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Zaccarini, F.;  Economou-Eliopoulos, M.;  Kiseleva, O.;  Garuti, G.;  Tsikouras, B.;  Pushkarev, E.;  Idrus, A. Platinum Group Elements Geochemistry and Mineralogy. Encyclopedia. Available online: https://encyclopedia.pub/entry/38971 (accessed on 15 November 2024).
Zaccarini F,  Economou-Eliopoulos M,  Kiseleva O,  Garuti G,  Tsikouras B,  Pushkarev E, et al. Platinum Group Elements Geochemistry and Mineralogy. Encyclopedia. Available at: https://encyclopedia.pub/entry/38971. Accessed November 15, 2024.
Zaccarini, Federica, Maria Economou-Eliopoulos, Olga Kiseleva, Giorgio Garuti, Basilios Tsikouras, Evgenii Pushkarev, Arifudin Idrus. "Platinum Group Elements Geochemistry and Mineralogy" Encyclopedia, https://encyclopedia.pub/entry/38971 (accessed November 15, 2024).
Zaccarini, F.,  Economou-Eliopoulos, M.,  Kiseleva, O.,  Garuti, G.,  Tsikouras, B.,  Pushkarev, E., & Idrus, A. (2022, December 19). Platinum Group Elements Geochemistry and Mineralogy. In Encyclopedia. https://encyclopedia.pub/entry/38971
Zaccarini, Federica, et al. "Platinum Group Elements Geochemistry and Mineralogy." Encyclopedia. Web. 19 December, 2022.
Platinum Group Elements Geochemistry and Mineralogy
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This entry is adapted from the peer-reviewed paper 10.3390/min12121565

Platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru) and osmium (Os) are known as the platinum group elements (PGE). These metals are characterized by peculiar physical and chemical properties, such as high melting points, resistance to oxidation and corrosion, strong conductivity and ductility. 

chromitite ophiolite platinum group elements

1. Introduction

Among the  platinum group elements (PGE), platinum and palladium have gained a great economic importance due to their use in oil refining, electronics, jewelry, the glass industry, medical applications and in the catalytic converter of vehicles to reduce the pollutants of exhaust gases [1][2][3]. Therefore, PGE and in particular platinum and palladium mine production have grown continuously since the second world war in response to the development of their modern applications.
Furthermore, considering the transformation towards a greener economy that started a few years ago and that PGE uses play an important role in reducing greenhouse gases, their price has increased rapidly [2]. The PGE are among the less abundant elements in the Earth [2]. However, local high PGE contents can be achieved through a combination of complex geochemical and geological processes that, in some cases, result in the formation of economic deposits [1][4]. Based on their geochemical behavior, the PGE have been divided in two subgroups: the Ir-subgroup (IPGE) consisting of Os, Ir and Ru and the Pd-subgroup (PPGE) consisting of Rh, Pt and Pd [5]. The IPGE are more refractory and less chalcophile than the PPGE. Therefore, the IPGE are more concentrated in chromitite, while PPGE tend to follow the sulfides [4]. For many decades, both the podiform and stratiform chromitites have been recognized as efficient collectors of the critical PGE. Chromitites start to precipitate at temperatures well above 1000 °C; therefore, most of them are enriched in the refractory IPGE, showing contents several orders of magnitude higher than their peridotites. Few chromitites contain economic amounts of the more valuable PPGE, such as the Upper Group 2 (UG2) stratiform chromitites of the Bushveld layered intrusion of South Africa, that still represent the only example of chromitite mined for the PGE recovery [1].
Additionally, the chromitites associated with the Ural–Alaskan concentrically zoned complexes are enriched in Pt and contain a great number of Pt-bearing minerals but, despite this enrichment, few of them are mined for the PGE recovery due to the small size of the mineralized bodies [6].
Two different types of chromitite have been reported within the ophiolites: (1) podiform chromitite that generally occurs in the mantle tectonite and (2) layered chromitite hosted at the base of supra-Moho cumulates, above the crustal sequence. About half a century ago, the podiform chromitites hosted in the mantle section of ophiolite complexes attracted the attention of economic geologists as a potential target for PGE recovery [7]. Several studies have shown that the majority of the ophiolitic chromitites are enriched in the refractory IPGE, and only a few of them show anomalous contents of the most economically significant PPGE.
It is generally agreed that the PGE enrichment in the podiform chromitites is due to the presence of the so-called platinum group minerals (PGM) that occur as tiny grains enclosed in the chromite crystals and, to a lesser extent, in the silicates of the chromitite matrix [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. This observation has promoted investigation of the chromitites in order to verify the mineralogical nature, size, morphology and textural position of PGM, with the aim to understand their origin and to provide useful information for their beneficiation during mineral processing. Consistently with their geochemical signature, the most abundant PGM described in the podiform chromitites consist of IPGE-minerals (IPGM), such as sulfides of the laurite-erlichmanite series, alloys in the Os-Ir-Ru system and sulfarsenides of Ir, Ru and Os [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The few ophiolitic chromitites enriched in Rh, Pt and Pd were also proved to contain abundant and complex PPGE minerals (PPGM). The main target of this contribution is to provide an overview of unpublished and existing data from the few ophiolitic chromitites enriched in PPGE and PPGM, in order to evaluate their economic potential. Their PGE geochemical and mineralogical aspects are discussed and combined with the chromite composition of their host chromitites, with the aim to understand the processes that caused their anomalous enrichment in PPGE and PPGM.

