© NATI Research JSC, 2005

       Introduction       Results      Concluding remarks

Authors

Top        Introduction

Zonal dunite-clinopyroxenite-gabbro Ural type massifs produce Pt, Ir, and Os placer deposits that actually have been mined in Russia since the nineteenth century. Chromite schliers in dunites are considered to be a source of placer platinum, the former "being the result of low temperature autoreaction-scarn process emerged in dunites after their consolidation" (Pushkarev et. al., 2002). However, the origin of platinum mineralization in dunite, outside chromite schliers as well as that of clinopiroxenites, which remained unaltered under the later thermal impact, is still not studied enough.

Fig. 1.
In numerous publications on geology and platinum-metal ore-formation of zonal Ural and Kamchatka complexes (Efimov, 1984; Koriak-Kamchatka…", 2002; Lazko, 1988; Nazimova et al., 2003; Pushkarev et al., 2002) the importance of the studies of both dunites and clinopyroxenites and the features of their mutual contacts have been emphasized.

To find out the special features of PGE mineralization at the dunite-clinopyroxenites intersection in Kytlym and Galmoenan zonal massifs (fig. 1) from four cross-sections have been collected and investigated 79 rock samples from nucleus dunites through by-contact dunites (or "metadunites", Efimov, 1984) to the clinopyroxenites. The obtained samples didn't contain the material of chromite schliers or recrystallized large- giantgrained dunites, with very high PGE content, that makes themselves having placer-forming significance. In opposite, the PGE content in dunite and clinopyroxenite samples is not more than 10-30 ppb that requires the usage of high-sensitive mineralogy studies technology: "ppm - mineralogy" (Knauf, 1996; see details on www. natires. com).

Top    Results

ROCK CHARACTERISTICS

Nucleus dunites consist of olivine (95-97 vol. %) and chromite (3-5 vol. %) and possess polygonal granoblastic texture. Olivine grains size is 0,3-0,7 mm, #Fe (Fe2/Fe2+Mg) - 8-10%. Chromite is represented by two morphological type grains: small euhedral grains inside olivine and bigger ahedral grains among olivine grains. Both types have similar composition: #Fe - 60.7-67.0, #Cr (Cr/Fe3+Cr+Al) - 52.7-58.4 and #Al (Al/Fe3+Cr+Al) - 20.2-22.5%. Olivine grains in metadunites are fragmentized and serpentinized. Olivine ferriferrousness is 10-11%. A part of chromite grains (#Fe - 50-58, #Cr - 62-64 and #Al - 15-20%) is replaced by magnetite (#Fe - 84-89, #Cr - 17-30 and #Al - 0-1%). Substitution takes place not only along grain borders (fig. 2b, c) but also inside the grains along defect zones of crystal lattice, followed by the formation of porous spots (fig. 2a), and it shows, that substitution taking place after fragile deformations in dunites (fig. 2b).

Fig. 2. Mineral alteration character
Abbreviations:   Ol – olivine, CPX – clinopyroxene, CRT – chromite, MGT – magnetite, SRP – serpentine, SLF – sulfides Cu, Ni.

Similar mineral transformation character is likely to be met in nucleus dunites as well, but it happens not so often. Clinopyroxenites are composed of diopside (85 vol. %, #Fe - 4-7%), altered olivine (up to 10 vol. %, #Fe - 12-13%) and magnetite (5 vol. %, #Fe - 89-99, #Cr - 0-9%). Clinopyroxene grains (1 - 5.5 mm) are euhedral. Olivine is represented by grains of two morphological types. The first type represented by euhedral relict serpentinized along the edges and crossing faults olivine grains among clinopyroxene grains (fig. 2d). The second type of olivine grains have an irregular form and partly serpentinized. The whole olivine-serpentine aggregation being replaced by later euhedral clinopyroxene (in fig. 2d the replacing area is shown in the circle). Shadow structure of replaced olivine grains is highlighted by thin magnetite inclusions inside new-formed clinopyroxene grains.

