Mineral. Deposita 28, 90-98 (1993)

i Iepineralium oslta (('; Springer-Verlag 1993

Syn-metamorphic gold mineralisation, Invincible Vein, NW Otago Schist, New Zealand R. Hay and D. Craw

Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand

Received: July 1, 1992 / Accepted: September 30, 1992

Abstract. The Invincible Vein fills a fault zone which strikes northeast and dips steeply southeast in the lower Rees Valley, NW Otago. The vein cuts north striking foliation in lower greenschist facies Otago Schist. Struc- tures associated with the fault zone are both brittle and ductile, and the fault zone has had a complex history of post-mineralisation reactivation. Mineralised vein mater- ial filling parts of the fault zone consist of quartz, albite, muscovite, chlorite, calcite, pyrite, arsenopyrite and minor gold. These minerals have been strained and locally re- crystallised during ductile deformation. Fluid inclusion homogenisation temperatures (140-175~ and ice melting temperatures (0 to - 1 ~ indicate that the min- eralising fluid was low salinity, low CO 2 water with a density between 0.88 and 0.93 g/cm 3. Arsenopyrite geo- thermometry implies a temperature of mineralisation of 370 +_ 70 ~ Mineralisation pressure lay between 2 and 5 kbar. Mineralisation pressure-temperature conditions and mineralogy are essentially the same as for meta- morphism of the host schist. Vein calcite oxygen isotope ratios (+ 12 to + 15 per mil) are similar to host schist values. Carbon isotope ratios of vein calcite ( - 3 to - 5 per mil) are distinctly different from ratios in host schist ( - 7 to - 10 per mil). Elevated vein Cr contents, and isotopically depleted carbon data, are consistent with some degree of equilibration with metavolcanic rocks. It is inferred that metavolcanic rocks of the underlying Aspiring Terrane were a significant source for rninerali- sing fluid and metals. Invincible mineralisation occurred in the latter stages of metamorphism, and is the earliest recognised gold-bearing vein system in the Otago Schist.

Gold-quartz vein deposits are commonly found in greenschist facies metamorphic belts around the world (Hodgson 1986; Cox et al. 1986; Kerrich and Wyman 1990). Formation of many of these deposits has been linked to metamorphic fluid generation and mobility within the host metamorphic belt (Henley et al. 1976;

Kerrich and Fyfe 1981; Bottrell et al. 1990). However, a common feature of most of these deposits is that the mineralisation post-dated the regional metamorphism of the host rocks (Bottrell et al. 1990; Forde 1991). Veins typically cut across the host schist structure, and a wall- rock alteration zone can extend for several metres into the host. Because of the time gap between metamorphism and mineralisation, many workers have suggested that miner- alisation cannot be linked directly to metamorphism. Instead, some post-metamorphic processes such as mag- matism or meteoric water circulation are invoked to cause mineralisation (Burrows et al. 1986; Nesbitt et al. 1986).

Recent work has suggested that metamorphic fluids can remain trapped within a metamorphic pile, because of the very low permeability of the rock, possibly for over 100 million years (Thompson and Connolly 1990). These fluids are released either by very slow percolation of fluids during uplift (Thompson and Connolly 1990), by rapid uplift of fluid-bearing rocks (Craw and Koons 1989; Koons and Craw 1991) or by a sudden increase in per- meability such as formation of fault zones (Goldfarb et al. 1991). Thus, the length of the time gap between meta- morphism and mineralisation is dependent on uplift rates and development of uplift-related structures.

If the hypothesis of slow metamorphic fluid release is correct, gold mineralisation should be expected to have occurred along any permeable features developed throughout the uplift history of a metamorphic belt. This expectation is demonstrably valid for many post- metamorphic structures (Craw and Norris 1991), but syn-metamorphic and late-metamorphic mineralisation is rarely documented (e.g. Barnicoat et al. 1991; Windle and Craw 1991; Hayward 1992). In particular, there is little evidence for gold mineralisation occurring in the very earliest stages of uplift from the metamorphic maximum, when metamorphic fluids were still in equilibrium with the host schist. The following paper reports one example of a syn-metamorphic mineralised vein system in the Otago Schist belt, New Zealand, a metamorphic belt in which there has been no regional syn-late metamorphic magma- tism to complicate interpretation of metallogenesis.

