Mobilization of gold in the deep crust: evidence from mafic intrusions in the Bamble belt, Norway

Lithos, 30 (1993) 151-166 Elsevier Science Publishers B.V., Amsterdam

151

Mobilization of gold in the deep crust: evidence from mafic intrusions in the Bamble belt, Norway Eion M. C a m e r o n a'b, Ersen H. Cogulu c and John Stirling a aGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A OE8 Canada bDepartrnent of Geology, University of Ottawa, Ottawa, Ontario. KIN 6N5 Canada CMinerco, 2506-1380 Prince of Wales Drive, Ottawa, Ontario, K2C 3N5 Canada (Received February 19, 1991 ; revised and accepted November 18, 1992 )

LITHOS

ABSTRACT The location of mesothermal gold deposits in the upper part of major transcurrent shear zones suggests that gold was derived from the deep, wider portions of the shears. The 30 kin-wide Bamble belt, south Norway, is a deep shear belt, metamorphosed to upper amphibolite and granulite grade at about I. l Ga. Preliminary studies showed that its rocks are strongly depleted in gold and other chalcophile elements. Mafic rocks were chosen for more detailed study of the mobilization of gold during metamorphism. Gold in mafic rocks largely partitions into the sulphide phase, which is mainly comprised of pyrrhotite. Modification of this phase was required to liberate gold. Bamble has two main groups of mafic rock. The first, metabasites, intruded before peak metamorphism, are uniformly low in gold. Theirfo2 at peak metamorphism, obtained from ilmenite-orthopyroxene equilibria, indicates a strongly oxidized mineral assemblage, well above FMQ. Pyrrhotite was stable at peak temperature of 800 ° C, but on cooling below 600 ~C, internal (mineralogical) buffering by the oxidized assemblage caused the fo2 to cross into the stability field of pyrite. Following conversion of pyrrhotite to pyrite, the latter was partly replaced by magnetite. These serial changes in the mineralogical form of iron sulphide was conducive to the extraction of gold, as was the oxidized nature of the mineral assemblage that would have buffered metamorphic fluids to high fo2- The second group of mafic rocks, "'hyperites", intruded after peak metamorphism, record intermediate steps in the extraction of gold. These intrusions, of coronitic gabbro, were metamorphosed, in whole or in part, to amphibolite. There is a decrease in gold and other chalcophile elements across a hyperite dyke from the pyrrhotite-bearing, coronitic gabbro interior to the pyrite-bearing amphibolite margin.

Introduction The source of gold in m a j o r m e s o t h e r m a l q u a r t z carbonate gold deposits r e m a i n s enigmatic. The largest q u a r t z - c a r b o n a t e gold camps have produced 1000 t Au or more. G i v e n that source rocks rich in gold have not been identified, gold must have been extracted from a large volume, at least 100 km 3, of rock or melt, c o n t a i n i n g the crustal average of 3 ppb Au. Veins occur along shear zones near or above the b r i t t l e - d u c t i l e t r a n s i t i o n , where the zone of deformation is relatively narrow. O n e possibility is that

the source of gold was the deep, wide portions of shear zones, with the fluids being focused upwards as the permeable shear zones n a r r o w e d into the b r i t t l e - d u c t i l e t r a n s i t i o n ( C a m e r o n , 1989a). Shear zones in the lower crust may be as wide as 40 km ( B a k e t al., 1975). A test of the hypothesis for the extraction of gold from a deep shear zone was made in the Bamble belt, south Norway. Rocks of this 30 km-wide shear belt of Proterozoic age are m e t a m o r p h o s e d to granulite a n d upper a m p h i b o l i t e grade a n d are substantially depleted in gold relative to its crustal a b u n d a n c e

0024-4937/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

152

E.M. C A M E R O N ET AL.

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PERCENTOF CRUSTALABUNDANCE Fig. 1. Abundances of Au, Sb and As and of Cu, Ni and Zn in Bamble metabasites, relative to average abundances in unmetamorphosed mafic rocks. Based on mean values for 16 samples from the granulite facies and 16 from the amphibolite facies (Cameron, 1989b). Sources for average abundances reported in Cameron (1989b). (Cameron, 1989a, b); metabasite intrusions contain only 0.04 of the usual abundance of gold in mafic rocks. Other chalcophile elements commonly associated with gold in veins, such as Sb and As, are also depleted, whereas base metals, including Zn and Ni. that are usually not enriched in gold deposits, are undepleted in the Bamble rocks (Fig. 1 ). These studies in the Bamble belt were preliminary; they did not consider processes that may have caused depletion. In this paper we consider the behaviour of sulphide and oxide minerals during metamorphism ofmafic rocks from Bamble. Mafic rocks were chosen because during crystallization gold is largely partitioned into sulphide minerals; its release, during metamorphism, requires modification of this readily observed fraction. The partition factor, sulphide melt/silicate melt, is about 1000 (Stone et al., 1990). Pyrrhotite is the principal primary sulphide mineral of mafic rocks, accompanied by chalcopyrite. Two types ofmafic rock were examined. The first, metabasites, were affected by high-grade metamorphism; all samples are substantially depleted in gold. The second, hyperites, were intruded after peak metamorphism; these are variably metamorphosed and variably depleted in gold.

Geology The Bamble shear belt is approximately 30 km wide and runs for 150 km along the southeast coast of Norway (Fig. 2). Its geology has recently been reviewed by Starmer ( 1991 ). The earliest rocks are sediments, including quartzites and graphitic units, overlain by a mixed volcanic-sedimentary sequence. The supracrustal rocks were deposited around 1.7 Ga (Starmer, 1991 ). Mafic dykes and sheets (metabasites) of tholeiitic composition are abundant throughout the belt. Tonalite-trondhjemite magma (tonalite gneiss) was intruded, mainly in the Arendal-Tromoy area. This sequence was metamorphosed at granulite and amphibolite grade. During metamorphism the belt was a broad shear belt between crustal blocks undergoing dextral slip; deformation is reflected in tight folding with steeply dipping foliation (Falkum and Petersen, 1980). Rocks in the highest grade zone (Zone D, Fig. 2) are depleted in large ion lithophile elements (LILE) (Field and Clough, 1976; Cooper and Field, 1977; Clough and Field, 1980). Anatectic granitoids are present in the upper amphibolite zone. Lamb et al. (1986) estimated granulite metamorphism at 800 + 60: C and 7.3 _+0.5 kbar, based on garnet-pyroxene and garnet-pyroxene-plagioclase compositions and Harlov (1992) estimated 785 + 60°C and 7.4_+ 0.7 kbar from garnet-ortho-

