Disturbed 40Ar-39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine

Pergamon Geochimica et Cosmochimica Acta, Vol. 61, No. 21, pp. 4655-4669, 1997

Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved

0016-7037/97 $17.00 + .oo

PII SOO16-7037( 97) 003504

Disturbed 40Ar-39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine, Western Australia: Evidence for argon loss associated with

post-mineralisation fluid movement

ADAM J. R. KENT’** and T. CAMPBELL MCCUAIG’.+ ‘Research School of Earth Sciences, The Australian National University, Canberra 0200, Australia

‘Deparhnent of Geological Sciences, University of Saskatchewan, Saskatoon, S7N 5E2, Canada

(Received May 6, 1996; accepted in revised form July 21, 1997)

Abstract-Hornblende and biotite that formed during gold mineralisation at the Scotia mine, Western Australia, have erratic 4oAr-3gAr release spectra and total gas ages that are -200-900 million year younger than the ca. 2600-2620 Ma minimum age of gold mineralisation, as given by 4oAr-3gAr plateau (muscovite) ages of crosscutting pegmatite dykes. Analysed homblendes are dominated by magnesio hornblende but also contain small domains of ferro-actinolitic hornblende, actinolitic hornblende, and actinolite. Biotite also appears to be substantially altered to chlorite along cleavage planes.

Relatively young apparent ages and high K/Ca ratios of argon released from homblendes at temperatures less than -1000°C are interpreted to be the result of degassing of contaminant biotite. However, this cannot totally explain the young ages of homblendes. Gas fractions released at furnace temperatures above lOOO”C, where the effect of biotite degassing is demonstrably negligible, still have apparent ages that are -200-900 million years younger than the age of muscovite from post-gold pegmatite dykes.

The close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples (less than a few metres in some cases) is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally-driven volume diffusion. Given a suitable thermal history, argon loss could occur preferentially in homblendes if ( 1) the closure (for slow cooling) and blocking (for reheating) temperatures of hydrothermal homblendes were lower than published estimates, as has been observed in structurally complex metamorphic homblendes and/or (2) the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated. However, neither of these interpretations can easily explain the large variation in hornblende ages.

It is instead suggested that argon loss occurred during mineral-fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite. There is abundant geological evidence for the passage of such a fluid at the Scotia mine, including the presence of numerous late brittle fractures containing retrogressive low-temperature mineral assemblages. Late fluid movement is probably related to Proterozoic erogenic activity along the nearby southeastern margin of the Yilgarn Craton.

The difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes (with respect to ore zones) preventing fluid egress to muscovite samples. The variations in ages of hydrothermal minerals are probably related to the extent of fluid/mineral interaction as this is a function of parameters, such as fluid/rock ratio, fluid P-T-X conditions, permeability, and mineral microstructures that may vary on short time and length scales. Recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 4oAr-3gAr systematics in environments, such as many hydrothermal ore deposits, where minerals may be exposed to fluids after crystallisation. Copyright 0 1997 Elsevier Science Ltd

1. INTRODUCTION

The 40Ar-39Ar technique constitutes one of the most widely applied isotopic dating techniques in the geosciences and is well suited to the investigation of many geological problems (e.g,. McDougall and Harrison, 1988). Commensurate with the wide application of the technique has been the detailed investigation of the behaviour of argon systematics in appro- priate minerals, such as hornblende, muscovite, biotite, and

*Present address: Department of Geological and Planetary Sci- ences, California Institute of Technology, Mail Stop 170-25, Pasa- dena, California 91125, USA ([email protected]).

+Present address: Etheridge Henley Williams, Suite 34A, 25 Wal- ters Drive, Osborne Park, Western Australia 6017, Australia.

feldspar. This has especially been with respect to the extent and nature of argon loss in these minerals during both laboratory (e.g., Hanson et al., 1975; Wartho et al., 1991; Lee, 1993) and geological (e.g., Turner et al., 1969; Harrison and McDougall, 1980; Wartho, 1995; Lister and Baldwin, 1996) processes and has provided a theoretical background on which to base interpretations of 4oAr-3qAr data.

To date, understanding of geological argon loss has been largely directed at that which occurs during slow cooling or episodic reheating of minerals. This has been described using theory based on volume diffusion whereby argon retentivity of a specific mineral is dependent on factors such as the diffusivity of argon and size of individual diffusion domains within the mineral in question and the time-temperature his-

4655

4656 A. J. R. Kent and T. C. McCuaig

tory experienced by the sample (e.g.. Turner, 1969; Dodson. 1973; McDougall and Harrison, 1988; Lister and Baldwin, 1996). For slowly cooled samples (subject to the sample experiencing a cooling rate that is linear with the reciprocal of temperature) the dependence of argon diffusion on temperature is conveniently expressed in terms of the closure temperature of the mineral (Dodson, 1973); at temperatures above the closure temperature diffusive argon loss is too rapid to allow accumulation of argon within the mineral. It is only at temperatures below the closure temperature that argon accumulation can occur, and the geological clock can commence to mark time. For minerals that have been held at high temperatures for extended periods of time or experienced transient reheating events. following the usage of Lister and Baldwin ( 1996), the term blocking temperature is used to describe the temperature above which significant argon loss occurs.

It is also known that argon loss can occur by mechanisms other than volume diffusion. During laboratory heating in vacua phase transitions and dehydroxylation (in hydrous minerals) are often more important than volume diffusion in dictating argon degassing behaviour (e.g., Hanson et al.. 1975; Gaber et al.. 1988: Lee et al.. 1991: Wartho et al., 1991; Lee. 1993). In natural samples it is more difficult to document argon loss by alternate mechanisms. However. there is emerging recognition that, in addition to volume diffusion, argon loss can be the result of chemical re-equilibration of potassic minerals (Wartho. 1995 )

This is especially germane to the interpretation of geochronological data from many hydrothermal ore deposits where the structures responsible for localising hydrothermal activity and mineralisation may often act as locii for further fluid flow after mineralisation. In this contribution we report an example of the disturbance of argon systematics in hydrothermal biotite and hornblende that formed during gold mineralisation at the Scotia gold mine, located adjacent to the southeastern margin of the Yilgam Craton in Western Aus- tralia (Fig. 1). In this location argon loss in biotite and hornblende was sufficient to produce variable apparent ages -200-900 million years younger than the time of gold mineralisation. However, this did not result in argon loss in nearby pegmatite muscovites. We suggest that, rather than argon loss occurring via volume diffusion controlled by the thermal history of this region. argon systematics of hydrothermal minerals can be best understood via argon loss during interaction between hydrothermal minerals and infiltrat- ing post-mineralisation retrograde fluids. In this situation the thermal history of the sample may play a subsidiary role to the hydrothermal history and parameters governing the extent of fluid-mineral interaction in controlling argon systematics.

2. THE SCOTIA MINE

The Scotia gold mine is located at the southern end ot the Norseman-Wiluna greenstone belt within the Norseman Terrane of the Archaean Yilgam Craton (Fig. I : Swager et al., 1990). The mine is situated about 70 km northwest of the Fraser Front, which delineates the southern boundary of the Yilgam Craton with the Proterozoic Fraser Province to the south (Fig. I; Gee, 1979).

........... ......... ......... ......... ........ . ........ ....... ....... ....... .......

.... . . ....... ....... ....... ....... .......

...... .

........ ......... ...... . .........

...... .........

....... ........... ....... .........

...........

.........

......... ........

3 Granit oid

fl Kalgoorlie Terrane greenstone sequences

Norseman Terrane greenstone sequences 1 Undiffe rent iated greenstone sequences

a Proterozoic Fraser Orogen

/Major fault

Maior lode oold deoosit

Fig. 1. Regional map of the Kambalda-Norseman region, showing the location of the Scotia deposit with respect to the location of the southeastern margin of the Yilgam Craton and other major gold deposits.

