The gold–vanadium–tellurium association at the Tuvatu gold–silver prospect, Fiji: conditions of ore deposition

Mineralogy and Petrology (2006) 87: 171–186DOI 10.1007/s00710-006-0128-6

The gold–vanadium–tellurium associationat the Tuvatu gold–silver prospect, Fiji:conditions of ore deposition

P. G. Spry and N. L. Scherbarth

Department of Geological and Atmospheric Sciences, Iowa State University,Ames, Iowa, USA

Received October 23, 2005; revised version accepted February 3, 2006Published online June 6, 2006; # Springer-Verlag 2006Editorial handling: N. Cook

Summary

The Tuvatu gold–telluride prospect is one of several epithermal gold systems along the>250 km northeast trending Viti Levu lineament, Fiji, which are genetically associatedwith alkalic magmatism. Vein structures contain a variety of sulfides, native elements,sulfosalts, and tellurides. Calaverite is intimately associated with various vanadium-bearing minerals: roscoelite, karelianite, vanadian muscovite, Ti-free nolanite, vanadianrutile, schreyerite, and an unnamed vanadium silicate. Thermodynamic calculationsfor the systems V–Al–K–Si–O–H (Cameron, 1998) and Au–Te–Cl–S–O–H at esti-mated conditions of formation of the telluride-native gold stage at Tuvatu (�250 �C,�Au¼ 1 ppb, �Te¼ 1 ppb, �S¼ 0.001m, �V¼ 0.0001m, and aK¼ 0.01), show thatthe stability fields of calaverite, roscoelite, and karelianite converge in pH-fO2 spacenear the hematite–magnetite buffer and at neutral to slightly acid conditions. Thermo-dynamic and textural data suggest that these minerals were deposited together at Tuvatuand likely explain the common coexistence of roscoelite and calaverite in epithermalgold systems elsewhere. The presence of magnetite with up to 0.7 wt.% V2O3 in theNavilawa Monzonite is consistent with the derivation of V from the alkalic intrusiverocks, which are also considered to be the source of Au and Te in the Tuvatu deposit.

Introduction

The intimate spatial relationship between vanadium minerals and gold, particularlygold-bearing tellurides, has long been known since the identification of roscoelite[K(V3þ,Al,Mg)2AlSi3O10(OH)2] by Blake (1876) in the Stuckslager gold deposit,Coloma district, California. Roscoelite and vanadium muscovite, are characterizedby the 1 M and 2 M structural types, respectively (Heinrich and Levinson, 1955) and

by roscoelite exhibiting >17 wt.% V2O3. They are the two most common V-bearingminerals in gold telluride deposits. Roscoelite is relatively common in epithermalgold telluride deposits where it has been identified in the Cripple Creek deposit,Colorado (Jensen and Barton, 2000), Boulder County deposits, Colorado (Lindgren,1907; Kelly and Goddard, 1969; Kurtz and Hauff, 1988; Saunders, 1991), Emperordeposit, Fiji (Ahmad et al., 1987), Spotted Horse, Maginnis, and Gies deposits, JudithMountains, Montana (Forrest, 1971; Zhang and Spry, 1994; Thieben and Spry, 1995),and Porgera deposit, Papua New Guinea (Cameron, 1998; Cameron et al., 1995). Inmost of these deposits, there is direct spatial association between the easy to iden-tify green roscoelite and gold mineralization, which is usually in the form of gold-bearing tellurides, particularly calaverite, and either native gold or electrum.