MOBILIZATION OF GOLD IN THE DEEP CRUST

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HIS~Y Fig. 2. Sample Locations, Bamble Shear Belt. Metamorphic zones A to D modified after Field et al. (1980). Shaded areas: enderbitic and charnockitic rocks.

pyroxene equilibria. These measurements are similar to estimates of 750-850°C and 6-8 kbar obtained from fluid inclusions in rocks of upper amphibolite facies (Touret and Dietvorst, 1983). This suggests that the transition to granulite was determined by the fluid changing from H z O - t o C O 2 rich (Touret, 1971 ). Metabasite samples were taken from the granulite facies on T r o m o y and from the upper amphibolite facies near Tvedestrand. The former contain plagioclase, orthopyroxene, ilmenite, magnetite +_hornblende +_clinopyroxene +_quartz, whereas at Tvedestrand the assemblage is plagioclase, hornblende, ilmenite, magnetite_+ biotite + quartz +_orthopyroxene. Mafic dykes and sheets (hyperites) were intruded in several stages during and subsequent to the main metamorphic event. They show two distinct phases of modification (Starmer, 1969; Brickwood and Craig, 1987). First, coronas formed around ferromagnesian minerals and there was alteration and recrystallization of plagioclase. Olivine was replaced by successive coronas, from inside out, of bronzite, amphibole _+spinel symplectite and

garnet; primary pyroxene was replaced by coronas of amphibole and garnet. The second phase was amphibolization, to create essentially andesinehornblende rocks. In some cases this affected the entire intrusion, in other instances, only the margin. Amphibolization was accompanied by gain in K, P, HzO and Fe3+/Fe 2+ and loss of Ca (Elliot, 1973) A hyperite dyke, showing a transformation from a coronitic gabbro interior to a thin, foliated amphibolite margin was sampled at Tvedestrand. The massive nature of this gabbro dyke contrasts with nearby metabasites, which have a strong subvertical foliation and are variably cut by veins of pegmatite. Less usually, hyperite intrusions were metamorphosed at granulite grade. Such a body was sampled on Tromoy. The timing of major events within the Bamble belt has been controversial. Formerly, the high-grade event was considered to have peaked at ~ 1540 Ma, based on R b - S r dating (Field and Rahiem, 1981; Field et al., 1985), although an early K - A r determination indicated ~ 1100 Ma (O'Nions et al., 1969). Recent S m - N d data and U - P b zircon data support the younger estimate (Table 1 ), with meta-

154

E.M. CAMERON ET AL.

TABEI Sequence of selected geological events in the high-grade, Tromoy sector of Bamble belt. Based on Starmer (1985) and Starmer (1991). Geochronological data: ( 1) O'Nions et al. ( 1969 ); ( 2 ) Field and Rahiem (1979); (3) Field et al. (1985); (4) Kullerud and Machado ( 1991 ) 1543_+ 10 Ma for zircon and 1105+_8 Ma for zircon overgrowth from granulite on Flosta Island, north of Tromoy; (5) Kullerud and Dahlgren ( 1991 ) Event Deposition ofsupracrustal rocks Intrusion of charnockite-enderbite Intrusion of metabasites Granulite facies metamorphism

Intrusion of early hyperites Intrusion of main hyperites Corona growth in hyperites Amphibolization of hyperites Intrusion of medium-grained granite Retrogression ofgranulite Intrusion of late hyperites

Date, Ma

1543+ l0 U-Pb (4) II00K-Ar(I) 1105_+8 U-Pb(4) 1070-1100 Sm-Nd (5) 1540_+21 Rb-Sr (3)

1063+20 Rb-Sr (2) 1060 Rb-Sr (3)

morphism spanning the interval I152-1095 Ma (Kullerud and Dahlgren, 1993). Hyperites are considered to have been intruded during or after peak metamorphism (Starmer, 1991 ), but de Haas et al. ( 1993 ) have obtained Sm-Nd dates that range from 1.1 to 1.8 Ga. For the hyperites sampled for this report, relative lack of structural deformation compared to adjacent metabasites is consistent with Starmer's interpretation. The last major event in the coastal portion of the belt at about 1060 Ma (Field et al., 1985) was intrusion of medium-grained granites, fluids from which caused local rehydration and retrogression ofgranulites at greenschist grade.

Analytical methods Potassium, Ti, P, Ba, Sr and Zr were analyzed in rock powders by fused disc XRF. Rubidium was analyzed by pressed pellet XRF to give a detection limit of 1 ppm. Analysis for Fe3+/Fe z+ was by the Pratt method: Co, Cr, Cu, Ni, Y and Zn were by inductively coupled plasma emission spectrometry; S by ion chromatography, to give a detection limit of 50 ppm. Scandium, La, Ce, Sin, Eu, Tb, Yb and Lu were determined by instrumental neutron activation analysis on 10-15 g samples. Arsenic and Sb, measured to detection limits of 20 ppb, were analyzed by hydride-atomic absorption spectrometry.

A high sensitivity method, with a detection limit of 0.1 ppb, was used for the analysis of Au. Samples weighing 15 g were dissolved in HF-aqua regia. This was followed by extraction of Au in small amounts of M1BK and multiple injection of MIBK into a graphite furnace-atomic absorption spectrometer. Data on precision and accuracy are given by Cameron (1989b). Spidergram plots are normalized to the mean of 16 samples of metabasite from Tromoy (Cameron, 1989b). These samples, strongly depleted in chalcophile elements and large ion lithophile elements (LILE), serve as a base to reference the less depleted hyperites. Mean compositions of the metabasites used for normalization are (in ppm ): Rb 4.8, K 4300, Ba 115, Sr 181, Sc 35. Y 24, La 7.2, Ce 13.4, Sm 3.0, Eu 1.0, Tb 0.9, Yb 2.1, Lu 0.37, Ti 7600, Zr 73, P 900, Cr 127, Co 51, Ni 88, Zn 130, Cu 33, As 0.30, Sb 0.028, Au 0.17 (ppb), S 355. Plots are arranged to show LILE to the left, with the least mobile, compatible elements in the centre, and chalcophile elements to the right.