Mineralisation at the Scotia mine is hosted within the Woolyeenyer Formation, a thick sequence of tholeiitic basalts. gabbroic dykes and sills. and high-MgO tholeiitic dykes. A gabbroic intrusion into the Woolyeenyer Formation at Norseman has a U-Pb age of 2714 5 5 Ma (Hill et al., 1992 ) . In the Scotia mine area this formation has been meta- morphosed to middle amphibolite facies (McCuaig et al., 1993 ) Hornblende-plagioclase geothermometry indicates peak metamorphic temperatures between 530-700°C for the rocks hosting the Scotia deposit (McCuaig et al., 1993). Although the timing of metamorphism is not well con- strained, it is probable that this either occurred broadly con- temporaneously with intrusion of large volumes of granitoids into the southern Norseman-Wiluna Belt between 2660-

Disturbed MAr/‘9Ar release spectra in a gold mine 4657

muscovite - garnet pegmatite

7/6/70 Ramp

Fig. 2. Field relations between a mineralised quartz-diopside-homblende-biotite bearing shear zone and garnet-muscovite pegmatite dyke underground at the Scotia gold mine (7/670 ramp).

2690 Ma (Binns et al., 1976; Hill et al., 1992) or during metamorphism within the lower crust of the Yilgarn Craton at ca. 2630-2650 Ma (Nemchim et al., 1994; Kent et al., 1996).

Gold mineralisation at Scotia occurs within a NNW to N striking, east-dipping ductile shear zone and extends discon- tinuously for 3 km along strike (Thomas et al., 1990). Ore is mostly hosted within multiple quartz-diopside-calcite veins. Within ore zones multiple veining events have produced complex composite veins with alternating bands of quartz, diopside, calcite, amphibole, biotite, and microcline (Mc- Cuaig et al., 1993 ) . Alteration selvages surrounding individual veins consist of an outer zone of biotite-homblende- plagioclase, which is replaced with increasing proximity to veins by hornblende-plagioclase-quartz, then diopside- quartz-calcite ( t microcline, zoisite, actinolite) assemblages. In composite veins, however, this zonation is often complicated by overlapping alteration envelopes. Micro- probe analyses reveal that hydrothermal amphibole compositions within diopside-quartz-calcite veins are actinolite to actinolitic hornblende, whereas hydrothermal amphiboles in altered basalt adjacent to veins are actinolitic hornblende to magnesio hornblende. These amphiboles are all coeval with vein emplacement and associated hydrothermal activity. Gold occurs as occasional small (<lo pm) grains, often in association with pyrrhotite and chalcopyrite. Vein and alteration assemblages are consistent with minimum temperatures for gold mineralisation in excess of 500°C (McCuaig et al., 1993).

Quartz-muscovite-albite +- biotite ? garnet pegmatites crosscut ore bodies in many locations and in all observed examples postdate mineralisation (Fig. 2; McCuaig et al., 1993). Ore zones and pegmatites were locally cut by E-W dolerite dykes dated elsewhere in the Norseman Terrane at ca. 2400 Ma (Fletcher et al., 1987). Both ore zones and pegmatites are disrupted by multiple generations of late brittle fractures (Thomas et al., 1990; McCuaig et al., 1993). These structures are variably filled with assemblages of chlorite-quartz-sericite-albite and albite-prehnite, fine-grained cataclasite, and gypsum. The brittle nature of these faults,

the multiple generations of these structures as indicated by crosscutting relationships, and the variable low-temperature alteration assemblages associated with them, collectively indicate that brittle deformation occurred after, and at lower pressures and temperatures, than gold mineralisation and pegmatite intrusion. Retrogressive minerals similar to those found in late fractures also occasionally occur as small flakes and grains (generally +5 pm) on grain boundaries and cleavage surfaces of prograde hydrothermal minerals.

3. SAMPLE DESCRIPTION

Muscovite was separated from pegmatite samples taken from two different localities: the 7/670 ramp (sample 91- 237) and 71440 north drive (sample ScPegS). In both these locations pegmatites occur as shallowly dipping dykes which crosscut ore zones. Field relations between pegmatite and ore zone at the 7/670 ramp are detailed in Fig. 2. Muscovite in pegmatites occurs mainly in books of large crystals, generally 0.5-2.5 cm across and up to 10 cm long. Garnet and albite were also separated from sample 91-237 and analysed for Sm-Nd, the results of which are detailed in Kent ( 1994) and Kent et al. ( 1996). The grain size of analysed muscovite samples was 400-200 brn for &Peg8 and 250- 150 pm for 91-237.

Hornblende and biotite were separated from three samples: 91-205, taken from within an ore zone on the 7/670 ramp; sample ScPvAm2, taken from the ore zone in the 7/ 440 north drive; and sample 91-233 from the 4/550 ramp. All samples consist of banded homblende-quartz-calcite- pyrrhotite-diopside-biotite-plagioclase altered metabasalt. In sample 91-205 hornblende and biotite rich bands alternate with quartz-plagioclase-calcite dominated bands. Both homblende and biotite form a foliation parallel to mineralogical banding although regions of decussate biotite and sheaflike whorls of hornblende are also occasionally apparent. Larger crystals of hornblende also crosscut foliated biotite. Electron microprobe investigations reveal that homblendes are predominantly magnesio hornblende (Table 1)) although there is some chemical variations, generally best expressed in A1203, MgO, and CaO contents. Typical compositions and overall compositional ranges are given in Table 1. K20 contents are typically 0.20-0.25 wt%. In thin section, biotite grains have pale brown to yellow-brown pleochroism and an irregular ragged birefringence commonly associated with partial retrogression to chlorite along cleavage planes. This can be observed in backscattered electron images which indicate that interlayer chlorite intergrowths are ubiquitous within biotite and are generally on the l-2 pm scale (Fig. 3). Consistently low K20 (5-7%; Table 1) from electron microprobe measurements also implies that biotite from this sample has suffered some degree of retrogression to chlorite. This is a common feature in many Archaean gold deposits formed at similar pressure-temperature conditions. The timing of chlorite retrogression is largely unknown.

Samples 91-233 and ScPvAm2 consist of banded homblende-quartz-calcite-pyrrhotite bearing altered metabasalt. Hornblende occurs as elongate prismatic crystals, largely in hornblende dominated bands, but also intergrown with quartz and calcite. As with sample 91-205, hornblende is


Table 1. Electron microprobe data for minerals from the Scotia mine.

111 91-205

Magnesio hornblende

Atom wt% prop.

PI 91-205

Ferri-actinolitic hornblende

Atom wt% prop.

131 91-205 Biotite

Atom wt% prop.

[41 91-233

Magnesio hornblende

Atom wt& prop.

I51 91-233

Magnesio hornblende

Atom wt% prop.

[61 91-237

Muscovite

Atom wt% prop.

[71 Amphibole

range

wt%

SiOz 48.08 6.900 52.28 7.416 34.13 5.220 48.95 7.044 45.27 6.550 47.18 6.280 45.27-52.28 Ti02 0.17 0.017 0.15 0.017 1.03 0.119 0.56 0.061 0.29 0.035 <0.08 0.13-0.56 Al&s 9.99 1.691 3.08 0.511 17.41 3.139 8.68 1.469 12.74 2.174 33.49 5.254 12.74-3.08 Cr20, 0.08 0.009 0.08 0.009 0.36 0.043 0.16 0.009 0.37 0.044 <0.09 0.08-0.37 Fe0 13.61 1.630 16.43 1.952 17.42 2.228 14.10 1.694 15.34 1.862 4.84 0.538 12.10-15.34 MnO 0.29 0.035 0.28 0.034 co.09 0.23 0.026 0.33 0.044 0.12 0.014 0.23-0.34 MgO 12.71 2.716 14.51 3.069 15.96 3.638 12.59 2.697 10.62 2.287 <0.08 10.62-15.36 CaO 11.77 1.812 11.05 1.679 <0.06 12.37 1.190 11.48 1.784 <0.07 11.05-12.37 Na,O 1.02 0.285 0.12 0.017 <O.ll 0.78 0.216 I .38 0.392 0.42 0.109 0.12-1.38 K20 0.22 0.043 0.08 0.034 7.01 1.368 0.21 0.035 0.24 0.044 11.21 1.904 0.08-0.24 Cl 0.00 0.01 na 0.02 0.03

9?26 14.099 0.00-0.02

Total 97.93 15.138 98.06 14.738 93.32 15.755 98.60 15.161 98.09 15.216

WCs 0.021 0.008 >134 0.020 0.025 0.008-0.035

Note: Atomic proportions calculated using twenty-two (biotite, muscovite) and twenty-three (hornblende) oxygen atoms. na-not analysed.

largely parallel to mineralogical banding but occasional sheaflike whorls are apparent. Magnesio hornblende is again dominant, although actinolitic hornblende and ferro actinolitic hornblende domains are present on many large magnesio hornblende crystals (Table 1).

