fluid evolution during metamorphism of the otago schist, new zealand: (i) evidence from fluid...

J. metamorphic Geol., 1999, 17, 173–186

Fluid evolution during metamorphism of the Otago Schist,New Zealand: (I) Evidence from fluid inclusionsM. P. SMITH* AND B. W. D. YARDLEYDepartment of Earth Sciences, University of Leeds, Leeds, LS2 9JT , UK

ABSTRACT Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt%NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenizationtemperatures from these inclusions range from 124 to 350 °C, with modal values for individual samplesranging from 163 to 229 °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting showa narrower range of T h from 86 to 170 °C with modes from 116 to 141 °C. These data have been usedin conjunction with chlorite chemistry to suggest trapping conditions of #350–400 °C and 4.1–6.0 kbarfor inclusions showing metastable melting from lower greenschist facies rocks, with the densities of manyother inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinitiesand Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, andthis suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modificationof the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydrationand hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that ofmodern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydrationof clays and zeolites. The contrast between the two terranes may be a result of the original sedimentaryprovenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caplesterrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydrationduring diagenesis, and hence may provide more fluid via dehydration at higher grades.

Key words: fluid inclusions; metamorphic fluids; Otago Schist; salinity.

the results of a study of fluid inclusions in quartz veinsINTRODUCTIONhosted by low- to medium-grade metamorphic rocksfrom the Otago Schist, South Island, New Zealand,The salinity of metamorphic fluids is of significance

for a number of reasons. Dissolved salts change the carried out in order to relate the conditions of the veinformation to the metamorphic evolution of the schist,activity of water, and hence the conditions for

dehydration reactions, but they also reduce the solu- and to examine the evolution of fluid salinity duringmetamorphism of sediments deposited in anbility of non-polar fluids in water, which may extend

the miscibility gap between water and carbonic fluids accretionary setting.Previous fluid inclusion studies in the Otago Schistto high temperatures and pressures (Hendel &

Hollister, 1981; Bowers & Helgeson, 1983; Yardley & have mainly concentrated on Au- and W-mineralizedveins which formed during exhumation and retrogradeBottrell, 1988; Johnson, 1991; Shmulovich et al., 1994).

The anion chemistry of a geological fluid dictates its metamorphism of the schist (Paterson, 1986; McKeag& Craw, 1989; McKeag et al., 1989; Craw & Norris,metal-carrying capacity through the formation of metal

complexes, and hence controls the potential for mass 1991; Craw, 1992; Hay & Craw, 1993; Mackenzie &Craw, 1993; Ashley & Craw, 1995), with limited datatransfer (e.g. Eugster, 1981) and ore deposition.

Metamorphic fluids can be defined either broadly relating to the prograde or peak metamorphic fluids(Yardley, 1982; McKeag & Craw, 1989; Craw &as fluids in chemical and isotopic equilibrium with

metamorphic host rocks, or more specifically as fluids Norris, 1991; Craw & Norris, 1993; Hay & Craw,1993). This study provides new data on progradederived from devolatilization reactions during meta-

morphism. Yardley (1996) has argued that metamor- fluids, and contrasts these fluids with those responsiblefor later, lower-P veining.phic fluids sensu stricto evolve in a continuum from

pore fluids in sedimentary protoliths, throughhydration and dehydration reactions during both

GEOLOGICAL SETTINGdiagenesis and metamorphism. In this paper we report

The samples analysed in this study are from the OtagoSchist belt of the South Island, New Zealand (Table 1).*Current and correspondence address: Department of Miner-The Otago Schists (sometimes also referred to in partalogy, The Natural History Museum, London, SW7 5BD, UK

(e-mail: [email protected]). as the Haast Schist) formed as a result of the

173© Blackwell Science Inc., 0263-4929/99/$14.00Journal of Metamorphic Geology, Volume 17, Number 2, 1999

174 M. P. SMITH & B. W. D. YARDLEY

Table 1. Description and locality of vein samples from the Otago Schist.

Location (1550 000

Sample no. sheet grid ref ) Terrane Vein mineralogy Host lithology (metamorphic gradea) Textural zoneb Relative agec

Lindis Pass

L1.2 G40 336 016 Torlesse Qtz–Chl Graphitic phyllite (PA.) TZ3A MS2

L2.1 G40 438 193 Torlesse Qtz–Cal–Chl Graphitic phyllite (PA) TZ2B MS2

L8.1 G40 338 995 Torlesse Qtz–Chl Graphitic phyllite (PA) TZ2B MS2

Lake Hawea

HA1 G39 118 254 Torlesse Qtz–Chl Quartzofeldspathic psammite (PA) TZ2B MP1

HB1A G39 118 275 Torlesse Qtz–Chl Semi-pelite/slate (GS–Chl zone)

TZ3A MP1

HD1 F39 098 324 Torlesse Qtz–Chl Quartzofeldspathic psammite (GS–Chl zone) TZ4 MP1/MS2

Lake Wakatipu

W3.1 F41 Nevis Bluff Torlesse Qtz–Chl Graphitic schist (GS–Bt zone) TZ4 MP1

W2.2 F42 Devils Staircase Caples Qtz–Ab Black phyllite (GS–Chl zone)

TZ3A MP1

W2.4 F42 Devils Staircase Caples Qtz–Chl Black phyllite (GS–Chl zone)

TZ3A MP1

Thompson Mountains

T1.2 E42 412 474 Caples Qtz Greywacke (PA) TZ1/2A LT

Balcutha Quarry

S1.5 H46 579 374 Caples Qtz–Ep Greywacke (PP) TZ1 N/A

a PA, pumpellyite–actinolite facies; PP, prehnite–pumpellyite facies; GS, greenschist facies. b Textural zones after Bishop (1972) and Mortimer & Roser (1993): TZ1, indurated, non-foliated

medium-grained sandstone; TZ2A, slightly foliated meta-sandstone with widely spaced cleavage; TZ2B, penetratively and well-foliated semischist; TZ3A, strongly foliated schist with

segregation lamellae 1–10 mm long; TZ3B, as 3A but segregation lamellae >10 mm long and <2 mm thick; TZ4, as for 3A but segregation lamellae >2 mm thick. c Relative ages: MP1,

postdates pervasive S1 fabric but is typically folded by D2; MS2, axial planar to D2 crenulations and hence contemporaneous with D2; LT, late tectonic. N/A, undeformed host rocks lack

pervasive fabric.

deformation and metamorphism of Upper Palaeozoicand Mesozoic rocks during the Cretaceous Rangitataorogeny (Harper & Landis, 1967; Adams, 1979). Themetasediments pass down grade to the north intoquartzofeldspathic pelites and psammites of theTorlesse terrane (Bishop, 1972, 1974), and to the southinto the andesitic greywackes, pelites and minorvolcanics of the Caples terrane (Bishop et al., 1976).The boundary between these two terranes lies withinthe Otago Schists, and has been the focus of muchrecent research (Turnbull, 1979; MacKinnon, 1983;Roser & Korsch, 1986, 1988; Frost & Coombs, 1989;Mortimer & Roser, 1992). The boundary shown inFig. 1 is from Mortimer & Roser (1992). The metamor-phic grade of the Otago Schist ranges from prehnite–pumpellyite to greenschist facies (Landis & Coombs,1967), but both Caples and Torlesse terrane rocks canbe traced to lower-grade, zeolite facies equivalents.Peak temperatures of 350–370 °C have been suggestedfor pumpellyite–actinolite facies metamorphism(Yardley, 1982), and the peak temperature and pressurefor the greenschist facies metamorphism in centralOtago have been inferred to be 390 °C at 4–5 kbar(Yardley, 1982; Jamieson & Craw, 1987). The rocksevolved along a clockwise P–T path with peak pressuresbeing attained before the peak temperature (Yardley,1982). Foliation-parallel W- and Au-mineralized veinshosted in greenschist facies rocks have been inferred

