The minerals of noble metals ( the elements of platinum group and gold) in chromitites of ultramafic complexes are well known in various tectonic structures of continents and oceans. Many researches are devoted to the investigation of chromitites and accessory PGM. But, despite of the low average contents of PGE in chromitites (n*10 - n*100ppb), the lists of noble metal minerals have dozens of mineral types.

The data given on photos 1 - 38 concern the Syum-Keu massif. These data in a combination to the data on mineralogy of noble metals of the Outocumpu ophiolite complex (see "PGMs in Outocumpu Ophiolite Complex"), allow to observe both common features and distinctions of PGE mineralization in these massifs.

Syum-Keu chromitites

Outocumpu chromitites

Crystallisation events            Primary euhedral PGMs:            Laurite (LR),            Erlichmanite (ER),            Sperrylite (SPR)            Fig. 1-6                                           Aftercrystallization                             thermal events                                         Replacement of primary                                         LR, ER, SPR by                                         Irarsite (IRS),                                         Hollingwortite (HGW),                                         Ni-sulphides                                          Fig. 7-14                                                        Native events I                                  Ir-Os, Ru, Rh, Pt-Ni, Ni-Fe            Fig. 9, 15-19                              Pd-As events                                                    The (PGE)(Te,Bi,Hg) appearance                                                                with arsenides.                                                                  Fig. 20-24                                                                                                                  Native events II              Native Au, Ag, Cu, Zn,                       Sulphide                             Fe, Pb, Sn, Bi            mineralization              Fig. 25-34                                    Fig. 35-38

Crystallisation events              Primary euhedral PGMs:              Laurite (LR),              Erlichmanite (ER),              Sperrylite (SPR)            Fig. 1-3                                           Aftercrystallization                             thermal events                                         Replacement of primary                                         LR, ER, SPR by                                         Irarsite (IRS),                                          Fig. 4-7                                                        Native events I                                  Not detected                                                           Pd-As events                                         Pd mineralization not detected                                          As-PGM - Anduoite: (Ru,Os)As2,                                                Gersdorffite: (Ni,Ru,Os,Ir)AsS.                                          Bi-Te phases- Hedleyite : Bi7Te3,                                                                   NinBim, NiAs,                                                 Fig. 16,17                                                                                                                 Native events II                  Native  Pb, Sn                            Sulphide                 Fig. 18                              mineralization                                                               (present, but not shown)

The data given in the table allow to allocate the main stages of formation of PGE mineralization in chromitites of Syum-Keu and Outocumpue massifs and to carry out their comparison.

First two stages - Crystallisation and Aftercrystallization thermal events - almost precisely coincide in chromitites of both massifs: in both massifs idiomorphic crystals of laurite, erlichmanite and sperrylite are present. They are formed during crystallisation of chromitites. The presence of laurite grains in chromite with the high contents of Al₂O₃ (Fig.1) testifies it.
Idiomorphic grains of laurite, erlichmanite and sperrylite are replaced by irarsite hollingwortite and Ni-sulphide. These changes, probably, are connected with autometamorphic stage of chromitites transformation. (The exact identification of Ni-sulphides is complicated and there is no complete reliance of their diagnostics. It is possible, that these Ni-sulphides are not mineral phases themselves, but the fragments of laurite, enriched with Ni).

Post-crystallisation history of Syum-Keu and Outocumpu chromitites has much more distinctions, than similar features. It demonstrates that on the further history of massifs the main influence is rendered by specific features of a geological structure and development of regions, but not the features of mantle sources and not the process of intrusion of mantle matter into the upper horizons of the Earth crust. In other words, the post-crystallisation and post-autometamorphic history of massifs is a history of ultramafic complexes in the conditions of the crust.

The subsequent stages of PGE mineralization are connected with the processes of transformation of already generated chromitite horizons.They proceed with supply of such chemical elements as Pd, Bi, Te, Hg and partly As, which are absent or not typical for chromitites.

The specific feature of PGE mineralization of Syum-Keu chromitites is the well expressed stage of native mineralization: Ir, Os, Ru, Rh, Pt-Ni and Fe-Ni alloys (Native events I) are present. The last one is not founded out in investigated Outocumpu chromitites. Pd and Hg mineralization is not founded out also, and the stage " Native events II" is also reduced considerably.

It is possible to compare stages of PGE mineralization in chromitites to regional geological processes resulting in occurrence of this mineralization. For that case it is necessary to carry out the detailed work not only on mineralogy of PGM, but also on both petrography and mineralogy of rock-forming and sulphide minerals.

For Outocumpu chromitites it is possible to try to link the occurrence of (Ir, Os, Ru, Rh)-gersdorffites with one of the stages of younger regional metamorphism. But the stages of PGM formation in Syum-Keu chromitites are very much risky to interpret owing to insufficient geological study of ultramafic massif itself.

In summary it is important to specify one more obvious conclusion: for reconstruction of a history and genesis of PGE mineralization in anyone ultramafic complex it is completely not enough to base only on definitions of PGE concentration in the rocks and to plot the chondrite-normalised diagrams. Without the data on mineralogy - the history and genesis of PGE is "a black cat in a black room".

The given material shows (and it is well known from the literature), that the formation of PGE mineralization takes place in some occurring at different times stages and is accompanied by differentiation of PGE which may be studied by mineralogy only. Therefore, the definition of concentration of elements in a sample gives the researcher only figures. But if to obtain and operate only with these concentration figures, the information on genesis and information on history of formation of PGE mineralization (and ultramafic complex as well), is completely destroyed.

If the purpose of research is a knowledge about the nature of any phenomenon, it is necessary to choose such technique, which does not destroy the information on this phenomenon during obtaining the information.

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