Variability in distribution ofmajor and trace elements in Lower Eocene siliceous sections of the Transuralian Region,Russia

P.V.Sm irnov·A.O.Konstantinov·G.A.Batalin·B.I.Gareev

Abstract This paper presents lithologic and geochem ical data from the sequence of the Eocene Irbit formation siliceous rocks(Transuralian Region)outcropping in a quarry in the Irbitdeposit(thicknessof15m)and in a large natural outcrop,Belaya Gorka(thickness of 13m).The data show that both outcrops are composed of diatomites and clayey diatom ites,both characterized by a certain degree of lithologic heterogeneity around their chem ical,granulometric,andm ineralogicalcompositions;m icrostructural features;and degree of diatom preservation.The values of indices important for the classif ication of siliceous rocks and determination of prospects for their industrial application—SiO2,Al2O3,Fe2O3,andclay fraction content—ranged from 66%to 77%,7%to 14%,3.00%to 5.60%,and 23%to 50%,respectively.In all studied lithologic varieties,element abundances of V,Cr,Mn,Co,Ni,Cu,Zn,Ga,Ge,and Sb were two to three timeshigher than their respective abundances in the Earth's crust.This is probably related to these elements'involvement in the biological cycle and favorable conditions for transport.Rb,Cs,Ba,and Sr,aswellas rareearth elements,are considered themost reliable indicatorsof lithologic and geochemical subdivision of a sequence of siliceous rocks,as they are associated mainly w ith clayey m inerals.Variations in these indicators have recorded,w ith great probability,even short-term cycles and sem i-cycles of silica?P.V.Sm irnovgeolog.08@mail.ru

Keywords Diatom ite·Clayey diatom ite·Irbit formation·Eocene·Geochemical variability

1 Introduction

Diatom ite and its clayey varieties,deposited in a sea basin during the late Paleocene and early Eocene,are one of the most widespread sedimentary rocks and occupy a vast portion of the Transuralian Region.In general,the siliceous depositsare considered from two basic pointsof view:asa valuable local resource for the production of building materials(Distanov 1976)and as a source of paleogeographic data for reconstructing conditions at themargin of WesternSiberiaat thePaleocene-Eoceneboundary(Sm irnov and Konstantinov 2017;Sm irnov et al.2017).The analysis of siliceous fossils plays a conventionally important role in studying siliceous deposits(Gurel and Yildiz 2007;Aleksandrova et al.2012;Elmas and Bentli 2013).M icropaleontological studies make it possible to reconstruct hydrodynamic,temperature,and hydrochem ical conditionsof the paleo—sea basin during the lifetime of silica-skeletoned organisms and the accumulation of their corresponding sediments,aswell as the dynam ics of these organisms'development(Glezer 1974;Smol and Stoermer 2010).Modern ideas about geochronology of specif ic stratigraphic units,paleogeographic reconstructions,and transgression-regression eustatic cycles are based on a detailed study of siliceous and othermicrofossils(Rubina 1973;Akhmetiev etal.2004;Akhmetiev and Beniamovsky 2006;Oreshkina et al.2008).However,data on the variability in the chem ical composition of diatom ites of theTransuralian Region are scarce.While thereare data on the variability in the contents ofmajor oxides(Distanov 1976;Generalov etal.1986;Ushatinskii1987;Sidorenkov 1989),there are no data on contents of rare earth and trace elements in the sequence of siliceous deposits in the Transuralian Region.This work examined variability in the chem ical composition of siliceous deposits of this region,specif ically diatom ites and clayey diatom ite in quarries of the Irbit Formation and in the Belaya Gorka outcrop.The results obtained can be useful for studying the formation stages of the sequence of siliceous deposits as well as evaluating the chem ical and lithologic homogeneity of large industrial deposits of diatom ites in the region.

