Magnetic felsic intrusions associated with Canadian Archean gold deposits

Keiko Hattori Ottawa-Carleton Centre for Geoscience Studies,

Department of Geology, University of Ottawa, Ottawa K1 N 6N5, Canada

ABSTRACT Felsic intrusions spatially and temporally associated with Archean Au deposits in the

Superior Province of the Canadian Shield are magnetic, due to high magnetite content, in comparison to felsic igneous rocks in most of the Archean terrain. Examples are found in Au camps of Hemlo, Red Lake, Harker-Holloway, Mishibishu Lake, Wawa-Missanabie, Gerald- ton, Matachewan, and Bouquet-East Cadillac. These Archean intrusions are comparable with Phanerozoic magnetite-series plutons accompanying metallic mineralization and magnetite- rich intrusions hosting Au-rich porphyry deposits. The evidence shows a distinct tectonic setting for the gold mineralization involving the generation of the particular magmas. The association of the two indicates either the derivation of the ore constituents from the intrusions or the emplacement of the magmas along dilation zones which also provided fluid flow from depth. The identification of magnetite-rich felsic intrusions may aid in targeting favorable exploration areas for Archean Au deposits.

INTRODUCTION Magnetite-series and ilmenite-series granitoids

are recognized in Phanerozoic orogenic terrains (Ishihara, 1977, 198 1). The magnetite-series in- trusions are characterized by the occurrence of magnetite, generally in a range between 0.1 and 2 vol%, and they display magnetic anomalies. The ilmenite-series plutons contain less oxides, <0.1 vol%, and no magnetite (Ishihara, 1977). The difference is attributed to a difference in oxidation of the magmas; magnetite-series magmas have higher oxygen fugacity. Phanero- zoic porphyry Cu deposits are exclusively hosted by magnetite-series intrusions, and base- and precious-metal mineralization may also accom- pany the intrusions (Ishihara, 1981). Ilmenite- series intrusions are generally barren, but some bear Sn and W mineralization, as in the Malay- sian and Bolivian Sn belts (Ishihara, 198 1).

Whereas felsic intrusive rocks, underlying large parts of Archean terrains, have generally low magnetic susceptibilities compared with surrounding mafic and ultramafic rocks, felsic intrusions associated with Archean Au min- eralization are abnormally magnetic. This paper presents examples and discusses the significance of magnetic plutons to Archean Au mineralization.

MAGNETIC FELSIC INTRUSIONS IN THE CANADIAN ARCHEAN GOLD CAMPS Hemlo

Au deposits occur in an intensely deformed part of an east-trending greenstone belt bounded by granitic rocks (Fig. 1); the Pukaskwa com- plex to the south, Cedar Lake pluton to the northeast, Heron Bay pluton to the southwest,

and Gowan Lake pluton to the northwest (Fig. 2; Muir, 1982a, 1982b). Porphyritic felsic dikes are abundant near the ore zones and were prob- ably derived from the Cedar Lake pluton (Corfu and Muir, 1986). The Au mineralization, which is bracketed by U-Pb zircon ages of dikes, was synchronous with the Cedar Lake pluton (Corfu and Muir, 1986). The Heron Bay and Cedar Lake plutons and the Pukaskwa complex have similar granodioritic compositions (Muir, 1982a, 1982b), but their magnetic susceptibilities are

very different (Fig. 2). The Heron Bay pluton is nonmagnetic, similar to the Pukaskwa complex (Ontario Department of Mines-Geological Sur- vey of Canada, 1963, Maps 21 566 and 2 167G). The Gowan Lake and Cedar Lake plutons dis- play distinct magnetic anomalies. In particular, the Cedar Creek stock, a satellite of the Cedar Lake pluton, and the western margin of the Cedar Lake pluton show concentric magnetic anomalies (Fig. 2). They are located only -300 m north of the Au mineralization.

Figure 1. Locations of some Archean gold deposits in Superior Province: 1 = Bousquet, 2 = Cameron Lake, 3 = Geraldton, 4 = Harker-Holloway, 5 = Hemlo, 6 = Kirkland Lake, 7 = Lac Shortt, 8 = Mishibishu, 9 = Missanabie, 10 = Matachewan, 11 = Red Lake, 12 = Shebandowan, 13 = Timmins, 14 = Wawa. L = Larder Lake Break, DP = Destor-Porcupine fault, striped area = Gren- ville Province, dotted area = Phanerozoic terrain.

