ISSN 1075�7015, Geology of Ore Deposits, 2011, Vol. 53, No. 6, pp. 455–473. © Pleiades Publishing, Ltd., 2011.Original Russian Text © A.A. Kremenetsky, N.A. Gromalova, E. Belousova, L.I. Veremeeva, 2011, published in Geologiya Rudnykh Mestorozhdenii, 2011, Vol. 53, No. 6, pp. 516–537.

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INTRODUCTION

It is known that the total proved reserves ofilmenite–rutile–zircon placers of the superlarge Ti–Zr deposits of the East European Platform and Scyth�ian Plate (East European and North Caucasus Ti–Zrprovinces) and the South Australian Platform (Fig. 1)are estimated at 2635 and 2688 Mt, respectively. Takentogether, they are 1.5 times greater than the totalresources of other provinces in the world (SouthAfrica, Mozambique, United States, Kazakhstan,etc). Although parts of these deposits are being activelymined as sources of 95%, 70% and 90% of the world’szircon, ilmenite, and rutile, respectively, considerablevolumes of placers are not yet utilized because of lowquality of ore, or remain undiscovered, because theTi–Zr placers were mostly formed under near�shoremarine conditions and are now overlapped by loosesediments up to 50 m thick. This, in turn, complicatesthe establishment of their links with source regionsand therefore decreases the efficiency of forecast,localization, and estimation of the resource potentialof the buried placers.

Corresponding author: A.A. Kremenetsky. E�mail:imgre@imgre.ru

Sources of ancient Ti–Zr placers are a subject oflong�term debate. Most researchers deny the possibil�ity to ascertain the relationships of these placers tospecific primary sources taking into account a set ofunfavorable circumstances: (1) remoteness of the bur�ied near�shore marine placers from the inferredancient primary sources; (2) the narrow stratigraphicinterval of placer formation (Paleogene–Neogene,less frequently Cretaceous, and still less, Jurassic)against the heterogeneity and wide chronologicalrange of their sources; (3) similar structure of orebod�ies in placers (productive layers about 5 m in averagethickness); and (4) similar mineral composition ofplacers (ilmenite > rutile ≥ zircon) and uniform grainsize of ore minerals (predominance of grain�size classof 0.10–0.044 mm).

As a consequence, in has not been possible to judgethe effect of bedrock and overlapping weathering man�tles on the early stage of placer formation. In otherwords, the role of feeding sources in the formation andlocalization of currently buried Ti–Zr placer is under�estimated.

A classic example of the uncertainty in identifica�tion of feeding sources is the Ti–Zr placers of theForecaucasus (Stavropol and Yergeni placer districtsof the North Caucasus Ti–Zr province). According to

Isotopic and Geochemical Features of Newly Formed Zircon Rims As a Criterion for Identification of Feeding Sources of Ti–Zr Placers

A. A. Kremenetskya, N. A. Gromalovaa, E. Belousovab, and L. I. Veremeevaa

a Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, ul. Veresaeva 15, Moscow, 121357 Russiab Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia

Received July 14, 2011

Abstract—Zircons were studied from the Ti–Zr placers of the Murray Basin (Mindarie and WIM�150 depos�its) and metamorphic rocks of the adjacent Kanmantoo Belt and the Ballarat Trough in Southeast Australia,and from Russian basins with Ti–Zr placers: the Cenomanian–Turonian and Poltavian basins in the EastEuropean Platform and the Sarmatian basin in the northern Caucasus, Stavropolye, and Kalmykia. Thestudy of the primary source weathering mantle intermediate reservoir rock economic placersystem includes (1) morphostructural and paleofacies reconstructions of the studied territories; (2) quantita�tive analysis of ore mineral distribution in each element of this system; and (3) study of zircon typomorphismfrom the feeding source to the basin, where Ti–Zr placers have been deposited. In all elements of the systemstudied, zircons were examined using optical and cathodoluminescence microscopy, an electron microprobe,mass spectrometry (laser ablation and SHRIMP II), including U–Pb dating, Lu and Hf isotopes, distributionof trace elements (REE, Y, P), and comparative analysis of indicative ratios: Th/U, HfO2/ZrO2, Y2O3/(Y2O3 +REE2O3), (La + Sm)/(Gd + Yb), etc. Newly formed rims replacing detrital zircon grains and marking thetime of late geological events were identified. The rims differ from the cores in distribution of trace elements.Genetic typification of diverse newly formed rims is based on discrimination of them by internal structure,isotopic and geochemical characteristics, which are used as a criterion of relationships between Ti–Zr placersand their inferred feeding sources. Based on these data, a prospecting model of the buried Ti–Zr placers withestimation of their resource potential has been produced.