2 Geological setting

Siliceous rocksof the Transuralian Region are attributed to three main stratigraphic units,which are composed of different rock varieties:the Paleocene Serov Formation,and two subformations(lower and upper)of the Irbit Formation(Nesterov etal.1984).They restunconformably on other Paleoceneand,locally,on Paleozoic deposits.The Serov Formationis widespread and is largely represented by opoka,clayey opoka,and opoka-like clays,as well as subordinate glauconite—quartz sands,sandstones with opoka and silica-opoka cement,and rarely diatomites(Sm irnov and Konstantinov 2016).Opoka isa compactand fractured rock(sometimes arenaceous)w ith a specif ic conchoidal structure,w ith a distinct horizontal,platy jointing locally.As usual,the lower boundary of the formation contains f ine-to coarse-pebble basal conglomerates with opoka and silica-opoka cement.The Irbit Formationis quite homogenous in terms of lithology and is largely represented by diatom ites and clayey diatomites,often sandy-silty.The thickness of the sediments varies from 0.6 to 13.0m.In themost complete sequence of the Transuralian Region,is subdivided into two subformations:lowerand upper.The depositsdip gently(regional dip angles are in m inutes)and the rocks are non-metamorphosed.The Lower Irbit ismainly composed of diatomitesand of diatomaceous,rarely tripoli-like,and opokalike clays(sometimes w ith intercalations of quartz and glauconite-quartz siltstones and sandstones).Diatom ites are pale gray,often w ith a greenish tint,clayey to different extents,sometimes silty,micaceous,light,weakly cemented,often loose,and earthy.The Upper Irbit is composed of gray to greenish-gray diatomaceous and montmorillonitebeidellite clays,clayey diatom ites,and diatom ites.The clayey diatomite is pale gray,non-lam inated,often silty,quite solid,and sometimesweakly loamy.There are plant detritus and worm tubes f illed w ith siltymaterial.In terms of lithology,the clayey diatom ite is close to diatomites of the Lower Irbit Formation:the former is characterized by a higher proportion of clayey and silty material and somewhathigherbulk density.Diatom itesof the Irbit Formation nearly always overlie opoka deposits of the Serov Formation.The boundary between diatom ite and opoka deposits is distinct;there are no signs of erosion(Distanov 1976).

In terms of structural geology,the area of study is an integral part of the Transuralian Uplift.In terms of the geomorphological zoning of the Urals and Transuralian Region,the study is in thewestern partof the continentalmarine accumulative plain,w ith a specif ic complex of sediments of both marine and continental origin.

3 M aterials and methods

3.1 Field studies and sam p ling sites

Samples for analyticalandmicroscopic studieswere taken from cleaned sides of the Irbit quarry and the natural Belaya Gorka outcrop;the sampling interval varied from 0.5 to 2.5meters,depending on the degree of uniform ity of the section.The outcrops,aswellasa simplif ied schematic of these sediments in the stratigraphic scale,are shown in Fig.1.

The Irbit quarry(57°39′33.16′′N,63°3′37.18′′E)is one of the largestand longest-exploited depositsof diatom ite in the region.The deposits are situated at the footand on the western slope of M t.Pushkareva Gora in the Irbit town area.Quarrying began in the 1920s(Kandykin 1930)for the production of lightweight bricks as well as dry constructionm ixturesand f ilter powders(Sm irnov etal.2017).In terms of the geology,the Irbit deposit is conf ined to horizontally lying strataof diatom ites,which havenotbeen traced to their full depth.According to survey data,the thicknesscan exceed 60m.Hydrogeological conditionsare favorable;the productive strata are not water-saturated.The Irbit deposit is a series of different-aged workings descending in succession down the slope of M t.Pushkareva Gora.Samples were taken from outcrops of three quarries located along themountain slope.