Nature of sulphide and oxide minerals minerals

Sulphides The least altered hyperites (coronitic gabbro) contain pyrrhotite and variable amounts of pyrite as the principal sulphide mineral, whereas amphibolitized hyperites and the metabasites contain mainly pyrite. Hyperites show transitions from a primary, pyrrhotite-bearing mineralogy to a pyrite mineralogy of metamorphic origin. Bleb pyrrhotite ( P O b ) - - IS mainly observed in hyperites as small, 2 to I0/~m, rounded blebs enclosed in minerals (Fig. 3a), such as hornblende, pyroxene and feldspar. Chalcopyrite may form part of the blebs and pentlandite is observed in some samples as exsolution flames in the pyrrhotite. The N i c o n t e n t o f POb is I o w , 0.1 tO 0 . 5 % . T h i s p y r r h o -

tite, accompanied by chalcopyrite and pentlandite, are interpreted to represent a F e - N i - C u sulphide liquid that separated from the silicate melt, the rounded form being derived from immiscibility of sulphide in silicate melt, Their invariable occurrence as inclusions within other minerals suggests that the host mineral protected pyrrhotite from change.

MOBILIZATION OF GOLD IN THE DEEP CRUST

15 5

Fig. 3. Photomicrographs of sulphide minerals in Bamble mafic rocks. (A) Interior Zone, hyperite dyke 40, sample 4009; bleb of pyrrhotite ( Pob ) in hornblende. (B) Interior Zone, hyperite dyke 40, sample 4009; grain of interstitial pyrrhotite (Poi), the right half of which has changed to a complex of Ni-rich pyrite (PyNi) (light gray) and magnetite (dark gray). (C) Metabasite dyke 39, sample 3901; composite grain of pyrite (PYim) and chalcopyrite, both with rims of magnetite. (D). Marginal Zone, hyperite dyke 40, sample 4001 ; pyrite (PYm~),with smaller grain of chalcopyrite, both rimmed by magnetite. Interstitial pyrrhotite (Poi) - - Occurs as larger, 20 to 100 ~m, polygonal grains. Since it is interstitial to metamorphic minerals, such as hornblende, it formed later than POo. It also differs from Po b in a distinctively higher Ni content, in the range 1.0 to 1.4%. In many samples Poi has been oxidized to a complex of Ni-rich pyrite (PYNi) and magnetite (Fig. 3b). Interstitial pyrite (Py~) - - Is present as polygonal grains, 30 to 250 ~tm in diameter. Unlike PYNi, which is derived from in situ oxidation of Poi, PYi has a low content of Ni, less than 0.1%. In all metabasite samples, and in some amphibolized hyperites, grains of Py~ are mantled by magnetite, the result of oxi-

dation of the rims of the pyrite; these are identified as mantled pyrite (PYlm) (Fig. 3c, d). Fracture pyrite (PYr) - - Occurs along fractures, particularly at the boundary or cleavages of Febearing minerals, such as orthopyroxene. Since this type of pyrite is often present with PYim, but is not mantled by magnetite, Pyf was introduced later than oxidation of PYi to PYim. Because it post-dates the processes of metamorphic oxidation that may be related to loss of gold, Pyf is not described in detail in this report. Chalcopyrite - - In addition to the pyrrhotite that is associated with bleb pyrrhotite, chalcopyrite occurs as interstitial grains, up to 200 ~tm. Chalcopyr-

156

ite frequently forms c o m p o s i t e grains with p y r r h o tite a n d pyrite. In rocks where the pyrite is m a n t l e d by magnetite, the c h a l c o p y r i t e is also m a n t l e d by m a g n e t i t e (Fig. 3c).

Oxides l l m e n i t e is the m o s t i m p o r t a n t o x i d e m i n e r a l in

E.M. CAMERON ETAL.

the mafic intrusions. It exceeds m a g n e t i t e in both the m e t a b a s i t e s a n d the hyperites, c o n t r a s t i n g with t o n a l i t i c gneiss on T r o m o y where m a g n e t i t e d o m i nates. Point c o u n t i n g o f sections from 12 s a m p l e s o f m e t a b a s i t e gave m e a n values o f 3.0 wt.% ilmenite a n d 1.1 wt.% magnetite, for an i l m e n i t e / m a g netite ratio o f 2.8. Sections from 12 s a m p l e s o f hyp e t i t e had a mean o f 2.90 wt.% i l m e n i t e a n d 0.64

Fig. 4. Back scatter images of ilmenite in Bamble mafic rocks. (A) Metabasite dyke 39, sample 3906, Tvedestrand; mainly IHMtype ilmenite. Grains show broad exsolution lamellae of ilmenite (dark) and hematite (light), with fine, second-generation lamellae also showing. Very fine lamellae of magnetite (white) are present, mainly within the hematite. In the lower right quadrant, a IO-type ilmenite grain is in contact with a magnetite grain. The magnetite is free of ulvospinel, which may have diffused into the ilmenite. The IO-type ilmenite contains very fine lamellae of hematite. (B). Metabasite dyke 45, sample 4502, Tromoy; 1HM-type ilmenite. Broad lamellae of ilmenite (dark gray) and hematite (light gray) and fine, second-generation lamellae of these minerals. Lamellae of magnetite (white) occur mainly in areas of hematite, but are bordered by a very thin zone ofilmenite. (C) Amphibolized Marginal Zone, hyperite dyke 40, sample 4001, Tvedestrand; IHM-type ilmenite grains, but with hematite lamellae (light) forming a smaller proportion of the grains than in metabasites. Lamellae of magnetite (white) are mainly within or near to the earlier lamellae of hematite. Note alteration to titanite (black) along the 0001 plane. (D) Coronitic gabbro, Intermediate Zone, hyperite dyke 40, sample 4003, Tvedeslrand; IM-type ilmenite. Straight, thin, lamellae of magnetite (white) with small, irregular blebs of hematite filling areas where magnetite is absent.

157

M O B I L I Z A T I O N OF G O L D IN THE DEEP CRUST

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wt.% magnetite, for a ilmenite/magnetite ratio of 4.5. Ilmenite grains take a number of forms, grouped here into three major types:

Ilmenite Only Type (I0). These grains are entirely ilmenite or contain sparse, thin lamellae of hematite along the 0001 plane. In metabasites the IO-type is infrequent, grains usually occurring close to magnetite grains (Fig. 4a), suggesting that they may have resulted from exsolution of ulvospinel from adjacent magnetite. In tonalitic gneiss, where magnetite is more abundant, IO-type grains form a higher proportion of the total ilmenite fraction.

llmen ite-Magnetite T.vpe (IM). These are similar to the IO-type but contain thin, straight lamellae of magnetite, usually less than 3 /zm in width, along the 0001 plane. Where present, lamellae of hematite are subordinate. This type is most characteristic of the hyperites. IM-type grains with no hematite lamellae are found in the least metamorphosed hyperites (coronitic gabbros), whereas blebs and lamellae of hematite become more prevalent within amphibolized hyperites (Fig. 4d).

llmenite-Hematite-Magnetite Type ( IHM). These are interpreted to have formed at high temperature as homogeneous grains of hemoilmenite.