In all samples amphibole compositions vary on small spatial scales. For example, in Fig. 3, within the space of a few microns at the grain boundary, magnesio hornblende with - 10 wt% A1203 changes to ferro-actinolitic hornblende with -3 wt% A&O3 (analyses 1 and 2 in Table 1) All hydrothermal hornblende analysed are predominantly magnesio homblende, although small regions of actinolite or actinolitic hornblende occur at grain boundaries, or as small regions (- 10 pm) in the interior of larger grains (Fig. 3). The observation that compositional variations within individual homblendes mirror those observed between different alteration zones (e.g., vein-distal hornblende-dominated assemblages and vein-proximal actinolite and actinolitic homblende; see above and McCuaig et al., 1993) suggest that fine-scale variations in amphibole compositions are largely related to superposition of different alteration facies during progressive hydrothermal alteration. Thus, magnesio homblende which originally formed at the margin of a quartz- calcite vein was converted to actinolite and actinolitic homblende at grain boundaries during emplacement of subsequent nearby auriferous quartz-calcite veins. This interpretation implies that variations in amphibole composition formed during the original hydrothermal mineralisation and alteration process, and not during pegmatite intrusion or the post- pegmatite fracturing and retrogression event described above. This is consistent with the observation that amphibole is not part of the retrogressive mineral assemblage (e.g., chlorite, sericite, prehnite, gypsum, albite). However, it must also be noted that given the complexity of amphibole compositional variations, it is impossible to rule out some degree of chemical modification of amphibole (similar perhaps to

that described by Wartho, 1995 ) during subsequent retrogression.

Grainsizes of analysed amphibole separates were 400- 200 km for ScPvAm2 and 180- 100 km for 91-205 and 91-233.

Stable isotope data is also reported for samples SC163- 66, SC358-33, X358-40b, SC-358-55, and SC358-99 from hydrothermal alteration assemblages from drill cores through the Scotia orebody. These samples are similar in appearance to those described above. Hydrothermal quartz, amphibole and clinopyroxene were separated from these samples, and preliminary stable isotope analyses obtained for these mineral separates as part of a larger study ( McCuaig, 1996) are reported here.

4. ANALYTICAL METHODS

Mineral separates from all samples were prepared to >99% purity, using standard heavy liquid, magnetic separation, and handpicking techniques. Mineral compositions were analysed at the California Institute of Technology using a JEOL JXA-733 Superprobe equipped with five crystal spectrometers and a Tracer Northern EDS detector using an accelerating voltage of 15 kV and beam current of 15 nA. Data was reduced with the CITZAF PRZ correction algorithms using natural and synthetic standards (Armstrong, 1995). Errors in major elements are on the order of 2% whereas errors for minor elements are in the order of 10%. Amphibole compositions were calculated using the EMP-AMP software. Backscattered electron and EDS X- ray maps were acquired with a Camscan Series II Scanning Electron Microscope operating at 15 kV.

Samples were analysed by the J0Ar-39Ar technique at two separate facilities. the Research School of Earth Science at the Australian National University, Canberra, Australia (samples 91-205, 91-233, 91-237) and Queen’s University, Kingston, Ontario, Canada (samples ScPeg8 and ScPvAm2). Although a detailed interlaboratory study between these two facilities has not been conducted, this does not influence the final conclusions of this study which is more con- cerned with resetting of argon systematics rather than a detailed comparison of the isotopic ages of these samples. Samples analysed at the Australian National University were irradiated in a cadmium-

Disturbed 40Ar/39Ar release spectra in a gold mine 4659

Fig. 3. SEM images of hornblende and biotite from sample 91-205. (a) Backscattered electron image of hornblende crystal with biotite (shown at the left and bottom of the image) showing a chlorite-filled fracture in top right comer and lighter-coloured ferri-actinolitic hornblende rim around the bottom of the crystal. Analysis 1 from Table 1 is from the core of this crystal and analysis 2 is from the rim. (b) Backscattered electron image of biotite crystal showing alteration to darker chlorite along cleavage planes. (c) and (d) EDS X-ray maps for aluminium and silicon for the same field of view shown in (a). Note Al-poor, Si-rich rim, and relatively Al-rich, Si-poor chlorite inclusions. The Si- and Al-poor regions in the bottom right and top left of the hornblende crystal are pits in the section.

lined aluminium canister for two 24 day cycles in the HIFAR reactor, Lucas Heights, Australia along with fluence monitor minerals GA- 1550 biotite for biotite and muscovite and 77-600 hornblende for hornblende. Argon was extracted from irradiated minerals in a dou- ble vacuum resistance furnace and analysed isotopically using a VG MM12 mass spectrometer. Analytical details are described in more detail in Kent and McDougall (1995). Samples analysed at Queen’s University were irradiated for 116 h in position 5c of the water- moderated, enriched uranium research reactor at McMaster Univer- sity, Hamilton, Ontario, Canada (samples ScPeg8 8 and ScPvAm2)

using international hornblende standard HB3GR as a fluence monitor. Argon was extracted using a Lindberg tubular furnace and ana- lyzed using a MS-10 mass spectrometer.

Argon isotopic ratios measured from both facilities were corrected for neutron induced production of argon isotopes from K and Ca. Ages from plateau-like segments in age spectra are calculated using gas-fraction weighting of individual steps and associated errors, and, in order to make ages from the two separate facilities comparable, ? 0.5% error in the irradiation parameter J is incorporated into all total gas and plateau-like segment ages. All ages were calculated


Table 2. ““Ar-39Ar data for minerals from the Scotia gold mine.

Temp (“C) 40Ar/3YAr 36Ar/39Ar

ScPeg8 Muscovite (400-200 pm) (0.0161 g, J = 0.02325)

“Ar/39Ar Vol 39Ar Fraction

(X10-* cm’) 39Ar (%) “OA@ 4oAr*/39ti Age t la’ WCs

500 208.04 0.2763 600 150.31 0.0121 700 140.80 0.0002 750 140.60 0.0008 800 140.43 0.0009 850 138.72 0.0010 900 139.79 0.0004 950 140.04 0.0010

1000 140.01 0.0011 1200 141.36 0.0138

91-237 Muscovite (250- 150 pm) (0.0014 g, J = 0.01963)

500 166.96 0.0460 600 174.92 0.0672 650 165.65 0.0131 690 168.79 0.0078 730 167.05 0.0051 770 165.69 0.0041 810 163.58 0.0033 840 162.84 0.0032 870 163.63 0.0017 900 163.54 0.0009 930 163.97 0.0021 960 164.26 0.0025

1000 163.91 0.0005 1050 164.45 0.0017 1100 169.24 0.0075 1350 168.79 0.0078

ScPvAm2 Hornblende (400-200 pm) (0.0504 g, J = 0.02323)

600 68.143 0.0183 2.054 0.196 4.2 92.1 700 71.673 0.0076 2.025 0.333 7.2 96.9 800 75.429 0.0024 1.082 1.044 22.5 99.1 900 88.613 0.0036 5.280 1.200 25.9 98.8 950 93.182 0.0040 4.891 0.919 19.8 98.7 975 99.367 0.0121 5.290 0.159 3.4 96.4

1000 100.228 0.0126 5.367 0.188 4.1 96.3 1050 104.837 0.0236 6.268 0.111 2.4 93.4 1200 123.412 0.0099 15.86 0.484 10.4 97.6

9 l-205 Hornblende ( 180- 100 pm) (0.0517 g. J = 0.01944)

600 87.069 0.0079 700 57.351 0.0237 750 67.406 0.0176 800 62.414 0.0088 850 57.663 0.0042 890 40.491 0.0027 930 62.260 0.0034 970 89.572 0.0072

