Sulfur isotopic systematics of granitoids from ...

Sulfur isotopic systematics of granitoids from southwestern New Brunswick, Canada: implications for magmatichydrothermal processes, redox conditions, and gold mineralization

Mineralium Deposita International Journal for Geology, Mineralogy and Geochemistry of Mineral Deposits ISSN 0026-4598 Volume 45 Number 8 Miner Deposita (2010) 45:795-816 DOI 10.1007/ s00126-010-0307-6

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Author's personal copy Miner Deposita (2010) 45:795–816 DOI 10.1007/s00126-010-0307-6

ARTICLE

Sulfur isotopic systematics of granitoids from southwestern New Brunswick, Canada: implications for magmatic-hydrothermal processes, redox conditions, and gold mineralization Xue-Ming Yang & David R. Lentz

Received: 27 June 2008 / Accepted: 1 August 2010 / Published online: 15 August 2010 # Springer-Verlag 2010

Abstract Bulk δ34Srock values, sulfur contents, and magnetic susceptibility were determined for 12 gold-related granitoid intrusions in southwestern New Brunswick, the Canadian Appalachians. The sulfur isotope compositions of sulfide minerals in some of the granitoid samples were also analyzed. This new dataset was used to characterize two distinctive groups of granitoids: (1) a Late Devonian granitic series (GS) and (2) a Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS). The GS rocks have a large range in δ34S values of −7.1‰ to +13‰ with an average of 2.2±5.0‰ (1σ), low bulk-S contents (33 to 7,710 ppm) and low magnetic susceptibility values (10−3 SI), indicative of oxidized magnetiteseries granites. The exceptions for the GMS rocks are the Lake George granodiorite and Tower Hill granite that display reduced characteristics, which may have resulted from interaction of the magmas forming these intrusions with graphite- or organic carbon-bearing sedimentary rocks. The bulk δ34S values and S contents of the GMS rocks are Editorial handling: A. Boyce X.-M. Yang : D. R. Lentz Department of Geology, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3 Present Address: X.-M. Yang (*) Ginguro Exploration Inc., 101-957 Cambrian Heights Drive, Sudbury, ON, Canada P3C 5S5 e-mail: [email protected]

interpreted in terms of selective assimilation–fractional crystallization (SAFC) processes. Degassing processes may account for the δ34S values and S contents of some GS rocks. The characteristics of our sulfur isotope and abundance data suggest that mineralizing components S and Au in intrusion-related gold systems are dominantly derived from magmatic sources, although minor contaminants derived from country rocks are evident. In addition, the molar sulfate to sulfide ratio in a granitic rock sample can be calculated from the δ34Srock value of the whole-rock sample and the δ34Ssulfide (or δ34Ssulfate) value of sulfide and/or sulfate mineral in the sample on the basis of Sisotope fractionation and mass balance under the condition of magmatic equilibrium. This may be used to predict the speciation of sulfur in granitic rocks, which can be a potential exploration tool for intrusion-related gold systems. Keywords Sulfur isotopic composition . Sulfate/sulfide ratios . Redox condition . Granitoids . Intrusion-related gold systems . Appalachians . Canada

Introduction The redox conditions of felsic magmas and associated fluids play an important role in the formation of ore deposits, in particular for controlling sulfur speciation in mineralized systems (Burnham and Ohmoto 1980; Lehmann 1982; Ohmoto 1986; Rye 2005). Porphyry copper–gold deposits are normally formed under relatively oxidizing conditions, whereas intrusion-related gold systems require relatively reduced conditions (McCoy et al. 1997; Thompson et al. 1999; Baker 2002; Kesler et al. 2002; Fan et al. 2003; Blevin 2004). The ferrous/ferric ratio of igneous rocks and their constituent ferromagnesian assemblages are commonly used

Author's personal copy 796

to constrain the redox state of the systems in question, together with data from experimental petrology (Wones and Eugster 1965; Czamanske et al. 1981; Kress and Carmichael 1991; Anderson and Smith 1995). The magnetic susceptibility of granitoids can reflect redox conditions at the time of their crystallization or subsequent alteration (Ishihara 1981, 2004; Ishihara and Sasaki 1989). Thermodynamic calculations may also provide constraints on the oxygen fugacity f (O2) of a specific system (Whitney 1984). Sulfur speciation in silicate melts is a function of f(O2)-T-PX conditions (Carroll and Webster 1994). The molar ratio of sulfate to sulfide in basaltic melts can be expressed as an empirical equation in terms of oxygen fugacity (Wallace and Carmichael 1992). Under oxidized conditions, anhydrite occurs as magmatic phenocrysts in volcanic rocks (Luhr and Logan 2002) or as magmatic inclusions within common rock-forming minerals from felsic dikes (Audétat et al. 2004), whereas under relatively reduced conditions, sulfide minerals of magmatic origin are common in igneous rocks with a broad composition ranging from ultramafic to highly silicic (Poulson and Ohmoto 1990; Poulson et al. 1991; Borrok et al. 1999; Lightfoot and Naldrett 1999; Larocque et al. 2000; Yang et al. 2006). Thus, if the sulfur speciation is determined, then the redox conditions of the system can be constrained to some extent. In addition, saturation of either a felsic or a mafic magma with respect to sulfide would result in significant depletion of Au and Cu, preventing the formation of a metal-rich volatile phase (Peach et al. 1990; Jugo et al. 1999; Yang et al. 2006). However, a small to moderate amount of pyrrhotite crystallization may not destroy a potentially ore-forming system (Candela and Holland 1986). Marini et al. (1994, 1998) realized that the sulfur isotope composition and sulfur content of magmas could be a potential tool in evaluating their average redox conditions, which was supported by subsequent studies (de Hoog et al. 2001; Luhr and Logan 2002). Sulfur speciation in the gas phase and associated magma is mainly controlled by the redox condition—f(O2) of the system (Ohmoto and Rye 1979; Taylor 1986; Hoefs 1987; Rye 1993, 2005; Carroll and Webster 1994). How to use sulfur isotopes to estimate redox state of granitic systems and associated gold mineralization, however, remains to be studied. Recent studies have shown a high potential for the existence of intrusion-related gold systems in southwestern New Brunswick (McLeod and McCutcheon 2000; Chi 2002; Lentz et al. 2002; Davis et al. 2004; Thorne et al. 2008; Yang et al. 2008), an area renowned for W-Sn-Mo-Bi-Sb mineral resources associated with granitoids (McLeod 1990; Whalen 1993; McCutcheon et al. 1997). Many gold occurrences in the area (e.g., Poplar Mountain, Lake George, Clarence Stream, Kedron, Jimmy Hill; Fig. 1) share similarities with intrusion-related gold systems elsewhere. This has stimulated exploration efforts for intrusion-related gold deposits in this