Fig. 1. Geological and metamorphic map of the Otago Schist,to form at temperatures from 350 to 390 °C andSouth Island, New Zealand, after Mortimer & Roser (1993),3–4 kbar on the basis of fluid inclusion homogenizationshowing sample localities used in this study and in Smith &

temperatures from veins, the above temperature esti- Yardley, 1999).mates and arsenopyrite geothermometry (McKeag &Craw, 1989; Craw & Norris, 1991; Ashley & Craw,1995). This suggests vein formation post-peak pressure

FLUID EVOLUTION, OTAGO SCHIST, NEW ZEALAND 175

conditions and close to the peak temperature. Post- minor apatite and titanite (Fig. 2a). Chlorite, albite,apatite and minor amounts of white mica, pyrite andpeak metamorphic W- and Au-mineralized veins,

which are typically hosted in extensional structures chalcopyrite are also present in some veins (Fig. 2b).Albite and apatite occur as eu- to subhedral crystals,formed during exhumation of the schist, have been

inferred to have formed at a range of conditions during apparently coeval with quartz. Chlorite is mainlypresent as individual inclusions in quartz, or as coarsethe retrograde evolution of the schist, from 200 to

300 °C and 0.6–3 kbar (Paterson, 1986; McKeag & plates lining vein margins (Fig. 2c). Late fracturescross-cutting the interiors of the veins are common,Craw, 1989; Craw & Norris, 1991).and are typically filled with chlorite and calcite(Fig. 2d).

Sample selection

The samples investigated for this study are examples Fluid inclusion petrographyof quartz-bearing veins from host rocks spanning arange of metamorphic grades and textural zones from

Quartzboth the Caples terrane and Torlesse terrane portionsof the Otago Schist. Locations, mineralogy, regional Fluid inclusions in quartz occurred in two distinct

settings. One setting (in which the majority ofgrade and relative age of formation of the veins aresummarized in Table 1. The lowest-grade rock samples inclusions studied occurred) was as isolated individual

inclusions or clusters. Inclusions in this setting wereare essentially undeformed volcanogenic greywackesthat have experienced intense burial metamorphism. generally larger in size than unequivocally secondary

inclusions, which are present as planar trails (Fig. 3).At somewhat higher grades (textural zone 2), there isgenerally a single pervasive fabric (S1), which is locally Both the isolated and trail hosted inclusions are either

two-phase (liquid plus vapour) or single phase (liquid)affected by D2 folds and crenulations. At the highestgrades studied (greenschist facies, textural zones 3 and at room temperature. The isolated inclusions generally

range in size from c. 5–10 mm, with some inclusions4), the second folding is sufficiently intense to producea marked second fabric. It is possible to distinguish up to 15 mm across (Fig. 3a,b). Rarely, apparently

vapour-dominated inclusions occur, but their failurepre-D1, post-D1 but pre-D2 (i.e. MP1) and post-D2veins from their structural relationships (criteria are to nucleate any liquid phase on cooling to <−130 °C

suggested that their vapour-dominated appearancegiven in Table 1). Veins at low grades generally post-date any pervasive fabric in their hosts, and preserve may be a result of the opening of aqueous inclusions

naturally or during the preparation of the wafers. Theprimary quartz growth textures of either fibre growthor coarse vuggy growth. Both types of growth may be precise identification of primary inclusions in meta-

morphic rocks is difficult as a result of a range ofpresent at a single locality, but fluid inclusions couldonly be investigated in the coarsely crystalline vein factors, the most critical being the possibility of

redistribution of inclusions of all generations duringquartz. At the higher grades studied here, many quartzveins are clearly metamorphic in that they pre-date or the annealing of quartz (Roedder, 1971; Swanenberg,

1980). However, inclusions isolated from planar trailswere synchronous with D2. These veins are generallyof coarsely crystalline quartz and may have patchy are likely to be the oldest inclusions in a given sample

(Crawford & Hollister, 1986), and limited microtherm-chlorite segregations at their margins or enclosedwithin them. Veins in the highest grade Otago Schist ometric data on unequivocally secondary inclusions

indicate lower densities (higher homogenizationrocks have commonly recrystallized to a fine sacchar-oidal quartz and the very small fluid inclusions present temperatures) than seen in the inclusions occurring in

isolated clusters, particularly when compared to thoseare probably not representative of peak metamorphicfluids (Craw & Norris, 1993). For this study, veins inclusions showing metastable ice melting (see below;

Table 2). This suggests the isolated inclusionswith coarse primary quartz textures were selected inthe field; the increasing re-crystallization of quartz (generally termed primary in this study) were trapped

at conditions closer to the vein formation conditionsveins with further metamorphism means that the workcannot be extended to the higher metamorphic grades than those trapped along healed microfractures. The

microthermometric properties measured in this studyof the western part of the schist belt.also correspond closely to the limited data oninclusions in matrix albite from the schist presented

Vein petrographyby Yardley (1982), giving further support for asyn-prograde metamorphism origin for the fluidThe veins studied are principally composed of quartz,

usually with varying amounts of chlorite and in some inclusions.instances carbonates (Table 1). In the majority ofcases, quartz appears to be of a single generation

Albitealthough highly strained crystals are common. Themajority of veins include wall rock fragments Isolated fluid inclusions in vein albite were observed

in sample W2.4. They show a similar range in size tocomprising phengite, chlorite, epidote, albite and


Fig. 2. Examples of vein petrography. (a) Back-scattered electron (BSE) image of vein hosted wall rock fragment. The fragment issurrounded by coarse grained chlorite. (sample L8.1). (b) BSE image of coarse grained, vein-hosted albite, chlorite and phengite(sample L2.1). (c) Photomicrograph of chlorite lining quartz vein margins taken in plane polarized light (sample L2.1). (d)Photomicrograph of chlorite and calcite filling a late-stage fracture in vein quartz. Taken with crossed polars (sample HB1A). Ab,albite; Cal, calcite; Chl, chlorite; Ep, epidote; Ms, muscovite; Qtz, quartz; Ttn, titanite.

those seen in quartz and occur in the central portions METHODSof eu- to subhedral crystals, which are generallyovergrown by an inclusion-free rim, indicating primary Analysis of fluid inclusionsor pseudo-secondary origin. The fluid inclusion assem-

Microthermometric measurements were made using ablage principally consists of two-phase, liquid plusLinkam TMS 90 temperature control unit and micro-vapour, inclusions, with single-phase inclusions formingscope stage, calibrated using natural CO2 inclusions ata much smaller proportion of the assemblage than in−56.6 °C, doubly distilled water at 0 °C, and a rangequartz. Secondary inclusions are rare.of pure solids including acetanilide at 114 °C andbenzanilide at 163 °C. Measurements were typicallymade of final ice melting temperature (Tm, ice) and

