Nurali Ophiolite Massif (the Southern Urals): Geological, Structural, and Mineralogical Features

Nurali Ophiolite Massif (the Southern Urals): Geological, Structural, and Mineralogical Features D.E. Saveliev, N.N. Ankusheva a Institute of Geology, Ufa Federal Research Center, Russian Academy of Sciences, 16/2 Karl Marks Str., Ufa 450077, Russia. E-mail: savl71@mail.ru b South Urals State University, Miass Department, Miass 456300, Russia E-mail: ankusheva@mail.ru (Paper accepted February 1, 2018)


Historical outline
The Nurali massif has been studied since the second half of the XIX century, after the first Cr ore mineralization discovery at the massif area. Prior to 1939, the extensive prospecting and exploration works have been carried out resulting in the discovery of more than 30 Cr deposits and ore mineralization occurrences. The largest of them are Mokraya Yama and Attestinskiy ore occurrences located at the northern part of the massif and described in reports of P.G. Farafont'ev, I.M. Parfenov, and S.A. Konyukhov. The systematic exploration of the area structure was started by V.S. Koptev-Dvornikov in 1932. Later, the massif was studied by the multi-scale geological (Muravyova, 1947;Sadrislamov, 1960;Anisimov, 1983) and complex geophysical survey (Vulfovich, 1963;Chursin, 1979). Time to time, the researche studies were carried out at the massif (Poirier, 1985;Prokin, 1962;Edelstein, 1964;Cherkasov, 1973). In 1978In -1980, in result of the prospecting works for Cr bearing ores, the Nuralinskiy and Kurmankulskiy deposits were discovered (Shumikchin, 1980).
The detailed structural and petrological study of the ultramafic rocks of the Nurali massif was perfomed in 1980 th by G.N. Savelieva, E.A. Denisova, the scientists from the Geological Institute. They compiled the structural and geological map of the mantle ultramafic rocks and transition wehrlitepyroxenite unit, which characterizes the chemical composition of the rock-forming minerals, and provides the new approach for interpretation of the origin of the massif as one of the typical example of the lherzolitetype ophiolite complexes at the Southern Urals (Savelieva, Denisova, 1983;Savelieva, 1987). The nature of the transition dunitewehrlite-clinopyroxenite complex of the massif was studied in (Pertsev, Savelieva, Nurali ophiolite massif (the Southern Urals) geological, structural and … 229 1997;Pertsev et al., 1997;Kovalev et al., 1998).

General geological characteristics of the massif
The Nurali massif is situated at the NE part of the Uchaly region in Bashkortostan Republic, near the Chelyabinsk region bor-der. The western part of the massif (Bolshoy Nurali Mountain) is a billowy area where the mountain slopes formed a variety of the small ridges interrupted with deep ravines ( Fig. 1-3). A maximal altitude is of 754.2 m, the relative elevations ranges from 250 to 300 m, and the slope angles are 25-40º.  (spinel-and spinel-plagioclase peridotites) To the East, the parallel ridge Maly Nurali is built with ultramafic rocks as well. Further, to the east it is followed by a hilly plane with polymictic mélange of blocks of the Paleozoic gabbroid, volcanogenic and volcanogenic-sedimentary rocks.
The ultramafic rocks are exposed at the area of about 120 km 2 which is of 30 km long and 4-5 km wide. They are confined to the Main Uralian Fault zone bordering the paleocontinental (at the west) and paleooceanic (at the east) parts of the Southern Urals. The massif is split into the blocks with the latitudinal faults. The largest faults are associated with Nizhniy and Verkhniy Eremel, Sherambay, and Shardatma River valleys. The riverhead of the Miass River is located within the massif as well.
According to the geophysical data, the massif is close to an inverted cone in shape. Along its borders at the east and west parts, significant linear positive magnetic anomalies with strong centripetal reduction of the rock magnetic susceptibility were observed. Above the central part of the massif, the intensive positive gravity anomaly was record-ed that showed the significant extent of the serpentinized ultramafic rocks to the great depth (Rudnik, 1965).
