Canadian MineralogistVol. 30, pp. 153-162 (1992)

ABSTRACT

In green mica schist from the main ore zone of theHemlo gold deposit, Ontario, a very fine-grainedpumpellyite-group mineral, occurring sparsely in closeassociation with vanadian titanite and barian tomichite,contains up to 25.7 wt.Y0 V2O3; the end-membercomposition is Ca6(V,Mg,Fe)a(V,Al)ssit2O56-n(OH)n, andvanadium is the dominant cation in both X and Ypositions. Vanadoan pumpellyite-(Mg) also occurs ingreen mica schist as prismatic grains in cross-cutting veinsand along foliation planes. The vanadoan pumpellyite-(Mg) contains a substantial amount of V (ranging from1.7 to 13.6 wt.9o V2O3), and, in addition, is characterizedby unusually high As, Mg, and F contents. Vanadium isincorporated into the pumpellyite-groupminerals mainlyby the direct substitution Alr+ : Vr*, with minorcontributionsfrom2Al3+ = V4+ + (Mg,Fe)2+ or 3Al3+= v5+ + 2(Mg,Fe)2+ (or both). In vanadoan pumpel-lyite-(Mg), As seems to substitute for Si in tetrahedralpositions. This is the first observation of pumpellyite-group minerals in the Hemlo gold deposit, and is furthersupport for a late, low- to veryJow-grade calc-silicatealteration within the main ore zone. The vanadium-richminerals of the pumpellyite group are considered to haveformed at about 300-400oC and < 2 kbar in an aqueousfluid of moderate to high salinity.

Keywords: vanadium, pumpellyite group, green micaschist, calc-silicate alteration, gold deposit, Henilo,Ontario.

Sovuernn

Nous avons d6couvert un min6ral i grains trds fins,du groupe de la pumpellfte, dans un schiste i mica vertde la zone mindralis6e principale du gisement d'or deHemlo, en Ontario; il est associ€ i une titanite et unetomichite vanadif€res, et contient jusqu'd 25.7a10 (enpoids) de V2O3. Le p6le vanadifbre ayant le vanadiumcomme cation dominant dans les positions X et Y auraitla composition Ca6(V,Mg,Fe)a(V,Al)sSil2O56_n(OH)n.Nous trouvons aussi la pumpelMte-(Mg) vanadifbre dansle m€me schiste, sous forme de grains prismatiques dansdes veinules recoupant le plan de foliation ou paralldlesd celui-ci. La pumpellfite-(Mg) vanadifbre contient entre1.7 et 13.60/o de V2O3 et, de plus, des quantit6sanormalement 6lev6es de As, Mg et F. La pumpellyiteaccepte le vanadium surtoul selon la substitution directeAl3+ = V3+, avec une contribution moins importante

153

VANADIUM.RICH MINERALS OF THE PUMPELLYITE GROUP FROMTHE HEMLO GOLD DEPOSIT, ONTARIO

YUANMING PAN nNn MICHAEL E. FLEETDepartment of Geologt, University of Western Ontario, London, Ontario N6A 587

descouples?Al3* = Va+ + (Mg,Fe)2+.1 3613+ = y5r+ 2(Mg,Fe)z+. Dans la pumpellfte-(Mg) vanadiflre, Assemble remplacer le Si dans les positions t6tra6driques.Cet article pr6sente les premiers indices de la pr6sence demin6raux du groupe de la pumpellyite d Hemlo, indicationde plus d'un 6pisode tardif d'altdration calco-silicat6e defaible i trbs faible intensit6 qui a affect6 la zonemin6ralis€e principale. Les min6raux du groupe de lapumpellyite riches en vanadium se seraient form6s ienviron 300-400'C i une pression inf6rieure i 2 kbar dansun milieu hydrothermal d salinit6 moyenne d 6lev6e.

(Traduit par la R6daction)

Mots-cl6s: vanadium, groupe de la pumpellyite, schiste imica vert, alt6ration calco-silicat6e, gisement d'or,Hemlo. Ontario.