Calcitehomogenization temperature (T h). Due to the smallsize and low salinity of the inclusions, the first iceVirtually all the samples studied contained calcite. In

samples L2.1, HD1 and W2.4, calcite with measurable melting was rarely seen reliably, but was recordedwhen possible. Tm, ice measurements were made atfluid inclusions occurred from varying stages in the

paragenesis. In samples L2.1 and HD1, calcite occurred heating rates of 0.5 °C/min and T h measurements weremade at heating rates of 5 °C/min. The precision ofas narrow veinlets occupying fractures in the primary

quartz, whereas in sample W2.4 calcite occurred lining Tm, ice varied from ±0.1–0.2 °C, and the precision ofT h was generally around ±1–2 °C (determined fromthe vein margin. Fluid inclusions from calcite in all

veins showed similar characteristics, being two-phase, repeated analyses of individual inclusions).Crush-leach analyses of fluid inclusions were carriedliquid and vapour, inclusions, frequently with well-

defined negative crystal shapes, ranging from 5 to out on six samples of vein quartz from the Torlesseterrane, using the modified technique described in15 mm in size.


LaCl3 solution, and a second split with doubly distilledwater. The acidified LaCl3 leach solution was analysedfor various cations using ICP-AES, and Na and Kusing AA-FES. The doubly distilled water leachsolution was analysed for anions using a Dionex ionchromatograph and for Na using AA-FES.

Mineral chemistry

Initial characterization of samples was carried outusing a Camscan Series 4 scanning electron microscope(SEM), equipped with a Link EDS detector. Mineralanalyses were carried out using a Cameca SX-50electron microprobe with a 15 kV accelerating voltageand 10 nA beam current, and usually with a 1 mmbeam diameter. For mica analyses, the beam diameterwas widened to 2–4 mm in order to minimize decompo-sition during analysis.

RESULTS

Fluid inclusion analysis

The results of fluid inclusion microthermometry aresummarized in Table 2. In the limited number ofexamples where the first ice melting temperature wasobserved with any confidence, it was usually withinthe range −21 to −30 °C, although in some instanceslower temperatures were measured. This suggests anNaCl-dominated fluid, possibly with minor KCl orCaCl2. In nearly every sample, the vapour bubble in anumber of inclusions was eliminated on freezing andreappeared on metastable ice melting at temperaturesabove 0 °C (Roedder, 1967). Such inclusions occurredalongside inclusions showing stable ice melting andare interpreted to be of the same initial generation.Tm, ice was therefore only used to estimate fluidinclusion salinity if the vapour bubble was visible priorto final ice melting.

Tm, ice

and fluid inclusion salinity

The final ice melting temperatures for all inclusionsshowing equilibrium melting ranged from 0 to −4.6 °C,corresponding to a salinity range of 0–7.3 wt% NaCl

Fig. 3. Transmitted light photomicrographs of fluid inclusions eq. (calculated using the equation of Potter et al.,from veins in the Otago Schist. (a),(b) Apparently primary

1978). Within individual samples the salinity range isliquid water plus vapour (Lw+V) and liquid water (Lw)usually very restricted (2s of the mean typicallyinclusions in quartz, adjacent to a secondary trail of aqueous

inclusions (arrowed). (c) Trails of secondary inclusions in vein ranging from 0.6 to 2.4; Table 2), suggesting that aquartz. single fluid was responsible for vein formation in each

case, despite the wide range in T h (see below).Comparison of the data from each terrane indicates aBottrell et al. (1988) and Yardley et al. (1993). Samples

of vein quartz, 2–5 g in weight, were first crushed and possible difference in fluid salinity (Fig. 4). Fluidinclusions from the Caples terrane veins have salinitiessieved to a grain size of 500–1800 mm, meticulously

cleaned, and, due to the presence of calcite in many ranging from c. 0.4–3.1 wt% NaCl eq. with a mean of1.5±1.0 (2s, n=128), whilst fluid inclusions from theveins, each sample was then boiled in 6 M nitric acid,

followed by washing in doubly distilled water. The Torlesse terrane have higher salinities ranging from 1.0to 7.3 wt% NaCl eq. with a mean of 3.4±2.6 (2s; n=samples were then crushed using an agate pestle and

mortar. One split of powder was leached with acidified 267). A further variation in salinity is evident in the


Table 2. Summary of fluid inclusion data for vein samples from the Otago Schist, South Island, New Zealand.

Salinity (wt% NaCl eq.) T h (°C)

Sample Mineral n Mean 2s Range Mean 2s Modea log Br/Cl

L1.2 Quartz 25 1.6 0.8 162–349 229 120 195 −2.79

15 Metastable 89–170 118 40 117

Secondaryb 7 1.4 0.6 214–344 253 100

L2.1 Quartz 42 3.2 1.6 135–>280 202 70 165 −2.80

5 Metastable 118–129 123 10 123

Calcite 19 1.6 1.3 138–327 209 121 145

L8.1 Quartz 44 3.3 0.7 143–320 196 108 155 −3.10

20 Metastable 86–138 116 24 129

HA1 Quartz 33 2.4 1.0 143–340 219 124 175 −2.33

4 Metastable 120–152 138 32 140

Secondary 3 2.8 0.8 190–285 223 107

HB1A Quartz 37 3.3 1.1 124–350 221 126 175 −2.90

8 Metastable 116–152 128 26 118

HD1 Quartz 11 2.6 2.3 152–201 163 36 152

8 Metastable 127–159 141 24 134

W2.2 Quartz 27 1.4 0.7 146–>350 210 106 172

5 Metastable 116–160 138 34 139

Secondary 4 1.5 0.2 221–256 231 33

Albite 26 1.6 0.8 145–177 163 18 168

W2.4 Quartz 11 2.0 0.9 135–231 164 78 –

11 Metastable 122–146 134 14 135

Calcite 9 3.9 0.0 154–188 174 20 176

W3.1 Quartz 82 4.9 2.3 132–257 181 54 178 −2.43

Secondary 13 4.1 0.8 157–199 176 35 168

T1.2 Quartz 53 1.6 0.9 138–325 179 72 151

Secondary 9 1.9 0.7 168–230 198 46

S1.2 Quartz 11 1.1 1.0 137–295 203 106 –

7 Metastable 95–157 136 48 147

a Where no clearly defined mode occurred, the value given is the median value of the most frequently occurring interval of 10. b Secondary inclusions in quartz.

Torlesse terrane samples between lower-grade samples(from pumpellyite–actinolite facies to the chlorite zoneof the greenschist facies), where the salinity range isfrom 1.0 to 4.5 wt% NaCl eq. with a mean of 2.86±1.6(2s, n=185), and a single sample from the biotitezone of the greenschist facies (W3.1), where the salinityrange is from 3.5 to 7.3 wt% NaCl eq. with a mean of4.9±2.4 (Table 2). In the absence of samples of higher-grade rocks containing primary fluid inclusions, it isimpossible to say whether this difference is a realreflection of grade or a local artefact. The lowestsalinities from the Torlesse terrane samples are thosefrom L1.2 which may represent a later stage (post-D?)than the other veins analysed.