2. The PPGE Enrichment in Ophiolitic Chromitites: How Was It Achieved

Although Pt and Pd can circulate through soils in aqueous solutions [24][25], under conditions of partial to complete serpentinization, the PGE are expected to behave inertly [5] and, eventually, to be remobilized only at a small scale [7][19]. Therefore, the effects of low-temperature metamorphism or alteration on the PGE distribution are considered negligible. Most of the overviewed chromitites have been affected by serpentinization to different extents, but their PGE contents have been broadly unaffected, as only redistributions may take place during alteration events, hence the overall PGE compositions represent high-temperature magmatic processes. According to the paper by Barnes et al. [26], the so-called “standard model” was proposed to summarize the hypothesis about the behavior the PGE in mafic and ultramafic melts formulated in the last years. The principles that have been widely accepted and summarized by Barnes et al. [26] are selected and listed in the following.
(1).
The predominant control of the distribution of PGE, being extremely chalcophile under almost all conditions, is the interaction of the magmas with magmatic sulfide liquids [26].
(2).
The partition coefficients of PGE from silicate into sulfide liquids are estimated to range from the order of thousands [26].
(3).
The melting of magmatic sulfides hosted in the mantle source is a critical control of the PGE contents of the resulting melts. The presence of trace amounts of residual sulfide is enough to induce PGE depletion in mantle melts that are sulfide-saturated at source or, alternatively, the degree of partial melting is enough to remove all of the source sulfide and to dissolve it in the silicate melt [26].
(4).
The refractory IPGE are retained in the mantle during partial melting, they decrease in residual melts during fractional crystallization and they are enriched in ultramafic magmatic rocks, independently from the presence of sulfides. On the contrary, the more chalcophile PPGE behave oppositely in the absence of a magmatic sulfide phase [5][26].
(5).
The PGE, under the appropriate conditions, can be fractionated from one another during differentiation or partial melting of sulfide magmas [26]. Pd solubility in silicate melts is orders of magnitude higher than that of other PGE, which may be responsible for Pd depletion of the residual rocks [27][28][29].
Despite the many factors summarized above, the two main mechanisms controlling the behavior of the PGE during the crystallization of the host chromitite are the partial melting that the mantle source underwent and the crystal fractionation process. Ophiolite complexes represent different oceanic environments characterized by different degrees of partial melting, from low percentages of melting at some mid-ocean ridges (MOR) to higher values in supra-subduction zone (SSZ) complexes [30]. In order to extract all the PGE, including the most refractory, from their mantle source, in which they may occur as alloys and sulfides, high degrees of partial melting, up to 30%, are required [5]. Lower melting degrees between 20 and 25% will dissolve all the preexisting sulfides leading to PGE liberation, especially the PPGE, into the melts [31]. Most of the mantle-hosted ophiolitic chromitite formed because of the reactions between the residual mantle and percolating magma such as hydrous high-Mg boninitic melts in an SSZ geodynamic setting and the aluminous-rich melt formed in the MOR or in the back arc basin (BAB) environments of the ophiolites [30][32]. The composition of the chromites is related to the nature of the percolating melts, thus most of the Cr-rich chromitites precipitated in the mantle sections of an SSZ ophiolite, whereas Al-rich chromitites are typical of the MOR region. The degree of partial melting of the SSZ mantle is higher than those of the MOR mantle, suggesting that the Cr-rich and Al-rich chromitites should be IPGE- and PPGE-rich, respectively. However, on the basis of available data, this model was successfully applied only to the Bracco chromitite [33]. The PPGE enrichment over IPGE in the Al-rich chromitite of the Bracco ophiolite was explained with the low degree of partial melting of their mantle source. The low degree of partial melting was not high enough to remove all the PGE from the mantle, especially the refractory IPGE. The most incompatible elements, such as sulfur and the PPGE, were partially removed from the mantle, concentrated in the melt and subsequently incorporated in the Bracco chromite forming system [33]. According to Table 1, enrichments in PPGE have been reported in both Cr-rich and Al-rich chromitites; therefore, the model of the low degree of the mantle source as the main factor to control the unusual PGE distribution in the Bracco chromitites cannot be applied to all the overviewed chromitites in this contribution. Melts produced from partial melting of the mantle are either saturated or undersaturated in sulfur. In particular, magmas