GEOLOGY, v. 15, p. 1107- 11 11, December 1987

Differences in oxidation conditions in the icate minerals due to high fo, (Cameron and plutons are also reflected in their alteration; the Camgan, 1987). margin of the Cedar Lake pluton displays hema- titic alteration, and the Heron Bay pluton shows Harker-Holloway no signs of oxidized alteration. The oxidized na- Several east-trending Au-bearing zones are ture of the magma of the Cedar Lake pluton is parallel to the Destor-Porcupine fault (Fig. 1). further confirmed by high Mg/Fe ratios in sil- The McDermott deposit is being developed with

Figure 2. Total intensity aeromagnetic contours and felsic intrusions in Hemlo area. Au on Trahs-Canada Highway HW17 indicates main orebody. BR = Black River. Plus signs indicate magnetic highs. Shaded areas are intrusions. A = Cedar Creek stock, B = Cedar Lake pluton, C = Pukaskwa complex, D = Gowan Lake pluton, E = Heron Bay pluton, F = batholith. Data sources: Ontario Department of Mines-Geological Survey of Canada, Maps 2156G, 2157G, 2167G, and 2168G (1963); Muir (1982a, 1982b); and Siragusa (1983).

reserves of 1.6 million tons at 0.263 oz/ton (Northern Miner [Toronto], May 5, 1986). Au- quartz veins occur in altered diorite and mafic volcanic rocks with abundant hematite (Work- man, 1986), suggesting that the mineralization took place under oxidized conditions.

The Harker stock and a stock on the bound- ary between Harker and Garrison townships, which were intruded near the Destor-Porcupine fault, have exceptionally high magnetic suscep tibilities, over 7.25 x lo3 emu/cm3, and they are prominent on the regional map covering 16 townships (Letros et al., 1983). Letros et al. (1983) called them "red magnetic syenites." Relatively unaltered samples indicate that they are magnetite-rich quartz monzonites with hematite and K-feldspar. The Garrison stock and a syenitic stock in the Michaud Township located farther away from the deposits are not magnetic (Ontario Department of Mines-Geo- logical Survey of Canada, 1970, Map 29563 1975, Map 20,140G).

Wawa-Missanabie Supracrustal rocks in the belt were intruded

by syntectonic tonalitic plutons and posttectonic granitic plutons and stocks. The Wawa area has several past producers within and near the Jubi- lee granodiorite stock, which is so magnetic as to attract hand magnets. Current producers in the Missanabie area (Renabie, Anglo-Dominion, and Canreos mines) are all located in the highly foliated Renabie pluton, which consists of quartz trondhjemite to granodiorite. The pluton con- tains abundant magnetite and exhibits distinct anomalies (Ontario De~artment of Mines-Geo- logical s u k e y of canaha, 1963, Map 2220G). Other foliated plutons with similar composi- tions, such as Marsh pluton (Bennett, 1978), are not magnetic (Ontario Department of Mines- Geological Survey of Canada, 1963, Map 22196) and are not known to accompany any significant mineralization.

Recent discoveries of economic Au deposits (Kremzar and Magino) are located near other magnetic intrusions in the area, the Finan gran- ite stock, Kremzar diorite, and Magino grano- diorite stocks (Ontario Department of Mines- Geological Survey of Canada, 1963, Map 21936). Less prominent but slightly magnetic intrusions in the area are the Ash Lake pluton (Ontario Department of Mines-Geological Survey of Canada, 1963, Map 22206) and Lachalsh Bay stock (Ontario Department of Mines-Geological Survey of Canada, 1963, Map 22076). Both are accompanied by Au showings.

r 7 - 1 0 5 10 KM

Mishibishu Lake Figure 3. Total intensity aeromagnetic contours and felsic intrusions in Mishibishu Lake area. A, mineralization occurs on the north shore Stars on north shore of Mishibishu Lake indicate gold deposits. Plus signs indicate magnetic of Mishibishu Lake near the contact between highs. Extremely magnetic band in northeast corner is due to occurrence of banded iron forma- tion. Data sources: Ontario Department of Mines-Geological Survey of Canada, Map 2176G mafic volcanic rocks and elastic sedimentary (1963); Bennett and Thurston (1977). rocks (Heather, 1985). The supracrustal rocks