Homogenization temperatures

All the vein quartz samples studied show an extremelywide range in T h, typically with a non-Gaussiandistribution (Fig. 5). This is illustrated by the differ-ences between mean and modal T h in Table 2. Thetotal T h range for fluid inclusions showing stable icemelting from both terranes is from 124 °C to in excessof 350 °C, with many of the samples showing almostthe entire range. The modal values, however, occurtowards the lower end of this range, ranging from 163to 229 °C. The wide range of fluid inclusion T h isalmost certainly due to post-entrapment modificationof fluid inclusion volume, as the extremely narrowranges in salinity data from individual veins points toFig. 4. Variation in salinity of primary inclusions in quartz anda single fluid in each case. Experiments by Vityk &albite with host rock metamorphic grade for veins of the

Otago Schist. Bodnar (1995) suggest that fluid inclusion salinities do


Fig. 5. (a),(b) Comparison of homogenization temperatures from fluid inclusions showing stable and metastable ice melting fromquartz veins in the Otago Schist. (c) Comparison of fluid inclusions salinities from quartz and albite in sample W2.2. (d)Comparison of fluid inclusion homogenization temperatures between quartz and albite in sample W2.2.

not change during re-equilibration of fluid inclusion sample W2.2 (Fig. 5). In the case of fluid inclusionswhich showed metastable ice melting, T h values aredensities. Such modification of fluid inclusion density

may result from leakage of the inclusions due to fluid uniformly lower than those recorded from inclusionsshowing equilibrium ice melting (Table 2; Fig. 5a,b).overpressures (>1 kbar) developed as a result of either

prograde heating after the maximum metamorphic The T h range for the entire metastable data set is86–170 °C, with a mean of 125 °C and a mode ofpressure (Crawford, 1981; Crawford & Hollister, 1986)

or a rapid decrease in confining pressure during 118 °C. The fluid inclusions showing metastable icemelting are therefore interpreted as preserving highexhumation (Sterner & Bodnar, 1989; Vityk & Bodnar,

1995). Little evidence for leakage of inclusions via densities, reflecting the conditions of vein formation.The correlation between metastable melting and highdecrepitation was seen in this study, but plastic

deformation (stretching) of fluid inclusions in quartz density probably arises due to the small vapour bubblesize in high-density inclusions, as this favours elimin-may also occur under similar conditions, again leading

to modification of the fluid inclusion volume (Vityk & ation of the vapour bubble due to ice expansion onfreezing (Roedder, 1967). In sample W2.2, fluidBodnar, 1996). Modification of inclusion density may

also occur as a result of deformation during or after inclusions in quartz and albite show directly compar-able salinity distributions (Table 2; Fig. 5c,d).vein formation. In this region there is intense folding

of many of the veins, and fine-grained, recrystallized Homogenization temperatures, however, show anarrow range in albite (145–177 °C), compared to thequartz (cf. McKeag & Craw, 1989) occurs alongside

coarser-grained strained quartz (the main host for fluid much wider range seen in quartz (146 to >350 °C).This is interpreted as the result of greater strain takinginclusions).

The main evidence for post-entrapment modification place in quartz (and hence modification in inclusionvolume) than in albite. It should be noted, however,of fluid inclusion density comes from the T h values of

fluid inclusions which showed metastable ice melting, that a limited number of measurements of T h frominclusions in quartz showing metastable melting haveand from comparison of microthermometric data from

fluid inclusions in cogenetic quartz and albite in a range of 116–160 °C and a mean of 138 °C (Table 2).


This suggests that despite the differences in T h between of pressure, temperature and a compositional variable,such as the ratio aAl3+/(aH+)3 which will be controlledinclusions in quartz and albite, both populations have

undergone some re-equilibration. by the mineral assemblage, and hence the bulk rockcomposition, assuming equilibrium with quartz and anFluid inclusions in calcite were studied from two

veins: W2.4 where calcite occurs as discrete crystals aqueous phase. The empirical geothermometer ofCathelineau (1988) was, however, calibrated usingalong the vein margins, and L2.1 where calcite occurs

lining fractures in the vein quartz. In sample W2.4, chlorite precipitated growing from aqueous fluids inandesitic and feldspathic sandstone host rocks, whichinclusions in calcite show a mean salinity of 3.4 wt%

NaCl eq., which is higher than the quartz mean from is a comparable situation to that of the Otago Schistchlorite, albeit at lower pressure. In the absence of anythe same sample of 2.00 wt% NaCl eq. The mean

homogenization temperature in calcite from this sample other reliable geothermometer for the Otago Schistveins, the temperatures of chlorite formation have beenis comparable to that for the associated quartz, but

does not show as wide a range (calcite mean T h= estimated using the method of Cathelineau (tempera-tures given in Table 3). These temperatures must be174±20 °C (2s), quartz mean T h=164±78 °C (2s),

Table 2). In sample L2.1, calcite shows lower salinities regarded with a degree of caution for the reasonsdescribed above, and because the calculation carriedthan the coexisting quartz (1.64 wt% NaCl eq.), but

the T h values are again comparable (Table 2). out takes no account of the oxidation state of iron inthe chlorite and involves an extrapolation of thecalibration outside the range of the original data set.

Crush-leach analysisThe calculated temperatures range from 353 to 386 °C,with little variation within any individual sample (2sThe complex fluid inclusion assemblages (Fig. 3) are

obviously a limitation to the value of bulk crush-leach ranges from 6 to 18 °C).analyses, but there are no systematic differencesapparent between the salinities of apparently primary

White micasand secondary fluid inclusions, and so results may wellprovide information about fluids present during meta- Vein-hosted white micas occur mainly in wall rock

fragments enclosed in vein quartz, and were analysedmorphism. The crush-leach analyses on the Torlesseterrane veins gave exceptionally poor charge balance, from samples L1.2, L2.1, L8.1, HA1 and HB1A. The

results are summarized in Table 4. It can be seen thatand typically showed excessively high Ca contents.This is almost certainly a result of failure to remove the micas are extremely homogenous in composition

within each sample, and are typically phengitic muscov-all the carbonate present despite careful preparation,perhaps as a result of carbonate being preserved along ite. The mean Si content (calculated to 24 (O, OH, F))

for each sample ranges from 6.7 to 6.8 atoms permicrofractures in quartz which were only exposed oncrushing of the cleaned samples. For this reason, we formula unit. This provides an indication of the

phengite content of the micas as Si generally entersdo not present the results of cation analyses here.There is, however, little potential for contamination of muscovite via the phengite substitution. Velde (1965),

Massone (1980) and Massone & Schreyer (1987) havethe Cl and Br contents of the inclusion leachates byany of the minerals present within the vein samples. studied the potential use of the phengite content of

white mica as a geobarometer. Massone & SchreyerFor this reason, log Br/Cl ratios are presented inTable 2 along with the microthermometric data. The (1987) noted that the use of the phengite geobarometer

was only valid in the presence of the limitinglog Br/Cl ratios vary from −3.10 to −2.33, with fourof the analyses lying within ±0.3 log units of the ratio assemblage K-feldspar–phlogopite–quartz, but pointed

out that minimum pressures could be estimated usingseen in modern sea-water (log Br/Cl=−2.8; Drever,1988). phengite from other assemblages. Using their data and

assuming temperatures of 350–400 °C implies minimumpressures for phengite formation in the approximate

Mineral chemistryrange of 6–9 kbar.