1010 91.870 0.0067 1040 89.235 0.0083 1080 119.95 0.0124 1150 96.162 0.0106 1300 89.751 0.0114 1500 149.03 0.2490

0.009 0.032 0.2 60.8 0.079 0.268 1.3 97.6 0.006 4.550 22.6 99.9 0.001 2.363 11.7 99.8 0.000 1.781 8.7 99.8 0.001 2.285 11.3 99.8 0.000 3.995 19.8 99.9 0.000 3.173 15.7 99.8 0.000 1.516 7.5 99.8 0.128 0.225 1.1 97.1

126.38 2473 2 74 146.66 2677 k 3 140.76 2620 k 3 140.36 2616 2 1 140.15 2614 ? 3 139.45 2607 2 1 139.65 2609 2 1 139.75 2610 ? 2 139.65 2609 k 3 137.23 2585 2 10

(140.51)d (2618 + 5)d

0.2503 0.0354 0.8 91.9 0.4599 0.0202 0.4 88.7 0.2438 0.0305 0.7 97.7 0.1245 0.0527 1.1 98.6 0.1493 0.1058 2.3 99.1 0.0630 0.1950 4.3 99.2 0.0596 0.2782 6.0 99.4 0.0481 0.3068 6.7 99.4 0.0552 0.4167 9.1 99.7 0.0529 0.7412 16.1 99.8 0.0412 0.7656 16.7 99.6 0.0509 0.6088 13.3 99.5 0.0530 0.6014 13.1 99.9 0.0446 0.3577 7.7 99.7 0.4182 0.0233 0.6 98.7 0.1245 0.0527 1.1 98.6

153.39 2506 + 8 155.13 2521 + 8 161.81 2579 t 5 166.49 2618 ? 8 165.53 2610 2 3 164.45 2601 k 2 162.60 2586 ? 3 161.86 2579 _’ 2 163.10 2590 k 2 163.24 2591 ? 2 163.33 2592 ? 3 163.51 2593 2 3 163.74 2595 2 3 163.94 2597 + 2 167.08 2623 ? 16 166.49 2618 ? 8

(163.34)* (2592 ? 4)d

62.97 1626 2 12 69.63 1736 2 7 74.81 1817 2 3 88.19 2011 2 2 92.63 2071 k 3 96.52 2122 + 11 97.22 2131 k 7 98.70 2150 1?1 7

122.87 2434 k 2 (89.43)d (2028 k 4)d

0.15 0.16 0.29 0.06 0.06 0.06 0.06 0.05 0.02

2.856 0.0430 1.2 73.4 64.04 1459 k 5 0.16 5.530 0.1199 3.4 88.6 51.01 1243 2 4 0.08

12.68 0.1109 3.1 93.9 63.91 1457 + 2 0.04 7.878 0.1376 3.9 96.9 60.85 1408 t 2 0.06 3.575 0.3077 8.6 98.3 56.86 1343 t 1 0.13 2.491 0.5503 15.6 98.5 39.97 1037 5 1 0.18 4.843 0.7513 21.2 99.1 61.90 1425 IIZ 1 0.09

13.96 0.5895 16.5 99.0 89.60 1819 -e 1 0.03 11.92 0.4068 11.4 99.0 91.73 1846 2 2 0.04 13.45 0.1189 3.3 98.6 88.85 1810 2 2 0.03 22.15 0.1562 4.3 98.6 120.17 2173 -+ 2 0.02 17.84 0.1855 5.2 98.4 95.85 1898 2 2 0.03 17.55 0.0787 2.2 98.0 98.08 1813 t- 3 0.03 13.43 0.0036 0.1 51.5 77.48 1657 k 60 0.03

(71.5)” (1571 2 2)”

Disturbed a.4r/39Ar release spectra in a gold mine

Table 2 (Continued)

4661

Temp (“0

Vol j9Ar Fraction (X lo-’ cm3) 39Ar (%) @A+ “Ar*P9A? Age ? la’ IUCa

91-233 Hornblende (180-100 pm) (0.0380 g, J = 0.01938)

600 113.95 0.1390 2.129 0.0485 1.8 64.1 700 81.842 0.0271 5.931 0.1349 5.1 90.8 750 97.089 0.0201 23.15 0.1040 3.9 96.0 800 88.655 0.0065 9.790 0.1386 5.2 98.8 840 86.375 0.0038 5.395 0.1781 6.7 99.2 880 89.464 0.0016 2.424 0.4398 16.5 99.7 920 95.672 0.0023 2.247 0.4909 18.5 99.5 960 116.26 0.0054 8.570 0.4108 15.5 99.3

1000 128.54 0.0056 11.93 0.2815 10.5 99.5 1040 129.15 0.0085 11.49 0.1741 6.5 98.8 1100 139.73 0.0140 27.02 0.0894 3.3 98.8 1300 118.34 0.0127 16.03 0.1626 6.1 98.6 1500 134.09 0.0668 17.65 0.0112 0.4 86.4

91-205 Biotite (150-90 pm) (0.0019 g, J = 0.01952)

600 55.376 0.0440 2.063 0.0321 3.6 76.8 650 39.567 0.0150 0.091 0.1019 11.6 89.0 700 77.614 0.0153 0.096 0.0529 6.0 94.3 730 121.91 0.0151 1.961 0.0433 4.9 96.5 770 104.16 0.0089 5.607 0.0469 5.3 97.9 810 97.917 0.0061 3.733 0.0599 6.8 98.5 850 87.566 0.0046 2.673 0.0957 10.8 98.7 890 84.104 0.0036 1.313 0.1353 15.4 98.8 930 94.407 0.0043 1.393 0.1111 12.5 98.7 970 99.734 0.0048 2.909 0.1200 13.6 98.8

1010 95.797 0.0061 3.451 0.0639 7.3 98.4 1050 95.076 0.0014 8.050 0.0199 2.2 96.3

73.14 1593 2 14 0.21 74.65 1614 2 4 0.08 94.81 1881 + 6 0.02 88.23 1798 t 2 0.05 86.06 1770 2 2 0.08 89.34 1813 Ifr 2 0.19 95.32 1888 t- 2 0.20

116.14 2127 ‘- 2 0.05 129.07 2261 ? 2 0.04 128.71 2257 2 2 0.04 140.77 2374 + 2 0.02 118.10 2148 2 2 0.03 117.42 2140 2 17 0.03

(103.79)d (1989 + 2)d

42.59 1092 2 6 35.22 944-c 6 73.20 1601 ? 4

117.76 2153 2 3 102.44 1982 + 4 96.69 1913 ? 3 86.58 1785 t 2 83.21 1741 + 2 93.32 1871 2 2 98.75 1938 +- 3 94.51 1886 + 5 92.13 1856? 9 (8.98)” (1751 2 4)d

a Calculated at 0°C 1 atm. b 4oAr* = (“A&,,, - 40Ar~,-phe#‘Ar,otll. ’ Quoted error does not include the error in the irradiation parameter J. ’ Calculated with weighting by mass 39Ar released per step.

using the decay constants of Steiger and Jager (1977), and final errors are given as 2a including the error in J. KlCa plots are given for amphibole only as the low contents of Ca in biotite and muscovite, coupled with the correction for 37Ar decay associated with the long irradiation (and subsequent 4-6 month storage period) for Archaean samples did not allow accurate determination of the KI Ca ratio for these minerals.

Oxygen and hydrogen isotope analyses were carried out at the University of Saskatchewan. Hydrogen isotope compositions were determined using the uranium method of Godfrey ( 1962) as modi- fied by Kyser and O’Neil ( 1984). Oxygen was extracted using the BrFS technique described by Clayton and Mayeda ( 1963). Duplicate analyses indicate a reproducibility of ? 0.2%0 for oxygen values and ? 5%0 for hydrogen isotope values. 6l*O values of 9.6%0 for NBS- 28 quartz and SD values of -65%0 for NBS-30 biotite were obtained using these techniques. S’*O and 6D values are reported relative to V-SMOW.

5. RESULTS

5.1. “APPEAR Analyses

4oAr-39Ar data for all samples is given in Table 2 and spectra and K/Ca plots (for homblendes only) are shown in Fig. 4.