Miner Deposita (2010) 45:795–816

region. However, an essential question about intrusionrelated gold systems is: do the intrusions supply mineralizing components (e.g., S, Au), and if so, can they be identified by a study of the sulfur isotopic composition of gold-related granitoid intrusions? This paper presents a new dataset on the bulk sulfur isotopic compositions and sulfur contents in 46 samples from 12 granitoid intrusions associated with gold mineralization and eight samples of the pertinent country rocks. Sulfide minerals from some of the granitoid samples were also analyzed for their sulfur isotope composition. On the basis of this dataset, sulfur sources and primary magmatic processes for these granitoids are evaluated. Then, a method to infer sulfur oxidation state is proposed by comparing the bulk sulfur isotopic compositions and sulfur contents of the granitoid rocks with the sulfur isotope compositions of their constituent sulfide minerals. This method can be used to estimate the relative redox conditions of the felsic magmatic systems and related hydrothermal systems, which may provide insights into the genesis of intrusion-related gold deposits.

General geology In southwestern New Brunswick, two major granitoid batholiths and associated satellite plutons were emplaced into the Gander and Avalon zones of the Canadian Appalachian Orogen (McLeod et al. 1994; Williams et al. 1999; Fig. 1a, b). The Pokiok Batholith cuts Cambrian to Middle Ordovician strata of the Gander Zone and Silurian strata of the Fredericton Basin (Fyffe and Fricker 1987). U– Pb zircon ages of the batholith are between 415 and 402 Ma (Whalen 1993). The Saint George Batholith (Cherry 1976; McLeod 1990; Whalen 1993) cuts Precambrian strata of the Caledonian Terrane, Ordovician strata of the St. Croix Terrane, and Early to Late Silurian strata of the Mascarene Basin (Fyffe and Fricker 1987; Fyffe et al. 1999). It is a composite batholith with isotopic ages (zircon U–Pb and 40 Ar/39Ar) ranging from 423 to 360 Ma (McLeod 1990; Whalen 1993; Davis et al. 2004). Although both batholiths are located near terrane boundaries, their emplacement occurred after amalgamation of the different crustal blocks. Two groups of granitoid intrusions associated with gold mineralization are recognized in this region: (1) a Late Devonian GS and (2) a Late Silurian to Early Devonian GMS in terms of field relations, geochemical and mineralogical characteristics, and age (Yang et al. 2003, 2006, 2008). The GS intrusions include Mount Pleasant, True Hill, Beech Hill, Kedron, Pleasant Ridge, Sorrel Ridge, and Mount Douglas (Butt 1976; Lentz and McAllister 1990; McLeod 1990; Lentz and Gregoire 1995; Whalen et al. 1996). According to the geochemical classification of Frost

Author's personal copy Miner Deposita (2010) 45:795–816

797

Fig. 1 Regional geological map of southwestern New Brunswick (modified from Chi 2002; Yang et al. 2006). a T3-J1: Upper Triassic to Lower Jurassic; D3-P: Upper Devonian to Permian; O3 to D1: Upper Ordovician to Lower Devonian. b Location of two series of granitoid intrusions, namely, Late Devonian Granitic Series (GS) granites: 1 Mount Pleasant granite suite (MPG), 2 True Hill granite (TRHG), 3 Beech Hill granite (BHG), 4 Kedron granite (KG), 5 Pleasant Ridge granite (PRG), and 6 Sorrel Ridge granite (SRG) on the northwest margin of Saint George Batholith (SGB), and 7 Mount Douglas granite, the eastern part of SGB; Late Silurian to Early Devonian Granodioritic to Monzogranitic Series (GMS) granitoids: 8 Poplar Mountain volcanic suite, 9 Lake George granodiorite (LG), 10 McDougall Brook granitoid suite, 11 Tower Hill granite (THG), 12 Evandale granodiorite (EG), 13 Magaguadavic granitoid suite (MGS), 14 Bocabec granitoid guite (BGS), and 15 Utopia granite (UP); 13, 14, and 15 constitute a western part of SGB. The location of the gold deposits and occurrences are shown: LG Lake George, CS Clarence Stream, PM Poplar Mountain, K Kedron, and JH Jimmy Hill

(a)

T3-J1 D3-P

Quebec

O3-D1

P.E.I

Ma

ine

New Brunswick

Humber Dunnage

PreCambrian to Middle Ordovician

Fig.1b location 100 km

Gander Avalon Meguma

(b)

CO

PM 8

9

LG

Pokiok Batholith 12

o 45 20’

Lower Paleozoic & Earlier

S1

4

Gander Avalon Middle Paleozoic Matapedia & Fredericton basins Mascarene Basin

5 11 TH

6

14

1

10

K JH 3 2 7 CS 13 St. George Batholith

15

N 20 km

Late Paleozoic Maritimes Basin Major faults Intrusions Silurian granites Early Devonian (gabbros)

Early Devonian (granites) Late Devonian (granites)

Gold occurrences PM = Poplar Mountain LG = Lake George CS = Clarence Stream JH = Jimmy Hill K = Kedron, TH = Tower Hill o