The natural outcrop,Belaya Gorka(57°36′34.62′′N,62°47′59.40′′E),is located on the right bank of the Irbit River,18 km southwestof the town of Irbit,in theoutskirts of the villageof Rechkalovo.A steep riverbank reveals the Eocene diatom ite sequence of the Irbit Formation.Relic pine forests prevailw ithin the adjacent territory.The total length of the outcrop is 300m.Due to the protected status of the territory and the proxim ity to the IrbitRiver channel,the Belaya Gora outcrop has never been considered a viable source for siliceous deposits in the Transuralian Region,although it ismentioned in some works by U.G.Distanov and I.N.Ushatinskii.

Fig.1 The location of the study area and position of the studied rocks on the geochronological scale

3.2 Analyticalm ethods

Studies of the chem ical,elemental,and m ineral compositions of deposits were performed at the Laboratory for Isotope and Element Analysis of the Institute of Geology and Petroleum Technologies of Kazan Federal University(Kazan).Lithologic and m icroscopic studies were performed at Tyumen IndustrialUniversity(Tyumen).Particle size distribution measurement of rock samples was performed w ith the standard method.

Concentrations of the major oxides were determ ined using an S8 Tiger high-end wavelength dispersive X-ray f luorescence spectrometer(Bruker,Germany),equipped w ith a rhodium anode 4 kW SST-m AX tube.Thismethod allowsmeasurement of the elements in solid,powder,and liquid samples in a range from B to U in a vacuum or helium atmosphere.Samples were ground in a planetary ballm ill for 10 m in to achieve particle size of less than 10μm.Samples weighing 0.5 g were placed in a ceram ic crucible and calcined at 1100°C for 2 h to determ ine loss on ignition(LOI).Then,4 g of each sample wasweighed on an analytical balance w ith an accuracy of 100 mg,m ixed w ith organic wax,and pressed into a substrate of boric acid w ith a force of 300 kN.The resulting pellet of each sample was placed in a spectrometer and analyzed using the standard Geo-Quant technique.The spectrum recorded was processed using the fundamental parameters(FP)calibration method;automatic recognition errors and spuriouspeakswereelim inated,diffraction phenomenaand matrix effectswere taken into account,and the LOIvalue was used to account for undetectable elements.

The elemental compositionsof the sampleswere studied using inductively coupled plasma—mass spectrometry(ICPMS,ThermoFisher Scientif ic,Germany).A 100-mg test sample was weighed in a Tef lon autoclave w ith an analytical scalew ith an accuracy of 0.1mg.Then,to obtain a reactant mixture,2m lofconcentratedhigh-purity hydrochloricacid(38%HCl),1m l of high-purityhydrof luoric acid(38%Hf),and 1m lof concentrated highpurity nitric acid(68%HNO3)were added successively to the autoclave.Before use,all acids were additionally purif ied.The blank solution was used for background correction.The sampleswere loaded in a Tef lon autoclave,which was sealed and placed in a Mars 6microwave oven(CEM Corporation,USA)in which the samples were heated to 210°C and kept there for 30min to be digested.After this,sampleswere treated w ith 10m l of 4.5%boric acid to dissolve rare earth element(REE)f luorides,insoluble in water.Autoclaveswere heated to 170°C and kept there for 30 m in.A fter cooling to room temperature,the autoclaves were opened and the resulting solution was diluted to 50m l with deionized water.An aliquot of 500 mcl of the resulting solution was taken and diluted with deionized water to 10m lw ith the addition of an internal standard element(In)w ith a f inal concentration of 5 ppb;hydrochloric acid was added to achieve a f inal concentration of all acids in the solution of 2%.The resulting solution was analyzed on a mass spectrometer pre-calibrated w ith multielement standards(1—100 ppb).Final concentrationswere calculated,taking into accountsample volume,sample weight,and dilution factor.M icroscopic studies were carried out using the JSM-6510LV-EDS electronm icroscope,theJEOLanalytical complex including Oxford Instruments INCA Energy 350 X-ray energy dispersive spectrometer,and the Hitachi TM-3000 scanning electron microscope w ith a Bruker Quantax 70 energy-dispersive X-ray spectrometer.

