RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES3002, doi:10.2205/2006ES000196, 2006
Conclusion
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Figure 21
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[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).
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Figure 22
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Figure 23
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[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].
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Figure 24
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Figure 25
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[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.

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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