International Geology Review PLATINUM-GROUP

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Jul 6, 2010 - TABLE 1. Types of Copper-Nickel Sulfide Ores of the Pechenga Ore Field. A. B. C. D. E. ..... minerals of the cobaltite-gersdorffite series rich in.
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PLATINUM-GROUP ELEMENTS IN THE COPPER-NICKEL ORES OF THE PECHENGA ORE FIELD a

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V. V. Distler , A. A. Fillmonova , T. L. Grokhovskaya & I. P. Laputina

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Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry [IGEM], USSR Academy of Sciences, Moscow Published online: 06 Jul 2010.

To cite this article: V. V. Distler , A. A. Fillmonova , T. L. Grokhovskaya & I. P. Laputina (1990) PLATINUM-GROUP ELEMENTS IN THE COPPER-NICKEL ORES OF THE PECHENGA ORE FIELD, International Geology Review, 32:1, 70-83, DOI: 10.1080/00206819009465756 To link to this article: http://dx.doi.org/10.1080/00206819009465756

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PLATINUM-GROUP ELEMENTS IN THE COPPER-NICKEL ORES OF THE PECHENGA ORE FIELD

V. V. Distler, A. A. Fillmonova, T.L. Grokhovskaya, and I. P. Laputina (Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry [IGEM], USSR Academy of Sciences, Moscow)

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From: "Platinovyye metally v medno-nikelevykh rudakh Pechengskogo rudnogo polya," Geologiya rudnykh mestorozhdeniy, 1989, No. 6, pp. 3-17.

The ores of the Pechenga field in the northwestern-most Soviet Union have been sub­ jected to metamorphism, resulting in redistribution of the platinum-group elements to give patterns of occurrence different from those typical of more purely magmatic deposits. The main carrier of the platinum-group elements are minerals of the cobaltite-gersdorffite series, paragenetically associated with serpentine-talc-calcite gangue minerals. These sulfarsenides incorporate platinoids released by the metamorphic transformation of primary sulfides.

The distribution of platinum-group elements dur­ ing copper-nickel ore deposition is governed almost entirely by the regime of sulfide deposition. From experience in many copper-nickel provinces, the concentration of platinum-group elements in the magmatic stage, proper, of ore deposition is now well understood. However, in some deposits of Archean-Early Proterozoic age, the time of major cop­ per-nickel ore deposition, post-ore metamorphism significantly affected not only the silicates but also the sulfides, causing redeposition of primary ore associations to form sulfide-rich types of coppernickel ores. This situation calls for reexamination of the processes of concentration of platinum-group elements to include all stages of the evolution of the ore parageneses.

Katsel'vaara and Kammikivi deposits in the west), and in the East Allarechensk ore node. Types of Copper-Nickel Ores in the Pechenga Ore Field and Evolution of the Sulfide Associations The present concept of copper-nickel sulfide for­ mation in the Pechenga ore field is based on the model of its tectonic evolution [6-8, 10] and general principles of the classification of ore associations of copper-nickel deposits. The deposits are related to volcano-intrusive igneous associations of intracontinental rift zones [4, 5, 10]. The main stages in the geologic history of the Pechenga ore field were: 1) formation of an intracontinental rift zone, with the development of a volcanogenic-sedimentary sequence and formation of a nickeliferous volcano-intrusive association, in­ cluding ultramafic volcanics of picrite and picritebasalt types, synvolcanic layered gabbroclinopyroxenite-wehrlite sulfide-bearing intrusions and deposition of high-grade copper-nickel sulfide ores; 2) formation of the Pechenga slice-block monocline, regional metamorphism in the greenschist and lower amphibolite facies with fracturetectonometamorphic and hydrothermal-metamorphic

Very little has been published on the state and be­ havior of platinum-group elements during the meta­ morphism of copper-nickel ores, in general, and nothing at all on the Pechenga ore field in particular. Our study of the behavior of platinum-group ele­ ments during ore formation in the Pechenga ore field is based on observations on the ore nodes of the eastern and western parts of the ore field (deposits of the Pilguryarvi, Zapol'yamoye, Tundra, Sputnik, and Kiyerdzhipori massifs in the east and the Kaula, 70 Copyright © 1990 by V. H. Winston & Son, Inc. All rights reserved.

V. V. DISTLER ET AL.

TABLE 1. Types of Copper-Nickel Sulfide Ores of the Pechenga Ore Field Textural-genetic types

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A. Disseminated in gabbroclinopyroxenite-wehrlite intrusives

A 1 . Slightly and moderately serpentinized wehrlite reretaining relict olivine A 2 . Serpentinized wehrlite A3. Talcized wehrlite A4. Talcized and amphibolized wehrlite

B. Densely disseminated in B1. gabbro-clinopyroxenitewehrlite and clinopyroxenitewehrlite intrusives B2. C. Brecciform and massive in tectonometamorphic zones

Mineral types

Host rocks

Serpentinized and amphibol­ ized wehrlite and clinopyroxenite Talcized wehrlite

Chalcopyrite-troilite pentlanditepyrrhotite (±cubanite), chalcopyrite pentlandite-pyrrhotite Chalcopyrite-pentlandite-pyrrhotite

Chalcopyrite-pentlandite-pyrrhotite

C 1 . Intrusives altered to serpentinite- Chalcopyrite-pentlandite-pyrrhotite, talc-amphibole-chloritepyrrhotite-pentlandite-chalcopyrite carbonate rocks Phyllite altered to amphibole2 chlorite-carbonate rock

C. D. Solid vein bodies in tectono­ metamorphic zones

D 1 . Metamorphosed intrusive rocks

Chalcopyrite-pentlandite-pyrrhotite, pentlandite-chalcopyrite

E. Veinlet-disseminated ores in contact-metamorphosed sedimentary rocks

E 1 . Contact hornfels and products of its hydrothermal meta­ somatism

Chalcopyrite, bornite-chalcopyrite, pentlandite-chalcopyrite

processes, accompanied by alteration of the primary magmatic mineralization.

