Doug MacKenzie

Structural and lithological continuity and discontinuity in the Otago Schist, Central Otago, New Zealand

Abstract The Otago Schist in Central Otago has undergone complex late metamorphic and post‐metamorphic deformation during uplift and exhumation. Two late metamorphic structural generations can be recognised and these may be genetically related. The earlier Manorburn Generation has been widely recognised and described previously. This generation has fold axes subparallel to a prominent syn‐metamorphic quartz rodding lineation. A later generation, herein named Poolburn Generation, has folds which superficially resemble Manorburn Generation, but has fold axes that are at a high angle to the quartz rodding lineation. Both generations occur in mappable fold zones (kilometre‐scale) that are generally not vergence boundaries, and some minor relative displacement may occur across fold zones. Fold zones occur within structurally and lithologically uniform schist domains. Abrupt changes in lithological sequences and orientations of structural elements such as Manorburn and Poolburn Generation fold axes and quartz rodding lineations occur at post‐metamorphic faults which separate different schist domains. Central Otago schist can be subdivided on the regional scale (tens of kilometres) into at least nine schist domains whose structural and lithologic continuity is disrupted by fault discontinuities. The domains and bounding discontinuities developed during Late Jurassic and Early Cretaceous uplift. Syn‐metamorphic compressive ductile deformation evolved to localised fold zones in the early stages of this uplift. Subsequently, regional extension caused juxtaposition of domains with different textural zones, and schists from slightly different structural levels. The Caples/Torlesse Terrane boundary is a composite feature, and different segments formed at different stages through the transition from ductile compression to brittle extensional deformation.

Supergene gold mobility in orogenic gold deposits, Otago Schist, New Zealand

Gold has been chemically mobilised by groundwater from host sulphide minerals in orogenic gold deposits of Otago. Mobilisation occurred near the Cenozoic Otago Schist erosional surface beneath a sedimentary cover. Initial Au mobilisation, on a scale of micrometres, occurred when solid solution and microparticulate gold in pyrite and arsenopyrite grains were liberated by sulphide oxidation to iron oxyhydroxide pseudomorphs. Larger-scale mobilisation involved leaching of gold from up to 100-m-thick zones which were the target of historic mining, with up to 10× enrichment of Au in reprecipitation zones. Gold in the supergene zones is commonly crystalline with octahedral shapes and nuggety forms which fill cavities and coat prismatic quartz crystals. This gold retains some or all of the Ag (typically 2–8 wt%) from the primary source gold. Oxidised groundwaters that have interacted with sulphides become enriched in dissolved sulphate, but retain high pH (7–8.5). Under these conditions, metastable thiosulphate ions can dissolve and transport Au and Ag to be precipitated later by either oxidation or reduction.

Hydrothermal alteration styles in ancient and modern orogenic gold deposits, New Zealand

Abstract Orogenic hydrothermal systems in the South Island of New Zealand were active during Mesozoic and late Cenozoic collisional deformation and metamorphism of greywacke/schist terranes. Observations on the currently active mountain‐building environment yield insights on processes occurring in the upper 5–15 km of the crust, and observations on an adjacent lithologically identical exhumed ancient mountain belt provide information on processes at 10–20 km in the crust. Hydrothermal fluids were mainly derived from metamorphic dehydration reactions and/or circulating topographically driven meteoric water in these mountain belts. Three geochemically and mineralogically different types of hydrothermal alteration and vein mineralisation occurred in these orogenic belts, and gold enrichment (locally economic) occurred in some examples of each of these three types. The first type of alteration involved fluids that were in or near chemical equilibrium with their green‐schist facies host rocks. Fluid flow was controlled by discontinuous fractures, and by microshears and grain boundaries in host rocks, in zones from metres to hundreds of metres thick. Vein and alteration mineralogy was similar to that of the host rocks, and included calcite and chlorite. The second type of alteration occurred where the fluids were in distinct disequilibrium with the host rocks. Fracture permeability was important for fluid flow, but abundant host rock alteration occurred as well. The alteration zones were characterised by decomposition of chlorite and replacement by ankeritic carbonate in zones up to tens of metres thick. The mineralising fluid was deep‐sourced and initially rock‐equilibrated, with some meteoric input. The third type of mineralisation was controlled almost exclusively by fracture permeability, and host rock alteration was minor (centimetre scale). This mineralisation type commonly involved calcite and chlorite as vein and alteration minerals, and mineralisation fluids had a major meteoric water component. The three mineralisation types can be traced spatially and/or temporally from one to another with some overlap. The first type is characteristic of the deeper parts of an orogenic hydrothermal system, and this type gave way to the second type formed at shallower crustal levels, locally near to the surface. The third type of alteration is typically a late‐stage, shallow‐level phenomenon.

The mercury and silver contents of gold in quartz vein deposits, Otago Schist, New Zealand

Abstract A regional study of gold from Otago Schist vein deposits from both Caples and Torlesse Terranes has revealed the presence of both Au‐Ag and Au‐Ag‐Hg alloys in both terranes. Almost all Hg‐bearing gold occurs in east Otago vein systems, and Hg‐free gold occurs in central and northwest Otago vein systems, irrespective of host terrane. One Central Otago mineralised zone has up to 1.7 wt% Hg in the gold. Au‐Ag alloy (3–8 wt% Ag) is typical for gold found in most Torlesse‐hosted vein deposits, and Au‐Ag‐Hg alloys with 1–4 wt% Hg are found in vein material from the Torlesse‐hosted Hyde‐Macraes Shear Zone. Au‐Ag‐Hg alloy (3–8 wt% Ag, 2–8 wt% Hg) is found in many Caples‐hosted vein deposits. There is no relationship between depth of vein emplacement and Hg content of gold, as both high and low Hg gold are found in shallow‐formed (<2 km) and deep‐formed (>6 km) deposits. There is no spatial relationship between Hg‐bearing gold and cinnabar‐bearing veins that occur in Caples Terrane rocks on the southern edge of the schist belt. Mercury‐bearing placer gold in eastern Southland may have been derived from erosion of east Otago veins. The source of Hg‐bearing placer gold in northern Southland is unknown as yet.