PLATINUM GROUP MINERALS

Two PGE mineral associations are being singled out (primary and secondary) for the following reasons: 1 - primary association minerals are presented by native minerals and alloys, and are developed in nucleus dunites, metadunites and clinopyroxenites, with grain morphology changing regularly along the cross-section from euhedral gains in nucleus dunite (fig. 2 e, f) to ahedral grains in clinopyroxenites (fig. 2 g). 2 - The secondary PGE association predominantly emerges in metadunites and is mostly developed in clinopyroxenites. Apart from native phases and alloys with Cu, Ni, Fe, Pd it includes Au, Ag and PGE minerals with S, As, Sb, Bi and O (12 mineral species) and is followed by alloys, sulfides and oxides of Fe, Co, Ni, Cu etc. (more than 20 mineral species). 3 - Secondary association minerals are developed over primary association minerals and don't forming euhedra grains. The primary association consist of intergrowth euhedral ferroplatinum alloys (Rh 0.7-1.5 mas. %) with Pt:Fe 3:1 and 2:1 ratios and native osmium. Native osmium is in acicular or lamellar forms with a clear boundarys (fig. 3 e, f).

Figure 3. Primary (e-g) and secondary (h-j) PGM associations.
Os – native osmium; FPT – ferroplatinum alloys; CRT – chromite; (Cu, Pt, Ni) – alloys; SLF – sulfide Cu, Ni; GV – geversit; SPR – sperrilite; Pt-Ox – platinum oxide; MGT – magnetite

The secondary association consist of platinum-copper (with platinum content range from 30.6 - 52.7 mas. %), nickel-platinum-cooper (~ 5%, Pt), tulameenite, tetraferroplatinum, Pd-Fe-Pt alloy, platarsite, geversite, hollingworthite sperrylite, erlichmanite, laurite, cooperite, sobolevskite, native gold and silver (fig. 3 g, k, i, j). Those mineral grains are of irregular form with rugged boundaries and they often develop over primary association minerals (fig. 3 h). Besides, the secondary association including different Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, sn, Sb, Au, Pb, Bi alloys, as well as Fe, Co, Ni, Cu sulfides. It is necessary to point out that the special features of the secondary association can be seen in nucleus dunites also, but it is very rarely and it doesn't change general trend of PGM transformation.

Top    Concluding remaks

1) As a result of cross-section investigations it has been stated that both rock forming and accessory minerals (including PGM as well), in dunite-metadunite-clinopyroxenite rock series are directly alterating, that giving reason to suppose the two-stage process of rock species formation in zonal complexes.
2) The recognition of a large number of PGM along dunite-metadunite-clinopyroxenite cross-sections with pointing out primary and secondary PGM associations gives basis to the usage of PGE accessory mineralization as an informative indicator of zonal complexes formation and transformation processes.


ACKNOWLEDGMENTS

The author is thankful to "Koryakgeoldobycha" JSC and it's employees for the opportunity of field sampling, to E.V. Pushkarev for his throughout assistance, to "NATI" JSC for the equipment kindly provided to us and for the laboratory research funding.

REFERENCES
1. Efimov A.A. Gabbro-ultramafic Ural complexes and ophiolite problem. Moscow, Science, 1984 (In Russ.).
2. Knauf V.V. On metrological maintenance of mineralogical works. Russian Mineralogical Society proceedings, part 75, № 6, 1996 (In Russ.).
3. Koryak-Kamchatka region - a new platinum-bearing province of Russia. Edited by V.P. Zaitsev, A.F. Litvinov, E.A. Landa,. SPb.: VSEGEI, 2002 ("Koriakgeoldobycha") (In Russ.).
4. Lazko E.E. Ultrabasic rocks of dunite-pyroxenite association. Igneous rocks, volume 5, Moscow, Science, 1988 (In Russ.).
5. Nazimova U.V., Zaitsev V.P., Mochalov A.G. Platinum group minerals of gabbro-pyroxenite-dunite Galmoanan massif of south Koryak highland (Russia). Geology of ore deposits, volume 45, № 6, pp. 547-565, 2003 (In Russ.).
6. Pushkarev E.V., Anikina E.V. Autoscarn nature of platinum mineralization in dunite - clinopyroxenite Ural-Alaska type complexes. Geology, genesis and the questions of complex precious metals assimilation. Moscow, IGEM RAS, 2002 (In Russ.).

       Introduction       Results       Concluding remarks