DOI: 10.1134/S1075701511060067

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(a)

M

W

0 100 km

Perth

AdelaideSydney

III

III

IV0 200 400 km

(b)

S

Y

Elista

Stavropol

Pyatigorsk

0 100 km

0 200 400 km

Moscow

VI

V

VII

VIII

9876

54321

Fig. 1. Ti–Zr placer provinces of (a) Australia and (b) Russia. (1) Placer province, (2) placer district or deposit, (3–5) metamor�phic belts: (3) Kanmantoo, (4) Ballarat and Melbourn, (5) Wagga; (6) pre�Alpine basement of the Greater Caucasus; (7) fault;(8) boundary of placer provinces (numerals in map) in Australia: I, Western; II, Eukla; III, Murray; IV, Eastern) and Russia: V,North Caucasus; VI, Central; VII, Ural; VIII West Siberian; (9) boundary of placer deposits (letters in figure) in Australia: M,Mindarie and W, WIM�150; placer districts in Russia: S, Stavropol and Y, Yergeni.

Boiko (2004), these placers are products of the erosionof northern massifs (Tokmokovo Rise) with the subse�quent transport of sediments along the channel of thepaleo�Don river toward the Caucasus Range. Thus,the Ti–Zr placers of the Stavropol region are regardedas a distal periphery of the East European placer prov�ince. In the opinion of Gurvich et al. (1968) and Ver�

emeeva et al. (2004), the North Caucasus is a sourcefor the Stavropol Ti–Zr placers. Our morphostruc�tural and paleofacies reconstructions in combinationwith isotopic and geochemical features of zircons(Kremenetsky et al., 2006, 2007; Mikhailov et al.,2007) supported the second hypothesis and allowed usto state that the weathering mantles of the pre�Alpine

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basement of the Greater Caucasus are the main sourceof Ti–Zr placers in the Stavropol krai.

This concept served as the basis for substantialgrowth of the resource potential of zircon in productsof erosion throughout the North Caucasus Ti–Zrprovince, and as a result, in the involvement of thisprovince in the program “Centers of Economic Devel�opment of Russia” (Mikhailov and Kimelman, 2010).

The advantageous geographic position of thisregion, shallow�seated placers, climatic conditionsfavorable for year�round mining, and developed infra�structure are convincing arguments for the develop�ment of deposits in the Stavropol placer district andcreation of a domestic source of Ti and Zr mineralcommodities in the south of Russia.

The enormous ore potential of this province, likeother Ti–Zr provinces of Russia, has yet to be realized,because in contrast to most recent near�shore marineplacers of Australia and South Africa, the placers inRussia are buried and therefore require more complexmining conditions, and are economically less effi�cient. As a rule, these placers require the removal of alarge overburden or the use of hydromining, for whichthe technology is so far insufficiently elaborated. Sec�ondly, the relatively high zircon grade of the Russianplacers, in some cases comparable with the best for�eign counterparts, is combined with lower ilmeniteand rutile grades. Third, fine�grained and clayey oresands in buried placers are characterized by lowerrecovery of valuable minerals. Fourth, it is not easy toproduce a conditioned concentrate at some of ourdeposits.

The enhancement of investment appeals to theseobjects requires, together with application of new con�centrating technologies (dry separation, etc.), thedevelopment of efficient prospecting criteria for prof�itable Ti–Zr placers in the overlapped territories. Thekey solution of this problem should be based on thecomprehensive geological and mineralogical study ofthe system comprising the primary source, weatheringmantle, intermediate reservoir rock, and economicTi–Zr placer. The study provides for (1) morphostruc�tural and paleofacies reconstructions of the permissiveterritories; (2) quantitative analysis of the distributionof ore, accompanying, and auxiliary minerals in eachelement of this system; (3) study of typomorphic fea�tures of zircon (±monazite), ilmenite, and rutile andtheir variations from the primary source to the areawhere Ti–Zr placers accumulate (Kremenetsky et al.,2010a). A special part is assigned to the optical andcathodoluminescence microscopy, electron micro�probe and mass spectroscopy (laser ablation,SHRIMP II).

The bedrocks studied and spatially associated Ti–Zr placers always contain a sufficient amount of detri�tal zircons transformed at the late stages of metamor�phism and dynamometamorphism. Such zircons areclearly identified in cathodoluminescent photomicro�graphs by newly formed rims (5–30 μm) superim�

posed on growth zones of premetamorphic detrital zir�cons differing in age and indicative geochemical ratios(Zr/Hf, Th/U, Y/Ho, etc.).

GENETIC TYPES OF ZIRCON RIMS

Unique properties of zircon (resistance to weather�ing, ability to recrystallize and regenerate, diverseappearance, habit, and internal structure, isotopic andgeochemical inhomogeneities displaying genesis andstaged evolution of host rocks) are well known andwidely used in geochronology and reconstruction ofgeological processes. Recently, with the appearance ofnew research methods, the number of publicationsconcerning zircon ages of geological complexes con�tinuously increases (Tables 1, 2). Recent publicationsdemonstrate a distinct departure from traditional sta�tistical methods of age estimation based on monomin�eralic fractions, because the estimates obtainedthereby are actually artificial mixtures of different agesand habitually lead to erroneous inferences about thesequence of geological events. The study of the inter�nal structure of zircon reflecting its crystallization andrecrystallization conditions is currently predominant.A key role is assigned to the thin (3–100 μm) outerrims at the margins of zircon crystals. As a rule, theserims differ in composition from the central zones andcorrespond to the latest stage of the studied geologicalevent.

