Geochemistry of the Cheremkhovo and Lower Prisayan

minerals

Article Geochemistry of the Cheremkhovo and Lower Prisayan Formations from the Jurassic Irkutsk Coal-Bearing Basin: Evidence for Provenance and Climate Change in Pliensbachian–Toarcian

Ekaterina A. Mikheeva * , Elena I. Demonterova and Alexei V. Ivanov

Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, 664033 Irkutsk, Russia; [email protected] (E.I.D.); [email protected] (A.V.I.) * Correspondence: [email protected]

Abstract: The Cheremkhovo formation (Pliensbachian) is the primary coal-bearing formation of the Irkutsk basin, Eastern Siberia. Still, few geochemical studies of the Jurassic sediments of the Irkutsk coal-bearing basin have been conducted, and there are no data on the geochemistry of the coal- bearing formation itself. This study presents geochemical data for 68 samples from the Cheremkhovo

formation and the overlying Lower Prisayan formation. The age of the former has been estimated by U-Pb dating of zircon from a tonstein (altered volcanic ash) layer as Pliensbachian, whereas the age of the latter is estimated as Pliensbachian–Toarcian according to regional stratigraphy. Major oxide Citation: Mikheeva, E.A.; Demonterova, E.I.; Ivanov, A.V. and trace element concentrations were obtained using X-ray ﬂuorescence spectrometry. Geochemical Geochemistry of the Cheremkhovo indicators showed diversity between the two studied formations. The indicators used show the and Lower Prisayan Formations from change in climate conditions, from warm and humid in the Cheremkhovo formation, to hot and arid the Jurassic Irkutsk Coal-Bearing during the deposition of the lower Prisayan formation. The provenance of the Irkutsk coal-bearing Basin: Evidence for Provenance and basin was mainly inﬂuenced by the source composition, not recycling, and sediments were mainly Climate Change in Pliensbachian– derived from felsic to intermediate igneous rocks with a mixture of other rock types. Toarcian. Minerals 2021, 11, 357. https://doi.org/10.3390/ Keywords: Irkutsk coal-bearing basin; geochemistry; Cheremkhovo formation; lower Prisayan min11040357 formation; paleoclimate; source; provenance

Academic Editors: Monika J. Fabia´nska,Magdalena Misz-Kennan and Justyna Ciesielczuk 1. Introduction

Received: 11 March 2021 The Irkutsk coal-bearing basin is located in the southern part of the Siberian Craton Accepted: 24 March 2021 (Figure1). A large fuel and energy complex has been created in the region; about 15 million Published: 30 March 2021 tons of coal are mined annually. On the Russian scale, it provides 6% of coal production [1]. The regional stratigraphic chart for the basin sediments was accepted in 1981 with the Publisher’s Note: MDPI stays neutral Cheremkhovo, Prisayan, and Kuda formations [2]. The Cheremkhovo formation (Fm.) is with regard to jurisdictional claims in the primary coal-bearing formation of the Irkutsk basin. Based on biostratigraphy, different published maps and institutional afﬁl- scholars attribute the Cheremkhovo Fm. to the Pliensbachian [3], Pliensbachian–Toarcian [2–4] iations. or Pliensbachian–Aalenian [5] intervals. The only radioisotopic date (187.44 +0.45/−1.60 Ma, Pliensbachian) was obtained by the U-Pb method using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on zircon of a tonstein (altered volcanic ash) layer from an industrial coal seam of the Azeysk deposit [6]. According to [3], whose Copyright: © 2021 by the authors. stratigraphic scheme is consistent with U-Pb data, the Prisayan Fm. and Kuda Fm. are Licensee MDPI, Basel, Switzerland. Pliensbachian–Aalenian and Aalenian–Bajocian, respectively. This article is an open access article Despite the large number of publications devoted to studies of the Irkutsk coal-bearing distributed under the terms and basin [3–5,7–11], there are too few modern analytical data for this area. In recent years, the conditions of the Creative Commons study of the Irkutsk coal-bearing basin was focused on stratigraphy, structural evolution, Attribution (CC BY) license (https:// and sedimentology [5,11–16]. Geochemical data were only obtained for coals of the Azeisk creativecommons.org/licenses/by/ coal deposit [17] and the uppermost Prisayan Fm. and Kuda Fm. [11,12,18]. 4.0/).

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Minerals 2021, 11, 357 years, the study of the Irkutsk coal-bearing basin was focused on stratigraphy, structural2 of 17 evolution, and sedimentology [5,11–16]. Geochemical data were only obtained for coals of the Azeisk coal deposit [17] and the uppermost Prisayan Fm. and Kuda Fm. [11,12,18].

Figure 1. Schematic map showing the distribution of the Irkutsk coal-bearing basin sediments in the southern part of the Siberian craton. Siberian craton.

On the the global global scale, scale, the the Toarcian Toarcian is mark is markeded by an by ocean an ocean anoxic anoxic event event(Toarcian-OAE) (Toarcian- [19],OAE) which [19], whichin turn in likely turn resulted likely resulted from Karoo-Ferrar from Karoo-Ferrar flood basalt ﬂood volcanism basalt volcanism [20,21]. [ To20, 21see]. Tothesee probable the probable changes changes in the in volcanic-relat the volcanic-relateded environmental environmental conditions, conditions, we we provide provide a detaileda detailed geochemical geochemical study study of of sediments sediments of of the the Cheremkhovo Cheremkhovo Fm. Fm. and Lower Prisayan Fm., i.e., those formations which, on the one hand, were formed before, at, and after the Toarcian-OAE, but,but, onon thethe otherother hand,hand, werewere notnot yetyet studied.studied.

