4.1. Petrographic Features of Ultramafic Rocks and Chromitites
Like in most ultramafic massifs of the ophiolite assemblages in the Southern Urals, completely serpentinized rocks dominate on the surface of the Kempirsay massif. At the same time, a low-temperature type of serpentinization (mesh texture) occurs almost universally, which allowed to preserve relic structures of ultramafic rocks (
Figure 2a,b). Primary silicate minerals (olivine, pyroxene) are completely replaced by serpentine, while the composition of accessory Cr-spinel remained mainly unaltered.
The primary composition of fully serpentinized rocks can be recognized by the presence of bastite pseudomorphs, which mark the presence of orthopyroxene grains in the protolith, as well as by the morphological and chemical features of accessory spinels. These features considered, lherzolitic and harzburgitic serpentinites predominate among the studied ones.
We managed to find the freshest samples of spinel peridotites in the core of boreholes 766, 809, 820, which will be discussed in more detail below. Among the studied samples, two structural types are distinguished: peridotite samples 809/357 and 820/300 are represented by a porphyroclastic structure, while samples 7087 and 8156 from deep borehole 766 show a granoblastic structure.
Peridotites of both types show clear signatures of high-temperature deformation of orthopyroxene and olivine, which are expressed in kink-band structures, undulose extinction, bending of planar structural elements: lamellae, orthopyroxene cleavage (
Figure 2c,d). Porphyroclastic peridotites consist of rare, relatively large, deformed enstatite grains surrounded by smaller grains of enstatite, diopside and olivine. Lherzolite samples with granoblastic structure (7087, 8156) contain almost no serpentine; olivine and enstatite grains are mainly equiaxial. Elongated olivine grains separated by deformation bands are rare here. In general, polygonal grains with triple junctions at angles close to 120° (
Figure 2e,f) prevail. It is typical of grains subject to secondary recrystallization, once the system tends to minimize grain boundary energy (e.g., [
34,
35]). A distinctive structural feature of large enstatite porphyroclasts is intensely developed numerous inclusions of other phases, i.e., diopside, pargasite and Cr-spinel (
Figure 3a–d). Thus, ultramafic rocks occur in fresh lherzolites from borehole 766 at the depth of more than 1000 m, showing traces of high-temperature deformation, syntectonic recrystallization and annealing recrystallization.
The morphology of Cr-spinel grains varies from amoeboid, vermicular and holly-leaf in lherzolites to subhedral in harzburgites (
Figure 4a–d). A similar pattern is observed in many ophiolite massifs and upper mantle xenoliths (e.g., [
17,
36,
37,
38]). Many accessory spinel grains contain inclusions of olivine, orthopyroxene, clinopyroxene, and pargasite. In addition, there are numerous fragments of silicate mineral grains incompletely captured by branches of xenomorphic spinel grains (
Figure 4a–d). We had formerly noted similar features in lherzolite massifs of the Southern Urals [
11,
23]. In dunites and dunitic serpentinites mainly found close to ore deposits, Cr-spinel grains show a nearly euhedral habit (
Figure 4g,h). However, their characteristic features are smooth boundaries and the presence of cracks along grains commonly perpendicular to foliation and banding of rocks.
Chromitites show a further change in the morphology of Cr-spinel grains. Poorly disseminated chromitites are composed of small grains of chromite (0.1–0.5 mm) with smooth contours (
Figure 5a). Many grains are broken by cracks that are usually perpendicular to the banding of ore bodies. Most chromite grains (80%–85%) have no inclusions; other grains (10%–15%) contain rare round or oval inclusions of olivine, which is in most cases replaced by serpentine, as well as tabular, prismatic or negative crystal inclusions represented by pargasite and rare phlogopite (
Figure 5b). Only few Cr-spinel grains (about 1%) contain numerous inclusions, which are mainly represented by pargasite (
Figure 5c,d), less often by diopside, enstatite and phlogopite.
