5.1. Position of Pd in the Pentlandite Structure
The highest concentration of palladium in Pd-rich pentlandites of the Southern-2 ore body corresponds to 11.26 wt.%, which is relatively close to the palladium content (12.96 wt.%) in synthetic pentlandite-like π (pi) phase [
29]. This phase has a cubic structure with a formula of Fe
4Ni
4MS
8, where M = Ru, Rh, Pd, although Fe:Ni:M ratio in π (pi) phase can vary within a wide range [
30]. The proximity of some Pd-rich pentlandite and the π (pi) phase in terms of Pd content suggests that they may have in principle an identical crystallographic structure.
Compositions of π-phase, ore-forming and Pd-rich pentlandites from various deposits are plotted on ternary diagrams of Pd-Ni-Fe (
Figure 10a) and Pd-Ni-S (
Figure 10b). The Fe content in Pd-rich pentlandites of the Southern-2 ore body varies within a narrow range. Thus, their compositional points form a trend, subparallel to the Pd-Ni side of the Pd-Ni-S ternary plots (
Figure 10a). Stillwater Pd-rich pentlandites contain more iron [
20] and form a parallel trend located closer to the Fe angle. Compositions of Pd-rich pentlandites from Norilsk-1 form a different trend, subparallel to Ni-Fe side (
Figure 10a), regardless of the fact that some compositions are similar to the Pd-rich pentlandites here presented. This may be due to the differences in the initial sulfide melt composition, possible difference in MSS and ISS composition and the conditions of crystallization. Thus, the studied Pd-rich pentlandites, as well as the ones reported in other works, do not correspond to the π-phase obtained in [
29], tending to have much less Pd and more iron than nickel.
It should be noted that when estimating the palladium concentration in pentlandite, a few intrinsic problems may arise. LA-ICP MS studies have shown that Pt and Pd can be present in sulfide minerals not only in a dissolved form (structural), but also in the form of microinclusions [
17,
31,
32]. This is especially important when assessing low PGE concentrations at the ppm level. The argument for the absence of the effect of microinclusions on the studied pentlandites from this study is that their composition is rather smooth with a regular change in Pd content along the microprobe profile (
Figure 9a).
The significant amount of palladium in pentlandite raises the question as to what position occupies in its structure. The structure of pentlandite is pseudocubic close packing with cations (Fe,Ni), occupying octahedral and tetrahedral positions [
33,
34]. The ionic radiuses of Pd, Fe and Ni differ by less than 15% [
35], and at the same time, palladium in pentlandite has a nominal oxidation state of +2, which theoretically makes it possible to replace Ni and/or Fe, in the structure of pentlandite, with Pd. Additionally, it has been established that palladium in pentlandite represents a homogeneous solid solution replacing Ni or Fe in the crystal lattice [
17,
19].
The composition of Pd-rich pentlandite from the Southern-2 ore body shows a strong correlation between Ni-Pd and Ni-Fe pairs, as well as between Fe-Pd (
Table 6). The correlation of cobalt with all other components is insignificant (
Table 6).
The correlation analysis involved 277 analyses of Pd-rich pentlandites containing less than 0.7 wt.% copper. Such a limitation on the copper content allows to partially or completely exclude the compositions of very small grains, the analysis of which may cause the detection of surrounding cubanite or chalcopyrite to occur. The data set included only compositions of pentlandite from the samples 32–11 and 31–11 (depths of 76.5 and 76.7 m, respectively).
The strong negative correlation between Ni and Pd indicates that Pd replaces Ni in the crystal structure of pentlandite (
Figure 11a). Positive correlation of Fe with Pd suggests that Pd does not substitute Fe. Moreover, a simultaneous increase in the content of Pd and Fe (
Figure 11b) indicates either a more complex isomorphism than the simple substitution of palladium for nickel, or two different processes that affect the pentlandite composition.
The first led to the enrichment of pentlandite in Pd by isomorphic replacement of Ni with Pd, whereas the second led to a change in the Ni/Fe ratio in pentlandite by replacing part of the Ni with Fe.