Allowing for mechanisms of crystallization andrecrystallization of zircon, including experimentaldata on this subject (Feisler et al., 2007; Fraser et al.,1997; Vavra et al., 1996; Watson, 1996; Hormannet al., 1980; Pidgeon, 1992; Putnis, 2002; Putnis et al.,2005) and our new data, the magmatic, hydrothermal,metamorphic, and regenerated rims enveloping andovergrowing zircon grains are recognized.

Magmatic rims of the late growth crystallize overzircon grains of early generations and xenogenic(detrital) grains captured by melts from country rocks.These rims develop as newly formed marginal growthzones (5–50 μm) typically conformable with oscilla�tory growth zones of primary zircon. As in the latter,the magmatic rims contain melt inclusions and arecharacterized by Th/U ratio typical of this geneticgroup of zircons (Table 1). The rims of this type areazonal or thin�banded and characterized by lower U,Th, REE, Y, Hf, Ti, Sr, and Ba contents, Th/U and

Eu/Eu*1 relative to the central zones. The Th/U ratio

in the late rims vary from 0.4 to 1.0; according toRubatto (2002), this range is characteristic of mag�matic zircons (Table 1).

The age of magmatic rims marks the late magmaticstage of the igneous complex formation. A minimaldifference in the age between the inner and the outermagmatic zones is 37–49 Ma (Table 1). Magmatic

1 Eu* is the content in chondrite

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Table 1. The U�Pb age of magmatic rims in zircon grains

Locality

Zircon

Source

central zone, Ma Th/U rim, Ma Th/U

Moore porphyry dike, Australia

Slightly zonal magmatic xenocryst, 2765 ± 17

– Thin banded magmatic with unconformable growth, 2765 ± 7

0.44–0.74 Baggott et al. (2005)

Granite gneiss of Cen�tral Tien Shan, China

Zonal detrital xenoc�ryst, 969 ± 11

0.17–0.25 Light and thin anatectic453 ± 11

0.14–0.44 Yang et al. (2008)

Eclogite of the Salma pluton, Belomorye

Dark to black zonal magmatic core, 2868 ± 35

0.8–1.5 Light gray metamor�phic, recrystallized at eclogitization, 1892–1822

0.0 Skublov et al. (2010); Mints et al. (2010)

Mafic and ultramafic rocks of the South Kovdor area, Kola Peninsula, Russia

Zonal crustal xenocryst,2754 ± 13

0.66–1.77 Light gray magmatic,2408 ± 8.0

0.46 Krivolutskaya et al. (2010)

Granitic rocks of the Strel'tsovka uranium district, the Argun�Bureya Median Mas�sif, Russia

Zonal magmatic core with melt inclusions and uraninite, 282 ± 1

– Dark magmatic with melt inclusions and ura�ninite, 245 ± 2

– Golubev et al. (2010)

Nepheline syenite of the Sakharjok pluton, Kola Peninsula, Rus�sia

Rhythmically zoned magmatic core, 2613 ± 35

– Porous hydrothermal,1810–1680

– Lyalina et al. (2010)

Granitic rocks of the Tyn'yar area, West Si�berian Plate, Russia

Zonal detrital core,2063 ± 29

0.6 Dark gray and homoge�neous magmatic,1542 ± 22

0.08 Ivanov and Erokhin(2011)

Amphibolite of the Karelian Craton, Russia

Zonal xenocryst, 2816 ± 22

0.70–0.84 Light and homogeneous magmatic, 2125 ± 15

0.42–0.87 Zlobin et al. (2010)

Granulite of the Khanka massif, Russia

Azonal magmatic core, 757.4 ± 4.4

– Zonal metamorphic,506.9 ± 2.6

– Khanchuk et al. (2010)

Tonalite, the Vyg River, southeastern Karelia

Zonal magmatic core with melt inclusions, 3122 ± 8

0.13–0.9 Fine�zonal metasomat�ic with fluid inclusions, 3073 ± 8

0.006–0.6 Sergeev et al. (2008)

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rims are characterized by concordant U–Pb ages,which are close to or younger than those of the centralzones. An inverse relationship established occasionallyis caused by partial loss of radiogenic lead from coresof zircon crystals, for instance, due to its metamictiza�tion.

Hydrothermal rims develop on both magmatic anddetrital zircons owing to the mechanism of dissolu�tion–redeposition under the effect of high�to low�temperature solutions (including metalliferous fluids).Rims of this type make up conformable or uncon�formable envelopes (3–40 μm) enclosing replaced zir�cons. As a rule, they contain fluid inclusions, are thin�banded or homogeneous, characterized by redistribu�tion of trace elements, and differ in Th/U,REE/REE*, and other indicative ratios depending oncomposition of hydrothermal solution and replacedzircon (Tables 1, 2). A zone of enrichment in trace ele�ments is displayed at the contact between hydrother�mal rim and replaced zircon in CL images. The con�centration of these elements in the rim itself is alwayslower. The partial or complete dissolution of marginalzones results in opening of the U–Pb isotopic system.