In densely disseminated chromitites, an average grain size increases (0.5–3 mm), the grain shape becomes angular, which indicates adjustment of individuals’ boundaries in consolidation settings (
Figure 5e). Transverse and radial cracks become more widespread (
Figure 5f), passing through inner parts of grains mainly. Such structural features in chromitites are known as “pull-apart” texture and attributed to the ore formation under tectonic stress conditions [
39].
An even greater consolidation is recorded in massive chromitites. It is expressed in the presence of anhedral segregations of vein minerals formed by “squeezing out” of weaker silicate minerals from gaps between rigid chromite grains (
Figure 5g). The consolidation is also reflected in sharp thinning and gradual vanishing of boundaries between Cr-spinel grains that become angular-shaped (
Figure 5g,h).
Main types of ores: massive and disseminated. Chromitites vary from fine-grained (<1 mm) to coarse-grained (>3 mm), the most typical are densely disseminated ores with 70%–90% chromite grains. Ore styles are banded, schlieren-banded, uniformly disseminated and spotty varieties. Massive chromitites (>90% chromite) are mainly composed of medium- and coarse-grained types (grain size more than 2 mm). Nodular ores are of subordinate importance and are usually found near massive ores, more often along the periphery of ore bodies. Nodules are usually composed of an aggregate of sub- and anhedral grains larger than 2 mm; the cement is completely serpentinized dunite.
4.3. Composition of Minerals
The main primary minerals of the studied ultramafic rocks are high-Mg olivine, orthopyroxene, and Ca-Mg clinopyroxene; Cr-spinel and amphibole are accessory phases. The main secondary mineral is serpentine; chlorite and carbonates are rare. Fe, Ni, Co, Cu, PGE sulfides, native minerals Ni-Fe, Cu and PGE also occur in small amounts as minor segregations in ultramafic rocks and chromitites. In this work, we will focus on description of the main minerals of the primary (upper mantle) assemblages only.
Olivine composes 75% to 100% of the rock volume in lherzolites and dunites, respectively. However, it is most susceptible to serpentinization and, therefore, is almost not preserved in the upper parts of the massif, giving way to mesh serpentine. In the studied samples, its content varies from complete absence to 10%–50%, and only in samples from borehole 766 is it almost completely preserved and makes up 75%–80% of the rock.
The olivine is exclusively magnesian and varies from 91 at.% to 95 at.% of the forsterite (Fo) end-member (
Table 2). Notably, grains included in ore chromite are highest in Mg, while some lherzolite samples are lowest in Mg. However, it should be noted that there is no clear relationship between the Mg content of olivine and the Cr-number (#Cr) of accessory spinel, as implied by fields in the OSMA diagram (
Figure 6b). Among the studied samples, the least magnesian olivines were found in peridotites of the MOF (Fo
91–92), while rather high-Mg olivines coexist with high-Al spinels (Fo
93–94) in fresh lherzolites of borehole 766.
Orthopyroxene is the second major mineral in lherzolites and harzburgites. In dunites it often occurs as a minor phase (less than 5%), such rocks are often called pyroxene- (or enstatite-) dunites [
14]. It is also subject to serpentinization with bastite pseudomorphs produced, but sometimes it is replaced by amphibole. Orthopyroxene is high-Mg with an enstatite end-member content of 91–93 at.% (
Table 3,
Figure 6c). The usual minor elements of Opx are Ca, Al and Cr. In fact, the following regularity is established: large enstatite porphyroclasts are higher in Al, Cr and Ca (2.5–3/0.6–0.8/0.5–0.8 wt.%, respectively) compared to their concentrations in neoblasts (0.9–1.5/0.2–0.5/0.3–0.6 wt.%, respectively) formed during syntectonic recrystallization of deformed grains.