In addition to the correlation analysis, a principal component analysis was performed for the data set of 277 Pd-rich pentlandites. This statistical method allows us to determine how many factors (principal components) affected the quantitative ratios of chemical elements in the pentlandite formula, as well as to assess the contribution of each factor. Each of these factors may be associated to some physicochemical process that affects the compositions of pentlandites in the data set. The main factor to be considered is the substitution of palladium for nickel, which provides the strong negative correlation of these components (
Table 6). Another possible factor to be taken into account could be the detection of cubanite matrix during the analyses, which would lead to a positive correlation between Cu and Fe. Changes in sulfur fugacity can lead to modification of the Ni/Fe ratio in pentlandite [
36]. These and other factors can act simultaneously but independently of each other and affect the final ratio of components (elements) in the pentlandite to variable degrees.
A covariance matrix was obtained as a result of the calculation. The respective eigen values of the covariance matrix are 0.1502, 0.002484, 0.001388, 0.000408, 0.00027, 0. The contribution of the first principal component is 97%. The impact of the other five factors is too small to have had noticeable effects on the composition of pentlandite. Therefore, they can be disregarded. The existence of a single significant principal component suggests that all isomorphic substitutions in pentlandites from the data set occurred within that particular process and are dependent on each other.
Ni, Fe and Pd are the main elements for isomorphic substitutions leading to the formation of Pd-rich pentlandite; hence, it was necessary to find a dependence in relation to these components.
By applying Deming regression, the possible error in the determination of both components was taken into account, which is necessary for assessing the relationship between the components in Ni-Pd and Fe-Pd pairs. The following equations were obtained:
where X
Ni, X
Pd and X
Fe are contents of the elements in apfu.
Equation (1) shows that the appearance of one atom of Pd in pentlandite formula leads to a decrease for nickel by 1.41 atoms. In other words, 0.71 of a palladium atom appears when one atom of nickel disappears. Equation (2) assumes that the addition of one atom of Pd is accompanied by the appearance of 0.43 atoms of Fe. That is, the addition of 0.71 of Pd atom leads to an increase in the number of iron atoms by 0.30. Thus, one atom of nickel is replaced by 0.71 atoms of Pd and 0.30 atoms of Fe, for a total of 1.01 (±0.05).
The increase in the Fe content upon enrichment of pentlandite with palladium raises some other questions. The Ni/ Fe ratio in pentlandite depends on the bulk composition of the sulfide liquid and the primary solid solution from which the pentlandite exsolved. This is in good agreement with the regular change in the content of nickel and iron in ore-forming pentlandite along the section of the ore body (
Table 2 and
Figure 5). The gradual changes in pentlandite composition can be explained by a gradual change in the composition of the sulfide liquid and solid phase during fractional crystallization.
Additionally, experimental data have shown that an increase in sulfur fugacity leads to an increase in the Ni/Fe ratio in pentlandite [
36,
37,
38,
39]. Thus, sulfur fugacity is also an important factor controlling the Ni/Fe ratio.
The selected data include 277 analyses of Pd-rich pentlandites from two samples of cubanite ores that are very close in terms of position in the section of the ore body (76.5 and 76.7 m) as well as in composition. Ore-forming pentlandites in these samples are very similar in iron and nickel content as well (
Table 2 and
Figure 5). Assuming that the role of palladium is completely equivalent to the role of nickel in the pentlandite structure (Ni* = Ni + Pd), the Ni*/Fe ratio in the Pd-rich pentlandites should be the same with Ni/Fe ratio in the ore-forming pentlandites. However, Ni*/Fe ratios turned out to be lower due to the replacement of nickel in Pd-rich pentlandite not only with palladium, but also with additional iron. Since the bulk composition of the ore, as well as the composition of chalcopyrite and cubanite hosts for the pentlandite grains, is the same, the change in the Ni*/Fe ratio in pentlandite may indicate that the physicochemical conditions have changed. This suggests that the crystallization of ore-forming pentlandite occurred under higher sulfur fugacity, giving as a result a higher Ni/Fe ratio. However, its transformation into Pd-rich pentlandite seems to have occurred at a lower sulfur fugacity, which caused a decrease in the Ni/Fe ratio due to the replacement of part of the nickel with iron. A different sulfur fugacity was assumed for the formation of several generations of pentlandite grains in the clinopyroxenite of the Platreef in the Bushveld Complex [
32]. However, in the ores here studied, there is only one generation of pentlandite, which underwent partial isomorphic replacement in the rim of the grains.
Another possible explanation for these variations denotes that if the Pd atom is not equivalent to the nickel in the pentlandite structure, then Pd-rich pentlandite and ore-forming pentlandite can be considered as different mineral phases. In this case, different (Ni + Pd)/Fe and Ni/Fe ratios in these phases can exist under the same conditions.