The subsequent crystallization of the newly formedrim closes this system, thus retaining a concordant U–Pb age of hydrothermal process superimposed onmagmatic or detrital zircons

Metamorphic rims develop on magmatic and detri�tal zircons under effect of high�pressure (eclogitefacies) or high�, medium�, or low�temperature meta�morphism of granulite, amphibolite, or greenschistfacies. Since eclogite� and granulite�facies metamor�phism is accompanied by partial melting of host rocks,the corresponding I�type metamorphic rims resemblemagmatic rims. Amphibolite� and greenschist�faciesmetamorphism is accompanied by solid�phase reac�tions in the presence of aqueous fluid and character�ized by development of metamorphic rims of H�typeresembling hydrothermal rims.

Metamorphic rims (3–100 μm) are homogeneousor thin�banded and contain melt (I�type) or fluid(H�type) inclusions. They are distinguished by sharplylowered Th/U ratio (0.002–0.08, less frequently0.10–0.66) (Table 2). According to Rubatto (2002),the composition of newly formed metamorphic rimsdepends on P–T conditions of metamorphism. For

Table 2. The U�Pb age of metamorphic rims in zircon grains

LocalityZircon

Sourcecentral zone, Ma Th/U rim, Ma Th/U

Paragranulite, southern Spain

Diffuse and zonal detrital core, 950–480

0.59–2.07 Light homogeneous metamorphic, 21.3 ± 0.3

0.08–0.39 Whitehouse and Platt (2003)

Metamorphic rocks of the Chupa Sequence, north�ern Karelia, Russia

Zonal primary magmatic core, 2900–2800

0.34–0.47 Homogeneous meta�morphic,1.2747 ± 6 2.1894 ± 17

0.53–0.66 Krylov et al. (2011)

Gold�sulfide�quartz ore from the Sukhoi Log de�posit, the Bodaibo Syn�clinorium, Russia

Heterophase detri�tal core,2600–500

0.60–1.55 Porous metamorphic�hydrothermal,447 ± 6

0.48–1.15 Yudovskaya et al. (2011)

Tonalite of the Orekhovo�Pavlograd Zone, the Ukrainian Shield, Ukraine

Zonal magmatic core, 3500 ± 13

0.4–0.6 Homogeneous meta�morphic, 2863 ± 22 2105 ± 40

0.01–0.13 Lobach�Zhuchenko et al. (2010)

Eclogite of the South Tien Shan, northwestern China

Detrital zonal core, 2450 ± 31

Š0.92 Homogeneous meta�morphic,319.5 ± 2.9

0.002–0.004 Su et al. (2010)

Tonalite of the Boras plu�ton, Sweden

Zonal magmatic core, 1680

– Metamorphic,940

– Cornell et al. (1999)

Granulite of the south�western Norway

Zonal primary magmatic core,1680–1570

0.1–0.7 Homogeneous meta�morphic,1560–1555

0.05 Hoskin and Hanchar (2003), Hoskin (2005)

Granulite of the Khanka pluton, Russia

Azonal magmatic core, 757.4 ± 4.4

– Zonal metamorphic, 506.9 ± 2.6

– Khanchuk et al. (2010)

Eclogite of the Sesia�Lanzo Zone, the Alps

Detrital core,264–390

0.03–0.6 Metamorphic, 65 ± 3

>0.01 Rubatto and Her�mann (2003)

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example, the composition of metamorphic rims onzircon from granulites is close to magmatic zircons(high U, Y, Hf, and P contents; enrichment in HREE,positive Ce anomaly, and negative Eu anomaly withEu/Eu* = 0.08–0.41). At the same time, the Th/Uratio is low (<0.07). Under conditions of eclogitefacies, metamorphic rims grow over detrital zircons atlimited supply of trace elements from outside. As aresult, the newly formed rims are characterized by lowTh, U, Y, Nb, P, and REE contents relative to zirconcores, a weak Eu anomaly (Eu/Eu* = 0.32–0.59), andlow Th/U ratio (<0.01). A difference in U–Pb agebetween central zones and metamorphic cores varyfrom 68 to 740 for magmatic zircons and from 694 to1884 Ma for detrital zircons (Table 2).

Regeneration rims arise on magmatic and detritalzircons under effect of deformation (folding, dyna�mometamorphism) by means of partial or completerecrystallization of marginal parts of zircon grainsowing to solid�phase diffusion. The rims of this type(2–30 μm) are homogeneous, typically unconform�able with the primary oscillatory growth zoning; con�tain relics of inherited solid�phase inclusions; arealways depleted in radiogenic lead and all trace ele�ments except for Hf. The distinguishing feature ofregeneration rims is the enrichment of their frontalzones in trace elements relative to the internal parts ofthe rims.