Ore types A and B (Table 1) are related to the magmatic process, proper, of ore deposition. Type A is typical of layered, fully differentiated, gabbroclinopyroxenite-wehrlite intrusives, and type B, of the lower endocontact zones of these intrusives plus thin, substratiform clinopyroxenite-wehrlite in­ trusives which apparently mark a later subphase of the ore-bearing magmatism. Typically, the clino­ pyroxenite-wehrlite intrusives are most saturated with magmatic sulfides.

These two stages together determined the present aspect of the mineralization of the Pechenga de­ posits. The multistage nature of the mineralization, as well as the pattern of ore deposition of the mag­ matic stage proper, were substituted by Gorbunov [2]. The findings of previous investigators and our own data enabled us to give the following classifica­ tion of the ores of the Pechenga field (Table 1).

Textural-genetic types C, D and E were formed in the second stage of formation of the ore field and are the products of tectonometamorphic transformation of the primary ores in fault zones. In addition, regional hydrothermal-metamorphic processes re­ lated to this stage had a substantial effect on the al­ teration of the disseminated and densely dissemi­ nated mineralization of the layered intrusions, affect­ ing the sulfide mineral associations as well as the silicate component of the ores.

The textural-genetic types of ores reflect the main natural associations of sulfide mineralization in the whole Pechenga ore field. Current ideas on the na­ ture and sequence of ore formation in the ore field are reflected in the distinction of ore types. We at­ tach very great importance to analysis of the evolu­ tion of coexisting minerals of the sulfide para­ geneses, as these reveal the sequence of ore-forming processes. 71

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Disseminated ores of type A, with well-expressed features of primary magmatic mineralization, are localized in the middle range of serpentinized wehrlite (with preserved or relict olivine). Dissemi­ nated sulfide (Fig. la) consists of xenomorphic and, less often, ovoid tri-mineral aggregates with the average quantitative proportion: 45% pyrrhotite, 40% pentlandite, 15% chalcopyrite. In such ores, minerals of the pyrrhotite group are present in three modifications—troilite, hexagonal pyrrhotite and monoclinic pyrrhotite, with well-expressed patterns of phase correspondence between these minerals, as well as of the pentlandite coexisting with them. Troilite is present either in the form of homogeneous grains or as intergrowths with hexagonal pyrrhotite; the maximum amount of troilite occurs in the top of an ore body, whereas in its bottom the relative proportion of hexagonal pyrrhotite in the inter­ growths increases. The distribution of nickel in pyrrhotites depends on the modification: troilite con­ tains up to 0.1 wt. % of nickel; hexagonal pyrrhotite intergrown with troilite contains no more than 0.2%; homogeneous hexagonal pyrrhotite contains from 0.3 to 0.7%, with the nickel content increasing from the top to the bottom of the ore body; hexagonal pyr­ rhotite intergrown with monoclinic contains up to 0.5%, with the latter containing up to 0.4%; and homogeneous monoclinic pyrrhotite contains up to 0.5%. It is typical that homogeneous monoclinic pyrrhotite is largely developed in ores localized in talcized wehrlite. The distribution of nickel between pyrrhotites and pentlandite has a known, almost direct proportionality to the concordant increase in contents (Fig. 2, I). At the same time, some spread of the contents between each class of values is a reflec­ tion of late metamorphic transformation of the ores.

redeposition of the ore components. In particular, when olivine is replaced by lizardite serpentine, no magnetite is formed, but instead a complex of secon­ dary sulfides including pyrrhotite and pentlandite intergrown with serpentine; in addition, larger mas­ sive pyrrhotite-pentlandite aggregates appear. Ap­ parently this phenomenon also is reflected in the in­ crease in the relative proportion of pentlandite in the ores at the bottom of the ore bodies compared to the less altered parts near the roof. Moreover, from the less to the more altered parts there is a general in­ crease in the number of morphological varieties of pentlandite, including the appearance of tabular and flame-like segregations of pentlandite in pyrrhotite, porphyritic pentlandite, and fine-grained monomineralic pentlandite intergrown with talc and other gangue minerals. In the ores that have been most transformed by metamorphism, the pattern of distribution of nickel between pentlandite and pyrrhotite is completely dis­ rupted. The distribution pattern (Fig. 2, II) is a field of points in which pentlandite of a particular com­ position coexists with pyrrhotite with a large spread of nickel contents, and pyrrhotite with a particular nickel content forms intergrowths with pentlandites with different nickel contents. We consider such a pattern of nickel distribution in coexisting sulfides to be one of the best indicators of metamorphic trans­ formation of magmatic ore associations. Cubanite is a typical mineral of the primary ores; as a rule, it is associated with troilite or hexagonal pyrrhotite. In these ores, cubanite is developed in the form of lamellae in chalcopyrite. As the extent of metamorphic transformation of the primary ores in­ creases, cubanite as a rule disappears from the sul­ fide parageneses. The mineral is typical of neither the densely disseminated nor the brecciform and massive ores.