Host rock

The host rock for the Invincible Vein is metasedimentary Otago Schist. The Otago Schist is a regionally extensive schist belt formed during the Mesozoic Rangitata Or- ogeny by collision, deformation and metamorphism of greywackes, argillites and basic metavolcanic rocks. Three distinct terranes have been amalgamated to form the schist belt: the quartzofeldspathic Torlesse Terrane in the northeast, the metabasite-rich Aspiring Terrane in the northwest, and the volcanogenic Caples Terrane in the southwest (Norris and Craw 1987). The Invincible Vein is hosted by pelitic and psammitic schists which are the metamorphosed equivalents of Caples Terrane argilli- tes and greywackes (Craw 1984). The Invincible Vein lies near the boundary between Caples and Aspiring Terranes, although this boundary is diffuse and coincides with a steep increase in metamorphic grade and degree of tex- tural reconstitution (Fig. 1B). Metabasites of the Aspiring Terrane crop out 4 km to the east of the vein and dip westward beneath the vein at about 40 ~

The Invincible Vein is hosted by greenschist facies rocks, a short distance (ca. 1 km) up-grade of the pumpel-

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Fig. 1 A-C. Locality maps, showing the Invincible Vein site in northwest Otago, New Zealand. Local geology is from Paterson (1982), Craw (1985), and Hay (1991). A Otago Schist belt in a regional context. B A portion of northwest Otago (see Fig. lA), showing the location of Invincible Vein (black square). Schist re- constitution textural zones I I , I l l , and I V are from Paterson (1982), and the Caples/Aspiring Terrane boundary approximately coincides with the I I I - I V boundary. Metamorphic facies: pa, pumpellyite- actinolite; gs, greenschist. C Local geology of the Invincible Vein

9I

lyite disappearance isograd. Rare relict grains of pumpellyite occur in some host schists. The host schist typically contains the assemblage: quartz-albite-muscovite-chlorite-epidote-titanite-graphite + actinolite _+ pyrrhotite _+ calcite. The schist has under-

gone intense polyphase ductile deformation, so that be- dding features are no longer preserved. Pervasive foliation is defined by mica rich lamellae and foliation-parallel quartz-albite ( + calcite) veins up to 1 cm wide. The foliation has been folded on a northerly axis, locally by mesoscopic scale kink folds, and regionally by the Earnslaw Synform. The Earnslaw Synform is part of the Moonlight Generation of structures, which formed as recently as Miocene (Craw 1985). The Invincible Vein lies on the eastern limb of the Earnslaw Synform, and fol- iation in the host schist generally dips about 50 ~ west.

Invincible Vein

The Invincible Vein occurs in a mountainous area, and was mined on a steep hillside (Fig. 1). The main mine was underground and active from 1882 to 1887, producing over 200 000 grams of gold from ore with an average grade of about 30 g/tonne. Since the mine closed, under- ground workings have collapsed and the Invincible Vein is no longer accessible. Some fresh sample material for this study was collected from mine dumps at the entrances to these workings. Structural data (Willett 1940; Figs. 1 and 2) were obtained from regional mapping, and from an exposure of the vein in McDougall's Creek. This exposure occurs in an active slip, and so some reasonably fresh

A . t r Y / I n v i n c i b l e Vein U / / / / / / ~ ~

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. . !i:;? ,i!!i iii i _ ii .... : l ~ v e i n quartz + sulphides 1 metre

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I brittle & ductile fault rocks ~EEE] soft grey fault gouge

Fig. 2 A, B. Structure of the Invincible Vein. A Block diagram, not strictly to scale, showing the relationships between the vein and the local and regional schistosity. B Detailed structure exposed in a trench across the vein in McDougalrs Creek (Fig. IC)

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sample material was obtained from outcrop and from trenches dug across the vein.