RESEARCH METHODS

The buried Ti–Zr placers in the Murray Basin(Mindarie and WIM�150 deposits) in Southeast Aus�tralia and the placer provinces of the North Caucasusand the East European Platform (Beshpagir, Central,Istobnoye, and other deposits) are objects of this study(Fig. 1).

After reconstruction of placer formation, the spa�tially separated varieties of primary rocks and the rocksof weathering mantles, paleochannels, and ore layersof Ti–Zr placers were sampled in the course of thisinvestigation. In the Ti–Zr province of the MurrayBasin two such systems have been sampled: (1) themetamorphic Kanmantoo Belt and the CambrianMindarie Ti–Zr deposit and (2) the metamorphicBallarat Trough and the Cambrian–Ordovician WIM�150 Ti–Zr deposit. In the North Caucasus and EastEuropean Ti–Zr provinces, the Voronezh CrystallineMassif in the north and the North Caucasus in thesouthwest were studied as inferred sources of theBeshpagir, Central, Istobnoye, and other Ti–Zrdeposits.

After petrographic and geochemical examinationof the primary rock samples, crushing, washing, andseparation of the heavy fraction, the quantitative min�eralogical analysis of ore minerals (zircon, ilmenite,rutile, etc.) and gangue minerals (garnet, sillimanite,etc.) was carried out, and zircon concentrate was sep�arated. The samples of loose rocks from the weather�

ing mantles, paleochannels, and Ti–Zr placers werepreliminarily panned; then the quantitative mineral�ogical analysis of gray heavy concentrate was per�formed with separation of a special zircon concen�trate.

At the first stage of the zircon study, we conductedstatistical analysis of the appearance and habit, color,and transparency of the grains; the degree of theirroundness; occurrence of inclusions, fractures, etc. Atthe second stage, 80–100 grains representative foreach sample, were placed into special preparations forfurther cathodoluminescence and optical examina�tion, and mass spectrometric measurements (laserablation, SHRIMP II) at the laboratories of the Rus�sian Geological Research Institute (VSEGEI), Insti�tute of Geochemistry and Analytical Chemistry, Rus�sian Academy of Science (GEOKhI RAS), Institute ofMineralogy, Geochemistry, and Crystal Chemistry ofRare Elements (IMGRE), and Macquarie Universityin Sydney.

To make preparations, the zircon grains no smallerthan 20–30 μm in size from the representative sam�plings were used. The grains were arranged on anadhesive under a binocular microscope and then cov�ered by epoxy to form a pellet, which was subsequentlypolished. The internal structure of zircons assigned forisotopic and geochemical study was examined underan AxioLab optical microscope with digital photo�camera and on a CamScan MX2500S SEM equippedwith EDS and CL detector at VSEGEI.

The points in unbroken parts of the grains withoutan admixture of common lead were chosen for analy�sis. Part of U–Pb analyses were carried out using aNew Wave Research UP213 laser at the Laboratory ofGEOKhI RAS. The local analysis using the laser abla�tion technique must meet strict requirements con�cerning the purity of sample preparation, analyticalprocedure, and also the purity of zircon with respect toalien mineral inclusions. About half of the analyzedzircons yielded concordant U–Pb isotopic data withinanalytical uncertainty limits, whereas others were dis�carded due to the presence of nonradiogenic lead.Another part of the U–Pb isotopic data, primarilyrelated to newly formed rims, was obtained on aSHRIMP II multicollector secondary�ion high�reso�lution mass spectrometer. The chemical compositionof internal inhomogeneities of zircon grains was deter�mined on a Camebax�microbeam microprobe at theIMGRE.

Ti–Zr PLACERS OF THE MURRAY BASIN, SOUTHEAST AUSTRALIA

The Murray Basin, situated in the south of the EastAustralian Orogenic Province (Fig. 1a) covers an areaof 750 × 550 km2 and borders on the metaterrigenousrocks of the Kanmantoo Belt in the west and on thevolcanic and terrigenous rocks of the Ballarat Troughin the south (Fisher and Warren, 1975). The rocks of

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the Kanmantoo Belt were formed in the Early Cam�brian, whereas the rocks of the Ballarat Trough aredated at the Cambrian–Early Ordovician. Theemplacement of granites and folding in the Kanman�too Belt are dated as Early Ordovician and in the Bal�larat Trough, as Late Ordovician.

Beginning from the Late Carboniferous and espe�cially in the Cretaceous, the slow but continuous ero�

sion of the South Australian Craton led to the forma�tion of sedimentary cover with marine regime deposi�tion of sands, including those containing heavyminerals, in the Murray Basin. The Mindarie Ti–Zrplacer deposit is located close to the Kanmantoo Belt,while the WIM�150 deposit is close to the BallaratTrough. The existence of spatially separated feedingsources for the single Ti–Zr paleobasin determined

80

70

60

50

40

30

20

10

03500500 1000 1500 2000 2500 3000

P, %

Age, Ma

80

70

60

50

40

30

20

10

03500500 1000 1500 2000 2500 3000

P, %

Age, Ma

1 2 3(a)

(b)

4 5 6

Fig. 2. Abundance frequency (P) of U–Pb (SHRIMP II) ages of zircons from Ti–Zr placer deposits and their inferred feedingsources: (a) Southeast Australia and (b) Russia. Panel (a). (1, 2) Ti–Zr placer deposits: (1) Mindarie, (2) WIM�150, (3) meta�morphic rocks of the Kanmantoo Belt and Ballarat Trough (curve). Panel (b). (4, 5) Ti–Zr placer deposits: (4) Istobnensky,Butovsky, Vysokonovsky; (5) Beshpagir, Gofit, Aigur; (6) metamorphic rocks of the Voronezh Crystalline massif and pre�Alpinebasement of the Greater Caucasus (curve).