1.1. Geological Setting The Jurassic Jurassic stratigraphy stratigraphy of of the the Irkutsk Irkutsk basin basin is compiled is compiled in Figure in Figure 2, using2, using data datafrom geologicalfrom geological surveys surveys [22,23] [22 and,23 Skoblo] and Skoblo et al. [3]. et al.The [3 ].basis The of basis the Jurassic of the Jurassic sediments sediments is rep- resentedis represented by the by 30 the m- 30to m-240 tom-thick 240 m-thick Cheremkhov Cheremkhovoo Fm. It Fm.is overlain It is overlain by the by135 the m- 135to 350 m- m-thickto 350 m-thick Prisayan Prisayan Fm. The Fm. Jurassic The Jurassic sediments sediments compete compete with the with up theto 145 up tom-thick 145 m-thick Kuda Fm.Kuda The Fm. alluvial, The alluvial, the alluvial–deluvial, the alluvial–deluvial, the floodplain, the ﬂoodplain, and lacustrine-boggy and lacustrine-boggy facies are facies dis- tinguishedare distinguished by the bybasin the sediments basin sediments [7,8]. [7,8].

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Figure 2. Lithology column of the Jurassic sediments of the Irkutsk coal-bearing basin. The black star marks the tonstein dated on zircon by U-Pb laser ablation inductivelyinductively coupled plasma massmass spectrometryspectrometry (LA-ICP-MS)(LA-ICP-MS) [[6]6]..

1.2.1.2. The The lower Lower Cheremkhovo Cheremkhovo Formation Formation TheThe lower lower Cheremkhovo Cheremkhovo Fm. is is mostly mostly coarse-grained coarse-grained and and composed composed of of sandstones, sandstones, gravelstones,gravelstones, conglomerates, conglomerates, and and thin layers of siltstones,siltstones, mudstones,mudstones, andand coal.coal. The subformationsubformation thickness thickness is is from from 100–130 100–130 m m to 30 m, up to fullfull thinningthinning [[24].24]. The lower CheremkhovoCheremkhovo Fm. indicates indicates the the transitions transitions from from widespread coarse-grained facies to clay-brecciatedclay-brecciated and sand facies [3,7,8,25]. [3,7,8,25]. Fa Faciescies depend on sedimentation conditions, the buried morphology, and provenance. The clay-brecciated facies is not more than 20 m [23]

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buried morphology, and provenance. The clay-brecciated facies is not more than 20 m [23] thick. The average thickness of the sand facies is 10–16 m, up to 30 m [23]. Both of these thick. The average thickness of the sand facies is 10–16 m, up to 30 m [23]. Both of these facies are limited within the Irkutsk basin. The widespread coarse-grained facies building facies are limited within the Irkutsk basin. The widespread coarse-grained facies building up the basal part of the Jurassic section extended over almost the entire area of the basin. up the basal part of the Jurassic section extended over almost the entire area of the basin. Samples were collected in order to cover the widespread coarse-grained facies of the lower Samples were collected in order to cover the widespread coarse-grained facies of the lower Cheremkhovo Fm. Cheremkhovo Fm. The lower Cheremkhovo Fm. is exposed in the area of Zalari town (53◦34009.1400 N, The lower Cheremkhovo Fm. is exposed in the area of Zalari town (53°34´09.14″N, 102◦32037.7400 E) (Figure3). The base of the section is unconformably overlying the Cam- 102°32´37.74″E) (Figure 3). The base of the section is unconformably overlying the Cam- brianbrian rocks.rocks. The The section section consists consists of ofwell-sorted well-sorted pebble pebble to boulder to boulder conglomerates. conglomerates. The con- The conglomeratesglomerates are areinterlayered interlayered with with beds beds of ofwell-sorted well-sorted sandstones, sandstones, varying varying from from fine- ﬁne- grainedgrained toto gravel.gravel. The composition of of the the pebbles pebbles is is controlled controlled by by quartzites, quartzites, quartz quartz with with granitoids.granitoids. PebblesPebbles are characterized by by a a good good degree degree of of roundness. roundness. The The polymict polymict sand- sand- stonesstones consist consist ofof badlybadly sorted minerals minerals (quartz, (quartz, cal calcite,сite, plagioclase, plagioclase, and and feldspar) feldspar) and and rock rock piecespieces (quartzite,(quartzite, gneisses,gneisses, and granite). С Clastlast roundness roundness is is from from medium medium toto good. good. The The matrixmatrix is is of of clay clay and and iron iron oxide oxide particles particles intermixed intermixed with with the the clay. clay. The The siltstone siltstone composition composi- istion close is close to that to ofthat sandstone. of sandstone.

FigureFigure 3. 3.Section Section ofof thethe lower Cheremkhovo formation formation (Fm.) (Fm.) sampling sampling in inthe the area area of ofZalari Zalari town. town.

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◦ 0 00 TheThe lower lower Cheremkhovo Cheremkhovo Fm. Fm. is is exposed exposed in in the the area area of of Kutulik Kutulik village village (53 (53°22´02.1422 02.14 ″N,N, ◦ 0 00 102102°47´02.3047 02.30 ″E)E) (Figure(Figure4 4).). The The deposits deposits are are composed composed of of sandstones sandstones and and siltstones. siltstones. The The polymictpolymict and and feldspar–quartz feldspar–quartz sandstones sandstones consist consist of of poorly poorly sorted sorted mineral mineral grains grains (quartz, (quartz, feldspar,feldspar, and and plagioclase) plagioclase) and and rock rock pieces pieces (quartzite, (quartzite, felsic felsic volcanic volcanic rocks, rocks, gneisses, gneisses, and and granite).granite). Mineral Mineral grains grains and and rock piecesrock pieces are characterized are characterized by varying by degrees varying of roundness,degrees of fromroundness, poor to from good. poor The to matrix good. is The composed matrix is of composed clay and iron of clay oxide and particles iron oxide intermixed particles withintermixed the clay. with The the siltstone clay. The composition siltstone composition is close to that is ofclose sandstone. to that of sandstone.

