ISSN 1028�334X, Doklady Earth Sciences, 2013, Vol. 452, Part 1, pp. 963–966. © Pleiades Publishing, Ltd., 2013.Original Russian Text © G.N. Savel’eva, V.G. Batanova, A.V. Sobolev, D.V. Kuz’min, 2013, published in Doklady Akademii Nauk, 2013, Vol. 452, No. 3, pp. 313–316.

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Harzburgite massifs of the ophiolite associationformed under supersubduction environments are tra�ditionally considered as chromite�bearing and per�spective for discovery of high�chromium ores [1, 2].However, the deposits of chromium ores are knownnot only in all dunite–harzburgite ophiolite massifs.The problem of significant differences between thecompositions of peridotites in massifs hosting largechromite deposits and those in massifs with small oreoccurrences is still open. The textures of residual peri�dotites (harzburgite and lherzolite) in ophiolites pre�serve traces of residual deformations that proceeded insubsolidus solid–plastic flow of mantle rocks [3]. Thelocation of ore bodies and host dunite in relation tomantle deformational textures and critical horizonshas been considered in some publications [4, 5], butthe revealed textural criterions may be mostly appliedas local for searching for bodies in potential ore fields.

Variations in the general petrochemical composi�tion of mantle peridotites of ophiolites are mainlycontrolled by the depletion degree of rocks in thecourse of partial melting of mantle matter. An increasein peridotite depletion results in an increase in themagnesium mole fraction of rocks and a decrease inthe contents of SiO2, CaO, and Al2O3 [6–8]. Thecompositions of minerals are widely variable evenwithin an individual massif, especially in relation torock�forming oxides (Cr2O3, Al2O3, and FeO) inpyroxenes and accessory chrome spinellide. Spatialvariations of pyroxene compositions are accompaniedby zoning of their large grains, which is reflected in adecrease in the concentrations of these oxides fromcenter to rim of neoblasts. The formation of zoning ofpyroxenes and neoblasts occurs at high�temperature

deformations of peridotites [9, 10]. Dunites includingore�hosting varieties are formed in the course of high�temperature deformations as well [9, 11]; the traces ofthese deformations are also registered in chromiteores. Migration of ore�forming oxides during defor�mations prompts to investigate the specific character�istics of ore�bearing mantle rocks reflected in the dis�tribution of rock�forming oxides composing chromiteore between minerals.

We compared the compositions of olivine,pyroxenes (enstatite and diopside), and accessorychrome spinellide from harzburgites collected in dif�ferent areas of the two largest ophiolite massifs of theUrals, Kempirsai and Voykar–Synyin. Chromites ofunique quality and resources have been mined in theKempirsai massif for a long time. The Voykar–Synyinmassif contains numerous chromite ore occurrencesof various compositions. The geological structure,petrology, and mineralogy of these massifs have beenconsidered in numerous publications [9, 11–14]. Thesamples for analysis were collected with account forthe available data [13] on geological structures,chromite�bearing zones, and ore chromite deposits(Kempirsai massif) and were analyzed on the sameapparatus.

The composition of all minerals discussed in thispaper was studied on a JEOL 8200 electron micro�probe at the Max�Planck Institute for Chemistry(Mainz, Germany). Olivines were analyzed by a spe�cial methodology [15] providing reliable estimationsof the contents of calcium, nickel, and manganese upto the third significant digit. Chrome spinellide, cli�nopyroxene, and orthopyroxene were analyzed at anaccelerating voltage of 20 kV and a current of 80 nA.Natural minerals and oxides were applied as standards.