X1.1. Hydrothermal minerals

91-205 Hornblende. The spectrum from this hornblende sample is more erratic than the other two homblendes analysed, although a general progression to older ages with progressive gas release is evident. The first 60% of argon released has apparent ages between 1000 and 1500 Ma, this is followed by steps with ages around 1800- 1900 Ma (with the exception of step 11). The total gas age of this sample 1572 ? 12 Ma. As with other hornblende samples K/Ca values are higher in early stages of gas release (maximum of 0.2) and have uniformly lower values around 0.02-0.03 for the remainder of gas release.

ScPvAm2 Hornblende. This sample shows increasing apparent step ages with progressive gas release, ranging from 1700 to 2434 Ma. The total gas age of this sample is 2028 t 14 Ma. K/Ca values are high for the first 35% of gas released (maximum 0.4) and then have uniform values around 0.1 for the next 60% of argon released. The final step has a K/Ca ratio of 0.02.

91-233 Hornblende. This sample is similar to ScPvAm2 hornblende with gradually increasing step ages with progres-


2500

2300

2100

1900

1700

1500

1300

1100

900 I

I-

1 T-

1.0

91-205 Hornblende

L 1,

0.2 0.4 0.6 0.6 1.0

Fraction 3sAr released

wca

ScPvAm2 Hornblende 2300-

2100-

isoo- I

ASP 1700. (W

1500

1300

1100 I

Q~Ook------ 0.2 0.4 0.6 0.8 1.0 Fraction 3sAr reteased

2500- 91-233 Hornblende

2300.

rj , , , , , , , I ,

0.0 0.2 0.4 0.6 0.6

Fractkm 39Al reb&sed Fraction 3gAr released

0.0 0.2 0.4 0.6 0.6

Frtiton 39AJ retensed

p-j _

o.“o~o bction 39~r releared

2500 91-205 Biotite

2300

Fig. 4. “Ar- ‘“AT spectra and K/Ca plot from hydrothermal biotite and hornblende and pegmatite muscovite samples analysed from the Scotia gold mine.

sive gas release. Apparent ages show a general trend of released and uniformly low (0.02-0.04) for the final 40% increasing age with progressive gas release from a minimum of argon released. of ca. 1600 Ma to a maximum age of 2373 Ma. The total 91-205 Biotite. Argon released in the early stages of gas gas age of this sample is 1989 t 12 Ma. K/Ca values are release ( 15%) has apparent ages in the order of 1000 Ma. generally higher (maximum of 0.2) in the first 60% of argon and the remaining steps form a saddle shaped spectrum. The

Disturbed 4oAr/39Ar release spectra in a gold mine 4663

2620

2590

2560

2530

1 25001 . m m I . . . . . 25ooJ * . I . , , * , ,

0.0 0.2 0.4 0.6 0.9 0.0 0.2 0.4 0.6 0.6

2590. 2615f14Ma

256Oj

2530.

3

Fraction 38~ released

Fig. 4. (Continued)

Fraction SAI released

total gas age for this sample is 175 1 + 12 Ma, and the saddle shaped portion of the spectrum (steps 4- 12) has an apparent age of 1878 2 12 Ma. The saddle shape of the spectrum is similar to that shown by spectra from biotite with interlay- ered chlorite alteration in Lo and Onstott ( 1989).

5.1.2. Pegmutites

In contrast to hydrothermal minerals, both pegmatite muscovite samples analysed exhibit 40Ar-3gAr spectra with well developed plateau-like segments which contain >90% of argon released. For sample 91-237, a plateau-like segment between steps 3 and 16, comprising 93% of total 39Ar released, corresponds to an age of 2593 + 12 Ma. Muscovite from sample ScPeg8 exhibits a plateau-like segment between steps 3 and 19 comprising 97% of 3gAr released, and this corresponds to an age of 2615 + 14 Ma. Within analytical error (at 20) both ages are indistinguishable.

5.2. Stable Isotope Analyses

Stable isotope results for hydrothermal minerals from the Scotia mine are shown in Table 3. Hydrothermal minerals

Table 3. Oxygen and hydrogen isotope data for minerals from the Scotia mine.

Water 6’*0 Temperature” yield

Sample and mineral 6”O 6D quartz (“C) (wt%)

&Peg8 muscovite 6.9 -58 2.0 ScPvAm2 hornblende 6.6 -70 11.5 510 1.2 91-205 hornblende 6.9 Sc163-66A amphibole 6.8 -65 11.6 510 - SC163-66B amphibole 6.8 11.1 560 SC358-99 amphibole -62 1.3 SC358-33 amphibole 6.4 -65 1.9 SC358-33 diopside 6.3 SC358-4OB diopside 6.4 11.0 500 SC358-55 diopside 6.4

All values reported in %o relative to V-SMOW. ’ Temperatures calculated using quartz-hornblende fractionation factor of Bottinga and Javoy (1973) and quartz-clinopyroxene fractionation factor of Clayton and Keiffer (1991).

show a limited range in 6 “0 values (diopside = 6.3-6.4%0; hornblende = 6.4-6.9%0; quartz = 11 .O- 11.6%0). Fraction- ations between quartz-hornblende and quartz-clinopyroxene are consistent with formation of gold-related alteration assemblages at temperatures between 500°C and 560°C. These calculated temperatures corroborate those estimated from phase equilibria considerations, which also indicate formation of Scotia gold-related hydrothermal alteration assemblages at temperatures > 500°C (McCuaig et al., 1993).

Hydrogen isotopic compositions of hydrothermal amphiboles also show little variation, with 6D values ranging from -70 to -61%0. Using a temperature range of 500-56O”C, the quartz-water fractionation curve of Clayton et al. (1972) for oxygen, and the hornblende-water fractionation curve of Suzuoki and Epstein (1976) for hydrogen, yields 6”O and SD values for the mineralising fluid of 8.8 to 9.%0, and -29 to -43%0, respectively. 6 ‘*OB,id and SDnuid estimates for other deposits within the Yilgam Craton are similar to these values, although less data is available for hydrogen isotopes (Golding et al., 1992). Calculated 6 180~uid values for deposits at Norseman and Kambalda, to the north of the Scotia deposit, range between 5 and 10%0 and SDauid for the Vic- tory-Defiance deposit at Kambalda is estimated at -30 t 12%0. Although the data available for the closest deposit to the Scotia mine (the Princess Royal deposit at Norseman) does suggest that this deposit has a different GDBuid value than Scotia (- 10 + 4%0), it is unsure as to the degree that minerals from Princess Royal are effected by the later metamorphic effects of a large mafic intrusion in this mine (Golding et al., 1992).

6. DISCUSSION

6.1. Tbe Age of Pegmatite Intrusion and Gold Mineraiisation

The 2593 + 12 and 2615 ? 14 Ma ages of plateau-like segments from pegmatite muscovites are within error of the 2620 + 36 Ma estimate of the age of sample 91-237 from Sm-Nd (garnet-albite) analysis (Kent et al., 1996). Intrusion of pegmatite dykes of comparable composition to those at Scotia occurred elsewhere within the southern Yilgam Craton at ca. 2630 Ma, and on a regional scale, post-mineralisation


pegmatite intrusion at this time was contemporaneous with an episode of partial melting, granitoid intrusion. and metamorphism within the lower-middle crust (Kent et al., 1996).

Field relations such as those detailed in Fig. 2 unambigu- ously demonstrate that pegmatite intrusion occurred after gold mineralisation at the Scotia mine, thus the ca. 2600- 2620 Ma 40Ar- j9Ar from pegmatite samples provide a minimum estimate of the timing of mineralisation. Gold mineralisation at Scotia prior to ca. 2600-2620 Ma is also consistent with the deposit forming during a widespread gold mineralisation event in the Yilgarn Craton at ca. 2630 Ma (e.g.. Groves, 1993a.b; Kent et al., 1996).

6.2. Interpretation of QAr-39Ar Spectra from Hydrothermal Biotite and Hornblende

‘0Ar-39Ar spectra from hydrothermal biotite and homblende are characterised by a lack of plateau-like features and erratic step ages in the earlier stages of gas release (Fig. 4). Isotope correlation plots for the hydrothermal minerals analysed are also erratic, and this, coupled with the radio- genie composition of argon released from these samples. precludes these from providing useful information.