As the majority of the ophiolite complexes, Nurali massif can be subdivided to the two assemblages: mantle unit occupies its western part and crustal unit is located at the eastern part. At the western part of the massif, spinel-plagioclase peridotites prevailed and followed by spinel peridotites and dunite-harzburgites forming a narrow belt at the East along the border of the "crustal section" (Fig. 1 The largest area is occupied by alternated spinel and prevalent spinel-plagioclase peridotites. Their structures are characterized by elongated schlieren consisted of fully saussuritized plagioclase and skeletal Crspinel grains. Harzburgites differs from them by absence of clinopyroxene and plagioclase, higher Mg# of olivine, and orthopyroxene, which constitute 70-90 % and 10-30 % of bulk rock, correspondingly. The average rock chemical composition is summarized in Table 1. The banding of harzburgites is more expressed and caused by irregular distribution Nurali ophiolite massif (the Southern Urals) geological, structural and … 231 of the enstatite. The flattened enstatite grains conform to a banding of NE strike and subvertical dip. All of peridotite species contain Cr-spinels (1-5 %). Their aluminous species are related to spinel peridotites. Harzburgites contain more the Cr# of accessory Cr-spinel, which is subhedral and shine with reddishorange colors.
Dunites contain olivine (95-98 %) and Cr-spinel (до 5 %). The oriented structures are characterized by elongated olivine grains and Cr-spinel chains. As a rule, they conform to each other and dunite-harzburgite banding at NE strike and subvertical dip. Olivine is rock-forming mineral and contains 8-9 % of fayalite. Cr-spinel is high-chromium and redbrown shine.
The main features of dunite-harzburgitelherzolite part of the massif are following: 1) the general development of the deformed structures indicated high-temperature plastic deformation of rocks, 2) the gradual increase of Mg# concentration from the west to east toward the contact with banding dunitewehrlite-pyroxenite complex.
The eastern part of the Nurali massif consists of wehrlite-pyroxenites and gabbroids. The ultramafic sequence includes websterites, orthopyroxenites, plagioclase pyroxenites, hornblendites, and cumulose harzburgites, and dunites (Kovalev et al., 1998;Pertsev, Savelieva, 1997;Smirnov, 1995). The presence of the stratiform ultramafic zone is an important peculiarity. The bands are from 0.5 cm to several meters thick, and "layers" reach the hundred meters in thickness. Clinopyroxenites and wehrlites prevail in stratiform zone, but essentially olivine bands are rare. This zone is 30-50 m thick with subvertical and steep western dip. Along with the banding, we observed a concordant flatness of olivine grains or sometimes lath-like aggregates. Wehrlites and clinopyroxenites are characterized by porphyroclastic structures.
East ultramafic rocks of duniteharzburgite-pyroxenite complex followed by gabbroids composing low areas near the Nurali Lake. Their bedrock is alternated with the outcrops and eluvium of serpentinites and volcanogenic-sedimentary rocks. Gabbro forms isolated block within serpentinite mélange and is represented by medium-and coarse-grained gabbro-diorites, banded hornblendite, fine-grained gabbro, and gabbroamphibolites.

Mineral resources of the Nurali massif
The Nurali massif includes numerous small deposits and Cr ore mineralizations. The most of them form lense and podiform ore bodies composed by massive and closedisseminated chromitites hosted in fully serpentinized ultramafic rocks. As a rule, prevalent apoperidotite serpentinites and Cr ores are separated by a thin apodunite serpentinite rim that is a typical peculiarity of ophiolite (Alpine-type) Cr deposits.
The Mokraya Yama is the largest deposit at the massif. According the archived data, in 1902 it produced more than 150 000 t of massive and close-disseminated chromitites (Kovalev, Salikhov, 2000). The quarry is currently remained and submerged. The destroyed bank consists of fully serpentinized ultramafic rocks. At the quarry flanges, apodunite and apoharzburgite serpentinites prevail, and an adjacent area is composed mainly by apoharzburgite serpentinites.
South from the Mokraya Yama deposit, the chain of small ore mineralizations of the Olgino ore cluster is situated with the largest Siyak-Tukan deposit and Petrovskiy quarry. They have been fully studied until now. Likewise, ore bodies of the Mokraya Yama deposit, which ore mineralization is lens-like, are composed of massive chromitites with thin apodunite serpentinite rims.
In addition to typical Alpine-type chromitites, several submeridional zones consisting of disseminated chromitites are located at the eastern part of the massif. They are confined with so called "margin dunites" separated spinel-and spinel-plagioclase peridotites from wehrlite-clinopyroxenitegabbro complex. The Nurali is the largest this type deposit. Attestinskiy and Kurmankulskiy deposits are situated and at the northern part of this dunite zone. According to the morphological classification , these deposits conform 232 D.E. Saveliev, N.N. Ankusheva to tabular bodies concordant to the hosted ultramafic rocks.