ChloriteDISCUSSION

Representative analyses of chlorite are shown inTable 3. A number of authors have proposed that the

The P–T conditions of vein formationcompositional variability of chlorite from a particularsetting may be a function of temperature, and this has Understanding the P–T conditions of vein formation

gives information on the relation of the veining to thelead to a number of attempts to calibrate a chloritesolid solution geothermometer (Cathelineau & Nieva, bulk rock metamorphism which is important for

relating the fluid inclusion composition to the composi-1985; Cathelineau, 1988; review in De Caritat et al.,1991). De Caritat et al. (1991) pointed out that tion of any prograde metamorphic fluids. Isochores

for the modal T h and the mean fluid inclusion salinityempirical chlorite geothermometers are not universallyapplicable, as the composition of chlorite is a function for inclusions showing equilibrium ice melting were


Table 3. Mean electron microprobe analyses of chlorite from veins in the Otago Schist.

L1.2 L2.1 L8.1 HA1 HB1A W2.2 W2.4 W3.1 T1.2

(n=10) (n=9) (n=14) (n=14) (n=10) (n=18) (n=18) (n=20) (n=12)

Mean 2s Mean 2s Mean 2s Mean 2s Mean 2s Mean 2s Mean 2s Mean 2s Mean 2s

SiO2 25.16 0.47 24.86 0.37 25.04 0.64 25.21 0.34 24.81 0.39 25.72 0.68 25.86 0.47 24.39 0.60 27.01 1.21

TiO2 0.03 0.06 0.03 0.06 0.03 0.05 0.02 0.05 0.03 0.06 0.01 0.03 0.02 0.04 0.04 0.06 0.08 0.17

Al2O3 20.50 0.40 21.34 0.34 20.38 0.68 19.63 0.37 20.05 0.40 19.88 0.61 19.70 0.46 20.97 0.88 20.86 0.99

Cr2O3 0.01 0.03 0.02 0.04 0.02 0.07 0.02 0.06 0.02 0.05 0.01 0.03 0.04 0.07 0.03 0.06 0.14 0.39

FeOa 31.77 0.81 31.05 0.81 31.55 1.05 32.57 0.66 32.28 0.52 28.32 0.70 28.65 0.59 31.30 0.74 19.60 2.53

MnO 0.50 0.09 0.41 0.16 0.79 0.19 0.47 0.17 0.52 0.10 0.50 0.17 0.62 0.14 0.49 0.22 0.38 0.14

MgO 10.58 0.17 10.67 0.42 9.88 0.49 9.92 0.28 9.82 0.25 12.92 0.46 13.14 0.44 10.33 0.62 17.67 0.58

CaO 0.02 0.03 0.09 0.21 0.02 0.06 0.11 0.28 0.02 0.04 0.04 0.07 0.02 0.06 0.02 0.04 0.10 0.07

Na2O 0.01 0.01 0.01 0.03 0.03 0.04 0.05 0.11 0.02 0.03 0.02 0.03 0.02 0.03 0.02 0.03 0.01 0.03

K2O 0.02 0.05 0.01 0.03 0.00 0.01 0.02 0.04 0.01 0.02 0.02 0.05 0.01 0.02 0.02 0.06 0.01 0.02

H2Ob 11.09 0.12 11.12 0.09 10.97 0.18 10.95 0.11 10.91 0.09 11.14 0.13 11.20 0.11 10.96 0.08 11.51 0.46

Total 99.71 99.62 98.74 99.00 98.54 98.59 99.27 98.56 97.36

Cations

to 14 (O)

Si 2.72 0.04 2.68 0.03 2.74 0.05 2.76 0.03 2.73 0.03 2.77 0.06 2.77 0.04 2.67 0.06 2.81 0.07

Ti 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01

Al 2.61 0.04 2.71 0.04 2.63 0.06 2.53 0.05 2.60 0.05 2.52 0.07 2.49 0.06 2.70 0.12 2.56 0.10

Cr 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.01 0.03

Fe2+ 2.87 0.07 2.80 0.06 2.88 0.07 2.98 0.04 2.97 0.04 2.55 0.07 2.57 0.06 2.86 0.07 1.71 0.17

Mn 0.05 0.01 0.04 0.01 0.07 0.02 0.04 0.02 0.05 0.01 0.05 0.02 0.06 0.01 0.05 0.02 0.03 0.01

Mg 1.71 0.02 1.71 0.07 1.61 0.09 1.62 0.04 1.61 0.04 2.07 0.07 2.10 0.07 1.68 0.10 2.75 0.11

Ca 0.00 0.00 0.01 0.02 0.00 0.01 0.01 0.03 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.01 0.01

Ni 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.02 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.00 0.01

K 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00

OH 8.00 0.00 8.00 0.00 8.00 0.00 8.00 0.00 8.00 0.00 8.00 0.00 8.00 0.00 8.00 0.00 8.00 0.00

Total 17.97 17.96 17.95 17.98 17.97 17.97 17.99 17.98 17.89

T(°C)c 368 10 386 6 377 18 355 11 365 8 353 7 342 6 380 11 381 12

Temperatures were calculated using the equation of Cathelineau (1988). See text for discussion. a Total Fe as FeO. b Calculated assuming full hydroxyl site occupancy. c Calculted using the

chlorite geothermometer of Cathelineau (1988).

calculated using the data of Zhang & Frantz (1987) estimated for greenschist facies metamorphism ofsphalerite at Moke Creek (Jamieson & Craw, 1987).and the FLINCOR program of Brown & Hagemann

(1994). The modal T h was used due to the asymmetric However, the chlorite temperatures correlate well withprevious estimates of peak temperatures for pumpelly-distribution of temperatures seen in inclusions showing

stable ice melting. Isochores were also calculated for ite–actinolite facies metamorphism of 350–370 °C(Yardley, 1982) which are probably appropriate forthe mean T h of inclusions showing metastable ice

melting in each sample, assuming a salinity equal to these rocks. Pressures calculated using the chloritetemperatures coupled with isochores derived from thethat measured for inclusions showing equilibrium ice

melting. The use of the mean in these cases was T h of fluid inclusions showing metastable ice meltingsuggest pressures in the range 4.1–6.0 kbar. This isjustified by the Gaussian distribution of temperatures,

and the close correspondence between the mean and more consistent with the previous estimates for pro-grade metamorphic pressures and may be a bettermode (Table 2). Fig. 6(a) shows the calculated isochores

for both sets of inclusions along with the P–T estimate of the vein formation pressure if inclusionsshowing metastable ice melting do truly preserve theconditions indicated by the calculated chlorite tempera-

tures for veins from the Torlesse and Caples Terranes. original fluid density. Which P–T conditions bestreflect vein formation depends upon whether chloriteAssuming chlorite formation was synchronous with

fluid inclusion entrapment, P–T conditions can be compositions relate to the conditions under which theoriginal higher-density inclusions were trapped, or tocalculated from chlorite temperatures coupled with

fluid inclusion isochores. Using the isochores for fluid conditions on the lower-P, higher-T , or retrogradesections of the P–T path when fluid inclusioninclusions showing stable ice melting suggests pressures

of 3–4.2 kbar for a temperature range of 355–386 °C re-equilibration took place. Since in many instancesthe chlorite analysed includes individual crystalsin the Torlesse terrane, and a range of 3–4.6 kbar for

a temperature range of 342–381 °C in the Caples trapped in quartz and others lining the vein margins,it seems likely in these instances that chlorite chemistryterrane. These conditions are comparable to or slightly

lower than the peak chlorite zone temperature and reflects primary vein growth and hence the initial,denser fluid conditions. However, chlorite lining frac-pressure of 390 °C and 4.6±0.6 kbar estimated on the

basis of oxygen isotope data and the homogenization tures in association with calcite show very similarcompositions to chlorite included in quartz, suggestingtemperatures of fluid inclusions in albite (Devereux,