Several factors may contribute to the unusual shape of the spectra from biotite and hornblende samples analysed: ( 1 ) The general increase of step ages with progressive degassing in hornblende samples may be due, at least in part, to the presence of an argon concentration gradient. However, given the erratic nature of the spectra and the evidence that argon release during hornblende step heating may not reflect inter- nal argon concentration variations (e.g., Hanson et al., 1975: Gaber et al., 1988; Wartho et al., 1991; Lee et al., 1991; Lee. 1993 ) , no attempt has been made here to model hornblende spectra in terms of diffusive argon loss. (2) Degassing of intergrown phases or exsolution lamellae with differing argon release behaviour, coupled with recoil of ‘9Ar during sample irradiation, is a documented cause of erratic spectra in biotite and hornblende (e.g., Harrison and Fitz Gerald. 1986; Lo and Onstott, 1989). This phenomenon can also significantly lower apparent closure temperatures (Harrison and Fitz Gerald, 1986; Lo and Onstott, 1989; Baldwin et al., 1990). Backscattered electron images of biotite from sample 9 l-205 reveal numerous intergrowths of chlorite along cleavage planes on the 1-2 pm scale (Fig. 3 ) . The saddle-shaped spectrum exhibited by biotite from sample 91-205 is also similar to spectra observed in other samples where biotite is known to be finely intergrown with chlorite (e.g., Lo and Onstott, 1989)) including samples from other Australian Ar- chaean gold deposits (A. J. R. Kent unpubl. data. 1994 ), Likewise backscattered images of hornblende from sample 91-233 and 91-205 show that small (generally 1 - 10 pm) domains of actinolite or actinolitic hornblende occur within larger magnesio hornblende dominated grains (Fig. 3 ). (3 ) Variable amounts of contaminating K-bearing phases in bulk mineral separates can also effect argon systematics, with observed spectra and apparent ages deriving from superim- posed degassing of the primary phase and the contaminant (e.g., Rex et al., 1993). For this study contamination of hornblende separates by biotite is most important, due to the difference in K contents of these minerals (Table 1) and the

2500

I

0

Field of amphibole _.--- K/Ca values A.

0 1000 -a

0.00 0.05 0.10 0.15 0.20 0.25 0.30 K/Ca

30 B.

25 t

0 4

n 91-205 / Hornblende

91-233 Hornblende

400 64nJ 800 loo0 12W 1400 1600 Temperature W)

Fig. 5. (a) Variation of apparent ages with K/Ca for gas release steps for hornblende samples from the Scotia gold mine. For each sample steps with low K/Ca tend to have older apparent ages. (b) Fraction of released “Ar with progressive heating for biotite and hornblende from sample 91-205 and hornblende 91-233.

common intergrowth of these phases in the analysed samples. Despite every precaution it is probable that some degree of contamination occurs in bulk mineral separates, and even small degrees of contamination of a hornblende mineral separate by biotite may have a marked effect on apparent ages during the lower temperature stages of gas release (< - 1000°C; Rex et al., 1993; Wartho, 1995). For the hornblende samples analysed in this study, high K/Ca values in the earlier, lower temperature, stages of gas release generally coincide with relatively young apparent ages (Fig. 5a, Table 2) and are probably due to degassing of contaminant biotite. However, as shown in Fig. 5b, at temperatures above


- 1000°C outgassing of biotite is largely complete and bio- studies of the age of muscovite with respect to other indica- tite-derived argon should not contribute to the apparent ages tors of cooling history, such as other mineral ages (e.g., of argon released from hornblende above this temperature Jager, 1979; Blankenburg et al.) 1989), or fluid inclusion (also similar to the findings of Rex et al., 1993). K/Ca ratios temperatures (Snee et al., 1988). Similarly calculations by calculated for argon released from hornblende samples at Lister and Baldwin ( 1996). using re-evaluated data of Rob- temperatures above 1000°C are in the range of primary val- bins ( 1972) for the diffusion parameters of argon in musco- ues for hornblendes measured by electron probe (Fig. Sa), vite, an infinite slab model for argon diffusion, and a range of further indicating that minimal biotite-derived argon has con- thermal histories and diffusion domain sizes (see discussion tributed to the argon released at these temperatures. For this below), suggest that the closure temperature of argon in study, contamination of hornblende by biotite may be most muscovite is between about 300-420°C (for pressures be- important for sample ScPvAm2, as the grainsize of analysed tween 0 and 5 kbar-commensurate with pressure estimates crystals is larger than that for the other hornblende samples for the Scotia deposits; McCuaig et al., 1993). Lister and analysed (400-200 pm compared to 200- 180 pm for 91- Baldwin ( 1996) also estimate the blocking temperature of 205 and 91-233), and the degassing temperature schedule muscovite to be in the order of 255-285°C from the basis provides minimal resolution of the gas released above of numerical calculations, again using a range of thermal 1000°C (Table 2). histories.

For sample 91-205 the > 1000°C fraction consists of five steps comprising 27% of the total argon released and has a mean age of 2246 t 12 Ma and K/Ca ratio of 0.03. For sample 91-233 the mean age of this fraction (6 steps and 27% of released 39Ar) is 1745 + 11 Ma with a K/Ca of 0.03. For ScPvAm2 the >lOOO”C fraction consists of only one step (10% of the total 39Ar released) with an age of 2434 & 14 Ma and K/Ca of 0.02.

6.3. Argon Loss in Hydrothermal Minerals

It is important to note that, irrespective of the causes of the erratic spectra, all hydrothermal minerals analysed appear to have suffered a considerable, but varying, degree of argon loss. Biotite from sample 91-205 is almost 900 million years younger than the ca. 2600-2620 Ma minimum age of gold mineralisation and hydrothermal alteration. For homblendes tbe ages of argon fractions released at temperatures above 1000°C are significantly younger (- 900-200 million years) than the minimum age of gold mineralisation. In addition to argon loss, the ages for the >lOOo”C fractions from the hornblende samples are also highly variable, with ages cov- ering a range of almost 700 million years.

Empirical and experimental studies suggest that for a given thermal history the closure temperature of biotite is slightly lower than that of muscovite (e.g., McDougall and Harrison, 1988; Lister and Baldwin, 1996). Thus, at the Scotia mine, biotite could have experienced thermally-driven diffusive argon loss without nearby muscovite being disturbed if the Scotia mine region was either: ( 1) reheated to a temperature above that of biotite but below muscovite blocking temperature (for a range of thermal histories the blocking temperature of biotite is estimated at - 230-255°C by Lister and Baldwin, 1996), or (2) the thermal history of the Scotia region involved extremely protracted cooling (albeit sufficient to generate a ca. 900 Ma difference in apparent ages of muscovite and biotite) between the muscovite and biotite closure temperatures (biotite closure temperature is generally thought to be in the range 300 5 50°C; McDougall and Harrison, 1988).

The close spatial association between of hydrothermal lodes, containing biotite and hornblende that have experienced argon loss, and pegmatites dykes containing apparently undisturbed muscovite provides an insight into the nature of the processes responsible for argon loss in the hydrothermal minerals. The similarity between tbe ages (either plateau or integrated ages) of muscovites from pegmatite dykes, and the Sm-Nd garnet-albite age of the pegmatite dykes from Scotia and from elsewhere in the southern Yil- gam Craton (Kent et al., 1996) suggests that little, if any, argon loss has occurred from muscovite within the pegmatites dykes. From this it can be inferred that these dykes cooled quickly through their closure temperatures and were not subsequently reheated to a temperature close to the blocking temperature for a time period sufficient to promote any significant degree of argon loss. Thus argon loss in hydrothermal minerals occurred at ambient temperatures that were below the closure and blocking temperature of argon diffusion in pegmatite muscovites.