with boninitic affinity represent sulfur-undersaturated mafic melts, because they derived from a strongly depleted mantle that has lost most, if not all, of its original sulfide phase [31][32]. As a consequence, sulfur saturation is rarely achieved during the precipitation of Cr-rich chromitites hosted in the mantle section of SSZ ophiolite. Examples of these chromitites include those from Kraubath, Newfoundland, Loma Caribe, Veria, Leka, Osthammeren, Ospa–Kitoi, Khara-Nur, Ulan-Sar’dag, Herbeira and Shetland. For this reason, Escayola et al. [34] proposed that sulfur saturation was achieved also in boninitic magma by reintroduction of sulfur into the residual mantle via metasomatic fluids. On the contrary, tholeiitic magma can be sulfur-saturated [32] and the reaction between this melt and mantle tectonites can be suitable for the precipitation of the chromitite enriched in the more chalcophile PPGE, after the formation of an immiscible sulfide liquid followed by a fractional crystallization process as reported for the Sebuku chromitite [35]. The chromitites from Bulqiza-Tropoja-Tropoja, Thetford Mine, Troodos, New Caledonia, Zambales, Al‘Ays and some of Kraka occur in the cumulate sequence, above the petrographic Moho, of the host ophiolite, whereas those of Moa Baracoa, Nurali, Berit and very likely also those of Vourinos are located in the MTZ (Table 1 and references therein). The PPGE enrichment in most of these chromites, which are not hosted in the deep mantle, is probably due to the presence of an immiscible sulfide liquid formed from a differentiated melt that, during its migration from the mantle upwards to the MTZ and the cumulus zone, underwent extensive fractional crystallization processes. The original composition of the melt was initially boninitic or tholeiitic and reached the sulfur saturation during its strong differentiation or by assimilation of sulfur form an external source. According to Proenza et al. [36], the local enrichment in PPGE in the chromitite of Moa Baracoa was caused by their interaction with pegmatitic olivine-norite dikes that introduce the PPGE and sulfur to the chromitite originally only enriched in IPGE. The local enrichment of PPGE in the Cliff chromitite of Shetland ophiolite was attributed to a secondary alteration process that was able to upgrade the tenor of PGE [37].
Table 1. World-wide occurrences of PPGE-PPGM-rich ophiolitic chromitites (listed in alphabetical order according to the host country).
Country Ophiolite Name of the Deposit Age Type of Chromitite Chromite Composition PPGE Data * PPGM Data Ref.
Albania Bulquiza-Tropoja   Jurassic mantle-cumulate n.a. no yes [18]
Austria Kraubath   Early Paleozoic mantle Cr-rich yes yes [11][38]
  Hochgrossen   Early Paleozoic mantle Cr-rich yes no [11]
Canada Newfoundland Middle Arm Brook Lower Ordovician mantle Cr-rich yes yes [34]
  Thetford Mine   Lower Ordovician cumulate Cr-rich yes yes [39][40]
Cuba Moa Baracoa Potosi Upper Jurassic-Lower Cretaceous MTZ Al-rich yes yes [36][41]
Cyprus Troodos   Cretaceous cumulate Cr-rich yes no [9]
Dominican Republic Loma Caribe Loma Peguera Jurassic-Cretaceous mantle Cr-rich no yes [21][42]
Greece Pindos Korydallos, Pefki Middle-Upper Jurassic mantle Al-rich yes yes [43][44][45][46][47][48]
  Vourinos Pefka, Rodiani Jurassic-Cretaceous mantle Cr-rich yes no [49]
  Otrhys Eretria-Tsangli Jurassic-Cretaceous mantle Al-rich yes yes [45][50]
    Aghios Stefanos            
  Skyros Island Achladones Jurassic-Cretaceous mantle Alr-rich yes yes [45][51]
  Veria Galaktos Jurassic-Cretaceous mantle Cr-rich yes yes [22][23]
Italy Bracco Ziona, Cima Stronzi, Jurassic mantle-cumulate Al-rich yes no [33]
    Canegreca, Mattarana,            
    Pian della Madonna,            
Indonesia Sebuku Island   Jurassic-Cretaceous mantle-cumulate Al-rich no yes [35]
New Caledonia New Caledonia Pirogue Cretaceous-Paleocene cumulate Al-rich yes yes [12][13]
Norway Leka   Lower Ordovician mantle-cumulate n.a. yes yes [52]
  Osthammeren   Lower Ordovician mantle n.a. no yes [10]
Philippine Zambales Acoje Eocene cumulate Cr-rich yes no [53]
Portugal Braganca Derruida Paleozoic n.a. n.a. yes yes [54]
Russia Nurali CHR II Paleozoic MTZ Al-rich yes yes [55][56][57]
  Ospa–Kitoi   Proterozoic mantle Cr-rich yes yes [58][59]
  Khara-Nur   Proterozoic mantle Cr-rich yes yes [58][59]
  Ulan-Sar’dag   Neoproterozoic mantle Cr-rich yes no [60]
  Kraka Wets Saksey,
Loginovskoe,
Babay		 Paleozoic cumulate Cr-rich yes yes [61]
    East Saksey,
Laktybash,
Khamitovskoe,
Maly Apshak,
Bol’shoy Apshak		   mantle Al-Cr-rcih yes ues  
Saudi Arabia Al‘Ays   n.a. mantle-cumulate Cr-rich yes yes [30]
Spain Herbeira   Paleozoic mantle Cr-rich yes yes [62][63]
Turkey Berit   Cretaceous MTZ Al-rich yes yes [64]
United Kingdom Shetland Harold’s Grave Early Paleozoic mantle Cr-rich yes yes [65][66]
    Cliff            
USA Rattlesnake Creek Pole Corral n.a. n.a. n.a. yes no [67]
* = published analyses with PPGE-IPGE ratio equal to or higher than 1; n.a. = not available; Ref. = References.

