5.1. Potential Sources of Deposits
The petrographic study of sandstones of the Sabantuy chromite paleoplacer shows that the main contributors in their composition were metamorphic and sedimentary complexes. Varied roundness of compositionally similar fragments suggests that the time and conditions of their transfer and sedimentation are also different. The Uralian orogenic belt was the main supply region of terrigenous material for these sedimentary basins during the whole Permian time [
6,
7,
22]. As it was established, the Ural Foredeep starts to form in the Early Permian and by the dawn of the Biarmian (Guadalupian) epoch, it had completely filled and detrital material was distributed further westwards, increasing the thickness of the sedimentary cover of the East European Platform (
Figure 16). Mizens and Maslov conducted lithological and geochemical studies of sandstones of the molasse formation in the southern part of the Ural Foredeep, which rocks of the Kazanian Stage are also attributed to [
6]. U-Pb isotope dating of detrital zircons from molasse sandstones yields the Proterozoic age for most zircon populations and the Early Paleozoic age for their minor part [
6]. Thus, the Riphean sedimentary and metamorphic complexes of the Bashkirian Meganticlinorium are considered the main supplier of detrital material for the studied sandstones [
22].
The appearance of the chromite-bearing horizon in the studied section of Kazanian sediments marks an exhumation and erosion of chromite-bearing complexes in the supply region. Notably, the boundary between the chromite-bearing horizon and underlying sandstones is rather abrupt. The amount of chromspinel in underlying rocks is not higher than 1 wt.%, whereas it is up to 30 wt.% in chromitolite bed and 10–15 wt.% in sandstones overlying the ore horizon. No one object with an economically important content of detrital chromspinel, such as the Sabantuy paleoplacer, had been ever known in the Ural Foredeep before [
36]. It was formerly suggested that detrital chromites in the Permian sediments of the Southern Pre-Urals were derived from oceanic and suprasubduction mafic–ultramafic complexes of Uralides [
6].
The noted class of chromspinel grain size (0.15–0.25 mm) is typical of placers [
17,
51,
52]. Grains with this size and a density of ca. 4–5 g/cm
3 seem to be the most transportable and transferred for a relatively long distance. Transportation of bigger grains is hardly possible; therefore, we believe that sources for the Sabantuy paleoplacer were quite remote, probably 100–200 km far from the placer. Such provenance could be ophiolite complexes of the Main Uralian Fault (MUF) zone and associated serpentinite mélange and volcanic rocks, as well as peridotite allochthones, tectonically moved westwards from MUF through the Ural-Tau dome structures [
29,
34,
46,
53]. For example, the Kraka ultramafic massif, being the largest peridotite allochtone (more than 900 sq. km of area) in the Southern Urals, is the nearest ultramafic body to the Sabantuy placer (
Figure 1b). Island-arc volcanic complexes of Paleozoic age in the Magnitogorsk zone (
Figure 1b), situated to the east of MUF, could be an additional source for chromspinel [
54,
55,
56].
The results of the geological survey indicate that within 100 km from the Sabantuy ore occurrence, there are no mafic–ultramafic or volcanic complexes that could be considered sources for chromspinel [
37]. The ore material was apparently concentrated in the Sabantuy paleoplacer due to intermediate collectors since a long direct transfer without precipitation of heavy minerals is hardly possible.
The study of lithology and petrography of sedimentary rocks in the Sabantuy paleoplacer revealed that olivine and orthopyroxene are absent among clastic minerals, whereas they are major for ophiolite peridotites. It is probably related to olivine and orthopyroxene’s less resistance in the weathering crust and exogenic environment in comparison with chromite [
57]. Clinopyroxene occurs as rare grains. Among them, augite, omphacite and diopside were identified. Only diopside is compositionally closest to clinopyroxene from ophiolite peridotites and inclusions in the Sabantuy chromites, and it can be a product of peridotite erosion. Besides, sandstones contain single fragments of serpentinites and debris of mineral grains (in order of decreasing amount): feldspar, serpentine, chlorite, amphibole, scapolite, biotite, garnet, epidote and tourmaline.
The bulk composition of studied melt inclusions in several chromspinel grains corresponds to basalt and andesite. Similar compositions have an interstitial glass preserved in some melt inclusions. This fact clearly indicates that volcanic rocks were one of the sources of detrital chromspinel. The dendritic architecture of facets in some chromite crystals (
Figure 6c,f) gives another piece of evidence for the volcanic origin of chromspinel [
58].