Although roscoelite is rare in mesothermal gold deposits, one exception beingthe Boulder Reef deposit, Golden Mile, Western Australia (Simpson, 1952), thepresence of other vanadium silicates or oxides is more common than in epithermalgold telluride deposits. For example, the giant Golden Mile district contains vana-dian muscovite, tomichite, nolanite, tivanite, vanadian hematite, vanadian magnetite,and vanadian tourmaline intergrown with gold tellurides, in particular calaverite(Nickel, 1977; Nickel and Grey, 1982; Gatehouse et al., 1983). This assemblage ofminerals constitute the well-known ‘‘green leader’’ gold lodes of the Golden Miledistrict (Nickel, 1977). Coulsonite (FeV2O4) was also reported at the Kalgoorlie de-posit by Spiridinov (1978). An even more diverse array of vanadian silicates andoxides is present in the large Hemlo gold deposit, Ontario, which contains vanadianmuscovite, roscoelite, vanadian rutile, barian tomichite, vanadian hematite, kare-lianite, hemloite, vanadian titanite, vanadian carfarsite, vanadian phlogopite, vana-dian allanite, vanadian pumpellyite, vanadian vesuvianite, vanadian epidote-groupminerals, and vanadian grossular (Harris, 1989; Pan and Fleet, 1991, 1992). In ad-dition, minor amounts of V (<2.5 wt.% V2O3) are present in tourmaline, chlorite,talc, chromite, and tremolite (Harris, 1989; Pan and Fleet, 1992). As with theKalgoorlie deposit, one of the best indicators to gold ore at Hemlo is the presenceof light to dark green colored vanadian muscovite (Harris, 1989).

At the epithermal Emperor and Tuvatu gold telluride deposits, Fiji, dark greenroscoelite is intimately intergrown with gold-bearing tellurides (Ahmad et al.,1987; Scherbarth and Spry, 2001; Pals and Spry, 2003). Here we report the resultsof a systematic mineralogical, electron-microprobe, and scanning electron micro-scope study of vanadium silicates and oxides in the Tuvatu deposit, noting that thepresence of vanadium minerals other than roscoelite and=or vanadian muscoviteare rare in epithermal gold telluride deposits. The major objectives of the study areto (1) Evaluate the reasons for the presence of vanadium silicates and oxides in theTuvatu deposit, and (2) Explain the intimate spatial relationship between calaverite(AuTe2) and vanadium minerals in the deposit. The latter objective is pertinentto understanding why the Au–V–Te association is so common in epithermal andmesothermal gold telluride ores, in general.

Geological setting

The Tuvatu gold–silver telluride deposit contains reserves of 480,000 ozs of goldand, in Fiji, is second in size to the 11.5 Moz Emperor gold telluride deposit.

172 P. G. Spry and N. L. Scherbarth

They are two of several epithermal gold deposits genetically associated withalkalic igneous rocks that are localized along the >250 km northeast trending VitiLevu lineament, Fiji. These alkalic igneous rocks formed during the breakup ofthe Vitiaz island arc in the south-east Pacific Ocean. The Tuvatu deposit is locatedin the Sabeto Valley, approximately 15 km northeast of Nadi. The basal unit ofthe rocks in the Tuvatu area is the 12–26 Ma Nadele Breccia (andesitic–basalticbreccias, pillow lavas, and sediments), which is a member of the WainimalaGroup (Fig. 1). The Wainimala Group is unconformably overlain by membersof the Sabeto Volcanics (interbedded andesitic volcaniclastics and flows), whichrepresent the basal unit of the 4.8–5.5 Ma Koroimavua Volcanic Group (Colleyand Flint, 1995; Hatcher, 1998). The Nadele Breccia was intruded by the 4.85 Ma(McDougall, 1963) Navilawa Monzonite, which is interpreted to be co-magmaticwith the Sabeto Volcanics, and is composed of a micromonzonite that envelopescoarse monzonite (Fig. 2). These monzonites, which host the Tuvatu deposit,are cut by several basaltic-andesite dikes and are considered to be the sourceof the ore-forming components including Au, Te, and V (Scherbarth and Spry,2000, 2006).