Accordingly, Pd isomorphically replaces only nickel in Pd-rich pentlandites of the Southern-2 ore body. The isomorphic substitution occurs as follows: XNi = 0.71 × XPd + 0.30 × XFe, where XNi, XPd and XFe are the contents of the elements in apfu. The isomorphic replacement of nickel with iron occurring simultaneously with the enrichment of pentlandite with palladium thus may indicate that the formation of ore-forming pentlandite and its replacement by Pd-rich pentlandite occurred under various physicochemical conditions and, perhaps, at different times.
5.2. Genetic Constraints and Mechanisms for the Enrichment of Pd in Pentlandite
Pentlandite is the only palladium concentrator among the base metal sulfide minerals of the nickel-copper ores, since palladium is a compatible element with pentlandite while incompatible to coexisting pyrrhotite, chalcopyrite and cubanite [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12]. Pentlandite is considered a late-forming mineral, within the base metal sulfides assemblage, that may be formed as a result of exsolution of MSS and ISS. Exsolution of MSS leads to the crystallization of more stable phases, at lower temperatures, like pyrrhotite, chalcopyrite and pentlandite, causing a redistribution of the majority of the palladium into pentlandite. However, this mechanism may not be enough to result in high concentrations of palladium in pentlandite since the overall concentration of Pd in MSS is relatively low. It has been established sufficiently that MSS is the first solid phase to crystallize from a sulfide liquid and that Pd is incompatible with it. For instance, the partition coefficient of Pd between MSS and liquid is 0.08–0.27 [
40,
41]. Therefore, Pd tends to accumulate in the residual sulfide liquid, and only one sixth of Pd actually dissolves into MSS. Consequently, pentlandite formed during exsolution of MSS may not yield more than 10 ppm of palladium. Crystallization of ISS begins right after the solidification of MSS and continues until consuming all the sulfide liquid. Exsolution of ISS not only leads to the formation of chalcopyrite and cubanite but also pentlandite, redistributing most of the palladium into pentlandite as with MSS. Considering that palladium is incompatible with ISS as in MSS, it then continues to accumulate in the remaining sulfide liquid. Hence, the amount of palladium dissolved in early ISS is also very low as in MSS. An important increment in the content of Pd could be expected only in the late ISS, which crystallizes from the very last portions of the sulfide liquid and may concentrate significant amounts of palladium.
In this study, we present 23 cases of reported Pd concentrations in pentlandite from different deposits and one experiment (
Table 7). Most authors, however, did not indicate the primary sulfide phase, MSS or ISS, from which Pd-pentlandite formed. Thus, we assumed a primary solid solution in each case based on the mineral association presented in the papers. Only three cases out of twenty-three, including our study, concern pentlandite formed during exsolution of ISS, whereas sixteen out of twenty-three studies introduce occurrences of crystallization of pentlandite from MSS (
Table 7). Four papers report concentrations of palladium in pentlandite lower than 10 ppm. (
Table 7). These are pentlandites from the Jinchuan deposits [
3], Kambalda [
14], Aguablanca [
42] and Creighton Mine in Sudbury [
15]. Such low concentrations are most likely the result of Pd partitioning under exsolution of MSS. Regarding the mechanism of Pd enrichment, some authors have suggested additional diffusion of Pd from contiguous ISS [
42]. However, these limited concentrations are considered normal within magmatic sulfide deposits as a result of processes that are well known. Higher concentrations of Pd, especially those reaching weight percentage levels, are rather atypical and are required to be further studied. In principle, such accumulations of Pd in pentlandite could not be regarded as the result of exsolution and redistribution of palladium among base metal sulfides from MSS nor early ISS. Therefore, authors encountering such enrichments were compelled to look for other sources of palladium and other mechanisms for enriching pentlandite with it.
Holwell and McDonald reported 70–200 ppm Pd in pentlandite from the Platreef in Bushveld, which they explained by means of a high Pd/semimetal ratio in the ores [
16]. They suggested that Pd binds preferentially to Bi and Te, which can lead to separation of an immiscible bismuth-telluride liquid at the latest stages of crystallization. Excess of Pd binds to sulfur and enters first into MSS and then into pentlandite [
16]. Godel and Barnes reported 0.2–6700 ppm of Pd in pentlandite from the J-M Reef in Stillwater. The authors assumed the transfer of Pd from outside by a fluid [
9]. On the other hand, Li and Ripley discovered pentlandite containing up to 9.8 wt.% Pd intergrown with altered Pt-Pd sulfide in the J-M Reef of Stillwater [
20]. A source of palladium from Pt-Pd sulfide, which was destroyed and replaced by Pt-Fe alloy, was suggested this time [
20].
