PGE- Copper

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Ni and Со w ith an admixture of PGE and hollingworthite. А wide range of miscibility between hollingworthite and Pd- and Rh-bearing cobaltite and gersdorffi te ...

Geology of Î ãå Depostts, VoL 38, No. 3, 1996, pp. 183- 196. ÒãàëüéèåÍ 1ãî ò Geologi ya Rudnykh hf estorozhtlentt, Vol 38, No. 3, 1996, pp. 211 225. Engli sh Trantlatton Copyri ght © 1996 by Ì À È Ê Í àóêà/ !èãåãðåã1î é ñà Publishi ng (Russi a).

P G E - C op p er - Ì

ñÊå1 M i n er al i z at i on

of t h e L ay er ed M assi f of t h e Ñ åï åãàà ÿÊàó à M ou n t ai n (P ech en ga O r e D i st r i ct , K ol a P en i n su l a) Ò . Ü . G r o k h o v sk a y a * , I . P . L a p u t i n a * , Ñ . S. K u z n et so v * , G . N . M u r a v i t sk a y a * , s . V . So k o l o v * * , V . À . T el ' ï î ~ * '", an d À . N .  î 1' ÿÜà Êî ÷ * * *1ò ï ~è 1å î / Î åî 1î óó î / Î ãå Î åð î þé ç, Ðåãòî ö òî ð Üó, Ì ò åãà 1î öó, àëÛ Ñ åî ñÁåò è é ó, ß èçè àï À ñø 1åò ó î ~ Áñ~åï ñåç, Sta romon etnyi p er. 35, M oscow, 10 90 17 R ussi a '"'"ÐåñÜåï ~à ï é å1' En terp ri se, Z ap oly a rnyi , 1844 15 R ussi a Received January 23, 1996 A b st r a c t — È å~÷ d at a o n t h e p et r o l o g y , p h a se , an d c r y p ti c l ay er i n g o f th e á å ï åãà à ÿ1ñàó à ( L u o st ar i ) m o u n t ai n m a ssi f as w e l l a s o n t h e g e o c h e m i st r y a n d m i n er al o g y o f i t s p l ati n u m - c o p p er - n i c k el m i n er al i z at i o n ar e ð ãåsen t e d . T h e i n t r u si v e r o c k s ar e d i v i d ed i n t o m ar g i n al a n d l ay er e d g r o u p s. T h e l ay er e d g r o u p i n c l u d es t h e u p p er an d l o w er g a b b r o n or i te z o n e s a n d th e m i d d l e r h y t h m i c al l y l ay er ed z o n e , w h er e m a s si v e g ab b r o n o r i t es i n te r c al at e w i th r o c k s o f c o n tr a st i n g c o m p o si t i o n . L o w - su l fi d e p l at i n u m m i n er al i z at i o n i s l o c a l i z e d w i t h i n se v er al ex te n d ed h o r i z o n s o f t h i n - l ay er e d , t a x i ti c , an d p o i k i l i ti c r o c k s o f t h e r h y th m i c al l y l ay e r e d z o n e . T h e g r ad e o f p l a ti n u m m e t al s i n o r e h o r i z o n s i s u p t o 5 g é w i th p a l l ad i u m p r ed o m i n at i n g o v er o t h e r p l ati n u m g r o u p e l em e n t s ( P G E ) . T h e P G E o c c u r i n th e m assi f i n mi ner al an d d i sp e r sed f o r m s . T h e p r ed o m i n a n t p l at i n u m - b e ar i n g a sso c i a t i o n i n t h e o r e h o r i z o n s o f th e Î å ï åãàà sk ay a m o u n t ai n m a ssi f c o n si st s o f m er e n sk y i t e a n d su l f ar sen i d e s o f N i a n d Ñ î w i t h an ad m i x t u r e o f P G E a n d h o l l i n g w o r t h i t e . À w i d e r a n g e o f m i sc i b i l i t y b e t w e e n h o l l i n g w o r t h i te a n d P d - an d R h - b ear i n g c o b al t i t e an d g er sd o r f fi t e i s d efi n e d . T h e P ec h e n g a î ãå d i st r i c t i s c h ar ac t er i z ed b y t h e d ev e l o p m e n t o f à p e c u l i ar m i n e r a l o g i c a l " su l f ar sen i d e " p r o v i n c e w i t h à fr ac ti o n a ti o n o f P G E i n th e i n d i v i d u al su l f a r sen i d e p ar ag e n eses o f e a c h i n t r u si o n .

I NTRODUCTI ON Precambrian rhythmically layered massif s, representing one of the maj or sources f or copper, nickel , and PGE production in the world, are al so widely developed in the B altic Shield. A t present in the K arelia-Kola region the byproduct PGE recovery during the processing of Cu-N i ores i s conducted only in deposits of the Pechenga ore di strict. Recently, proper PGE minerali zati on was di scovered in some rhythmically layered massifs (Fedorovo-Panskii , L ukkul ai svaara, and Burakovskii massif s). A s à result, the eastern part of the Baltic Shield was di stinguished as à new platinum-bearing metal logenic province [M itrofanov et al ., 1995] . One of such massif s with à potential for the pl atiï ø ï ~ î ððåã—ø ñ1ñå1 mineralization i s the Early Proterozoic layered intrusion of the Î åï åãàÃskaya mountain (also called the Luostari massif ) located in the Pechenga ore di strict near the northeastern closure of the Pechenga structure. Sulfi de copper—ø ñ1ñå1mineralization in the massif was recognized f or the fi rst time by N .V. K arpinskaya. T he detailed geological mapping of the Luostari massif and the di scovery of horizons of the copper—ø ñ1ñå1 mineralization was carried out by the Pechenga geological exploration party (Yakovlev, 1971), and its PGE mineralization was evaluated by Ì .À . Sotnikova.