Fig. 1. Geology of Viti Levu, Fiji (modified after Begg, 1996; Rodda, 1967). The locationof the Tuvatu prospect with respect to the Viti Levu lineament is indicated

The gold–vanadium–tellurium association 173

Mineralogy and conditions of ore-formation

Gold mineralization in the Tuvatu deposit is generally hosted in sub-vertical, north–south trending and north northeast–south southwest trending veins as well as shal-low south dipping veins, and appears to be intimately related to the emplacementof the Navilawa Monzonite (A-Izzedin, 1998; Scherbarth and Spry, 2000). Hatcher(1998) suggested that gold was deposited in three different lode types, ‘‘steep-dipping veins’’ striking northeast (e.g., Nasivi and Upper Ridges lodes), shallowlydipping veins or ‘‘flatmakes’’ representing reactivated oblique thrust faults (e.g.,Murau lode), and irregular brecciated bodies or ‘‘shatter zones’’ (e.g., SKL lode)that occur at the intersection of the other two lode types (Fig. 3). Gold mineraliza-tion overprints porphyry-style copper mineralization in the Navilawa Monzonite inthe northern parts of the deposit in the H and Tuvatu lodes where it occurs inbiotite–magnetite–K feldspar dikes. However, most of the gold occurs in epither-mal-style veins that cover an area of approximately 1 km by 0.25 km.

Mineralogical and paragenetic studies Ashley and Andrew (1989), Hatcher(1998), and Scherbarth and Spry (2000) suggest a five stage paragenetic

Fig. 2. Simplified geological map of the Tuvatu prospect


sequence (Fig. 4). A magmatic stage, which is best developed in the H andTuvatu lodes, is followed by three primary hydrothermal stages characterizedby different alteration assemblages (potassic, propylitic, phyllic), and a latesupergene stage (or post-mineralization stage). The most important gold-bearingstage is the phyllic stage (stage 4) where native gold is spatially associated withbase metal sulfides (galena and sphalerite), bismuthinite (Bi2S3), and varioustellurides [calaverite (AuTe2), krennerite ((Au,Ag)Te2), sylvanite ((Au,Ag)2Te4),petzite (Ag3AuTe2), st€uutzite (Ag5-xTe3), hessite (Ag2Te), altaite (PbTe), andcoloradoite (HgTe)]. Altaite is the most common telluride in the Tuvatu deposit.Compositions of native gold, tellurides, and sulfides are given in Scherbarth andSpry (2006).

Fig. 3. Plan view of the Tuvatu prospect lode structures


By combining the fluid inclusion date of Ashley and Andrew (1989) andScherbarth (2002), it can be concluded that primary fluid inclusions in stage 1apatite formed over a wide range of temperatures (276� to >500 �C), with mosthomogenizing at temperatures >450 �C. These fluids boiled and were hypersaline(>50 eq. wt.% NaCl), and were followed by stage 2 variably saline, boiling fluids(5 to >40 eq. wt.% NaCl), which formed at ca. 310 �C. Stage 3 fluids were gen-erally less saline (1 to 10 eq. wt.% NaCl) than stage 1 and 2 fluids and formed at ca.300 �C. However, in places, stage 3 fluids were also locally boiling and very saline(up to 37 eq. wt.% NaCl). Stage 4 non-boiling, moderately saline fluids (mean¼8.4 wt.% NaCl equiv) formed between approximately 325� and 100 �C (mean¼257 �C) and accompanied telluride and base metal sulfide deposition, as well asthe formation of vanadium-bearing minerals. The pressure correction to stage 4fluids was likely small (<10 �C) with lithostatic conditions persisting through stage1 to 3 until hydrostatic conditions dominated during stage 4. Values of �34S forsulfides in the porphyry and epithermal veins range from �15.3 to �3.6ø andreflect an increase in the �SO4=�H2S ratio of a boiling magmatic fluid (Scherbarthand Spry 2001, Scherbarth, 2002).

Oxygen isotope compositions for water in equilibrium with stage 2 orthoclase,magnetite, and phlogopite, and stage 3 quartz and muscovite range from 4.4 to10.2ø, whereas values of �18O for water in equilibrium with stage 4 quartz rangefrom 7.8 to 11.5ø (Ashley and Andrew, 1989; Scherbarth and Spry, 2001; Scherbarth,2002). These values, coupled with �18O and �D values of fluids in equilibrium withphlogopite of 6.5 to 9.8ø and �25.9 to �9.9ø, respectively, overlap with watersfrom the arc magmas and subduction-related volcanic vapor boxes and are consistentwith a magmatic source and permissive of a meteoric water contribution. The Tuvatudeposit appears to have originally developed as a porphyry copper system that wassubsequently overprinted by epithermal gold–silver telluride mineralization. Thegold–silver veins are considered by us to be the late stages of the same hydrothermalsystem that involves the porphyry copper-style mineralization.