Mansur and coauthors published pentlandite with 0.005–0.1 wt.% of Pd from the Norilsk-1 deposit and 0.08–242 ppm at the Merensky reef in Bushveld [
43]. Pentlandite layers between pyrrhotite (former MSS) and chalcopyrite (former ISS) were studied. The authors proposed that Pd-rich pentlandite formed by a peritectic reaction of MSS + Liquid = PnSS, where PnSS is a high-temperature solid solution from which pentlandite is obtained upon cooling [
43,
45]. In addition, Brovchenko and coauthors found 0.04–4.62 wt.% Pd in pentlandite from Norilsk-1 deposit and similarly came up with a peritectic reaction as plausible explanation for this atypical pentlandites [
12]. Accordingly, there are four main hypotheses, which have been proposed to explain the high concentrations of palladium in pentlandite. These include (1) high Pd/semimetals ratio in sulfide liquid, (2) transfer of Pd into pentlandite by a fluid from outside, (3) destruction and element redistribution of Pd-bearing PGM and (4) a peritectic reaction of MSS + Liquid = PnSS. Nonetheless, most of them are not appropriate in explaining the high concentrations of Pd in pentlandites of the Southern-2 ore body.
The examined section of the ore body, in this study, exhibits a layered structure (
Figure 2), where the pyrrhotite layer is the result of crystallization and subsolidus transformation of MSS, and the cubanite and chalcopyrite layers are the product of crystallization and subsolidus transformation of ISS. The boundary between the pyrrhotite and cubanite layers is a potential place for the peritectic reaction MSS + Liquid = ISS to occur. A reaction of this type has been repeatedly reproduced in experiments with different initial compositions of the sulfide liquid [
46,
47,
48,
49]. If Pd-rich pentlandite forms as a result of another peritectic reaction MSS + Liquid = PnSS like the one proposed by Mansur et al. [
43], then we should expect to see such a layer of PnSS immediately above the pyrrhotite layer. However, there is no accumulation of pentlandite grains at or near the contact between pyrrhotite and cubanite ores. Furthermore, Pd-rich pentlandite in this study was found at different levels of the cubanite and chalcopyrite layers and, in particular, very far from the top of the pyrrhotite layer (
Figure 5). In addition, high concentrations of palladium occur in the pentlandite lamellae within cubanite (
Figure 8f), which completely excludes the formation of such pentlandite as a result of the peritectic reaction as described before.
The hypothesis of imbalance between Pd and semimetals [
16] assumes that Pt and Pd are more likely to form complex compounds with semimetals such as Te, Bi, Sb and As, than with S [
50,
51,
52,
53]. This would lead to the appearance of PGE-bismuthotellurides at the latest stages of the sulfide liquid crystallization [
54,
55,
56] or separation of an immiscible liquid consisting of PGE and semimetals [
51,
52]. Therefore, the excess of palladium will bond with sulfur and thus can enter MSS and ISS, and finally into pentlandite, as a result of the exsolution of those primary solid solutions. If there was to be an excess of palladium atoms associated with sulfur during the crystallization of the Southern-2 ore body, then it should have been dissolved in MSS and ISS and then it would enter into the ore-forming pentlandite. The occurrence of Pd-rich pentlandites together with ore-forming pentlandites in this case is inexplicable.