66 30’



et al. (2001), most GS rocks are ferroan, whereas some are magnesian granite within a narrow silica range (Fig. 2). They are calc-alkaline (Yang 2007; Yang et al. 2008) and collectively display the characteristics of highly evolved Itype granites with features of ilmenite series (Ishihara 1981, 2004) exhibiting relatively reduced conditions (Seal et al. 1987; Yang and Lentz 2005). They intrude Ordovician to Devonian sedimentary rocks (e.g., quartzite, slate, siltstone, and greywacke) along NE-trending fault zones (Fig. 1b). These intrusions show a variety of textural types ranging from equigranular to seriate and porphyritic and variations in accessory mineralogy (Table 1). They are all sub-solvus granites reflected by the presence of discrete sodic plagioclase and K-feldspar crystals, suggesting that they formed under moderate to high volatile pressures. The GMS granitoids constitute the western part of the Saint George Batholith, Tower Hill, McDougall Brook, Evandale, and the Pokiok Batholith (Butt 1976; Cherry 1976; McLeod 1990; Yang et al. 2008) (Fig. 1b). Most of the rocks in this series are granodiorite, but a few are monzogranite (i.e., the Bocabec granitoid suite; Whalen 1993; Table 1). The GMS granitoids are calc-alkaline (Yang et al. 2008) and mostly magnesian, although some highly differentiated phases are ferroan (Fig. 2). They intrude Cambrian to Upper Silurian greywacke, siltstone, shale, and volcaniclastic rocks, which have been deformed and regionally metamorphosed to middle to upper greenschist facies during the Acadian Orogeny (Fyffe and Fricker 1987). The GMS granitoids are either magnetite series or ilmenite series following the classification of Ishihara (1981, 2004). They are typically less evolved in composition than the GS rocks and appear to have been emplaced at variable structural levels (Yang and Lentz 2005). Hornblende is present, and plagioclase is more abundant than orthoclase in sub-solvus GMS granitoids. 1.0 MPG

FeOtot/(FeOtot+MgO)

0.9

TRHG BHG KG

0.8

PRG

ferroan

Intrusion-related gold mineralization A number of gold deposits and occurrences occur around these granitoid batholiths and associated satellite intrusions (Fig. 1b). The Lake George Sb-W-Mo-Au deposit, formerly the largest Sb producer in North America, is spatially and temporally associated with an unexposed Early Devonian granodiorite stock, a part of the Pokiok Batholith (Seal et al. 1987; Lentz et al. 2002; Yang et al. 2002, 2004; Leonard et al. 2006). Gold-bearing quartz-carbonate veinlets and stockworks are widespread in drill core around the property and ore-grade zones (up to 11.7 g/t Au) are locally developed (Lentz et al. 2002), which are associated with W-Mo mineralization within the granodiorite stock and the proximal metamorphic aureole. Multiple lines of evidence suggest that Au mineralization is related to the hydrothermal system associated with the intrusion (Seal et al. 1987). The Clarence Stream Au deposit occurs as Au-bearing quartz veins in the sheared contact zone of the Magaguadavic granite, a phase of the Saint George Batholith. Visible gold, aurostibite, and gold-antimony intergrowths in the quartz veins, are associated with a low ƒ(O2) sulfide mineral assemblage consisting of pyrrhotite, arsenopyrite, and berthierite. The mineralized zones, about 2 km along strike, trend NE and dip NW. Diamond drilling indicates several relatively high-grade intersections, including 15.06 g/t Au over 21 m in the East Zone, 21.51 g/t Au over 9.5 m in the Central Zone (Chi 2002). The link between gold mineralization and felsic magmatism is manifested by the presence of aplitic and granophyric granitic dikes that laterally change into auriferous sulfide-bearing quartz veins (Thorne et al. 2002). These dikes are late fractionates of the Magaguadavic granite. A recent study reveals that the characteristics of the Clarence Stream deposit are consistent with intrusion-related gold deposits (Thorne et al. 2008). The other Au occurrences (Fig. 1), such as Poplar Mountain, Jimmy Hill, Kedron, are reported recently and exhibit various features of intrusion-related gold systems (Chi 2002; Chi et al. 2008; McLeod et al. 2008; Yang et al. 2008).

SRG

0.7

LG THG

0.6

EG

magnesian

Petrography of sulfide minerals

MGS BGS

0.5 0.4 50

UG

60

70

80

SiO2

Fig. 2 FeOtot/(FeOtot +MgO) vs. SiO2 (wt.%) plot of granitoid samples. The boundary between ferroan and magnesian granitoids is from Frost et al. (2001). Shaded area denotes the Late Devonian GS granites. Symbols as in Fig. 1

Sulfide minerals are common in both groups of granitoids, although of low abundance and patchy distribution (i.e., rare disseminations), similar to I-type granites from the Lachlan fold belt (Whalen and Chappell 1988). The sulfide mineral assemblage (e.g., pyrite, pyrrhotite, and chalcopyrite) in the GS and GMS granitoids is relatively simple compared to that of the granitoids in the Bingham-Park City belt, Utah (Borrok et al. 1999), although all of the minerals are not necessarily present in each of the intrusions. Pyrite is

Rock

GRIII, topaz-bearing monzogranite

NMP90-1-1861

Porphyritic biotite monzogranite



TH80-6-120

TH80-6-389

TH82-10-181

Aplite

BH135

Li-mica topaz monzogranite


BR84-4-63

BR84-4-99



C82-5-63

C82-5-75



C80-10-260

C81-10-209

45°20′55.6″

45°20′55.6″

45°20′55.6″

45°20′53.0″

45°22′45.6″

45°22′45.6″

45°22′45.6″

45°25′17.6″

45°25′17.6″

45°25′17.6″

45°24′38.4″

45°24′32.6″

45°26′30.3″

45°25′56.6″

45°25′56.6″

45°25′53.6″

45°26′56.3″

45°26′22.6″

LG78-18-1190

Porphyritic granodiorite

Lake George Granodiorite (LG) 45°51′33.2″

Late Silurian to Early Devonian Granodiorite to Monzogranite Series (GMS) Rocks



C80-9-266

C80-10-37

Sorrel Ridge Granite (SRG)


C82-4-14

Pleasant Ridge Granite (PRG)


BR84-4-39

Kedron Granite (KG)


DBH-01-111

Beech Hill Granite (BHG)


DTRH-01-121

True Hill Granite (TRHG)

GRIII, topaz-bearing monzogranite

PRL95-1-1914

45°24′53.8″

45°26′22.6″

GRII, Biotite monzogranite


PRL95-2-1962

45°26′22.6″

Latitude


NMP89-1-1759

PRL95-2-2019

Mount Pleasant Granite Suite (MPG)