The subsequent investigati ons revealed that the intrusion of the General 'skaya mountain is mainly made up of metagabbroids with occasional unaltered gabbronorites and ol ivine gabbronorites (Fedotov et al ., 1974), and that it contains continuous horizons of di sseminated and veinlet-di sseminated Cu-N i mineral izati on and some PGE mineral s kotulskite and merenskyite (B akushkin, 1979) . Detailed mineralogical investigations of the massif were resumed only in the beginning of the 1990s, when hollingworthite, telluropalladinite, michenerite, and sperryl ite were i dentifi ed in addition ñî the PGE mineral s di stingui shed earlier in horizons of Pt—Cu- N i mineralization by the researchåãç of the Kola scienti fi c Center (Barkov et al ., 1994). This paper describes the rocks and petrology of the massif on the basi s of cumulus terminol ogy (Wager and B rown, 1968), and the f ormati on conditions of the CuN i PGE mineralization. GEOL OGICA L POSIT I ON AND TECTON IC CONDITIONS OF ÒÍ Å I NTRUSI ON EM PL A CEM ENT The General 'skaya mountain ò àçèÊ belongs to à large group of Early Proterozoic platinum-bearing rhythmical ly layered intrusions localized in two maj or rift systems of the eastern Baltic Shield— the Northern 183

~

184

GROKHOVSKAYA et al.

K arelian (Olanga-K oi li smaa-Kemi ' ) and PechengaImandra-Varzuga system s (M edno—Ì é å1å÷óå..., 1985; A l apieti et al ., 1990) (Fig. 1). The f ormation of sublati tudinal riftogenous zones and the intrusi on of tholeitic magmas, f orming layered massif s, are supposed (î present the culminati on of the Early Proterozoic extension of the A rchaean crust and the partial melting of the upper mantle (Gaal , 1990). A ll massif s were f ormed approximately 2450 Ì à ago, and they are situated in relatively small local extension zones, associ ated with transf orm faults. The subsequent geological history of the PechengaI mandra—Varzuga rif t system, especial ly of its northern unit containing the Î åï åãàÃÿ1ñàóà mountain massif , was more complicated and prolonged in compari son (î that of the Northern Ê àãåÃian system. A fter the intru-

~

sion of the layered massif s, the continued extensi on led (î the opening of the basin with the oceanic crust and to several phases of basic-ultrabasic and then intermedi ate-acid magmati sm. The rift trough was fi l led with volcanogenic—sedimentary complexes; at the end of the M iddle Proterozoic the col li sion closure of ri ft basins took place with their transformati on to suture imbricated zones. The Proterozoic tectonic movements were most intensive ø the Pechenga ore district and determined the specifi c f eatures of its metallogeny (M ints, 1990; K azansky and L obanov, 1996), including its PGE mineral izati on. Four intrusive basic ultrabasic complexes are associated spati ally with the Î åï åãàÃâ1ñàóà mountain massif . They are the M iddle Proterozoic layered intrusions of the Pechenga î ãå fi eld, the M iddle Proterozoic diabase



~6

F i g . 1. Ì àð of th e l oc at i o n of p l ati nu m -beari n g l ay ered b asi c- è1ñãàÜàÿ ñ i n tr usio ns of the easter n p ar t of t he B al ti c Shi el d (A l api eti et a l ., 19 90 ; w ith am en d m en ts and al ter ati o n s) . 1- C al ed on i an f o l ded st r u c tu r e; 2- Pal eozoi c al k al i n e m assi f s; 3—Svecok arel i an gr an i toi d s; 4 E ar l y Pro ter ozoi c sedi m ent ar y - v o l can o geni c com p l ex es; 5 E ar l y Prot er ozoi c l ay ered m assi f s î é ë å ri f t sy stem b asem ent and th e M i dd l e Pr oterozo i c r i f t- asso ci ated m assi f s; 6 A rch ean an d E ar l y K ar el i an co m p l ex es o f r i f t sy stem f r am ew o rk s. PG E -b eari n g l ay er ed m assi f s: À - Ðåñéåï äà- I m an dr a- V ar zu g a r i f t sy st em : 1- Î åï åãàÃç1ñàó à m o u n tai n (L u ostar i ) , - Sak en , 3 —Ê Û ï y ar v i , 4 K ari k y av r , 5 m assi f s o f t he Pech eng a î ãå fi el d , 6 M on ch ego r sk , - F edorov sk , 8- Pan sk ;  - Sev ero- K arel i an r i f t sy st em : 9- L uk k u l ai sv aar a, 10 T si pr i ng a, 11 K i v ak k a, 12—N ar ank avaar a, 13 K oi l l i sm aa, 14 Port i m o , 15- Pen i k at , 16- K em i , 17 —Òî ãø î , 18- K oi t i l ai nen .

GEOL OGY OF ORE DEPOSIT S

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PG E —C O PPE R N I C K E L M I N E R A L I Z A T I ON

dikes and ñÈ ñå-l ike gabbro-peridotite intrusions of the Nyasyukka type, and the complex of layered intrusions of basic ultrabasic composition, the age and tectonic position of which remains unclear (Fedotov et al ., 1974). This last complex includes about 30 small magmatic bodies located to the northeast of the Pechenga structure (K arik ' yavr, Saken, Khiriyarvi , and î áæåã massif s), contacting only the A rchean gneisses. By both their petrology and platinum mi neralizati on, these massif s have much in common with the General 'skaya mountain intrusion. A ccording to determinations by Óè.À . B alashov, Ò. . Bayanova, and others, the Sm—Nd isochrone of basic and ultrabasic rocks of the massif s corresponds to an age of 1939 + 60 Ì à, and that of the cross-cutting aplite-like granites of 1900 + 70 Ì à. These datings most likely correspond to the phase of superimposed regional metamorphi sm (Balashov et al ., 1993). GEOL OGICA L STRUCT URE A ND PETROL OGY OF THE INTRUSI ON The Î åï åãàÃç1ñàóà mountain massif i s situated at the northeastern closure of the Pechenga volcano-tectonic structure at the Pitkayarvi and Yuzhno-Pal ovarsk regional f aults. The intrusion penetrates gneisses of the Archean Kola series, and it i s overlain by basal conglomerates of the L ower to M iddle Proterozoic Pechenga sedimentary volcanogenic series, which contain pebbles and boulders of altered gabbronorites (Fedotov et à1., 1974). The age î ã " ãäå massif , determined with the Sm- Õé method, is 2453 + 42 Ì à with Nd (T) = 2.3 + 0.4 (Balashov et aI., 1993). The massif i s extended in the submeridional direction (from 10' to 20' NE) and its outcrop area i s approximately 3.5 õ 1.5 km (Fig. 2). The massif thickness increases f rom the northeast t o the southwest from 200—300 m to 1700 m due to the gentle dipping (from 30' to 35' ) under the Pechenga series. The intrusion contacts dip towards each î áæåã at angles î ã' ããî ãï 60' to 65' f or the eastern contact and from 30' to 50' for the western contact. Several systems of faults divide the intrusion into separate blocks. D ikes of diabases and öèàããê~ àãÜî ï àãå veins are associated with cataclastic fault zones of the northeastern (40' ) and northwestern (300' and 340' ) strikes (Yakovlev, 1971). The internal structure of the Î åï åãàÃç1ñàóà mountain intrusion diff ers f rom that of similar massif s of the Imandra-Varzuga and Northern K areli an rif t systems. Basic composition rocks prevail . The rhythmically layered section of the intrusion is dominated by basic rocks, while ultrabasic rocks, represented mainly by olivine-plagioclase cumulates, are sharply subordinate. The ultrabasic rocks are local ized in the central most differentiated section of the intrusion, whereas massive gabbronorites are devel oped in its peripheral parts. Plagioclase preserves its cumulus position even in parageneses with predominating pyroxenes and olivine in contrast to rocks of ultrabasic groups of most of the GEOL OGY OF ORE DEPOSIT S