INrnooucrtoN

Pumpellyite-group minerals occur commonly inmetamorphic rocks, particularly metabasitesrecrystallized in the subgreenschist facies (Deer e/al. 1986, Liou et al. 1987). They are well knownfor their chemical complexity, which involves botha large diversity and wide range of cationsubstitutions (Deer et al, 1986). Passaglia &Gottardi (1973) presented a general formulaWsX4YBZDOso-n(OH)n, where W = Ca, K, Na; X: MB, Mn2+, Fe2*, Al , cr ; r : A l , Fe3*, cr ,Mn3+, Ti; and Z : Si, and a scheme ofclassification and nomenclature based on thepredominant cation in the I position. Passaglia &Gottardi (1973) further distinguished pumpellyite-group minerals by a suffix, indicating thepredominant cation in the Xposition. To date, thefollowing end-member mineral species and theirnames have been reported: pumpellyite-(Mg),pumpellyite-(Mn), pumpellyite-(Fe3 + ), pumpellyite-(Fd*), julgoldite-(Fez+), julgoldire-(Fe3+), ok-hotskite-(Mn2+), okhotskite-(Mg) and shuiskite-(Mg) (Palache & Vassar 1925, Moore 1971,Passaglia & Gottardi 1973, Ivanov et ol. 1981,Togari & Akasaka 1987, Dasgupta et al. 1991,Nickel & Nichols l99l). In the present paper, wedescribe two unusual compositions of the pumpel-

t54 THE CANADIAN MINERALOGIST

lyite group the main ore zone of the Hemlogold deposit, Ontario, a new end-member with Vas the dominant cation in both I and X sites, anda vanadoan pumpellyite-(Mg) with significantsporadic enrichments in As and F.

GEoLocrcAL Srrrnc aNoTHE HEMLO GOI.D DBposTT

The Hemlo gold deposit, located approximately35 km east of Marathon, Ontario, is situated withinthe late Archean (2800-2600 Ma; Corfu & Muir1989) Hemlo - Heron Bay greenstone belt of theWawa subprovince of the Superior Province (Muir1982). The geology of the Hemlo - Heron Baygreenstone belt, and particularly of the Hemlomining district, has been described in numerousstudies (Muir 1982, Macdonald 1986, Corfu &Muir1989, Harris 1989, Pan & Fleet 1990), and mostagree that all supracrustal rocks have beensubjected to a complicated history of deposition,magmatism, deformation, metamorphism and al-teration.

The Hemlo gold deposit contains a total

published reserve of approximately 80 Mt at 7.7g/t Au (Harris 1989) and hosts the three biggestgold mines in current production in Canada,namely, the Williams, Golden Giant, and DavidBell mines. The geological complexity of the Hemlogold deposit, particularly the high intensity ofdeformation, pervasive alteration (mainly potassic)and association with host rocks of medium-grademetamorphism, have led to a lack of consensusamong investigators regarding the style ofmineralization. As a result, almost all knownore-forming processes have been proposed for itsorigin (Macdonald 1986).

OccunnsNce AND PETRocRAPHY

Pumpellyite-group minerals occur in green micaschist from the main ore zone of the Golden Giantmine. Green mica schist is one of the importanthost rocks of the Hemlo gold deposit and ischaracterized by a well-developed schistositydefined by a combination of preferred orientationof vanadian muscovite and an alternation ofquartzofeldspathic, micaceous and pyritiferous

Frc. l. Photomicrograph (a) and electron back-scattered images (b, c and d) of the pumpellyite-group minerals fromthe Hemlo gold deposit: a) prismatic vanadoan pumpellyite-(Mg) with polysynthetic twinning in cross-cutting vein,b) chemical zonation in vanadoan pumpellyite-(Mg) enclosing a quartz grain (Qtz), c) the V-rich pumpellfte-groupmineral (Vp) and vanadoan pumpellyite-(Mg) (Vbp) associated with vanadian titanite (Ttn) and barian tomichite(Tm), and d) the V-rich pumpellyite-group mineral along grain fractures of vanadian titanite. Scale bar in a) is50 pm, and in the rest, 10 pm,

VANADIUM.RICH MINERALS FROM THE HEMLO GOLD DEPOSIT r55

layers. Pumpellyite-bearing green mica schist con-tains quartz, alkali feldspar, vanadian muscovite,barite, pyrite and molybdenite, with minoramounts of barian tomichite, vanadian titanite,epidote, allanite, antimonian vesuvianite and gar-neq this assemblage is typical of green mica schistfrom the Hemlo gold deposit (c/. Harris 1989).Harris (1989) and Pan & Fleet (1991) recognizedthat some examples of green mica from the Hemiogold deposit are chromium-bearing and do not havea detectable vanadium content. Pumpellyite has notbeen observed in green mica schist that shows anenrichment in Cr.