The source of palladium is one of the main problems in elucidating the origin of Pd-rich pentlandite. Li and Ripley found Pd-rich pentlandites in the J-M Reef at the Stillwater complex and suggested that they are the result of decomposition of Pt-Pd sulfides as a result of interaction with an S-undersaturated hydrothermal fluid, forming Pt-Fe alloys and releasing of Pd [
20]. The spatial relationship between Pd-rich pentlandites and PGE-bismuthotellurides has also been noted for other deposits [
16,
17]. Four out of the six studied areas containing Pd-rich pentlandite in the Southern-2 ore body are situated in the vicinity of large multiphase PGM grains, as shown in
Figure 6. The other two areas have no evident spatial relationship with any PGM. This means that, even though PGMs are absent nearby some occurrences of Pd-rich pentlandites, on the investigated polished surface, we cannot assure there is no PGM above or below these Pn grains. Therefore, it can be assumed that some PGMs could be a source of palladium for the studied Pd-rich pentlandites. The large multiphase PGM grains, as in our case, are thought to be formed because of crystallization of an immiscible PGE-semimetal liquid. The separation of a PGE-telluride liquid at the latest stages of sulfide liquid crystallization has been confirmed experimentally [
52]. In natural ores, this immiscible liquid can be of a more complex composition [
16,
17]. Sn, Bi and Pb are the main semimetal anions in the PGE-immiscible liquid in the ores of Southern-2 ore body, whereas Te is present only in small amounts. Pd and subordinate Pt are the main cations in these multiphase PGM grains. Other PGEs do not form their own minerals and are usually not detected as impurities in the minerals of Pt and Pd. The rounded or irregular-complex shape of the multiphase PGM grains (
Figure 6), the intricated internal structure, and the large number of different PGM phases may indicate that this type of PGM composites are the result of crystallization of droplets of an immiscible liquid with complex composition. Although the large multiphase PGM grains are important reservoirs of palladium, they cannot be regarded as the source of palladium for Pd-rich pentlandites for the following reasons.
Pt and Pd are more strongly bonded to semimetals such as Te, Bi, Sb and As than to S [
50,
51,
52,
53]. Thus, if a bond of palladium with a semimetal atom is formed, the breaking of such a bond and the new formation of a bond between palladium and sulfur is very unlikely.
Liquid immiscibility implies equilibrium between coexisting liquids and crystallizing phases. If separation of a PGE-semimetals liquid from a sulfide liquid ever occurs, PGE and semimetals would be concentrated in the former. Upon cooling, the separation of components between the liquids will also increase, and consequently, the partitioning of palladium from the PGE semimetal liquid back into the sulfide liquid or solid sulfide matrix is very unlikely.
As opposed to the metamorphic reaction suggested for the J-M Reef, Stillwater, including Pd release [
20], in our case, there is no single evidence of breakdown or dissolution of any PGM phases or multiphase PGM grains. Thus, a source of palladium from PGM grains is not manifested.
If PGM grains were to be a source of palladium, then the highest Pd contents in pentlandite would be those occurring in very close proximity or direct contact with them. However, Pd content in pentlandite does not depend on the proximity or contact with PGM grain (
Figure 8f and
Figure 9b).
The above stated arguments do not allow the multiphase PGM grains to be considered a source of palladium for studied Pd-rich pentlandites. Thus, the considered hypotheses cannot fully explain the occurrence of Pd-rich pentlandite in the ores of the Southern-2 ore body, and a new mechanism must be proposed.
5.3. Origin of Pd-Rich Pentlandite in the Southern-2 Ore Body
According to the petrographic and geochemical features revealed in this study, four aspects are considered crucial in the interpretation of the genesis of Pd-rich pentlandite in the Southern-2 ore body: (1) the rather small size of the areas containing Pd-rich pentlandite (
Figure 6b); (2) the extremely high concentrations of Pd, reaching up to 11.26 wt.% (
Table 3); (3) the pretty sharp variations in the content of palladium in pentlandite in short distances (
Figure 6b); (4) the irregular (non-zonal) distribution of palladium in individual grains of pentlandite (
Figure 8f and
Figure 9b). Nevertheless, the main problems concern the source of palladium as well as the mechanism of enrichment.
The hypothesis claiming that residual sulfide liquid could be a source of palladium seems very compelling. Most of the areas containing Pd-rich pentlandite are spatially associated with multiphase PGM grains, which are the result of the separation of an immiscible PGE-semimetal liquid from a residual sulfide melt at the latest stages of crystallization of sulfide liquids [
51,
52]. Since large droplets of such immiscible liquid occur evenly along the section of cubanite and chalcopyrite ores, the residual liquid could have been remained among the ISS cumulate in the form of enclaves during crystallization of the ISS. Therefore, most of the palladium should have been contained in the immiscible PGE-semimetal liquid. However, at a high Pd/semimetal ratio, part of the palladium would form a bond with sulfur and thus would be in the coexisting sulfide liquid [
16]. Crystallization of such a liquid could lead to the occurrence of Pd-rich ISS and consequently palladium-rich pentlandite. In this case, the map in
Figure 6b shows the enclave of the crystallized residual sulfide liquid together with the immiscible PGE-semimetal liquid. However, this completely contradicts the morphological features of pentlandite and the distribution of palladium in it.