A sem i-quantitative analysis of the m ineral composition—both for bulk samplesand for the clay fraction—was performed using a DRON-2 X-ray diffractometer at the West-Siberian Geological Center(Tyumen).The processing of the diffractograms was carried out using the Geo-Quant software,which eliminated automatic detection errors and parasitic peaks.

Statistical data processing was performed using the software complex Statistica 10.For sesquioxides and basic particle size fractions,pairw ise Spearmancorrelation coeff icients were calculated.Dendrograms were plotted(Ward'smethod)to identify and distinguish associationsof elements,and to group the samples by lithologic features.The geochem ical index of light-/heavy-REEs(LREE/HREE);the chem ical index of alteration(CIA)and chem ical index ofweathering(CIW);and the paleosalinity indicator,Sr/Ba,were also calculated.

4 Results and discussion

4.1 Lithologic and structural features of the studied outcrops

The siliceous deposits from the two sections under consideration are characterized by a certain degree of heterogeneity.The colors range from lightgray w ith a yellow ish tinge to brown due to ferruginous spots,and they are weakly cemented.The Irbit deposit is characterized by alternating clayey and purer diatom ite varieties—the basis for distinguishing four lithologically homogeneous strata:

1.Stratum1(Unit I-A).Interval 0—1.5 m:clayey diatom ites;grayish-brown,w ith a uniformtexture and signs of ferrugination,possibly redeposited due to slope landslide processes;

2.Stratum2(Unit I-B).Interval 1.5to5.5—6m:diatom ites of lightgray to cream colorw ith amassive texture.W idely distributed local ferrugination along the stratif ication of structural units and zones with pronounced fracturing as well as rare small dark inclusions;

3.Stratum 3(Unit I-C).Interval 6.0 to 8—9.0 m:clayey diatom ites,brownish-gray,w ith a heterogeneous texture;a noticeable amount of coarse inclusions,sandy material,and indistinct signs of local ferrugination.

4.Sequence 4(Unit I-D).Interval 9—13m:brownish to

reddish pale yellow w ith greenish and brownish tints in the upper part,to grayish-brown to gray in the lower partwhen passing tomore pure varieties.The deposits are characterized by a massive homogeneous texture and local ferrugination along the facesof the structural units.The lower part of the stratum is composed of common pale gray and cream-gray diatomites w ith a massive homogeneous texture.

The Belaya Gorka outcrop includes three successive strata:

1.Stratum1(Unit BG-A).Interval 0—6.5m:clayey diatom ites,creamy-gray to light gray w ith a welldef ined homogeneousmassive texture and rare,small,dark-colored impregnations.

2.Stratum 2(Unit BG-B).Interval 6.5—11.5m:diatom ites,brown,heterogeneous due to the occurrence of lenses and ferruginous spots,w ith a well-def ined massive texture;

3.Stratum 3(Unit BG-C).Interval 11.5 to 13—14.0m:clayey diatomites creamy-gray and pale gray w ith a non-uniform massive structure,indistinct layering,and rare lenticular ferruginous intercalations.It is problematic to make a comprehensive lithologic description of the deposits of the lower part of the sectionw ithout additionalm icroscopic studies.Structural and textural featuresof the depositswere also conf irmed by variations in the particle size composition(Table 1).

4.2 Scanning electron m icroscopy

M icroscopic structures of diatomites from the lithologically different strata of the Irbit deposit and the Belaya Gorka outcrop are shown in Fig.2.The diatomites of the Irbit deposit displayed rich and diverse species composition corresponding mainly to the Lower Eocene Coscinodiscus payeri Zone:Pyxidicula moelleri(A.Schm idt)

Strelnikova et Nikolaev,Coscinodiscus payeri Grunow,Moisseevia uralensis(Jouse′)Strelnikova,Stephanopyxis turris(Greville in Gregory),etc.(Sm irnov et al.2017).Apart from diatom algae,samples contained abundant spicules of siliceous sponges and to a lesser extent radiolarian shells.