There are two distinct modes of occurrence ofpumpellyite-group minerals in the green micaschist. Firstly, prismatic pumpellyite occurs aseither concordant or discordant aggregates inmicaceous or pyritiferous layers, but only rarely inquartzofeldspathic layers. Concordant aggregatesappear along foliation planes, and discordantaggregates occur in cross-cutting veins @ig. la).Locally, the prismatic pumpellyite is rather abun-dant and constitutes up to 3 volume go of rocksamples. In thin section, the prismatic pumpellyiteis characterized by strong pleochroism, with acolorless to weakly red, B brown or deep brownwith a violet tint, and ? pale brown. Locally,prismatic grains of pumpellyite display anomalousinterference colors. Zoning is not uncommon andgenerally is characterized by a rim and a distinctcore (Fig. lb); oscillatory zoning also is locallydeveloped. Polysynthetic twinning, with [100] as

the twin plane, is common (Fig. la; c/. Coombs1953).

A second pumpellyite-group mineral occurs assparse, very fine grains (less than 15 pm inmaximum dimension) and locally in close associa-tion with vanadian titanite and barian tomichite(Figs. lc, d). This very fine-grained pumpellyite-group mineral is greenish brown, vitreous,transparent, and brittle. In thin section, it isstrongly pleochroic (a colorless, B brown to deepbrown, and 7 pale brown), biaxial negative withstrong dispersion (r < r) and an estimated 2tl of50 +5 ' .

Cururcar CoprposrrroN

Chemical compositions of the pumpellyite-groupminerals were investigated with a JEOL JXA-8600Superprobe electron microprobe fitted withautomated wavelength-dispersion spectrometers atthe University of Western Ontario (operatingconditions: 15 kV, l0 nA, beam diameter 2-3 pm,2O-second counts, with minerals, synthetic glassesand metals as standards). Analytical results aresummarized in Figures 2a, b and 3, and repre-sentative compositions are reported in Tables I and2. The two texturally distinct pumpellyite-groupminerals differ significantly in chemical composi-tion. In particular, the prismatic pumpellyite inveins and along foliation planes is vanadium-bear-ing (Table l), whereas the very fine-grained varietyassociated with vanadian titanite and barian

E(Ms + FE)

Ftc. 2. Diagrams illustrating range in composition of pumpellyite-group minerals from the Hemlo gold deposit: a)Al-E(Mg + Fe*)-V and b) D(Al + V)-Mg-Fe*. Open star, Cas(Mg2Alr4Al8Sir2O42(OH)r+; solid star,Ca3(V2Mg)aV6Sil2O56-n(OH)n; crosses, the V-rich pumpellyite-group mineral; solid circles, vanadoan pumpellyite-(Me).

I(Al + V)

156 THE CANADIAN MINERALOGIST

TABLE l. COMPOSITIONS OF VANADOAN PUMPELLTTTE-(Mg)FROM T}IE HEMIO GOLD DEPOSIT

SiQ (wt 7a)Tio,Alro3cr0,v'o,Aso!sbro,

MsoMnOCaOBaON&ro

Toal

Amlysis

>(X + Y)

1

32.450.m1352

trd13.624:f70.m

0.162r,97

nd0.0t0.?3031

93.78

35.49 v.46 3635 35J3 3639 37.150.16 0.u 0.08 0.m 0.m 0.m1533 1522 1594 t7.49 ?!37 &

nd dd 0.14 od nd 0,0311.14 1t.12 !0,61 8J7 5.03 1.67029 2m d 030 0.40 trd0.10 0.06 nd 0.05 0.00 ad2-93 r.64 2.70 3.16 2.6t 0J73:79 4.89 435 4,01 457 4.45o.il 0.m 0.07 0.09 0.10 0.06

nffi 22io 229 22.93 2356 23nd !d 0.06 nd trd 0.00

0.01 0.03 0.03 0.04 0.03 0.u2o.u 0J4 0.70 0.61 0.97 0.000.19 031 029 0 0.41 0.m

,2.47 .44 93.08 .62 9.62 E233

t1.94

0.06

chmical f@d@ calculotqt on fte bqsis of 49 atorc of dySo

AI

Ftc.3. Compositions of pumpellyite-group minerals fromthe Hemlo gold deposit on a plot of V versas A1. Notethat the compositions of the V-rich pumpellyite-groupmineral (crosses) plot close to the substitution of oneAl for each atom of V, whereas those of the vanadoanpumpellyite-(Mg) (solid circles) deviate toward sub-stitutions involving 2Al for each atom of V and 3Alfor each atom of V, especially at low V contents.

tomichite is vanadium-rich and approaches thevanadium end-member composition (Table 2, Fig.2a).

Knowledge of Fd+/Fd* is essential in theprocedure ofPassaglia & Gottardi (1973) for cationassignment and nomenclature, but this ratio cannotbe determined by electron-microprobe analysis.The limited material available for either of thepresent pumpellyite-group minerals prevents adetermination of this ratio by other analyticaltechniques, and estimation based on charge-balancecalculations is not possible either, owing tounceftainty regarding the valence states of othercations, such as V and As, and substitutionsinvolving F or OH- for O2-. In this study, the totaliron content was calculated as Fd*. This isprobably justified by the low Fe3* content ofcoexisting epidote [Fe3+/(Al + Fe3+) < 0.2; Pan& Fleet, unpubl. datal, because Fe3* is expected tostrongly partition into epidote, as demonstratedexperimentally by Schiffman & Liou (1983).Moreover, the total iron content is generally lowin both pumpellyite-group minerals from the studyarea (Tables l, 2, Fig.2b), and therefore wouldnot have a significant effect on the presentdiscussion.

12.00 12.0J 12fi3 r2,r3 1233 l2.m 12.00

siAs

z

uAl

Cr

sbFeMgMn

530 6.110.m 0.04

3:7r 3r20.m 0.0r0.49 0.83L6 r.9r0.05 0.03

t2.t5

8.m

0.01

E.0l

039

BaNa

11.04 12.@ 1lJ6 t2.13 1139 [.91035 0.05 0.47 0.14 0.o7o.il 0.m 0.m 0.m 0.m o.v)

825 8.r8 8.03 8.r7 8 8.03

o.41 0.78 0,74 0.64 I.00

6.84 183 9330.m 0.m 0.m

0.01236 132 0.430.01 0.m0.88 0J2 02r1.99 22i 2.130.03 0.03 0,t2

6.V2 6270.03 00'2

0.0429 2,V0.010.46 0.752.45 L170,4 0.02

11.98 r2.l l

8.16 7.90.01

0.02 0.03

n.95

a2A

0.01

12.ffi

8.02

0.01

12.10 t2.r3

8.15 824

o.a 0.o2

f is gnin 5 strdied by x-ny diff@tion (s Tablo 3): m 7, b reight pe@nqFeO, is total iron @teoq nd is @t dctermine4 W, X, Y. md Z m s delircd byPasglia & Codadi 0973).

The composition of prismatic pumpellyite incross-cutting veins and along foliation planes isquite variable, ranging from 32.5 to 37.2 wt.o/oSiO2, 13.5 to 24.8 wt.qo Al2O3, 1,7 to 13.6 wt.vloV2O3, 3.8 to 5.1 wt.9o MgO, 0.9 to 3.2 wt.q0 FeO',and 22.0 to 23.6 wt.9o CaO (Table l). Thesecompositional variations exist not only from grainto grain but also as pronounced zonation withinindividual grains (Fig. lb). Such zoned grains showan increase in V, accompanied by (Mg + Fe"), anda decrease in Al, from core to margin. With rareexceptions, color in plane-polarized light directlyreflects the Y content, i.e., the deeper the color,the higher the V content. Minor amounts of Astypically are present in the prismatic pumpellyite.In a few cases, exceptionally high As contents (upto 4.8 wt.9o As2Or) have been obtained in grainsofrelatively hieh V contents (Table l). In addition,appreciable amounts of fluorine (up to 1.0 wt.VoF) are not uncommon (Table 1). Titanium, Cr, Mn,Sb and Na are locally present in minor amounts;

VANADIUM-RICH MINERALS FROM THE HEMLO GOLD DEPOSIT t57

TABLE 3. UNrcEI' PARAMETERS OF VANADOAN PT'MPELLYTTB-(Mg)FROM THE HEMLO GOLD DEFOSIT.

AND CPMPARISON WTTU PUMPELLYTIE-CROUP MINERALS

Ysib a(A) (A) PO Efem€

TABLE 2. COMPO$IIONS OF THE V.RICH PUMPELLYTIE-GROUP MINERALFROM TIIE IIEMLO GOLD DEPOSTT

1 2 3 4 5 6 7

,3.t3 33,32 34.52 U.52 v.A 3.58 34;12c(A)

sior(s. %)TlO,ALqc&qvro,A&orsbrqF€O*MgoMnOCaOBeoNsro&oO=Fl m

r. l3 0;13 0.63 0.11 0.m 0.18 0.2r4.55 5.23 6.09 7.12 10.72 10.48 12.4 p@pellyrre{MC)0.56 O.47 0.,{O 0.43 0.90 !d 0.78 p@peuyi&-(Mg)