During cooling, the hematite-ilmenite solvus was intersected (Fig. 5 ) causing the exsolution of broad lamellae of ilmohematite along the 0001 plane (Fig. 4b). With further cooling and expansion of the twophase region, there was exsolution of further generations of hematite in ilmenite, and ilmenite in hematite, these both forming fine lamellae (Fig. 4b). Finally, there was formation of lamellae of magnetite, also along the 0001 plane (Fig. 4b). The magnetite lamellae are interpreted to be late, since they occur mainly within broad lamellae of ilmohematite, indicating that magnetite formed after phase separation of hematite. This is consistent with the explanation of Buddington et al. (1963) that magnetite exsolution results from the subsolidus reduction of the hematite component of ilmenite: 6Fe2 03 ~4Fe3 04 + 02 Magnetite lamellae in ilmohematite are bordered by a thin zone of hemoilmenite: since magnetite contains less Ti than the parent ilmohematite, the excess Ti formed a border zone. The IHM-type is the principal form of ilmenite in the metabasites and is also present in the tonalitic gneiss. In hyperites, it only occurs within the most strongly amphibolized samples (Fig. 4c). Some IHM ilmenite grains (which were avoided for the analytical work described below) show late replacement by titanite, particularly on fractures along the 0001 plane (Fig. 4c). Where titanite is present in areas containing magnetite lamellae, the magnetite is not replaced, indicating that formation of the magnetite preceded titanite replacement. IHM grains, similar to these described here, have been described by Kretschmar and McNutt ( 1971 ) from the Whitestone anorthosite, Ontario and by Amcoff and Figueiredo (1990) for metamorphosed noritic layers in the Serrote da Laje deposit, Brazil. Magnetite grains present in metabasite and hyperite samples contain negligible amounts of ulvospinel. As will be discussed below, ulvospinel continued to exsolve from grains (but not from magnetite lamellae in ilmenite) down to low temperatures. In the mafic rocks, a minority of magnetite grains contain ilmenite lamellae. These lamellae are usually broader than 10 ,um, or occur in a cloth weave texture. Ilmenite lamellae in magnetite are more common within the tonalitic gneiss.

158

Results from metabasites

The three-phase IHM-type ilmenite, which dominates the metabasite samples, was derived from a single phase, hemoilmenite, present at peak metamorphic temperatures. In order to reconstruct the primary composition of the hemoilmenite an image analyzer attached to a scanning electron microscope was used to estimate proportions of the three phases. Then, using microprobe analyses of these phases, the composition of the original grains was determined. This was done for 2 samples ofgranulite-facies metabasite from Tromoy (4502, 4507) and 2 samples of amphibolite-facies metabasite at Tvedestrand (3902, 3907). Thirteen grains per sample were measured by image analyzer, together with 4 microprobe analyses of each phase per sample (Table 2). The reconstructed average compositions of the primary ilmenite in the samples range from 0.55 to 0.68 mole fraction ilmenite, which at 800 °C lie directly above the consolute point of the hematite-ilmenite solvus (Fig. 5). In Fig. 6 a isopleth band representing this range of composition is plotted info2-T space. The intersections info2-T space of isopleths for ulvospinel in magnetite with isopleths for ilmenite in hemoilmenite are often used to estimate thefo2-Tat which the two oxides equilibrated (Buddington and Lindsley, 1964). In the metabasites it is not possible to measure the primary content of ulvospinel in magnetite, because ulvospinel has migrated out of the grains during slow cooling. Magnetite grains from these rocks typically contain < 0.1% TiO2 and lack ilmenite lamellae. As an alternative for estimatingfo2 and Tat peak metamorphic temperatures, isopleths representing the composition of orthopyroxene have been drawn to intersect the isopleth band for ilmenite (Fig. 6). This has the advantage that their intersection estimates f o ~ - T of oxide-silicate equilibration, which may be tested by comparing T to independent measures of temperature for silicate equilibration. The isopleth for orthopyroxene shown in Fig. 6 was derived from the composition of orthopyroxene in five samples of metabasite from Tromoy and one sample (3902) of metabasite from Tvedestrand (Table 3 ), using the empirical formulation of Fonarev and Grafchikov (1984). Upper amphibolite facies metabasites at Tvedestrand, near the amphibolite/granulite transition, contain sparse grains of orthopyroxene. Orthopyroxene isopleths for the

E.M. CAMERON ET AL.

mean of the Tromoy samples and for the one sample from Tvedestrand are so close that they are plotted as one in Fig. 6. The intersection of the orthopyroxene isopleth with the band representing ilmenite brackets 800°C, which is the temperature independently estimated for peak metamorphism in the granulite and upper amphibolite facies (e.g., Touret and Dietvorst, 1983; Lamb et al., 1986). The fo2 at the point of intersection is about 2.7 log units above the FMQ buffer Afo 2 --2.7, equivalent to fo2 o f - 11.8 at 800°C). The estimate of Afo2 at 800°C defines the start of the cooling path for the metabasites. During cooling, ilmenite grains exsolved successive generations of hemoilmenite and ilmohematite. Finally, lamellae of magnetite formed within the grains. These lamellae exsolved in equilibrium with adjacent hemoilmenite, permitting estimation ofJo2-Tat which reduction of hematite to magnetite occurred. For the two samples of granulite-facies metabasite and two samples of amphibolite-facies metabasite listed in Table 2, these points straddle the pyrrhotite-pyrite-magnetite (PPM) buffer (Fig. 6), as does an additional sample of metabasite. The estimated temperature range for the five metabasite samples is 435 to 504°C, with a mean of 470°C. Using the program QUILF (Andersen et al., 1993), estimated temperatures are significantly higher, averaging 560°C, but remain along the PPM buffer. Magnetite-ilmenite pairs from IHM ilmenite grains were also measured for a sample of tonalite gneiss from Tromoy and a thin mafic band in quartzite from Hisoy. These also plot close to the PPM buffer (Fig. 6). One explanation for clustering of points near the PPM buffer is that reduction of hematite to magnetite served as the 02 donor required for the conversion ofpyrrhotite to pyrite: 6FeS + 12Fe203 ~ 3FeS2 + 9Fe3 0