The earliest stage of transformation of the mag­ matic sulfides was greenschist-facies regional metamorphism, during which sulfides were replaced by magnetite with conjugate replacement of coexisting olivine by an aggregate of serpentine and magnetite.

The economically valuable mineralization of the tectonometamorphic zones consists of brecciform and massive ores that have a number of specific fea­ tures that were determined by the conditions of their formation. Above all, we should stress the fact that the brecciform ores, and the massive ores associated with them, always appear in those places where the earlier ores were the densely disseminated type.

In the hydrothermal-metamorphic processes re­ lated to tectonometamorphic zones, a sharply dif­ ferent character of mineral reactions is observed. With increasingly intensive metamorphism (serpentinization, development of talc, carbonatization), there is an increase in the extent of replacement of primary sulfides by an association of metamorphic silicates rather than by magnetite (Fig. lb), and neogenic sulfides developed as the result of

Detailed petrographic study of sections across zones of brecciform ores makes it possible to state 72

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FIGURE 1. Structural relationships of sulfides and silicates in copper-nickel ores of the Pechenga ore field: a) interstitial magmatic sulfides (white) in disseminated ores in wehrlite with preserved olivine (1) in serpentine (2), polished section, x50; b) partial replacement of sulfides (white) by metamorphic minerals (1) and magnetite (2), forming shadow structures in serpentinized and talcized wehrlite, polished section, x50, c) replacement of sulfides (white) in densely disseminated ores in intensively metamorphosed wehrlite, with development of a tremolite + actinolite + serpentine association (gray), polished section, x100; d) relict patch of intensively replaced sulfides (in center of photo) in neogenic sulfides of brecciform ores (sulfides light, gangue minerals gray), polished section x100.

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TABLE 2. Volcanogenic-Sedimentary Rocks Chalcopyrite veinlet mineralization in essentially quartzose rocks with albite and chlorite in silicified phyllites Copper-nickel brecciform ores in chlorite-carbonatetalc plus quartz rocks, formed by replacement of phyllite

Copper-nickel brecciform ores in wehrlite altered to chlorite-carbonate-actinolite-talc rock; densely dissemi­ nated mineralization in wehrlite altered to serpentinechlorite-carbonate-talc rock

Chalcopyrite veinlet-disseminated mineralization in silicified phyllite

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Volcanogenic-sedimentary rocks

definitely that the sulfides in the ores with brec­ ciform texture were deposited after metamorphism (or metasomatism) of the intrusive (with its magmatic sulfides) and volcanogenic-sedimentary rocks. The ore zoning thereby produced can be represented schematically as follows:

distribution of nickel between pyrrhotite and pent­ landite in the brecciform ores, and the relationship of the distribution of this component is identical to that shown in Figure 2, II. It should be emphasized that pentlandite in the brecciform ores is somewhat more nickel-rich than that in the disseminated ores.

We emphasize that when the metamorphic rock complex was being formed, the primary volcano­ genic-sedimentary and intrusive rocks were most profoundly transformed within immediately the tec­ tonic zones, the transformation dying out relatively in their peripheral parts. In the latter, relatively fresh fragments or relicts of the primary rocks are in­ variably preserved and bear witness that profound metamorphic transformations and later ore deposi­ tion involved both volcanogenic-sedimentary and intrusive rocks.

The facts clearly indicate the polygenetic nature of the mineralization in the Pechenga ore field. The results of studying the evolution of the sulfide parageneses are very important for analyzing the trends of concentration of the platinum-group ele­ ments. Distribution of Platinum-Group Elements in Polygenetic Copper-Nickel Ores The patterns of concentration of individual platinum-group elements in the deposits of the Pechenga ore field are typical of the copper-nickel sulfide ore association. Palladium and platinum are predominant compared to the rare platinoids, with the accumulation of all the platinum-group elements generally related to the sulfide-rich parts of the deposits. At the same time, the relationship of the concentration of platinum-group elements to sulfide enrichment is rather complex. It has been established that the distribution of nickel, copper and precious metals are different for the individual texturalgenetic types of ores (Fig. 3). For type A there are no significant paired correlations of concentrations of platinum-group elements with each other or with nickel and copper. For type B the correlations are more definite. The concentrations of nickel are al­ ways related to those of all the platinum-group ele­ ments or of palladium and platinum in particular. Highly significant correlations appear between the

Deposition of the sulfides after formation of the metamorphic rocks is reflected in the ore para­ geneses of these zones. The main mineral of the pyrrhotite group is monoclinic pyrrhotite, largely typi­ cal of places with densely disseminated and breccia mineralization. No troilite was found in these ores. The nickel content of the pyrrhotite is 0.11-0.83 wt. %, and it varies unsystematically. The pentlandite content is as much as 20% in areas with densely disseminated mineralization; in the brecciform ores it increases to 35%, and it is also high in the massive ores—up to 30-35%. The last apparently is due to the fact that the source of the nickel was not only primary sulfides, but also mobilization of silicate nickel in the ultramafic rocks and its fixation in sulfide form. It is very im­ portant that no systematic pattern was found in the 74

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FIGURE 2. Distribution of nickel between coexisting pyrrhotite and pentlandite in disseminated ores in wehrlite retaining olivine (primary magmatic mineralization, type A, Table 1)(I),and in disseminated, densely disseminated and brecciform ores (types A2-A4, B, D, Table 1) (II). 1-2) Pyrrhotite: 1) hexagonal, 2) monoclinic; 3) troilite.

platinum-group elements. In type C there is no cor­ relation between platinum-group elements and pre­ cious metals, but the relationships of concentrations between all the platinoids are retained.