8.61 25.53 ,57 4,6 2l.ro 19.67 18.t9 pmpelyits-(Mg)trd id nd nd nd O.2O !d pupellyite-(Mg)nd sd !d id nd 0.39 nd pmfBlyile-(Mg)

3.34 2.n 3.59 2-29 0.89 2.7O 0.87 JutgoL,ti@{Fd)2-lO 2.11 2.35 2.17 2.U 2.47 2.9 Sh,,iseire-(Mg)0.50 0.44 0.43 0.62 0.04 o.37 0. I I oh&ctie-(IAf)

2t.56 20.65 m39 20.12 2r.6 2r.s3 2r.1r

Ar 8.82-(3) 592.<3)Ar 8.82i4) 5.9(1)Al 8830) 5.950)Ar 8.83 5.90Ar 8.812 5.895F{ 8.9n 6.ffi1Cr 8.897 5&3M!' 8.887 6.0@

19.11(4) 97J1Q) l19.15(2) 97.69(' 219.15(3) 9730(6) 319.17 91,t2 4t9.lr6 97,41 519432 97.& 619,14 98 7r9J3 97.08 8

0.09 0.04 sd 0.06 0.m nd 0.000'14 0'19 0'o7 0'r0 9'9 0'll 9'9 I to 3 e v@dm pmpeuyns(t"tg) ftom &is srdy Q3,4.6 ai 8.9 vt Eo v'oe0'09 o'D' 0'06 o'2n 9'f - 4- 9'91 rcspedrdy): 4, cali & Albsti (1969); 5. Yoshim & lfarsmoo (19851 6, Allrmm0'm 0'm 0'm 0'00

tn 9.t^7 9.t^? *.ix*y0s7:);r,r@eralo98l);s,TosdiaAkasraog{a?.0.o 0.m 0.m 0.m 0.m 0.05 0.05

91.96 91.75 93.10 91,79 t2.94 '2.75 92.n

chmiel fomlas elola&d @ tho basis of 49 ffi of oryge

si

rAl

ztAl

MgMn

CaBsNaK

w

11.94 11.89 r2.t1 12.19 11.86 11.96 il.r60.04

0.06 0.ll 0.00 0.m 0.14 0.m 0.14

l2.m u.m 12.11 r2.r9 12.00 l2.m 12.6

12.00 11.9 12.15 11.93 12.05 11.98 12.18

2,3, 4.6 and 8.9 wt.Vo V2O3, respectively, wereremoved from thin sections for X-ray-diffractionstudy using a Gandolfi c€unera (CrKcv radiation).The calculated unit-cell parameters do not showany systematic variation, despite the large differen-ces in the V content (Table 3). The grain with 8.9wt.9o V2O3 (and also containing 0.8 wt.Vo As2Or;Table l) yielded unit-cell parameters of a 8.83(1),, 5.95(l), c 19.15(3) A, p 97.70(6)o, which arealmost identical to those of pumpellyite-(Mg) (Galli&Alberti 1969, Yoshiasa & Matsumoto'1985; Table3). A similar X-ray-diffraction study on a titanitefragment containing the very fine-grained V-richpumpellyite-group mineral yielded only the lines oftitanite.

DrscussroN

Enrichments in V, As, Mg ond F

All grains of the pumpelhite-group minerals ingreen mica schist from the main ore zone of theHemlo gold deposit are characterized by highvanadium contents (Tables l, 2, Fig. 2a). Inparticular, the extremely fine-grained aggregates inassociation with vanadian titanite and bariantomichite have as much as 25.7 wt.slo V2O3 andrepresent a possible new mineral with end-membercomposition Ca6(V,Mg,Fe)a(V,Al)sSir2O5Gn(OH)n,and vanadium as the dominant cation in both Yand Xsites (Table 2, Fig. 2a). However, the limitedamount of material available and its very finegrain-size prohibit a more complete charac-terization; therefore, this phase is reported here asan unnamed possible new mineral. The prismaticvariety in veins and along foliation planes alsocontains a significant amount of V (1.3 to 13.6wt.9o V2O) and thus is a vanadoan pumpellyite-(Mg), because Mg and Al are known to preferen-tially partition into X and Y sites, respectivelyCYoshiasa & Matsumoto 1985), and V is assumed(below) to be mainly in the trivalent state.