Vein petrography

Mineralised material consists principally of quartz and sulphides, particularly pyrite and arsenopyrite, with rare chalcopyrite grains. Sulphides are irregularly distributed through the quartz, in sulphide-rich patches, as dissemi- nated grains, and as sulphide stylolites (cf Dowling and Morrison 1989). Gold occurs as free grains up to 100 ~tm across in white quartz, and as 1 10 ~tm blebs within sulphides. Albite is a common accessory in quartz-rich vein material, and chlorite, calcite and muscovite occur along quartz vein margins. Sulphides (and gold) occur in the immediate host rock schist, up to 1 m from the vein, as scattered grains associated with recrystallised muscovite and chlorite, locally crosscutting the schist fabric.

(0.01 0.1 mm) recrystallised silicate vein material (Fig. 3B). Vein quartz generally shows undulose extinction and numerous deformation lamellae, and quartz subgrain formation increases towards microshear zones. Albite grains develop undulose extinction towards microshears, and within microshears albite grain boundaries are highly irregular, grains are fractured, and grain size is markedly reduced (ca. 0.01 mm). Sulphides are commonly fractured and disaggregated adjacent to microshears, and the frac- tures are filled by fine-grained quartz, muscovite and chlorite. Muscovite and chlorite in microshears bend around sulphide microlithons, and fill pressure shadows adjacent to sulphide grains.

Zones of black cataclastic fault rock up to 5 cm wide cut mineralised rock and host schist. The cataclasite contains angular fragments of schist, quartz and sulphides in a very fine-grained matrix which includes granulated

Vein structure

The Invincible Vein occurs in a well-defined fault zone which strikes northeast and dips very steeply southeast (Fig. 1). The fault crosscuts schist foliation at a high angle, but the foliation bends towards the fault, and is locally kink folded adjacent to the fault (Fig. 2A). The vein consists of quartz and fault rocks, between sheared host schist margins (Fig. 2B). A combination of ductile and brittle fault rocks occur within the vein, with brittle features generally overprinting ductile features. There is evidence for both normal and reverse fault movement in the fault rocks on the hand specimen scale, indicating a complex history. The fault zone has presumably been reactivated by regional tectonics since the fault's incep- tion,

Ductile fault rocks consist of partially recrystallised schist crosscut by anastomosing muscovite-chlorite-titan- ire microshear zones spaced 1 mm to 2 cm apart. These microshear zones intersect relict foliation at about 30 50 ~ (Fig. 3A), and schist microlithons between microshear zones have been rotated. The sense of shear implied by microlithon rotations is ambiguous and may reflect the complex structural history. Schist quartz grains have serrated margins near microshear zones, and some sub- grains have formed. In the microshear zones, the quartz has recrystallised to very fine (ca. 0.005 mm) anhedral strain-free grains. Microshears which cut across quartz- albite segregation veins consist of narrow (0.1 ram) zones of fine grained (0.005 0.01 mm) recrystallised quartz. Mi- ca-rich microlithons have strained and folded chlorite and muscovite with cleavages locally bent into parallelism with adjacent microshear zones. Both chlorite and mus- covite in microshears have recrystallised to fine grained (ca. 0.1 ram) microshear-parallel aggregates.

Silicates and sulphides in mineralised rock have been deformed and recrystallised to finer grain size where microshear zones extend from adjacent ductile fault rock into the veins (Fig. 3B). Microlithons of less deformed, relatively coarse grained (up to 1 ram) vein material are surrounded by microshear zones consisting of fine grained

Fig. 3 A, B. Photomicrographs of Invincible Vein zone, under crossed polars; horizontal field of view is about 4 mm. A Deformed wall rock, with schistosity (trending top left to lower right) disrupted by ductile microshears (horizontal) in which muscovite and chlorite have been reoriented and recrystallised. Quartz in schist segre- gations (top and bottom) has subgrain development and local grain size reduction. B Mineralised vein material, dominated by quartz which has undulose extinction and deformation lamellae (lower portion), and grain size reduction ( central microshear). Arsenopyrite (upper centre) and albite ( immediately to right of arsenopyrite) have been fractured, fragmented, and locally recrystallised in the micro- shear

sulphides. Black slickensided surfaces are common in cataclasites where sulphides have been sheared and smeared along discrete movement planes. Thin ( < 5 mm) bands of pseudotachylite and ultramylonite cut the cata- clasite. These later fault rocks have been cut in turn by brittle microfaults with detectable but conflicting senses of offset on the scale of millimetres.