Analytical procedures

Surface and underground specimens were obtained from underground locationsand drill core from several epithermal veins in the Tuvatu deposit. Samples wereexamined with a dual transmitted-reflected light Olympus BX 60 petrographicmicroscope and Hitachi S-2460N and JEOL JSM-35 scanning electron micro-scopes possessing EDAX area mapping and back-scattered imaging capabilities.Mineral compositions were obtained using an ARL-SEMQ electron microprobe atIowa State University. The instrument was operated under the following condi-tions: 15 kV, a sample current of 10 nA, and 2 mm beam diameter. Standards usedwere natural (orthoclase for K, kyanite for Al, albite for Na and Si, hornblende forCa, gahnite for Zn, chromite for Cr, ilmenite for Ti, scapolite for Ca) and syntheticminerals (knebelite for Mn, fayalite for Fe, forsterite for Mg, V2O5 for V).

1Fig. 4. Paragenetic sequence for the mineralogy of the Tuvatu prospect (derived fromAshley and Andrew, 1989; Hatcher, 1998; Scherbarth, 2002)

P. G. Spry and N. L. Scherbarth: The gold–vanadium–tellurium association 177

Tab

le1

.Representative

electronmicroprobeanalysesofV-bearingsilicatesandoxides

12

34

56

78

91

01

11

21

3

Ele

men

t(w

t%

)