Under the m icroscope,a well-def ined biomorphic texture was observed:rocks were composed mainly of fragments and whole tests of diatoms ranging in size from several to 30—70μm w ith an adm ixture of radiolariansand spicules of sponges.The terrigenousmaterial(～5%—7%of the area)was represented predom inantly by angular,sem i-rounded,isometric and elongated quartz grains,and feldspars w ith a size from＜0.005 to 0.076mm.The quartz grains were pure w ith point-like silty inclusions,microfractures,and inclusions of hydromica f lakes.Single grainsweresurroundedbyultra-thin,discontinuous regeneration ridges.The binding(cementing)materialwas composed of small,probably crushed,fragments of diatomsmeasuring 0.001—0.005 mm or less sometimes w ith an insignif icant adm ixture of clay m inerals as well as an admixture of authigenic silica and clay minerals such as kaolinite,chlorite,and hydromica.A numberof differences were identif ied in themicrostructure of separate horizons within the Irbit deposit section(Fig.2a—c).In diatomites from I-A,large honeycomb fragments of diatom shells up to 50—80μm in sizewerequite abundantw ithin pore space,whichwas quitewell purif iedfromclaym inerals.However,in I-B,large fragments of biogenic material rarely occurred in the groundmass represented by small diatom detritus.Samples from I-C and I-D were characterized by an especially high degree of preservation and a certain uniform ity of the forms of diatoms observed under the electronm icroscope.

Table 1 Distribution of granulometric fractions of the studied sediments,%

Fig.2 M icroscopic structure of diatomites from the lithologically different strata:a—c Irbit deposit;d—f Belaya Gorka

The diatomite samples of the Belaya Gorka section consisted of numerous fragments and whole tests of diatoms composed mainly of opal(Fig.2d—f).There was an insignif icant adm ixture of a silty material consisting of fragments of quartz,chlorite,and hydrom ica.Rocks contained rare sponge spicules.The diatom assemblage was quite uniform and,most likely,can be attributed to the early Eocene Coscinodiscus payeri(Grunow)Zone.Diatomsw ith valves of round and often isometric shape were widely represented.The centric diatoms of the genera Triceratium and Pyxidicula were commonly observed.Trinacria and sim ilar Pseudotriceratium diatoms of thetriangular form were less common in the rock.They varied in size from 20 to 70μm.Siliceous m icrofossils were characterized by a good degree of preservation,w ith the porous spacesof the valves purif ied from claym ineralsand silica neoformation.Diatoms ranging in size from 0.02 to 0.17mm were clearly visible even at low magnif ication,amounting to 60%of the f ield of view(Fig.2).Poreswere very small and intraskeletal w ith a size of 0.05—0.5mm,and they occupied about5%of the f ield of view.Deposits of units BG-A and BG-C were characterized by a higher proportion of clay m inerals and grains of detritalmaterial.

4.3 X-ray diffraction

Table 2presents the results of the semi-quantitative determ ination of the mineral composition for the most representative samples.

In all the samples studied,based on the results of X-ray diffraction analysis,opal/opal-CT were dominant,quartz subordinate;feldspars,hydrom ica,and m ixed-layer m inerals of the illite—smectite group were adm ixtures.An‘‘opal halo''typical of diatom ite and clayey diatom ite as a structurelessmaximum in the range of 2Θfrom 20°to 26°was much better distinguished in purer rocks.In clayey diatom ites,especially in the eroded deposits of the upper partof the Irbitquarry,clear peaksof quartzwere strongly f ixed.The clay fraction was also dominated by opal(bioclastic)and m ixed-layer minerals(especially in clayey varieties),and,to a lesser extent,quartz and hydrom ica.Signif icant differences between the rocks of the two outcrops have not been identif ied,indicating a high homogeneity of the Eocene silicic rocks in the region.