1968; Yardley, 1982) and the pressure of 4.5 kbar that fracturing and secondary chlorite growth occurred


Table 4. Mean electron microprobe analyses of white micas from included wall rock fragments in veins from the Otago Schist.

L1.2 L2.1 L8.1 HA1 HB1A

(n=11) (n=16) (n=13) (n=11) (n=4)

Mean 2s Mean 2s Mean 2s Mean 2s Mean 2s

SiO2 49.72 0.80 49.74 1.10 49.66 0.61 49.83 0.57 50.36 1.32

TiO2 0.04 0.06 0.03 0.05 0.05 0.07 0.06 0.08 0.03 0.08

Al2O3 27.82 0.47 29.64 0.77 28.19 0.80 26.00 1.11 26.79 1.25

FeOa 3.27 0.71 2.30 0.35 3.26 0.24 4.43 0.44 3.87 0.87

MnO 0.08 0.06 0.06 0.07 0.09 0.07 0.06 0.07 0.08 0.09

MgO 2.52 0.11 2.19 0.21 2.31 0.24 2.54 0.12 2.51 0.28

CaO 0.02 0.02 0.19 0.17 0.05 0.15 0.06 0.05 0.05 0.03

Na2O 0.18 0.05 0.30 0.10 0.16 0.05 0.22 0.41 0.17 0.08

K2O 10.61 0.37 9.94 0.69 10.81 0.32 10.68 0.38 10.38 0.18

H2Ob 4.32 0.09 4.36 0.09 4.36 0.06 4.28 0.07 4.33 0.12

F 0.21 0.14 0.21 0.15 0.15 0.10 0.17 0.13 0.16 0.14

Cl 0.02 0.03 0.03 0.03 0.02 0.02 0.04 0.02 0.04 0.02

total 98.81 1.11 98.99 0.91 99.11 0.76 98.39 0.67 98.76 1.34

O=F 0.09 0.09 0.06 0.07 0.07

O=Cl 0.00 0.00 0.00 0.01 0.00

Total 98.72 98.89 99.04 98.31 98.68

Cations to 24 (O, OH, F)

Si 6.74 0.06 6.67 0.09 6.72 0.07 6.84 0.09 6.84 0.14

Ti 0.00 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.00 0.01

Al 4.45 0.05 4.69 0.12 4.50 0.12 4.20 0.16 4.29 0.21

Fe2+ 0.37 0.08 0.26 0.04 0.37 0.03 0.51 0.05 0.44 0.10

Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

Mg 0.51 0.02 0.44 0.04 0.47 0.05 0.52 0.03 0.51 0.05

Ca 0.00 0.00 0.03 0.02 0.01 0.02 0.01 0.01 0.01 0.00

Na 0.05 0.01 0.08 0.03 0.04 0.01 0.06 0.11 0.04 0.02

K 1.84 0.07 1.70 0.13 1.87 0.05 1.87 0.07 1.80 0.04

OHb 3.91 0.06 3.91 0.07 3.93 0.04 3.92 0.06 3.92 0.06

F 0.09 0.06 0.09 0.06 0.06 0.04 0.07 0.06 0.07 0.06

Cl 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.00

Total 17.97 17.87 17.98 18.02 17.93

a Total Fe as FeO. b Calculated by difference assuming a full hydroxyl site occupancy.

at very similar temperatures but lower pressures than that the kinetics of re-equilibration of phengite areextremely sluggish, and that phengites of severalthe original trapping. We interpret the conditions

indicated by the isochores for metastable ice melting compositions may be preserved within a individualrock. Bearing this in mind it is possible that theand chlorite temperatures as the most likely indication

of the environment of vein formation. The range of phengite represents compositions formed during earlyhigh-pressure metamorphism (Yardley, 1982) prior todensities shown by inclusions with stable T m, ice almost

certainly represents a combination of inclusions which vein formation at peak metamorphic temperatures.The relation of the P–T estimates for vein formationstretched under different conditions during exhumation

of the schist, and the extent to which stretching of to previous estimates of the P–T –t path of the OtagoSchist is shown in Fig. 6(b). On the basis of isochoresinclusions resulted in measured fluid densities which

corresponded to the ambient conditions at the time. from the inclusions showing metastable ice melting,this suggests that the veins were formed duringThe internal overpressure under which any particular

inclusions will decrepitate is dependent on both the size prograde pumpellyite–actinolite to lower greenschistfacies metamorphism, prior to the peak greenschistand shape of the inclusions, and on the fluid composition

(Bodnar et al., 1989; Vityk & Bodnar, 1995). These facies temperatures of 380–420 °C (Yardley, 1982) andbelow the peak pressure observed in the schist.factors also probably influence the conditions under

which stretching of inclusions occurs, and hence may The density of the majority of inclusions thenre-equilibrated via stretching, due to internal overpres-lead to a range of fluid densities that form on stretching

and which depend on the properties of individual sures developed during exhumation of the schist. Itshould be noted, however, that the uncertainties in theinclusions. Assuming fracturing of the vein and second-

ary chlorite formation accompanied at least some chlorite temperatures, and also in the peak conditionsof the samples investigated, make it impossible to rulere-equilibration, fluid inclusions homogenizing around

the modal temperature may have stretched at pressures out the interpretation of the veins as truly peakmetamorphic.1–2 kbar lower than the original trapping conditions.

All these pressure estimates are lower than theminimum pressure estimated from the phengite content

The evolution of salinity in the metamorphic fluidof white micas. These micas represent fragments of thewall rock, rather than phases which have precipitated It has been suggested by Yardley (1996) and Yardley

et al. (1997) that sedimentary pore fluids evolvefrom the vein fluid. Massone & Schreyer (1987) noted


derived fluids from sub-aerial portions of the complexalong highly permeable fault zones. Such sedimentarypore fluids must form the precursors of metamorphicfluids, although the cation, and to a much lesser extentthe anion, composition of the fluid will be modified byreactions taking place during both diagenesis andmetamorphism as the fluids re-equilibrate with theirhost rocks under changing P–T conditions (Eugster,1981).