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18 5

î áæåã layered intrusi ons of the area, where plagioclase i s an intercumulus mineral. The layered and marginal groups are di stingui shed within the á åï åãàÃç1ñàóà mountain massif . The layered group of the intrusion consists of three zones different in structure— ï ððåã gabbronorite, rhythmical ly layered, and lower gabbronorite. The upper gabbronori te zone i s about 400 m thick, and it is composed of intercal ating medium-grained gabbronorites and gabbros, occasionally quartz-bearing, and more rarely of anorthosites, orthopyroxenites, and microgabbronorites, represented by pl agi oclaseorthopyroxene, plagi oclase—Ü÷î ðóãî õåï å, plagioclase, and orthopyroxene cumulates with intercumulus augite and bronzite. The characteristi c rock textures are gabbric, gabbroophitic, and poikilophitic. Rocks are strongly altered by metamorphism; however, they preserve their initial textures and relicts of the primary mineral s. The rhythmi cally layered zone is distingui shed by possessing the most complicated structure and cumulate diversity. Its thickness is insignifi cantly increased from 350 m to 400 m as the ò àçÛ dips to the southwest. The zone includes several cyclic units with à contrasting set of cumul ates f rom 5 to 30 m thick alternating with homogeneous gabbroid rocks (Fig. 3). The upper and lower boundaries of the zone are sharply distinguished by the appearance and disappearance of olivine—ð1àð î ñ1àçå cumulates. Cyclic units are f ormed by the alternation of layers from few centimeters to several meters thick composed of gabbronorites, olivine gabbronorites, norites, anorthosites, bronzitites, websterites, pyroxene troctolites, and, more rarely, plagioperidotites. The diversity of rocks corresponds to that of cumulus associations, represented here by plagioclase; plagioclase and olivine; olivine, orthopyroxene, and plagioclase; orthopyroxene; and orthopyroxene and plagi oclase. Intercumulus associations include bronzite, augite, diopside, pigeonite-augite, and invert ed pigeonite. Plagioclase and olivine-plagi ocl ase cumulates with intercumulus ortho- and clinopyroxene with sporadic cumulus apatite prevai l ø the rhythms. This part of the section i s characterized by à di stinct phase layering determined by the alternation of olivinebearing and olivine-f ree cumulates (Fi gs. 3, 4à, 4Ü). Rocks of micro-grained, taxitic, poiki liti c, and poikilophite textures are developed in each unit. Porphyry orthopyroxenite with cumulus bronzite among micro-grained pl agi oclase- É|ðóãî õåï å intercumulus, composed also of idiomorphic crystals (Fig. 4c), i s typomorphic f or the massif . T he origin of such rocks, actually represented by cumulates of two generations, is not clear yet . Specifi c rock textures characteristic f or critical zones of l ayered massif s are repeatedly observed in cyclic units of the rhythmically layered zone. These textures are dominated by small, rounded,

PGE COPPER- Ì ÑÊÅ1. MINERALIZATION

18 7

F i g . 3 . V ari ati o n s o f co m posi ti on s of o l iv i ne , b ron zi te, an d p l agi ocl ase and con tents o f N i and PG E i n the secti on o f th e G en åãàà ç1ñàó à m ou n tai n m assi f (b o r eh ol e 34 6 1) . 1 L ay er ed g r ou p ; à- ñ- zones: à—ï ðð åã g ab br ono r i te, b rhy th m i cal l y l ay er ed , ñ- l ow er g abb r on ori te ; Ï m ar gi n al g r ou p . 1—5- ñè ï ø l u s asso ci at i on s: 1 p l agi ocl ase + o r th o py r ox ene + cl i n opy ro x ene, - o r th opy r ox en e, 3 pl agi oc l ase, 4- o rth o py rox ene + p l agi o cl ase, 5- p l ag i ocl ase + o l i v i ne , ol i v i ne + p l ag i o cl ase + o rt hopy ro x en e ; 6 —gn ei sses.

quartz-bearing gabbronorites with occasional interlayers of orthopyroxenites, anorthosites, trachytoid microgabbronorites, and granophyre gabbropegmatites. Plagioclase cumulates with intercumulus inverted pigeonite and augite are predominant . The margi nal group, up to 100 m thick, i s represented by fi ne-grained contaminated porphyry-like quartz-bearing gabbronorites of the gabbroophite and dolerite textures. GEOL OGY OF ORE DEPOSIT S

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A NA LY TICA L M ETHODS OF M INERA L COM POSITI ON DETERM INATI ON Rock-forming minerals were analyzed with the M S-46 " Cameca" Õ-ray microanalyzer. The analysi s of PGE, sulfi des, and Ni —Ñî - ãå—ÐÎ Å sulfarsenides was conducted with the three-channel "ÑÀÌ ÅÂÀ Õ-M I CROBEA M " microanalyzer. M icrodeterminations of PGE were carried out ø sulfi des according to à technique proposed by I .P. L aputina. The detecti on limit f or Pd,

~

PGE —C O PPE R —Õ ?Ñ Ê ÅË M I N E RA L I Z A T I ON

Rh, and Ru is 0.007 wt %: f or Pt and Os, from 0.009 to 0.0 1 wt %; and for Ir, 0.0 1 wt % (in the presence of Pt and Os it is 0.025 wt %).