The inhomogeneity of the relationships of the platinum-group elements is governed by the uneven distribution of their concentrations in the different types of sulfide ores. In Figure 4, which reflects the 75

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However, this relationship is not a simple line. When nickel concentrations reach about 2%, there is no further increase in the amount of platinum-group elements, although the amount of nickel (or sulfide enrichment of the ores) may increase. Moreover, it is important that the maximum content of platinumgroup elements found in disseminated ores (field I) are not reached in the high-grade ores. These facts clearly indicate the polygenetic nature of platinum-group distribution in the sulfide ores. This distribution reflects: 1) a tendency for the platinoids to accumulate as the total sulfide satura­ tion of the ores increases, as is manifested for all types of ores that retain features of their primary magmatic nature; 2) a tendency for redistribution of platinoids upon metamorphic transformation of the primary ores, which leads to locally unsystematic ac­ cumulation of metals and causes a wide spread of contents in each type of ore. In this connection, we turn our attention again to the disproportionality in the concentrations of nickel and platinoids in dis­ seminated and high-grade ores, which is an essential argument in solving problems of their genetic relationships.

FIGURE 3. Correlation matrix of relationships of nickel, copper, and precious metals in copper-nickel ores of types A, B and C (224 samples). Correlation coefficients are sig­ nificant (K ≥ 0.4) for ore types corresponding to letters in sectors; no significant correlation was found for ore type corresponding to missing letters.

paired relationships of nickel (which is a fairly strict normalization index of ore type) and the total platinum-group elements, the set of values of metal concentrations breaks down into several fields or linear zones, with individual trends of variation of the metals of the platinum group with varying con­ centrations of nickel in the ores. Field I, correspond­ ing to type A ores, has a platinum-group element dis­ tribution that is practically independent of nickel and has maximum spread of concentration values for a particular nickel content.

Mineral Carriers of Platinum-Group Elements Regardless of the region to which deposits of the copper-nickel sulfide association belong, there is an association of platinum-group elements which is fairly stable in respect of species and includes more than 60 mineral species. The main phases in this association, tellurides, telluro-bismuthides, arsenides and some intermetallic compounds and, less often, sulfides of palladium and platinum, are observed virtually always and their frequency of occurrence depends only on the bulk concentrations of the metals in the ores.

Field II has a distinct tendency toward an increase in concentrations of platinum-group elements with increasing nickel enrichment of the ores (or their saturation with sulfides). The high-grade dissemi­ nated and densely disseminated ores fall in this field. The correctness of distinguishing this field is con­ firmed by the relationship of increasing content of total platinum-group elements to increasing nickel concentration, brought out in a single vertical section of the same ore body (curve 1-2 in Fig. 4).

The experience of many years' study has revealed that although the platinum-metal minerals typical of copper-nickel ores occur in the deposits of the Pechenga ore field, the number of finds of them amounts to only a few isolated grains. For instance, we found sperrylite, PtAs2, and atokite, (Pd, Pt, Cu)3Sn, having their usual chemical compositions. Finds of merenskyite (PdTe2), mangerite (PdBiTe), and antimony-rich mangerite (Pd(Bi, Sb)Te), asso­ ciated with hessite, melonite and altaite, in a chalcopyrite-rich sector on the east flank of the Pechenga

The most clear-cut relationship between platinumgroup elements and nickel concentrations is mani­ fested for all types of high-grade ores, as is ex­ pressed by the relationship of curve 3-4 in Figure 4. 76

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FIGURE 4. Paired correlation between total platinoids and nickel in ores of the Pechenga ore field. Explanation in text.

ore field have been described [9]. Altaite is a rela­ tively abundant accessory mineral of the ores.

skeletal crystals, 100-150 μm (usually 200-400 μm) in size (Fig. 5a-d). The grains of platinoid-bearing cobaltite-gersdorffite are localized in sulfides that have been subjected to substantial replacement by late metamorphic minerals, but the sulfarsenides themselves were not involved in this replacement. The crystals have a massive aspect and, what is most important, they themselves intersect tabular and acicular grains of metamorphic phases. The relation­ ships of the sulfarsenides to the ores and rock-form­ ing minerals indicate they are metacrysts formed during the metamorphic transformation of the primary ores.

Minerals of the cobaltite-gersdorffite series—the main form of occurrence of platinoids in the ores. The frequency of occurrence of platinum-nickel mineral phases, proper, definitely shows that these compounds are the main carriers of the platinumgroup elements in the ores. We might expect a cer­ tain uniqueness in the platinum mineralization of the ores of the Pechenga deposits, because of the very substantial role metamorphic transformation plays in their development. The first data on the unusual forms of occurrence of platinoids in the deposits of the Kola area were obtained in a study of a number of copper-nickel prospects in the gneiss complex on the southeast rim of the Pechenga structure, where minerals of the cobaltite-gersdorffite series rich in platinum-group elements were found [5].

It should be emphasized that the frequency of oc­ currence of sulfarsenides correlates to a certain ex­ tent with the intensity of metamorphic replacement. These minerals are rare or absent in disserninated ores in which the structural-textural and paragenetic characteristics typical of magmatic sulfides have been preserved (Table 1, type A). The frequency of occurrence of sulfarsenides is greater in the densely disseminated, massive and brecciform ores (Table 1, types B and C).