FigureFigure 4. 4.Section Section of of the the lower lower Cheremkhovo Cheremkhovo Fm. Fm. sampling sampling in in the the area area of of Kutulik Kutulik village. village.

1.3.1.3. The The Upper upper CheremkhovoCheremkhovo FormationFormation TheThe upper upper Cheremkhovo Cheremkhovo Fm. Fm. is is composed composed of of siltstones, siltstones, mudstones, mudstones, industrial industrial coal, coal, andand sandstones. sandstones. All All coal coal deposits deposits known known in in the the Irkutsk Irkutsk basin basin are are associated associated with with the the upper upper CheremkhovoCheremkhovo Fm. Fm. The The Irkutsk Irkutsk basin basin coals arecoals mostly are lowermostly rank; lower metamorphism rank; metamorphism increases inincreases the northwest in the direction northwest to thedirection southeast to [the23]. southeast The second-highest [23]. The stagesecond-highest of metamorphism stage of ismetamorphism characterized by is coalscharacterized of the southeastern by coals of part the of southeastern the basin, but part there of arethe no basin, coal depositsbut there thatare wouldno coal be deposits economic that to would mine. be economic to mine.

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TheThe upper upper Cheremkhovo Cheremkhovo Fm. samplesFm. samples were collectedwere collected at the Cheremkhovo, at the Cheremkhovo, Golovinsk, Bozoy,Golovinsk, and Azeisk Bozoy, coal and deposits. Azeisk Thecoal sampled deposits. sections The sampled of the Cheremkhovo sections of the (53 Cheremkhovo◦15015.2700 N, 102(53°15´15.27◦57009.0600″E) N, and 102°57´09.06 Golovinsk″ (53E) ◦and26051.21 Golovinsk00 N, 102 (53°26´51.21◦53058.4800 E)″ coalN, 102°53´58.48 deposits are″ almostE) coal thedeposits same are (Figure almost5). the The same deposits (Figure are 5). composed The deposits of coals, are composed mudstones, of coals, siltstones, mudstones, and sandstones.siltstones, and The sandstones. arkose and feldspar–quartzThe arkose and sandstonesfeldspar–quartz consist sandstones of badly sortedconsist minerals of badly (quartz,sorted minerals feldspar, and(quartz, plagioclase) feldspar, and and rock plagioclase) fragments and (volcanic rock fragments rocks, gneisses (volcanic and gran-rocks, ite,gneisses and quartzite). and granite, Mineral and quartzite). grains and Mineral rock pieces grains are and differentiated rock pieces byare roundness, differentiated from by poorroundness, to good. from The poor matrix to good. is composed The matrix of clay. is composed The siltstone of clay. composition The siltstone is close composition to that ofis sandstone.close to that of sandstone.

FigureFigure 5. 5.Section Section ofof thethe upper Cheremkhovo Fm. Fm. samp samplingling in in the the area area of of the the Cheremkhovo Cheremkhovo and and GolovinskGolovinsk coal coal deposits. deposits.

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◦ 0 00 ◦ 0 00 TheThe BozoyBozoy section section (52 (52°52´01.1652 01.16 N,″ 105N, 105°02´28.3702 28.37 E)″ isE) represented is represented by coals, by mud-coals, stones,mudstones, siltstones, siltstones, and sandstones and sandstones (Figure6 ).(Fig Theure feldspar–quartz 6). The feldspar–quartz sandstones sandstones are composed are ofcomposed badly sorted of badly minerals sorted (quartz, mine feldspar,rals (quartz, and plagioclase)feldspar, and and plagioclase) rock pieces and (volcanic rock rocks,pieces gneisses,(volcanic granite, rocks, andgneisses, quartzite). granite, The and fragments quartzite). are differentiatedThe fragments by are varying differentiated degrees ofby roundness,varying degrees from poorof roundness, to good. The from matrix poor isto composed good. The of matrix iron oxide is composed particles of intermixed iron oxide withparticles the clay. intermixed The siltstone with the composition clay. The siltstone is close to composition that of sandstone. is close to that of sandstone.

FigureFigure 6. 6.Section Section of of the the upper upper Cheremkhovo Cheremkhovo Fm. Fm. sampling sampling in in the the area area of of the the Bozoy Bozoy coal coal deposit. deposit.

1.4.1.4. TheThe Lowerlower Prisayan Formation AccordingAccording to to Skoblo Skoblo et et al. al. [ 3[3],], the the lower lower Prysayan Prysayan Fm. Fm. isis Pliensbachian–Toarcian.Pliensbachian–Toarcian. The The lowerlower Prisayan Prisayan Fm. Fm. isis composed composed ofof sandstones, sandstones, siltstones,siltstones, andand gravelstonesgravelstones withwith a а thin thin inter-bedinter-bed of of mudstones, mudstones, conglomerates, conglomerates, and and coals. coals. TheThe lower lower Prisayan Prisayan Fm. Fm. was was sampled sampled along along the the Angara Angara river’s river’s starboard starboard side side below below ◦ 0 00 ◦ 0 00 thethe village village of of Ust-Baley Ust-Baley (52 (52°37´34.5137 34.51 N,″ 103N, 103°58´08.2758 08.27 E;″ Figure E; Figure7). The 7). section The section is repre- is sentedrepresented by sandstones by sandstones with beds with of pebblesbeds of andpebbles siltstones. and silt Thestones. composition The composition of pebbles isof dominatedpebbles is bydominated felsic effusive by felsic rocks, effusive with a fewrocks, granitoids with a few and granitoids metamorphic and rocks. metamorphic Pebbles

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arerocks. characterized Pebbles are by characterized a good degree by of a roundness. good degree The of quartz–feldsparroundness. The andquartz–feldspar feldspar–quartz and sandstonesfeldspar–quartz consist sandstones of badly sorted consist minerals of badly (quartz,sorted minerals feldspar, (quartz, plagioclase, feldspar, and plagioclase, mica) and rockand piecesmica) (felsicand rock volcanic pieces rocks, (felsic granite, volcanic gneisses, rocks, andgranite, quartzite). gneisses, Clasts and differquartzite). by varying Clasts roundness,differ by varying from poor roundness, to good. from The matrix poor to is composedgood. The ofmatrix iron oxideis composed particles of intermixed iron oxide withparticles clay, andintermixed clay. The with siltstone clay, and composition clay. The is siltstone close to thatcomposition of sandstone. is close At to different that of stratigraphicsandstone. At levels different of the stratigraphic Irkutsk basin, levels especially of the Irkutsk in the Prisayanbasin, especially Fm., beds in the or nodulesPrisayan withFm., abeds high or calcite nodules concentration with a high occur. calсite concentration occur.