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50 μm

50 μm

CLOpt Opt CL Opt CL

50 μm

50 μm

50 μm50 μm50 μm

200 μm

50 μm

50 μm

50 μm100 μm50 μm

AR�02.3AR�02.4 AR�16.1

100 μm 100 μm 100 μm

Md.7 Md.4

Md.1

100 μm

2800

2683

579

1044

580

980

AR�095

525

595

Md.6

3110

3600

452

2400 1060

627234

491

(c)

(b)

(a)

(d)

(e)

Fig. 3. Zircons from: (a) Ti–Zr placers of the Murray Basin, (b) metamorphic rocks of the Kanmantoo Belt and (c) BallaratTrough in the transmitted light (Opt) and cathodoluminescence images (CL). Numeral in boxes are U–Pb ages, Ma of cores andnewly formed rims in zircon grains from (d) source rocks and (e) Ti–Zr placers.

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our choice for performing the study of the primarysource–weathering mantle–intermediate reservoirrocks–economic placers system.

The comparative analysis of mineral compositionof primary sources (slates, metasandstones, granites),weathering mantles, and Ti–Zr placers shows similar�ity in mineral species (zircon, monazite, ilmenite, leu�coxene, rutile, tourmaline, garnet, etc.) with obvious(1000 times and more) accumulation of these mineralsin the placers. Comparison of the main attributes ofzircons (habit, degree of roundness, color, transpar�ency, elongation, etc.) shows that they vary within awide range and make up polymodal curves as a resultof statistical processing. The clearly expressed peaks ofthese curves are typical of primary rocks and placers.The U–Pb age of 50–100 zircon grains reveals a simi�lar tendency. As is shown in Fig. 2a, the placers of the

Mindarie and the WIM�150 deposits are characterizedby polymodal peaks within a range of 3000 to 200 Ma.

The aforesaid indicates that the studied zircons aredetrital and accumulated in the Archean–Paleozoic(primary rocks) or in the Mesozoic–Cenozoic(weathering mantles, placers). Only one statisticallysignificant difference between the compared pairs con�cerns the grain size. The zircon grains in the KanmantooBelt–Mindarie placer pair are larger (> 0.14 mm) than inthe Ballarat Trough–WIM�150 placer (>0.14 mm).This discrepancy cannot be admitted as a significantdistinguishing criterion; therefore, attention wasmainly paid to the internal structure of zircon from thecompared samplings and their isotopic and geochem�ical characteristics.

The images of the studied zircons in reflected andtransmitted light, as well as CL images, show thatdespite their different habit and degree of roundness,

0.55

0.45

0.35

0.25

0.15

0.050 122 4 6 8 10

2600

2200

1800

1400

1000

600

206Pb/238U

207Pb/235U

I (c)0.15

0.13

0.11

0.09

0.05

0.030.1 1.50.3 0.5 0.7 0.9 1.1

850

750

550

450

350

250

206Pb/238U

207Pb/235U

II (d)

1.3

650

0.07

0.30

0.25

0.22

0.18

0.14

0.100.5 1.5 2.5 2.5

1400

1000

800

206Pb/238U

207Pb/235U

I (a) 1600

1200

0.13

0.12

0.11

0.10

0.08

0.070.1 0.5 0.9

206Pb/238U

207Pb/235U

II (b)

1.1

0.09

780

740

700

600

620

580

540

500

460

Fig. 4. Diagrams with concordia (SHRIMP II data): (a, b) zircon grains from metamorphic rocks of the Kanmantoo Belt and (c, d)Ti–Zr placers of the Mindarie deposit in the Murray Basin, Australia; (I) central zone and (II) regeneration rim of zircon grains.

464

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

KREMENETSKY et al.

most grains reveal three zones: the central zone withoscillatory zoning typical of magmatic zircons (Fig. 3a),the metamorphic rim (5–50 μm), which conformablyenvelops the central zone or less frequently overgrowsit unconformably (Fig. 3b), and the regeneration rim(3–30 μm) developing at grain margins discordantlywith respect of preceding zoning (Fig. 3c).