However, such simple models of thermally-driven diffusive argon loss based on published closure and blocking temperature estimates cannot explain the observed argon loss in hydrothermal hornblende from the Scotia deposit. Esti- mates of the closure and blocking temperature of homblendes are 500 2 50°C (e.g., Harrison, 1981) and 400- 430°C (Lister and Baldwin, 1996), respectively. Given that both the estimated blocking and closure temperatures of hornblende are in excess of that of muscovite, it is difficult to explain the observed argon loss in hornblende from the Scotia mine via a model of diffusive argon loss during slow cooling and/or post-mineralisation reheating. Any thermal disturbance or period of protracted cooling capable of re- sulting in argon loss from hornblende would also be expected to completely reset tbe adjacent muscovite.

In light of this, two explanations for argon loss at the Scotia mine are discussed below. Tbe first involves variations in the blocking and closure temperatures of the analysed minerals away from estimated values, and the second involves argon loss during post-mineralisation fluid flow along ore-hosting structures.

6.3.1. Variations in closure and/or blocking temperatures

Most estimates of muscovite closure temperature are in the range - 275-400°C and are largely based on empirical

One explanation for tbe contrasting argon systematics of muscovite and hornblende at the Scotia mine could be that (assuming argon loss in hornblende samples was a volume-


diffusion driven process) the actual closure and/or blocking temperatures for muscovite and hornblende differ consider- ably from the estimates given above for these minerals. For example, if the closure temperature of hydrothermal homblendes were sufficiently less than 500 t 5o”C, and/or the closure temperature of pegmatite muscovites were sufficiently greater than 350 +- 50°C then, given a suitable thermal history, diffusive argon loss in hornblende could potentially occur during slow cooling, without argon loss in muscovite.

Variations in closure temperature are certainly possible. For example, in amphiboles it is well documented that exsolution can result in lower apparent closure temperatures in metamorphic homblendes (Harrison and Fitz Gerald, 1986; Baldwin et al., 1990). The experimental results of Baldwin et al. ( 1990) suggest that the depression of closure temperature in highly exsolved metamorphic homblendes is on the order of lOO- 150°C (these authors calculate closure temperature between 360 and 435°C for their samples of metamorphic hornblende). In the analysed hornblende samples from Scotia, the presence of structural defects, small domains of actinolitic hornblende observed within larger magnesio homblende grains, and the presence of small inclusions of impuri- ties, such as chlorite or biotite, may similarly have lowered the closure temperature of this mineral (Baldwin et al., 1990: Lee, 1993).

Another alternative is that the closure and/or blocking temperature of the pegmatite muscovite analysed was significantly higher than that of the estimated values. In contrast to the infinite plane geometry used by Lister and Baldwin (1996) for calculations of the muscovite closure and blocking temperatures, Hames and Bowring ( 1994) recom- mend a cylindrical argon diffusion geometry for this mineral. Further, these authors, on the basis of recognised argon concentration gradients in large muscovite crystals, suggest that the effective diffusion dimension in such grains may be controlled by the physical grainsize of the mineral. If this is the case for muscovite grains the size of those used in this study (0.5-2.5 cm), then muscovite closure and blocking temperatures could be significantly higher than estimates for small grains. This is illustrated in Fig. 6 where the calculated closure temperature of muscovite is plotted against the effective diffusion dimension for a range of cooling rates (note that the calculations for this plot use a cylindrical diffusion geometry and the diffusion parameters for muscovite recom- mended by Hames and Bowring, 1994 from re-appraisal of the data of Robbins, 1972). However, although high closure temperatures may be possible, it is by no means certain that the effective diffusion dimension is controlled by the physical grain size in such large muscovite crystals. Argon concentration gradients have been observed in grains only up to a few millimetres in diameter, not the 50-250 mm size of the muscovite crystals analysed in this study. Further, Lister and Baldwin ( 1996) argue against the interpretation that the argon concentration gradients observed represent diffusion within a single diffusional domain. Also, the authors of this study are unaware of any empirical estimate for muscovite which independently assess the closure temperature as significantly in excess of 400°C.

In summation, it is considered unlikely that the observed

SW-

dT/dt = lO”C/Ma

----- dT/dt=S’C/Ma

zw 1 ,,,,; ; _,_,“‘“;t’““,_ ,,,,,,

.(x)1 .Ol .l 1

a (an)

Fig. 6. Variation of calculated closure temperature in muscovite with increasing effective diffusion dimension calculated from the formulation of Dodson ( 1973) using cylindrical geometry and diffusivity data from Robbins ( 1972) and Hames and Bowring ( 1994).

argon loss in hornblende in solely the result of thermally driven argon loss via volume diffusion. Further, even if it were feasible that a combination of lowering of hornblende and elevation of muscovite closure/blocking temperatures could produce argon loss in hornblende and not pegmatite muscovite, it is difficult to explain the differences in the apparent ages of hornblende samples by such a model. The apparent ages of gas released from hornblende samples at temperatures above 1000°C differ by almost 700 Ma. If the closure or blocking temperature, either during slow cooling, or during a reheating event, provided the only control on argon loss then similar ages for the different hornblende samples would be expected, unless there were also dramatic variations in closure/blocking temperatures and/or thermal history between samples from different locations. It is difficult to imagine that variations in the thermal history or closure/blocking temperature would be sufficient over the few hundred metres range over which samples were collected to generate ages that differ by up to 700 million years.

6.3.2. Argon loss during fluid jiow

An alternative explanation for the disturbed argon systematics in hydrothermal minerals from the Scotia mine is that argon loss occurred by non-volume diffusion processes. As discussed in the introduction to this paper, there is considerable evidence that argon loss during laboratory step heating in hornblende and biotite is the result of nonvolume-diffusion processes such as dehydroxylation and in vacua phase transitions. Likewise other studies have suggested that, in some situations, argon loss over geological time may also be the product of nonvolume-diffusion processes, such as mineral-fluid interaction and/or chemical re-equilibration (e.g., Miller et al., 1991; Wartho, 1995).

The suggestion, therefore, is that argon loss in hydrothermal hornblende, and possibly also biotite, at the Scotia mine


occurred during movement of a retrograde fluid along the ore-hosting structures. Argon loss was associated with mineral-fluid interaction (and possibly also concomitant chemical re-equilibration) during this retrograde event (or events). Further, it appears that argon loss in hornblende occurred at temperatures below that of the closure and blocking temperatures of pegmatite muscovite, which if these are similar to published estimates, suggests that argon loss in hornblende occurred at temperatures below about 250-300°C.

This scenario is consistent with the abundance of field and mineralogical evidence that show that the deposit has been the site of major retrograde (post-mineralisation) fluid flow. In all parts of the mine numerous brittle fractures crosscut and often displace both the ore-hosting structures and post-ore pegmatites (e.g., Fig. 2; Thomas et al., 1990; Mc- Cuaig et al., 1993). The brittle nature of deformation associated with fractures and retrograde assemblages (quartz-chlorite-albite-calcite-sericite; albite-prehnite; gypsum) infer that fractures formed at pressure-temperature conditions substantially less than those at which gold mineralisation occurred (McCuaig et al., 1993). This is consistent with the suggestion that argon loss in hornblende occurred at relatively low temperatures. In some areas small flakes (91-5 pm) of retrogressive minerals (predominantly chlorite) are also found along cleavage planes, grain boundaries, and in micro- fractures in hydrothermal minerals (e.g., Fig. 3; McCuaig et al., 1993) suggesting that retrograde fluid flow also occurred along grain boundaries within ore zones. Further, as documented already, hornblende samples from the Scotia mine show marked chemical variation, and although this is largely interpreted to have occurred during gold mineralisation, the possibility that some of the chemical variations in homblende are the result of post-crystallisation chemical re-equilibration cannot be ruled out.

The actual mechanisms of argon loss during fluid activity or chemical re-equilibration, remain less well understood than argon loss via volume diffusion. Wartho ( 1995) suggested that argon loss in amphiboles from the Rameka gab- bro in New Zealand, and from Bayan Obo in Inner Mongolia were most likely related to partial chemical re-equilibration during post-ctystallisation thermal events. Miller et al. ( 1991) argued that reset K-Ar ages in homblendes from the Connemara region of Western Ireland were related to excess structural water and anomalous 6D values, interpreting this to indicate that exchange between the hornblende and fluid was the most probable cause of argon loss.