The model for metamorphic fluid evolution in anaccretionary environment outlined above is consistentwith the salinities and Br/Cl ratios of fluid inclusionsfrom veins in rocks from prehnite–pumpellyite faciesto the chlorite zone of the greenschist facies in theTorlesse terrane. In these samples, fluid inclusionsalinities are very close to that of sea-water (modernsea-water salinity c. 3.3 wt% NaCl equivalent), andthe Br/Cl ratio of inclusion leachates also deviatesvery little from the modern sea-water value. This leadsus to suggest that the metamorphic fluid has evolveddirectly from a marine sedimentary pore fluid withlittle modification of its salinity and anion signature,although the cation composition may have beenextensively modified during equilibration with the hostrock. The single vein sample taken from the biotitezone of the greenschist facies (W2.2) shows highersalinities and a lower Br/Cl ratio than those seen inthe other veins, suggesting some process must haveoccurred to add chloride to the fluid. In the absenceof evaporites and under conditions where phaseseparation is impossible, possible mechanisms for this

Fig. 6. (a) Isochores for modal T h for fluid inclusions showing increase in salinity include the loss of Cl from hydrousequilibrium and metastable ice melting. The range of minerals in the vein host rocks, particularly apatiteconditions derived on the basis of temperatures calculated from

(Smith & Yardley, 1999), and the loss of water fromchlorite coexisting with the host quartz is shown in each case.the pore fluid due to hydration reactions. All these(b) Comparison of the P–T conditions from (a) with previous

estimates of prograde metamorphic conditions and inferred mechanisms require low water5rock ratios and iso-P–T paths for the Otago Schist. Paths 1, 2 and 3 refer to lation of the pore fluid from a large fluid reservoir indifferent areas of the schist from Yardley (1982): 1, Routeburn-

order to have a significant effect on the pore fluidFohn Lakes district (cf. Craw & Norris, 1991—hypotheticalchemistry. Such conditions are likely to have occurredburial path for Otago and Alpine Schists); 2, Queenstown-

Kawarau district; 3, Haast River. See text for discussion. during the metamorphism of the Otago Schist, as theporosities of sediments in accretionary settings may beas low as 5% once depths of 3–4 km have beencontinuously from formation waters in deep basins to

metamorphic fluids during burial, heating and meta- reached (Bray & Karig, 1985), and may be muchlower under metamorphic conditions. The loss of watermorphism, with marked contrasts in fluid chemistry

occurring between rocks with sedimentary protoliths from the pore fluid due to hydration reactions can beruled out as being unlikely during prograde metamor-from continental margin and accretionary settings.

Tectonically, South Island, New Zealand, represents phism of a pelitic schist, although retrogression fromhigh-pressure assemblages to greenschist facies assem-an exhumed accretionary complex, with the Torlesse

and Caples Terranes resulting from the accretion and blages and hydration reactions in metabasites withinthe sedimentary package may have had some effectmetamorphism of continental and arc-derived sedi-

ments and volcanic rocks during the Cretaceous. Pore (Yardley, 1981). It seems likely, therefore, that theaddition of Cl due to Cl–OH and Cl–F exchangefluids within a modern accretionary complex near

Barbados (ODP Leg 110 Scientific Party, 1987; Gieskes reactions between the fluid and hydrous minerals,especially relict detrital Cl-apatite, at low water5rocket al., 1990) show a decrease in salinity with depth,

resulting from mixing of the sea-water-derived pore ratios could be the main mechanism contributing tomodification of the fluid salinity.fluid with low-salinity fluids derived from the dehy-

dration of clays and other hydrous minerals deeper in In vein samples from the Caples terrane fluidinclusions, salinities are uniformly lower than sea-the complex. Further dilution of the pore fluid may

take place as a result of the channelling of surface- water salinities. Craw & Norris (1991) suggested that


Table 5. Summary of previous data on fluid inclusions in Au- and W-mineralized veins from the Otago Schist.

Salinity Inferred Inferred

Deposit Reference Mineralization Fluids (wt% NaCl eq.) Th (°C) trapping T (°C) P (kbar)

Greenschist facies metamorphic veins

Lake Hawea (1) W H2O–NaCl 0 180–260 350–390 3–4

Macraes (2) Au (+sulphides) H2O–NaCl >2 130–160 300–350 2.5–3.5

W

Invincible vein (3) Au H2O–NaCl 0 140–175 300–370 2–5

Post-peak metamorphic veins

Bonanza (2) Au (+sulphides) H2O–NaCl >4.3 175–270 200–250 0.6–1

H2O–NaCl–CO2 #9 200–270

Nenthorn (2) Au (+sulphides) H2O–NaCl >5 195–270 200 <0.3

H2O–NaCl–CO2 >5 >230

Barewood (2) Au (+sulphides) H2O–NaCl 0.5–2.8 170–200 300 1–2

W

Bendigo (4) Au (Sb) H2O–CO2–NaCl >5 258±20 290 0.25

W 233±13 260

Glenorchy (5) Au H2O–NaCl >0 140–180 260–320 1.5–3.5

W

Shotover (6) Au H2O–NaCl 1.7–2.7 145–175 150–200 0.4–1

H2O–CO2–NaCl <1

References: (1) Craw & Norris (1991); (2) McKeag & Craw (1989); (3) Hay & Craw, 1993; (4) Paterson, 1987; (5) Craw & Norris, 1991; (6) Craw (1989).

the fluid responsible for the formation of gold–scheelite metamorphic veins in this study, suggesting a directrelationship between these deposits and the metamor-(Au–W) veins was derived from metamorphic dewater-

ing of either the vein host rocks, or rocks deeper in phic vein fluid studied here. The pressures inferred forthe formation of these veins are lower than thosethe sedimentary pile. Such fluids were of very low

salinity (Tm, ice >−1 °C), and hence fluids derived inferred from inclusions showing metastable ice meltingin this study, but this may be due to the effects of fluidfrom the dewatering of clays, zeolites and chlorite may

have contributed to the dilution of fluids of sea-water inclusion stretching and re-equilibration recognized inthis study. These deposits are relatively minor, however,salinity to give the low-salinity fluids seen in Caples

terrane veins. This difference in fluid salinity between in comparison to vein deposits formed after peakmetamorphism, which are typically hosted in exten-the two terranes may relate to the original provenance

of the host sediments. Craw & Norris (1991) reported sional structures that cut the schistosity at a highangle. Reported fluid salinities for post-metamorphica review of analyses of the water content of quartzofeld-

spathic schists and metabasites, and inferred a decrease veins range from low values, similar to those reportedhere from syn-metamorphic veins, to relatively highof 4 wt% in the water content of metabasic rocks on

metamorphism to the oligoclase zone of the greenschist values (up to >9 wt% NaCl eq.). The main difference,however, is the presence of CO2 as a major componentfacies, and a decrease of 2–3 wt% in the water content

of quartzofeldspathic rocks. This suggests that there in post-metamorphic veins, whilst in the syn-metamorphic veins analysed in this study the CO2may be a greater potential for dilution of the pore

fluid by dewatering in metabasic terranes, or in terranes contents are low enough to be undetected by microther-mometric techniques. Post-metamorphic veins in thepredominantly composed of immature sediments with

a basic to intermediate sedimentary provenance, and Alpine schist to the west of the area studied areinterpreted to have formed from circulation of meteoricmay account for the differences in fluid salinity between

fluid inclusions in veins from the Caples and Torlesse waters during exhumation of the schist, with the CO2contained in fluid inclusions potentially derived fromterranes.the oxidation of graphite (Jenkin et al., 1994), as wellas, or instead of, metamorphic reactions at deeper