The morphology of ore-forming Crspinels is described as follows: small rounded euhedral grains in disseminated ores; and strongly fractured anhedral grains in massive ores. Middle-chromous spinels (Cr-picotite) are exposed at the Sredne-Nuralinskiy ore mineralization located within peridotites at the central part of the massif, and serpentinite mélange at the eastern part of the massif. The contents of the main components are (wt. %): Cr2O3 41-47, Al2O3 20-23, MgO 13-15, FeO 15-18. The admixture elements are MnO (up to 0.1 %), NiO (up to 0.15 %) and TiO2 (up to 0.24 %).
Most of peridotites are strongly fractured that prevents to observe the orientation of the main structural elements. 'Fresh' rocks are rare. We observed orthopyroxene grains (yellowish-green enstatite), clinopyroxene (lightgreen diopside) and Cr-spinel (Fig. 6). In the olivine groundmass, "negative" micro relief is revealed. Monomineral olivine parts are characterized by a smooth homogeneous light-brown surface.
The differences between the amounts of pyroxenes and olivine in groundmass specified the banding, which is an important structural element of peridotites. The orientation of pyroxene grains and Cr-spinels indicate the mineral foliation and lineation. During the study of peridotites with an optical microscope, we observed the rock microstructure and measured the orientation of olivine grains. According to the measurements, we determined a statistical position of structural rock elements to map the special symbols.
On the west part of Nurali Ridge, spinel peridotites prevail (lherzolites and clinopyroxene-containing harzburgites), but toward the watershead ridge area, spinel-plagioclase peridotites are growing. The ratios between pyroxenes, olivine, and Cr-spinels are the same, but plagioclase is always occurs associated with Cr-spinel (up to 10 %). Typically, the schlieren-shaped aggregates composed of the anhedral to dendritic Cr-spinel exposed usually at the central parts, and plagioclase rim are observed (Fig. 7).  (Savelieva, Denisova, 1983 Fig. 7 we can see the mantle section structure of the latitudinal area of our route described in (Savelieva, Denisova, 1983;Denisova, 1990). At the scheme and outcrops, we observed the banding, flatness, and lineation. Although, we did not observe any clear structural elements in lumps and outcrops, but, if interpolate the large number of data, we can examine a folded structure of the mantle section of the massif.
Studying the peridotites with an optical microscope, we observed numerous indicators of high-temperature plastic deformation of the main rock-forming olivine and orthopyroxene ( Fig. 8-10). These minerals react to the expended stress in different ways. Olivine forms fragments of grains (polygonization) with developed substructures, and significantly elongated grains divided into blocks with small-angle borders. Occasionally, a disorientation is revealed within large grains that indicated by the presence of small olivine inclusions in the same groundmass. These events are caused by the intragranular or rotary recrystallization (Carter, 1976;Poirier, 1985).
On the contrary, an orthopyroxene is characterized by the adversely oriented grains bend to form kink-bands (Fig. 9), an intensive nucleation that means the formation of numerous centers of recrystallization at the points with the most distorted crystal lat-tice, and the formation of clinoenstatite and diopside lamella inside the bended grains.

Fig. 10. Petrographical peculiarities of spinelplagioclase and spinel peridotites. Transmitted light. a-b, e-fthe same areas: leftcrossed nicols, rightparallel nicols; csmall twinned plagioclase therein olivine groundmass (crossed nicols), d, g, hplastically deformed olivine crystals with fault lines (crossed nicols). A scale is 0.1 mm
The polished sections were studied using an universal stage (Fedorov's stage) to reveal the preferred orientation of the main crystallographic sides of olivine and orthopyroxene (Savelieva, Denisova, 1983). Regarding the structural rock elements, enstatite crystals are conformed to the translational sliding along (100) [001] (Fig. 11). In olivine, two fabric patterns were observed. The first of them is similar to the translational sliding along (010) [001] with a partial [100] peak. The second one has peaks of lineation at a small angle to Nm, and conform to the sliding of slip system (010)[001] and recrystallization (Fig. 12). Thus, micro-and petro structural data indicate that spinel-and spinel-plagioclase peridotites of this section are mantle tectonites formed due to plastic flowing at high temperature and pressure.