The youngest major deformation phase to affect the Rees Valley schist is the Miocene Moonlight Generation (Craw 1985). This deformation involved primarily brittle structures, including kink folds and cataclasis, and at least some of the brittle structures in the vicinity of the Invinc- ible Vein are due to that phase of deformation (Craw 1985). However, the ductile structures, and some of the brittle deformation, presumably predate the Moonlight Generation, and some may have been locally reactivated. There are too few constraints on structural evolution and timing of deformation to unravel the age and order of superposition of structures in the Invincible Vein within a regional context.

M i n e r a l i z a t i o n t e m p e r a t u r e and pressure

Mineralogy and structure

The silicate mineralogy of vein material (quartz-albite- muscovite-chlorite-calcite), is essentially the same as that of the host schist. These minerals have also recrystallised into new crosscutting textures in the wall-rock during mineralisation. Hence, there is no wall-rock alteration as is found in most other hydrothermal veins in Otago (McKeag et al. 1989; Craw et al. 1991), other than addition of sulphides and gold. Thus, the mineralisation appears to have occurred in mineralogical equilibrium with the greenschist facies host rock, i.e. under greenschist facies temperature and pressure conditions.

The vein has clearly formed in a brittle fault structure, but this structure has associated ductile strain as well. Mineralised vein material has been deformed in a ductile manner, resulting in grain-size reduction and recrystall- isation of the greenschist facies assemblage. Mineralis- ation presumably occurred in the region of the brittle- ductile transition (BDT) in the middle crust. The BDT coincides approximately with the greenschist facies in most metamorphic belts (Sibson 1977; Craw and Koons 1989). Hence we conclude that the Invincible Vein formed under greenschist facies temperature and pressure condi- tions. This qualitative argument cannot precisely deter- mine temperature or pressure, but the observed structures and mineralogy are typical of rocks deformed and meta- morphosed at 300-400 ~ and 2-5 kilobars (Sibson 1977; Holm et al. 1989).

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study. The inclusions are typically 2-5 lam across, and contain two phases: liquid water and water vapour. Microthermometric measurements were made using the techniques described by Craw (1988). The inclusions homogenise between 140 and 175 ~ with a pronounced mode at about 150~ (Fig. 4A). The ice melting temper- atures for these inclusions range from near 0 ~ down to - 1 ~ (Fig. 4B). These data confirm that the inclusions

are water with very low concentrations of dissolved ma- terial such as salts and CO2. The inclusions have a density of about 0.88-0.93 grams/cm 3.

The composition of arsenopyrite coexisting with pyrite can be used as an approximate geothermometer, although the calibration of this geothermometer is not well known (Kreschmar and Scott 1976; Scott 1983; Sharp et al. 1985). The geothermometer is best applied to minerals which have equilibrated under greenschist facies conditions (Sharp et al. 1985), and is considered valid in this study. Arsenopyrite compositions were determined by micro- probe analysis on four samples with arsenopyrite coexis- ting intimately with pyrite, and three samples without pyrite. Methods and standards were those described by McKeag and Craw (1989) and Windle and Craw (1991).

Fluid inclusions and arsenopyrite geothermobarometry

White quartz surrounding gold-bearing arsenopyrite in the Invincible Vein contains scattered primary fluid inclu- sions and secondary inclusions along healed planes. The following descriptions apply to primary inclusions only, as apparent secondary inclusions were avoided in this

Fig. 4 A-D. Compositional data from the Invincible Vein used in geothermobarometry. A Homogenisation temperatures for primary fluid inclusions in vein quartz. B Ice-melting temperatures in pri- mary fluid inclusions in vein quartz. C Compositions of arsenopyrite with (black) and without (stipple) coexisting pyrite (see text). D Estimate of temperature and pressure of mineralisation determined from arsenopyrite and fluid inclusion density (see text)

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Arsenopyri te composit ions range from 29 32 weight % As (Fig. 4C) in all samples studied whether or not pyrite was present. This composi t ional range implies a temper- ature range of 300-440~ (Kreschmar and Scott 1976), and it is not possible to further constrain the temperature without a more definitive sulphide assemblage. This ar- senopyrite temperature can be used with fluid densities derived from fluid inclusions to estimate mineral isat ion pressure. The same method has been used for several other Otago vein systems (McKeag and Craw 1989; Craw and Norris 1991), and can provide useful relative pressure condit ions of mineralisat ion, in spite of the many un- certainties. Pressures derived by this method range from 2 to 5 kilobars (Fig. 4D).