The relation of the metamorphic fluid to ore-forminglevels in the sedimentary pile as suggested by McKeag

solutions& Craw (1989) and Craw (1992). Whatever the originof the carbonic component in the late-orogenic fluids,A number of mesothermal deposits of gold, or gold

and scheelite, in the Otago Schist have been interpreted they apparently represent a separate stage of fluidmigration, unrelated to that studied here.as forming from metamorphic fluids (Paterson, 1986;

McKeag & Craw, 1989; McKeag et al., 1989; Craw &Norris, 1991; Craw, 1992; Hay & Craw, 1993;

CONCLUSIONSMackenzie & Craw, 1993). Their characteristics aresummarized in Table 5. It can be seen that those veins Syn-metamorphic quartz veins in the Otago Schist

were formed during prograde pumpellyite–actinoliteinterpreted as syn-metamorphic (Lake Hawea scheel-ite-bearing veins, Macreas Au-bearing veins and the to lower greenschist facies metamorphism, at sites of

fracturing due to high pore fluid pressures. SubsequentInvincible gold vein) contain fluid inclusions which arevery similar to those reported from unmineralized lower-pressure, higher-temperature greenschist facies


Bishop, D. G., 1972. Progressive metamorphism from prehnite–metamorphism and decreases in pressure duringpumpellyite to greenschist facies in the Dansey Pass area,exhumation of the schist led to the development ofOtago. Bulletin of the Geological Society of America, 83,

overpressures in fluid inclusions trapped in quartz, and 3177–3198.consequent re-equilibration of the fluid inclusion Bishop, D. G., 1974. Stratigraphic, structural and metamorphic

relationships in the Dansey Pass area, Otago. New Zealanddensities. The major result of these processes is theJournal of Geology and Geophysics, 17, 301–335.occurrence of a high-density fluid inclusion population

Bishop, D. G., Bradshaw, J. D., Landis, C. A. & Turnbull, I. M.,showing metastable ice melting, representing conditions 1976. Lithostratigraphy and structure of the Caples Terraneclose to the trapping conditions of the fluids, and a of the Humboldt mountains, New Zealand. New Zealand

Journal of Geology and Geophysics, 19, 827–848.second population of inclusions with a wide range inBodnar, R. J., Binns, P. R. & Hall, D. L., 1989. Synthetic fluiddensity (all lower than the density observed from the

inclusions – VI. Quantitative evaluation of the decrepitationfirst population) which show stable ice melting, rep- behaviour of fluid inclusions in quartz at one atmosphereresenting inclusions which have re-equilibrated under confining pressure. Journal of Metamorphic Geology, 7,

229–242.later metamorphic conditions.Bottrell, S. H., Yardley, B. W. D. & Buckley, F. B., 1988. AThe salinities of fluid inclusions from veins from the

modified crush-leach method for the analysis of fluid inclusionTorlesse terrane of the Otago Schist are very similarelectrolytes. Bulletin Mineralogique, 111, 279–290.

to those of modern sea-water, as are their Br/Cl ratios. Bowers, T. S. & Helgeson, H. C., 1983. Calculation of thethermodynamic and geochemical consequences of non-idealThis suggests that the metamorphic pore fluid wasmixing in the system H2O–CO2–NaCl on phase relations inderived from the pore waters of the sedimentarymetamorphic systems: metamorphic equilibria at high tem-protoliths of the rocks with little modification of theperatures and pressures. American Mineralogist, 68, 1059–1075.

anion content, and hence total dissolved salts. The Bray, C. J. & Karig, D. E., 1985. Porosity of sediments incation composition is likely to have been modified accretionary prisms and some implications for dewatering

processes. Journal of Geophysical Research, 90, 768–778.considerably by interaction with the host rock. TheBrown, P. E. & Hagemann, S. G., 1994. MacFlincor: a computermain cause of changes in the anion content of the fluid

program for fluid inclusion data reduction and manipulation.is believed to be due to the release of halogens from In: Fluid Inclusions in Minerals: Methods and Applications (edsdetrital minerals such as Cl-apatite (Smith & Yardley, De Vivo, B. & Frezzotti, M. L.), pp. 231–250. VPI Press.

Cathelineau, M., 1988. Cation site occupancy in chlorites and1999). In contrast, fluid inclusions from the Caplesillites as a function of temperature. Clay Mineralogy, 23,terrane show salinities uniformly lower than modern471–485.

sea-water. This may be an effect of the different original Cathelineau, M. & Nieva, D., 1985. A chlorite solid solutionsedimentary provenance of the two terranes. The geothermometer. The Loz Azufres (Mexico) geothermal

system. Contributions to Mineralogy and Petrology, 91,Torlesse terrane consists primarily of quartzofeld-235–244.spathic sediments, which have less potential to contrib-

Craw, D., 1989. Shallow level, late stage gold mineralisation inute to the dilution of the pore fluid than have the Sawyers Creek, Shotover valley, northwest Otago, Newextensively hydrated andesitic and more basic sedi- Zealand. New Zealand Journal of Geology and Geophysics,

32, 385–393.ments of the Caples terrane.Craw, D., 1992. Fluid evolution, fluid immiscibility and gold

deposition during Cretaceous–Recent tectonics and uplift ofthe Otago and Alpine Schist, New Zealand. Chemical Geology,ACKNOWLEDGEMENTS98, 221–236.

Craw, D. & Norris, R. J., 1991. Metamorphogenic Au–W veinsThis work was supported by NERC grant GR3/9002.and regional tectonics: mineralisation throughout the upliftB.W.D.Y. would like to particularly acknowledge thehistory of the Haast Schist, New Zealand. New Zealand

help of S. Cox, who helped greatly with the field Journal of Geology and Geophysics, 34, 373–383.sampling, took him to some of the localities and Craw, D. & Norris, R. J., 1993. Grain boundary migration of

water and carbon dioxide during uplift of garnet-zone Alpineprovided considerable logistical support. B.W.D.Y. isSchist, New Zealand. Journal of Metamorphic Geology, 11,also indebted to D. Norris, R. Sibson and D. Coombs371–378.(University of Otago) for their advice and stimulating Crawford, M. L., 1981. Fluid inclusions in metamorphic rocks—

discussions. At Leeds, D. Banks assisted with the fluid low and medium grades. In: Short Course in Fluid Inclusions:Applications to Petrology. (eds Hollister, L. S. & Crawford,inclusion crush-leach analyses and E. Condliffe helpedM. L.), pp. 157–181. Mineralogical Association of Canada.with the microprobe analyses. We would also like to

Crawford, M. L. & Hollister, L. S., 1986. Metamorphic fluids:thank R.C. Burruss and P.M. Ashley for their construc-the evidence from fluid inclusions. In: Fluid–Rock Interactions

tive reviews, and D. Robinson for editorial comments During Metamorphism. (eds Walther, J. V. & Wood, B. J.),on an earlier version of this manuscript. Advances in Physical Geochemistry, 5, 1–35. Springer–Verlag,

New York.De Caritat, P., Hutcheon, I. & Walshe, J. L., 1991. Chlorite

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