4

This reaction does not require the introduction of an fluid-borne oxidant and is indicative of closed system redox reactions during cooling. In hyperites, the conversion of pyrrhotite to pyrite and magnetite may be observed mineralogically (Fig. 3b). Another reaction that can consume 02 is the oxyexsolution of ulvospinel from magnetite (Buddington and Lindsley, 1964):

4 0.02 10.72 0.13 0.15 80.93 0.06 0.07 0.35 0.03 0.03

4 0.00 47.72 0.03 0.00 49.65 0.28 0.90 0.16 0.13 0.07

98.94 0.91

No. anal. SiO~ TiOz AI2Os Cr2Os FeO MnO MgO V2Os NiO ZnO

Total Xilm X ulvo

92.49 0.21

0.22 0.11

0.04

94.70

4 0.03 1,46 0.22 0.08 92.40 0.12 0.05 0.23 0.08 0.03

0.23 0.06

96.52 0.55

0.0t 28.81 0.09 0.05 66.48 0.19 0.52 0.22 0.t0 0.05 100.85 0.90

4 0.03 48.29 0.06 0.06 51.12 0.17 0.85 0.15 0.10 0.02

0.54 0.06

93.03 0.20

4 0.04 10.18 0.17 0.18 81.49 0.05 0.06 0.62 0.08 0.16

0.31 0.10

0.04

95.05

4 0.03 1.36 0.19 0.25 92.74 0.04 0.04 0.28 0.10 0.02

0.15 0.10

97.55 0.56

0.04 29.28 0.12 0.13 66.91 0.11 0.48 0.32 0.09 0.07

llmenite Hematite Magnetite Total

llmenite Hematite Magnetite Total

Wt. prop. 0.55 Std. dev. 0.10

Sample 3906

Sample 3902

99.31 0.92

4 0.00 48.05 0.08 0.00 49.21 0.99 0.50 0.35 0,05 0.08

0.66 0.15

93.20 0.20

4 0.00 10.06 0.18 0.20 81.02 0.12 0.03 1.28 0.09 0.22

0.33 0.16

0.05

94.19

4 0.01 1.59 0.18 0.00 91.48 0.00 0.02 0.91 0.00 0.00

0.01 0.02

97.25 0.68

0.00 35.22 0.11 0.07 59.97 0.70 0.34 0.66 0.06 0.12

Ilmenite Hematite Magnetite Total

Sample 4502

100.34 0.92

4 0.00 48.96 0.09 0.08 49.30 0.49 0.90 0.32 0.07 0.13

0.49 0.14

93.40 0.19

4 0.02 9.43 0.12 0.34 82.56 0.03 0.06 0.66 0.04 0.14

0.50 0.15

0.04

95.31

4 0.06 1.29 0.12 0.44 92.30 0.10 0.05 0.34 0.10 0.51

0.01 0.02

96.94 0.56

0.01 28.86 0.11 0.21 66.33 0.26 0.48 0.50 0.05 0.13

llmenite Hematite Magnetite Total

Sample 4507

Analyses of three-phase ilmenite-hematite-magnetite grains from granulite-facies metabasite (4502, 4507 ) a n d amphibolite-facies metabasite ( 3902, 3906 ). The area of each phase, measured in 13 grains per each sample, was converted to weight proportion, averaged, and the standard deviation between grains calculated. Using analyses for each phase, the original composition (Total) of hemoilmenite prior to phase separation was obtained

TABLE 2

,,..-I

¢3

m

.-e m

C~ © t"-

7 ©

t--'

©

160

E.M. CAMERONET AL.

a z~

IHM-TYPE ILMENITE Metabasite: • Tonalite: • M a f i c B a n d in Q u a r t z i t e : • Amphibolized Hyperite: •

4

~

J

~,¢

i

Lil" # ..~t.,.;'~ ''

II

+

3

'I::";" •

0 .,--I

m

r~

o

]

,

............................. I

400

500

600 Temperature

I

I

700

800

900

°C

Fig. 6. Top half of plot shows the location of Bamble metabasites in fo2-T space at peak metamorphism and the path of subsequent cooling. The location at peak metamorphism is derived from the intersection of an isopleth band for ilmenite with an orthopyroxene isopleth, llmenite isopleths were derived from the data for 4 metabasite samples from the amphibolite and granulite facies (Table 2 ) computed by a program of M.S. Ghiorso (see Ghiorso and Sack, 1991 ). The orthopyroxene data, representing the mean of 6 samples (Table 3), was applied to the empirical formulation of Fonarev and Grafchikov (1984). The termination of the cooling curve is obtained from the composition of lamellae of magnetite and ilmenite in lHM-type ilmenite grains, with computation by the Ghiorso program. The points cluster along the pyrrhotite-pyrite-magnetite (PPM) buffer (Kishima, 1989 ); additional points are shown for a sample of tonalite and a marie band in quartzite, both at granulite grade. The position of the graphite buffer at 3 kb ( Frost and Chako, 1989 ) shows that the rocks were outside the field of stability of graphite. Also shown is the primary ilmenite isopleth for hyperite dyke 40, which is much less oxidized than the metamorphic assemblage of the metabasites. However, the position defined by IHM-type ilmenite grains from the amphibolite margin of this dyke indicates an increase in J/o2 during amphibolization. ~fo2 is relative to the FMQ buffer ( Myers and Eugster, 1983 ).

TABLE 3 Analysesby microprobeofothopyroxene in samplesof metabasite from Tromoy and Tvedestrand Tromoy

Tvedestrand

Sample No. anal.