Late Devonian Granite Series (GS) Rocks

Sample

67°2′51.1″

67°2′55.6″

67°2′55.6″

67°2′55.6″

67°2′55.7″

67°3′15.5″

67°3′15.5″

67°3′15.5″

66°57′9.5″

66°57′9.5″

66°57′9.5″

66°54′15.1″

66°54′15.4″

66°52′37.5″

66°51′31.5″

66°51′31.5″

66°51′23.4″

66°48′44.5″

66°49′39.6″

66°45′56.0″

66°49′39.6″

66°49′39.6″

Longitude

23

31

30

30

31

36

35

36

34

35

34

40

27

31

30

33

31

35

34

33

34

33

Quartz

Table 1 Modal mineralogy of granitoid samples from southwestern New Brunswick

25

32

35

35

33

43

43

40

44

43

44

40

32

33

35

35

35

43

43

40

43

42

K-feldspar

45

33

31

31

31

20

21

23

20

20

20

18

33

30

30

27

28

20

21

24

20

23

Plagioclase

4

3

3

3

4

1

1

1

1

5

5

4

4

5

1

1

2

2

1

Biotite

Muscovite

2

Amphibole

Pyroxene

Apatite, zircon, titanite, ilmenite, pyrrhotite, pyrite, chalcopyrite

Ilmenite, zircon, monazite, pyrite, pyrrhotite

Ilmenite, zircon, monazite, pyrite, pyrrhotite

Ilmenite, zircon, pyrite, pyrrhotite

Ilmenite, zircon, pyrite, pyrrhotite

Rutile, monazite, zircon, ilmenite, columbite, xenotime, Li-mica, pyrite

Rutile, monazite, zircon, ilmenite, columbite, cassiterite, Li-mica, pyrite

Rutile, monazite, zircon, ilmenite, columbite, Li-mica, pyrite

Li-mica, topatz, zircon, ilmenite, pyrite, pyrrhotite

Li-mica, topatz, zircon, ilmenite, pyrite, pyrrhotite

Li-mica, topatz, zircon, ilmenite, pyrite, pyrrhotite, chalcopyrite

Monazite, zirocn, ilmentite, pyrite, chalcopyrite

Zirocn, aptite, ilmenite, pyrite

Zircon, monazite, titanite, cassiterite, ilmenite, pyrite

Zircon, monazite, titanite, ilmenite, magnetite, pyrite

Zircon, monazite, titanite, cassiterite, ilmenite, magnetite, pyrite

zircon, monazite, titanite, cassiterite, ilmenite, magnetite, pyrite

Ilmenite, zircon, fluorite, monazite,topatz, Libearing mica, pyrite

Ilmenite, zircon, monazite,topatz, columbite, Li-bearing mica, pyrite

Ilmenite, zircon, fluorite, monazite, pyrite

Ilmenite, zircon, fluorite, monazite, pyrite

Ilmenite, zircon, fluorite, monazite, topatz, pyrite

Accessory minerals

Author's personal copy

Miner Deposita (2010) 45:795–816 799






LG80-36-1647

LG81-14-1291

LG81-19-1351

LG83-2-1995

LG83-2-2461

Muscovite-biotite granodiorite




C80-3-136

C80-3-277

C80-3-499

C80-3-466

Coarse-grained granodiorite


Fine-grained monzogranite

Aplite

Pyrite-bearing quartz vein

DM-01-123

DM-01-130

DM-01-135

DM-01-139

DM-01-137

SU-01-129

01-134

Country rocks

Siltstone (OSSB)

Very coarse-grained granite

Fine-grained biotite granite

Medium-grained biotite granite

DB-01-97

DB-01-103

Utopia Granite (UG)

45°12′18.6″

Medium-grained gabbro

DB-01-92

45°23′31.4″

45°13′55.6″

45°11′39.1″

45°12′29.4″

Fine-grained gabbro

SDB-01-84

45°12′29.4″

45°23′31.4″

45°23′31.4″

45°23′31.4″

45°23′31.4″

45°19′17.0″

45°21′12.9″

45°35′58.2″

45°35′57.9″

45°17′9.9″

45°17′9.9″

45°17′9.9″

45°17′9.9″

45°16′49.6″

45°51′46.1″

45°51′46.1″

45°52′11.3″

45°52′11.3″

45°51′47.5″

45°51′46.3″

Latitude

66°39′45.0″

66°56′59.4″

67°8′8.8″

67°7′29.5″

67°8′7.3″

67°8′7.3″

66°39′45.0″

66°39′45.0″

66°39′45.0″

66°39′45.0″

66°44′51.7″

66°57′21.4″

66°4′5.7″

66°4′5.4″

67°14′3.7″

67°14′3.7″

67°14′3.7″

67°14′3.7″

67°13′49.6″

67°4′7.7″

67°4′7.7″

67°4′15.7″

67°4′15.7″

67°2′57.6″

67°4′8.0″

Longitude

27

22

23

3

2

95

33

23

21

23

22

23

21

22

21

25

23

22

23

23

22

25

20

21

Quartz

44

15

19

50

21

19

18

20

22

20

21

20

20

21

23

25

26

25

20

26

25

K-feldspar

25

50

45

48

50

15

50

55

51

50

48

50

52

55

49

50

50

46

45

47

49

48

47

Plagioclase

3

7

10

1

4

3

6

5

4

5

3

2

3

3

3

4

3

5

4

3

5

Biotite

1

1

2

2

1

Muscovite

5

2

7

10

1

1

1

2

2

3

1

2

1

2

1

Amphibole

41

37

Pyroxene

pyrite

Monazite, zircon, ilmenite, magnetite, pyrite

Magnetite, ilmenite, zircon, apatite, pyrite

Magnetite, ilmenite, zircon, apatite, pyrite

Magnetite, pyrite

Magnetite, pyrrhotite, pyrite

Pyrite

Zircon, ilmenite, magnetite, pyrite

Magnetite, ilmenite, apatite, zircon, monazite, rutile




Apatite, zircon, magnetite, ilmenite, pyrite

Apatite, zircon, magnetite, ilmenite, pyrite

Apatite, zircon, ilmenite, tatanite, pyrite, pyrrhotite











Accessory minerals

800

Bocabec Granitoid Suite (BGS)


DM-01-87

Magaguadavic Granodiorite Suite (MGS)

Granodiorite

Granodiorite

SEV-01-125

SEV-01-128

Evandale Granodiorite (EG)


TH-01-75

Tower Hill Granite (THG)


Rock

LG79-6-612

Sample

Table 1 (continued)


Author's personal copy

*Foliated gabbro from the eastern contact zone of the Tower Hill Granite intrusion.

