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

Results and Discussion

[5]  The results of our long-term fieldwork and of the experimental and theoretical research of the mineralogy, petrography, and metallogeny of Uralian Alpine-type ultrabasites and the analysis of literature data on analogous rocks elsewhere led us to the following conclusions.

[6]  The evolution of the mineral assemblages of the Alpine-type ultrabasites occurred in six major stages: mantle, regional metasomatic, pegmatitic, retrograde and prograde metamorphic, and hypergenesis [Makeyev, 1992b; Makeyev and Bryanchaninova, 1999].

[7]  The Alpine-type ultrabasites evolved during the metamorphic stage in a closed system and could exchange only energy with the environment.

[8]  The driving forces of the evolution of the mineral assemblages were variations in the intensive parameters of the mineral-forming medium, namely, a temperature and pressure decrease, variations in the fluid regime and in the pH and Eh. Certain ranges of thermodynamic parameters, and thus, certain types of mineral assemblages, corresponded to the typomorphic rocks of each stage.

[9]  The lherzolites, diopside harzburgites, plagioclase lherzolites, bronzitites, and plagioclase-bronzite veins complimentary to the gabbro were produced during the first stage in the mantle. The P - T crystallization conditions of minerals and rocks during the first stage were evaluated by the pyroxene geothermometer at T =1200 pm 100o C and P =20 pm 5 kbar [Makeyev, 1992b; Makeyev and Bryanchaninova, 1999; Mercier, 1980].

[10]  The rocks of the dunite-harzburgite complex (dunites, harzburgites, and websterites), the dunite-wehrlite-clinopyroxenite complex, and the high-quality metallurgical Cr ores were produced during the regional metasomatic stage, which occurred at T = 900 pm 50o C and P = 12 pm 5 kbar.

[11]  Regional metasomatism resulted in the olivinization of the ultrabasites, with MgO and SiO2 becoming the major mobile components. The origin of large dunite masses was related to the accumulation of MgO and the removal of SiO2. The olivinization of the harzburgites and lherzolites was associated with the synchronous origin of clinopyroxenite and websterite veins in which excess components (CaO, Al2O3, and Na2O) were accumulated. Regional metasomatism facilitated the development of large economic deposits of Cr, whose ore mineral is chromite. The metasomatic genesis of the chromite follows from the existence of analogous metasomatic zoning (from top to bottom): harzburgite dunite chromite; an increase in the contents of ore Cr-spinel from the periphery to the cores of chromite bodies; the symmetric inner structures of the lens-shaped chromite bodies; the aureoles of lower Fe# of olivine around the chromite bodies; and other features.

[12]  Under the original P - T conditions ( T = 900 pm 50oC, and P = 12 pm 5 kbar), contact metasomatism proceeded along contacts of ultrabasites and gabbro. This Mg-Ca metasomatism produced the dunite-wehrlite-clinopyroxenite complex. Thus, the clinopyroxenite developed after gabbro and the dunite and wehrlite replaced harzburgite and lherzolite.

2006ES000196-fig06
Figure 6
[13]  We were the first to identify a pegmatite stage in the evolution of the ultrabasite rocks. This stage was responsible for the origin of pegmatoid dunite with early antigorite, whose water is likely of juvenile origin. The Cr-bearing silicates (kemmererite, kochubeite, amesite, uvarovite, and pargasite) crystallized in the Cr ores during this stage in fractures and small cavities. The rare mineral association of corundum with plagioclase should also be attributed to the pegmatite stage. This association with oligoclase, ruby, phlogopite, fishmanite (Sr-bearing mica), pargasite, Fe-Al chromite, and ferroaluminochromite occurs most widely at Rubinovyi Log in the central part of the Rai-Iz Massif. The rocks are ruby-bearing pegmatites (Figure 6). The ruby plagioclasites were dated by the K-Ar method on phlogopite at 320 pm 20 Ma. The typomorphic Cr-spinel of this association is ferrochromite (Fe6Mg2 )8 (Cr12.5Al3Fe0.5 )16O32. The occurrence of water-, chlorine- and fluorine-bearing minerals in this mineral assemblage suggests their crystallization in the presence of a pegmatite fluid [Bryanchaninova et al., 2004b; Makeyev, 1992b; Makeyev and Bryanchaninova, 1999].