FigureFigure 7. 7.Section Section of of the the lower lower Prisayan Prisayan Fm. Fm. sampling sampling in in the the area area of of Ust-Baley Ust-Baley village. village.

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2. Materials and Methods The studied material comes from natural outcrops and sections of the Cheremkhovo, Azeysk, Golovinsk, and Bozoy deposits. The sampling locations are shown in Table S1. The petrographic analysis was carried out on sedimentary rocks and conglomerate pebbles. Sample preparation and the analytical procedures were performed at the Center for Geodynamics and Geochronology of the Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences (Irkutsk). The samples were crushed, split, and pulverized to a powder. Major element oxide (SiO2, Al2O3, Fe2O3, TiO2, MnO, MgO, CaO, K2O, Na2O, P2O5) and trace element (Ni, Cu, Ga, Pb, V, Cr, Co, Ba, La, Ce, Nd, Sm, Ta, Sc, Cs, As, Br, Nb, Zr, Y, Sr, Rb, Th, U) concentrations were analyzed by wavelength dispersive X-ray ﬂuorescence (XRF) with an S8 TIGER (Bruker AXS GmbH, Germany) X-ray spectrometer using SPECTRAplus software after the procedures described in [26,27]. This method was chosen because of the low cost and high speed of the total analytical procedure, compared to other multielement analytical methods.

3. Results This study presents geochemical data on the Jurassic deposits of the Cheremkhovo and lower Prisayan formations from the Irkutsk coal-bearing basin. Major element oxide and trace element data for the samples are given in Table S1. The Irkutsk basin’s sediments reveal a wide range of major element oxides, with SiO2 and Al2O3 being the dominant constituents. The concentration of SiO2 is from 40.93 to 88.24% in the Cheremkhovo Fm. and from 53.12 to 75.57% in the lower Prisayan Fm. The concentration of Al2O3 is from 3.92 to 23.13% in the Cheremkhovo Fm., and from 12.91 to 20.92% in the lower Prisayan Fm. The concentration of Fe2O3 ranges from 0.69 to 19.32% in the Cheremkhovo Fm., and from 1.75 to 13.99% in the lower Prisayan Fm. In sporadic samples the concentration of CaO is very high, more than 6.60%. We associate such CaO concentrations with nodules that occur at different stratigraphic levels of the Irkutsk basin section. Other major element oxides, such as K2O, TiO2, MgO, Na2O, and P2O5, are present in low concentrations (≤4.05%). In order to geochemically characterize the studied deposits of the Irkutsk coal-bearing basin, we did not use carbonate-rich sediments (CaO ≥ 10%), metasomatized or metamorphosed sediments [28].

3.1. Chemical Classification The samples from the Cheremkhovo and lower Prisayan formations have different ratios of SiO2/Al2O3, and geochemical classification suggests that they cluster tightly in the graywacke, arkose, and litharenite fields. The Cheremkhovo Fm. samples fall in the graywacke, arkose, and litharenite fields [29]. The lower Prisayan Fm. samples fall in the graywacke field. The low SiO2 and ratios of SiO2/Al2O3 content demonstrate the immatu- rity of the Irkutsk basin sediments and suggest short transport distances between the major source regions and the sedimentary basin. The samples exhibit differences in Na2O/K2O ratios, especially in the lower Prisayan Fm. The group of the lower Cheremkhovo Fm. samples (six points) has very low log (Na2O/K2O) values from −1.44 to 1.26, and was not included in the classification diagram of Pettijohn [29] (Figure8). Minerals 2021, 11, 357 10 of 17 Minerals 2021, 11, x FOR PEER REVIEW 10 of 17

Figure 8. Chemical classification of sandstones in the Irkutsk coal-bearing basin, based on their Figure 8. Chemical classiﬁcation of sandstones in the Irkutsk coal-bearing basin, based on their major major element compositions (after [29]). element compositions (after [29]).