In the deposits of the Pechenga ore field, platinoid-bearing nickel-cobalt sulfarsenides are relatively unevenly distributed, but on the whole are more abundant than any other platinoid-bearing phases. Usually they are isolated grains confined to sulfide aggregates, less often clusters of up to 5-7 grains on the surface of a standard polished section. As a rule these minerals form perfect idiomorphic or

On the basis of chemical composition of the main mineral-forming components, the platinoidbearing sulfarsenides are an isomorphous series with 77

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FIGURE 5. Metacrysts (a, b) of platinoid-bearing sulfarsenides (white) in sulfides.

relatively wide variations of the proportions of the cobaltite (Cob), gersdorffite (Grs) and arsenopyrite (Asp) end members, although the absolute majority of compositions lie in the field of cobalt-rich varieties (Table 3). The limiting members of the compositions of the Pechenga sulfarsenides (in atomic %) are Cob48Asp36Gers16—Cob44Gers40 Asp15—Cob89Asp7Grs5. The absolute majority of analyses lie below values of Asp 10-12 . The predominant varieties, in content of the gersdorffite end member, are phases with a composition of Grs 16-20 , and of the cobaltite end member, CoD60-75.

that the contents given in Table 3 characterize the most frequently found concentrations of platinoids in the sulfarsenides, but they are not the extreme values for a given deposit. In individual deposits of the ore field, cobaltites containing 14,500 ppm of platinum, 18,300 ppm of iridium and 6000 ppm of osmium have been found. Some rare minerals containing platinoids. A very interesting phase containing iridium is the leadnickel sulfide shandite, found by Spiridonov. The mineral, whose composition corresponds to the crystallochemical formula Ni 3 Pb 2 S 2 , is confined to serpophite veinlets, and thus is comparable to the metamorphic sulfarsenides in its conditions of formation. The iridium content of the mineral is 79009600 ppm. At the same time, this mineral is of no real importance in the iridium balance of the ores, because its abundance is extremely low.

Platinum, rhodium, iridium, ruthenium and osmium have been identified in the sulfarsenides; no palladium has been found (Table 3). The platinumgroup elements are unevenly distributed between grains. Sulfarsenides containing platinoids and grains devoid of them (at a sensitivity 0.01 wt. %) may be present in the same section (for instance, Table 1, samples 2, 5, 6). Moreover, the distribution of individual platinum-group elements between closely situated grains is random. For instance, in one crystal two or three platinoids may be present, and in another only one, but one that is absent in the first grain. Finally, a zonal distribution of metals between the center and the edges of a single crystal is observed. Wide variations in absolute concentrations of each of the five platinum-group elements and in their sum have been established. It is noteworthy

Maucherite, in which palladium was found, is of somewhat more interest. This mineral apparently also belongs to the metamorphic association. Its palladium content ranges from 1000 to 2100 ppm and is unevenly distributed, but there also are maucherite grains in which no palladium was found. It is very important that the platinoids show selective fractionation between cobaltite-gersdorffite and maucherite. The former contains all platinoids 78

V. V. DISTLER ET AL.

TABLE 3. Minerals of the Cobaltite-Gersdorffite Series, Containing Platinoids (x-ray microanalysis, in wt. %)* Sample No. 1

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2

3 4

5

6

7

8 9

Number of grains 1 center 2 center 1 edge 2 edge 1 2 3 4 5 6 7 8 9 10 1 1 2 3 4 5 center 5 edge 1 2 3 4 1 2 3 4 1 edge 1 center 2 3 4 5 6 7 1 1 2 3

ΣPt

Ni

— — — — — — — —

0.05 0.05 0.03 0.03 0.07 — 0.83 0.03

0.17 — 0.02 — 0.02 — — — —

— — — — — — — — —

0.24 0.06 0.08 0.08 0.06 0.72 0.09 0.09 0.08







0.08

— — — —

0.04 0.05 0.01 0.03

— — — —

— — — —

0.04 0.05 0.06 0.03

— — 0.02 — 0.81

0.11 0.04 — 0.04 —

— — — — —

0.08 0.12 — — —

0.22 0.16 0.07 0.05 0.81

— — — — — — 0.19 —

0.05 — — 0.07 — — — —

— — 0.02 — — 0.04 — 0.15

— — 0.05 — — — — —

0.05 0.05 0.07 0.07 0.03 0.04 0.19 0.15

8.88 6.86 7.51 5.04 6.92 8.09 8.42 8.24 6.91 7.57 7.71 6.20 8.43 6.22 12.61 2.18 2.66 2.55 2.11 2.50 2.64 1.99 2.43 2.54 2.34 9.67 9.29 10.51 10.16 11.22 15.93 7.66 7.17 6.69 6.74 7.80 7.05 7.47 10.69 9.56 12.01

Pt

Rh

Ir

— — 0.03 0.03 — — — 0.03

— — — — — — 0.80 —

0.05 0.05 — — 0.07 — 0.03 —

— — — — — — — —

0.04 0.03 — 0.08 — — 0.07 0.07 0.03

— — 0.02 — — 0.72 0.02 0.02 0.02

0.03 0.03 0.04 — 0.04 — — — 0.03

0.08



— — 0.05 — 0.03 — 0.05 0.01 — — — 0.05 — — 0.03 — — —

Ru

Os

*Analyst I. P. Laputina.