[14]  X-ray diffraction data demonstrate that the antigorite forms syntactic aggregates with olivine. These syntactic intergrowths suggest that the antigorite and olivine crystallized in equilibrium with each other. This is the highest temperature generation of the antigorite, which likely crystallized at temperatures of no less than 700 pm 50oC. The contents of the primary antigorite in the pegmatoid dunites vary from 1% to 5%, and, hence, the water concentration in the fluid can be evaluated at 0.13 pm 0.65%. This was most probably juvenile water, which belonged to the ultrabasite system itself. Conversely, the serpentinization of olivine in the ultrabasite rocks proceeded under the effect of seawater [Bryanchaninova et al., 2004a].

[15]  The pegmatites were formed after the regional metasomatic stage, simultaneously with the solid-state (subsolidus) transformations of the rocks, their recrystallization with the participation of a fluid, and the accumulation of Ca, K, Na, Al, and Sr in the reaction products. All minerals of the ultrabasite rocks contain isomorphous Cr and are enriched in Al. We determined that the Rubinovyi Log pegmatites bear a new Sr-bearing mica, which contains 6% SrO [Bryanchaninova et al., 2004b].

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Figure 7
[16]  The main indicator of the mineral assemblages of Alpine-type ultrabasites is Cr-spinel, a mineral occurring in practically all rock varieties and having a composition and structure typomorphic of each of these varieties. The compositional evolution of the Cr-spinel (Figure 7) proceeded from Cr-picotite to chromite and then to magnetite and was associated with the depletion of Mg, Al, Zn, and Ni and enrichment of Fe (both Fe2+ and Fe3+ ), Mn, Ti, and V.

[17]  Cr-spinel is one of the most widely spread typomorphic minerals of ultrabasic rocks and is contained in each of them (it is a so-called telescoped mineral). Cr-spinel is also a major mineral of the Cr ores, and its contents vary from 0.1% to 2% in various rocks: dunites, harzburgites, lherzolites, wehrlites, websterites, pyroxenites, troctolites, and metamorphic ultrabasites. The contents of Cr-spinel in dunites often increases to 10%.

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Figure 8
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Figure 9
[18]  Cr-spinel grains broadly vary in shape and size: from dust-sized (0.001 mm) inclusions in silicates to large (5-25 mm) grains and their aggregates (Figures 8 and 9). Their shapes and color also vary. Anhedral transparent grains of Cr-spinel of color varying from yellowish green to brown were found only in the primary lherzolites and harzburgites. Equant reddish brown transparent grains are typical of the aluminochromite ores. Euhedral Cr-spinel crystals of octahedral habit, opaque (usually black) are contained in the secondary metasomatic rocks: dunites, pyroxenite veins, and high-Cr ores. Anhedral and skeletal crystals, which are sometimes replaced by serpentine and chlorite, are typical of accessory Cr-spinel in the metamorphic ultrabasites. Metamorphic Cr-spinel is usually black and opaque and often contains multiphase inclusions.

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Figure 10
[19]  The reflectance indices ( R ) of polished Cr-spinel grains and their microprobe analyses indicate that these grains are either homogeneous or zonal, with cores of primary composition and younger ferro-ferri Cr-spinel margins (Figure 10). Such grains are often surrounded by kemmererite rims (rinds) and consist of three zones: (i) cores, (ii) peripheral rims of ferrochromite, ferrichromite, and Cr-magnetite, (iii) and rinds of chlorite with magnetite dust.

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Figure 11
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Figure 12
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Figure 13
[20]  The composition of Uralian Cr-spinel varies within fairly broad ranges. We have compiled a database of analyses of Cr-spinel (more than 1000 chemical and microprobe analyses; Figures 11, 12, and 13). Most of these analyses characterized ultrabasite massifs in the Polar Urals [Makeyev, 1992b; Makeyev and Bryanchaninova, 1999]. The database also includes approximately 200 analyses of accessory and ore Cr-spinel from the Kempirsai, Sarany, Nizhnii Tagil, and Veresobyi Bor massifs, from the Western Sayan Range, and from some other territories [Makeyev and Bryanchaninova, 1999; Pavlov and Grigorieva-Chuprynina, 1973]. These analyses were used for comparison.

[21]  The composition of the ore Cr-spinel usually varies from Cr-picotite to chromite, and the composition data points plot near the left corner of N. V. Pavlov's triangular diagram [Pavlov and Grigorieva-Chuprynina, 1973]. Each of the massifs is characterized by its own variability ranges of the composition and physical properties of its Cr-spinel. We established two isomorphic trends for the ore Cr-spinel: Mg, Al Fe3+ and Mg, Al Fe2+, Fe3+. The former and latter trends characterize the primary and metamorphic ores, respectively. The accessory Cr-spinel is even more variable in composition. In Pavlov's triangular plot, the data points of the accessory Cr-spinel scatter not only in the left-hand part (as the ore Cr-spinel points do) but also extend into the right-hand part (Figures 11, 12, and 13). The most broadly variable composition is characteristic of Cr-spinel from the Syum-Keu, Rai-Iz, and Nizhnii Tagil massifs. Their metamorphic Cr-spinel is ferrochromite, ferrialuminochromite, ferrichromite, Cr-magnetite, and magnetite. The Voikar-Syninskii and Kempirsai massifs contain primary picotite, Cr-picotite, aluminochromite, and chromite. This reflects the intensity of metamorphic transformations of the ultrabasites of these massifs. The compositional variations of the accessory Cr-spinel were traced from picotite to chromite according to the following tendency: Mg, Al, Zn, Ni Cr, Fe2+, Fe3+, Mn, Ti, V. The Cr-spinel in the rocks of the mantle stage is picotite, Cr-picotite, and low-Cr aluminochromite; the rocks of the regional metasomatic stage contain high-Cr aluminochromite and chromite; the pegmatite-stage rocks bear ferro-aluminochromite and ferrochromite; and the rocks of the contact metamorphic stage contain ferro-feri Cr-spinel (Figure 7).

[22]  Microprobe analyses of one grain of accessory Cr-spinel from dunite of the Rai-Iz Massif revealed a zonal structure of this grain: it had a core of aluminochromite (Mg3.5, Fe4.5 )8 (Cr10.8 Al4.8 Fe0.4 )16 O32 and peripheral portions consisting of chromite (Mg3.2 Fe4.8 )8 (Cr12.0 Al3.5 Fe0.5 )16O32. This grain clearly illustrates the Mg, Al Cr, Fe compositional trend of the Cr-spinels and the original high-Al composition of the Cr-spinel with respect to the chromite. Other grains often display evidence of the exsolution of unstable ferrichromite and chrome-magnetite phases into ferrochromite and magnetite. This process can be expressed by the scheme: ferrochromite (Cr, Zn) ferrichromite, Cr-magnetite (Al, Fe2+, Mg - const) magnetite (Fe3+, Mn, V, Ti).

[23]  It is now quite obvious that ferrochromite (Fe6 Mg2 )8 (Cr12 Fe3 Al)16O32, magnetite (Fe6 Mg2 )8 (Fe12-15 Cr4-1 )16O32 could crystallize in ultrabasite rocks only during prograde metamorphism, at relatively low temperatures.

[24]  The series of spinel MgAl2O4 and magnetite FeFe2O4 is known to have a miscibility gap.

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Figure 14
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Figure 15
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Figure 16
[25]  Together with L. D. Zaripova of the Kazan State University [Makeyev, 1992a, 1992b; Makeyev and Zaripova, 1984], we examined the Mössbauer spectra of approximately 100 samples of natural primary and metamorphosed Cr-spinels from the Rai-Iz Massif. The composition and some other characteristics of some of these samples are listed in Table 1. The ferrichromites and chrome-magnetites were determined to consist of two phases and display a broad compositional miscibility gap in the chromite-magnetite series (Figures 14, 15, and 16). The dependence of the quadrupole split ( D E) of iron ions at three sites ( BFe3+, BFe2+, and AFe2+ ) on the Al mole fraction [Al/(Al + Cr)] in the Cr-Al Cr-spinel series (chromite-picotite, where Fe3+<1 f.u.) can be expressed in the form of the following two linear and one quadric regression equations:

DE = 1.075 [Al/(Al + Cr)] + 0.578  mm s-1 (B Fe3+),

DE = 0.345 [Al/(Al + Cr)] + 0.469  mm s-1 (B Fe2+),

DE = -3.736 [Al/(Al + Cr)]2

+ 4.047 [Al/(Al + Cr)] + 0.572  mm s-1 (AFe2+).