Unlike argon loss via volume diffusion, argon loss associated with mineral-fluid interaction can potentially explain the observed differences in argon systematics of hydrothermal minerals compared to pegmatite muscovite at the Scotia mine. If argon loss depends on fluid interaction, then the thermal history of a particular sample becomes subordinate with respect to the hydrothermal history. The degree of mineral re-equilibration, and thus also presumably the degree of argon loss, will then depend on parameters such as the local permeability, fluid P-T-X conditions, fluid/rock ratios, and the extent of fluid-mineral reaction. Compared to thermal history variations, fluid parameters can vary on small spatial and temporal scales. Thus the large differences in ages of the > 1000°C argon fractions from hornblende samples from

Scotia could be explained by differing degrees of fluid-mineral interaction, controlled by localised variations in fluid parameters. The apparent lack of argon loss in muscovite may be a function of the reduced permeability of the pegmatites in comparison to the relatively more fractured and dis- placed ore zones. Field observations support this. Compared to ore zones, pegmatite are less fractured and show no indica- tion of grain-boundary retrograde phases evident in the minerals in lode zones. Where brittle fractures do occur within pegmatites, they are more widely spaced and are marked by thin zones of cataclastic deformation, with retrograde minerals restricted entirely to these spaced fractures. This suggests that fluid flow within pegmatites during the fracturing was restricted entirely to these fractures.

If fluid-mineral interaction is the cause of the observed argon loss in hydrothermal minerals then, unlike the study of Miller et al. ( 199 1) , this appears to have not significantly affected stable isotope systematics. Both the hydrogen and oxygen isotope values of hydrothermal minerals from Scotia are consistent with those estimated for nearby gold deposits (albeit with a small dataset for 6D values) and temperatures calculated from oxygen isotope fractionation of 500-560°C (Table 3) are the same as the estimates of the temperature of mineralisation from mineral equilibria (McCuaig et al., 1993). Further careful work and an enlarged regional data- base is required to investigate whether hydrogen isotopes at Scotia have been perturbed during argon loss (cf. Kerrich and Cassidy, 1994). Fluid-mineral interaction may have occurred at temperatures less than those required for re-equilibration of oxygen isotopes, however, it is also plausible that any retrograde fluid involved in mineral-fluid equilibration and argon loss had already attained oxygen and hydrogen isotope equilibrium with hydrothermal minerals during movement along the ore structures from deeper crustal lev- els. Also, microscale investigations may be required to recognise disturbances of oxygen and hydrogen isotopes if mineral-fluid re-equilibration occurred within limited spatial domains.

Although it is difficult to constrain the precise timing of argon loss with the available data, the range of apparent ages in hornblende and biotite samples is consistent with movement of retrograde fluids throughout the Scotia deposit in response to Proterozoic erogenic activity along the adjacent margins of the Yilgam Craton. Scotia is situated less than 100 km from the southern and eastern boundary of the Yilgam Craton (Fig. 1) and is within a zone of deformation, as determined from aeromagnetic data, which is attributed to Proterozoic interaction between the Yilgam Craton and the Albany-Fraser Province (Whitaker, 1990). Several major erogenic episodes, involving high grade metamorphism and granitoid emplacement, occurred within the Albany-Fra- ser Province between ca., 1900- 1100 Ma (Gee, 1979). This activity may have caused successive pulses of fluid activity along Archaean structures adjacent to the cratonic margin, including those which host gold mineralisation at Scotia, in a way analogous to that documented for Archaean faults in the Superior Province (Kerrich, 1994; Powell et al., 1995). In the Yilgarn Craton these retrogressive events are also recorded in other isotopic systems. For example, lead isotope results from ore sulfides associated with gold mineralisation


in the Norseman and Kalgoorlie Terranes record at least two Proterozoic events: one at ca. 2000 Ma and a later event at ca. 1100 Ma, that have altered lead isotopic compositions of gold-related galena (Perring and McNaughton, 1990). Craton-wide intrusion of mafic dykes at ca. 2400 Ma (Fletcher et al., 1987), including some in the Scotia mine vicinity, may also be related to some argon loss events.

6.3.3. Ramfications for interpretation of 10Ar-.‘yAr results

The possibility that argon loss may be attributed to post- mineralisation fluid movement through the Scotia deposit has important ramifications for the interpretation of results from 40Ar- 39Ar analysis of minerals that may have interacted with fluids after formation. Where isotopic resetting of this style has occurred the criteria required to assess the validity of isotopic ages may differ from those used to detect thermally-driven argon loss. The thermal history of a sample becomes largely secondary in favour of the hydrothermal history of a sample, and factors such as permeability, fluid/ rock ratios, fluid P-T-X conditions, and crystal microstructures (to facilitate fluid-mineral exchange reactions ) deter- mine the degree of isotopic disturbance (e.g., Miller et al., 199 1) As these parameters can vary on short length and timescales, it is envisaged that argon loss via this process can be much more variable than that driven by thermal history variations, which are generally regional in scale. This is especially germane to the studies of hydrothermal ore deposits as, by definition, many ore deposits of this type are located within structures that act as locii for the passage of hydrothermal fluids. Many mineral deposits, such as Archaean gold deposits, are associated with major crustal structures that have experienced long post-mineralisation fluid movement histories (e.g., Kerrich and Cassidy, 1994; Powell et al., 1995). In this situation there is considerable opportunity for argon loss to have occurred within hydrothermal minerals. Interpretations from previous studies on Archaean gold deposits that have entertained the possibility of argon loss only under purely thermal regimes (e.g., Hanes et al., 1992: Zweng et al., 1993) will require reassessment in light of the results outlined herein. Although some studies have recom- mended the use of stable isotope systematics to recognise activity of retrograde fluids (e.g., Miller et al., 1991; Kerrich and Cassidy, 1994). the conclusions of this study (albeit limited by restricted data for comparison) are that argon loss can occur without obvious disturbance of the bulk oxygen (and possibly also hydrogen) isotope systematics of coexist- ing silicates.

7. CONCLUSIONS

Argon loss in hydrothermal hornblende (and possibly also biotite) at the Scotia gold mine, Western Australia is interpreted to have occurred during fluid/mineral interaction associated with post-gold movement of a retrograde fluid along ore-hosting structures. Explanations for the observed argon systematics whereby argon loss occurs via volume diffusion and is controlled by the thermal history of the Scotia region (such as during protracted cooling or post-mineralisation reheating) are considered improbable.

If argon loss is controlled by fluid/mineral interactions, then factors such as fluid/rock ratio, fluid P-T-X conditions, permeability, and mineral microstructures may be more important in governing 40Ar-39Ar systematics than factors known to influence volume diffusion. Thus this study recom- mends caution for assessing jOAr- 39Ar behaviour in environments, such as many hydrothermal ore deposits, where fluids may have interacted with minerals after crystallisation. Inter- pretations based exclusively on the thermal, rather than hydrothermal, history of analysed samples may be insufficient. Although the example outlined herein is an extreme case, more subtle forms of this phenomenon may be difficult to recognise.

Acknowledgments-AJRK acknowledges the receipt of Australian Postgraduate Research and ANUTECH Supplementary Scholar- ships, and TCM acknowledges receipt of NSERC and University of Saskatchewan Postgraduate Scholarships. The Australian Institute of Nuclear Science and Engineering (AINSE) provided financial assistance for sample irradiation in Australia. J. Fedorowich is thanked for argon analysis of samples at Queen’s University. West- ern Mining Corporation and Central Norseman Gold Corporation provided access to the Scotia mine, logistical support during field work. and financial assistance for analytical work. K. Johnson, S. Peters, and R. Waugh are thanked for assistance and inspiring discussions during field work. P. Carpenter is thanked for help with electron microprobe and SEM analyses. Additional financial and logistical support by BHP-Utah, Normandy Poisedon, Pancontinental Mining, Placer Exploration, and Western Mining Corporation to AJRK is also gratefully acknowledged. Discussions with J. Eiler and reviews by I. McDougall, T. Spell. J. Lee, K. Cassidy, R. Kerrich, K.V. Hodges, and T.K. Kyser significantly improved the manuscript.

Editoriul handling: T. K. Kyser

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Disturbed 40Ar-39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine