t50'7

The Canadian Mineralogistvol. 37. pp. 1501-1524 r19991

NEW Pd-Pb AND Pb-V OXIDES FROM A BONANZA.TYPE PGE-RICH,NEARLY BMS.FREE DEPOSIT IN THE PENIKAT LAYERED COMPLEX. FINLAND

ANDREI Y. BARKOVS

Deparnnent of Earth and Planetary Sciences, McGill University, 3450 University Steet, Montreal, Quebec H3A 2A7, Canada

TAPIO A.A. HALKOAHO

Geological Suwey of Finland, Regional Office for Mid-Finland, P.O. Box 1237, FIN-70211 Kuopio, Finland

ANDREW C. ROBERTS

Geological Suney of Canada, 601 Booth Street, Ottah)a, Ontario KIA 0E8, Canada

ALAN J. CRIDDLE

Depanment of Mineralogy, the Natural History Museum, Cromwell Road, London SW7 5BD, U.K.

ROBERT F. MARTIN

Depanment of Earth and Planetary Sciences, McGilI University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada

HEIKKI PAPUNEN

Depanment of Geology, University of Turku, FIN-20014 Turku, Finlanrl

ABSTRACT

The Kirakkajuppura platinum-group-element (PGE) deposit, in the Penikat layered complex, Finland, is associated with thelow-sulfide Sompujiirvi PGE reef, located near the contact between ultramafic and gabbroic cumulates, close to the countryrocks. The deposit is unique among PGE deposits in layered intrusions as it displays, on a relatively small scale, very high bulk-rock concentrations of PGE (up to >0 5 kg/t total PGE) aLong with a low content of S and Cr, e.g.,\fcn 224 ppm. 108 ppm Sand 0.8 wt.% Cr; (Pd + Pt)/(Os + Ir + Ru) = 93.9 and Pd/ft = 3.0 (mean of four whole-rock analyses). The platinum-groupminerals (PGM), predominantly Pt-Ni-poor vysotskite-braggite, zvyagintsevite and a Pd-Pb oxide, mainly occur in alteredpyroxenite, among grains of Mg-rich actinolite and clinochlore, as unusually large veinlerlike or chain-like aggregates up to-1 cm in length. In agreement with the whole-rock data, the PGM do not show a close relationship with base-metal sulfides

(8M,9) and accessory chromite, occur in virtually BMS-free chromite-poor samples, and are mainly responsible for the bulk-rockS. Unnamed PdePbOlg is the product of oxidation of the zvyagintsevite. It is anhydrous, is a very poor diffractor of X rays [theshongest two lines arc 2.69(l0vb1and 2.35(3) Al, anO trai a low reflectance (ai SaO nm in air: k, ZO.Z-ZO.I% and ni Z0.6-21 3%o).Unnamed PbaV2Oe also occurs in the hydrous silicate association. The Kirakkajuppura PGE deposit cannot have formedas a result of the collection of the PGE by a magmatic sulfide liquid in situ. Tlrclate-stage PGM, responsible for the high PGEconcentration in this deposit, precipitated from a hydrothermal fluids

Keywords: platinum-group elements, platinum-group minerals, Pd-Pb oxide, Pb-V oxide, unnamed minerals, mahc-ultramaficrocks, Kirakkajuppura PGE deposit, Penikat complex, layered intrusion, Fennoscandian Shield, Finland.

Souuanr

Le gisement de Kirakkajuppura, enrichi en 6l6ments du groupe du platine (EGP),est situd dans le complexe stratiforme dePenikat, en Finlande. I1 est associ6 au banc de Sompujiirvi, ir EGP et d faible teneur en sulfures, situd prds du contact entre lescumulats ultramafiques et gabbroiques, tout prds de I'interface avec les roches encaissantes. Le gisement est unique parmi lesgisements de EGP des complexes stratiformes en atteignant, sur une dchelle relativement restreinte, des teneurs extrOmement

I E-mail address:barkov@eps.mcgill.ca

1508 THE CANADIAN MINERALOCIST

dlevdes en EGP dans les roches totales (jusqu'd >0.5 kg/t) et une faible teneur en S et Cr, e.9., 224 ppm des EGP, 108 ppm de SetO.Swt.Eo de Cr; (Pd + ft)/(Os + Ir + Ru) =93.9 etPdlPt = 3.0 (moyenne de quatre analyses de roches totales). Les min6rauxdu groupe du platine (MGP), surtout vysotskite-braggite d faible teneur en Pt et en Ni, zvyagintsdvite et un oxyde Pd-Pb, setrouvent suftout dans une pyrox6nite altdrde, parmi des grains d'actinolite magndsienne et de clinochlore, sous forme d'aggr6gatsen veinules ou en chaines atteignant environ 1 cm en longueur Tout comme f indiquent les donndes obtenues sur roches totales,les MGP ne montrent pas de liens 6troits avec les sulfures des mdtaux de base ou la chromite accessoire; ceux-ci se trouvent dansdes roches essentiellement sans sulfures, et rendent donc compte de la teneur de ces roches en soufre. Un oxyde sans nom, decomposition PdePborg, serait le produit d'oxydation de la zvyagintsdvite Ce mindral est anhydre, et sa pibtre cristallinitd en faitun faible diffracteur de rayons X. Les deux raies les plus intenses du spectre de diffraction sont A 2.69 (intensitd 10, trbs floue) et2.35(3) A.Il possede une faible rdflectance (d 580 nm dans I'air: \20.2-2O.7Vo et R220.6-2l.3Vo). Un autre oxyde sans nom,PbaV2Oe, est aussi associ6 aux silicates hydratds. Le gisement de Kirakkajuppura ne s'est pas form6 par collection des.EGP parun liquide magmatique sulfur6 lz slla. Plut6t, ces mindraux tardifs enrichis en 6l6ments du groupe du platine auraient 6t6 prdcipitdspar voie hydrothermale.

(Traduit par la Rddaction)

Mots-cl6s: dldments du groupe du platine, min6raux du groupe du platine, oxyde de Pd-Pb, oxyde de Pb-V, min6raux sans nom,roches mafiques et ulramaflques, gisement de Kirakkajuppura, complexe de Penikat, massif intrusif stratifome, bouclierfennoscandien. Finlande.

h.nnooucrroN

Oxides of the platinum-group elements (PGEI nrelyoccur in nature. Clark et al. (1974) documented a Pd-(Hg)-bearing oxide at Itabira, Brazil, and suggested thatit might be a mercurian variety of "palladinite" (not yetapproved by the CNMMN, IMA), which was originallydescribed from Brazil by Johnson and Lampadius asearly as 1833 (Jedwab & Cassedanne 1998, and refer-ences therein). Davis et al. (1917) recorded a relatedPd-Cu oxide atltabira. Recently, "palladinite" (ideallyPdO) has been rediscovered by Jedwab et al. (1993),and Olivo & Gauthier (1995) and Olivo er al. (1995)have repofted related Cu-Fe-bearing PdO-type phasesfrom Brazil. Various natural PGE oxides have been re-ported from a number of localities worldwide (Nixon etal.1990, Weiser 1990, Augd & Legendre 1994, Jedwab1995, Garuti et al. 1997, Krstid & Tarktanl99T).

We focus here on the occurrence and properties ofan unnamed Pd-Pb oxide, which is the only naturalPb-rich PGE oxide so far known; it is a platinum-groupmineral(PGI\4) of potential economic signifrcance in theKirakkajuppura PGE deposit, Penikat layered intrusion,Finland. We also present the characteristics of this un-usual deposit, exceptionally enriched in PGE althoughit is nearly free of base-metal sulfides (BMS) and rela-tively poor in chromite. In addition, an unnamed Pb-Voxide is reported from this mineral deposit. The Pd-Pboxide and associated minerals were studied both in situand in healy-mineral concentrates obtained from explo-sion dust.

Geor.ocrcer- BecrcnouNo

The geology, igneous stratigraphy, and mineralogyof the Penikat intrusion, one of the Early Proterozoicmafic-ultramafic layered complexes in the Fenno-scandian Shield (Fig. 1), were described by Alapieti &

Lahtinen (1986, 1989), Halkoaho et al. (1989a,b, 1990a, b), Huhtelin et al. (1989a, b, 1990), Halkoaho(1993) and Alapieti & Halkoaho (1995). The intrusionis about 23 km long and from 1.5 to 3.5 km wide(Fig. 2). The footwall rocks are mainly late Archeangranitic rocks, and the wallrocks are younger tholeiitrcflows, subvolcanic sills, and subordinate polymicticconglomerates. The intrusion is composed of five largeblocks (namely Ala-Penikka, Keski-Penikka,Yli-Penikka, Kilkka and Sompujlirvi) and shows char-acteristic features of layered intrusions, including thepresence ofmarginal and layered series, and cryptic andmegacyclic layering. The marginal series consists of afine-grained chilled margin, which has a sharp contactwith the footwall rocks, non-cumulate-textured gabbroicrocks, and some gabbronoritic and orthopyroxenitlccumulates. The layered series (cd. 2 km in total thick-ness) was divided into five megacyclic units (Alapieti& Lahtinen 1989, Halkoaho 1993), which are designatedMCU I to MCU V. The MCU I unit is composed oforthopyroxene * chromite orthocumulate, containingintercumulus plagioclase and augite; it is overlain byplagioclase-augite gabbroic cumulates. The MCU II andIII units display quite similar geological structures; theirlower parts are composed predominantly of layers ofperidotitic and pyroxenitic rocks and chromite-dominantinterlayers, whereas the upper parts are composed ofplagioclase - augite - orthopyroxene cumulates ofgabbroic affinity with some interlayers ofplagioclase-bearing peridotite, websterite and anorthosite. The rocksof the upper part of the MCU III unit have a pegmatitictexture, and the MCU IV unit hosts the main PGEdeposits (Fig. 2). The MCU IV unit consists of lowerultramafic olivine- and orthopyroxene-rich cumulates(about 20 m), which are overlain by gabbronoritic andanorthositic rocks about I km thick. The MCU V unit iscomposed of a lower layer of orthopyroxene cumulate(up to 7 m) and an overlying sequence of various

Frc. 1. Location of Early Proterozoic and Proterozoic mafic-ultramafic layered intrusions in the Fennoscandian Shield (basedon various data from the literature).

Pd-Pb AND Pd-V OXIDES, PENIKAT COMPLEX, FINLAND

Boren\s Seo

o

ll'i."'f-.49-flruunvnnrsx

USSIA/.,

rr7,fi(5uoncnesor{St\!

_.-*_Fe

tmandra . T{

L a y e r e d I- C o m o l e x

+ + + +l + + + ++ + + +

c l ec i l

%7:&

* .Nft;fu?t{- H',.J,""l

lliliI..Tll :1 i : r : i : : ; i l

=

EEI

rtl

I - o*\

4?o

rX

0 5 0 l O O k m

Palaeozo ic a lka l i rocks

Jothnian sedimentarv rocks(13OO-14O0 m.y . )Svecokarelian qranitoids(1900-1750 m.-y.)Pro terozo ic sed imentaryand volcanic rocks

Layered intrusions Q.4 -2.5Ga)

M a i n l y A r c h a e a n r o c k s LokeOnego

\

t 5 l 0 THE CANADIAN MINERALOGIST

N

BLOCK

gabbroic rocks, which are plagioclase - orthopyroxene,plagioclase - orthopyroxene - clinopyroxene, andplagioclase - clinopyroxene cumulates.

Limited cryptic composit ional variat ions areobserved within the layered series (Halkoaho 1993); itsupper contact is erosional, however, and the uppermostpart of the section seems to have been removed (Alapieti& Lahtinen 1989). The olivine is completely altered in

Muginal series

Megacyclic uit I (MCU I)

Megacyclic mit II (MCU II)

Megacyclic mit III (MCU III)

Megrcyclic mit IV (MCU IlJ

Megacyclic uit V (MCU V)

AP R@f (MCU I\D

SJ Reef (at the contrct between MCUs Itr md IV)

PV Reef (at the contact between MCUS IV md V)

Kirakkajuppua deposit

Average layering

this complex. Ca-poor pyroxene varies from Ensl at thelowermost stratigraphic level of the MCU I unit to EnTein the MCU V unit. Clinopyroxene varies within theranges Ca41 t-ounMg+r6-s05Fess 1a9, with the mostmagnesian augite found in the middle part of the MCUII unit. The cumulus plagioclase most strongly enrichedin the anorthite end-member (Ans2) is found in themiddle MCU III unit. The intercumulus plagioclase

X

LAYEFJ,D INTRUSION

, II j : ' . , ]ti.!;i

'WEH

+ao!

COUNTRYROCKS

lilE cabbroic Loljmu dyke

lI Lot" Ar.h"- grmitoiclsm Proterozoic metasedimentary(ZZ and retzyolcmic rocks

FIc. 2 Generalized geological map of the Penikat layered intrusion, Finland (Alapieti &Lahtinen 1986, Alapieti & Halkoaho 1995, and references therein), showing location ofthe Kirakkajuppura PGE deposit

5

Pd_Pb AND Pd-V OXIDES, PENIKAT COMPLEX, FINLAND

Poikilitic plagioclase-orthopyroxeneorthocumulate with intercumulus ausite

1 5 1 I

Interlayers ofanorthosite

I o o o o o o i

ili#*;;P;1;ooo.%;ffi

,:'":+'"{,f"^:}

Aoo".i r

ffi re.iaotitic-pyroxenitic cumulates

ml'.t'.t Pyroxenitic cumulates l//lPlagroclase-orthopyroxenemesocumulate f-Jl Fragmentsof

| / , / lwi thntercumulusausi te l+ lanorthosi te

Plagioclase-au gite-orthopyroxeneadcumulate

from the lower MCU I unit and cumulus plagioclasefrom the MCU V unit are the poorest in anorthite (An56and An66.61, respectively).

TsB Knerraruppuna PGE Dpposrr

Diverse assemblages of PGM, including the un-named Pd-Pb oxide and the associated Pb-V oxide,occur in the Kirakkajuppura PGE deposit, at the north-ern end of the intrusion, relatively close to the country

l-b * l 6"66ro1" pegrnatoids

FIc. 3. Geological map of the Kirakkajuppura area, Penikat layered complex (Alapieti & Lahtinen 1989), showing the outlineof an exploratory open pit.

l-* loarnor"

rocks (Fig. 2). The deposit is located at the contactbetween peridotitic-pyroxenitic and gabbroic cumulates(Figs. 3, 4), and is spatially associated with the low-sulfide Sompujiirvi (SJ) PGE reef, which is situated atthe boundary between the MCU III and MCU IV units(-0.5 to 1 km above the base of the complex) andextends along the entire length of the intrusion(Halkoaho 1993). Primary rock-forming minerals atKirakkajuppura have been extensively replaced byhydrous silicates. Clinopyroxene (augite and diopside),

t 512 THE CANAD]AN MINERALOGIST

1 0 m

ffi Pyroxenitic cumulates- Plaeioclase-ausite-onhoowoxme[f ]adcimulate

F-i3 Poikilitic plagioclase+rthopyroxenetgd orthocmulate wth lnter(muls ausrte

@ Interlayer of morthosite

fpg-l Gabbroic pegmatoids

FFI overbwden

N Kitukkulupp*u PGE-minemlization (SJ Ree{)

FIc 4. Cross-section (A-B) though the PGE-mineralized zone, showing the main geo-logical features of the Kirakkajuppura PGE deposit (see Fig. 3 for location and refer-ences). The outline of the exploratory open pit is drawn in bold.

A

TABLE I REPRESENTATI\IE COMPOSITIONS OF HYDROUSSILICAIESFROMTIIE PGEDEPOSN,

PENIKAT COMPLEX FINLAND

preserved in rocks in the vicinity of the ore deposit,exhibits signif icant composit ional variat ionsWo:o r-+r zEn+g l-4e 9Fs9 7-14 3. Accessory chomite, richinCr (34.043.3wt.Vo CrzO) and in Al (11.3-16.2wt.%AlzO:), strongly varies in terms of i ts l00Mg/(Mg + Fez*) and 100Fe3*/(Fe3* + Al + Cr) values, 3-30and1J2,respectively, and mostly plots within the com-positional field of spinel from stratiform intrusionsflrvine 1967).

The accessory ilmenite is strongly enriched in Mn(3.4-1 l. 1 w t.Vo MnO ; i. e., 7 .5 -25.3 mol. 7a pyrophaniteend-member), and differs from most examples of il-menite in layered intrusions, which, with a few excep-tions, contain less than I wt.Vo MnO. The apatiteanalyzed is poor in Cl (<0.4 wt.Vo Cl) and rich in F (>2wt.Vo F), and the mica (phlogopite) is relatively rich inMg (Mg# 60.9-'72.4; Mg# = 100Mg/(Mg + Fe + Mn),and does not contain detectable Cl.

The principal PGE mineralization at Kirakkajuppurais erratically distributed in a completely altered pyrox-enite, which now consists predominantly of calcicamphibole (>70 modal 7o), subordinate chlori te(clinochlore), and various amounts (<10 modal 7o) ofdisseminated chromite, which is heterogeneously dis-tributed in the rock. The PGE-mineralized zone is not,however, restricted to the pyroxenite; it extends to theadjacent gabbroic and peridotitic rocks (Fig. 4). In the

Amphibole

SiOzvr%o 57 15 57.47T iO, nd ndAlros 0 2l 0.25CrO. nd ndF€O 930 t07MnO 013 0 18MgO 19 19 70NiO na n&CsO 12'19 1268I],O 2 r5 215

56 41 57 3 t 56 58n d n d n d0 6 3 0 1 7 0 3 8a d 0 0 5 0 2 58 1 8 7 1 2 7 1 90 19 023 025

1942 1932 1952t rd 017 O.22

1276 1244 t2402 r 3 2 \ 3 2 1 2

8 0 7 8 0 0

0 0 3 0 0 6<001 003

0 8 4 0 8 50 0 3 0 0 34 0 6 4 1 1192 I 88

1 0 8 0 9 4 1 0 3002 002 0 0 l4 0 0 4 w 4 0 0l 9 t 1 8 9 1 8 7

Toal

si@fuNAIuAl

CrFeI\,tnMSCa

Ms#

r@ 97 100 50

7 9 7 8 0 00 0 3

<001 004

56 E8n d0 5 9n d8 8 2ot2

19 150 1 7

12 422 1 4

t00 29

7 9 70 0 30 0'l

2974o07

16 85D &

t7 55o l 2

U M021

' 0 0 7l l 95

2 9 ?1 0 30 9 6

1 4 70 0 13 5 9

<0 01

7 0 8

9972 9894 98 91 100.62

E l 0 7 9 4

o o 70,04

0.96o 0 2407t @

8 0 6

The wvelength{isposion cl€ctrof,-miqoprobe ualy*s wre ffiied out usiog 8JEOL733 insEumqt AlFesFeO;K Na, Cl, mdFwue rcught, but not dstectedn a : not aahm4 n d : not detwted HrO (wt o/o) cal@lated AoB charge balm@Forunrlre elculaed on the ba6is of E(O + OID = 24 for mphibole ed t(O + OH)- 16 for ahlorite Mg# = looMy(Mg + Fe + Mn)

Pd_Pb AND Pd_V OXIDES, PENIKAT COMPLEX, FINLAND r513

Frc 5. Unusually large aggregates (A) and a chain-likeveinlet (B) of platinurn-group minerals (PGM), predomi-nantly vysotskite Pt-poor braggite (altered to various ex-tent) and zvyagintsevite, among hydrous silicates (sil).Kirakkajuppura PGE deposit. Back-scattered electron(BSE) images. Scale bars: 1 mm (A) and 100 pm (B)

PGE-ich altered pyroxenite, both the amphibole (acti-nolite) and chlorite are Mg-rich (Mg# 80 and >70,respectively; Table 1), indicating a relatively high-magnesium precursor, the relict textures of which implya coarse to pegmatitic grain-size prior to alteration.

The PGE mineralization

In terms of petrography , the PGE-ich pyroxenite isquite similar to altered mafic-ultramafic rocks in otherlayered intrusions. It can virtually be devoid of BMS.Cut specimens as large as -10 cm may not contain anyrecognizable BMSbult, instead, contain unusually large(up to -1 cm in the longest dimension), megascopicallyvisible aggregates of PGM among the hydrous silicates.The PGM aggregates are typically veinlet-like and areheterogeneously distributed in the rock. The PGM donot show a close relationship with accessory chromite.For instance, unusually large aggregates of PGM(Fig. 5A.) are found in a polished section (-3 x 2.5 cmin size) virtually devoid of both BMS and chomite. Inagreement with this observation, the maximum concen-trations of PGE in the ore deposit were documented ina S-Cr-poor sample (250 and 9840 ppm of S and Cr,respectively). However, trace to accessory BMS arepresent in the rock, as chalcopyrite, bornite, millerite,and secondary chalcocite (grains <0.3 mm) are occa-sionally encountered in the concentrates ofPGE-bearingminerals. Similarly, a chalcopyrite - bornite - milleriteassemblage is associated with various PGM in PGE-ichmafic rocks of the neighboring layered intrusion, one ofthe layered intrusive bodies of the Oulanka complex(Fig. 1); these rocks are, in contrast, rich in BMS(Barkov et al.1996).

FIc. 6. Typical grains of vysotskite - braggite at Kirakkajuppura A. Anhedral braggite, bg (Pda 7sPt0 leNio osSo sa), a part ofPGM aggregates in Figure 5A., which is associated with Mg-rich calcic amphibole (am) and is replaced by very fine-grainedaggregate of zvyagintsevite-related phases (white). B. Subhedral braggite, bg (Pd677Pt623NioozSoss; center of grain) withinclusions ofFe-rich cuprorhodsite, cpr [(Cu6 65Fes 36)q1 01(Rh1 sllrs 12Pt6 sTCos s2)12 6253 e7]. Black: epoxy. BSE images. Scalebars are 10 um in each casc.

l5l4 THE CANADIAN MINERALOGIST

TABLE 2 PLATINUM.GROIJP MINERALS IN TIIEKIRAKKAJUPPIIRA PGE DEPOSIT

subordinate

Vysotskit+braggiteZryagintswitePd-Pb oxid{s)Cuprorhodsit+mslmiteLuitffilictm@iteIrusiteKeithoomitePd-dch kond€ritePalladim gold*

Rh(Ni,Fe,Cu)5S.(Feo.,Cu" )F.h,S,i*PqPb,s,(Pd,Pt)Cu

{ Up to 9 wt % Pd (WDS electron-microplobe data)*+ Fmorhodsite - oprcrhodsite - (maleite) sries (sm Bukov et al 20ffi)

TABLE 3 REPRESENTATIVE WHOLE-ROCK COMPOSITIONSOF PGE-RICH, S-POOR ALTERED PYRO)GMTES

FROMTHE PGEDEPOSTT.PENIKAT LAYERED COMPLEX FINLAND

tltEE

EE

EEo!!tEEEtll

SiO, wt %Tio,AtqFeOMnOMsoCoONaroKrOcrro,PrO,

Total

46 03022

1 t .35o.2l

201,28 ,f00 0 30 0 10 4 50 0 3

9428

39 690 0 8

l l 9 6l0 850 1 4

24 594 5 00 0 1

<0 010 0 4n d

91 86

51 04026

8 3 90 1 9

21 959 8 0n d

<0 0l0 l E0 0 3

95 61

41 960 1 9620

12 34025

19 4 l7 5 4o 0 2

<0 0l4 6 20 0 1

92 54

No Avqage

Cr (ppm) 2Mo 3300Ni 500 760Cu 10 10s 6 0 7 0

122 610420 1900t96 1020

2300 740021 100 60600s4700 203000

600 1340

26900 1120 8345850 580 673100 250 93160 140 108

780 94 401 52600 330 r3r251000 I20 584

11200 1'170 5667 5116000 17100 537@334000 56700 162100

1170 530 910

4656 761 2238288 332 302

128 5 171 8 123 5

102.7 135 7 93 90 1 2 0 4 3 0 1 4

os (ppb)kRuRhftPdAu

EPGE (ppm) 78,8 274 5Pd,iPt 2 59 3.35Pd/Ir 1lo2 106 6(P1+Pd) (Qs+k+Ru)

1027 747o 02 0_01

Numbor$ 1-4 ed 5-8 refq to difsflt wples from the Kitakkejuppua dePosit;

howwq, smples I to 4 @fltain apprcxi@tely tbe me lwel ofPGE ro wples 5-8ppm: puts pq million; ppb: parts pq billiotr Results for smples 5-8 re Fl@t€d

Aom thow reported by Halkoaho et al (1989a)

Frc. 7. A Chain-like veinlet of zvyagintsevite and minorvysotskite - braggite, Pd-(Cu-Fe-Pt)-Pb oxide, andkeithconnite (PGM) among hydrous silicates (sil). B. Mag-nification of part of the veinlet (outlined in Fig 7A), show-ing numerous, minute crystals of zvyagintsevite (white)within Mg-rich calcic amphibole (arn; actinolite) and chlorite(chl; clinochlore); ttn: secondary titanite. Note subparallelorientation of the zvyagintsevite, titanite and amphibole.C. Fufther magnification of area shown in Figure 78. Notesubparallel orientation of the zvyagintsevite (zv, white),including the grains less than 0.5 pm in size; am: hostamphibole. BSE images. Scale bars: 100 pm (A), 10 pm(B) and I pm (C)

Pd_Pb AND Pd_V OXIDES, PENIKAT COMPLEX, FINLAND 15 15

Pt-poor members of the vysotskite - braggite series,zvyagintsevite, and the Pd-Pb oxide are the main PGMat Kirakkajuppura. The large aggregates of PGM arecomposed mainly of vysotskite - braggite, which is al-tered and probably oxidized to various degrees, andcommonly replaced by very fine-grained aggregates ofzvyagintsevite and related secondary Pd-(Pt-Cu)-Pb-rich PGM (Fig. 6,4.). Such unusually large grains ofbraggite (8 mm) were previously reported from theStillwater complex (Criddle & Stanley 1985). Braggitealso occurs as subhedral grains, which may displayabundant fractures and late alteration along these frac-tures (Fig. 68). There are also thiospinel PGM (malanite- cuprorhodsite - ferrorhodsite series) in these aggre-gates. Various PGM, identifred in the mineral deposit,are listed in Table 2; a more detailed description will begiven separately.

Bulk-rock compositions (Table 3) are consistent withmineralogical observations, indicating that, in general,the Pd-Pt sulfides predominate over all other PGM inthe deposit. Owing to this predominance, bulk-rock con-centrations of Pd, ft and S (atom Vo) inthe BMS-free orvery-low- sulfide samples (e.g., no. 6 and7, Table 3)give atomic proportions of vysotskite - braggite; itshould be noted that, unlike other localities, Ni is notessential in the Kirakkajuppura vysotskite - braggite.For instance, bulk-rock composition number 6 (Pd 203,Pt 60.6, and S 70 ppm) yields a nearly ideal braggite-type formula (Pda azPto r+)>r orSo se @asis: I atoms = 2),and number 7 gives a similar chemical formula (Pd6 72Pto r+)>o s6St 14. Some of the PGE-rich samples exhibitan excess of S relative to vysotskite - braggite stoichi-ometry, indicating that other sulfides (trace BM,9) alsoare present in the rock.

The presence of unusual chainlike veinlets of PGM,which cut the hydrous silicates (e.g., Figs. 5B, 7A), is aremarkable feature. These veinlets are up to -1 mm inIength, and the number of PGM grains in such chainsexceeds 300, whereas the individual grains are, in gen-eral, very small (<1 to 50 pm; mainly <10 pm). Withina chain (Fig. 7A), a subparallel orientation of micro-crystalline PGM, hydrots silicates and secondarytitanite is locally observed (Fig. 7B). This textural rela-tionship suggests that deposition of these late-stagePGM was controlled by both fractures and cleavages inthe hydrous silicates. Scanning electron microscopy(SEM), coupled with quantitative energy-dispersionspectrometry (EDS, frnely focused beam, 100-s countperiods, and Link ISIS revision 3.2 program) were ap-plied to characterize the components ofthe chains. Theyare principally composed of zvyagintsevite, subordinatevysotskite - (Pt-poor) braggite, Pd-(Pt-Cu-Fe)-pboxide, and a very rare Pd-rich telluride, probablykeithconnite, Pd2s 1a(Te6 52Bio:+)>o so @asis: I atoms =27). Interestingly, the chain shown in Figure 7A isnearly monomineralic, and includes numerous submi-croscopic, mostly subhedral to euhedral crystals ofzvyagintsevite. The SEM-EDS technique allowed us to

FIc 8. Unnamed Pb-V oxide (V-Pb) filling cleavage rnMg-rich calcic amphibole, am (actinolite). BSE image.Scale bar: 10 pm.

analyze grains as small as <1 pm, and acceptable totals(lO0 t2 wt.Vo) were obtained. EDS analyses of selectedgrains of zvyagintsevite (n = 34) were carried outthroughout this chain (Fig. 7A) and gave the followingmean (with ranges): Pd 58.75 (57.5-60.0), Pt l.15 (0.7-1.6), Pb 40.10 (38.9-41.0), Sn not derecred (0.0-0.6),total (not normalized) of 100.0 wt. 7o (98.0-101.8), cor-responding to (Pdz saPto 03)>2 srPbr o:. In addition, abouttwenty subparallel grains of zvyagintsevite, less than0.8 pm in size (Fig. 7C), were analyzed semiquantita-tively with the EDS and their (Pd + ft)/(Pb + Sn) valueis close to 3. Results of wavelength-dispersion (WDS)electron-microprobe analyses are in good agreementwith the EDS results, and indicate that, unlike the typeoccurrence at Noril'sk (Genkin et al. 1966), thezvyagintsevite at Kirakkajuppura is poor in Pt and Sn,and is close to end-member Pd3Pb. Electron-microprobeanalyses (EDS and WDS; n = 18) of the vysotskite -

TABLE 4 CIIEMICAL COMPOSITION5 OF UNNAMED Pb-V O)flDE4FROM TIIE PENIKAT LAYERED COMPLEX FINLAND

PbO wt %V.o'

Total

Pbryfu

E3 31 84 1216 16 1639

99 47 100 51

t425 83 5l 842516 19 16 16 t6 57

100 44 99 67 100 82

Alomic propoftions (O = 9)

e Quentitative EDS-SEM electron-mimprobe malyres Cu, Ni' Co, M4 Ti, @d Cr

wqe rcught, but not detected As a tes! this PFV oxide alrc wa malyzed for Si

Low content of Si itr ths€ rsults (0 4-0 5 wt % SiOr, €rcluded from th€ total)

indicates very miaor ontamination by the host (actitrol-ite) @/l: atoms per fomula

mit + Veirlat shom in Figue 8

4 1 1 4 1 01 96 196

4 1 3 4 1 2 4 0 81 9 5 1 9 5 1 9 7

1 5 l 6 THE CANADIAN

braggite in different textural associations yreld 13-31,mean -20 mol.7o PtS. These compositions agree withthose derived from the bulk-rock analvses (14-25 mot.% PtS).

FIc 9. Grains ofthe unnamed Pd-Pb oxide, ox (mounted inepoxy, black), associated with zvyagintsevite (zv), Pt-poorbraggite (bg) and vysotskite (vs). Note the subhedral (pre-sumably pseudomorphous) outlines of the oxide, its frac-tured appearance (A), clear replacement relations withzvyagintsevite (B), and intergrowth relationships withnearly end-member vysotskite, Pd1 6sNi6 62Pta6 stSo sr (C)BSE images. Scale bars: 10 pm (A and C), and 100 pm (B).

MINERALOGIST

UNNeueo Lsa>VeNaoIUM OXIDE

An unusual Pb-V-rich phase (Fig. 8) was identiftedin spatial association with the late-stage PGM andhydrous silicates It fills a cleavage in a Mg-rich calcicamphibole (actinolite; Mg# 80.4). As in the case of themicrocrystalline zvyagintsevite (Fig. 7), it is a very latephase, whose deposit ion was control led by theactinolite's cleavage. Examination under SEM-EDSreveals V, Pb and O as the constituents. EDS analysesof the phase (Table 4) were carried out using pure metalstandards (and SiOz for Si) and a finely focused beam(-1 pm). A low content of Si (0.4-0.5 wt.% SiOz)encountered in these analyses indicates very minorcontamination by the host (actinolite). The electron-microprobe results indicate that the phase differs in com-position from chervetite (the only Pb-V oxide mineralknown: PbzYzOt; Bariand et al. 1963) and appears torepresent a new mineral. The ideal formula, suggestedby the analytical results, is PbaVzOq and requires Pb2*and V5* in order to maintain charge balance. This PFVoxide may be a natural equivalent of synthetic PbaVzOq(PDF 26-1163), but its small grain-size precludes amore detailed examination.

UNxalreo Paueotul,elEAD OxIDE

Appearance

The Pd-Pb oxide occurs as both subhedral andanhedral grains (up to 0.2-0.3 mm), which typicallydisplay abundant fractures (like syneresis cracks;Fig. 9A). An alteration along late linear fractures also isobserved. Textural evidence suggests that the oxide is areplacement product of zvyagintsevite (Fig. 9B), imply-

Frc. 10 Aggregate of very fine-grained Pd-Pb-(Cu-Fe)oxide [(Pds o:Cuo soFee 13Pt0 e5);e e1Pb0 eaOl6], ox (gray)'which is surrounded by a narrow rim ofnearly end-memberzvyagintsevite (Pd2 e6-2 eePk6 0rPbt or r o:), zv (white); tel:Pd-(Pb)-rich telluride. Note subhedral outlines ofthe com-posite grain. Host: actinolite. BSE image Scale bar: 10 pm.

ing that the subhedral habit is pseudomorphous. Oxidegrains may contain relics of zvyagintsevite and alsovysotskite - braggite crystal(s); the latter have remainedwell preserved during a selective replacement ofzvyagintsevite by the oxide. The oxide grains are com-monly heterogeneous. Single, rather homogeneousgrains also are present, however (e.g., Fig. 9A), butX-ray powder-diffraction indicates that they are cryp-tocrystalline. The oxide is found partly mantlingvysotskite (Fig. 9C); it may well have formed at the

t5r'7

expense of pre-existing zvyagintsevite, rather than thevysotskite, however.

Figure 10 shows some unusual textural relationshipsinvolving the Pd-Pb oxide. Very f ine-grainedPd-(Cu,Fe)-Pb oxide is surrounded by a thin rim ofzvyagintsevite. The outline of this composite grainindicates its subhedral habit before alteration. Two dif-ferent interpretations can be proposed. The first is thatthe rim represents relics of a pre-existing subhedral grainof zvyagintsevite. Owing to a particular feature (e.g.,

Pd_Pb AND Pd-V OXIDES, PENIKAT COMPLEX, FINLAND

R%

400 450 500 550 600 650 70c2nm

Frc. 1 1. Reflectance spectra for two grains of the unnamed Pd-Pb oxide from Penikat,grains 1 and 2, both measured in air and in oil, compared with the spectra fol"palladinite" [(Pdne3Cus66Hges65Fee663);100O; basis: O = 1] from the type locality,Itabira,Braz1l (Jedwab et al.,unplbl. dala).

-'E-'Grain 1 Rl-.8-"Grain 1 R...4.''Grain2 Ri- .r- .Grain2 R2--*-- Pdo Rt* poo nz

-* +i* *

1 5 1 8 THE CANADIAN MINERALOGIST

l , , m

TABLE 5 REFLECTANCE DATA FOR TWO GRAINS OF TJNNAMEDPd_Pb O)CDEFROM THE PENIKAT LAYERED COMPLEX FINLAND

Grain I Crain2 Grain I Graitr2R,% R% R% Ryo Rr% R% R% Ry.(air) (air) (air) (air) (oil) (oil) (oil) (oil)

and anisotropic (J. Jedwab et al., unpubL data), thePd-Pb oxide has a distinctly lower reflectance, and, incontrast, shows no biref lectance and anisotropy(Fig. 11). Reflectance data obtained for two grains ofthe Pd-Pb oxide indicate that it is isotropic or nearly so(Table 5).

The existence of a synthetic PdO..xH2O phase (PDF9-254) stimulated us to test whether or not the Pd-Pboxide also is hydrous. Owing to the small grain-size,infrared-absorption spectroscopy studies of natural PGEoxides are complicated, and we are not aware of resultsof similar studies in the literature. The infrared spec-trum of the Penikat Pd-Pb oxide was obtained using aSpectra-Tech IR-Plan infrared microscope interfaced toa Bomem Michelson MB-120 Fourier-transform infra-red spectrometer, which utilizes a 0.25 mm liquid-ni-trogen-cooled mercury-cadmium telluride detector. Avery small amount of the pure material (from a grain-0.2 mm in size) was dug out of polished section andmounted in a diamond-anvil microsample cell. Pressurewas then applied to crush the mineral and cause it tospread as a randomly oriented powder; after crushing,the pressure was released. The diamond cell was thenpositioned in the microscope accessory, and the pow-dered sample masked, so as to isolate a circular areameasuring 100 pm in diameter and between 1 and 5 pmin uniform thickness. Two hundred scans were collectedand added together, at 4 cm I resolution, from 700 to4000 cm-r. The transmittance spectrum was producedby taking the ratio of the single-beam spectrum of thespecimen in the diamond-anvil cell against a single-beam spectrum of the empty diamond-anvil cell col-lected with the same parameters. The spectrum obtained(Fig. l2) is virtually devoid of any bands; there is abso-

TABLE 6 CHEMICAT COMPOSMON OF TJNNAMED Pd PbO)flDE FROM TIIEPENIKAT LAYERED COMPLEX, FINLAND

PdOwtYo El 91Pbo 17 44

Total 99 35

Pdqfu 8 9s5Pb I 045

80 29 76 56 79 86 80 83 82 85 80 3818 l t 2 t39 19 l0 l8 59 rE 89 t892

98 40 97 95 98 96 99 42 101 74 99 30

Alomic Foportioos (O = l0)

8 8 4 8 8 8 8 8 9 8 8 61 1 6 l l 2 l l l 1 1 4

Atomic proportions (O - l)

P d q f u 0 9 0 0 8 9 0 8 7 0 8 8 0 8 9 0 E 9P b 0 1 0 0 1 1 0 1 3 0 1 2 0 1 1 0 1 1

$ JEOL 733 electlon microprobe, m ac@lerating voltage of 15 kV md a probecwent of 15 DA The following lires (standuds) wue usd: PdZa, PtIc nd PbZa(pure metals md PbTe)* Cmeca Cmebu electron mioroprobq il rcc€lqatitrg voltagp of 20 kV md aprobe Ment of 23nA The following lines (standuds) were used: PdZc,PMa aldPb,l.1d (pue metals ed PbS) h w6 sught, but mt det@ted** Avsage result of six malyses (1 to 6)

9 39 2E 98 6E 3E O

' 7 5

7 27 06 8665

6 4

4004204404604E0500520540560580600620640660680100

23 6 243 22823 6 243 22923 5 242 22723 3 23 ,8 223228 234 2202 2 4 2 3 0 2 1 62 1 9 2 2 5 2 t 22 1 5 2 2 1 2 0 921 1 2r7 20452 0 7 2 1 3 2 0 22 0 5 2 1 0 1 9 9202 20 '7 t9 61 9 9 2 0 4 1 9 3t 9 7 2 0 r 1 9 11 9 5 t 9 9 1 8 81 9 4 1 9 7 1 8 6

10 8 1121 0 ' 7 1 1 11 0 5 1 0 81 0 3 1 0 69 9 1 0 29 5 9 89 2 9 58 8 9 l8 5 E 88 2 8 58 0 8 37 8 8 07 6 7 A 57 45 7'.17 2 1 4'7 1 725

2 3 2a t o

2 2 5t , 1

2 1 72142102062042 0 11 9 81 9 61 9 31 9 0

9 79 69 49 0t 78 4E I7 97 6

7 l7 06 9

6 66 4

COLOR VALUES: C ILLIMINANTx y Ya/. P.Y. Ld

o 290o 294

Grain 2 (aioRr 0299R2 0299

0 3 0 6 2 r 20 3 0 6 2 l t

0307 2060307 21 1

GEin 1 (an)RrR2

Grain I (oil)R,R2

5 4 4 7 95 4 4 7 9

5 0 4 7 95 0 4'79

0 288 0295 E 61 l0 6 4790288 0296 889 105 479

Grain 2 (oil)Rr 0 2E9 0296 7 3lR2 02E9 0296 767

l0 1 47El0 0 478

The spectra ws€ obtained with a Z€iss Axiotroo MPM8O sDectroDhoxometq. Theobjectives used wce Epiplan N@flws X50, ef;N.A. 030, with Zeiss oil forimersion ND = 1 515, and the reflela@ stmdrd wm Z€iss SiC (rc 472\

'Ine

dimeter ofthe mwing spot was set at 8 miqometeE Grain l: re ;epresetrtaweoompositions I ud 5; grain 2 trmbs 3, Table 6

skeletal habit), an oxidizing fluid may have had accessto the central part of this crystal and, thus, the grainbegan to oxidize, and the Pd-Pb oxide crystallized, fromthe center to the periphery. The oxidation was incom-plete, and relics of the grains are preserved as the rim.The second interpretation is that the pre-existing crystalof zvyagintsevite was completely replaced by thePd-Pb oxide; however, its subhedral (pseudomorphous:cf. Fig. 9,A.) outlines remained. At a later stage, theoxide reacted with a reducing fluid (e.g., containing H2as reducing agent) and was reduced back to thezvyagintsevite.

Reflec tanc e and infrared-ab s orption s pectro s copy

Under reflected light in air, the Pd-Pb oxide is grayand has a low reflectance. Compared with "palladinite"from the type locality, which is distinctly bireflectant

890 86 ' tI l0 133

0 8 90 1 1

lutely nothing in the regions where one would expectbands ascribable to H2O or OH groups. For this reason,we infer that the Pd-Pb oxide is anhydrous.

Composition

Relatively large grains of the Pd-Pb oxide were ex-amined and, analyzed with the SEM-EDS, and werefound to contain Pd, Pb and O. A large number of quan-titative EDS analyses reveal relatively narrow compo-sit ional ranges for Pd and Pb, which are ratherhomogeneously distributed. Results of two sets of WDSelectron-microprobe analyses of the oxide (large grains),obtained using different instruments, standards and ana-lytical conditions, are in good agreement with each other(Table 6).

In addition to the large grains, small grains of thePd-Pb oxide (from a chainlike veinler) and the micro-crystalline aggregate (Fig. 10) were analyzed with EDS.Essential concentrations of Cu (up to 4.8 wt.% CuO),Fe (0.7-1.6 wt.7o FeO) and Pt (up to 1.7 wlVo PtO2)were recorded. The compositional variation observed[(Pd7 s 6 sCu6 6-6.sFe6 1-6 3Pt6 65-0 lo):8 8-e 0Pb0 e I 1010]suggests that elements other than Pd (Cu, Fe, and Pt)may replace -1 Pd atoms per formula lunit (apfu). Theelevated Cu concentration cannot be ascribed to con-tamination by the host; the hydrous silicate is totallydevoid of Cu. Zvyagintsevite crystals from this oredeposit are poor in Cu; thus, the Cu in the oxide wasprobably contributed from a fluid during oxidation. Asa more remote possibility, it may reflect an impurity inthe fine-grained material analyzed. We recall that Olivo

t 519

& Gauthier (1995) reported up to 8.4 wt.Vo CuO (andup to 1.3 wt.Vo Fe2O3) in the "palladinite" from Brazil.

Electron-microprobe results suggest a generalizedformula PdqPbOro or (Pd,Pb)O for the Penikat oxide.The phase PdqPbOro has apparently never been synthe-sized. The only Pd-Pb oxides reported are PdPbO2,whose X-ray powder pattem (PDF 38-1357) is substan-tially different from that of the Kirakkajuppura mate-rial, and PdePbOle (Machida er al. 1984). No XRD dataare available for the latter phase, which also displaysthe ratio Pd:Pb of 9 but is richer in O and differs in theoxidation state of Pd (1.e. ,Pd+ versusPd2+). A relation-ship between the natural Pd2*ePb2*O1s and syntheticPd4+ePb2+ole may exist.

Pd and Pb are commonly divalent, and both the for-mulae (Pd6ePbo r)O and PdePbole are thus possible interms of charge balance. Three major arguments favorthe choice of the latter formula. (l) Isomorphous rela-tionships between Pd and Pb are highly unusual. (2) All

oo

d

F

Pd-Pb AND Pd-V OXIDES, PENIKAT COMPLEX, FINLAND

i f ii D @

-/*udh\-

Wavenumber (c ; r )

Ftc. 12 Infrared-absorption spectrum of the unnamed Pd-Pb oxide from Penikat.

TABLE? X-RAYP TIONDATAFORTTIE UNNAMED Pd_Pb OXDE

FROM THEPENIKAT COMPLEX FINLAND

I (est )

The pattem obtaircd using a 57 3 m Debyeschtrs @en, Cu ndiatioq Ni filts(l CuKa = I 54178 A), intqsitio esrinared vizually Not conBcted for sbdnkage mdrc intemal stedtrd 6: broad line, vD: very broad line

d(L) r(est) d(A) r(est) d(A)

2 0 4 t b 1 5 5t82 | t441 6 8 l r b 1 3 3

l0vb 269 I3 2 3 5 0 50 5 2 1 7 l v b

r520 THE CANADIAN MINERALOGIST

the Pd-Pb oxide grains analyzed so far contain aboutthe same (high) level of Pb, i.e. -1 opf, Pb (O=10); thischaracteristic implies that Pb occupies a separate site inthe crystal structure. (3) None of the related natural PdO-

0

type oxides reported to date shows as high a content ofa semimetal as does this Pd-Pb oxide. Nevertheless, theformula (Pd,Pb)O is still a possibility to be tested. -zo

No(6

ctt- -40

-60

-40

X-ray study

X-ray powder data for the Pd-Pb oxide are presentedin Table 7. The mineral is cryptocrystalline, as was re-vealed in an attempted precession single-crystal study,and produces rather broad diffuse arcs on the powderfilm. For this reason, we chose to measure the patteffiobtained with a 57.3 mm Debye-Scherrer camera. Itexhibits important differences with respect to both thetetragonal and cubic polymorphic modifications of PdO(cf PDF 43-1024 and46-1211).

DrscussroN

The Kirakkajuppura PGE deposit of the Penikat lay-ered complex is rather unique among PGE-mineralizedoccurrences in layered intrusions, even those of the low-sulfide type. The deposit shows examples of unusuallyPGE-rich mineralization, which occurs in nearly BMS-free rock and does not display a close relationship withaccessory chromite. In many related complexes, for in-stance at Bushveld (Von Gruenewaldt et al. 1986) andRathbun Lake (Rowell & Edgar 1986), most of the PGMare enclosed by hydrous silicates, although their closespatial association with BMS or chromitite (or both) ischaracteristic. Furthermore, the deposit contains exceed-ingly high (e.g., -0.5 kg/t total PGE; Table 3) maxr-mum concentrations of rhe PGE, mainly Pd and Pt, invery S-poor (and relatively Cr-poor) samples. The maxi-mum enrichment in PGE is at least one order of magni-tude higher than that reported from the Merensky-typeand related ores worldwide, and is quite comparablewith PGE grades found in dunite pipes of the Bushveldcomplex, in which the highest concentrations of PGEamong layered intrusions are recorded (e.9., Naldrett1981). The relative abundance of late-stage vysotskite- braggite in the form ofunusually large aggregates (Fig.5A), the general absence or scarcity of BMS atKirakkajuppura, and the atomic ratio (Pd + Pt)/S closeto 1 in the bulk-rock compositions indicate that nearlyall S in the PGE-rich samples is contributed by the PGE-bearing sulfides. This contrasts with observations fromother parts of the Sompujlirvi reei in which S-free PGMpredominate (Alapieti & Lahtinen 1986, Halkoaho1993).

Another distinctive feature of the Kirakkajuppuramineralization is the relative abundance of Pb-ich PGM(zvyagintsevite, unnamed Pd-Pb oxide, Pd-richkonderite and previously unknown Pd3Pb2S2). The pres-ence of Cr-FI-PGE thiospinels (Barkov et al. 20OO)

- - l 2 3 4

1000 /T ,K

FIc. I 3. Log a(Oz) - T diagram showing position of the Pd-PdO buffer, compared with magnetite-hematite (Mag-

Hem) and qtaftz - fayalite - magnetite (QFM) buffers(Olivo & Gammons 1996, and references therein)

also is unusual, as these are typically associated withUral-Alaska-type complexes. In other layered intru-sions, a few minor occuffences of zvyagintsevite havebeen reported (e.g., unpubl. report of Laflamme 1976,cited in Szymafiskiet al. 1997,Barkov et al. 1991); thisPGM rs rather characteristic of other types of PGE de-posits, such as the Noril'sk (Genkin et al. 1966, Cabn& Traill 1966) and the Konder alkali-ultramafic com-plex, in which, in related placers, unusually large (8 mm)crystals of zvyagintsevite occur (Cabri & Laflamme1997).The location of the Kirakkajuppura deposit rela-tively close to the wallrocks (Fig. 2) may imply thatcontamination of the magma was responsible for therelative abundance of the Pb-rich PGM in the deposit.A test ofthis possibility awaits isotopic data. The onlyrelevant available data concern another Finnish layeredintrusion, Suhanko-Konttijiirvi (Portimo complex,Fig. l); there, the Pb isotopic composition of galenafrom the marginal series provides evidence for contami-nation (Alapieti et al. 1989).

Textural relationships (Fig. 7) suggest a low-tem-perature crystal l izat ion for the Kirakkajuppurazvyagintsevite, after associated Pd-rich vysotskite -

braggite, which itself is of low-temperature origin. Ac-cording to experimental data (Cabri et al. 1978),vysotskite can only crystallize at a subsolidus tempera-ture. The last PGM to form in this mineral assemblageis, in general, the Pd-Pb oxide.

The Penikat PGE oxide contains a significantamount of an element other than Pd and O. Similarly,other natural PGE oxides contain a wide variety of mi-nor elements, such as Cu, Fe, Ni, Mn and Hg (Clark et

Pd-Pb AND Pd_V OXIDES, PENIKAT COMPLEX, FINLAND r521

al. l9'74, Jedwab et al. 1993, Aug6 & Legendre 1994,Olivo & Gauthier 1995, Garuti et al. 1997, Krsti6 &Tarkian 1997). Apart from Mn and some base metals,most of these elements in PGE oxides are inherited fromtheir precursors, the primary PGM. The incorporationof these elements may well stabilize PGE oxides (cf.Westland 1981).

The oxidation reactions

"Palladinite", the related Pd-(Cu-Fe) oxide, formedduring recent weathering via oxidation of palladseite(PdrzSers) to metallic Pd and subsequent oxidation ofthe Pd to PdO (Olivo & Gammons 1996). Although thePd-PdO reaction requires a very high fugacity of oxy-gen, a lower/(O2) would be needed to form PdO at lowertemperatures (Fig. 13). In New Caledonia, the complexPGE oxides formed in lateritic conditions (Aug6 &Legendre 1994). However, the Penikat Pd-Pb oxidemay have had a different origin. It appears to haveformed during a very late-stage event of deuteric hy-drothermal alteration, as a result of oxidation ofzvyagintsevite by strongly oxygenated fluids at low tem-peratures. A change in volume during the conversion ofthe zvyagintsevite may have resulted in the stronglyfractured appea.rance ofthe Pd-Pb oxide (Fig.9A).

Various reactions to produce the Pd-Pb oxide maybe proposed. It may have been formed in two stages.The first stage would have resulted in the formation ofpure Pd, which, owing to further oxidation by the fluids(second stage), would have then been converted to thePd-Pb oxide. Two possible mechanisms of oxidationexist, with H2O or without H2O, respectively:

6Pd3Pb + 2O2+ 2H2O = l8Pd + 6PbO + 2H2

The release of hydrogen would lead to locallyreducing micro-environment.

18Pd + 9O2 + 2PbO = 2PdsPbOro.

The availability of H2O is consistent with theintimate association of the ore minerals with hydroussilicates. The rest of the PbO (l.e.,4PbO) may have beenincorporated in the associated Pb-V oxide, accordingto the simple reaction 4PbO + VzOs (fluid) = Pb+VzOs,which took place during a late-stage oxidizing eventwhen a fluid infiltrated the cleavage in the amphibole(Fie. 8).

The second possible mechanism of conversion maybe represented as:

6Pd3Pb +3O2= l8Pd+6PbO

and

l8Pd + 9O2 + 2PbO = 2PdsPbOro.

No remnants of the pure Pd were observed, however,and the zvyagintsevite might thus be directly convertedto the Pd-Pb oxide, according to a hypothetical reaction:

3Pd:Pb + 6oz= PdqPbOro + 2PbO.

Petrogenetic aspects

A strong partitioning of PGE into a segregating sul-fide liquid is believed to have played an important rolein the formation of stratiform PGE deposits in layeredintrusions (e.g., Campbell et al. 1983, Naldrett et al.1990). The partition coefficients of the PGE in favor ofsulfi de liquid over basaltic liquid are high ; e. 5., 17 (X 7 )X 103 and l0 (t 4) x 103 for Pd and Pt, respectively(Fleet et al. 1996), and, in the case of-a high Ni contentin sulfide fraction, 28 (X12.5) X 10r and 16.5 (1 6.3)x 103 (Crocket et al. 1997), respectively. In spite ofthese values, the extremely PGE-ich and S-poor min-eralization at Kirakkajuppura could not have formed asa consequence of the collection of PGEby a magmaticsulfide liquid lz .rifu. Textural relationships (e.9.,Figs. 5, 7) strongly suggest the hydrothermal deposi-tion of the PGE minerals from a volatile-rich phasewhich, at least locally, may have been mobile. The vein-let-(chain)-like aggregates of PGM that we describedlikely precipitated directly from the fluid phase, in sitescontrolled by systems of micro fractures in the rock andby the cleavage of the hydrous silicates. The stronglyheterogeneous distribution of the PGM in the ore depositand the abundance of pegmatitic rocks in the area(Figs. 3, 4) also support this suggestion.

Theoretical considerations suggest that Pd and Ptmay be mobile in the form of bisulfide complexes rnH2S-rich aqueous solutions under reducing conditionsand at various temperatures (e.9., Gammons & Bloom1993,Part & Wood 1994, Wood et al. 1994), and thatthe relevant Pd-Pt sulfides are relatively soluble asbisulfide complexes even at temperatures as low as 25"C(Wood er al. 1989). Recently, Pd and Pt sulfides havebeen synthesized at temperatures below 100oC underreducing conditions (Tarkian et al. 1996). Chloridecomplexing of PGE also may be of importance in trans-porting them under hydrothermal conditions; however,this would require a relatively hot, saline, and acidicf luid and a strongly oxidized environment (e.9.,Gammons et al. 1992). Chlorine is interpreted to havebeen a likely agent for transport and deposition of PGEin some ore deposits (e.9., Boudreau & McCallum1992,Li & Naldrett 1993, Olivo & Gammons 1996). No evi-dence for a Cl-rich environment at Kirakkajuppura hasbeen found, however. Three subhedral crystals of apa-tite (up to 20 pm), found in the section containing thelargest aggregates of PGM (Fig. 5), are very poor in Cl(30.2wt.% Cl). The amphibole associated withthe PGMis nearly Cl-free (<0.05 wt.Vo Cl), but its composition(poor in Fe, K and A1) indicates that this feature may

1522 THE CANADIAN MINERALOGIST

well be due to crystallochemical constraints (cl Obertiet al. 1993), rather than to a Cl-depleted environment.

The efficient preferential incorporation of Pd, rela-tive to Pt and to other PGE, into hydrothermal fluids isimplied by the Pd-rich character of the Kirakkajuppuramineralization and by the high-Pd compositions of theIate-stage PGM (vysotskite - braggite series, zvyagint-sevite and keithconnite). The predominance of theS-rich PGM and their intimate association with thehydrous silicates are consistent with the theoretical pre-dictions that Pd and Pt may be mainly transported asbisulfide complexes and deposited from H2S-rich,weakly oxidized or reduced aqueous fluids at relativelylow temperatures. Most of the late-stage PGE sulfidesin the ore deposit could be formed under such condi-tions, during an early stage ofhydrothermal activity. ThePd-rich composition of the late-stage vysotskite -braggite is in good agreement with the experimental data(Cabi et al. 1978), which indicate a higher-tempera-ture character of Pt-rich members of the series relativeto vysotskite (Pd-rich). In general, the local hydrother-mal system at Kirakkajuppura may have evolved towarda S-poor and more oxidized environment. We contendthat at a later stage of crystalli zation, zvyagintsevite andrelated phases partly replaced the vysotskite - braggite,the zvyagintsevite chains were formed, and thesubmicrometric zvyagintsevite precipitated as sub-parallel grains from fluids that percolated along thecleavage in the hydrous silicates (Fig. 7). At the finalstage of hydrothermal alteration, the Pd-Pb oxide ap-peared as a result ofthe interaction of the zvyagintsevitewith the latest, more strongly oxygenated fluids.

AcrrqowLpocevsNrs

This study was supported by research grants fromthe Natural Sciences and Engineering Research Coun-cil of Canada and from the Academy of Finland. Wethank E. Moffatt (Canadian Conservation Institute)for the IR spectra of the Pd-Pb oxide, J. Jedwab forunpublished reflectance and compositional data for"palladinite" from the type locality, and M. Tarkjan andB. Cornelisen for additional electron-microprobe analy-ses. AYB thanks the organizing committee of the IGCPProject 336 Symposium in Rovaniemi (Finland) in1996, for assistance in collecting samples from theKirakkajuppura PGE deposit. Thanks are due to L.J.Cabri and G. Garuti for their helpful reviews, and to ourcolleagues at the Outokumpu Mining Oy for their coop-eration and permission to publish this paper.

RspBneNcBs

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Received March 28, 1999, revised manuscript accepted Octo-be r21 ,1999 .