1.87 2.25 2,52 2.96 4.18 4.n 4.950.30 0.m 0.17 0.03 0.m 0.05 0.050.16 0.13 0.11 0.12 0.25 0.217.39 7.31 6.91 6.81 5.85 5.45 5.r71.00 0.84 1.08 0.68 0.2s 0.78 0.a1.13 1.13 \.X 1.14 1.50 Ln rs20.15 0.13 0.13 0.19 0.u 0.l l 0.03

x x + Y )

7.93 7.t3 1.67 7,62 7.tt0.01 0.01 0.m 0.01 0.000.!0 0.13 0.01 o.Cr/ 0.030.04 0.10 0.m 0.09 0.m

E.ot t.06 7.68 7.19 7.91

0.m 0.00 0.m 0.m 0.m

7.98 1.730.00

0.07 0.030.@

8.05 7.76

0.13 0.13

wL% is edgh !€o€G f.Ot i" oat t- o"tFl !d is d deaqDiG4 If, tg y dZ @ I d€dled by Peastia & co0ald O97JI

Ba, K and Cl are generally below detection limits(Table l).

The composition of the very fine-grained pum-pellyite-group mineral in close association withtitanite and barian tomichite from the Hemlo golddeposit ranges from 33.2 to 34.9 w't.s/o SiO2, 4.6to 10.0 wt.Vo Al2O3, 18.9 to 25.7 wt.Vo Y2O3,2.1to 3.2wt.Vo MgO, 0.9 to 3.3 wt.9o FeO', and 20.1to 21.5 wt..9o CaO (Table 2). In addition, minoramounts of Ti, Cr and Na are common, but Asand F are typically low in abundance (Table 2). Incontrast to the prismatic pumpellyite, individualgrains of this V-rich pumpellyite-group mineral aregenerally homogeneous in composition (Fig. ld).In a few cases, however, it occurs only as a thinmargin to a core of considerably lower V content(Fig. lc), which is similar to that of the prismaticpumpellyite.

UNrr-Cell DnrsNsroNs

Three grains of the prismatic pumpellyite with

158 THE CANADIAN MINERALOGIST

The high concentration of V in pumpellyite-group minerals from the Hemlo gold deposit isreadily explained by an adequate local source ofthis element, because the Hemlo gold deposit is wellknown for a high V enrichment, and a large numberof V-bearing minerals (including vanadian mus-covite, barian tomichite, vanadian garnet, vanadianepidote-group minerals, vanadian titanite,vanadian rutile, karelianite, cafarsite, and hem-loite) have been documented in the main ore zone(Harris 1989). Also, octahedrally coordinatedvanadium cations (V3+, V4+ and V5+) all aresimilar in ionic radius to vIAl3+ (0.64, 0.58, 0.54versus 0.535 A, respectively; c/. Shannon 1976)and, therefore, can readily substitute for the latterin octahedral positions (discussed below).

Yoshiasa & Matsumoto (1985) recognized thatMg atoms do not occupy more than half of theavailable X sites [M(2)] in pumpellyite. This isapparently the case in the V-rich pumpellyite-groupmineral from the study area (Table 2, Fig. 2b).However, many of our analyses of vanadoanpumpellyite-(Mg) reveal more than 2 Mg atoms performula unit, the highest value being 2.6 andequivalent to 654/o occupancy of X sites (Table l,Fig. 2b). Therefore, vanadoan pumpellyite-(Mg)from the study area is also unusually Mg-enrichedand cannot be considered to be an intermediatecomposition between pumpellyite-(Mg) and theV-rich pumpellyite-group mineral (Figs. 2a, b). Ithas been noted above that vanadoan pumpellyite-(Mg) and the Y-rich pumpellyite-group mineralfrom the study area also differ in other con-stituents, such as As and F.

Substantial amounts of As and F have not beenreported previously in pumpellyite-group minerals(c/. Deer et al. 1986). In fact, neither of these twoelements is routinely sought in most studies of thepumpellyite-group minerals. The high As and Fcontents (up to 4.8 wt.Vo As2O5 and 1.0 wt.qo F,respectively) in vanadoan pumpellyite-(Mg) of thestudy area may be unusual. Arsenic is charac-teristically high in abundance in the main ore zoneof the Hemlo gold deposit, and is present mainlyin arsenopyrite, barian tomichite and native arsenic(Harris 1989). Similarly, substantial halogen con-tents have been obtained in several vein minerals,such as phlogopite, titanite, garnet, apatite,synchysite, and antimonian vesuvianite, from themain ore zone (Pan et ol. l99l).