All these data testify the heterogeneous sources of sandy rocks and chromspinel in particular and, possibly, quite a remote transfer of chromspinel from supply regions to the placer.
5.2. Potential Sources for Chromspinels
Chromspinels are reliable petrological indicators of P-T-fO
2 conditions, geotectonic settings and composition of host rocks. They are widely applied for petrogenetic and tectonic reconstructions [
1,
2,
3,
4,
45,
48,
59]. Our research showed that chromspinel of the Sabantuy paleoplacer had broad variations ruled by substitution of trivalent Al
3+ and Cr
3+ and bivalent Fe
2+ and Mg cations. The range of these variations is clearly displayed on the Cr#–Mg# diagram (
Figure 10). The diagram shows fields of chromspinels from peridotites of different geotectonic settings, layered intrusions, Uralian-Alaskan-type intrusion and chromitites, MORB basalts, boninites and metamorphic complexes. Most fields have significant overlap areas, whereas compositions of the studied chromspinels are also distributed along several fields at once, which does not allow suggesting a single source for chromites.
The results of the compositional analysis of monomineralic, polymineralic and melt inclusions (
Section 4.5) indicate that chromites could have a volcanogenic origin in addition to their presumed peridotite source. However, since inclusions in chromites are rare, it is impossible to get a general statistic view and define the main type of rocks that supplied chromites to the Sabantuy paleoplacer. To solve this problem, we can consider histograms of the distribution of different components in the studied chromites compared to typical objects (
Figure 17). We used two components of chromspinel Al
2O
3 and Cr
2O
3 with the widest range of values for this purpose. For comparison, we chose peridotites of the Uralian ophiolite complexes without chromitites (778 analyses) [
29,
34,
46,
60,
61], the Uralian ophiolites with chromite ore (1954 analyses) [
34,
46], ophiolite complexes worldwide (5453 analyses) [
42], island-arc volcanic rocks of the Magnitogorsk zone in the Southern Urals (142 analyses) ([
54,
55,
56] and our unpublished data) and MORB basalts worldwide (296 analyses) [
42].
As shown in
Section 4.4., all histograms for chromspinel from the Sabantuy paleoplacer show a unimodal distribution of components with distinctly identify maximums (
Figure 8). It allows defining the prevalent composition of chromspinel in the placer fairly confidently and comparing it with compositions of chromspinels from typical geological objects, which can be potential sources. It is clear that the distribution pattern for Al
2O
3 and Cr
2O
3 in the studied chromspinels with the maximums of 12 wt.% and 52–56 wt.%, respectively (
Figure 17a), differs greatly from the distribution model for these components in island-arc volcanic rocks in the Southern Urals, where low-Al and extremely high-Cr chromites prevail dramatically, whereas chromites with the elevated alumina content are almost absent (
Figure 17f). We reach a similar conclusion comparing the studied chromites with chromspinel from oceanic basalts, where far more high aluminum and fewer chromium compositions dominate (
Figure 17e). The comparison with chromspinels from the Uralian ophiolite complexes excluding associated chromite ores shows an essential difference akin to that for oceanic basalts. The maximum of Al
2O
3 for peridotites is recorded at the level of 25–30 wt.%, the maximum of Cr
2O
3 is in the range of 40–45 wt.% (
Figure 17b). The model of Al
2O
3 and Cr
2O
3 distribution in chromspinels of worldwide ophiolite complexes is essentially closer to the chromspinels we studied (
Figure 17d). Thus, the statistical maximum of Al
2O
3 in worldwide ophiolites is registered in the interval of 10–20 wt.%, whereas the maximum of Cr
2O
3 is in the range of 40–45 wt.% (harzburgite-type chromites) with a significant amount of high-Cr chromites in the interval 50–60 wt.% of Cr
2O
3 in the histogram. It is specified based on a much more comprehensive sampling of chromite ores associated with ophiolites. This pattern is similar when we study the distribution of Al
2O
3 and Cr
2O
3 in the Uralian ophiolite complexes, taking into account the compositions of chromite ores (
Figure 17c). In this case, histograms for the studied detrital chromites and ophiolites show comparable maximums in the interval of 10–12 wt.% Al
2O
3 and 50–60 wt.% Cr
2O
3, while less outstanding maximum typical of chromspinel from peridotites is preserved. This comparison fairly confidently suggests that peridotites of the Uralian ophiolite complexes and associated chromite ores were one of the main sources of chromspinel in the Sabantuy paleoplacer. The influence of ophiolite ore chromite and associated dunite can be prevalent in the Sabantuy paleoplacer, which explains the position of maximums in histograms of the Al
2O
3 and Cr
2O
3 distribution.