4.4 M ajor element distribution

The content of major oxides helps characterize both differences between specif ic lithologic varieties of siliceous rocks and possibilities for their industrial use(Fuya et al.1995;Hassan etal.1999;Ilia etal.2009;Wang etal.2009;Yildiz et al.2016).For example,ratios of SiO2:A l2O3:Fe2O3:CaO are the basis—accepted by the post-Soviet countries—for industrial classif ication of siliceous rocks(Distanov 1998).Both outcrops under consideration were characterized by a high degree of correlation between major oxides and rock lithology(Table 3,Figs.3,4).

The SiO2content in siliceous rocksof the Belaya Gorka outcrop varied insignif icantly:from 72%to 77%.Alum inum and Fe oxides also showed insignif icantvariability:7.4%—7.9%and 3.4%—4%,respectively.Other oxides—K2O,MgO,MnO,TiO2,CaO,Na2O,and P2O5—returned low values:1%—2%for K2O and MgO and＜1%for the remaining oxides.The higher SiO2contentswere recorded in the interval of 6.5—11.5m(BG-B),which is composed of the purestvarietieswith ahigh degreeof preservation of diatom valvesand the predom inanceofa silty fraction over a pelite one.The ratios of K2O/Na2O,SiO2/Al2O3,and Al2O3/TiO2,aswell as CIA and CIW of rocks,showed no signif icant variability through the section.A more diverse distribution pattern of sesquioxides characterized the Irbit deposit section.The SiO2content varied in aw ider range:66%—77%.A high SiO2content(74%—77%)was recorded in the common varieties of diatom ites(interval 9—15m,I-D),w ith 70%—76%in clayey diatom ites from the upper part(0—6m)of the section(I-A,B).The lowest contents(66%—68%)were found in clayey diatomites from the intervalof 6—9m(I-C).The A l2O3contentvaried from 7%to 8%in diatom ites and from 10%to 14%in clayey diatom ites.The contents of the remaining oxides in rocks of the Irbit deposit section,aswell as in rocks of the Belaya Gorka outcrop,were subordinate.

Itshould benoted thatalthough contentsofmajoroxides in both sections showed a high degree of uniform ity,the SiO2:Al2O3:Fe2O3ratio was directly related to variations in lithologic and m icrostructural features of rocks.++++—dom inant(＞50 w t%),+++—abundant(20—50 w t%),++—subordinate(5—20 w t%),+—low(1—5 w t%),tr—traces(＜1 w t%)

Table 2 M ineral composition of bulk and clay fraction of selected samples from lithologically different intervals;Q—quartz,O—opal,F—feldspars,M l—mixed-layers(illite—smectite),H—hydrom icas;

Table 3 Major element concentration of the samples from Irbit deposit and Belaya Gorka outcrop;all values in w t%oxide

Based on the results of the samples from the two outcrops,there was an average positive correlation(0.46)between SiO2content and the silty fraction;an average negative correlation between SiO2content as the major rock-form ingcomponent andFe2O3(-0.67),MgO(-0.69),P2O5(-0.42),and LOI(-0.54);and low correlation w ith A l2O3(-0.39).

It isevident that this relationship isbecausemore clayey varietiesw ith a low content of SiO2can be enriched w ith organicmatter.The Al2O3content showed an average and high positive correlation w ith all basic oxides:Fe2O3(0.49),K2O(0.80),MgO(0.63),TiO2(0.65),CaO(0.60),Na2O(0.41),and P2O5.In addition,the positive correlation between the contents of Na2O and K2O(0.66),Na2O and TiO2(0.54),and K2O and TiO2(0.70)should be noted.