OSSB Ordovician-Silurian Sand Brook Formation; OK Ordovician Cookson Group

Strata codes are after McLeod et al. (1994)

45°17′28.0″ Foliated gabbro* TH-01-83

67°11′44.7″

45°16′42.2″ Muscovite schist (OK) C80-2-523

67°14′6.1″

45°16′42.2″ Graphite slate (OK) C80-2-240

67°14′6.1″

45°17′27.2″ Garnet-muscovite schist (OK) TH-01-86

67°13′53.4″

45°18′29.6″ Graphite slate (OK) TH-01-85

67°10′44.0″

45°14′40.0″ Graphite slate (OK) TH-01-72

67°12′46.1″

45°23′31.4″ Greywacke (OSSB) 01-138

66°39′45.0″

Latitude Rock Sample

Table 1 (continued)

Longitude

Quartz

K-feldspar

50

Plagioclase

Biotite

Muscovite

30

Amphibole

19

pyrite

pyrite

pyrite, prrhotite

pyrite

pyrite, prrhotite, arsenopyrite

pyrite, prrhotite

pyrite

Pyroxene

Accessory minerals


801

enriched in aplites or some of the pegmatite dikes associated with the granitoid intrusions, where it occurs interstitially between quartz and feldspar crystals, or as narrow veinlets cross-cutting these minerals, suggesting that the pyrite is of hydrothermal origin. In both GS and GMS rocks, pyrite is the dominant sulfide phase. It is commonly associated with biotite, either distributed along cleavage planes or entirely included within it (Fig. 3a, b). Some pyrite crystals occur interstitially between quartz and feldspar. Euhedral pyrite is also occasionally included in plagioclase. A euhedral pyrrhotite replaced by magnetite (Fig. 3c) was observed within an orthoclase crystal, consistent with post-magmatic processes that modified the original magmatic pyrrhotite (Keith et al. 1997). Partly irregular, subrounded pyrrhotite is found mainly included in plagioclase (Fig. 3d); pyrrhotite is much less common than pyrite. Sparse chalcopyrite occurs in some of the GMS intrusions (Fig. 3e, f, g); it is more common in aplite dikes, such as those associated with the Beech Hill granite (Fig. 3h). Textural relations suggest that the sulfide minerals are primary, although those occurring interstitially among silicates must have crystallized after the silicates and, therefore, must be late-stage. However, the sulfides included in biotite and plagioclase must have formed earlier. Experimental data show that pyrrhotite can remain stable up to temperatures of 1,190°C, so it may have been present in the original magmas, if they were S-saturated. In contrast, pyrite is able to crystallize in magmas at temperatures only below 750°C (Craig and Scott 1974), which is above the 1 kbar haplogranite minimum inferred for many intrusions in this region (Butt 1976). Higher pressure can expand the stability field of pyrite; at 5 kbar, pyrite is stable to over 800°C (Sharp et al. 1985). In addition, the pyrite in our samples contains up to 830 ppm Ni and 1,788 ppm Co (Yang et al. 2006), consistent with previous findings that trace elements in pyrite increase its stability to higher temperatures (Barton and Skinner 1979). Although some of the pyrite may be hydrothermal, much of it seems to be magmatic, particularly those crystals that occur as inclusions in biotite and feldspar (Fig. 3a, b). Chalcopyrite with exsolved PbS inclusions (Fig. 3e–g) is most probably secondary, whereas that replaced by late pyrite more likely formed by exsolution of primary sulfides.

Analytical methods Approximately 1 kg of material for each sample was reduced to 0.5–2.0 cm chips in a steel jaw crusher. The chips were then pulverized to powder finer than 200 mesh in a soft-iron shatter box. Sulfur was extracted as H2S from the powdered bulk-rock samples by reaction with tin(II)strong phosphoric acid (“Kiba reagent”) at 280±10°C in a



Fig. 3 Photomicrographs of mineralogy and textural relations of sulfide minerals in granitoids. a Irregular pyrite associated with magnetite, which occurs as an inclusion in a biotite phenocryst from a seriate to porphyritic granite sample (NMP89-1-1759) in Mount Pleasant granite suite; b Rounded pyrite included in biotite crystal in the Sorrel Ridge granite, sample C81-9-258; c Hexagonal pyrrhotite crystal replaced by magnetite, both of which are included in orthoclase in a porphyritic monzogranite (sample 193A) from the McDougall Brook granitoid Suite; d Irregular pyrrhotite included partially in plagioclase from the Tower Hill granite, sample C80-2-580; e Chalcopyrite crystals interstitial to quartz and K-feldspar in sample C80-3499 from the Tower Hill granite; the tabular and cubic chalcopyrite crystals display dissolved or resorbed margins and contain irregular galena (PbS) inclusions (bright patches); f Irregular PbS inclusions on the left tabular chalcopyrite grain in (e); g Irregular PbS inclusions in chalcopyrite zoomed in the right dissolved grain in (e); h Chalcopyrite crystal that was replaced by late pyrite in sample BH135 from an aplite dike associated with the Beech Hill granite. a–c RPL (reflected plane light); d– h SEM-BSE image. Bt biotite, Cpy chalcopyrite, G galena, Kfp K-feldspar, Mt magnetite, Mus muscovite, Pl plagioclase, Po pyrrhotite, Py pyrite, and Qtz quartz

nitrogen gas flow (Kiba et al. 1955; Sasaki et al. 1979). Using this technique, the sulfur in the rocks, originally present as both sulfide, and sulfate, is completely reduced to H2S. This is then collected as Ag2S for weighing and is finally converted to SO2 based on the method of Robinson and Kusakabe (1975). Sulfide minerals separated with a micro-drill from the slab samples were processed using a combustion technique (Robinson and Kusakabe 1975), which converted the sulfide-sulfur to SO2. The isotopic analyses were carried out on a Finnigan Delta plus mass