3. PGM from the Magmatic Stage to Their Alteration History

Although serpentinization has not significantly changed the PGE content in the overviewed chromitites, the PGM, after their magmatic precipitation, can be altered and transformed by low-temperature hydrothermal fluids. In podiform chromitites, two different occurrences of PGM have been described: (1) tiny and polygonal inclusions in chromite mainly consisting of Os-Ir-Ru alloys, sulfides such as laurite and erlichmanite and sulfarsenides, namely irarsite and hollingworthite; (2) interstitial PGM at contacts with chlorite and serpentine, generally characterized by an irregular shape and complex mineralogical assemblage, including Os-Ir-Ru alloys and oxides in the IPGE-rich chromitites and a great variety of Rh-Pt-Pd phases in association with Fe-Ni-Cu sulfides and awaruite in the PPGM-rich chromitites. The PGM which are included in chromite crystals are more easily preserved from the alteration processes, whereas the interstitial PGM can be modified by low-temperature secondary hydrothermal fluids and oxidized during weathering. Based on their composition, morphology and mineralogical association, an order of crystallization of the PGM associated with ophiolitic chromitites was suggested. Minerals in the Os-Ir-Ru alloys system, followed by sulfides of the laurite-erlichmanite series and part of the sulfarsenide irarsite and hollingwhortite are the first to crystallize at temperatures above or around 1000 °C, prior or concomitantly with the host chromite. The precipitation of these PGM is mainly controlled by the sulfur ƒ(S2) and arsenic ƒ(As) fugacities in the mantle [16][17][68]. In particular, the ƒ(S2) during the precipitation of chromitites is expected to increase with decreasing temperature. Although at this stage the sulfur saturation is not achieved, minerals of the laurite-erlichamnite series start to precipitate [16][17][68][69][70]. The presence of abundant PPGM associated with magmatic Ni-Cu-Fe sulfides occurring in the silicate matrix of the host chromite in the PPGE-rich ophiolitic chromitites can be considered the witness of the presence of an immiscible sulfide liquid during their crystallization. When the magma reaches sulfur saturation, an immiscible sulfide liquid may segregate from the silicate melt, collecting the more chalcophile PPGE available in the system. Therefore, the researchers can argue that the enrichment in PPGE in most of the overviewed ophiolitic chromitites was achieved thanks to the formation of an immiscible sulfide liquid becoming trapped interstitially to chromite crystals at a magmatic temperature, as a consequence of a crystal fractionation process. During the cooling of the PPGE-rich immiscible sulfide liquid, the PPGM start to precipitate together with Ni-Cu-Fe sulfides that may contain PPGE in solid solution as described for the Braganca chromitite [54]. The same model was previously applied to explain the PPGE enrichment in UG2 stratiform chromitites of the Bushveld layered intrusion of South Africa [4], and later also to the Korydallos [46], Al‘Ays [30], Newfoundland [34], Nurali [55], Sebuku [35] and Zambales [53] ophiolitic chromitites overviewed in this contribution.
After their crystallization at the magmatic stage, the PPGM and their associated sulfides, being hosted in the silicate matrix of the host chromitite, are more vulnerable to secondary alteration processes [7]. During serpentinization, the magmatic PPGM and Ni-Cu-Fe sulfides underwent desulfurization, generating awaruite and alloys characterized by variable Pt-Pd-Rh-Cu-Ni-Fe assemblages. The occurrence of secondary PPGM containing Sb, As, Bi, Te, Sn, Hg, Pb and Au (Table 2 and references therein) suggests that these elements were originally present in the differentiating magmatic sulfide liquid or, alternatively, they have been introduced by an external source transported by serpentinizing hydrothermal fluids that caused alteration of the host ophiolite [34][37]. Where alteration was most intense, magmatic PGM were transformed to PGE-bearing oxides, which reflects a further alteration stage in the sequence of low-temperature genesis of PGM, as documented in the chromitites from Pindos, New Caledonia, Sebuku, Nurali, Al‘Ays, Herbeira and Berit [12][30][35][46][56][62][64]. Despite the evident mineralogical reworking and alteration of the PGM, the data summarized in this contribution suggest that the low-temperature secondary processes that affected the overviewed chromitites caused only a small scale redistribution of PPGE, without changing the whole-rock magmatic PGE contents.
Table 2. PPGM identified in the PPGE-enriched ophiolitic chromitites.
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Rhodium                                          
Approved minerals                                          
Bowieite Rh2S3 x x   x               x                  
Cuprorhodsite CuRh2S4 x x   x               x                  
Kingstonite Rh3S4 x                                        
Hollingworthite RhAsS x x         x x   x   x x x       x x x x