Clinopyroxene occurs as an accessory mineral in some samples of dunites and harzburgites (up to 5%), while in lherzolites it is present mainly as small grains (10–100 µm) in the amount of 5%–8%. Large grains of clinopyroxene are very rare. It is often found in association with Cr-spinel grains or close to large orthopyroxene porphyroclasts. The studied clinopyroxene grains are represented by the Ca-Mg variety (
Table 4). On the triangular diagram their compositions fall into the field of diopside mainly and only a small part of the analyses is interpreted as augite (
Figure 6c). The main impurities are aluminum (0.74–3.26 wt.% Al
2O
3) and chromium (0.25–1.29 wt.% Cr
2O
3), single analyses show the presence of sodium (up to 0.2 wt.% Na
2O) and titanium (up to 0.3 wt.% TiO
2).
Amphibole is found in minor amounts (0.n–2%) in lherzolites, usually occurring inside deformed enstatite grains as lamellae or as small prismatic grains along their periphery in association with small grains of olivine, ortho- and clinopyroxene. Amphibole typically shows a very consistent composition and corresponds to the Mg-Ca variety—Pargasite (
Table 5), which constant minor elements are sodium and chromium. Their amount ranges from 0.8 to 3 wt.% of the corresponding oxide.
Cr-spinel is a constant accessory mineral in all varieties of ultramafic rocks and the major mineral of chromitites. Its content in lherzolites and harzburgites varies from tenths of a percent to 3%–5%, while in dunites there are wider variations up to the formation of ore concentrations (disseminated chromitites contain > 20% chromite).
Cr-spinel grains are characterized by widely varied compositions (
Table 6;
Figure 7a,b) expressed both in the change of ratios between trivalent cations (mainly Cr/Al) and divalent cations (Mg/Fe). The content of trivalent iron in accessory spinel grains is insignificant and slightly increases in host dunites and chromitites. The further growth of Fe
3+ is associated with the metamorphism of ultramafic rocks and chromitite, which is no typical phenomenon for the Kempirsay massif.
Varied compositions of Cr-spinel indicate a clear link to mineralogical and chemical compositions of ultramafic rocks. Thus, the most high-Al spinel grains are characteristic of lherzolites (#Cr = 0.2–0.45), intermediate values of Cr/Al are recorded in harzburgites (#Cr = 0.45–0.62) and the most high-Cr are spinel grains from dunites (#Cr = 0.7–0.85) and chromitites (#Cr = 0.8–0.9). Compositions of Cr-spinel in lherzolite-harzburgite-dunite assemblages show a positive correlation between the Cr/Al and Fe/Mg ratios. However, the transition from dunites to chromitites is accompanied by a slight increase in #Cr and a significant decrease in the content of Fe (
Figure 7b).
4.4. Estimates of PT-fO2 Formation Conditions of Mineral Assemblages
To determine formation conditions of primary mineral assemblages of ultramafic rocks, we used several versions of olivine–spinel [
42,
45,
46,
47] and two-pyroxene geothermometers [
43,
44,
48,
49], oxygen barometer from [
42] and geobarometers from [
44].
It is well known that systematic discrepancies exist between the two mentioned types of geothermometers, which are generally considered as the result of different rates of equilibrium in olivine-spinel and orthopyroxene-clinopyroxene pairs (e.g., [
50]). Some researchers believe that it allows us to reconstruct the history of rock cooling, on the one hand, and roughly estimate initial and final conditions of high-temperature processes stopping in the respective area of the upper mantle (e.g., [
51]).
The estimates we obtained indicate that equilibrium temperatures in the Ol-Spl and Opx-Cpx pairs differ by 150–250 °C on average, and in both cases reflect temperatures of subsolidus reactions. Temperatures estimated according to the program after [
44] fall in the range of 850–1150 °C. Combined with the pressure calculation, they indicate the formation of ultramafic rocks at the level of the upper mantle depths from garnet to plagioclase facies (
Figure 7c). At the same time, there are two clusters that correspond to the depth ranges of 40–70 km and 15–30 km, respectively.