SiO

24

6.9

35

2.1

64

7.6

14

7.3

04

7.8

90

.07

0.1

80

.30

0.4

50

.13

0.0

20

.45

41

.49

TiO

20

.00

0.0

00

.08

0.0

00

.13

0.0

00

.00

0.0

00

.00

94

.10

95

.78

62

.40

0.0

0A

l 2O

39

.63

5.9

97

.47

7.9

01

2.5

00

.51

0.6

30

.00

0.0

00

.11

0.0

70

.30

1.8

4C

r 2O

30

.00

0.0

30

.03

0.1

60

.00

0.1

60

.63

0.3

10

.53

0.0

00

.00

0.0

00

.14

V2O

32

7.4

12

8.3

63

0.2

83

2.7

12

4.4

38

2.1

78

6.6

69

9.8

99

8.6

15

.24

1.8

63

7.1

95

2.9

0F

eO0

.16

0.0

00

.11

0.3

60

.24

6.6

61

.46

0.0

80

.03

0.1

00

.56

0.5

50

.06

Mn

O0

.08

0.0

80

.05

0.0

00

.06

0.0

10

.00

0.0

00

.08

0.0

00

.00

0.0

10

.00

Mg

O0

.97

1.0

51

.39

0.6

61

.38

0.0

40

.11

0.0

80

.00

0.0

00

.00

0.0

00

.06

Zn

O0

.00

0.0

10

.03

0.0

60

.00

0.0

00

.00

0.1

70

.07

0.0

00

.13

0.0

00

.00

CaO

0.1

40

.06

0.0

20

.09

0.1

60

.02

0.1

70

.01

0.0

10

.24

0.0

90

.09

0.0

4N

a 2O

0.1

10

.04

0.4

50

.03

0.0

00

.00

0.1

70

.00

0.0

00

.04

0.0

00

.00

0.2

6K

2O

8.7

87

.98

9.2

49

.02

9.3

10

.00

0.1

40

.00

0.0

00

.03

0.1

10

.00

0.5

9

To

tal

94

.20

95

.77

96

.75

98

.29

96

.09

89

.64

90

.15

10

0.8

49

9.7

89

9.9

99

8.6

21

00

.99

97

.38

No

.o

fca

tio

ns

22

22

22

22

22

16

16

22

22

91

2S

i6

.82

27

.36

16

.82

06

.70

06

.75

90

.01

00

.02

70

.00

70

.01

10

.00

20

.00

00

.02

93

.30

1T

i0

.00

00

.00

00

.00

80

.00

00

.01

40

.00

00

.00

00

.00

00

.00

00

.95

20

.97

93

.00

60

.00

0A

l1

.64

90

.99

61

.26

21

.31

92

.07

90

.09

10

.11

00

.00

00

.00

00

.00

20

.00

10

.02

20

.17

2C

r0

.00

00

.00

30

.00

40

.01

80

.00

00

.01

90

.07

40

.00

60

.01

00

.00

00

.00

00

.00

00

.00

9V

3.1

95

3.2

09

3.4

78

3.7

15

2.7

65

9.9

72

10

.26

81

.97

91

.97

20

.05

70

.02

01

.89

03

.37

5F

e0

.02

00

.00

00

.01

30

.04

30

.02

80

.84

30

.18

00

.00

20

.00

10

.00

10

.00

60

.03

00

.00

4M

n0

.01

00

.01

00

.00

60

.00

00

.00

70

.00

10

.00

00

.00

00

.00

20

.00

00

.00

00

.00

00

.00

0M

g0

.21

00

.22

00

.29

70

.13

90

.29

00

.00

90

.02

40

.00

30

.00

00

.00

00

.00

00

.00

00

.00

7Z

n0

.00

00

.00

10

.00

30

.00

60

.00

00

.00

00

.00

00

.00

30

.00

10

.00

00

.00

10

.00

00

.00

0C

a0

.02

20

.01

00

.00

30

.01

40

.02

40

.00

30

.02

70

.00

00

.00

00

.00

30

.00

10

.00

40

.00

3N

a0

.03

10

.01

10

.12

50

.00

80

.00

00

.00

00

.04

90

.00

00

.00

00

.00

10

.00

00

.00

00

.03

9K

1.6

32

1.4

37

1.6

88

1.6

30

1.6

76

0.0

00

0.0

26

0.0

00

0.0

00

0.0

01

0.0

02

0.0

00

0.0

60

1R

osc

oel

ite

(00

TV

-23

F);2

Rosc

oel

ite

(PS

99T

V-6

9F

);3

Rosc

oel

ite

(00T

V-5

5);4

Ro

sco

elit

e(P

S9

9T

V-6

8F

);5

Ro

sco

elit

e(9

9T

V-7

);6

No

lan

ite

(99T

V-6

8a)

;7

No

lan

ite

(PS

99

TV

-68

F);8

Kar

elia

nit

e(9

9T

V-6

8a)

;9

Kar

elia

nit

e(P

S99T

V-6

8F

);10

Ru

tile

(00

TV

-4);11

Ru

tile

(00

TV

-10

0);12

Sch

rey

erit

e(0

0T

V-4

);13

Unnam

edV

-sil

icat

e(9

9T

V-6

8F

);det

ecti

on

lim

its

are<

0.1

wt.

%


Fig. 5. Photomicrographs of a Radiating needles of roscoelite (Rc) in quartz (Qtz).Masses and needles of intergrowths of karelianite (Kr) and nolanite (Nl) occur in ros-coelite (plane polarized light); b Radiating cluster of roscoelite (Rc) needles in quartz(Qtz). Nolanite (Nl) occurs as masses and as replacements of roscoelite needles (planepolarized transmitted light); c Euhedral and subhedral karelianite (Kr) crystals in massesof roscoelite (Rc). Calaverite (Ct) occurs as isolated crystals in roscoelite and as veinletscross cutting roscoelite (plane polarized reflected light); d Karelianite (Kr) crystals inter-grown with and replaced by nolanite (Nl) within roscoelite (Rc) and quartz (Qtz). Notethe presence of a calaverite (Ct) grain within roscoelite (plane polarized reflected light);e Karelianite (Kr) intergrown with calaverite (Ct). Nolanite (Nl) has replaced karelianite(Kr) and roscoelite (Rc). Small bladed subhedral crystals of an unnamed vanadiumsilicate (Un) occur in quartz (plane polarized reflected light); f Blades of an unnamedvanadium silicate (Un), nolanite (Nl) and calaverite (Ct) in quartz (Qtz) (back-scatteredelectron image)