1108 GEOLOGY, December 1987

are intruded by several batholithic granites and stocks. The oval-shaped Mishibishu Lake stock consists of porphyritic monzonite (Bennett and Thurston, 1977). Abundant magnetite causes a prominent magnetic anomaly, noted by Bennett and Thurston (1977), and the rocks commonly exhibit hematite and K-feldspar alteration. The abundance of magnetite, the occurrence of primary titanite in plagioclase, and the lack of ilmenite implies high fo, for the magmas (Noyes et al., 1983). The magnetic nature of the stock contrasts with surrounding nonmagnetic granitic plutons which are barren of Au (Fig. 3; Ontario Department of Mines-Geological Survey of Canada, 1963, Map 21766).

Red Lake The supracrustal rocks of the Red Lake

greenstone belt are dated at 2992 to 2733 Ma, and they were intruded by several batholiths and stocks of granodioritic to monzonitic composi- tion at 2700 and 2731 Ma (Andrews et al., 1986). Detailed zircon age determination of dikes suggests that the mineralization was syn- chronous with the intrusive activity (Andrews et al., 1986).

The central mining area, including the Dick- enson and Campbell mines, is surrounded by the s rout Lake batholith to the east, the Dome and McKenzie stocks to the south, and the Hammell Lake batholith to the west. A southern Au zone,

including the Madsen and Starratt-Olson mines, is bounded by the Killala-Baird batholith and Faulkenham Lake stock to the west, the Dome stock to the north, and the Keg Lake stock to the east.

Iron formation and ultramafic intrusions ob- scure the magnetic nature of the felsic intrusions. However, the Dome, McKenzie, and Howey stocks, the southern part of the Little Vermillion Lake batholith, the Hammell Lake batholith, and the southern part of Trout Lake batholith display high total magnetic fields (Ontario De- partment of Mines-Geological Survey of Can- ada, 1960, Map 8526; Ontario Geological Survey, 1978, Map P1579). The Trout Lake

TABLE 1. ARCHEAN GOLD CAMPS AND MAGNETITE-BEARING FELSIC INTRUSIONS I N SUPERIOR PROVINCE, CANADA

Au* Oxidized p lu tons Z i rcon age L i t h o l o g i e s Remarks x 106 y r s

Heml o 534 (1) Cedar Creek s tock Cedar Lake p l u t o n 2690 (2 ) Gowan Lake p l u t o n

Missanabie 65 (8 ) Renabie p l u t o n Magino stock Cl i ne Lake complex Ash Lake p l u ton Lachalsh Bay stock

Wawa J u b i l e e stock Mi sh ib i shu a5 (14) Mish ib ishu monzonite s tock Red Lake 401 (1) Dome stock 2718 (17)

McKenzie s tock Howey stock Trout Lake b a t h o l i t h 2700 (17) L i t t l e V e r m i l l i o n L. b a t h o l i t h 2731 (17) K i l l a l a - B a i r d bath01 i t h 2704 (17) Hammell L. b a t h o l i t h 2717 (17) Faul kenham stock

Matachewan 23 (1) Cai ro and o the r , minor s tocks K i r k l and Lake 720 (1) "Syeni t i c " i n t r u s i v e complex

Harker- >13 (25) Harker s tock Hol l oway Stock on Harker IGarr ison

boundary Beardmore- 66 (1) C r o l l Lake stock

Geraldton Coyle Lake stock

El rnhi rs t Lake stock Kaby Lake stock

Cameron Lake >8.7 (31) Stephen Lake p l u t o n 2699 (32) Nolan Lake stock

Shebandowan Moss Lake p l u t o n Burchel l Lake p l u ton 2683 (30)

Bousquet- 73 (1) Mooshla complex East M a l a r t i c

Lac Shor t t 12 (36) I n t r u s i v e complex

Hb-bt gd i t e Hb-bt gd i te . l o c a l l y p. (3) Q-mz, l o c a l l y p. (5) Fol . tdmi te . gd i t e (9.38) Fol. hb g d i t e Fol. d r i t e , f o l . q - f -p (38) Tdmite, hb-bt g d i t e (13) Gdi te (13) Hb-bt g d i t e (38) Hb-px mz, Hb q-mz, l o c a l l y p. Gdi te (18) Q-d r i t e (18) D r i t e , tdmi t e (18) Gdi te , t d ~ n i t e , d r i t e (18)