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Co

Fe

As

S

Σ

4.61 44.85 21.5 101.95 22.6 22.04 4.80 43.94 20.73 98.42 26.37 4.37 44.56 21.58 104.92 24.92 4.21 45.89 21.01 101.10 24.79 5.61 43.30 20.82 101.51 24.64 5.74 42.84 21.26 102.57 23.38 5.67 42.70 20.43 101.48 23.62 5.28 43.03 20.78 100.98 24.26 6.07 42.42 20.90 100.56 24.92 5.10 43.03 21.10 101.96 22.56 5.09 43.18 21.37 99.97 19.04 13.28 37.36 27.27 103.23 22.69 5.17 42.04 18.83 97.24 15.68 13.31 37.35 27.35 99.97 19.62 5.80 41.40 19.58 99.83 32.67 3.70 42.14 19.47 100.25 30.33 3.37 42.63 19.02 98.10 30.75 2.25 43.57 19.27 98.47 30.85 3.02 41.83 19.42 97.23 1.68 43.36 19.22 98.41 31.65 30.47 2.01 42.99 16.21 94.40 31.69 2.17 43.18 19.25 98.28 30.98 2.00 42.33 18.77 96.55 30.95 1.96 43.34 18.70 97.54 29.91 2.22 41.16 19.11 94.80 20.39 8.02 41.46 20.18 99.72 22.78 7.71 36.85 19.12 99.75 20.84 7.04 41.30 19.70 99.61 20.24 7.59 40.86 20.25 99.26 18.67 5.39 43.62 20.79 99.76 18.31 5.53 43.55 20.70 104.07 24.37 5.81 42.78 20.49 101.82 24.12 5.63 42.71 21.18 100.81 23.46 4.60 42.94 20.77 98.51 23.64 4.93 43.28 20.51 99.15 23.64 4.97 42.96 20.76 100.20 24.00 5.14 43.13 20.66 100.05 24.43 6.28 42.10 21.43 101.74 19.27 6.26 42.42 20.74 99.45 16.08 4.63 41.69 18.96 91.11 19.20 5.96 43.15 21.51 102.28

INTERNATIONAL GEOLOGY REVIEW

except palladium, whereas the latter contains only palladium.

than about 500°C, corresponding to the conversion of serpentine to olivine. As has been shown experi­ mentally [6], the solubility of platinoids in sulfides depends strongly on temperature and is greatest at conditions where a monosulfide solid solution is stable at magmatic temperatures. As the temperature falls to 800-600°C, the solubility of platinoids de­ crease abruptly, and at 500°C is either absent or ex­ tremely low for the individual platinum-group ele­ ments. Thus, even with a rough estimate of the temperature of recrystallization and redeposition of sulfides during metamorphic transformation of the primary ores based on the upper limit of stability of serpentine, the process of elimination of ad­ mixed platinoids from pyrrhotite and pentlandite is obvious.

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Conditions of Formation of the Platinum Mineralization The concept of solid solutions of platinoids in the ore-forming sulfides explains the general evolution of the state of platinum-group elements during magmatic sulfide ore deposition [4]. On the basis of the theoretical postulates of the concept, we have no reason to deny that solubility of the platinum-group elements in sulfides played the leading role in the stage of magmatic ore deposition proper in the Pechenga ore field. At the same time, we are unable to characterize the values of platinoid solubility in pyrrhotite and pentlandite by direct methods, for practical reasons, primarily involving direct iden­ tification of the small and ultrasmall amounts of dis­ solved platinum-group elements. However, the fac­ tual data may serve as substantiation of primary solution in the sulfides of the Pechenga deposits. There is a well-expressed direct dependence of platinum-group element enrichment of the ores on their total sulfide saturation, manifested in the parts of the intrusive massifs that were least meta­ morphosed and have retained their magmatic silicate and sulfide parageneses. It is typical that in such ores the frequency of occurrence of the platinoidmineral carrier minerals proper, including sulfarsenides, is minimal. A convincing argument in favor of the essential role of solid solution in the primary sulfides is the general finding in all types of ore of platinoid minerals typical of deposits of that par­ ticular association.

At the same time, it seems important to estimate more precisely the conditions of stability of the platinoid-bearing phases of the cobaltite-gersdorffite series. For this, we can proceed from estimates of the stability of the complex of coexisting silicate and carbonate phases; the relationships of the sulfides and platinoid-bearing sulfarsenides to these phases are fairly clear. These relationships are manifested in the fact that the ore association as a whole was formed after the parageneses of autometamorphic and regional metamorphism, represented by lizardite serpentine, actinolite and tremolite. Intensive meta­ morphic and metasomatic transformations of serpentinite and serpentinized wehrlite and pyroxenite immediately preceded ore deposition, and according to [1], the associations ferroserpentine + antigorite, ferroantigorite + talc, chlorite + talc, talc + ferroantigorite + ferroserpentinite + calcite were formed suc­ cessively. We have shown that in the tectonic zones where hydrothermal-metamorphic processes were most extensive, ore minerals were deposited as the critical calcite + talc + tremolite paragenesis was being formed. Taking into account that the platinoidbearing sulfarsenides form metacrysts in sulfides and undoubtedly were later than the chlorite + talc gangue paragenesis, it is obvious that the tempera­ ture of formation of the sulfarsenides was close to that of the stability of the talc + carbonate or talc + calcite + tremolite associations. At a constant partial pressure of carbon dioxide (P C O 2 = 50 bar, PH2O = 1 kbar), these associations are stable [11] in the temperature range of 440-375°C (talc + calcite + tremolite + quartz paragenesis) and below 410375°C (talc + calcite + quartz paragenesis).