Vanadium minerals

Stage 4 calaverite is intimately associated with fine-grained masses of greenmica. EPMA of the green mica shows that it mostly consists of roscoelite (upto 32.71 wt.% V2O3, which is among the highest reported vanadium values forroscoelite from an epithermal Au–Ag–Te deposit. (Table 1, Fig. 5a and b). Vana-dian muscovite is the other green mica present (Fig. 6).

Five vanadium-bearing oxide minerals were also identified in various veins:karelianite (V2O3), Ti-free nolanite ((V,Fe,Al)10O14(OH)2, with between 65.2 and87.3 wt.% V2O3), schreyerite (V2Ti3O9), rutile (with up to 5.2 wt.% V2O3) and mag-netite (with up to 0.7 wt.% V2O3). Representative microprobe analyses are reportedin Table 1. Of the vanadium-bearing oxides, karelianite is the most common. It occursas euhedral to subhedral crystals (Fig. 5c), elongate laths (Fig. 5d) and irregularmasses up to 2.5 mm in length. Nolanite forms as irregular masses with karelianite(Fig. 5d) where it appears to have replaced the latter mineral. In these masses, markedvariations occur in the Fe to V ratio of nolanite. It is essentially devoid of Ti, andcontrasts in composition with nolanite from, for example, the Kalgoorlie gold deposit,which contains minor amounts of Ti (Nickel, 1977). Elsewhere in the Tuvatu deposit,nolanite occurs as fine elongate crystals that replaced mats (Fig. 5a) and individualneedles of roscoelite (Fig. 5b). Where it replaced roscoelite it gives the roscoelite abrown hue rather than the more characteristic green color generally associated withthis mineral. Schreyerite, although rare, forms as fine flakes (<0.04 mm) intimatelyassociated with granular masses of rutile (<0.16 mm) in colloform quartz, roscoeliteand pyritohedral (single or masses) and platy pyrite grains. Magnetite is common ascoarse euhedral grains in early porphyry-style mineralization and to a lesser extent assubhedral to euhedral minor magnetite in pyrite within epithermal veins. Magnetitein both settings contains up to 0.7 wt.% V2O3.

Fine needles (up to 150 mm in length) of a vanadium silicate within quartzoccurs on the margins of small intergrowths of karelianite, nolanite and roscoelite

Fig. 6. Chemical variation dia-gram of V content (per formulaunit) versus Al content (in theoctahedral site) of vanadian micaat Tuvatu. Note that the mostcommon mica at Tuvatu is roscoe-lite rather than vanadian muscovite


in the Upper Ridges veins. These grains were first identified in back-scattered elec-tron images (Fig. 5d and e) and yielded qualitative analyses using an SEM thatyielded an approximate formula of VSiO3. Electron microprobe analyses yieldedthe same 1:1 V:Si ratio, but totals were generally <100 wt.%. It is likely that themineral is VSiO3(OH), where V3þ is partly replaced by Al3þ (Table 1). The com-position of this silicate does not correspond to that of any named mineral, and itmay therefore represent a new mineral species.

Discussion

The source of V, Au, and Te is unknown in gold telluride deposits but it is oftenlinked to alkalic igneous rocks spatially associated with the gold ores (e.g.,Ahmad et al., 1987; Zhang and Spry, 1994; Pals and Spry, 2003). Jensen andBarton (2000) pointed out that owing to the chalcophile nature of tellurium, itis most abundant in mantle and crustal rocks, and particularly enriched in alkalichydrothermal systems. In alkalic igneous rocks, tellurium and gold constitutes upto �10 ppb. Such settings also contain magnetite-bearing mafic igneous rocks thathost vanadium-bearing ores. It should be noted that the Navilawa Monzonite, SabetoVolcanics, and the basaltic dikes in the Tuvatu mine area contain 280–387 ppm V(mean¼ 317 ppm V), 292–355 ppm V (mean¼ 329 ppm V), and 235–465 ppm V(mean¼ 356 ppm V), respectively, and that V is located in magnetite and phlogopitein these rocks (Scherbarth and Spry, 2006). These values are higher than theaverage crustal abundances values for V of 151 ppm (Rudnick and Fountain,1995) and 230 ppm (Taylor and McLennan, 1995). Like those at Tuvatu, magnetitefrom alkalic rocks associated with the Golden Sunlight (Zhang and Spry, 1994)and the Porgera gold telluride deposits (Richards, 1990) are also enriched invanadium.