spectrometer at the University of Ottawa, Canada. The results are expressed as δ34S values relative to the Cañon Diablo Troilite sulfur. The uncertainty of the reported value is within ±0.2‰ (1σ) based on replicate analysis of international standards IAEA-S1 (−0.3‰) and IAEA-S2 (+21.7‰). Bulk-rock sulfur contents were determined gravimetrically, assuming the product Ag2S (obtained via the Kiba technique) to be stoichiometric with an uncertainty 20‰) and low sulfur contents of contaminants (i.e., country rocks) are used in the modeling. Such country rocks have not yet been found in the study area, although this type of metasedimentary rock is common in the Meguma Terrane (Poulson et al. 1991), which has a different geology from the Gander and Avalon zones (Whalen et al. 1996; Williams et al. 1999). Magma degassing, however, is an effective process for generating a large range in δ34S values in fractionating magmas (Taylor 1986; Mandeville et al. 1998; Marini et al. 1998; de Hoog et al. 2001; Luhr and Logan 2002). Estimate of magma temperatures, based on models of zircon, apatite, and monazite solubility in felsic melts, are less than 800°C for the Pleasant Ridge granite and Beech Hill granite magmas (Yang and Lentz 2005; Yang et al. 2008). Such temperatures are too low for significant sulfur diffusion in felsic melts (Baker and Rutherford 1996), suggesting that sulfur would be retained in the magma during early-stage degassing. However, available data indicate that the Pleasant Ridge granite magma was water oversaturated and had a high fluorine content (up to 9,800 ppm; Taylor 1992; Yang and Lentz 2005), which may have raised sulfur diffusivity in the magma. A low-temperature and lowpressure degassing process that results in a sulfur isotopic shift has not been described in the literature for such highly evolved and high-level granite systems. Figure 8 shows how the change in sulfur isotopic composition of the magma could be caused by a loss of sulfur during degassing processes as a function of the fraction of sulfur remaining in the melt (F), compared to its

20 Closed-system Open-system

15

PRG

Smelt( ‰)

0.1

34

that the country rock has a very high S content (up to 20,000 ppm) and a δ34S value of about 0‰. This is not consistent with the composition of the local greywacke and siltstone country-rock (1,100 ppm and −7.5‰). The sulfur isotope compositions and sulfur contents of the other GMS rocks are best modeled by SAFC processes. In addition, SAFC modeling of other GMS intrusions suggest that the δ34S values of the parental magmas may have been as high as +7.0‰ with sulfur contents up to 300 ppm, similar to common I-type granitoids. Interintrusion variations in δ34S values and sulfur concentrations might reflect not only slight differences in their sources but also differences in their country rocks and in the degree of incorporation of country rock sulfur.


BHG SRG

10

MPG 0.3

5

0.5

0.1 0.3

0.5

0.7 0.7

0.9

0

-5 10

100

1000

Fig. 8 Degassing models for the Pleasant Ridge granite and Beech Hill granite, showing the change in sulfur isotopic composition of the magma due to loss of sulfur during degassing processes as a function of the fraction of sulfur remaining in the melt (F), compared to its initial composition. Two sets of modeling are shown: closed-system and open-system. Modeling was performed assuming a total pressure of 1 kbar, water saturation, a water fugacity coefficients of 0.75°C at 760°C (Burnham 1979), and a corresponding water fugacity of 0.75 kbar. The initial δ34S value and S concentration of the melt are assumed to be 2.2‰ and 700 ppm, respectively. The f(O2) condition is expressed as ΔNNO form; when ΔNNO equals to +0.5 log unit, the samples from the Pleasant Ridge granite and Beech Hill granite are confined within the field defined by the open-system and closedsystem degassing processes. Samples from Sorrel Ridge granite and Mount Pleasant granite suite are plotted for comparison, suggesting that low initial δ34S and lower temperatures are required for the degassing modeling (i.e., −4‰, 720°C, ΔNNO=+0.7), denoted by dashed lines. Symbols as in Fig. 1

initial concentration. Two cases are shown (see Appendix for detailed degassing modeling): one for a closed-system and the other for an open system. Modeling was performed assuming a total pressure of 1 kbar, water saturation (Butt 1976; Taylor 1992), a water fugacity coefficient of 0.75 at 760°C (Burnham 1979), and corresponding water fugacity of 0.75 kbar. The initial δ34S value and S concentration of the melt are assumed to be +2.2‰ and 700 ppm (Clemente et al. 2004). The f(O2) is expressed in ΔNNO notation that is the log unit of oxygen fugacity relative to that of the nickel–nickel oxide buffer, i.e., ΔNNO=log f(O2)sample–log f(O2)NNO (Huebner and Sato 1970). When ΔNNO equals to +0.5 log units, the samples from the Pleasant Ridge granite and the Beech Hill granite are confined within the field defined by the open-system and closed-system degassing processes. It is clear that open-system degassing is more effective in shifting the δ34S values of the melt to high positive values. Samples from the Sorrel Ridge granite and the Mount Pleasant granite suite are plotted for comparison, suggesting that a low initial δ34S value and even lower temperatures are required for the degassing modeling (i.e., −4‰, 720°C, ΔNNO=+0.7) denoted by dashed lines in


809 10

Figure 8. Calculations demonstrate that degassing could drive δ34S values up to +13‰ (Fig. 8). This is adequate to cover the range of variation of δ34S values observed in the Pleasant Ridge granite and Beech Hill granite samples.

o

In terms of mass balance (Taylor 1986), the following equation can be obtained.

where δ34Srock, δ34Ssulfide, and δ34Ssulfate represent the isotopic compositions of the rock (bulk), sulfide, and sulfate, respectively; x denotes the mole fraction of sulfate in the rock, which is related to oxygen fugacity (Wallace and Carmichael 1992; Yang and Lentz 2009). The sulfur isotope fractionation factor between sulfate and sulfide is only temperature dependent under crustal pressure (Ohmoto and Rye 1979; Ohmoto 1986; Hoefs 1987; Ohmoto and Goldhaber 1997) and can be expressed by the following equations. ð2Þ

(a)

-10 -10

-5

0

5

10

34

Srock (‰)

10 o

5

0

34

$sulfatesulfide ¼ d 34 Ssulfate d 34 Ssulfide

MPG TRHG KG SRG THG

-5

ð1Þ

Ssul e(‰)

d 34 Srock ¼ ð1 xÞd34 Ssulfide þ xd 34 Ssulfate

0

34

Sulfate to sulfide ratios in granitoids

S sul e(

)

5

-5

1000lnasulfatesulfide ðasulfatesulfide 1Þ 1000 $sulfatesulfide

ð3Þ -10 -10

(b) -5

0

5

10

34

Srock (‰)

