Chromite-PGE mineralization in the Uktus Alaskan-Type Complex (Central Urals, Russia) Giorgio Garuti1, V. Evgeny Pushkarev2, Federica Zaccarini1, Roberto Cabella3 and Elena Anikina2 1Department of Earth Sciences, University of Modena, (Italy) 2Institute of Geology and Geochemistry, Ekaterinburg (Russia) 3Department for the study of the Territory and its Resources, University of Genoa, (Italy) e-mail: [email protected] Introduction 60° 40’ It has been proposed that chromitites owe 60° 64° their high Pt (and Pd) content to the segregation of Pt-Belt magmatic sulfides during chromite precipitation (Naldrett and Von Gruenewaldt 1989). However, 56° 45’ 58° chromitites from Alaskan-type dunites apparently UKTUS contradict this assumption since they characterize for high Pt contents, although there is no evidence for the attainment of sulfur saturation during their 52° formation. In Alaskan-type chromitites, Pt is usually accompanied by Ir (i.e. St.Louis et al. 1986), thus emphasizing the anomalous behavior of Pt that is forced to precipitate with the refractory Ir, Dunite Gabbro being decoupled from the low-melting allied 2 km Clinopyroxenite-wehrlite elements Rh and Pd. The reason for the behavior of Chromite localities Pt in Alaskan-type complexes still remains poorly understood, being in apparent contrast with common mechanisms of PGE fractionation in Figure 1. Geological sketch map of the Uktus complex stratiform and podiform chromitites. This article (from Pushkarev 2000). provides evidence that the PGM assemblage and PGE distribution in the dunite-chromitite Composition of chromite association from the Uktus Alaskan-type complex Compositions of disseminated and massive (Central Urals) are strictly related with chromite from the Uktus dunite overlap the field of compositional changes in chromite and coexisting other Alaskan-type chromites of the Urals, and olivine, possibly reflecting variations of oxygen clearly distinguish from ophiolitic chromitites fugacity or silica activity below sulfur saturation. because of a remarkable iron enrichment in both the bivalent and trivalent sites (Fig.2). Some Geological Setting compositions may enter the “spinel gap” (Roeder The Uktus complex is located close to the 1994), thereby the chromite shows unmixing of Fe- southern end of the so-called “Platinum-bearing spinel lamellae inside Cr-magnetite as a result of Belt” of the Urals (Fig. 1). It covers an area of equilibration (Pushkarev 2000). The Fe3+# increases about 50 km2 and consists of dunite, up to figures of 0.45, and defines a positive 2+ 2+ clinopyroxenite, wherlite, olivine- and amphibole covariation trends with Fe # and TiO2. The Fe # gabbro arranged in a concentrically-zoned structure. of the chromite correlates negatively with the Details of the geology and petrology of the Uktus forsterite content of the coexisting olivine (Fig. 3), complex can be found in Pushkarev (2000). so that magnesiochromitite (Fe2+# = 0.32) is Chromitite in the form of tiny vermiculations and associated with Mg-dunite (Fo = 92-95%) in the wispy aggregates of a few millimetres is common southern dunite body, whereas the most Fe-rich in the dunites. Chromitite also occurs as veins and chromites (Fe2+# = 0.72) are found associated with schlieren of about 0.5-3 x 15-30 cm in size, and at the Fe-rich dunite (Fo = 80-84%), in the marginal one locality inside the southern dunite body, it zone of the northern dunite body. The oxygen forms lenses and layers extended up to some meters fugacity (fO2) calculated according to O’neil and along strike. Wall (1987) exceeds the FMQ buffer, and is remarkably in excess compared with values from ophiolites of the Urals (Pushkarev 2000). 1 2.5 2 0.9 1.5 2 0.8 1 TiO wt% 0.7 0.5 0 0.6 0 0.2 0.4 0.6 3+ # 0.5 Fe # Cr 0.4 0.5 0.3 Sp 0.4 ine l g 0.2 ap 0.3 3+ 0.2 0.1 Fe # 0.1 0 0 0 0.2 0.4 0.6 0.8 1 0.30.50.70.9 2+ 2+ Fe # Fe # Figure 2. Chemical composition of chromite from chromitite (dots) and dunite (circles) of the Uktus complex. Cr# = Cr/(Cr+Al), Fe2+# = Fe2+/(Fe2++Mg), Fe3+# = Fe3+ /(Fe3++Al+Cr). Fields of chromitites from the Urals, dark grey: ophiolites, light grey: Alaskan type. Relationships between chromite composition 0.8 and PGE mineralization Chondrite-normalized PGE patterns of the 2+ 0.7 Uktus dunites (Fig. 4) are characterized by a sharp 0.6 positive anomaly of Pt, as it is typical of Alaskan- 0.5 type dunites of the Urals (Garuti et al. 1997). The PGE profiles of chromitites vary from negative, 0.4 Chromite FeChromite # with the Os, Ir, and Ru predominating over Pt, Rh, 0.3 and Pd (Type Ia, b), to saw-like, with variable 80 85 90 95 anomalies of Pt and Ir (Types II and III). These Olivine Fo % PGE profiles correspond to three different types of PGM assemblage, and conspicuously are correlated 5 with