Minakawaite RhSb                                   x      
Zaccariniite RhNiAs     x     x       x x         x   x      
Unnamed minerals                                          
RhTe x                                        
Rh2As x                                        
Rh2S3   x                                      
Rh5S4                       x                  
Rh2SnCu       x                       x          
Unidentified minerals *                                          
Rh-Pd-Sb     x                                    
Rh-As                                   x      
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Platinum                                          
Approved minerals                                          
Braggite (Pt,Pd,Ni)S       x     x                       x    
Cooperite (Pt,Pd,Ni)S x     x       x       x     x   x        
Genkinite (Pt,Pd)4Sb3                                         x
Geversite Pt(Sb,Bi)2                               x   x      
Hongshiite PtCu                                         x
Isoferroplatinum Pt3Fe x     x               x       x          
Malanite CuPt2S4 x     x               x                  
Platarsite PtAsS x x           x   x     x x              
Platinum Pt x                                        
Sperrylite PtAs2   x     x   x   x     x x     x   x x x x
Tetraferroplatinum PtFe x     x                     x       x    
Tulameenite Pt2FeCu x                     x                  
Unnamed minerals                                          
Pt(Ni,Fe)3           x x                            
Pt6Cu                                   x      
Unidentified minerals *                                          
Pt-Cu x     x                       x          
Pt-Pd-Cu                           x   x          
Pt-Pd-Au                                       x  
Pt-Pd-Cu-Au                                         x
Pt-Fe-Cu                       x   x     x        
Pt-Pd-Cu-Ni-Fe     x x                     x   x x      
Pt-Fe   x         x       x   x   x   x     x  
Pt-Ir                           x              
Pt-Ir-Fe-Ni           x                              
Pt-Ru-Rh                       x                  
Pt-Ir-Ru-Os                               x          
Pt-Pd-S                             x            
Pt-Pd-Rh                                   x      
Pt-oxides                       x     x     x x    
Cu-Pt oxides                                       x  
  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Palladium                                          
Approved minerals                                          
Atheneite (Pd,Hg,Pt)3As                           x              
Cabriite Pd2SnCu       x                                  
Froodite PdBi2                           x              
Keithconnite Pd3−xTe x x                                      
Mertierite II Pd8(Sb,As)3   x                                      
Nielsenite PdCu3             x                            
Paolovite Pd2Sn             x                            
Plumbopalladinite Pd3Pb2                                 x        
Potarite PdHg x x                       x   x     x   x
Skaergaarite PdCu             x                            
Sobolevskite PdBi                           x              
Stibiopalladinite Pd5Sb2   x                   x x               x
Vincentite (Pd,Pt)3(As,Sb,Te)                                       x  
Zvyagintsevite Pd3Pb             x                       x    
Unnamed minerals                                          
Pd3Fe   x                                      
Cu6Pd                                   x      
PdAs2 x                                        
Unidentifined minerals *                                          
Pd-Cu x                                        
Pd-Cu-Sb                                   x      
Pd-Rh-Sb     x                                    
Pd-Pt-Fe-Cu-Ni                             x            
Pd-Pt-Cu-Sn-Pb             x                            
Pd-Sn                           x           x  
Au-Pd             x                            
Pd-Te                                   x      
Pd-As                                   x      
Pd-As-S                                       x  
Cu-Pd-Au             x                         x  
Pd-Pt-Au                                          
Pd oxides             x                     x x    
1 = Bulquiza-Tropoja [18] (Albania); 2 = Kraubath [38] (Austria); 3 = Newfoundland [34], 4 = Thetford mine [39][40] (Canada); 5 = Moa Baracoa [41] (Cuba); 6 = Loma Peguera [21][42] (Dominican Republic); 7 = Pindos [43][46][47], 8 = Othrys [50], 9 = Skyros [51], 10 = Veria [23] (Greece); 11 = Sebuku [35] (Indonesia); 12 = New Caledonia [12][13]; 13 = Osthammeren [10] (Norway); 14 = Braganca [54] (Portugal); 15 = Nurali [55][56][57], 16 = Ospa–Kitoi, Khara-Nur [58][59], 17 = Kraka [61] (Russia); 18 = Al‘Ays [30] (Saudi Arabia); 19 = Herbeira [62] (Spain); 20 = Berit [64] (Turkey); 21 = Shetland [65][66] (UK). * = PGM without a precise stoichiometry.