[26]  The exsolution of the Cr-spinel solid solution in the chromite-magnetite series was also confirmed by the data of IR spectroscopy, magnetic susceptibility, and, for some samples, by their microprobe analyses. Occasional finds of ferrichromite and chrome-magnetite, which are identified by chemical analyses, should be attributed to natural mixtures with variable proportions of two members of the ferrochromite-feromagnetite series on a microscopical (cluster) scale.

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Figure 17
[27]  This newly discovered miscibility gap in the chromite-magnetite series is shown in a spinel-hercynite-magnetite-magnesiomagnetite-magnesiochromite-chromite diagram (Figure 17). As can be seen from this Figure, only three of the end members (spinel, hercynite, and magnetite) were found in nature, and none of the other end members is known to occur in natural rocks. Hence, we were the first to constrain the stability region of the limited solid solution of natural Cr-spinels. This region is bounded by a complicatedly curved surface within a tetrahedral compositional diagram and includes two miscibility gaps.

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Figure 18
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Figure 19
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Figure 20
[28]  We examined the typomorphic features of the composition and properties of natural Cr-spinels (Figures 18, 19, and 20). The following regression equations can be proposed to relate the composition of this mineral to its unit-cell parameter a 0, wavelength of the IR spectrum n1, and the reflectance R:

a0 (Å) = 8.162 - 0.0098times Mg2+ + 0.0146times Cr3+ + 0.0139 times Fe3+,

n1 ( cm-1) = 671.8 + 0.580times Mg2+ - 3.268 times Cr3+ - 1.607times Fe3+,

R (%) = 9.872 - 0.406times Mg2+ + 0.369times Cr3+ + 0.605 times Fe3+.

[29]  Other indicator minerals of Alpine-type ultrabasites are olivine, clinopyroxene, plagioclase, pentlandite, and some others. Their composition and physical characteristics vary during the evolution of the rocks similarly to those of the Cr-spinel. These minerals can also be utilized for the purposes of genetic analysis and petrological reconstructions.

[30]  The results of our research allowed us to identify new mineralogical indicators and propose new criteria for the assessment of the occurrences and deposits of Cr ore mineralization related to Alpine-type ultrabasites. The first of them is a correlation between the compositions of the accessory and ore Cr-spinels. This criterion makes it possible to predict the composition of Cr-spinel in concealed and unidentified orebodies. The composition of the ore Cr-spinel is correlated with the composition of the accessory Cr-spinel, a mineral that is contained in practically all of the rocks. The other criterion is the concentration of the fayalite end member of the rock-forming olivine (or the index of refraction Ng of this olivine), which decreases toward orebodies and zones with Cr ore mineralization [Bryanchaninova and Makeyev, 1988]. The third criterion is the mineral assemblages of the rocks, the compositions of the typomorphic minerals, and the spatial distribution of ultrabasite complexes, with this criterion making it possible to assay the possible reserves of the Cr ores.

[31]  Our data on accessory and ore Cr-spinel in ultrabasites from the Polar Urals indicate that the composition of this mineral seems to be a sensitive indicator of prograde metamorphism but is almost insusceptible to retrograde metamorphism. The prograde metamorphism of ultrabasites to the amphibolite and greenschist facies transforms and recrystallizes their accessory Cr-spinel and is favorable for its depletion in Cr and Al, the oxidation of its Fe2+ to Fe3+, and, eventually, in enrichment of the mineral in the magnetite end member. The final metamorphic product of the accessory Cr-spinel is newly formed magnetite and kemmererite, which concentrates some Al and Cr removed from Cr-spinel.

[32]  The analysis of our data led us to identify two trends in the correlations between the compositions of ore and accessory Cr-spinel in chromite-bearing ultrabasites.


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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