3.2.3.2. Weathering Weathering and and Paleoclimate Paleoclimate TheThe rate rate of of chemical chemical weathering weathering of of rocks rocks and an thed the erosion erosion rate rate of weatheringof weathering profiles profiles is controlledis controlled by by climate climate as wellas well as as source source rock rock composition composition and and tectonics. tectonics. It It is is well well known known thatthat chemical chemical weathering weathering strongly strongly affects affects the the major major element element geochemistry geochemistry and and mineralogy mineralogy ofof siliciclastic siliciclastic sediments sediments [ 30[30–32].–32]. Several Several chemical chemical indices indices have have been been proposed proposed to to quantify quantify thethe intensity intensity of of weathering weathering [28 [28,30–36].,30–36]. The The chemical chemical index index of alterationof alteration (CIA) (CIA) proposed proposed by Nesbittby Nesbitt and Youngand Young [30] is[30] widely is widely used toused check to check the degree the degree of chemical of chemical weathering weathering in rocks in androcks as aand marker as a of marker palaeoclimate. of palaeoclimate. The CIA ratio The can CIA be calculatedratio can as:be CIAcalculated = Al2O 3as:/(Al CIA2O 3 = 1 +CaOAl2O3/(Al+ Na2O2O3 +CaO + K2O)1 +× Na1002O [30+ K].2O) The × plagioclase100 [30]. The index plagioclase of alteration index (PIA) of alteration is a CIA modifi-(PIA) is 1 cationa CIA [ 32modification]. This can be[32]. calculated This can as:be PIAcalculated = 100 ×as:(Al PIA2O =3 100− K ×2 O)/(Al(Al2O3 2−O K32O)/(Al+ CaO2O+3 Na+ CaO2O 1 1 −+ KNa2O).2O CaO− K2O).represents CaO1 represents the quantity the quantity of CaO integratedof CaO integrated in the silicate in the fraction,silicate fraction, and can and be 1 determinedcan be determined using the using McLennan the McLennan et al. method et al.[ 31method], where [31], CaO where= CaO CaO−1 (10/3= CaO× − P(10/32O5). × P2OAs5). shown in Figure9, the CIA values for all samples varied from 51.47 to 90.23. The CIA valuesAs shown of the in lower Figure Cheremkhovo 9, the CIA values Fm. for samples all samples ranged varied from 51.47from 51.47 to 84.27 to 90.23. (average The valueCIA values 75.68), of from the 64.14lower to Cheremkhovo 90.23 (average Fm. value samp 80.81)les ranged for the from upper 51.47 Cheremkhovo to 84.27 (average Fm., andvalue from 75.68), 81.78 from to 66.90 64.14 (average to 90.23 value (average 72.17) value for the80.81) lower for Prisayanthe upper Fm. Cheremkhovo We see that theFm., Cheremkhovoand from 81.78 Fm. to 66.90 samples (average show intensevalue 72.17) weathering for the andlower the Prisayan lower Prisayan Fm. We Fm.see that shows the moderateCheremkhovo weathering. Fm. samples The PIA show values intense for the weat Cheremkhovohering and the Fm. lower were quitePrisayan high, Fm. ranging shows frommoderate 51.93 toweathering. 98.65, and The offering PIA highvalues chemical for the Cheremkhovo weathering with Fm. hot were and quite humid high, conditions. ranging Thefrom smallest 51.93 averageto 98.65, PIA and is seenoffering in the high lower chemical Prisayan weathering Fm. (80.74). Thewith PIA hot values and ofhumid the studiedconditions. samples The suggestsmallest intense averag chemicale PIA is weathering.seen in the lower For most Prisayan samples, Fm. the (80.74). PIA and The CIA PIA valuesvalues match of the wellstudied and samples indicate suggest a comparable intense weathering chemical weathering. grade (Figure For9). most In the samples, group ofthe thePIA lower and CIA Cheremkhovo values match Fm. well samples, and indicate the PIA a andcomparable CIA values weathering do not match grade well. (Figure This 9). groupIn the also group has of very the low lower log (NaCheremkhovo2O/K2O) values Fm. samples, in the classification the PIA and diagram CIA values (Figure do8). not match well. This group also has very low log (Na2O/K2O) values in the classification diagram (Figure 8).

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Figure 9. The plot of the chemical index of alteration (CIA) versus the plagioclase index of Figure 9. The plot of the chemical index of alteration (CIA) versus the plagioclase index of Figurealteration 9. The (PIA). plot of the chemical index of alteration (CIA) versus the plagioclase index of alter- alteration (PIA). ation (PIA). Th/U ratios rise with increasing weathering. The Th/U ratios of the lower Th/U ratios rise with increasing weathering. The Th/U ratios of the lower CheremkhovoTh/U ratios Fm. rise withsamples increasing ranged weathering. from 0.6 to The 4.0 Th/U (average ratios ofvalue the lower2.1), from Cheremkhovo 1.9 to 4.5 Cheremkhovo Fm. samples ranged from 0.6 to 4.0 (average value 2.1), from 1.9 to 4.5 Fm.(average samples value ranged 3.0) for from the0.6 upper to 4.0 Cheremkhovo (average value Fm., 2.1), and from from 1.9 1.9 to to 4.5 4.0 (average (average value value (average value 3.0) for the upper Cheremkhovo Fm., and from 1.9 to 4.0 (average value 3.0)2.8) for for the the upper lower Cheremkhovo Prisayan Fm. (Figure Fm., and 10). from At 1.9the toplot 4.0 of (average Th versus value Th/U, 2.8) we for can the see lower that 2.8) for the lower Prisayan Fm. (Figure 10). At the plot of Th versus Th/U, we can see that PrisayansamplesFm. from (Figure the Irkutsk 10). At basin the plot did of not Th suffer versus from Th/U, strong we can weathering. see that samples Low Th/U from ratios the samples from the Irkutsk basin did not suffer from strong weathering. Low Th/U ratios Irkutskmay characterize basin did not coal-bearing suffer from samples, strong weathering.with the wide Low U content Th/U ratios bonded may with characterize an organic may characterize coal-bearing samples, with the wide U content bonded with an organic coal-bearingagent [37,38]. samples, The Th/U with ratios the wide to set U contentweathering bonded degrees with for an organicthe Irkutsk agent basin [37,38 samples]. The Th/Uagent ratios [37,38]. to setThe weathering Th/U ratios degrees to set for weat thehering Irkutsk degrees basin samples for the seem Irkutsk to be basin problematic, samples seem to be problematic, but such values in the Irkutsk basin sediments may indicate a butseem such to valuesbe problematic, in the Irkutsk but basinsuch values sediments in the may Irkutsk indicate basin a simple sediments cycling may history. indicate a simple cycling history. simple cycling history.