The ophiolite mantle section of the Voykar–Synyinmassif in the Polar Urals is represented by spinelharzburgite with small areas of lherzolite, numerousdunite bodies, and veins of dunites and pyroxenites.Restites demonstrate evidence of the complex historyof partial melting, plastic deformations, multistagemelt migration, and its interaction with the rock [9, 11].Spinel harzburgite collected far from dunite and

Minerals of Mantle Peridotites:Indicators of Chromium Ores in Ophiolites

G. N. Savel’evaa, V. G. Batanovab, Corresponding Member of the RAS A. V. Sobolevb, and D. V. Kuz’minc

Received March 25, 2013

DOI: 10.1134/S1028334X13090183

a Geological Institute, Russian Academy of Sciences, Moscow, Russiab Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russiac Max�Planck Institute for Chemistry, Mainz, Germanye�mail: savel2@yandex.ru

GEOCHEMISTRY

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pyroxenite bodies consists of olivine (75–80%), ensta�tite (15–20%), diopside (1–4%), and chrome spinel�lide (~1.5%); the concentration of diopside in lherzo�lite increases up to 7%. The prevailing size of olivinegrains is 4–6 mm; pyroxene grains, 1–2 mm. Thecomposition of harzburgite including the concentra�

tions and distribution of rare�earth and highly sidero�phile elements allows us to consider this rock as restiteafter 14–16% partial melting of the mantle matter[11]. The areas of sampling cover a significant part ofthe harzburgite section of the Voykar–Synyin massif,from the lower part of the allochthonous plate in thewest to the upper part of the section at the contact withrocks of the dunite–wehrlite–pyroxenite complex inthe east. Sampling was carried out far from the knownchromite occurrences, as well as within 200–300 m ofchromite ore bodies of various compositions.

Mantle rocks of ophiolites of the Kempirsai massifin the South Urals are mostly represented by spinelharzburgite with a small portion of lherzolite in thesoutheastern part of the massif, dunite bodies, andwebsterite veins in the south [12, 13]. The mineralcomposition of harzburgite is similar to that of theVoykar–Synyin massif: olivine (75–80%), enstatite(15–20%), diopside (1–4%), and chrome spinellide(~1.5%). However, the microtexture of harzburgite ofthe Kempirsai massif is distinguished by the size of oli�vine grains (1–2 mm), mainly smaller than that ofpyroxenes (2–4 mm), whereas harzburgite of theVoykar–Synyin massif is characterized by the oppositesize relationships. Pyroxenes demonstrate clear zoneddistribution of Cr, Al, and Ca (Fig. 1). The sampleswere collected in the southern part of the Kempirsaimassif, where the largest deposits of high�chromiumores of the Don group are located, as well as in thenorthern part, where small occurrences of aluminifer�ous chromites are known.

Оl Оl

En

En

Di

Di

En

En

Di

DiDi

Chr

Amph

Serp

0.67

1.07

0.74

0.50

0.31

0.340.42

0.73

1.31

En serp500 µm

Fig. 1. Photomicrograph (reflected light) of harzburgite from the southeastern part of the Kempirasi massif demonstrating zoningof large enstatite (En) and diopside (Di) grains. Circles denote points of analyses; numerals near circles indicate the concentra�tion of Cr2O3, wt %; Ol, olivine; Amph, amphibole; Chr, chrome spinellide; En serp, serpentine (bastite) after enstatite; Serp,serpentinite.

0.45

92.5 91.5

NiO, wt %

92.0

0.40

0.35

0.30

12

91.0 90.5 90.0 89.5Fo, %

Fig. 2. Relationship between NiO and the forsterite com�ponent (Fo) in olivine from harzburgite of the Voykar–Synyin (1) and Kempirsai (2) massifs.

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MINERALS OF MANTLE PERIDOTITES 965

Olivine from harzburgite of the Voykar–Synynmassif contains 89.71–92.27% forsterite (Fo),whereas the compositional range of olivine fromharzburgite of the Kempirsai massif is 90.76–91.55%Fo (Fig. 2). Variations in the CaO concentrations arewider in the first massif in comparison with theKempirsai massif: 0.001–0.022 and 0.003–0.010 wt %,respectively. The concentrations of NiO are slightlyhigher in the Kempirsai massif (0.387–0.422 wt %, onaverage ~0.400 wt %) in comparison with the Voykar–Synyin massif (0.344–0.387 wt %). The relationshipbetween the Cr2O3 and Al2O3 concentrations in ensta�tite and diopside from harzburgite of the Voykar–Synyin massif is strongly different from that forharzburgite of the Kempirsai massif. In the first mas�sif, as well as in many other ophiolite massifs world�wide, the concentration of these oxides is significantlyhigher in diopside (Fig. 3a); in the second massif it ishigher in enstatite from harzburgite from the southernpart of the massif hosting the deposits of the Don

