluid inclusions and arsenopyrite geothermometer). The Bonanza system formed from immiscible H 2 O-CO 2 -NaCl fluids at about 1,000 bars. Nenthorn formed under near-surface conditions from boiling low-salinity water, with some CO 2 present. Barewood most closely resembles Macraes but is thought to have formed at slightly shallower levels. The veins formed at different times during Mesozoic-Cenozoic uplift of the Otago Schist belt. C and O isotope data and the high CO 2 content of some fluids suggest that the vein-forming fluids were metamorphogenic. A component of downward-percolating meteoric water was probably involved in shallower vein formation. Vein formation is considered to be an inevitable consequence of devolatilization of uplifting metamorphic rocks, with focusing of fluids into available structural features. Similar fluid flow has occurred within the last 1 m.y. in the northwestern extension of the Otago Schist, where Alpine fault uplift has induced gold mineralization on joint surfaces.