changes in chromite composition and oxygen 0 MH fugacity (Fig. 4): Type Ia, b profile corresponds to PGM 2 -5 FMQ IW assemblages consisting of laurite (Ia), or kashinite -10 and cuproiridsite (Ib) with accessory irarsite and -15 tolovkite. These assemblages do not contain Pt Log (FOLog ) -20 minerals, and occur associated with -25 magnesiochromite having oxidation ratio lower 600 800 1000 1200 1400 than 0.35. Type II profile refers to PGM assemblages T°C dominated by Pt-Fe alloys with isoferroplatinum- type composition (Pt3Fe) and Ir-PGM (iridium, Figure 3. Chromite-olivine compositional relationships cuproiridsite, cuprorhodsite), along with accessory and variation of the oxygen fugacity (fO2) as a function of tulameenite, erlichamanite, malanite, cooperite, and temperature (T°C), according to the equation of O’Neill, very accessory geversite, potarite and prassoite. H. and Wall, V. (1987). Chromitite: dots, dunite: circles, Type II assemblage is associated with grey field : ophiolitic chromitites of the Urals (Pushkarev magnesiochromite having the highest oxidation 2000). MH: magnetite-hematite, FMQ: fayalite-magnet- ite-quartz, IW: iron-wustite. ratios (0.40-0.44). 0.7 10 0.6 III 0.5 II 0.4 II 1 3+ 3+ 2+ Ib Fe /(Fe0.3 +Fe ) Ia 0.2 0.3 0.4 0.5 0.6 0.7 0.1 Fe2+ # III 0.7 Samp le/chondrite 0.6 III 0.01 0.5 I a,b 0.4 II 3+ 3+ 2+ Ib 0.3 Fe /(Fe +Fe ) 0.001 Ia Os Ir Ru Rh Pt Pd Au 0.2 -7 -6 -5 -4 -3 -2 log (fO2 ) Figure 4. (Left) chondrite-normalized PGE patterns for chromitites and dunites (grey field) from Uktus. The PGE were determined by ICPMS after Ni-sulfide assay at the Modena University. Chondrite values from Naldrett and Von Gruenewaldt (1989). (Right) variation of the chromite oxidation ratio Fe3+/(Fe3++Fe2+) as a function of Fe2+# = Fe2+/(Fe2++Mg) and oxygen fugacity (fO2). Labels I (a,b), II, and III indicate type of PGE profile and the corresponding PGM assemblage (see text for explanation). Type III profile corresponds to a PGM isoferroplatinum with magnesiochromite at the assemblage consisting of irarsite, tulameenite, Rh- maximum state of oxidation would suggest that a Sb-S compound, Pt-Pd-Cu and Pd-Cu alloys and sharp drop of the Pt solubility in the silicate melt Pd-rich copper. It is associated with chromian was triggered by an increase in fO2, while fS2 was magnetite characterized by oxidation ratio of 0.59. keeping well below the sulfur saturation threshold The mineralogical data consistently indicate a slight (Amossé et al. 1990). The fact that Pt minerals are increase of Rh- and Pd- with respect to Pt-phases, absent in the magnesiochromite with low oxidation that causes attenuation of the Pt anomaly in Type ratio, and the PGM assemblage is dominated by III PGE profile. laurite and Ir-sulfides, would support this Most of the PGM occur as euhedral grains conclusion. included in the chromite, the texture indicating that 2) An alternative explanation is based on they are early magmatic phases mechanically the assumption that the low silica activity in entrapped in growing chromite crystals. However, Alaskan-type magmas favors crystallization of there is textural evidence that part of the PGM olivine instead of orthopyroxene and produces an assemblage (i.e. geversite, tulameenite, irarsite, increase of the Fe2O3 activity, so that the coexisting potarite, prassoite) formed at some late stage as a chromite has relatively higher oxidation ratios result of PGE re-mobilization by the action of compared with chromites derived from tholeiitic hydrothermal fluids. and high-alumina basalts (Irvine, 1967). The high iron activity has also a very strong effect in Concluding remarks enhancing stabilization of Pt-Fe alloys (Borisov and 1) The Uktus occurrence supports the Palme 2000), thereby considerable drop of the Pt contention that high concentrations of Pt in the solubility is to be expected in the magma. This Alaskan-type chromitites are originally due to process would be independent from variations in extensive stabilization of Pt-Fe alloys during fO2. chromite precipitation. The close co-precipitation of 3) The positive Pt anomaly observed in chromitites and dunites of the Alaskan-type Irvine, T.N., 1967, Chromian spinel as petrogenetic complexes may reflect a distinctive feature of their indicator. Part 2. Petrologic implications. parent melts, possibly inherited from a mantle Canadian Journal of Earth Sciences, 4: 71- source that had been previously enriched in Pt. This 103. Pt enrichment in the source region of the Alaskan- Naldrett, A.J. and Von Gruenewaldt G., 1989, type magmas might have been produced in the Association of Platinum-group elements course of second-stage melting of a depleted and with chromitites in layered intrusions and metasomatised mantle below the subduction zone ophiolite complexes.