0222 4

3805 3

4502 6

4503 4

4507 4

SiO2 TiOz A1203 FeO MnO MgO CaO Na20 K20 Total

51.07 0.24 2.29 23.62 0.60 19.27 2.44 0.02 0.12 99.67

51.13 0.09 1.60 25.87 0.47 19.27 0.55 0.02 0.00 99.00

52.16 0.09 1.06 24.45 0.70 20.95 0.48 0.00 0.03 99.92

53.71 5 2 . 7 2 0.05 0.07 7.10 1.04 18.33 2 2 . 6 4 0.73 0.51 15.80 2 2 . 5 6 2.31 0.55 1.86 0.03 0.10 0.00 99.99 100.12

3902 4 50.28 0.12 3.31 24.53 0.54 20.58 0.26 0.04 0.00 99.66

magnetite within the same sections have negligible TiO2 contents, < 0.1%, as a result of loss of ulvospinel during cooling below the - 4 6 6 ° C estimated for the magnetite lamellae in ilmenite. Many granulite terrains equilibrate or re-equilibrate near to the graphite-CO2 buffer (Frost and Chako, 1989). The metabasites formed and reequilibrated atfo2'S well above this buffer ( Fig. 6 ). However, the Bamble rocks are not unique. Granulites that contain ilmohematite include those of Proterozoic age from Lofoten-Vestralen, Norway (Schlinger, 1985 ); a lower crustal shear zone of Archean age from the Labwor Hills, Uganda (Nikon et al., 1973; Sandiford et al., 1987) and granulites retrograded to amphibolite and uplifted along the Red Sea Rift (Seyler and Bonatti, 1988).

6Fe2 TiO4 + 0 2 ~6FeTiO3 + 2Fe304

Results from hyperites

Magnetite lamellae in IHM ilmenite contain appreciable amounts of ulvospinel, unlike grains of

In Fig. 7 distribution curves for Au, plotted using a log scale, are compared for metabasite and hyper-

MOBILIZATION OF GOLD IN THE DEEP CRUST

161

99

._1- 9o >-

METABASe~ •

,, j

,,~ O n" 70 13_

LU 50 _> I--

•~

30

~

10

0

1

I 0.1

, 0.3

, 0.5

I 1

, 3

, 5

AU, PPB (LOG) Fig. 7. C u m u l a t i v e probability distribution curves for Au in 32 metabasite s a m p l e s and 24 s a m p l e s o f hyperite.

ire samples. Metabasites have a low, uniform distribution. By contrast, the curve for hyperites is comprised of two distributions. The lower distribution, which is depleted relative to crustal abundance, is composed of hyperite samples that contain pyrite as the main sulphide mineral. The upper distribution, with values for Au extending into the normal range of crustal abundance, mainly comprises samples where pyrrhotite is important. Hyperite dyke 40 at Tvedestrand, with an exposed width of 22 m, shows a transition from an interior of coronitic gabbro to a foliated, amphibolitized margin. The gabbroic Interior Zone, about 3 m thick near the eastern edge of the outcrop (samples 4008 and 4009 ), has a granoblastic texture, with plagioclase, orthopyroxene, clinopyroxene and hornblende. A relict corona structure is shown by spinel symplectites. Orthopyroxene, which is strongly pleochroic, occurs in three forms: as relicts within clinopyroxene; as sub-idioblastic to xenoblastic grains; and as equant and triple-jointed crystals interstitial to plagioclase. Clinopyroxene replaces the first two types oforthopyroxene. A second type of clinopyroxene is large xenoblasts of igneous diallage. Plagioclase is zoned, with a spinel-rich inner core, and a margin free of inclusions. Oxide minerals are spinel relicts from corona symplectires, plus equant grains of ilmenite and magnetite. Samples have lesser hornblende than other parts of

the dyke. In this zone, pyrrhotite comprises approximately two thirds of iron sulphide minerals, the remainder being pyrite. Between the Interior Zone and the foliated Marginal Zone there is an Intermediate Zone (samples 4002-4007) where hornblende is more abundant than in the Interior Zone, at the expense of pyroxene. In this zone pyrite, as Py~ and PYNi, is more abundant than pyrrhotite (POb and Poi), but there is no mantling of pyrite by magnetite. At the western end of the exposure there is a 1 m thick Marginal Zone (samples 4001, 10402) of amphibolite with a sub-vertical foliation. This contains plagioclase, amphibole, ilmenite and magnetite; but no pyroxene remains. Pyrite as Py~ and PYN~ is the dominant sulphide, with only trace pyrrhotite. It is only in this Marginal Zone of the dyke that mantling of both pyrite and chalcopyrite by magnetite occur (Fig. 3d). Although rims of magnetite are narrow or incomplete and are not present in all grains of pyrite, this condition simulates that of the metabasites affected by the high grade metamorphic event. In addition to these sulphides, PYr is present in variable amounts in all three zones. Magmatic sulphide deposits are associated with some Bamble hyperites; the primary mineralogy is pyrrhotite-chalcopyrite-pentlandite. In deposits where the host intrusion is amphibolized, Brickwood (1986) found that this primary assemblage has been changed to a pyrite-magnetite-chalcopyrite-pentlandite. These changes are similar to that which occurred during amphibolization of dyke 40. Samples from the interior zone, through an intermediate zone, to the amphibolite margin decline in gold and related chalcophile elements, whereas other elements remain relatively constant (Fig. 8). The Interior Zone averages 0.8 ppb Au, which likely reflects a loss from the primary content of the gabbro, given the partial transformation ofpyrrhotite to pyrite. The Marginal Zone averages 0.3 ppb Au, somewhat greater than the average of 0.16 ppb Au for 16 nearby samples of metabasite from Tvedestrand. Ilmenite within dyke 40 is mainly 1M type. In the Interior Zone, where pyrrhotite is most abundant, IM grains have negligible amounts of hematite as blebs or lamellae. Across the dyke, into the amphibolized margin, hematite becomes more abundant in the ilmenite. In metabasites, exsolution of hematite preceded the formation of magnetite lamellae in ilmenite; the converse seems to have occurred

162

E.M. CAMERON ET AL.

TABLE4 uJ I-

'1'1'1'1'1'1'1'1'1'1'1'1'

~)

23~0107 (D LU ira

5

GABBRO INTERIOR ~ - - - - ~ (PYRRHOTITE) / I AMPHIBOLITE MARGIN ~ - - -

Analyses of IM- and IHM-type ilmenite from the Interior Zone (4009) and Marginal Zone (4001) of hyperite dike 40 at Tvedestrand. Analyses of IM-type grains were by a broad, 40 tzm, microprobe beam to include thin lamellae of magnetite and blebs of hematite IM-type ilmenite

IHM-lype ilmenite

Sample No. anal.