Figure 10. Th versus Th/U plot (after [39]). FigureFigure 10. 10.Th Th versus versus Th/U Th/U plotplot (after(after [[39]39]).). VariationsVariations in in Rb/Sr Rb/Sr and and Sr/Cu Sr/Cu ratios ratios are appliedare applied to represent to represent palaeoclimatic palaeoclimatic condi- Variations in Rb/Sr and Sr/Cu ratios are applied to represent palaeoclimatic tions.conditions. The Sr/Cu The Sr/Cu ratios ratios of the of lower the lower Cheremkhovo Cheremkhovo Fm. samplesFm. samples ranged ranged from from 2.6 to 2.6 5.2 to conditions. The Sr/Cu ratios of the lower Cheremkhovo Fm. samples ranged from 2.6 to (average5.2 (average value value 4.2), and4.2), fromand from 9.2 to 9.2 15.3 to (average15.3 (average value value 12.9) 12.9) for the for lower the lower Prisayan Prisayan Fm. 5.2 (average value 4.2), and from 9.2 to 15.3 (average value 12.9) for the lower Prisayan samplesFm. samples (Figure (Figure 11). Rb/Sr 11). ratiosRb/Sr demonstratedratios demons minortrated diversities,minor diversities, with all with samples all havingsamples Fm. samples (Figure 11). Rb/Sr ratios demonstrated minor diversities, with all samples

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valueshaving from values 0.1 tofrom 2.7 0.1 (Figure to 2.7 11 (Figure). The smallest 11). The Rb/Sr smallest values Rb/Sr are values seen at are the seen lower at Prisayanthe lower Fm.Prisayan (average Fm. value(average 0.3). value Sr/Cu 0.3). ratios Sr/Cu rise ratios under rise drier under conditions drier conditions [40–42]. [40–42]. Rb/Sr ratios Rb/Sr reduceratios underreduce drier under conditions, drier conditions, and rise underand rise cold under conditions, cold conditions, just as low just ratios as representlow ratios warmrepresent conditions warm [conditions43,44]. We [43,44]. can see We that can Sr/Cu see that ratios Sr/Cu increase ratios and increase Rb/Sr and ratios Rb/Sr decrease ratios fromdecrease the upper from the Cheremkhovo upper Cheremkhovo Fm. to the Fm. upper to the part upper of the part lower of the Prisayan lower Prisayan Fm. section Fm. (Figuresection 11 (Figure). This 11). can This reﬂect can changingreflect changing climate climate conditions conditions from warmfrom warm and humid and humid to hot to andhot arid.and arid.

FigureFigure 11. 11.Sr/Cu Sr/Cu versusversus Rb/SrRb/Sr plot. The The trends trends of Sr/Cu and and Rb/Sr Rb/Sr ratios ratios are are from from [43,44] [43,44]..

TheThe geochemical geochemical data data have have shown shown the effectthe effect of climate of climate ﬂuctuations fluctuations within thewithin deposi- the tiondeposition of the Irkutsk of the coal-bearingIrkutsk coal-bearing basin deposits. basin deposits. In the Cheremkhovo In the Cheremkhovo Fm. a warm, Fm. a humid warm, climatehumid prevailedclimate prevailed and we see and a highwe see grade a high of chemical grade of weathering chemical weathering of the source of rocks. the source The indicatorsrocks. The used indicators represented used changes represented in climate changes conditions in climate within conditions the deposition within of thethe lowerdeposition Prisayan of the Fm. lower At this Prisayan time, climate Fm. At conditions this time, climate changed conditions from warm changed and humid from to warm hot andand arid. humid to hot and arid.

3.3.3.3. Source Source Composition Composition and and Provenance Provenance TheThe geochemical geochemical characteristicscharacteristics ofof sedimentarysedimentary rocksrocks cancan preservepreserve provenanceprovenance in-information,formation, despitedespite thethe destructiondestruction ofof primary primary structures structures and and alteration alteration of of minerals minerals by by sedimentarysedimentary processes processes [ 39[39,45–47].,45–47]. The The ratio ratio Th/Sc Th/Sc does does not not change change considerably considerably during during sedimentarysedimentary recycling recycling [39 [39],], whereas whereas the the ratio ratio Zr/Sc Zr/Sc increases increases significantly. significantly. Thus, Thus, the the ratios ratios Th/ScTh/Sc cancan bebe appliedapplied inin tracing tracing sedimentary sedimentary provenance, provenance, and and high high Zr/Sc Zr/Sc ratiosratios can can be be consideredconsidered indicators indicators of of zircon zircon enrichment. enrichment On. On a Th/Sc–Zr/Sca Th/Sc–Zr/Sc diagramdiagram (Figure(Figure 12 12),), the the CheremkhovoCheremkhovo Fm. Fm. and and lower lower Prisayan Prisayan Fm. Fm. samples samples reflect reflect a a positive positive correlation correlation between between Th/Sc and Zr/Sc. The average Zr/Sc and Th/Sc ratios of the Cheremkhovo and lower Th/Sc and Zr/Sc. The average Zr/Sc and Th/Sc ratios of the Cheremkhovo and lower Prisayan formations are 22.21 and 0.95. This pattern suggests that the provenance of these Prisayan formations are 22.21 and 0.95. This pattern suggests that the provenance of these rocks was influenced by the source composition, not by sediment recycling [39]. rocks was influenced by the source composition, not by sediment recycling [39].