The temperature and pressure inferred from the above methods are consistent with the mineralogical and struc- tural observat ions (above), which indicate that mineral- isation occurred near to the BDT under greenschist facies conditions. The arsenopyri te temperature range includes an oxygen isotope temperature of 390~ derived for metamorphism of schist near the pumpellyite-actinoli te facies/greenschist facies boundary at Glenorchy, 10 km to the south (Yardley 1982; Paterson 1982). Greenschist

facies metamorphic pressures inferred elsewhere in the Otago Schist lie between 4 and 5 kilobars (Yardley 1982; Jamieson and Craw 1986), which coincides with the upper end of the Invincible pressure range (Fig. 4D). Considering the many uncertainties, we conclude that the estimates of temperature (370 _+ 70 ~ and pressure (2 5 kbar) derived above are reasonable.

Whole rock geochemistry

A suite of altered rocks from immediately adjacent to the Invincible Vein, and unaltered schists from the nearby host rock, were selected for further examinat ion of the subtle wall-rock alteration. Twenty- two samples were pre- pared and analysed by X-ray fluorescence spectrometry, as described by Craw et al. (1991). Representative analyses are presented in Table 1, and all the data are presented by Hay (1991). Altered and unaltered samples have similar major and trace element chemistry, except for the higher As and Au contents of altered samples, confirming petro- graphic observations. Variations in most other elements are such that unaltered and altered composi t ions overlap.

Table 1. Whole rock X-ray fluorescence analyses, Invincible Vein

63218 63219 63220 63212 63213 63215 63216 63227 63217 63214 63228 Unalt Unalt Unalt Alt Alt Alt Alt Alt Alt Alt Alt (P) (P) (P) (P) (P) (P) (P) (P) (P) (~) (if)

SiO2 61.63 62.39 65.14 51.48 63.79 65.74 53.36 55.48 57.84 63.74 60.98 TiO2 0.70 0.74 0.64 1.88 0.77 0.63 2.25 1.69 1.73 0.79 0.75 A1203 18.49 17.66 16.56 19.78 17.57 16.54 18.85 18.91 17.52 17.49 18.22 Fe203 5.58 5.87 5.09 9.87 5.87 4.80 10.62 9.48 8.82 5.70 6.27 M nO 0.09 0.08 0.07 0.14 0.09 0.07 0. l 7 0.15 0.15 0.10 0.08 MgO 1.79 1.99 1.64 3.21 1.82 1.51 2.76 3.27 2.92 1.93 2.18 CaO 1.59 1.58 1.43 1.29 0.52 0.52 1.14 1.14 1.60 0.90 1.26 Na20 3.87 3.46 3.90 5.19 3.46 3.74 3.93 4.19 4.48 4.06 3.89 K20 3.77 2.83 2.70 2.24 3.04 2.96 2.52 2.21 1.79 2.74 3.07 P205 0.18 0.18 0.17 0.35 0.17 0.14 0.53 0.32 0.35 0.18 0.19 LOI 2.67 2.88 2.88 3.54 2.68 2.41 3.54 3.46 3.57 2.80 3.29

Total 100.36 99.66 100.22 98.97 99.78 99.06 99.67 1 0 0 . 3 0 1 0 0 . 7 7 100.43 100.18

V 112 141 103 236 131 110 248 223 205 149 140 Cr 53 60 48 270 91 59 282 245 226 70 62 Ba 789 659 612 176 584 562 474 405 344 405 487 La 39 38 39 49 40 43 55 43 44 55 46 Ce 81 71 79 546 165 152 155 165 104 361 289 Pr 19 15 19 13 20 20 19 19 12 18 18 Nd 25 22 28 32 27 29 31 27 24 35 34 Nb 14 12 12 19 13 13 26 17 19 15 11 Zn 99 96 89 144 97 76 154 138 131 105 95 Cu 28 29 23 21 35 24 93 41 37 28 30 Ni 22 22 18 69 24 21 62 76 89 29 25 Ga 20 22 21 25 20 18 25 23 20 22 19 Rb 133 114 107 92 125 116 102 88 71 109 116 Sr 188 265 199 350 94 143 151 221 160 387 601 Y 25 22 25 27 25 22 44 33 32 23 23 Zr 190 159 198 212 168 190 210 188 196 204 184 Pb 12 21 16 14 18 21 17 9 17 25 16 Th 13 1! 13 10 12 10 5 8 6 15 9 U 3 3 3 2 3 3 2 1 1 2 2