Fig. 12. Typical Cr-spinels grains associated with pelitizated plagioclase in spinel-plagioclase (а) and spinel peridotites (b, c). Plate-polarized light, parallel nicols; a scale is 0.1 mm
Microscopically, olivine in dunites indicates the high-temperature translational sliding, and frequently we observed the plastic fracture bands and subgrain borders (Fig. 14) expressed in heterogeneous grain extinction. Olivine grains are from micrometers to 3-5 mm in size that caused by an intensive polygonization and recrystallization of rocks in a plastic deformation envinronment. Olivine is highly magnesian and contain 90-92 % of a forsterite. Cr-spinel forms small grains (0.0n-0.5 mm), usually with a euhedral habit. They are highly chromous with Cr2O3 more than 50 wt. % ( Table 2). The primary olivine is altered and replaced mainly by a low-temperature loop-shaped serpentine preserved the primary rock structure. The extent of serpentinization ranges between 40 and 90 %.
Dunites and dunite-harzburgites compose Nurali poor impregnated Cr ore deposit discovered by E.A. Shumikhin in 1980 (Fig.  15). At Nurali deposit, up to 700 m long and 20-35 m thick 5 ore zones are revealed. These ore zones have NE strike (50-60º), and NW dip (70-80º), and conformed the banding of host rocks. Inside the zones, 130 m long and 2.5-12 m thick 4 ore bodies were explored. Ores are poorly impregnated with Cr content ranged between 5 and 30 %. The estimated Cr2O3 resources are 1.6 mil tons with average 5-7 wt.%.

The transition wehrlite-clinopyroxenite complex
We consider the framework of this complex on two examples (see Fig. 1, outcrops 3  and 4). There is Zapadno-Sherambayskiy chromite ore mineralization inside the transition mantle-crustal complex. In addition to this name listed on the map compiled by E.A. Shumikhin [1980], other names are used in literature: " KN-192" (Smirnov, 1995) and "CHR-II" . The rocks have lens-like and banded structures with different content of olivine and clinopyroxene. Furthermore, there are the areas enriched with orthopyroxene and Cr-spinels.
Olivine is intensively altered and replaced by serpentine veinlets with abundant powdered secondary magnetite. The latter indicates increase of Fe# content in the primary olivine compared with that of mantle section peridotites. The section is composed by wehrlites and olivine clinopyroxenites. Websterites are rare.
Alternating wehrlites and websterites are unevenly enriched with Cr-spinels with content from the first percentages up to almost monomineral chromitite veinlets and lenses of several millimeters to 1-2 cm thick conformed to a general banding of host rocks. According to S.V. Smirnov (1995), the chromitite formation is related to "apowehrlite high-temperature metasomatic websterites" with a persistent body thickness. They are conformed to the wehrlite jointing. Further, it was established that "metasomatic orthopyroxenes" are formed in wehrlites with content increase toward the contact with websterites (Smirnov, 1995).
1) The fractionally alternated layers and lenses of clinopyroxenites, olivine clinopyroxenites and wehrlites are 1-10 cm thick. They have fine-and medium-grained structure, "fresh" clinopyroxene (diopside) and an intensive olivine serpentinization with rare olivine relics. Apoolivine serpentine is βlizardite with abundant secondary magnetite grains. At the lower (eastern) part of this horizon, ore mineral is magnetite. Cr-spinel is identified at the upper part. Among the homogeneous section of the mainly fine-grained rocks, we observed the coarse-grained clinopyroxenite schlieren of 50-70 m thick.
2) The fractionally alternated olivine clinopyroxenites and wehrlites contain rare orthopyroxene grains, lenses of monomineral olivine rocks and clinopyroxenites, and thin layers and schlieren of websterites, and orthopyroxenites. The amount of clinopyroxenites increases to the top of the horizon, along with amphibole enrichment. Additionally, chromitite layers and lenses 3×10 m in size are confined to this horizon of 100-150 m thick (Priozernyy ore mineralization).