4001 llmenite 35

4009 llmenite 21

llmenite 2

4001 Hematite 2

SiO2 TiOz A1203 Cr203 FeO MnO MgO V203 NiO Total Xilm X ulvo

0.02 45.34 0.06 0.04 52.16 0.97 0.46 0.20 0.02 99.27 0.86

0.02 45.65 0.05 0.05 51.38 0.75 0.54 0.21 0.03 98.68 0.87

0.00 50.21 0.04 0.17 48.70 0.90 0.70 0.47 0.12 101.31 0.94

0.05 9.00 0.05 0.08 82.96 0.10 0.02 0.37 0.07 92.70 0.18

~ ~ TI ~J~>~

3

Rb Ba Y Ce Eu Yb Sc Zr Cr Ni Cu Sb S K Sr La Sm Tb Lu Ti P Co Zn As Au Fig. 8. Spidergram plot for elements in Marginal Zone (pyrite-dominant ) and in Interior Zone (pyrrhotite-dominant) ofhyperite dyke 40. Data represent the mean of 2 samples for each zone, normalized to the mean composition of 16 metabasite samples from Tromoy.

within the hyperites. Blebs of hematite fill areas of the ilmenite grain that are free of magnetite (Fig. 4d). Presumably, formation of magnetite lamella depleted the surrounding area in hematite present in solid solution. To obtain the primary composition of IM-type ilmenite in coronitic gabbro and amphibolized equivalents, samples from the Interior Zone (4009) and Marginal Zone (4001) of dyke 40 were analyzed by broad beam (40/zm) methods and the average of a large number of such analyses averaged (Table 4). The beam included the hematite and magnetite phases, which are too thin for independent analysis. The results for the two samples, which are similar, plot as an isopleth that approaches the FMQ buffer at temperatures of magmatic crystallization (Fig. 6). This is lower than the primary ilmenite isopleths representing the metabasites. Magnetite lamellae and adjacent ilmenite were analysed in grains of IHM-type ilmenite in samples 4001 and 10402 from the Marginal Zone. The results (Fig. 6) suggest an increase in Afo2 during amphibolization. Magnetite in the form of grains continued to requilibrate to low temperatures, expelling Ti. Grains in sample 4001 from the Marginal Zone contain < 0.1% TiO2, contrasting with 3.4% TiO2 (Table 4 )

Magnetite 2 0.00 1.91 0.13 0.06 91.07 0.00 0.06 0.17 0.00 93.40 0.06

for magnetite lamellae in ilmenite grains from the same sample. Even magnetite grains from the coronitic gabbro interior of this dyke have retained little Ti, averaging 0.35% TiO2 in sample 4009. In the Marginal Zone, magnetite seems to have formed during amphibolization. Two samples from the Marginal Zone average 1.25 wt.% magnetite, compared to 0.64 wt.% for 7 samples from the other two zones. Many of the magnetite grains from the Marginal Zone are strongly elongate along the foliation, suggesting that they formed during deformation and amphibolization. As for the metabasites, re-equilibration of grains of magnetite at low temperature precludes their use in constructing a path in fo2-T space for metamorphism of dyke 40. Metamorphism of hyperite intrusions was mostly at amphibolite grade. However, on Tromoy, a hyperite dyke (dyke 50) is metamorphosed to hornblende granulite. It has a granoblastic texture with, in places, a weak foliation. Veins of hornblendite occur peripheral to the dyke. The mineral assemblage is plagioclase, hornblende, orthopyroxene, clinopyroxene, ilmenite and magnetite. Large porphyroblasts of garnet, up to 5 cm, occur irregularly through the rock. There are no relict corona textures. The sulphide assemblage is POb, Poi, PYNi, PYr and chalcopyrite, with pyrrhotite dominant over pyrite. Neither PYi nor PYimare present. There are flames of pentlandite in Po~. Ilmenite is of the IM-

MOBILIZATION OF GOLD IN THE DEEPCRUST U.I I--

0

&.

0 0 u.I I< m < t-U.I

10

'1'1'1

I'1'1'1'1'1'

163

'

'1'

7

5

3

1

o

0.7

Lu

0.5

>a

0,3 ' Rblg, I ;,l

K

' 1 ' 1 ' ,lEu' 1 y. sc

' ' c, N?cJsb I

1 ' 1 '

Sr La Sm Tb Lu 77 P Co Zn As Au

Fig. 9, Spidergram plot for elements in hyperite dyke 50, metamorphosed to granulite grade. Data represent mean of 3 samples normalized to the mean composition of 16 samples of metabasite from Tromey.

Pyroxenes and other Fe-bearing silicates tend to preserve their peak metamorphic compositions during cooling, whereas oxides tend to re-equilibrate. This led Frost et al. ( 1988 ) and Frost ( 1991 a, b) to suggest that the relative proportions and compositions of oxide minerals have a strong influence on the path taken byfo2 during metamorphic cooling. Rocks in which magnetite is dominant will tend to decrease in Afo2 because exsolution of ulvospinel consumes 02. Where ilmenite is dominant, and particularly where ilmenite is rich in hematite, cooling is sub-parallel to the ilmenite isopleth, increasing in Afo2. The path taken during cooling of the metabasites is sub-parallel to the FMQ buffer and intersected the PPM buffer at < 600°C (Fig. 6). The maximum temperature of entry into the pyrite field, if cooling had been along the ilmenite isopleths, is about 620°C. On reaching the PPM buffer there was reduction of hematite within the iimenite grains to magnetite, permitting the complementary oxidation of pyrrhotite to pyrite: 6FeS + 12Fez 03 ~ 3FeS2 + 9Fe3 04

type, with negligible blebs or lamellae of hematite. This rock, although metamorphosed to granulite grade, has retained pyrrhotite. Except for As, the spidergram (Fig. 9) for dyke 50 shows a strong peak for chalcophile elements, relative to the metabasties. But Rb is strongly depleted (mean ratio K/Rb, 2225), more so than the metabasites (mean ratio K/Rb, 1090).

The fo2 path taken during cooling from the metamorphic peak, together with oxidation of pyrrhotite to pyrite, can be accounted for by internal buffering of the rocks, with no need to invoke an introduced fluid phase. Following this, there was partial replacement of pyrite to magnetite. This required the introduction of a fluid, which served as an oxidant, and which removed the S released by the reaction. Cameron (1989b) suggested the reaction:

Discussion

2FeS + 2 H 2 0 + 202 = 2H2 S +

Prograde metamorphism of the shear belt, which affected the metabasites, resulted in an oxidized mineral assemblage. Equilibration between the oxide and silicate fractions of the metabasites at peak metamorphic temperatures was at ~ 2.7 log units above FMQ, equivalent to afo2 of - 11.8 at 800°C. Other lithologies, such as tonalite, also had oxidized assemblages. Applying the empirical formulation of Fonarev and Grafchikov (1984) to orthopyroxene compositions at 800°C and 7.3 kbar, Cameron ( 1989b ) obtained averaged fo2 values of - 11.8 for metabasite and - 12.2 for tonalite gneiss, both groups from the granulite facies on Tromoy. Harlov ( 1992 ) estimated - 12.4 at 830 ° C for granulite-facies tonalite gneiss from Tromoy and Hisoy.