Gran i te (21) Hb sye, q-sye, l o c a l l y p. Px-mz, hb-bt q-mz, q-sye

hb f-p, (38) Q-mz (38) SYe (26)

Hb-bt g d i t e Tdmite, Hb-gdi te,

l o c a l l y p. (28) Hb-gdite, l o c a l l y p. (28.32) Hb-gdi te , tdmi t e (38) Hb-gdi t e MZ Q-sye, sye Hb-bt g d i t e , l o c a l l y p. Fol. q - d r i t e , q - d r i t e , q- f -p

Sye, d r i t e

Mg, t i t (3.4). mgntic (6) Mg, t i t (3.38). mgntic (6 ) My, t it (37), mgntic (7) My, t i t (38), mgntic (10) Mg (38). weak mgntic (11) Mg (12.38) Mg, t i t. (13) Mg, t i t. (13) Mg. tit. (38) Mg, t i t . (15,38), mgntic (16) Mg (18 ) , mgntic (19) Mgntic (19) Mgntic (20) Mgntic (20) Mgntic (20) Mgntic (20) Mgntic (20) Mg, T i t (21) Mg, t it (4,22,38), mgntic (23) Mg, t i t (24.38)

Mg (38), mgntic (26) Mgntic (26)

Mgntic (NW p a r t s ) (27) Mg, tit, g r - b t (28,29),

mgntic (27, 28) Mg (38), g r - b t (28), mgntic (27) Mg (38), mgnt ic (27) My (38), mgnt ic (33) Mgntic (N, W & S p a r t s ) (33) Mg, mgntic (34) Mg (38), weak mgntic (34) Mg (35,381

Mg (38), mgntic (37)

Note: Numbers i n parentheses r e f e r t o the f o l l o w i n g references: (1) Co lv ine e t a l ., 1984; (2) Cor fu and Muir , 1986; (3) Muir, 1982a; (4) Cameron and Carrigan, 1987; (5) Muir, 1982b; (6) On ta r io Department of Mines-Geological Survey o f Canada, Map 21676, 1963; (7 ) On ta r io Oepartment o f Mines-Geological Survey o f Canada, Map 21566. 1963; (8) Northern Miner (Feb. 16, 1987); (9) Bennett, 1978; (10) On ta r io Oepartment o f Mines-Geological Survey o f Canada, Map.2220G, 1963; (11) Ontar io Department o f Mines-Geological Survey o f Canada, Map 21936, 1963; (12) Bruce, 1942; (13) Sr ivastava and Bennett, 1978; (14) Nor thern Miner ( A p r i l 27, 1987); (15) Bennettand Thurston, 1977; (16) On ta r io Oepartment o f Mines-Geological Survey o f Canada. Map 21766, 1963; (17) Andrews e t a l ., 1986; (18) Horwood, 1945; (19) On ta r io Geological Survey, Map P1579, 1978; (20) On ta r io Department o f Mines-Geological Survey o f Canada Map 8526, 1960; (21) Ferguson, 1965; (22) Lovel I, 1967; (23) Ontar io Department o f Mines-Geological Survey o f Canada, Map 287G, 1970; (24) K.D. Hicks, personal commun., 1987; (25) Northern Miner (May 5, 1986; McDermott depos i t on1 y) ; (26) Le t ros e t a1 . , 1983; (27) On ta r io Department of Mi nes-Geol og ica l Survey o f Canada, Map 30,0026. 1974; (28) Mackasey and Wallace, 1978; (29) Mackasey, 1976; (30) S t o t t , 1985; (31) M e l l i n g e t dl., 1986; (32) Davis and Edwards, 1986; (33) On ta r io Department o f Mines-Geological Survey o f Canada, Map 11696, 1962; (34) Ontar io Department o f Mines-Geological Survey o f Canada, Map 11126, 1961; (35) V a l l i a n t , 1981; (36) Cormier e t al., 1984; (37) Quebec Oepartment o f Natura l Resources-Geological Survey o f Canada, Map 70966, 1966; (38) K. H a t t o r i (personal observat ion) .

b t = b i o t i t e , d r i t e = d i o r i t e , f o l . = f o l i a t e d , f - p = fe ldspar porphyry, g d i t e = g ranod io r i te , g r -b t = green b i o t i t e , hb = hornblende, mg = magneti te, mgntic = aeromagnetic anomaly, mz = monzonite, p. = p o r p h y r i t i c , px = pyroxene, q = quar tz , q - f -p = quar tz fe ldspar porphyry, q-mz = quar tz monzonite, sye = syeni te , t i t = t i t a n i t e , tdmi te = trondhjemite.