$sulfatesulfide ¼ 6:463 106 T 2 þ 0:56 ð0:5Þ ð200 400 C; Ohmoto and Lasaga 1982Þ

ð4Þ

Fig. 9 Bulk-rock δ34Srock values (‰) of granitoids vs. constituent sulfide δ34Ssulfide values (‰) plotted on a calculated diagram showing the relationship of the mole fraction (x) of sulfate to sulfur isotopic composition of bulk-rock and sulfide in a magmatic equilibrium system. Calculated lines for x ranging from 0.0 to 1.0 are shown at 850°C (a) and 650°C (b). Symbols as in Fig. 1

$sulfatesulfide ¼ 7:4 106 T 2 0:19 ð600 1000 C; Miyoshi etal: 1984Þ

ð5Þ where Δsulfate–sulfide stands for the degree of fractionation between sulfate and sulfide in the system of interest; α sulfate–sulfide is sulfur isotope fractionation factor between sulfate and sulfide; T is temperature in Kelvin. At a given temperature (e.g., magmatic temperatures of 650°C to 850°C, common for granitoids), the δ34S value of sulfate in equilibrium with sulfide can be calculated, based on Eqs. (1–3) and (5) (Figs. 9a and b). Using Fig. 9a (T= 850°C) as an example, it can be predicted that both the fields below x=1.0 and above x=0.0 lines are prohibited for magmatic rocks (magmas), given sulfide and sulfate species are equilibrated isotopically. If located on the x=0.0 line (theoretically), the rock does not contains any sulfate

species and must have been formed under very reduced conditions. In contrast, if plotted along the x=1.0 line, the rock does not contain any sulfide phase and must have been formed under moderately to highly oxidized conditions. Most S-bearing magmatic rocks are constrained between these two extremes, and their exact locations depend upon their redox conditions that are related to their source region, magma composition, nature of country rocks, and the magmatic to hydrothermal processes involved. If crystallized at magmatic temperatures (Fig. 9), δ34Ssulfide value is consistently lower than that of bulk δ34Srock, given any sulfate is present in the system; the δ34Ssulfide value decreases with increasing x values that relate to redox conditions as the contribution of sulfide to the whole-rock δ34Srock values decreases. This is because oxidized compounds of sulfur have a greater tendency to retain 34S than


besides pyrite. The granite sample PRL95-2-1962 (Table 2) from the Mount Pleasant granite suite display a high x value (Fig. 9), and the δ34S value of sulfate in equilibrium with sulfide at 700°C was also calculated for this sample to be +5.3‰ according to Eq. 5 (Miyoshi et al. 1984), and the mole fraction of the sulfate (x) is required to be 0.645 in order to maintain mass balance (Eq. 1; δ34Srock =+2.6‰, δ34Ssulfide =−2.3‰; Fig. 10). With such high sulfate mole fraction, discrete sulfate minerals would be expected in the sample; however, no detectable sulfate minerals were identified. This may be explained in two ways. The temperature (700°C) used in the calculation is not realistic or magmatic equilibrium was not achieved. We did iterative calculations, selecting different temperatures to match the mass balance requirements. For example, at 300°C (certainly not a magmatic temperature), using Eq. 4 (Ohmoto and Lasaga 1982) to perform the computation, the calculated δ34S value of sulfate is +17.9‰, and x=0.242. At 933°C (also not reasonable for the Mount Pleasant topaz-bearing granite, as magma temperatures are not so high in terms of constraints from zircon, apatite, and monazite solubility models and other petrologic evidence; Yang et al. 2003), the calculated δ34S value of sulfate is +2.6‰, equal to the value of bulk-rock δ34Srock; thus, x is 50

o

‰

40 30

34 sulfate

o

5.3‰

Ssulfate(‰)

20

34

reduced forms, i.e., SO42− > SO3 > SO2 > S > H2S = S2− (Ohmoto and Rye 1979). It can thus be predicted that the more oxidized a magmatic system becomes, the lower the expected δ34Ssulfide. Otherwise, the magmatic system is either not in equilibrium or has been disturbed by later processes, if the δ34Ssulfide value is not low as expected. Moreover, if an igneous rock sample plots in the field above the x=0.0 or below x=1.0 lines, the implication would be that the sample experienced processes resulting in disequilibrium between sulfate and sulfide, which suggest that it may have interacted with sub-solidus hydrothermal fluids. Alternatively, external sulfur may have been introduced into the system via assimilation (Ohmoto and Goldhaber 1997). The Tower Hill granite samples, being GMS, show very low x values (10−3 SI), indicative of oxidized, magnetite-series granites. The exceptions for GMS rocks are the Lake George granodiorite and Tower Hill granite that have low magnetic susceptibility values and reduced characteristics, which may result from interaction (hybridization) of the intrusion with graphite or reduced organic carbon-bearing sedimentary rocks and a related CH4–CO2-bearing hydrothermal system (Yang et al. 2004). Reduced GMS intrusions, such as Lake George, may have high potential for generating magmatic hydrothermal gold mineralization (Lentz et al. 2002). The sulfur isotope composition and sulfur contents of GMS rocks are best interpreted by selective assimilationfractional crystallization processes (SAFC). Initial δ34S values of granitoid melts may range from −1.0‰ to +7.0‰ and initial sulfur contents are between 150 and 300 ppm, similar to common I-type granitoids. Interintrusion variations in δ34S and sulfur concentration may not only correspond to subtle differences in the magma sources, but also may reflect intrusion of the magmas into distinct country-rocks of variable composition. The initial δ34S values and sulfur concentration of the GS rocks are between −4.0‰ and +2.2‰, and 500 and 700 ppm, respectively. Low-temperature and low-pressure


