RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3002, doi:10.2205/2006ES000196, 2006

Conclusion

2006ES000196-fig21
Figure 21
[33]  Our materials on the rocks of the lherzolite-harzburgite, dunite-harzburgite, and dunite complexes (which are either not altered at all or were affected only by retrograde metamorphism) suggest a weak correlation between the compositions of their ore and accessory Cr-spinels: dunites contain high-Cr accessory Cr-spinel-chromite, and lherzolites and harzburgites contain, respectively, high-Al aluminochromite and Cr-picotite (Figure 21). A striking example of the aforesaid is offered by the Kempirsai Massif [Pavlov and Grigorieva-Chuprynina, 1973]. Other ultrabasites metamorphosed during the retrograde stages show some inconsistencies between the compositions of their ore and accessory Cr-spinels. This can be illustrated by the examples of the Tsentralnoe and Zapadnoe economic deposits of high-Cr ores of the Rai-Iz Massif, whose orebodies consisting of chromite and subferrialuminochromite are hosted by rocks with metamorphic accessory spinel of high-Fe composition: ferro-ferri Cr-spinel (Figure 22).

2006ES000196-fig22
Figure 22
2006ES000196-fig23
Figure 23
[34]  An efficient method for the evaluation of the Cr2O3 concentration in spinel is infrared spectroscopy. The position of IR absorption bands in the spectra depends on the composition of the Cr-spinel, and this makes it possible to rapidly and accurately enough evaluate the Cr2O3 concentrations in large collections of samples. In the course of studying the Polar Urals, we have obtained and interpreted more than 10000 IR spectra of Cr-spinels. Maps were prepared (Figures 22 and 23) that show the variability of this parameter over an area of approximately 2000 km2 for all of the three massifs in the Polar Urals [Makeyev, 1990, 1992a, 1992b; Makeyev and Bryanchaninova, 1999].

2006ES000196-fig24
Figure 24
2006ES000196-fig25
Figure 25
[35]  We conducted mineralogical mapping at these three ultrabasite massifs in the Polar Urals: Syum-Keu, Rai-Iz, and Voikar-Syninskii. Figures 22, 23, 24, and 25 demonstrate the potentialities of large-scale mineralogical mapping, as illustrated by the example of application of these techniques at the Rai-Iz Massif. The reader can find all other mineralogical and prediction maps for the whole Polar Uralian ultrabasite belt in the monograph [Makeyev and Bryanchaninova, 1999]. The criteria used for the mapping included the dunite constituent (Figures 22a, 23b, and 24) in the rock complexes (this was the basis used to prepare the geological maps of the ultrabasite massifs), the Fe mole fraction (Figures 22b and 25b) of the rock-forming olivine (this parameter was evaluated from the indices of refraction of this mineral or was calculated from its microprobe analyses and makes it possible to reliably localize orebodies), and IR spectroscopic data (Figures 22d, 23d, 25a, and 25b) on the accessory and ore Cr-spinel (these data allowed us to assay the composition the quality of the Cr ores). The materials were generalized in the form of a prediction map of the chromite potential of the massif (Figure 25d).

[36]  Areas highly promising for exploration for chromite ores are those where the dunite-harzburgite complex with high dunite contents contains accessory Cr-spinel corresponding to chromite or ferrichromite in composition (with the n1 parameter of the IR spectra ranging from 620 cm-1 to 635 cm-1 ). The Cr concentration in the accessory Cr-spinel of the dunites increases toward chromite orebodies. Conversely, the areas (fields) of the harzburgite and dunite-harzburgite complexes with low dunite contents and high Al concentrations in the accessory Cr-spinel (with the n1 parameter lower than 600 or higher than 640 cm-1 ) are not promising as exploration targets for Cr ore mineralization.

[37]  Blocks of ultrabasite rocks most promising for exploration for large deposits are those with large dunite bodies and with adjacent fields of the dunite-harzburgite complex containing much dunite. These areas are characterized by low contents of pyroxene in the harzburgite (10-12%), accessory Cr-spinel high in Cr, and relatively Fe poor olivine from the dunite composing the ore zone.


RJES

Citation: Makeyev, A. B. (2006), Typomorphic features of Cr-spinel and mineralogical prospecting guides for Cr ore mineralization, Russ. J. Earth Sci., 8, ES3002, doi:10.2205/2006ES000196.

Copyright 2006 by the Russian Journal of Earth Sciences

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