Di

189

(à)



Hd

Q • • • •• •

II

C R Y PT I C L AY E RIN G

The maj or rock-f ormi ng si licates in the GenåãàÒç1ñàóà mountain massif are plagioclase, pyroxenes, and olivine. Irrespective of the relatively wide ÷àï àtions of chemical compositi on of these minerals in diff erent types of cumulates, their vertical trends are manifested only in the insignifi cant increase of the M g/Ì g + Fe ratio in dark-colored silicates and in the anorthity content in plagioclase from the top to the bottom of the intrusion. T he greatest vari ations in silicate compositions are characteristic f or horizons of the thin intercalation of cumulates and sulfi de enriched rocks of the rhythmical ly layered zone (Fig. 3). Plagi oclase is present in practically all types of rock , and it is relatively insignifi cantly al tered in compari son to dark-colored silicates. The plagioclase composition varies from bitownite to andesine from A n455to À

Ï

7 ç 2 ~ w

i t h

l a b r a d o r

p r e d o m

i n a t i n g

( T a b l e

1 , F i g s . 3 , 5 ) .

Plagioclase f orms elongated smal l laths of the composition ÀÏ 7à 73 in the marginal group. The greatest compositi on variations and the lowest anorthite contents (from À ï 7~to A n455) are characteristic f or plagi oclases of gabbronorite zones. Í åãå plagioclase f orms crystal s with direct and reversed zonality. In gabbronorites and norites of the rhythmical ly layered zone, the plagioclase composition is simi lar to that of the gabbronorite zone. In olivine-bearing and orthopyroxene cumul ates, plagioclase (Àï 7ç 59) reveals ï î zonality, or it has direct zonality with the increase of Na content to the grai n periphery. Oli vi ne is observed only in rocks of the rhythmical ly layered zone. In al l parageneses olivine i s without zonality, and it diff ers by à greater variability of the M g and Fe (Fo71 7ç~) concentrations and à smaller M g/M g + Fe ratio than f or coexi sting orthopyroxene (Table 1, Fig. 5). In horizons of sulfi de mineral ization, the iron content of olivine is always greater than that of the oreàããåå 1î å 1~ç ( F o 67 .8—7 1.4)

Pyroxenes of the layered group are represented by ortho- and clinopyroxene occupying both cumulus and intercumulus positions. 0 rthopyroxene possesses à lower M g/M g + Fe rati o in the upper gabbronorite zone (Åï ~, sW o62Fs2211). In the remaining horizons, the low calcium pyroxenes (bronzite and inverted pigeonite) manifest insignifi cant zonality with à decrease in the M g and Ca content f rom the center to the periphery of grains, and they are characterized by the f ol lowing composition: Åï 72ç-çî .4 W o1.7-5.ç4Ðç16ç-ãõç. Cl inopyroxenes are represented by augite and di opside (Åï 40~5~ 7.~W o41,7 ~ç. Fss,7- 12.ç5) (Table 1, Fig. 5). Pigeonite-augite (Åï 56-59÷÷029ÐÁ12- 15) occupies the cumulus position, and it i s observed relatively rarely. GEOLOGY OF ORE D EPOSIT S

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• = -

av e

l

À

Ü

Fi g. 5. Vari ati ons of composi tions of coexi sti ng (à) orthoand .cli nopyroxene, (Ü) ol ivine, and (ñ) pl agi ocl ase of the layered group of the massi f . 1- IV - fi elds of pyroxene compositi ons: - di opsi de, I I- augi te, Ø - pi geonite, I V - bronzi te; D i- di opside, Hd—Üåñ1åï bergite, Åï å ï ç(àé å, Fs- ferrosil ite, Fo f orsteri te, Fa—f ay al ite, A n- anorthi te, A b- albite.

Very l ow con tents of chr om ium i n py rox enes are character i sti c . M i ner al s of the spi n el group are represented by occasi onal sm al l i di om orphic grain s (f r om 5 to 15 ð ò ), som eti m es c on centr ated i n the r ock i n tercu m u l u s. Tw o gr oup s of spi nel com p osi ti on s ar e d i st i n gu i shed . H ercy ni te i s the m ost di stri buted : (Fep û -î 9ç M go07-Î .39) (A l O.û -î 9î Ñãî ëî -î .çç 1 åî -î .î 9)ã0 4. C r- sp i nel i s characteri zed by l ow M g contents and hi gh contents of bi val ent Fe. V ari ati ons i n the chrom ium and al um i ni um contents are in signi fi cant , and that of the tri v al ent Fe i s considerable: ( (F~ 91-î .98 M goîã-î î9)(Ñ~î .çü-î .6ã À 1îëî -î .4 F ep p,~ 53) ~0 4) . A l l m ineral s of thi s group contain adm ixture of V (0 .5 1 to 1.27 w t %), N i (0 .0 1 to 0.33 w t %), and Z n (0 .14 to 2 .0 8 w t %) . T i contents v ar y f r om 0 .0 15 to 5 .15 w t % . Ap ati te i s l ocali zed ø sul fi de-bear i n g pl agiocl aseol i vi ne and p l agi ocl ase cumul ates of th e rhy thm i cal l y l ay ered zone, w here i t f or m s nu m er ou s el ongated and short pri sm ati c grai ns i n si l i cates and su l fi des. A pati te has increased contents of T R , C l , and F (Tabl e 2) . A mp hibol es are represented by actin ol i te and ferr opargasite. I n or e hor i zons th e C l contents i n p argasi te are

0.15 to 0.38 wt %. ÐÂÅ-Ñ Î ÐÐÅÊ.—È ÑÊ Å?. M INERA L I ZATI ON PGE- Cu—Ì mi neralization of the General 'skaya mountain massif is localized in the rhythmically layered zone, and it f orms from 3 to 7 horizons of variable thickness (f rom 1 m to 20 m) and length (from 0.5 km to 1.0 km). These horizons are concordant with the layering, and they are confi ned to rhythmically layered, taxitic, and poikilitic rocks. No signifi cant increases in the thickness and number of ore-bearing horizons al ong

GROKHOVSKAYA et al.