Mechanisms of substit ution

Vanadium may be present in silicates as Vi*va+ and yti leuans r*s). au compositions of thiV-rich pumpellyite-group mineral from Hemlo plotclose to the line for a simple substitution of Al3+: V3+ (Fig. 3). At low V contents, the vanadoan

pumpellyite-(Mg), however, shows a significantdeviation toward substitutions involving 2Al foreach V and 3Al for each V (Fig. 3). Additionalelements, such as Cr, Ti and Fe3+, in the vanadoanpumpellyite-(Mg) may have contributed to thisdeviation, but cannot be entirely responsible owingto their low concentrations. Figure 4a illustratesthat vanadium in vanadoan pumpellyite-(Mg) hasa positive correlation with (Mg + Fe*), indicatingcoupled substitutions of V and (Mg,Fe) for Al,which may be written as: 2Al3+ V4+ +(Mg,Fef+ and 3Al3+ = V5* + 2(Mg,Fe)2+. Thepresence of Va+ is supported by the characteristicweak red absorption color of vanadoan pumpel-lyite-(Mg) (c/. Staples et al. 1973).It is noteworthythat the V contents (cations p.f.u.) of vanadoanpumpellyite-(Mg) generally plot above values forthese coupled substitutions (Figs. 3, 4a), and that(Mg + Fe*) does not significantly exceed 3 atomsp.f.u. (Fig. 4a). These compositional featuresindicate that the simple substitution Al3+ : V3+also operates in vanadoan pumpellyite-(Mg) andpredominates where concentrations of V are high.

In respect to As contents, there are clearly twopopulations of vanadoan pumpellyite-(Mg): As-bearing and As-poor (Figs. 4c, d). Vanadoanpumpellyite-(Mg) with a substantial amount of Asis invariably Si-deficient (Table l), and there is anegative correlation between As and Si (Fig. b).These observations indicate that As in vanadoanpumpellyite-(Mg) probably substitutes for Si.Arsenic also tends to correlate negatively with Al(Fig. 4c), and is elevated only where the (Mg -Fe*) content is high (Fig. d). It is possible,therefore, that As is incorporated into vanadoanpumpellyite-(Mg) by a coupled substitution of Si4*+ Al3+ - As5* + (Mg,Fe)2+, assuming As to bepresent as As5* (c/. Donnay & Allmann 1968).

The substitutions involving V and As are mostlikely responsible for the unusually high Mg contentof vanadoan pumpellyite-(Mg) from the study area.Fluorine can be incorporated into vanadoanpumpellyite-(Mg) by either a simple substitution for(OH)- or a coupled substitution for O2-. We favorthe latter scheme, which is probably related to theunusually high Mg contents, e.g., Al3* + 02- =(Mg,FeF* + F, similar to that in epidote-groupminerals (c/. Peacor & Dunn 1988). The formersubstitution is probably less important, becausepublished data on pumpellyite-group minerals donot indicate a high F content despite its highstoichiometric (OHf content (c/. Yoshiasa &Matsumoto 1985).

Substitution of As for Si

The substitution of As for tetrahedrally coor-dinated Si has been postulated in several rare

a oa

a a oa

aaa

aa

aa

1 O

a a

VANADIUM.RICH MINERALS FROM THE HEMLO GOLD DEPOSIT

3 0.5

0.0

159

1 .04.0

3.0

> 2.0

1 .0

2.5

r(ug + F4

3 o.s at r t

a a

.:.

manganoan arsenosilicates, such as ardennite,holdenite, parwellite, tiragalloite, and kolicite, butwas found to be unlikely in all cases on the basisof crystal-structure determinations (Donnay &Allmann 1968, Allmann & Donnay 1971, Moore &Araki 1977a, b, Gramaccioli et sl, 1980, Peacor1980). These authors recognized that As in theseminerals is accommodated in a separate tetrahedralposition. For example, Allmann & Donnay (1971)reported that ardennite is structurally related toepidote, in which chains of A106 and AIO4(OH)2octahedra are bridged by SiOa and Si2O, groups.

3.00.0 k

2.0 | 1 .0 1 1 . 5

sl

1 .01 .0

1 O

a

loO 3

oot o .

0.0

3 0.s

0.02.0

At Xus + F5)FIc. 4. Diagrams indicating possible mechanisms of substitution in vanadoan pumpellyite-(Mg) from the Hemlo gold

deposit: a) \ versus D(Mg + Fe*), b) As yerszs Si, c) As versus N, and d) As versas E(Mg + Fet).

3.02.51 0

However, ardennite differs slightly in structuraltopology, and contains Si3Or6 groups and isolatedSiOa and (As,V)Oa tetrahedra (Donnay & Allmann1968, Allmann & Donnay 1971).