The U–Pb (SHRIMP II) dating of each of thesezones gave the following age ranges: 3600–1050 Mafor the central zone; 600–500 Ma for the metamor�phic rim; and 500–450 Ma for the regeneration rim(Figs. 3d, 3e). The U–Pb ages of the central zones andthe outer rims in zircons from metamorphic rocks ofthe Kanmantoo Belt and Ti–Zr placers of the Minda�rie deposit are given in Table 3; the same for the rocks ofthe Ballarat Trough and Ti–Zr placers of the WIM�150are given in Table 4. The diagrams with concordiadrawn on the basis of these data show that the centralzones of zircons from metamorphic rocks of the Kan�mantoo Belt and from the placers of the Mindariedeposit are close in concordant age—mean values are1000 and 1190 Ma, respectively (Fig. 4a, 4c), whereasthe outer rims are dated at 580–600 Ma (Figs. 4b, 4d).

Zircons from the Ballarat Trough and the WIM�150deposit display the same mean concordant age of cen�tral zones (1100–1000 Ma), whereas the outer rimsare somewhat younger (425–450 Ma).

The above data lead to the conclusion that despitewidely scattered attributes of detrital zircons, theirmetamorphic and regeneration rims mark the age ofthe last geological event in the feeding source andretain it in all other elements of the placer formation

system: weathering mantle–intermediate reservoirrocks, and Ti–Zr placer.

These relationships allowed us to trace directgenetic links of the Ti–Zr placers localized thereinwith different feeding sources. The Mindarie deposit isa product of erosion of metamorphic rocks of the Kan�mantoo Belt (age of regeneration of zircon rims is580–600 Ma and relatively large (>0.14 mm) zircongrains). The WIM�150 deposit is a product of erosionof metamorphic rocks of the Ballarat Trough (age ofregeneration of zircon rims is 425–450 Ma and rela�tively small (<0.14 mm) zircon grains) (Fig. 5).

Taking into account practical importance of iden�tification of alternative feeding sources of Ti–Zr plac�ers of the Murray Basin, we consider briefly the mech�anisms of redistribution of trace elements in detritalzircons during the formation of the late metamorphicor regeneration rims.

In the case of regional metamorphism of rocksfrom feeding sources, the development of metamor�phic rims on detrital zircons of varying ages isexpressed in the formation of homogeneous outerzones with a minimal amount of inclusions. In photo�micrographs, they are, as a rule, white (deficiency inDy) or dark (excess of Dy). Figure 6a (point 3) clearlydemonstrates a decrease in the concentration of alltrace elements (except for Hf) with inheritance oftheir set from the central zone.

The late regeneration rims as products of dyna�mometamorphism of rocks from the feeding sourcedisplay another pattern. These rims develop owing torecrystallization of marginal parts of detrital and

100

80

60

40

20

00.14 0.25

1400

1200

1000

810

600

400

200

0

P, %

mm

WIM�150Mindarie

KanmantooBelt

BallaratTrough

Metamorphic rocks of

Mindarie deposit

WIM�150deposit

Metamorphicrocks of the Ballarat

Murray Basin

(a) (b)

Ma

1.1

1.3

0.6

0.4

the Kanmantoo BeltTrough

Fig. 5. Abundance frequency (P) of (a) grain size and (b) U–Pb age of central zone (upper lines) and regeneration rims (lowerlines) of zircon grains from feeding sources of Ti–Zr placers in the Murray Basin, Australia.

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

ISOTOPIC AND GEOCHEMICAL FEATURES 465

metamorphic zircons propagating inward from theirmargins with formation of a relatively wide (3–30 μm)regeneration rim in the back growth zone and a narrow(a few micrometers) frontal rim. As follows from the

microprobe data, this process is accompanied by asharp increase in the concentrations of all trace ele�ments and complementary redistribution of Hf, REE +Y, P, and Th between the depleted reaction rim and

0.80

0.70

0.400.300.200.10

01 2 3 4 5 6

0.600.50

Points

IV

ThO2

UO2

ThO + UO2

0.800.70

0.400.300.200.10

01 2 3 4 5 6

0.600.50

Points

IV

7 8

ThO2

UO2

ThO + UO2

4.00

3.70

2.00

1.30

1.00

0.50

0

3.00

2.50

III

Y2O3

TR2O3

P2O5

CaO

(Y, REE)2O3

4.00

3.70

2.00

1.30

1.00

0.50

0

3.00

2.50

III

Y2O3

TR2O3

P2O5

CaO

(Y, REE)2O3

1.60

1.40

1.20

1.00

0.80

2.00

1.80

II

HfO21.60

1.40

1.20

1.00

0.80

2.00

1.80

II

HfO2

I

(a)

1105 ± 30 Ma

564 ± 8 Ma

1

23

45

6

I

1123 ± 34 Ma

450.3 ± 8 Ma

87

6 5

432

1

(b)

Fig. 6. Boundaries between (a) central zone and metamorphic rim (sample AR�26III) and (b) central zone and regeneration rim(sample AR�51a) in zircons; points of U–Pb dating and electron microprobe analysis (I) and concentrations of trace elements(II–IV).

466

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

KREMENETSKY et al.