190

T a b l e 1 . C o m p o s i t i o n o f c o e x i st i n g s i l i c a t e s i n r o c k s o f t h e G e n e r a l 's k a y a m o u n t a i n m a s si f , w t %

A nal y- M ineral D epth, si s ï î . Phase m

Cum u l ate

~ ' ~->ã ~~ Æ ç )ÑããÎ ç) T i O q

ÐåÎ ( MgQ ~Ì ï Î ~ÑàÎ

NiO ~Nà~Î ~

T otal

97.37

442.5 Pl

0.37

7.16 13.98

0 .0 6

442.5 Pl

0.| ç 13.4 1 24.57

0 .0 8

442.5 Pl

0.42

464.8 Î ðõ

î ì ~ î ÷àã 12.79

464.8 Î ðõ

0.19

567.5 0 1 + Î ðõ + Pl

î .î ç ~ î .ã

567.5 Ol + Opx + Pl 567.5 OI + Î ðõ + Ð1

97.9

0 .0 8

11.67 26 .68

0.06

25.23 34 .9 3

î .ç1

99.10

607.4 Pl + 0 1

î .î ~ 0 .0 7 10.66 28.93

0 .0 8

607.4 Pl + 0 1

19.86 4 1.02

î .ç ~

0.12

79 1.5 0 1+ Î ðõ

î .îü î .|ã

79 1.5 Ol + Î ðõ

0.62

79 1.5 0 1 + Î ðõ 79 1.5 0 1+ Î ðõ 802.6 Pl + Opx + Ñðõ

13.10

~2 7 .8 5

7.08

15 .9 7

26.18

3 7 .2 9

0 .24

~4 . 8 9

0 .0 8

802.6 Pl + Î ðõ + Ñðõ

6.05

î .î ç

99.14

3 .05

99.11

98.8

î .î ~ 101.68 1î î .ãç 3 . 8 6 ~ 1î ã.î ç 99.97

0 .0 8

î .ç

0.27

0.16~ 0.23

99.47 4 . 15 ~ 10 1.77

27. 17

0.32

607.4 Pl + 0 1

0 .44

100.05 100.82

3.14 100.85

0.39

î .ã

98.46

3.26 100.07

N ote: L egend: Ñðõ~ 11ï î ðóãî õåï å, Î ðõ—oããhopyãoõeï e, Pl —plagi oclase, Î l- î éÌ ï å.

the gentle southwestern dip of the massif are observed. The host cumul ates are represented by both fresh unaltered and highly altered rocks. The structure and composition of sul fi de horizons were thoroughly descri bed ø previous papers (Yakovlev, 197 1; B akushkin, 1979). Di sseminated and veinlet-di sseminated ores predominate. They contain f rom 1- 2 to 15—20 vol % of sulfi des, which are characterized by extremely irregular di stributions. Rather ãàãå bodies of rich epigenetic ores are confi ned to the concordant and crosscutting faults, and do not extend beyond the intrusion boundaries.

Di sseminated and veinlet-disseminated sulfi de mi nerali zation i s mostly observed in coarse-grained and taxitic plagioclase cumul ates. It i s often confi ned to contacts of ol ivine pl agioclase and plagioclase cumulates. Sulfi des are localized in interstitions of cumulus plagioclase (Fig. 4Ü), or they f orm intergrowths with amphiboles and other secondary silicates. The grade of secondary alteration of silicates i s increased in sulfi debearing horizons. Horizons with di sseminated sulfi des, as à rule, contain greater amounts of apatite, biotite, and amphiboles. In ultramafi c rocks, sulfi des f orm either ãàãå rounded, sometimes layered, inclusions of up to 2 cm in size or dense dotted disseminations.

Table 2. Composition of apatite in sulfide-bearing horizons of the á åï åãà1'skaya mountain massif , wt%

The mineral composition of sulfi des i s relatively homogeneous. The ores are composed of pentlandite, monoclinal or more rarely hexagonal pyrrhotite, and tetragonal chalcopyrite. Pentlandite has à medium iron content with variations of the Fe/N i ratio from 0.81 to 0.92 and with à Ñî content from 0.28 to 0.90 wt %. Pyrrhotite is relatively stable in compositi on. The ratio of total metal to sulfur in pyrrhotite varies from 0.85 to 0.9 1, and the Ni content varies from 0.2 to 0.4 wt %. Chalcopyrite contains ï î admixture of N i and Ñî . The subordinate ore mineral s are pyrite, ãï àñÛ ï àú ite, and violarite (replacing pyrrhotite and pentl andite), bornite, mi llerite, hessite, sphal erite, galenite, chalcosine, covellite, argentopentl andite, arsenopyrite, mi nerals of the cobaltite—gersdorffi te group, electrum, and PGE minerals. GEOL OGY OF ORE DEPOSIT S

Vo l . 3 8

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199 6

~

PGE CO PPE R —ÜÏ Ñ Ê Å1 M I N E R A L I Z A T I ON

19 1

Di stri buti on î ~ ÐÎ Å, Ni , and Ñè In the á åï åãàÃç1ñàóà mountain intrusion, increased PGE contents are confi ned to hori zons of disseminated and veinlet-disseminated sul fi de mineral izati on, and they are always associated with this mineral ization. The sulfi de horizons contain from 0.2 to 2.0 wt % of N i and Cu (Ni/Cu = 1.0 — 1.13). Total PGE grades vary f rom 1.0 to 5.0 äé , with the background Ni and Cu contents not exceeding 0.3—0.7 wt %. That allow s us to relate the PGE minerali zation to the low-sulfi de type (Fig. 6). À positive correlation between PGE, Cu, and Ni i s observed (see Fig. 3). A s in other massif s with low-sulfi de PGE minerali zati on, palladium prevai ls in the ores. The PGE fractionation degree ø the GenåãàÃç1àóà mountain massif i s slightly lower than in the layered massi f s of the I mandra-Varzuga and Northern K arel ian systems, and it i s higher than in the layered intrusions of the Pechenga ore fi eld and K arikyavr massif (Fi g. 6). PGE Occurrence Forms Elements of the platinum group occur in the GenåãàÃskaya mountai n massif in mineral and di spersed f orms. M ineral s of noble metal s are represented by sperrylite, hollingworthite, kotul skite, michenerite, minerals of the merenskyite-mel onite group, and electrum. Elements of the platinum group also f orm solid soluti ons in sulfi des and sulfarsenides. PGE mineral s are local ized in pyrrhotite, pentlandite, more rarely ø chalcopyrite at the contact of sul fi des with silicates, and ø rock-f orming silicates. PGE mineral s are associ ated with sulfarsenides of nickel and cobal t ; however, their paragenetic relationship remains unclear, because in most cases it i s impossible to deterï àï å the deposition sequence of PGE mineral s and minerals of the cobaltite-gersdorffi te group. Intergrowths of merenskyite, kotul skite, and sperryl ite with sulf arsenides and inclusions î é þ 11ø ð ÷î ãé éå and bismuthotellurides of pal ladium in cobaltite and gersdorffi te are observed. Someti mes the fi nest aggregates of sperrylite, N i—Ñî -sulf arsenides, and bismuthotellurides of palladium are localized in amphiboles and other secondary mineral s within the sulfi de-bearing horizons. Sulf arsenides of PGE, N i, and Ñî are of ten associated with sphalerite. In contrast to î ë åã contemporaneous rhythmical ly layered intrusions of the region with very diverse PGE minerals phases (Grokhovskaya et aL, 1988 and 1992), in the Î åï åãàÃç1ñàóà mountain ï è çÛ é å combination of PGE mineral s is very limited. We analyzed 80 grains of palladium bithmuthotel luride and distinguished only f our grains of kotul skite and the mineral of the Pd3(Te, Â1)~ composition. The remaining grains were represented by merenskyite and nickel merenskyite. M erenskyite f orms numerous monomineral aggregates and intergrowths with minerals of the cobaltitegersdorffi te group, and it i s often associated with pyrGEOL OGY OF ORE DEPOSIT S