group (Fig. 3b). Diopside of this area is remarkablydepleted in Cr2O3 and Al2O3, whereas in the northernpart of the massif containing small chromite bodies,the low concentrations of Cr2O3 and Al2O3 are not soclear. Our previous data on the composition of miner�als from this massif [13] are completely consistent withthe results obtained. Independently of the location ofthe harzburgite sample in relation to the known orebodies of the Voykar–Synyin massif, the concentra�tion of rock�forming oxides (Cr2O3 and Al2O3) isalways higher in diopside than in enstatite. As a whole,the composition of some pyroxenes (diopside andenstatite) from harzburgite of the Kempirsai massif ischaracterized by a quite low concentration of Cr2O3

and Al2O3 and a high magnesium mole fraction ofenstatite. Accessory chrome spinellide of harzburgitefrom the Voykar–Synyin massif has a lower magne�sium content at the same chromium concentrationthan the accessory minerals of harzburgite from theKempirsai massif (Fig. 4). Accessory spinellides fromdunite and ore spinellides of the Kempirsai massifhave a high magnesium content at the same chromiumconcentration as well. Ore spinellide is characterizedby a higher chromium content.

Thus, there are significant differences in the com�position of olivine and chrome spinellide and mostlyin the concentration and distribution of major ore�forming oxides between ortho� and clinopyroxenes inmantle complexes with large chromite deposits and inmantle complexes hosting relatively small bodies ofchromite ores. With regard to the huge volume of ana�lytical data on the composition of pyroxenes in mantleperidotites demonstrating higher concentrations of

0 1

1.8

1

2

2 3 4 5Al2O3, wt %

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

(b)

0 1

2.0

2 3 4

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Cr2O3, wt %

(a)

65

1.8

Fig. 3. Relationship between Cr2O3 and Al2O3 in enstatites(1) and diopsides (2) of the Voykar–Synyin (a) andKempirsai (b) massifs.

80 50

Cr#

70

100

0

12

40 30 10 0Mg#

3456

60 20

90

80

70

60

50

40

30

20

10

Fig. 4. Relationship between the chromium mole fraction(Cr# = 100Cr/(Cr + Al)) and magnesium number (Mg# =100Mg/(Mg + Fe)) in accessory chromites from dunite (1and 4), harzburgite (2 and 5), and chromium ores (3 and 6)of the Kempirsai (1–3) and Voykar–Synyin (4–6) massifs.

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Cr2O3 and Al2O3 in diopsides than in enstatites, weconsider that Ca pyroxene (diopside) from harzburgiteof the Kempirsai massif was not primarily depleted inCr2O3 and Al2O3. The compositions present the finalresult of removal of these components from diopside.We assume that in this case a significant portion ofmantle rocks underwent high�temperature recrystalli�zation accompanied by redistribution of Cr and Alfrom silicates (pyroxenes) to the ore mineral. Theextremely narrow range of compositional variations ofhigh�magnesium olivine from harzburgite of theKempirsai massif and the relatively small size of itsgrains are consistent with this conclusion. Recrystalli�zation of peridotites and redistribution of the compo�nents most likely resulted from migration of a hugevolume of reacting melt/fluid through harzburgite [12,14] and the high rate of mantle deformations (decreaseof olivine grain sizes) accompanying melt migration.Chromium ores of the Kempirsai massif were formedat the expense of passing melts/fluids, as well as migra�tion of ore components from silicates primarilyenriched in Cr and Al.

ACKNOWLEDGMENTS

This study was supported by the Russian Founda�tion for Basic Research (project no. 11�05�00011).

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Translated by A. Bobrov