Extraction of gold from mafic rocks during metamorphism requires the modification of the sulphide fraction, which is the principal host to gold. Examination of the metabasites has shown that sulphide minerals were very readily modified, continuing to adjust to changing conditions after peak metamorphism. This is in contrast to the stability of the silicate assemblage. Loss of gold may occur by dissolution of sulphides; by phase change, such as pyrrhotite to pyrite; or by recrystallization without phase change. The cooling curve (Fig. 10) suggests several opportunities for the modification of sulphides and release of gold. The first of these (A, Fig. 10) was at peak metamorphism when pyrrhotite was a stable phase and the estimated fo2 was

SO 2

Jr Fe304

164

E.M. CAMERON ET AL.

5

o

,~s~,:,.-,~,'~, "

ii +

~

......~ ' ~ ' ~ (

'*~0

A

O M ETAMORPHISIv O ...I

a

AMPHIBOLIZATION (HYPERITE)

O

I

I

1

I

I

400

500

600

700

800

Temperature

900

°C

Fig. 1O. Summary diagram offo2-T path for metabasiles metamorphosed to amphibolite and granulite grade and an approximation to the path taken during amphibolization of hyperite dyke 40. The path taken by the metabasites was internally buffered by the oxidized mineral assemblage formed at peak metamorphism, whereas that for the hyperite was externally buffered, probably by the fluid associated with amphibolization. The points A, B and C along the metabasite path define the periods when Au may have been lost from primary sulphide phases (see text for discussion ). close to the H2S= SO2 boundary. A position close to this boundary favours dissolution of sulphide minerals (Whitney, 1988 ) and may also account for the depletion of S and Cu, in addition to Au, Sb and As, in granulite-facies metabasites from Tromoy (Cameron, 1989b ). The second opportunity for the release of gold came when the cooling path crossed into the stability field of pyrite (B, Fig. 10). The mineralogical change from pyrrhotite to pyrite may have made the contained gold accessible for extraction by a fluid phase. However, whether gold was extracted from the rocks at this time is moot. As argued above, the transformation of pyrrhotite to pyrite was internally buffered by the mineral assemblage and did not require the introduction of a fluid oxidant. The third period for the extraction of gold was during the partial replacement of pyrite and chalcopyrite by magnetite (C, Fig. 10). This would have been an opportune time for the extraction of gold from the shear belt, since, as noted above, the presence of an oxidized metamorphic fluid is indicated. Dissolution of metallic gold is a two-stage process: first oxidation of Au ° to Au +, followed by the complexing of Au ° by a ligand. The introduction of a fluid oxidant would facilitate the Au ° to Au + reaction and the dissolution of sulphide could have provided a sulphide ligand (Cameron, 1989b). Hyperite dyke 40 provides more definitive evidence for the processes leading to the loss of gold,

Amphibolization of the coronitic gabbro led to the transformation of pyrrhotite to pyrite, followed by the partial replacement of pyrite by magnetite. Samples representing these progressive mineralogical changes also show a progressive fall in the contents of gold and related chalcophile elements. The coronitic gabbro equilibrated near the FMQ buffer, in the field of stability of pyrrhotite. Amphibolization of the gabbro resulted in oxidation, including the formation and equilibration of IHM-type ilmenite grains, and the crossing of the PPM buffer into the stability field of pyrite. Unlike the metabasites, this path was not buffered by an oxidized, high-temperature mineral assemblage, but required the introduction of a fluid phase. It is possible, but unproven, that this fluid phase was the same as that which caused the partial oxidation of pyrite and chalcopyrite to magnetite in the metabasites and other rocks metamorphosed at high grade that host the hyperite intrusions. Amphibolized hyperites are foliated, which would have facilitated the passage of fluid. Hyperites that remain as coronitic gabbros, lack a marked foliation. The requirement for a fluidbased oxidant is most apparent for the magmatic segregation ores in hyperites described by Brickwood (1986). These are massive bodies, dominantly of pyrrhotite, which has been largely changed to pyrite where the host intrusion is amphibolized.

MOBILIZATIONOFGOLDIN THEDEEPCRUST Hyperite dyke 50 was metamorphosed to hornblende granulite, but this was not accompanied by oxidation and the rocks retain a pyrrhotite-dominant sulphide mineralogy. This dyke retains a relatively high tenor of chalcophile elements but is depleted in Rb. The data from this dyke supports the evidence given above for the role played in loss of gold by oxidative metamorphism and transformation of a primary pyrrhotite mineralogy.

Conclusions The Bamble shear belt is distinguished by abundances of gold and related chalcophile elements that are substantially below crustal abundance values. Since this is expressed by a variety of protolith lithologies (Cameron, 1989a, b), it is probable that depletion of these elements was the result of metamorphic processes. Metabasites both from the granulite and upper amphibolite facies acquired an oxidized mineral assemblage during prograde metamorphism, well above the FMQ buffer. Other lithologies also experienced oxidative metamorphism, including tonalite gneiss. Oxidized conditions, both during and after peak metamorphism, were favourable for mobilization of gold by (a) allowing dissolution or phase change in the sulphide minerals that are a principal host to gold; (b) encouraging the oxidation reaction Au ° to Au ÷ and; and (c) by dissolution of sulphide ligands to complex with Au +. Intermediate stages in the loss of gold are recorded in the hyperites, which were intruded and then partially amphibolized after peak metamorphism of the belt. Decrease of gold and other chalcophile elements is coincident with mineralogical changes that accompany amphibolization of coronitic gabbro within a hyperite dyke, principally the transformation of pyrrhotite to pyrite. Preliminary analyses by ion microprobe (S.L. Chryssoulis and E.M.C) of sulphide grains from a hyperite dyke showing the partial replacement of pyrrhotite by pyrite (Fig. 3b) indicate a sharp drop in gold content from pyrrhotite to pyrite.

Acknowledgements Gwendy Hall and Judy Valve carried out the the analyses of gold. Tom Andersen, Arne Bjorlykke,

165 Edgar Froese, Daniel Harlov and Ron Frost all contributed constructive comments on the manuscript. We are most grateful to all these persons.

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