×î 1. 38

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19 96

Os

Ir

Ru

Pd

F i g. 6 . C ont en ts of PG E , no rm al i zed by C l chond ri tes, i n h ori zon s o f th e sul fi d e m i ner al i zati o n of l ay er ed i ntru si o n s o f th e easter n p ar t of th e B al ti c Shi el d . 1- á- òï û ÿ é : 4- Î åï åãàÃç1ñàó à m ou n tai n , Ç- L uk k u l ai sv aar , 1- B u r ak o v sk , 4- M o n c h eg o r sk , 6 - Pec h en g a m assi f s, 5K ari k y av r.

rhotite and pentlandite (Fig. 7). Considerable variations of the Òå, Bi, Pd, and N i contents are observed in merenskyite. The Pt admixture in merenskyite does not exceed 1.12 wt %, the A g admixture does not exceed 3.3 wt %, and Sb is constantly present (up to 1.0 wt %) (Table 3). Sperrylite i s widely di stributed in the î òåÿ. It f orms numerous fi ne monomineral aggregates in ãî ñ1ñ4 î ï ï ing silicates and sul fi des. Furthermore, sperrylite is present in intergrowths with bi smuthotellurides of pal ladium, with sulfarsenides of Ni , Ñî , and Fe, and as inclusions in the latter. The sperrylite composition i s relatively constant, and an admixture of Rh (to 0.94 wt %), Pd (to 1.2 wt %), and Os (to 0.76 wt %) is obser ved. The paragenesis of hollingworthite and sulf arsenides of Ni and Ñî with the admixture of PGE i s widely di stributed in ore horizons of the General 'skaya mountai n massif . Hollingworthite i s observed in the intergrowths with sperrylite, bi smuthotel lurides of palladium, and sulfarsenides of Ni , Ñî , and Fe. T he size of the grains does not exceed 5 to 25 pm . Zonal metacrystal s are discovered, in which hollingworthite composes the central part, and cobaltite composes the peripheral part . The holl ingworthite composition varies over wide ranges. The iridium, osmium, and platinum varieti es of hollingworthite are observed. The typomorphic f eature

~

192

GROKHOVSKAYA et al.

of this mineral is the constant presence of Pd (to 6.89 wt %), N i , and Ñî (Table 4). Sulfarsenides of nickel and cobalt constitute idiomorphic, rounded, or irregular aggregates with sizes from 5 to 200 ðò in pyrrhotite, pentlandite, occasionally chalcopyrite, and rock-forming silicates. M etacrystal s of sul far senides contain inclusi ons of replaced pyrrhotite and pentlandite and silicate minerals (Fig. 7Ü). Sulfarsenides were f ormed later than sulfi des. Hollingworthite, merenskyite, and kotulskite are intergrown with sulf arsenides and form fi ne inclusions in them (Fig. 7à). Sulfarsenides are represented by the intermediate members of the i somorphic cobaltite—äåãÛ î ãÈ 1å row with wide vari ations in the Ñî and Ni contents and insigni fi cant variations in the Fe contents (Table 4, Fig. 8à). Transitional varieties (cobal t gersdorffi te and nickel cobaltite) are characteristic, and the end members of the row (CoA sS and N iA sS) are not observed. M ineral s

are stoichiometric, with the A s/ $ ratio cl ose to one. The distribution of Ñî and Ni within some grains of cobaltite and gersdorffi te i s very irregul ar. Grains with stable contents of Ñî and Ni are regi stered; however, zonal crystals predominate with the enrichment of central zones with Ni and marginal zones (from 1 to 10—15 pëï wide) with Ñî (Figs. 9à and 9Ü). Very of ten the core of à grain i s composed of cobalt gersdorffi te, and the marginal zone i s composed of nickel cobalti te. In contrast to earlier studied K arikyavr, Saken, and Khiriyar vi massif s and Æî çå of the Pechenga ore district, where I r, Os, Ru, Pt, and Rh are di stingui shed only in separate grains of # i Co-sulf arsenides (Di stler and L aputina, 1979; Grokhovskaya and ?.àðèéï à, 1988), in the General 'skaya mountain massif almost al l grains of sulfarsenides contain fractions or several percents of PGE. The microprobe analyses of 120 grai ns of mineral s of the cobaltite—äåãÛ î ãé3ñå group from different ore horizons revealed that Ni and Ñî sulf arsenides are enriched to diff erent degrees by Pd (to 6.87 wt %), Rh (to 14.5 wt %), Os (from 0 to 2.09 wt %), Pt (from 0 to 1.57 wt %), Ir (f rom 0 to 0.29 wt %), and Ru (from 0 to 0.17 wt %). Typical compositions of PGE-bearing sulfarsenides and the mean values f or all 120 grains studied of cobaltite and gersdorffi te are illustrated in Table 4. The constant enrichment of minerals of the cobaltitegersdorffi te group by Os (on average about 0.29 wt %) explains the increase in Os bulk content in ore horizons of the massif . Pd and Rh in sulf arsenide grains, enriched in PGE, are distributed relatively uniformly with à slightl y increased concentration in the central part of à grain, where, as à rule, Ni contents exceed Æî çå of Ñî (Figs. 9b—9ñ). The positive correlati on of Pd and Rh with Ni û ÷å11 pronounced in the CoA sS—(XPGE)A sS N iA sS diagram (see Fig. 8b). High grades of Pd and Rh in cobaltite and gersdorffi te and increased contents of Ni , Ñî , and Fe ø hol lingworthite, determined in the GenåãàÃü1ñàóà mountain massif , indicate à great miscibility of PGE and transitional metals in sulf arsenides and help to di stingui sh à continuous i somorphi c row of the compositions (Ñî ,Ì ,Fe)A sS—(Pd,Rh)A sS from Rh,Ðd-cobal ti te and gersdorffi te to Ì ßÑî ,Fe-hollingworthite. The wide mi scibi lity of Ñî Ni —ÐÎ Å- sulfarsenidesis also confirmed experimentally (Ò. L . Å÷ç® -neeva et al ., 1992). The analysi s of PGE di stribution in ore-f orming sulfi des (pyrrhotite, pentlandite, chalcopyrite, and pyrite) indicated that there was ï î signi fi cant sul fi de enrichment by platinum group metals. I n pentlandite f rom sulfi de horizons enriched by sulfarsenides, f rom 0.007 to 0.026 wt % of Pd was determined. Such à low concentration of Pd in pentlandites of the General 'skaya