As shown by the data above, for the high-grade sulfide ores, the main carrier of platinoids obviously is sulfarsenides of the cobaltite-gersdorffite series. Recrystallization of the principal ore-forming sul­ fides led not only to disruption of the systematic dis­ tribution and redistribution of nickel and cobalt be­ tween pyrrhotite and pentlandite, but also obviously helped to eliminate admixtures from both phases, specifically platinoids in solid solution. It is per­ fectly obvious that the differences in nickel dis­ solved in pyrrhotites of the same composition and the presence of low concentrations of nickel in highsulfur pyrrhotite are related precisely to the redis­ tribution of nickel under relatively low-temperature conditions of metamorphism. The maximum tem­ perature of that process could not have been more 80

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V. V. DISTLER ET AL.

FIGURE 6. Experimental diagram of miscibility of phases in the system CoAsS-NiAsS-FeAsS (after Klemm [13]). Composi­ tion fields of platinoid-bearing sulfarsenides are entered in the diagram. 1 -3) Compositions of sulfarsenides of: 1) Pechenga ore field, 2) Karikyavr deposit and ore shows in Archean gneisses on the northeast margin of the Pechenga ore field (from Distler and Laputina [5]), 3) deposits in Sudbury district [10, 14]; 4-7) fields of sulfarsenides with different contents of platinoids (in wt. %): 4) 1, 5) 15, 6) 3-5, 7) 1-3.

Experimental data [12] on the temperature sta­ bility of nickel, cobalt and iron sulfarsenides are on the whole consistent with the above. Maximum miscibility of the cobaltite, gersdorffite and arsenopyrite end members is observed in the temperature range of 500-650°C, in which very high-iron cobaltite-gersdorffite can exist [13]. At lower tempera­ tures, a distinct tendency appears for the miscibility of the three end members to decrease, and phases in which one of them is dominant become increasingly stable (Fig. 6). An absolute majority of the actual composition fields of the low-iron cobaltitegersdorffite of the Pechenga ore field on the ex­ perimental diagram, given in [13], lie below 600°C isotherm, and only isolated extreme iron-rich com­ positions may correspond to higher isotherms.

Moreover, in some copper-nickel deposits, for in­ stance Sudbury [10], as well as in the sulfide-bearing massifs on the Archean rim of the Pechenga struc­ ture and some others, sulfarsenides very rich in platinum-group elements are known. The sul­ farsenides of the Karikyavr deposit and several nearby prospects are especially rich—the cobaltitegersdorffite contains up to 15 wt. % of total platinum-group elements, with a substantial overall range of concentrations. In the Sudbury ore district, sulfarsenides containing up to 5-7 wt. % platinumgroup elements have also been found [10, 14].

A summary of the data on all known shows of platinoid-bearing sulfarsenide ores indicates that the solubility of platinum-group elements in them is fairly limited and does not exceed 20 wt. %. The solubility of nickel, cobalt and iron in sulfarsenides of the platinum-group elements also is limited, with maximum total concentrations of not more than 20 wt. % [5].

The temperature dependence of the solubility of platinoids in minerals of the cobaltite-gersdorffite series has not been investigated experimentally. As is known, sulfarsenides proper of each platinum-group element exist in nature.

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INTERNATIONAL GEOLOGY REVIEW

The stability fields of nickel, cobalt and iron sulfarsenides with different contents of platinoid estab­ lished experimentally by Klemm [13] (Fig. 6) show a fairly definite dependence of the variations in platinoid content on variations in major component composition of the minerals. In the set analyzed, the maximum contents of platinum-group elements are typical of the intermediate members of the cobaltitegersdorffite series with maximum iron content. The field of such compositions on the diagram cor­ responds to fairly high-temperature conditions of stability of the sulfarsenides. In the lower-tempera­ ture fields, where phases nearer one end member, either cobaltite or gersdorffite, are stable, a definite reduction in the concentrations of platinoids in the sulfarsenides is observed. For instance, gersdorffite from the Sudbury province is very poor in platinumgroup elements with total contents substantially below 1 wt. %. In the deposits and prospects in the Archean complex surrounding the Pechenga ore field, the platinoid content of the phases also decreases upon passing from cobaltite-gersdorffite to cobaltite. The decrease in concentration from phases with a total platinoid content of 10-15 wt. % to those with less than 1 % is very considerable.

textural and paragenetic features of the ores, but also by the variations in the equilibrium distribution of nickel between coexisting sulfides. 2. The distribution of platinum-group elements reflects their incorporation during an increase in overall sulfide saturation in all types of ores that retain features of their primary magmatic nature. Metamorphic transformations of the primary ores caused locally unsystematic redistribution of platinum-group elements and disruption of the proportional relationships of nickel, copper and precious metals. 3. For the first time, it is shown that the main car­ riers of platinum-group elements in the ores are nickel and cobalt sulfarsenides of the cobaltitegersdorffite series, containing platinum, rhodium, iridium and osmium, unevenly distributed between individual metacrysts. The presence of platinumgroup elements in some rare sulfides (shandite) and arsenides (maucherite) is established. Platinumgroup minerals typical of copper-nickel sulfide deposits—intermetallic compounds, tellurides, tellurobismuthides, arsenides, etc.—have been found throughout the history of study of the Pechenga deposits, but only in isolated grains. It is shown that the platinoid-bearing sulfarsenides were formed due to liberation of platinoids from solid solution in magmatic sulfides during metamorphism.