The assessment of redox conditions for the formation of olivine–Cr-spinel assemblages demonstrates a fairly wide range of values, from −2 to +2.7 Δlog FMQ (fO
2), with the largest range of values typical of lherzolites. The lowest values of oxygen fugacity were found in fresh lherzolites from borehole 766, while zero and positive values prevail in host rocks of the Almaz-Zhemchuzhina deposit. In general, most of the values are comparable with the field of abyssal peridotites, as well as with those we obtained earlier for the rocks of the lherzolite massifs of the Southern Urals (Kraka, Nurali, Mindyak), ranging from −1 to +1.5 Δlog FMQ(fO
2) (
Figure 7d).
4.5. Textural Features of Olivine and Orthopyroxene According to EBSD Data
Undulose extinction of olivine and orthopyroxene grains is attributed to the distortion of the crystalline lattice as a result of plastic deformation and to the presence of low-angle grain boundaries—LABG (or subgrain boundaries) in case of recovery. Optical microscopy shows that the nature of LABG can be different, i.e., in some cases, formation of a series of parallel elongated crystal blocks (subgrain walls) is recorded, in other cases, LABGs separate approximately equiaxial areas (chessboard-type subgrains) [
52]. Both types of boundaries are widespread in serpentinized ultramafic rocks of the MOF and in fresh lherzolites from borehole 766.
We studied the latter using EBSD and obtained data on the lattice preferred orientation (LPO) of olivine and orthopyroxene (
Figure 8). The fabric of both minerals indicates that the rocks are mantle tectonites that were subject to high-temperature deformation in the dislocation creep regime accompanied by syntectonic recrystallization. At the same time, various types of fabric were established in the studied samples. In sample 7087, the [001] axis coincides with the lineation (L) of olivine, and the other two form girdles perpendicular to the foliation (S). This pattern is transitional between B and C fabric types of olivine and is characteristic of “wet” conditions of plastic deformation [
53]. In orthopyroxene, the [001] axis also coincides with the lineation, and the (010) plane coincides with the foliation plane, which is typical of the BC type fabric of orthopyroxene [
53].
In sample 8156, the maximum of the [100] olivine axis is close to lineation, the [010] axis form the maximum almost perpendicular to the foliation, and the [001] axis is either bunched near it or inclined at an acute angle to foliation. A similar pattern is characteristic of fabric type A [
35,
53], which is usually formed during slip by [100] (010) system. The axes of orthopyroxene from sample 8156 produce numerous dispersed maxima on the pole figure, which makes unambiguous interpretation difficult. The observed fabric pattern is closest to the ABC type, which was established in some experiments on the deformation of orthopyroxene [
54].
4.6. Mineral Inclusions in Olivine and Orthopyroxene
Fine segregations of other phases, i.e., diopside, pargasite and Cr-spinel, with their size ranging from fractions of a micrometer to a few tens of micrometers are observed in almost all of the studied ultramafic rock samples with well-preserved relics of primary silicates, in plastically deformed orthopyroxene grains (
Figure 9 and
Figure 10). They usually occur in parallel swarms in large porphyroclasts (
Figure 9a,b). In the rim of large Opx grains, fine segregations of pargasite, spinel and diopside are associated with fine equiaxial olivine and enstatite grains depleted in minor elements. Inside the deformed olivine grains, there are usually rod-like or vermicular segregations of Cr-spinel and tiny segregations of pargasite (
Figure 9c,d).
In some cases, there is a regular change in the morphology and size of segregations with a change in the host mineral structure and the distribution style of inclusions. In particular, while abundant parallel lamellae of diopside, pargasite and spinel prevail in the central parts of large deformed orthopyroxene grains (
Figure 10a,b), rare equiaxial inclusions of the diopside, pargasite and spinel with no inclusions around them are observed in the rims of the same grains (
Figure 10c,e). In some cases, we observed branches from those precipitates that were equally oriented regarding lamellae in the main crystal volume (
Figure 10d–f).