F i g. 7. N i ck el- Ñ î

P G E —su l f ar sen i d e s.

( à ) I n t er g r o w t h o f ( 1) n i c k e l c o b al t i t e , ( 2 ) m e r e n sk y i t e , a n d ( 3 ) h o l l i n g w o r th i t e at t h e c o n t ac t o f p y r r h o t i t e ( g r e y ) w i t h si l i c a t e s , s c a l e r u l e r i s 2 5 It m ; ( Ü) m e t a c r y s t a l o f ( 1) g e r sd o r f fi t e i n ( 2 ) p e n t l a n d i t e a n d ( 3 ) p y r r h o t i t e , sc a l e r u l e r i s 2 5 It m .

mountain massif in compari son with pentlandites of low sulfide mineralization of other l ayered intrusions of the K arelia-Kola regi on, in which up to 0.5 wt % of Pd is observed, are probably due ñî the redi stribution of Pd from sulfi des into sulfarsenide associations. GEOL OGY OF ORE DEPOSIT S

× î 1. 38

N o. 3

199 6

P G E —C O P P E R —ÜÏ Ñ Ê Å 1. M I N E R A L I Z A T I O N

19 3

T ab l e 3. Chem i cal com p o si t i on of b i sm uthotel l ur i d es of pal l adi um i n the á åï åãàÃç1ñàóà m ount ai n m assi f , w t %

Analysis ï î .

Borehole/depth, ò Ê -42

ã ç 4 5 6 7 8 9

1î 11

2 194/4 14.3 3461/459.9 3461/778.5 346 1/782.9 3461/784 .4 3461/787.9 3461/792.0 3463/ 1424.3 3463/ 1449.9 3463/ 1589.5 3461/787.5

Pd

Ni

28.00 15.68 25.40 2 1.80 25.72 23.93 24.00 25.07 24.53 18.93 23.24 4 1.67

0 .40 8.38 0.69 2.36 0.42 3.42 2.15 1.22 1.16 5.78

Òå

0 .4 9

0 .79 0 .36 0 .0ç 0 .02 0 .52 | .| ã 0 .25

î .çç ~ã Note: A nal yses: 1- 11 are merenskyite and N i-merenskyi te, 12 i s kotul skite.

GEOL OGY OF ORE DEPOSIT S

V o l . 38

No. 3

19 9 6

50.70 66.90 55.10 67.30 55.46 6 1.91 68.12 56.16 55.18 70.19 32.08 40.57

â

sb

20.12 8.25 18.10 4.81 17.43 10.29 5.02 15.54 14.25 3.77 43.78 16.10

0 .2 8 0 .6 8

î .ç~ 0 .39 0 .2 8

î .çã 0 .65 0 .66 0 .8 9

T otal

~

194

G R O K H OV SK A Y A et al . (PG E )A sS

7 ~

( à)

î ~

• ~



•2 • ç •



ñ • •

î ~î î

* À î ä

/

3,



îî

Ôî î '

• •

î

CoA sS v



a v l s == = J • Ý ÜgÌ • ~ ~~ ó • ~» • ~•Ú••ô•ìå



Ë

àî

C o A sS

X/

v

Ì

Ú/

%/

\ /

N i A sS

N iA sS F i g . 8 . D i a g r a m s o f c o m p o si t i o n s o f m i n er a l s o f t h e p se u d o - t er n ar y sy st e m ( N i - Ñ î —ã å- (K P G E ) - À â—S .

( à) V ar i a t i o n s o f N i , Ñ î , a n d F e i n P G E - b e ar i n g m i n e r a l s o f t h e c o b a l t i t e —8 åãçñ1î ï ï ñå g r o u p : 1- 3 - m a s si f s : 1- á å ï å ãàà â1ñàó à ï þ è ï t ai n , 2 - K a r i k y a v r , S ak e n , an d K h i r i y ar v i , 3 —Su d b u r y ; ( b ) v ar i a ti o n s o f c o m p o si t i o n s o f ( 1) h o l l i n g w o r t h i t e a n d ( 2 ) m i n e r a l s o f t h e c o b al t i t e - g er sd o r fi t e g r o u p i n o r e h o r i z o n s o f t h e G e n e r al ' sk ay a m o u n t ai n m as si f .