The substitution of As for Si in tetrahedralpositions might bb expected on crystal-chemicalgrounds: the ionic radii of tetrahedrally coor-dinated As5+ and Sia* are 0.335 and 0.26 A,respectively (Shannon 1976). It is noteworthy thatmore than 20t/o of the tetrahedrally coordinatedAs5+ in manganostibite, MnrSbAso,2, is replacedby Si (Moore 1970, Dunn 1987). Pumpellyite-group

'\.a \

\ tra,

160 THE CANADIAN MINERALOGIST

minerals also are structurally related to epidote andcontain Si2O6(OH,O) groups and isolated SiOatetrahedra (Galli & Alberti 1969, Allmann &Donnay 1973, Yoshiasa & Matsumoto 1985). Thecalculated chemical formula for vanadoan pumpel-lyite-(Mg) clearly indicates a substitution of Si byAs, most likely involving the isolated SiO4tetrahedron (c/. Allmann & Donnay 1971).

In ardennite, V is also accommodated in anindependent tetrahedral site, and the completesolid-solution involves As (Donnay & Allmann1968). A minor substitution of V for As occurssimilarly in tiragalloite (Gramaccioli et ol. 1980).However, there is no evidence for either asignificant substitution of As by V or incorporationof a significant amount of V in tetrahedrallycoordinated positions in the pumpellyite-groupminerals from the study area. A weak positivecorrelation between V and As (not presented here)in vanadoan pumpellyite-(Mg) is readily at-tributable to the negative correlation between V andAl (Fie. 3).

P a ro gen e t ic s i g n ific a nce

This is the first observation of pumpellyite-groupminerals in the main ore zone of the Hemlo golddeposit. Pumpellyite-(Mg) is one of the charac-teristic minerals of a late, low- to very-low-grade,calc-silicate alteration in mafic metavolcanic rocksin the vicinity of the Hemlo gold deposit (Pan &Fleet 1990). Walford et al. (1986) also reported alate, pervasive calc-silicate alteration in the hang-ing-wall metasedimentary rocks of the Williamsmine. Harris (1989) noted that calc-silicates arerare in the main ore zone, except for some partsof the David Bell mine. However, Fleet & Pan(1990) and Par. et al. (1991) reported a largenumber of calc-silicate minerals, including epidote,titanite, tremolite, prehnite, allanite, garnet, an-timonian vesuvianite, and armenite in the main orezone of the Golden Giant mine. With rareexceptions, these calc-silicate minerals occur eitherin cross-cutting veins or along foliation planes. Inaddition, tremolite (with calcite, phlogopite,dolomite, clinopyroxene, tourmaline, talc andtitanite) occurs in the diffuse westward extensionof the orebody, and is abundant in the North zoneof the Golden Sceptre Resources Limited propertyand the C zone of the Williams mine (Fleet & Pan1990, Pan et al. l99l). These observations have ledto our suggestion that the late calc-silicatealteration event also occurred in the Hemlo golddeposit (Fleet & Pan 1990, Pan et al. 1991). Thepresent report of pumpellyite-group minerals in themain ore zone is, therefore, further support for thislate calc-silicate alteration in the Hemlo golddeposit.

Preliminary results of mineral equilibria and afluid-inclusion study indicate that the late calc-sili-cate alteration event in the Hemlo gold depositoccurred at about 300-400'C and less than 2 kbarin an aqueous fluid of moderate to high salinity.The V-rich minerals of the pumpellyite group areconsidered to have formed under similar condi-tions, largely through reaction of constituentsalready present or locally remobilized. We believethat the initial enrichment in V in the host rocksof the deposit occurred in an earlier metasomaticevent.

ACKNOWLEDGEMENTS

We thank J. Gray and G. Skrecky of theWilliams mine (Williams Operating Corporation)and D.G. Mcllveen and P. Johnson of the Goldenciant mine (Hemlo Gold Mines Inc.) for logisticalsupport in the field, R.L. Barnett, Y. Cheng, J.Forth, and D.M. Kingston for technical assistance,and R.E. Goad for collecting one sample used inthis study. We also thank P.J. Dunn and P.Robinson for constructive reviews, J.A. Mandarinofor suggestions, and R.F. Martin and S.E.Haggerty for editorial assistance. This study wassupported by an Ontario Geological Survey Geo-science Research Grant (No. 393 to M.E.F.).

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VANADIUM-RICH MINERALS FROM THE HEMLO COLD DEPOSIT l 6 l

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Willowdale. Ontario.

162 THE CANADIAN MINERALOGIST

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Received February 12, 1991, revised manusqiptYosnnsa, A. & Mersutvtoro, T. (1985): Crystal acceptedJune2T' 1991.