At the same time, in the Pechenga ore field proper, the effect of fractionation of platinum-group elements in sulfarsenides of different compositions is not very distinct. Primarily, this is related to the fact that the sulfarsenides of this deposit are very poor in platinoids, and the distribution of con­ centrations of the elements between individual grains is very uneven. However, considering the overall conditions of solubility of platinum-group elements in sulfarsenides, we should certainly take into account the fact that the cobaltite-gersdorffite of the Pechenga ore field was formed under greenschist-facies conditions, while the sulfarsenides of the Karikyavr deposit and similar prospects, clearly richer in platinum-group elements, were meta­ morphosed under amphibolite-facies conditions.

4. The temperature conditions of stability of platinoid-bearing sulfarsenides can be estimated on the basis of the stability of the coexisting hydro­ thermal-metamorphic talc-tremolite-calcite associa­ tion. The relatively low solubility of platinoids in nickel and cobalt sulfarsenides is related to their formation under greenschist conditions, whereas sul­ farsenides formed under amphibolite-facies condi­ tions in other areas of the Kola region are richer in platinoids. References

Conclusions 1. Godlevskiy, M. N., Yudina, V. V., and Batashev, Ye. V., 1984, The role of original composition in the metamorphism of ulcramafic rocks: Zap. Vsesoyuz. mineral. o-va., Pt. 93, No. 2, pp. 137-151. 2. Gorbunov, G. I., 1968, Geologlya i genezis sul'fidniykh mednonikelevykh mestorozhdeniy Pechengi (Geology and Genesis of the

1. The distribution of platinum-group elements in the polygenetic copper-nickel sulfide ores of the Pechenga ore field was controlled by the behavior of these elements during the two main stages of ore formation: the magmatic stage proper and the hydrothermal-metamorphic stage. The multi-stage nature of ore deposition is witnessed not only by the 82

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fundamente Vostochno-Yevropeyskoy platformy (Zones of Metamafic Rocks in the Basement of the East European Platform): Nauka Press, Moscow. 9. Spiridonov, G. V. and Kravtsova, O. A., 1987, Novyy dannyye o telluridakh mednonikelevykh mestorozhdeniy Pechengi (New Data on Cop­ per-Nickel Tellurides of the Pechenga Deposits): Moscow, 9 pp.—Dep.: TsNIItsvetmet ekonomiki i informatsii 09.02.88, No. 1989-tsm 88. 10. Cabri, L. J. and Laflamme, J. H. G., 1976, The mineralogy of the platinum-group elements from some copper-nickel deposits of the Sud­ bury area, Ontario: Economic Geology, Vol. 71, No. 7, pp. 1159-1195. 11. Distler, V. V. and Genkin, A. D., 1980, Deposits of sulfide copper-nickel of the USSR and their connection with cratonic volcanism: Proc. 6th IAGOD Symp. Schweizerbart,, Stuttgart, pp. 275-295. 12. Hellner, E. and Schürmann, K., 1966, Stability of metamorphic amphiboles: The tremoliteferroactinolite series: Jour. Geol., Vol. 74, No. 3, pp. 322-331. 13. Klemm, D. D., 1965, Synthese und analyzen in dem Dreiecksdiagrammen FeAsS-CoAsSNiAsS und FeS2-CoS2-NiS2: Neues Jahrb. Mineral. Abh., Vol. 103, No. 3, pp. 205-255. 14. Platinum-Group Elements: Mineralogy, Geol­ ogy, Recovery, 1981: Canadian Inst. Min. and Metal., Toronto.

Pechenga Copper-Nickel Sulfide Deposits): Nedra Press, Moscow. 3. Distler, V. V., 1980, Solid solutions of platinoids in sulfides. In Sul'fosoli, platinovyye mineraly, rudnaya mikroskopiya (Sutfosalts, Platinum Minerals, Ore Microscopy) (pp. 191-200): Nauka Press, Moscow. 4. Distler, V. V. and Dyuzhikov, O. A., 1988, For­ mation of copper-nickel sulfide deposits. In Rudoobrazovardye i geneticheskiye modeli endogennykh rudnykh formatsiy (Ore Deposition and Genetic Models of Endogenous Ore Forma­ tion) (pp. 166-172): Nauka Press, Novosibirsk. 5. Distler, V. V. and Laputina, I. P., 1979, Nickel and cobalt sulfarsenides containing platinumgroup elements: Doklady AN SSSR, Vol. 248, No. 3, pp. 718-721. 6. Distler, V. V., Grokhovskaya, T. L., Yevstigneyeva, T. L., et al., 1988, Petrologiya sul'fidnogo magmaticheskogo rudoobrazovaniya (Petrology of Magmatic Sulfide Ore Deposi­ tion): Nauka Press, Moscow. 7. Kazanskiy, V. I., Genkin, A. D., and Glagolev, A. A., 1983, The Pechenga ore district. In Glubinnoye stroyeniye i usloviya formirovaniya endogennykh rudnykh rayonov, poley i mestorozhdeniye (Deep Structure and Conditions of Formation of Endogenous Ore Districts, Fields and Deposits) (pp. 208-226): Nauka Press, Moscow. 8. Novikova, A. S., 1975, Zony metabazitov v

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