4. Are the PPGE- and PPGM-Rich Ophiolitic Chromitites Economic for PGE Recovery?

The PGE are listed among the rare, noble, critical and most valuable metals in nature. Presently, more than 90% of the PGE production comes from two countries, Russia and South Africa. Economically mineable PGE deposits are also found in the layered intrusions of the Great Dyke (Zimbabwe) and Stillwater Complex (USA) as well as in the Sudbury Basin in Canada [1][2]. PGE production in Russia is dominated by palladium, largely occurring in the Cu-Ni sulfide ore deposits of the Norilsk-Talnakh district, which belongs to the Siberian traps province [71]. The Bushveld layered intrusion in South Africa is known for its large platinum and palladium resources and, according to Cawthorn [1], there are enough PGE reserves to supply world demands for centuries using the current mining techniques. In the Bushveld, the PGE are recovered from three very different ore bodies, namely the Merensky Reef, the UG2 chromitite and the Platreef. Now, the stratiform UG2 chromitites, which extend for nearly the entire 400 km length of the eastern and western limbs of the Bushveld complex, are the only example of chromitites that are mined also for the PGE recovery, being extremely enriched in PPGE. In the UG2 chromitites, the PPGM generally occur associated with sulfides in the interstitial magmatic silicates, and are only rarely enclosed in fresh chromite, similarly to those described in the PPGE- and PPGM-rich ophiolitic chromitites. Although the PGE content may be relatively high, up to 81,867 ppb as detected in one sample from Shetland, presently, the ophiolitic chromitites are not economic considering that: (1) few podiform chromitites are enriched in PPGE and (2) the PPGE enrichment is only local and randomly distributed in chromitites with a small size.

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Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Federica Zaccarini
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Maria Economou-Eliopoulos
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Olga Kiseleva
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Giorgio Garuti
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Basilios Tsikouras
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Evgenii Pushkarev
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Arifudin Idrus
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