The calculations of pairw ise Spearmancorrelation coeff icients showed no strong relationships between the contentsofsesquioxidesandgranulometricsizecomposition.However,there was an average positive correlation between the coarse-grained sand fraction and TiO2(0.42);a positive correlation between the f ine-grained sand fraction and Na2O,K2O,and TiO2,and between the silty fraction and Na2O(0.50),K2O(0.49);as well as a moderate negative correlation between the ultraf ine fraction and K2O(-0.50)and P2O5(-0.46).The proportion of clay fraction was negatively correlated w ith Na2O(-0.67),K2O(-0.54),and TiO2(-0.49)but positively correlated w ith LOI(0.61).These correlations indicate an increase in the proportionsof claym ineralsand terrigenous material.Such relationshipshavebeen previously noted for siliceous deposits in northwestern Siberia(Generalov etal.1986).

Fig.3 Distribution ofmajor oxides in studied sections of Irbit deposit

4.5 Trace and rare earth elem ents

Elemental compositionsof rocks from the Irbitdepositand the Belaya Gorka outcrop are presented in Table 4 and Figs.5 and 6.The contentsof large ion lithophile elements(LILEs)such as Rb,Cs,Ba,and Sr in rocks of both outcrops were generally 1.5—2 times lower than in Earth's crust(Wedepohl 1995).They varied greatly from 51.6 to 81.4 ppm(Rb),27.8 to 148 ppm(Sr),and 163 to 337 ppm(Ba).

The distribution of these elements appears to be an important parameter for subdivision of the strata of siliceous rocks:LILEs are largely associated w ith clayey m inerals(W ronkiew icz and Condie 1987;Nyakairu and Koeberl 2001;Lisboa et al.2015).Accordingly,their variability along the section correlatesw ith the proportion of clay m inerals and,accordingly,pelite fraction.Such relationships are characteristic of the rocks studied:the contents of Rb,Cs,Ba,and Sr in clayey diatom ites of the Irbit deposit(Stratum 3)and clayey diatomites of the BelayaGorka sectionwerealmost two timeshigher than in the purer rock varieties.The Sr/Ba ratio varied from 0.16 to 0.5,indicating that the purest varieties of siliceous rocks accumulated in a slightly saline water basin.

Elements such as Y,Th,U,Pb,Zr,Hf,Nb,and Ta(high f ield strength elements)are traditionally characterized by low involvement in sedimentation processes and association w ithminerals stable to weathering;their contents as a whole were found to be two to three times lower than in Earth's crust and did not show signif icant variation in the section.This is quite logical since the accumulation of biosiliceous rocks took place under conditions when the Ural Mountains were not,in fact,a provenance area.The higher contents of these elements are also characteristic of less pure siliceous rock varieties,the formation of which apparently occurred during phases of weak activation oferosion processes and increased transportation of terrigenousmaterial(Chirva and Lyubom irova 1973).

Fig.4 Distribution ofmajor oxides in studied sections of Belaya Gorka

The elements V,Cr,Mn,Co,Ni,Cu,Zn,Ga,Ge,and Sb behaved quite differently;on thewhole,their contentswere 1.5—5 times higher than in Earth's crust.This can be explained as follows:these elementsare characterized by a high level of biogenic accumulation,and they canmigrate intensively in a form of organom ineral colloids through inland weathering under hum id,near-tropical,climatic conditions w ith w ide development of weathering crusts(Dupre′et al.1996;Oliva et al.1999).Conditions of this sortexisted during the accumulation of the sequence of the biogenic siliceous deposits in the Transuralian Region(Leclaire 1974;Barron et al.2015).

The total contentof rare earth and trace elementsvaried from 72 to 132 ppm in the section of the Irbit depositand from 84 to 124 ppm in the Belaya Gorka outcrop section.In the section of the Irbit quarry,the LREE/HREE ratio varied highly from 4 to 7,but in the Belaya Gorka outcrop,it varied insignif icantly,from 5.5 to 6.Higher contents of rare earth and trace elements are characteristic of clayey varieties.