degassing processes could account for the δ34S values and sulfur concentrations of some of these granitic rocks (e.g., the Pleasant Ridge granite, Beech Hill granite, Sorrel Ridge granite, and Mount Pleasant granite suite). The other GS intrusions (e.g., the True Hill and Kedron granites), however, are consistent with SAFC processes. The data for the bulk-rock δ34S values of the granitoids, together with their constituent sulfide sulfur isotopic compositions, can be used to estimate their redox conditions. δ34Srock > δ34Ssulfide indicates a relative oxidized state, whereas δ34Srock < δ34Ssulfide suggest a reduced condition. Using the magmatic equilibrium model proposed in this study, the ratio and origin of sulfate in granitoids are evaluated (Figs. 9 and 10). Similarly, if the δ34Ssulfate and δ34Ssulfide values of magma are known, the bulk δ34Srock value and sulfate ratio may be readily estimated at a certain temperature assuming magmatic equilibration. This method can be applied to estimating the relative redox conditions of felsic magmatic systems and related hydrothermal systems, which may provide insights into intrusion-related gold deposits. Acknowledgements We thank Wendy Abdi (the University of Ottawa) for assistance in determination of sulfur isotopes and Dr. Douglas C. Hall (the University of New Brunswick) for help in the SEM analysis. Dr. Steve McCutcheon, Dr. Kay Thorne, Les Fyffe, and Malcolm McLeod (New Brunswick Department of Natural Resources) are thanked for their insightful discussions on regional and local geology. Dr. G. Chi (the University of Regina) is acknowledged for his support at the initiation of this study. Dr. S. Ishihara (the Geological Survey of Japan) is thanked for kindly sending us his research papers and for his comments on the early version of the manuscript. The first version of the manuscript was reviewed by Drs. Greg Arehart, Phil Candela, Ron Frost, and Marjorie Wilson. Two anonymous reviewers are gratefully acknowledged for their critical and constructive comments that help significantly improve the manuscript. We thank Prof. Dr. Bernd Lehmann and Dr. Adrian Boyce for their comments and suggestions that classified some important issue of this paper. The project was funded by a grant from the Geological Survey of Canada and a NSERC Discovery grant to D. L. and the New Brunswick Innovation Fund.

Appendix Selective assimilation and fractional crystallization modeling It is necessary to briefly describe the AFC model before SAFC is introduced. AFC processes were described in detail by Taylor (1980) and DePaolo (1981) and generally termed bulk AFC, which are simplified by Rollinson (1993) as the following equations applied to predicting trace element and isotope evolution in a magma system being assimilated by country rocks. CL =Co ¼ f 0 þ

r CA ð1 f 0 Þ ðr 1 þ DÞ Co

ð6Þ


813

where f 0 ¼ F ðr1þDÞ=ðr1Þ , r is the ratio of the assimilation rate to the fractional crystallization rate, F is the fraction of magma remaining, D is the bulk distribution coefficient between crystals and melt; Co, CL, and CA are concentrations of the trace element in initial magma, residual magma, and the assimilated wall rock, respectively. dL do ¼

r C DΔ A dA do ð1 F z Þ r 1 zCL zðr 1Þ

r C DΔ A ln F 1 r1 r 1 zCL ð7Þ

where δo, δL, and δA are δ values of the isotope in the initial magma, residual magma, and the assimilated wall rock, respectively; z ¼ ðr þ Δ 1Þ=ðr 1Þ; Δ ¼ dminerals d L (≅ 1000lnαx-L, where lnαx-L is the isotopic fractionation factor between the crystallizing minerals and the magma). The AFC model was modified to describe a system where sulfur is transferred from country rocks to the magma through a hydrothermal fluid (about 500°C) buffered with a graphite–pyrite–pyrrhotite assemblage in the country rocks (Poulson and Ohmoto 1989). The f(H2S) of the fluid is greatly elevated, due to heating by the magma, and is higher than that of a fluid in equilibrium with the magma. The concentration gradient allows the transfer of sulfur from country rock into the magma on a very local scale, not requiring melting of the country rocks. This is termed SAFC process that can be described by the following equations (Poulson et al. 1991). CL =Co ¼ F

z0

þ

r0 QCA 0 r QCA 1

1 0 1 F z 0 z Co

ð8Þ

where z0 ¼ ðr0 QCA þ Δ 1Þ=ðr0 QCA 1Þ, r′ is rate of heating of country rock/crystallization rate, Q is fraction of sulfur in country rock transferred from country rock into magma. dL do ¼

r0 QCA 1 0 ðdA do Þ 1 F z 0 0 r QCA 1 z CL

ð9Þ

for the case when Δ=0.

whereby all exsolved vapor is immediately removed from the system: ð10Þ d 34 Sf ¼ d34 Si ð1000 þ d34 Si Þ 1 F ða1Þ and closed system, or batch degassing, whereby exsolved vapor continues to re-equilibrate isotopically with the melt. d 34 Sf ¼ d34 Si þ ðF 1Þ 1000ln a

ð11Þ

Subscripts f and i refer to the final and initial states, respectively; F is the fraction of S remaining in the magma, and α is the gas-melt equilibrium S-isotope fractionation factor (Marini et al. 1998). Following Sakai et al. (1982) the fractionation factor between coexisting gas (g) and silicate melt (m) can be expressed by: 1000 ln a ﬃ d 34 Sg þ d 34 Sm

¼ YGS d 34 SSO2 þ ð1 YGS Þd34 SH2 S h i YIS d 34 SSO2 þ ð1 YIS Þd34 SS2 4 ¼ YGS 1000 ln aSO2 H2 S þ YIS 1000 þ 1000 ln aH2 SS2 ln aS2 SO2 4

ð12Þ

where YGS ¼ XSO2 =ðXSO2 þ XH2 S Þ, which denotes the SO2 fraction in total gaseous S; YIS ¼ XSO2 =ðXSO2 þ XS2 Þ (see 4 4 2 Marini et al. 1998), which represents SO4 fraction in total ionic S in melt and is related to magma redox condition suggested by Wallace and Carmichael (1992). logðYIS Þ ¼ 0:48ΔNNO 0:70

ð13Þ

The fractionation factors in Eq. (12), dependent on temperatures, are approximated by the following equations (Taylor 1986). 3 3 10 10000 ln aSO2 H2 S ¼ 0:42 T 3 2 10 þ 4:367 T 3 10 0:105 0:41 ð14Þ T (for 400°C to 1,300°C)

Degassing Modeling

1000 ln aS2 SO2 4

The sulfur isotope composition δ Sf of magma due to loss of sulfur during degassing, relative to its initial composition δ34Si, is expressed as a function of the fraction of sulfur remaining in the magma (F). Two extreme cases are considered (Taylor 1986; de Hoog et al. 2001): open system, or fractional degassing (Rayleigh distillation),

103 ¼ 7:4 T

2 þ 0:19

ð15Þ

34

(for 600°C to 1,000°C) 3 2 10 0:19 1000 ln aH2 SS2 ¼ 1:1 T (for 600–1,000°C)

ð16Þ


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