Unalt, unaltered schist; Air, altered schist; (p), pelitic schist; (fr), fault rock; Major elements in wt%; Trace elements in ppm; LOI-Loss on ignition

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The only element to show consistent differences is Cr which is distinctly more concentrated in altered rocks (50-280 ppm) compared to unaltered rocks (30-60 ppm). Chromium is typically concentrated in chlorite, implying a direct relationship between Cr content and modal chlor- ite content, or MgO content since chlorite is the main magnesium mineral in the host schist. Unaltered Invinc- ible rocks, and other quartzofeldspathic rocks from the nearby Aspiring Terrane, do show a straight line relation- ship between Cr and MgO (Fig. 5A). Altered Invincible rocks show a broad spread from this line towards distinct Cr enrichment.

Graphite

Most of the Invincible host rocks contain very fine- grained ( < 1 lam) dusty material scattered through the micaceous layers, which is suspected to be graphite. Graphite-rich schist has been shown to have an effect on gold deposition from metamorphogenic hydrothermal flu- ids (McKeag et al. 1989), so the carbon content of some Invincible and associated rocks has been determined for comparison. Analyses were conducted at the Micro-ana- lytical Laboratory, Chemistry Department, University of Otago. Three ca. 1 mg portions of powdered rock for each sample were pre-treated with hydrochloric acid to remove carbonate. The powder was vapourised in a Carlo Erba Elemental Analyser EA 1108 and the resultant carbon dioxide content of vapour was measured with a chromato- graph. The results (Table 2) show that the Invincible rocks contain small but significant quantities of graphite, similar to the graphitic schists which host the Macraes gold mine in east Otago (McKeag et al. 1989).

Carbon and oxygen isotopes

Carbon and oxygen isotopic ratios of calcite from the Invincible area were determined by the University of Waikato Micromass 602 mass spectrometer. Analyses were conducted at 50~ using phosphoric acid for dis- solution, and standards NBS-19 and Te Kuiti limestone. Four samples of calcite-rich selvedge from foliation-para- llel metamorphic veins were obtained from unaltered host rock, and five samples of vein calcite were obtained from within the Invincible Vein (Hay 1991). The resultant data (Fig. 6) all lie in or near the field of Otago and Alpine Schist carbonates defined by Blattner and Cooper (1974). The oxygen isotopic ratios are similar for all calcites analysed: between + 12 and + 15 per mil. There is a distinct difference in carbon isotopic ratios between host schist ( - 7 to - 10 per rail) and vein calcites ( - 3 to - 5 per mil).

Oxygen isotopes in calcite are sensitive to temperature of mineral deposition (Bottinga and Javoy 1973), so the overlap of host and vein oxygen isotopic ratios is con- sistent with Invincible Vein formation at metamorphic temperatures (as deduced above) from the same fluid as the host schist veins. In contrast, data from the Macraes mine (Fig. 6; McKeag et al. 1989) show that a lower temperature ofmineralisation caused a shift towards more

A

MgO 0//

100 200 I I

B

o I ( eight '~) / A / .

Z ;20" I

A

Cr ( p p m ) 3OO

i

Cr ( p p m ) 400

I

Fig. 5 A, B. Chromium vs. MgO plot to show variation of (A) Invincible Vein mineralised (squares) and unmineralised material (diamonds), in comparison to nearby host pelitic schist (triangles). An approximate linear relationship (indicated) exists for the unmineral- ised vein rocks and host rocks. B Comparison of Invincible Vein data (dashed field) and line from Fig. 5A, to Aspiring Terrane data (open triangles; Palmer et al. 1992)

enriched oxygen. A similar effect was noted for oxygen isotopes in quartz at Glenorchy (Paterson 1982).