5 Discussion and interpretation of lithologic and geochem ical data

Even insignif icant variations in the rock lithology of the studied section have been clearly recorded in the features of themicroscopic textureaswellas in the variations in the contentsofmajor oxidesand rare earth and trace elements.Themore clayey varietieswere characterized by higher A l and Fe oxide contents,LOI,and individual groups of LILEs.

The structure of the sequence of siliceous deposits in the Transuralian Region ismarked by a considerable variety even w ithin relatively small intervals(13—15m):there are‘‘clear''diatomites w ith a high degree of preservation of siliceous fossils,a high SiO2content,and a small admixture of mostmetals as well as different clayey varieties.The jointuse of lithologic and geochem icaldata allowsone to signif icantly extend the representation of the tim ing of accumulation of siliceous deposits in the region under consideration during the period ofmaximum transgression in the early Eocene.The results obtained indicate that during this period there existed probable activation of tectonic activity,as well as possible variations in the salinity of the sea basin and in sedimentation conditions more generally.

Table 4 Trace and REE concentration of the samples from Irbit deposit and Belaya Gorka;all values in ppm

Table 4 continued

The results of the cluster analysis illustrate the clear relationship between the lithology of siliceous rocks and the contentsofmetalsand rare earth and trace elements.In the dendrogram(Fig.7),the rock associations of I-0.5,I-1.5,I-7,and I-8(clayey varieties from the section of the Irbit deposit)as well as I-4.5,I-12,I-13,and I-14(the purest varietiesw ith a high degree of preservation of diatoms and a lower content of the clayey fraction)wereclearly distinguished.These regularities for rocks of the Belaya Gorka outcrop were less evident due to its homogeneity.In any case,the variability in the lithology and chem ical composition of the siliceous deposits of the Transuralian Region should be considered before development of the deposits,especially if these rocks are intended for use in the chem ical,medical,and high-tech industries.

Fig.5 Correlation between geochem ical indexes for Irbit deposit

Fig.6 Correlation between geochem ical indexes for Belaya Gorka

Fig.7 Hierarchical clustering analysis(HCA)of studies samples based on the trace and rare earth elements composition(Dendrogram using Wardmethod)

6 Conclusions

Based on the studies carried out,a number of conclusions and generalizations can be drawn:

1.The studied sections are characterized by certain cyclicity,recording the reversible process of sedimentation,the periodicity of oscillating movements,and the dynam ics of transgressive—regressive cycles.

2.Themainm ineral componentsare opal,and to a lesser extent quartz,feldspar,m ixed-layer minerals of the illite—smectite group and hydromica.The clay fraction is dom inated by opal andm ixed-layerm inerals,which play an important role for clay differences.

3.The clayey varieties in biogenic siliceous rocks are characterized by the highest contents of lithophilic,rare earth,and trace elements.

4.The variability of the chemical composition of rocks w ithin the limits revealed doesnotposeany constraints in the processing of raw materials in accordance w ith the existing consumption pattern(construction and thermal insulation materials).However,these constraints should be considered when expanding the application of siliceousmaterials,in particular,for the production of f ilter materials,catalysts,f illers,and otherapplications,where the elemental composition of raw materials can signif icantly affect the quality of marketable output.

Acknow ledgem entsAuthors are sincerely grateful to Professor,Doctor in Geology and M ineralogy,and Corresponding Member of the RAS I.I.Nesterov for valuable advice and to Research specialist,Educational Center for Geology of Oil and Gas,K.A.Faizieva for assistance in carrying out studies.The authors are grateful to D.V.Voroshchuk for assistance in preparing the manuscript in English.The reported study was funded by RFBR according to the research project entitled Ge/Si ratio in opal-cristobalite rocks as a tool for diagnosing the sources of silica's inf lux the the Paleocene-Eocene sedimentation basin ofWestern Siberia(Project No.18-35-00034).