Gold is transported in hydrothermal solutions as Auþ and is carried as aque-ous complexes with sulfur-bearing (e.g., AuHS(aq), HAu(HS)2(aq), AuðHSÞ2

�) and

halogen-bearing ligands (e.g., AuCl2�) (Seward, 1991; Cooke and Simmons,

2000) whereas Te is carried as a variety of complexes in the system Te–O–H(e.g., H2Te(aq), HTe�, Te2

2�) (Zhang and Spry, 1994; McPhail, 1995). Althoughaqueous telluro-gold species are unknown in nature, Seward (1973) speculatedthat AuðTe2Þ

�, Au2(Te2)0, and AuðTe2Þ2

3�may be important in the formation of

gold telluride deposits. Saunders and May (1986) also suggested that AuðTe2Þ�

may be the gold complex in gold telluride systems. Other potential aqueous tell-uro-gold species that have been considered are AuHTe(aq) (McPhail, 1995) andAuðHTeÞ2

�(Cooke and McPhail, 2001), as well as structurally bound gold

species such as Fe(SAs)–Au(HTe)0 and Fe(SAs)–Au2Te0 (Pals et al., 2003)However, there is an increasing body of thermodynamic, experimental, andfield evidence that Te (McPhail, 1995; Cooke and McPhail, 2001; Larocqueet al., 2006) as well as Au and Ag can also be transported in the vapor phase(Williams-Jones et al., 2002). Although vanadium is transported in hydrothermalsolutions, it is unclear whether it can be transported in the vapor. The stability ofvarious species in the system V–O–H were calculated at 25 �C by Wanty andGoldhaber (1992) and under hydrothermal conditions up to 300 �C by Cameron(1998).


By combining the results of the thermodynamic calculations of Zhang and Spry(1994) in the system Au–Te–Cl–S–O–H and those of Cameron (1998) for thesystem V–Al–Si–O–H, the stability of the minerals calaverite, roscoelite, andkarelianite can be evaluated at conditions of ore-formation at the Tuvatu depositassuming that these minerals deposited from a hydrothermal solution. Details ofthe assumptions made for the thermodynamic calculations and the sources of ther-modynamic data are given in Zhang and Spry (1994) and Cameron (1998). Calcu-lations for both systems, in large part, employed the isocoulombic method (e.g.,Gurney, 1938) to calculate equilibrium constants of reactions. Uncertainties of upto 1 unit in log K (equilibrium constant) may be expected using this method (e.g.,Mountain and Wood, 1988). Note that thermodynamic data for nolanite andschreyerite are unavailable.

The stability fields of roscoelite, karelianite, and calaverite in f O2-pH space at300� and 250 �C for �Au¼ 1 ppb, �Te¼ 1 ppb, �S¼ 0.001m, �V¼ 0.0001m,and aK¼ 0.01 are shown in Figs. 7 and 8, respectively. The temperatures chosenare those that span the deposition of tellurides in stage 4 at Tuvatu (meanTh¼ 257 �C, with a trapping temperature <10 �C higher than the mean). Valuesof �Au¼ 1 ppb and �Te¼ 1 ppb are typical of those found in modern geothermalsystems whereas those for �S¼ 0.001m, �V¼ 0.0001m, and aK¼ 0.01 are appro-