However, some issues have no rational explanation still. Thus, it is not clear why no one grain of high-Al chromspinel with Al
2O
3 > 50 wt.% was found among chromspinels of the Sabantuy paleoplacer, though it is typical of lherzolites widespread in the Main Uralian Fault zone in the Southern Urals and, in particular, in the largest Kraka peridotite allochthone closest to the Sabantuy placer [
29,
34,
46]. Another intriguing fact is that sediments of placer contain almost no main rock-forming minerals of peridotites, i.e., olivine, orthopyroxene and clinopyroxene. Though we obtained some evidence that volcanic chromspinel participated in chromite placer, we do not know their provenance yet. It is possible, however, that they were derived from island-arc-related volcanic rocks of the Magnitogorsk zone.
We provided analyses using discrimination diagrams to define geotectonic settings, composition and facial conditions of protolith of rocks that were sources for detrital chromspinels. The TiO
2 content is one of the well-known indicators of the chromspinel origin, which is commonly used for dividing between peridotite and volcanic chromspinels and for identification volcanic rocks of different origin [
11,
16,
45]. The TiO
2 content in the most studied chromspinels is less than 0.2 wt.%, and it corresponds to peridotite-type chromites. The TiO
2 content in chromites comprising melt inclusions is yet commonly higher than 0.4% and reaches 1.6%, which confirms their volcanic origin.
The Fe
2+/Fe
3+ ratio is an additional criterion to divide chromspinels into the mantle and volcanic types [
45,
52]. Thus, chromite of mantle affinity usually has a high Fe
2+/Fe
3+ ratio, and, vice versa, chromite from volcanic rocks has a low ratio. On the TiO
2–Fe
2+/Fe
3+ diagram, most Sabantuy chromite falls into the peridotite field (
Figure 18a). Chromspinel with a higher degree of oxidation state Fe
2+/Fe
3+ < 5 corresponding to volcanic-type chromite is in a subordinate amount. However, an essential part of compositions lies in the uncertainty area, where the fields overlap.
Compositions of chromspinels matching to peridotites were plotted in the Mg#–Cr# diagram (
Figure 18b). It shows that most figurative points overlap with the field of fore-arc peridotites, while the minor part overlaps with the field of abyssal peridotites. In the Al
2O
3–TiO
2 diagram (
Figure 18c), almost all points are in the field of chromspinel from suprasubduction peridotites that commonly overlap with the field of oceanic peridotites. Provided that chromspinels with TiO
2 > 0.4% have a volcanic origin, which is validated by the presence of melt inclusions, the compositions in the Al
2O
3–TiO
2 diagram are distributed between island-arc and oceanic basalts with minor overlap area (
Figure 18d). However, this conclusion is only probabilistic.
The presence of titanomagnetite with TiO
2 > 15 wt.% in the heavy fraction of the Sabantuy sediments indicates that clastic material could be also derived from ancient platform layered mafic–ultramafic intrusions or basalts, such as those known in the Archean and Proterozoic complexes of the Western Slope of the Urals and, in particular, in the Bashkirian Meganticlinorium [
28].
The obtained data indicate that the most Sabantuy chromite could be derived from peridotites and chromitites of ophiolite complexes widespread in the Southern Urals, including huge peridotite allochthones transferred to the west from the Main Uralian Fault through the Ural-Tau anticline structures [
22,
29,
34,
46]. One such allochthon is the Kraka peridotite massif, hosting numerous chromite deposits of different scales [
34,
46]. In the modern coordinates, it is closest to the Sabantuy placer [
38]. Mafic and ultramafic volcanic rocks of the oceanic and island-arc stage of the Urals development could be another source for chromites [
53,
54,
55,
56]. It cannot be excluded that ancient Archean and Proterozoic picrite and basalt dykes or layered mafic–ultramafic intrusions, widespread in the western slope of the Urals [
22,
28], contributed additional chromite in sedimentation basins in Permian time.