CONCL USI ON Thus, the General 'skaya mountai n massif belongs to the complex of the Early Proterozoic (- 2.45 Ga) layered PGE-bearing massif s, developed in the PechengaVarzuga and N orthern K arelian rift systems of the eastern part of the B altic Shield. The rock associations of the Î åï åãàÃÿ1ñàóà mountain massif are di stinguished among contemporaneous layered intrusions of the K arelia-Kol a region by the predomination of plagioclase-olivi ne cumulates. The formation of these cumulates was determined by the early crystallization of plagioclase, as rel ative to the remaini ng silicates, and, probably, by its intratelluric origin (M iller and Weiblen, 1990). Platinum mineral izati on of the massif bel ongs to the low-sulfi de type. Such mineralizati on in rhythmically layered plutons i s usually confi ned to " criti cal zones" (Càãnðbåll et aL, 1983, etc.), but ø the GenåãàÃâ1ñàóà mountain massif several cyclic units of rocks are developed instead of à single critical zone. I n addition to common cumul ates, these rhythms include micrograined, taxitic, and poikilitic rocks often represented by unstable parageneses and enriched by fl uidbearing phases. D isseminated and veinlet-disseminated Cu—Ì 1—ÐÎ Å-mineral ization is confi ned to such rhythms. The mineralization is localized in the intercumulus of the primary silicates, with plagi oclase and plagioclase—olivine cumulates being the most enriched by sul fi des. The analysis of the PGE di stribution, normalized by C1 chondrite, indicates that, Úó the degree of Pt and Pd concentration, the Î åï åãàÃâ1ñàóà mountain massif is close to intrusi ons of the eastern f ramework of the Pechenga structure (K arikyavr, Saken, and K hiriyarvi) and massif s of the Pechenga ore fi eld. Thi s regional ly separated group of non-contemporary intrusi ons of the

Pechenga di strict i s characteri stic of the wide development of sulf arsenides (including PGE-bearing) and consti tutes à peculi ar " sulf arsenide" mineralogical province. Sulfarsenides in all massif s of the di strict are close ø compositi on and are represented by transitional varieties of the cobaltite—gersdorffi te ãî ú with wide variations of Ni and Ñî contents and different concentrations of PGE. The highest concentrations of PGE in sulfarsenides are characteristi c for the Î åï åãàÃç1ñàóà mountain, K arikyavr, Saken, and K hiriyavri rnassif s. Sulfarsenides of the Î åï åãàÃç1ñàóà mountain massif contain high concentrations of Pd and Rh and sl ightly lower concentrations of Os, whil e in the K arikyavr, K hiriyarvi, and Saken massif s the concentrations of Ir, Os, Pt, and Ru predominate. Cobaltites and gersdorffi tes of the M iddle Proterozoic intrusions of the Pechenga ore fi eld contain decimal fracti ons of wt % of PGE. The individual f ractionati on of PGE in sulf arsenides f or each intru sion indicates, probably, the predominating role of local interchamber fl uid and hydrothermal processes during the formati on of platinum-bearing sulfarsenides w ith the addition of the only element (arsenic) on the regional scale. The close paragenetic association of PGE mineral s with Ni —Ñî —Ðå—ÐÎ Å— sulf arsenides confi rms their synchronous deposition, most probably, during the fl uid-hydrothermal stage of magmatic chamber development due to the removal of PGE from sulfi des and their redi stribution in the proper minerals of pl atinum group elements and PGE-bearing arsenic- sulfarsenic associations. The prol onged polyphase development of PGE mineralization in massif s of the Pechenga district— 6 î ò high-temperature magmatic processes to relatively lowtemperature fl uid and metamorphic alterations— is evidenced by the rather complete fr acti onation of platinum GEOL OGY OF ORE DEPOSIT S

× î 1. 3 8

No . 3

199 6

GROKHOVSKAYA et al.

196

group elements, manifested in à very limited number of PGE mineral forms and in insignifi cant variations of their composition. This f eature of the Pechenga district massif s distinguishes them from the " classic" Early Proterozoic layered massif s of the Baltic Shield containing, as à rule, PGE mineralization of various mineral and chemical compositions. The other characteristic f eature of the PGE mineralizati on of the Pechenga di strict massif s i s their enrichment by heavy and ãàãå PGE that indicates à higher melting temperature of the magmati c source of these intrusions. B oth these f eatures are in good agreement with the prol onged geodynamic activity of the Pechenga di strict . T he evolution of rifts is determined fi rst of all by thermal factors; theref ore, the diff erences menti oned above are explai ned by the é é åãåï ( thermal and, correspondingly, geodynamic environments of the Early Proterozoic development of layered massif s of the area, namely, by active rif ting the Pechenga district located, probably, above the actual hot spot, and by passive rifting ø î áæåã areas of the K arelia-Kola regi on. This work was supported by the Russian Foundation f or Basic Research (Proj ect ï î . 95-05- 15359.) R EF E R EN CES A l api eti , Ò.Ò., Fi l en, Â .À ., L ahti nen, Û ., L avr ov, Ì .Ì ., Smol kin, V.F., and Voitsekhovsky, S.N ., Early Pr oterozoic L ayered Intrusi ons i n the N ortheastern Part of the Fennoscandi an Shi eld, Mi ner al. Petrol ., 1990, vol . 42, nos. 1 4 ,

pp. 1- 22.

Bakushki n, Å.Ì ., Sulfi de Copper- % ñ1ñå1 M i neral i zati on of the á åï åãàÃç1ñàóà M ountai n Intrusi on (L uostari M assi f), Novye dannye po mi neralogi i medno- ni kelevykh kolchedannykh r ud Kol skogo poluostrova (New D ata on the M ineral ogy of Copper-N i ckel and M assive Sulfi de Ores of the Kol a Peninsula), A patity, 1979, ðð. 79- 84. Bal ashov, Yu.À ., Bayanova, Ò. ., and M i tr ofanov, F.Ð., Isotope D ata on the A ge and Genesi s î ã'1.àóåãåñ1Basi c—1Ë ããàÜàsi c I ntrusions in the K ol a Peni nsula and N or thern K areli a, N ortheastern Bal ti c Shi el d, Precambrian Res., 1993, vol . 64, ðð. 197- 205. Barkov, À Õ è., L ednev, À .I ., and Bakushki n, Å.Ì ., M ineral s of Elements of the Plati num Group from the Î åï åãàÃç1ñàóà M ountain M assif , Kol a Peni nsula, D okl. Akad. Nauk, 1994, vol . 338, ï î . 6, ðð. 785- 788. Campbell , 1.Í ., N aldrett, À Ç., and Barnes, $ .J., À M odel for the Origin of the Plati num-Rich Sul fi de H orizons i n the Bushvel d and Sti ll water Complexes, J. Petrol., 1983, vol . 24, ðð. 133- 165.

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