Fig. 7. Log fO2-pH diagram for the system V–Al–Si–O–H superimposed onto the systemsAu–V–Te–Cl–S–O–H and Fe–S–O at 300 �C highlighting the overlapping stability fieldsof calaverite with roscoelite, muscovite, and karelianite. Conditions used in construc-tion are �Au and �Te¼ 1 ppb, �S¼ 0.01m, �V¼ 0.0001m, aK¼ 0.01. Details of calcu-lations of the system Au–V–Te–Cl–S–O–H and Fe–S–O are given in Zhang and Spry(1994) whereas those for the system V–Al–Si–O–H are reported in Wall et al. (1995) andCameron (1998).


priate for compositions associated with alkalic igneous rock gold telluride systemsand were the compositions used by Cameron (1998) in his calculation of the sta-bility of aqueous species and minerals in the system V–Al–Si–O–H associatedwith the formation of the Porgera gold deposit. Values of �S¼ 0.001m andaK¼ 0.01 were also proposed by Scherbarth and Spry (2006) and are consistentwith stabilities of members in the systems K–Al–Si–O and Fe–S–O and sulfur iso-tope compositions when plotted in log fO2-pH space.

At 250� and 300 �C for neutral to moderately low pH conditions, V4þ and V5þ

complexes predominate with V3þ being incorporated into roscoelite and karelia-nite. As was shown by Wall et al. (1995) and Cameron (1998), vanadium is trans-ported by an oxidizing fluid and is fixed as roscoelite and karelianite uponreduction at near neutral and moderately acid conditions respectively. At neutralto slightly acid conditions, near the hematite–magnetite buffer, calaverite is depos-ited from solution and overlaps the stability fields of karelianite and roscoelite. Theoverlap is greater at 250 �C than at 300 �C suggesting that the field of overlap inpH-fO2 space increases with decreasing temperatures. The coexistence amongkarelianite, calaverite, and roscoelite at 250 �C (approximate temperature of stage4 mineralization at Tuvatu) suggests pH conditions of 4.5 and a value of log fO2 of�35 to �30. Nolanite formed after karelianite but the conditions of its formationare unknown.

Conclusions

The elements, Au, Te and V, likely accompany each other throughout the entire oreforming cycle at Tuvatu. Each of these elements was likely derived from the

Fig. 8. Log fO2-pH diagram for the system V–Al–Si–O–H superimposed onto the systemsAu–V–Te–Cl–S–O–H and Fe–S–O at 250 �C highlighting the overlapping stability fieldsof calaverite with roscoelite, muscovite, and karelianite. Conditions used in constructionare �Au and �Te¼ 1 ppb, �S¼ 0.01m, �V¼ 0.0001m, aK¼ 0.01


Navilawa Monzonite and then subsequently carried in a magmatic hydrothermalsolution in the aqueous phase and possibly in the supercritical vapor phase, in thecases of Au and Te. Deposition from these fluids resulted in the formation ofprecious metal tellurides, including calaverite, and the precipitation of variousvanadium silicates and oxides. The coexistence of calaverite, roscoelite, and kar-elianite suggests that the ore fluid at 250 �C had a pH of 4.5 and a value of log fO2

of �35 to �30. While roscoelite is the most common vanadium phase at Tuvatu,the incorporation of the other vanadium minerals in roscoelite warrants closerinspection for similar phases in other epithermal ores.

Acknowledgements

This study was financed by Emperor Gold Mining Company. We thank Tony Woodward,David A-Izzedin, Saimoni, Don Milella, and Charles Barclay for their assistance while inFiji. Greg Cameron is kindly thanked for providing a copy of part of his Ph.D. thesispertaining to the stability of vanadium in hydrothermal systems. Diana Oshun read a pre-liminary draft of the manuscript while Alfred Kracher and Warren Straszheim are thanked fortheir assistance with the electron microprobe and SEM analyses, respectively. The manuscriptwas improved by the reviews of Nigel Cook, Yuanming Pan, Johann Raith and Jim Saunders.

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186 P. G. Spry and N. L. Scherbarth: The gold–vanadium–tellurium association

Documents

The gold–vanadium–tellurium association at the Tuvatu gold–silver prospect, Fiji: conditions of ore deposition