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Figure 12. The Zr/Sc versus Th/Sc plot (after [39]). FigureFigure 12. 12.The The Zr/Sc Zr/Sc versusversus Th/Sc Th/Sc plot (after [[39]39]).). Major element contents record sediment recycling processes and the changing proportionsMajorMajor element elementof sedimentary contents contents recordand record first-cycle sediment sediment source recycling recycling rocks processes which processes are and shown theand changingby the the changing index pro- of portionsproportions of sedimentary of sedimentary and and ﬁrst-cycle first-cycle source source rocks rocks which which are are shown shown by by the the index index of of compositional variability (ICV) [48]. The ICV can be calculated as: ICV = (Fe2O3 + K2O + compositionalcompositional variability variability (ICV) (ICV) [48 [48].]. The The ICV ICV can can be be calculated calculated as: as: ICV ICV = (Fe= (FeO2O+3 + K KO2O + + Na2O + CaO + MgO + MnO + TiO2)/Al2O3 [48]. The ICV values of the upper Cheremkhovo2 3 2 NaFm.Na2O2 Orange + + CaO CaO from + + MgO MgO 0.30 + + MnOto MnO 0.98, + + TiO withTiO2)/Al2 )/Alan 2average2OO33 [[48].48]. valueThe ICVICV of values0.62.values The of of the theICV upper upper values Cheremkhovo Cheremkhovo of the lower Fm.CheremkhovoFm. range range from from Fm. 0.30 0.30 range to to 0.98, 0.98, from with with 0.57 an an to average average1.30, with value value an ofaverage of 0.62. 0.62.The valueThe ICV ICV of values0.77. values The of of ICV the thelower values lower CheremkhovoofCheremkhovo the lower Prisayan Fm. Fm. range range Fm. from from are 0.57higher 0.57 to to 1.30, and1.30, withrange with an anfrom average average 0.69 valueto value 1.52, of ofwith 0.77. 0.77. an The The average ICV ICV values values value of the lower Prisayan Fm. are higher and range from 0.69 to 1.52, with an average value of of 0.97.the lower The samples Prisayan mostly Fm. are lie higher between and and range along from two 0.69 trends, to 1.52, on eitherwith an side average of the valuefresh 0.97. The samples mostly lie between and along two trends, on either side of the fresh basalt basaltof 0.97. line. The samplesDecreasing mostly weathering lie between to andmoderate along twovalues trends, in onthe eitherlower side Prisayan of the freshFm. line. Decreasing weathering to moderate values in the lower Prisayan Fm. demonstrates an demonstratesbasalt line. Decreasing an increase weathering in the ICV valuesto moderate to immature values parametersin the lower (ICV Prisayan > 1, by [46])Fm. increase in the ICV values to immature parameters (ICV > 1, by [46]) (Figure 13). Samples (Figuredemonstrates 13). Samples an increase with ICVin the > 1ICV were values deposi toted immature in tectonically parameters active (ICV settings. > 1, byOn [46]) the with ICV > 1 were deposited in tectonically active settings. On the other hand, ICV < 1 are other(Figure hand, 13). SamplesICV < with1 are ICV mature > 1 were and deposi wereted deposited in tectonically in a activetectonically settings. quiescent On the mature and were deposited in a tectonically quiescent environment [48]. environmentother hand, [48].ICV < 1 are mature and were deposited in a tectonically quiescent environment [48].

Figure 13. The plot of the index of compositional variability (ICV) versus CIA. The trends of FigureFigure 13. 13.The The plot plot of of the the index index of of compositional compositional variability variability (ICV) (ICV) versus versus CIA. CIA. The The trends trends of granite,of granite, andesite, and basalt are from [49]. andesite,granite, andandesite, basalt and are basalt from [49are]. from [49].

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KK2O2O andand Rb are are substantially substantially sensitive sensitive to to sedimentary sedimentary cycling cycling and and are aregenerally generally used used as indicators of the source rock composition [50]. The average K O values are 2.67 as indicators of the source rock composition [50]. The average K2O values2 are 2.67 and and 2.37% in the lower and upper Cheremkhovo Fm., respectively. The average K O 2.37% in the lower and upper Cheremkhovo Fm., respectively. The average K2O value2 in valuethe lower in the lowerPrisayan Prisayan Fm. is Fm. 2.99%. is 2.99%. The Theaverage average RbRb values values of of the the lower andand upperupper CheremkhovoCheremkhovo Fm. Fm. and and lower lower Prisayan Prisayan Fm. Fm. areare 71.28, 71.28, 78.55, 78.55, and and 77.39 77.39 ppm, ppm, respectively. respectively. The plot of Rb–K O reveals that most samples have K/Rb ratios near 230, which deﬁnes The plot of Rb–K2 2O reveals that most samples have K/Rb ratios near 230, which defines magmaticmagmatic trend trend [50 [50],], and and is similaris similar to thatto that of the of averagethe average Earth’s Earth’s crust crust [51] (Figure[51] (Figure 14). The 14). high Rb concentrations (>40 ppm) indicate that the studied samples were derived from an The high Rb concentrations (>40 ppm) indicate that the studied samples were derived acidic–intermediate igneous source (Figure 14). from an acidic–intermediate igneous source (Figure 14).