Minor fractionation of carbon isotopes in neutral pH fluid between calcite and fluid at high temperatures (Bot- tinga and Javoy 1973) indicates that the difference in carbon isotope ratios between host schist and vein is probably not due to a temperature differential. The host schist calcites have relatively depleted carbon, similar to that of the Macraes calcites which owe their depleted carbon to interaction with graphite (McKeag et al. 1989; Craw et al. 1991) which is typically < - 2 0 per mil (Kreulen 1988). Calcites from the depleted end of the schist carbonate field (Fig. 6) are mainly from graphite- bearing hosts in the Alpine Schist (Blattner and Cooper 1974; Craw and Koons 1989). These calcites have oxygen isotopic ratios affected by meteoric water, but their car- bon isotopic ratios are apparently controlled by the host rock graphite. We infer that the small graphite content of the Invincible host rocks has had a similar effect on the metamorphic calcites in those rocks.

The Invincible Vein calcites are clearly not in isotopic equilibrium with the host rock calcites, and some external control is assumed. The fluid which deposited the vein calcites was relatively enriched isotopically, and had little or no interaction with graphitic schist at the site of deposition or in the fluid pathway. The rock with which this fluid last equilibrated contained carbon in a more oxidised form, possibly as calcite.

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Table 2. Carbon analyses

Locality Sample no. Rock Type wt % C Ref.

Invincible OU63218 Pelitic 0.14 This 0.13 study 0.17

OU63219 Pelitic 0.20 0.20 0.23

OU63186 Psammitic 0.11 0.12 0.10

OU63189 Psammitic 0.23 0.15 0.15

Glenorchy OU35605 Pelitic 0.26 This 0.22 study 0.27

OU35630 Pelitic 0.35 0.21 0.25

OU31872 Psammitic 0.13 0.11 0.12

OU31876 Psammitic 0.07 0.07 0.07

Macraes Drill hole 46 Wall rock 0.24 McKeag Wall rock 0.17 et al. 1989 Wall rock 0.15 Wall rock 0.15 Mineralised 0.41 Mineralised 0.31 Mineralised 0.34 Mineralised 0.10 Mineralised 0.06 Wall rock 0.12

Discussion and conclusions

Mineralisation and metamorphism

Gold mineralisation occurred throughout the Mesozoic- Recent uplift of the Otago Schist (Craw and Norris 1991). The structurally earliest vein system hitherto recognised at Macraes has a distinct zone of wall-rock alteration, in which some of the pre-ore host schist mineralogy has been destroyed (McKeag et al. 1989). This alteration, and some isotopic data, suggest that the mineralising fluid was not in chemical or thermal equilibrium with the host rock (McKeag et al. 1989). The Invincible Vein has wall-rock alteration mineralogy essentially identical to the meta- morphic mineralogy, and geothermobarometry data are consistent with greenschist facies metamorphic condi- tions. Syn-mineralisation structures at Invincible have a more ductile character than the Macraes deposit which is thought to have formed near to the BDT (Teagle et al. 1990). Hence, we conclude that the Invincible Vein formed at a structurally earlier stage than the Macraes deposit, in one of the earliest brittle structures formed during uplift of the Otago Schist.

Hydrothermal activity and metal concentrations oc- curred during fully ductile stages of deformation and metamorphism of the Otago Schist, and the resultant veins have been tightly folded in a ductile manner. This mineralisation was of limited extent, and involved tungsten concentration only (Craw and Norris 1991). There is no evidence for gold deposition in these syn- metamorphic veins. The Invincible Vein therefore repre- sents the earliest recognizable stage of gold mobility and concentration in the Otago Schist. The vein may represent a Rangitata Orogeny equivalent of modern (Kaikoura Orogeny) gold-bearing veins which have greenschist facies vein mineralogy and cut greenschist facies schist in the actively rising Southern Alps (Craw et al. 1987; Craw and Koons 1989; Koons and Craw 1991).

f Invincible calcite //0t o and ._.~# vein ~ ag ,A, schist /AIp.,ne Schist / 0 _ _ ca,oo

Ota0o _ greenschist 4 '