*Au produced p lus reserve i n m e t r i c tonnes.

GEOLOGY, December 1987 1109

batholith near the border between Ange and Shaver and between Shaver and Ranger town- ships displays strong anomalies, over 1000 gammas above background. The petrographic descriptions of dikes and stocks by Horwood (1945, p. 41,42,45) and Ferguson (1965, p. 13) confirm high magnetite content of the intrusions.

Beardmore-Geraldton This Au camp contains 18 past producers and

is in an east-trending greenstone belt east of Lake Nipigon (Fig. 1). The supracrustal rocks were intruded by felsic plutons, stocks, and nu- merous porphyry sills, and the intrusions host a part of the ore in the camp. Au deposits in the northern camp occur near the contacts of felsic stocks: the Quebec Sturgeon mine in and near the Elmhirst Lake stock and Coyle Lake stock, the Mitto and Greenoaks showings in the Elm- hirst Lake stock, and the Dik-Dik (Orphan) mine in and near the Kaby Lake stock. These stocks show prominent magnetic anomalies (Ontario Department of Mines-Geological Sur- vey of Canada, 1974, Map 30,002G). Mackasey (1976) noted the magnetic character of the Coyle Lake stock. Mackasey (1976) and Mack- asey and Wallace (1978) reported the occur- rence of magnetite, titanite, and green pleochroic biotite in these stocks and in minor porphyry sillsand dikes. Green biotite generally has high ~ e ~ + content relative to total Fe (Hayama, 1959).

DISCUSSION Table 1 lists some Canadian Archean Au

camps and the associated plutons. The composi- tions vary from tonalite to quartz syenite, and the margins of the intrusions commonly display foliation parallel to that in the host rocks. Their distribution along structural breaks and the de- formation in and around the intrusions are taken as evidence for syntectonic to late tectonic em- placement of the intrusions (e.g., Colvine et al., 1984; Stott, 1985). All the intrusions contain primary magnetite, suggesting that their magmas were oxidized. These intrusions are analogous to Phanerozoic magnetite-bearing felsic plutons in Southeast Asia, Alaska, western North America, and Chile, which commonly accompany base- and precious-metal mineralization (Ishihara, 1977, 1981). The fertile nature of the intrusions is further demonstrated in southern Pacific por- phyry copper belts (Mason and McDonald, 1978; Whalen et al., 1982). Chivas (1981) and Mason (1978) showed that the intrusions host- ing porphyry deposits were crystallized from oxidized magmas and that the magmas were progressively more oxidized during the crystallization.

Among porphyry Cu deposits, those rich in Au appear to be associated with even more highly oxidized intrusions. Sillitoe (1979) noted that the host intrusions for Au-rich porphyry Cu

deposits contain abundant magnetite. This ob- servation is confirmed in the Philippines (Sillitoe and Gappe, 1984) and South America (Sillitoe et al., 1982). The porphyry Au deposit at Por- gera, Papua New Guinea, is also hosted in a highly magnetic dioritic complex. Pitcher (1983) suggested that Au-rich porphyry Cu deposits are hosted in M-type granitoids, which are intrinsi- cally more oxidized than I-type.

In Archean greenstone belts, several workers have noted the spatial association of felsic intru- sions with Au mineralization (e.g., Hodgson, 1983). The temporal association was recently pointed out from detailed zircon age data of pre-ore and crosscutting dikes (Colvine et al., 1985; Andrews et al., 1986). This study shows that the intrusions associated with Archean Au mineralization are all magnetic and oxidized in the magmatic stage, equivalent to Phanerozoic magnetite-series intrusions. The oxidized condi- tion of the intrusions may be attributed to the intrinsically oxidized nature of the source mag- mas or vapor evolution from the magmas. If the magmas had high water content relative to Fe and if they were emplaced in dilation zones, preferential degassing of reduced gases, such as H2 and H2S, may have caused the oxidation of the remaining magmas (e.g., Candela, 1986). The occurrence of the intrusions along tectonic breaks is consistent with both possibilities, but the moderately high content of Fe in the intru- sions and the lack of reduced alteration near the intrusions may favor the intrinsically oxidized source magmas. In either case, the occurrence of magnetite-rich intrusions manifests a distinct tec- tonic setting involving the generation of particu- lar magmas and the gold mineralization.