Structure and geochemistry of the Rise & Shine Shear Zone mesothermal gold system, Otago Schist, New Zealand

Abstract The Rise & Shine Shear Zone is a late metamorphic deformation zone developed in biotite zone Textural Zone 4 schist in Central Otago. The shear zone has been hydrothermally altered, with addition of gold associated with replacement of schist minerals by pyrite and arsenopyrite. Hydrothermal alteration of schist during mineralisation involved replacement of titanite by rutile, recrystallisation of metamorphic quartz, muscovite and chlorite, and addition of ankerite. Mineralised schist has abundant microshears that have developed parallel and subparallel to the pervasive schist foliation, and these microshears contain much of the hydrothermal sulfides and gold. Microshears have been deformed locally by upright syn‐mineralisation brittle reverse faults and angular folds that have a southerly axial trend. These more brittle deformation zones are confined to the Rise & Shine Shear Zone. Gold‐bearing veins and mineralised breccias, made up of quartz, albite, pyrite, arsenopyrite, calcite, and chlorite, fill extensional sites associated with upright fold zones. Calcite δ18OVSMOW from these late‐stage mineralised veins ranges from +7 to +15‰ and δ13CPDB from ‐5.3 to ‐6.6‰, are similar to many other gold‐bearing vein systems in Otago, but are distinctly different from the Macraes deposit. Mineralisation occurred near to the brittle/ductile transition, at 200–400°C. The upper part of the shear zone was truncated by a shallow northeast‐dipping normal fault, the Thomsons Gorge Fault, which juxtaposed shear zone rocks against unmineralised Textural Zone 3 chlorite zone rocks in the middle Cretaceous. The Rise & Shine Shear Zone has some structural and geochemical features in common with the Hyde‐Macraes Shear Zone, but also some important differences, and is not a simple strike‐extension of that structure.

Near‐surface secondary gold mobility and grain‐size enhancement, Barewood Mine, east Otago, New Zealand

Abstract Primary gold at Barewood, east Otago, New Zealand, occurs in quartz veins in the Otago Schist. The gold typically occurs as 1–3 pm grains enclosed in arsenopyrite, pyrite, or chalcopyrite. Secondary gold mobility occurred during formation of a zone of kaolinitisation (up to 20 m deep) in the Otago Schist immediately beneath an early Tertiary unconformity. Oxidation of the sulphides released the gold and allowed local migration, possibly as thiosulphate or reduced sulphur complexes. Silver was slightly more soluble than gold, and secondary gold has higher atomic fineness (930) than primary gold (870). Secondary gold is coarse grained (up to 1 mm), and was deposited in fractures, shears, and cavities in quartz veins and associated sheared schist. The secondary gold was precipitated with goethite and kaolinite, and locally with scorodite and/or pharmacosiderite. Dissolution, migration, and redeposition of gold is due to fluid composition evolution from local moderately reduced, sulphur rich, acid co...

Structural control of gold‐scheelite mineralisation in a major normal fault system, Barewood, eastern Otago, New Zealand

Abstract Gold and scheelite bearing quartz veins in Otago Schist at Barewood, eastern Otago, are located in a regional scale zone of faults, which is traceable for at least 20 km along strike. Individual mineralised faults in the zone can be mapped up to 10 km along their northwest strike. The faults have been normal faults since their inception. Early movement involved warping of adjacent schistosity up to 5 m from the faults, and shearing of the schist in the faults, which dip about 50° northeast. Later movement was entirely brittle, and was characterised by some shears and a set of steeply dipping (up to 80° northeast and southwest) conjugate fractures. Minor late‐stage movement occurred along moderately northeast dipping shears. Slickensides suggest that later stages of movement involved significant strike‐slip motion as well as a normal component. Two stages of quartz veins occur in many localities and both quartz vein types contain gold, scheelite, and sulphides. The early quartz veins are massive w...

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

Abstract 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.

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 (< 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.

Geochemical signatures of mesothermal Au-mineralized late-metamorphic deformation zones, Otago Schist, New Zealand

Hydrothermal processes along two regional-scale shear zones in the Otago Schist were dominated by structurally controlled fluid flow and mineralization in the host schist, with relatively minor quartz vein formation, and mineralized rocks are only subtly different from unmineralized rocks. Most Au in the shear zones is associated with sulphide minerals (pyrite and arsenopyrite) disseminated through the host schist or along microshears. Minor enrichment of Sb, Mo and Bi (ppm level) is detectable in the Hyde-Macraes Shear Zone (HMSZ). Hydrothermal muscovite is slightly more aluminous (1–2 wt%) than metamorphic muscovite in both shear zones. HMSZ muscovite averages >900 ppm N, in contrast to metamorphic muscovite that averages c. 200 ppm N. In both shear zones, rutile has replaced metamorphic titanite and epidote has altered to carbonate and phyllosilicates, but these reactions were nearly isochemical. Structurally controlled hydrothermal graphite in the HMSZ occurs in microshears (up to 3 wt%, above background <0.2 wt%). Alteration in the Rise & Shine Shear Zone (RSSZ) was accompanied by addition of abundant ankerite. The two shear zones have subtly different geochemical signatures and are not directly genetically related. However, As enrichment is a key exploration target for both shear zones.