-12 -t "Alpine Schi | ca/~o r~ c influenced I +4 +18 +1~2 +~6

618 0 SMOW

Fig. 6. Carbon and oxygen isotope variation of calcites in the Invincible Vein (larqe circles) and immediate host rock (larqe trian- .qles), compared to principal Otago Schist carbonates (Blattner and Cooper 1974), including metavolcanic carbonate (Devereux 1968; diamonds). Fields for Alpine Schist (Craw and Koons 1989) and Macraes gold mine data (McKeag et al. 1989) are indicated also

Source of fluid and metals

Fluids which form metamorphogenic gold deposits are generally presumed to come from dehydration reactions within the metamorphic pile (Norris and Henley 1976; Kerrich and Fyfe 1981). The observed fluid composition at Invincible is similar to metamorphic fluids observed elsewhere in the Otago Schist (Yardley 1982; Holm et al. 1989). There is less agreement on the sources of the metals, which may come from leaching of trace amounts from the entire metamorphic pile (e.g. Paterson 1982), or from specific metal-enriched zones such as metavolcanic hori- zons (e.g. Henley et al. 1976). Recent work in the Otago Schist has implicated the metavolcanic-rich Aspiring Ter- rane, which is presumed to underlie much of the schist belt, as a potent source of both fluid and metals (McKeag et al. 1989; Craw and Norris 1991). Gold solubility is sufficiently high under metamorphic conditions that source of water, rather than source of gold, is the limiting factor (Craw and Norris 1991). One indicator of the oceanic Aspiring Terrane source is distinct Cr enrichment

97

in the wall-rock alteration zone at Macraes (McKeag et al. 1989).

The Cr enrichment at the Invincible Vein may be an indicator of a connect ion to a Cr-rich source such as the Aspiring Terrane. Chromium is enriched in almost all Aspiring Terrane metavolcanic rocks, compared to quartzofeldspathic schists from Otago. The M g O - C r plot (Fig. 5B) depicts a broad spread of Cr enrichment in metavolcanic rocks away from the typical quartzofeld- spathic M g O - C r line to enclose the Invincible altered rocks (Fig. 5B). We tentatively interpret this as evidence for the Invincible mineralising fluid having closer ap- proach to equilibrium with metavolcanic rocks than with the host quartzofeldspathic schist. Aspiring terrane meta- volcanic rocks dip beneath the Invincible Vein in the Earnslaw synform, and are projected to lie about 2-3 km below the vein on the basis of regional structure (Craw 1984 1985).

Fur ther evidence for the connect ion to the underlying metavolcanic rock is found in the carbon isotope data. The Invincible Vein calcite carbon isotopic ratios imply equilibration with a graphite-free rock mass (see above). Whereas the quartzofeldspathic schists contain pyrrhoti te which can coexist with graphite, the greenschists contain pyrite and magnetite, implying oxygen activities above the stability field of graphite (cf. Johnstone et al. 1990). Calcite is the stable ca rbon mineral in greenschists, and graphite is not observed (Craw 1984). Carbon isotopic ratios of calcite from greenschists reflect the absence of graphite, and typically range from 0 to - 6 per mil (Devereux 1968; Fig. 6). The metavolcanic rock carbon isotopic ratios overlap the Invincible Vein data (Fig. 6).

The above two arguments for a link between the Invincible Vein mineralising fluid and Aspiring Terrane metavolcanic rocks provide only circumstantial evidence for that link. However, both the Cr anomaly and the C isotope data are distinctive and difficult to explain in the context of the immediate host rock. The greenschists are also distinctive in composi t ion with respect to C isotopes and Cr content compared to other Otago Schist rock types, and resemble the Invincible Vein material. There- fore, we conclude that the Invincible mineralising fluid was derived from Aspiring Terrane greenschists, and the gold may have the same source. The fluid rose rapidly along the Invincible fault as the fault became brittle during late metamorphic uplift, so that the fluid remained in equilibrium with the source greenschists at least with respect to Cr and C isotopes.

Acknowledgements. The work on which this paper is based benefited from discussions with R. J. Norris and R. H. Sibson. The manuscript was improved by reviews by R. Kerrich and an anonymous referee. Financial support of the James Park Memorial Fund and the University of Otago Research Committee is gratefully acknow- ledged. Department of Conservation staff kindly allowed access to a historical site for this study.

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