Figure 14. The Rb versus K2O plot (after [50]). Figure 14. The Rb versus K2O plot (after [50]). 4.4. Discussion Discussion TheThe CheremkhovoCheremkhovo Fm.Fm. isis consideredconsidered toto belongbelong toto eithereither thethe PliensbachianPliensbachian [3[3],], Pliensbachian–ToarcianPliensbachian–Toarcian [ 2[2,4],4] or or Pliensbachian–Aalenian Pliensbachian–Aalenian [ 5[5]] intervals intervals by by biostratigraphy. biostratigraphy. TheThe upper upper Cheremkhovo Cheremkhovo Fm. Fm. marksmarks the the lacustrine-swamp lacustrine-swamp sedimentation sedimentation in in the the wide wide territoryterritory of of the the Irkutsk Irkutsk coal-bearing coal-bearing basin. basin. There There is is evidence evidence of of the the presence presence of of tuffaceous tuffaceous interlayersinterlayers in in the the Cheremkhovo Cheremkhovo Fm. Fm. sediments sediments [ 17[1,7,23,52].23,52]. An An ash ash interlayer interlayer (tonstein) (tonstein) is is presentpresent in in the the industrial industrial coal coal seam seam of of the the Azeisk Azeisk deposit deposit in in the the northwest northwest part part of of the the basin. basin. TheThe tonsteins tonsteins were were formed formed due due to theto the transformation transformation of felsic of felsic pyroclastic pyroclastic material material [17]. The[17]. ageThe of age the of coal-bearing the coal-bearing deposits deposits of the of Cheremkhovo the Cheremkhovo Fm., represented Fm., represented at the at Azeisk the Azeisk coal deposit,coal deposit, was established was established as 187.44 as 187.44 +0.45/ +0.45/−1.60−1.60 Ma Ma using using the the LA-ICP-MS LA-ICP-MS U-Pb U-Pb method method onon accessory accessory zircons zircons from from tonstein tonstein [6 [6].]. The The age age of of the the upper upper Cheremkhovo Cheremkhovo Fm. Fm. places places the the upperupper limit limit of theof tectonicthe tectonic quiescence quiescence period period before thebefore intensiﬁcation the intensification of tectonic of processes tectonic inprocesses the southern in the mobile southern framing mobile of theframing Siberian of the craton. Siberian craton. InIn the the context context of of regional regional stratigraphy, stratigraphy, the obtainedthe obtained age ofage 187.44 of 187.44 Ma is Ma consistent is consistent with thewith valid the stratigraphic valid stratigraphic scheme [2scheme] and the [2] research and the of Skobloresearch et al.of [3Skoblo] that supplements et al. [3] that it. Thesupplements coarsening it. of The the coarsening basin’s sediments of the basin’ is upwards sediments through is upward the Prisayan through and the Kuda Prisayan Fm. Accordingand Kuda to Fm. the According consistent schemeto the consistent [3], the Prysayan scheme Fm. [3], andthe KudaPrysayan Fm. areFm. Pliensbachian– and Kuda Fm. Aalenian and Aalenian–Bajocian, respectively. From the lower Prisayan Fm. there are are Pliensbachian–Aalenian and Aalenian–Bajocian, respectively. From the lower no industrial coal seams in the Irkutsk basin. The lower Prisayan Fm. is Pliensbachian– Prisayan Fm. there are no industrial coal seams in the Irkutsk basin. The lower Prisayan Toarcian [3]. Fm. is Pliensbachian–Toarcian [3]. Petrographic studies of the Irkutsk basin rocks are fully consistent with the data of Petrographic studies of the Irkutsk basin rocks are fully consistent with the data of previous studies [3,7,8]. The geochemical studies generally conﬁrm this but give more de- previous studies [3,7,8]. The geochemical studies generally confirm this but give more tailed information. The available data indicate three major provenance areas of sediments: detailed information. The available data indicate three major provenance areas of the Siberian Craton, the Caledonian complexes bordering the craton, and the Transbaikalia sediments: the Siberian Craton, the Caledonian complexes bordering the craton, and the

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region [12,18]. The evolution of the relief near the southern part of the Siberian Craton depended on the subduction of the Mongol-Okhotsk oceanic slab [53]. Data in the Irkutsk basin confirmed the coarsening of sediments upward through the Prisayan and Kuda Fm. This fact, with an increase in 143Nd/144Nd ratios and the Jurassic-age detrital zircons, was associated with the closure of the Mongol-Okhotsk Ocean [12,54]. To mark the provenance area for the Cheremkhovo Fm., isotope-geochemistry and geochronological studies are required. An understanding of coal deposits demands wider perspectives of the processes in sedimentary settings. Coal-depositional environments significantly influence coal’s characteristics [55]. Different geochemical parameters have been used as indicators for the depositional environment during or shortly after coal accumulation. The climate change in Toarcian led to an almost complete stop in the processes of coal accumulation in the Irkutsk basin.

5. Conclusions This study presents geochemical data on Jurassic deposits of the Cheremkhovo and lower Prisayan formations from the Irkutsk coal-bearing basin. Based on the chemical composition of siliciclastic rocks, the Cheremkhovo and the lower Prisayan formations are classiﬁed as graywacke, arkose, and litharenite. CIA and PIA, Rb/Sr, and Sr/Cu ratios showed the event of climate change within the sedimentation of the Irkutsk coal-bearing basin deposits. The Cheremkhovo Fm. prevails in a warm, humid climate, and we see a high grade of chemical weathering of the source rocks. The indicators used revealed the change in climate from warm and humid to hot and arid during the deposition of the lower Prisayan Fm. The provenance of the Irkutsk coal-bearing basin was inﬂuenced by the source composition, not recycling. The studied sediments were deposited due to the destruction of mainly felsic to intermediate igneous rocks.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/min11040357/s1, Table S1: The table with the sampling locations, major element oxide, and trace element concentrations for the Cheremkhovo Fm. and lower Prisayan Fm. from the Irkutsk coal-bearing basin. Author Contributions: Conceptualization, A.V.I., E.I.D., E.A.M.; methodology, E.I.D. and E.A.M.; software, E.A.M.; validation, E.I.D. and A.V.I.; formal analysis, E.I.D. and E.A.M.; investigation, E.I.D. and E.A.M.; resources, E.I.D. and E.A.M.; data curation, E.I.D. and E.A.M.; writing—original draft preparation, E.A.M.; writing—review and editing, E.I.D. and A.V.I.; visualization, E.A.M. and E.I.D.; supervision, E.I.D. and A.V.I.; project administration, E.I.D.; funding acquisition, E.A.M. All authors have read and agreed to the published version of the manuscript. Funding: The reported study was funded by RFBR and the Government of the Irkutsk Region, project number 20-45-383008. Data Availability Statement: Not applicable. Acknowledgments: Laboratory investigations were carried out at the Center for Geodynamics and Geochronology of the Institute of the Earth’s Crust of Siberian Branch of the Russian Academy of Sciences (Irkutsk, Russia). The authors are grateful to the reviewers’ valuable comments that improved the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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