3) Olivine rocks with uneven (banded and lense-banded) enstatite varies to pegmatoid harzburgite contained long-prismatic enstatite (Pertsev, Savelieva, 1997). The thickness of this unit ranges from 1 to 3 m.  (Saveliev et al., 2017): anative palladium Pt inclusion in Cr-spinel; bcrystals of pentlandite and cuprous Pt in Cr-spinel; enative palladium Pt inclusion in pyroxene; fferroplatinum crystal in Cr-spinel; c, d, g, haccessory mineral inclusions in serpentinite; ddetailed photo g, awaruite inclusions associated with native Pt microinclusions This is followed by the turfy area composed of serpentinites with different size gabbroid blocks. The one of themoutcrop 4b (see Fig. 18) is situated south from Nurali Lake.
Near the ridge, at the elevation of 536.4 m, Priozerny chromitite ore mineralization is located. It is hosted by wehrlites and pyroxenites (outcrop 4c, see Fig. 18). This mineralization was discovered by (Smirnov, 239 Volchenko, 1992;Smirnov, 1995). In publications, different names are used: CHR-I , and "South Miass" . The ore mineralization is located at the Malyy Nurali Ridge with elevation of 536.4 m b.s.l. The ore mineralization is a lens-and schlieren-shaped ore body composed of coarse-grained thick impregnated chromitites hosted in wehrlite-pyroxenite rocks. Chromitites are confined to significantly serpentinized and rodingtized orthopyroxene rocks (Smirnov, 1995). The main lens is submeridionally elongated with 10×3 m of size. There are several small chromitite bodies up to 1×0.5 m in size. Furthermore, several more chromitite bodies hosted in clinopyroxenites, wehrlites and olivine-enstatite rocks at the adjacent area are described in literature .
At the Priozerny ore mineralization, Crspinels are widespread and geochemically similar to such in peridotites and middlechromous ores of the central part of the massif.  NiO (up to 0.29 %). Cr spinels form euhedral and subhedral coarsegrained aggregates (1-10 mm in size).

The origin of the ultramafic rocks of the Nurali massif
The formation of rocks at the massif has been interpreted in different ways. Until the 1980 s the hypothesis about the magmatic formation of ultramafic rocks and gabbros had been prevailed (Rudnik, 1965;Shumikhin, 1980). After the investigation of G.N. Savelieva and E.A. Denisova (1983), the massif has been considered as the typical example of ophiolite assemblage with mantle lherzolite ultramafites in the basis. Currently, it is generally accepted that the modern structural setting of the Nurali massif is resulted from the multiple tectonic movements. According to the model of G.N. Savelieva (1987), the peridotites of a mantle section are a weakly depleted upper mantle material ex-posed to a plastic flowing and metamorphic differentiation under a decompressive rise. At the later stages, the strain rate was increased sharply, and the partial melting became more intensive resulting in the melt separation and formation of strongly depleted dunite-harzburgite zone The problems of the geodynamic setting and formation of the transitional wehrliteclinopyroxenite complex and gabbroids are particularly discussed. According to literature (Smirnov, 1995;Moloshag, Smirnov, 1996), the transition wehrlite-clinopyroxenite complex was formed due to an interaction between gabbroids and mantle ultramafic rocks. In publications (Pertsev, Savelieva, 1997;Savelieva, 1987), it is argued that the transition zone of Nurali massif was formed due to a multiple integration of melts derived from a progressive depleted mantle source during the spreading at Р = 6-8 kbar.
However, based on the REE and PGE geochemical study, Fershtater and Bea (1996), and  examined the differences between the ultramafites and gabbros of the transition zone and spreading environments. Particularly, amphibole gabbro and diorites are calc-alkali with elevated K and large-ionic lithophyle elements (LILE) comparing to oceanic tholeiites. It testifies that the Nurali massif is compatible with "orogenic lherzolites" of the subcontinental mantle of the Western Alps, Pyrenees and Betic Cordillera.  (Saveliev et al., 2017): a -PGE sulfides and heaslewoodite intergrowth; b -Ir-Os-Ru_S and NiS mineral intergrowth in Cr-spinel; cplatinoid inclusions in Cr-spinelide; dlaurite inclusions in Crspinel; emillerite inclusions in Cr-spinel; f-laurite inclusions in Cr-spinel The structural and geochemical data indicate the massif was formed within the undercontinental mantle, most likely within the rift structure (Chashchukhin et al., 2007;Saveliev et al., 2008). The structural and geochemical data of dunites from the Nurali massif (Denisova, 1990) and similar Kraka massif (Saveliev, Blinov, 2015) indicate a reomorphic origin of dunites and related Crspinelides.