The association of the oxidized intrusions and the gold mineralization suggests that the intru- sions are genetically related to the mineraliza- tion, or that the loci of the intrusions were the focus of the fluid flows. At present there is no strong evidence to negate the latter option, but I favor the derivation of ore fluids from or through the magma system on the basis of the evidence of the release of oxidized hydrothermal fluids from some of the intrusions (Hattori and Cameron, 1986; Cameron and Hattori, 1987).

Felsic intrusions, which are oxidized either intrinsically or extrinsically, have the potential to be fertile for Au for the following reasons. (1) Oxidized magma can incorporate Au from source and surrounding rocks into melt by oxi- dizing Au to Au+ and Au3+ and dissolving en- closing sulfides. (2) Oxidized magmas will not produce sulfide melts due to the high solubility of S (Carroll and Rutherford, 1985); therefore, Au would remain in the magmas. (3) Oxidized magmas will produce oxidized hydrothermal fluids. Au is more soluble in more oxidized fluids as ions or as chloride complexes. Au in thiocomplexes has maximum solubilities at fo2 near the boundary between reduced and oxi-

dized S species (Seward, 1973). Au, which parti- tioned into the fluid phase, would not precipitate immediately. Oxidized magmas thus efficiently extract Au, transport it to upper crustal levels, and concentrate it into hydrothermal fluids.

The above summary does not include the well-known Pearl Lake porphyry hosting the Hollinger-McIntyre deposits in Timmins, which does not display profound magnetic anomalies (Ontario Department of Mines-Geological Sur- vey of Canada, 1970, Map 2936). When the mineralization takes place within a stock, in- tense alteration directly related to the minerali- zation destroys magnetite to form pyrite and hematite. However, pervasive hematitic altera- tion with anhydrite (e.g., Mason and Melnik, 1986) and anhydrite of magmatic-hydrothermal origin in the stock (Hattori and Cameron, 1986) indicate the oxidized nature of the magma. This is also the case at the Kirkland Lake Au camp. Intensive replacement of magnetite by hematite causes less prominent aeromagnetic anomalies (Ontario Department of Mines-Geological Sur- vey of Canada, 1970, Map 289G), whereas rela- tively fresh hand specimens of the intrusions are strongly magnetic due to magnetite content up to 5 ~01%. The occurrence of celestite in rela- tively fresh intrusions and barite in veins with magmatic signature indicate the oxidized nature of the magma. Thus, the two largest Archean Au camps in North America, Hollinger-McIntyre and Kirkland Lake, are hosted in and adjacent to felsic intrusions that crystallized from ex- tremely oxidized magmas.

CONCLUSIONS Felsic plutons spatially and commonly tem-

porally associated with several major Archean Au deposits in the Abitibi greenstone belt are magnetic, equivalent to magnetite-series intru- sions in Phanerozoic orogenic terrains as defined by Ishihara (1977). The fertile nature of oxid- ized plutons proven in Phanerozoic terrains is also characteristic of Archean terrains. The pres- ent study has an important implication for the exploration for Archean Au deposits; the identi- fication of magnetite-rich intrusions and the rec- ognition of oxidized magmatic hydrothermal alteration may assist in selecting the favorable areas.

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ACKNOWLEDGMENTS Funded by a National Science and Engineering Re-

search Council Operating Research Grant and an Ontario Geoscience Research Grant. I thank S. Ishihara (Geological Survey of Japan) for guiding this study; K. Heather and S. Buck (Ontario Geological Survey) for samples from the Stephen Lake and Mishibishu Lake stocks, respectively; S. Butorac (Citadel Mines) for core from the Jubilee stock; R. Calhoun (Noranda Exploration), G. Yule (Canamax Resources), A. Bath, and H. Lovell (Ministry of Northern Development and Mines) for assistance in collecting samples; and G. K. Czamanske (US. Geo- logical Survey), K. Card, B. E. Taylor, and E. M. Cameron (Geological Survey of Canada), T. Urabe (Geological Survey of Japan), and A. C. Colvine (Ontario Geological Survey) for helpful comments on the manuscript.

Manuscript received June 23, 1987 Revised manuscript received September 9, 1987 Manuscript accepted September 23, 1987

GEOLOGY, December 1987 Printed in U.S.A.