It may be assumed that large crystals of pentlandite (
Figure 4e,f) were formed as a result of cotectic or eutectic crystallization of ISS and PnSS. Most of palladium thus should enter into the PnSS during the crystallization of PnSS from the liquid. Additionally, the distribution of palladium should be uniform in general, due to the high diffusion rate of the elements at magmatic temperatures. If this homogenization did not happen during crystallization from the melt, then it should have been completed during subsolidus re-equilibrations by diffusion. Pentlandite crystals have a network of chalcopyrite lamellas, and large areas of pentlandite are replaced by either cubanite or chalcopyrite (
Figure 4f and
Figure 8a,d). This may indicate an intense cation exchange of Cu, Fe and Ni within the PnSS crystal below the solidus. Furthermore, microprobe studies have shown that the composition of ore-forming pentlandite is the same within one sample. Consequently, the homogenization of the concentrations of the main components was rather effective. If Pd entered the PnSS at the magmatic stage of crystallization, then its concentration should have been equally distributed within the studied grain as well as the concentration of Fe, Ni and Cu. In other words, those PnSS crystals that crystallized from the residual sulfide liquid should have a slightly higher concentration of palladium than others and an even distribution of palladium in the grain. However, the distribution of palladium in large crystals of pentlandite is markedly uneven (
Figure 9). Moreover, the presence of lamellae of Pd-rich pentlandite in cubanite (
Figure 8f) contradicts this assumption. If PnSS were to be crystallized from the melt, then palladium would be consumed during its crystallization. After that, there should be no source of palladium for the Pd-rich pentlandite lamellae.
In our opinion, both lamellae and granular pentlandite in cubanite and chalcopyrite ores occurred as a result of ISS exsolution. If a portion of the ISS crystallized from the residual Pd-rich sulfide liquid, then the exsolution of such ISS should lead to the occurrence of Pd-rich PnSS. Nonetheless, a long sequence of subsolidus transformations of PnSS into pentlandite, chalcopyrite and cubanite should have led to a homogenization of the composition with respect to palladium as well as Cu, Fe and Ni.
Because of the extremely irregular distribution of palladium in pentlandite grains (
Figure 8f), palladium enrichment occurred once PnSS subsolidus transformations were completed. Therefore, the enrichment of pentlandite in palladium is a superimposed process, which is not associated with either crystallization of the sulfide liquid or subsolidus transformations of sulfides. The map of palladium distribution (
Figure 8f) confirms the superimposed nature of the process. This map shows that dramatic enrichment in palladium occurs in local areas associated with neighboring pentlandite grains or closely spaced parts of these grains, regardless of whether the lamellae or large crystals were enriched. One could easily assume the direction of the diffusion along which the palladium was supplied. Arrows in
Figure 8f show these directions. However, there is a problem with the source of palladium for a late superimposed process which needs to be addressed.
Godel and Barnes proposed the possibility of Pd transfer, from a source outside, by a fluid into pentlandite for the J-M Reef in Stillwater [
9]. However, in our study, it seems challenging to invoke for an outsider Pd-rich source in the Southern-2 ore body. Many ore bodies comprising massive sulfide ores of the Talnakh and Oktyabr’sky deposits have aureoles of metasomatically altered rocks containing sulfide and PGE mineralization. These metasomatic changes, including the formation of sulfide impregnations, are caused by fluid released during the crystallization of the same ore body. The concentration of copper, Pt, Pd and other incompatible elements in the 100% sulfide calculation [
23] of altered rocks is higher than in the massive sulfide ores. Sulfide impregnation is clearly visible in igneous and sedimentary rocks near the contact with the ore body. However, the influence of such metasomatic fluid on the sulfide ores of the ore body itself is unrevealed because sulfide minerals deposited from this fluid are difficult to detect among the primary massive sulfides. Fluid inclusions in sulfide minerals are evidence of the effect of such a fluid (
Figure 12). There are large groups of inclusions (
Figure 12a), single inclusions or chains of inclusions (
Figure 12b). Some of them are euhedral in shape (
Figure 12b) and contain Pd-bearing daughter mineral phases (
Figure 12a,b). The presence of these indicates the impact of the fluid, which could have been the source of palladium for the enrichment of some pentlandite grains in palladium. Currently, there is no way to determine the source of this fluid with supporting evidence. This could well be either a neighboring ore body or some part of the Southern-2 ore body that solidified later than the studied section of the EM-7 borehole. An important fact, however, is the time gap between the occurrence and subsolidus transformations of ore-forming pentlandite and the process itself of enrichment with palladium.