Opt CL Opt CL

Opt CL

100 μm

200 μm200 μm100 μm

2120 2696 1925

2466

1698

100 μm 100 μm

1515

100 μm100 μm

1395

234491

306300

1977

100 μm

100 μm 100 μm100 μm

Fig. 7. Zircons from: (a) metamorphic rocks of pre�Alpine basement of the Greater Caucasus and (b) Ti–Zr placers of the NorthCaucasus province in the transmitted light (Opt) and cathodoluminescence images (CL); (c, d) U–Pb ages, Ma of cores andnewly formed (c) metamorphic and (d) regeneration rims in zircon grains.

(a)

(b)

(c)

(d)

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

ISOTOPIC AND GEOCHEMICAL FEATURES 467

322

0.054

0.053

0.052

0.051

0.050

0.049

0.0480.32 0.420.34 0.36 0.38 0.40

207Pb/235U

206Pb/238U

Sample О9T = 318.2 ± 2.5 Ma, N = 8 (2σ)

338

334

330

326

318

314

310

306

Fig. 8. U–Pb isotopic data on zircons from orthogneisses of the North Caucasus plotted on diagram with concordia.

enriched frontal zone, as well as by loss of 206Pb and rela�tive gain in uranium in the regeneration rim (Fig. 6b).These data indicate a transition from the closed condi�tions of the U–Pb isotopic system during formation ofdetrital zircons to the open formation conditions ofrecurrent metamorphic rims.

Such a distribution of trace elements between theinner zones and the outer rims can be used as a crite�rion for recognizing metamorphic and regenerationrims.

Ti–Zr PLACERS OF THE NORTH CAUCASUS PROVINCE IN RUSSIA

In addition to the known Beshpagir deposit, anumber of new economic Ti–Zr placers have beenrevealed in the North Caucasus province (Veremeevaet al., 2004; Kremenetsky et al., 2007). Their total areais about 18000 km2 (Fig. 1b). Their approved resourcesare 9.1 Mt TiO2 and 2.1 Mt ZrO2. The placers arelocalized in the Lower Neogene fine�grained sandy

sediments with predominant concentration of heavyminerals in fraction 0.14 + 0.071 mm. The thicknessof productive sand varies from 2.7 to 6.8 m; their depthfrom ground surface does not exceed 35–40 m. TheNorth Caucasus province is situated 150–200 km northof the Greater Caucasus Range and 350–400 km southof the Voronezh Crystalline Massif (VCM). Mean�while, some researchers suggest that the cover of theEast European Platform and the Precambrian base�ment of the VCM were a feeding source of the Ti–Zrplacers of the Stavropol region. The rocks of the pre�Alpine basement of the Greater Caucasus are assumedto be an alternative source. We studied both possiblesources: primary rocks and weathering mantles of theVCM and the North Caucasus. The results obtained(Fig. 2b) are consistent with the data pertaining to theMurray Basin. Only zircons from the Precambrianrocks of the VCM and the retained weathering mantlescovering them are exclusions. Both contain a signifi�cant amount of zircon grains (>0.25 mm) with abun�dant fractures and various degree of metamictization.

468

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

KREMENETSKY et al.

Such zircons, moving away from a source, are com�monly broken and dispersed and thus do not reach aregion of accumulation of near�shore marine placers.

The metamorphic rims of zircons from (a) bed�rocks of the VCM and (b) newly formed rims of zir�cons from the North Caucasus (b) are shown in Fig. 7.The former are slightly zonal and often unconform�ably superimposed on the oscillatory magmatic zoningin the grain cores, whereas the latter are azonal andthinner (3–30 μm).

The following features have been revealed from thedata on U–Pb (SHRIMP II) age of the aforemen�tioned zircons. First, the ages of central zones andmetamorphic rims of the compared zircons rangefrom 2800 to 1300 Ma (Figs. 7c, 7d), so it is impossibleto identify the sources using statistical methods. Sec�ond, the occurrence of regeneration rims with an aver�age concordant age of 318 ± 2.5 Ma for zircons fromrocks of the pre�Alpine basement of the Greater Cau�casus (Fig. 8) and from the Ti–Zr placers of theStavropol and Yergeni ore districts (Fig. 9) clearlyindicates that precisely these rocks fed the placers.Third, in the basement rocks of the VCM as an alter�

native source, young newly formed rings do not occurat all. Only ancient metamorphic shells dated at1800–1600 Ma enveloping older cores (2.8–2.0 Ga)are widespread in both bedrocks and placers (Fig. 9).Thus, the paleo�Don hardly can be regarded as achannel feeding the placers in the North Caucasusprovince. The fourth feature is concerned with thespecific evolution of the gneiss–migmatite complex ofthe Greater Caucasus. As is known, this complex istraditionally dated as Proterozoic (Somin et al., 2006).The occurrence of zircon grains with the outer rimsdated at 318 Ma enveloping the older detrital cores(1500–2000 Ma), as well as findings of metamorphiczircons having an age of 310–320 Ma in orthogneissesand migmatized paragneisses allowed Somin to statethat the gneiss–migmatite core of the Elbrus Subzoneof the Central Caucasus was formed during theVariscan tectonic epoch due to reworking of the Prot�erozoic and Paleozoic rocks.

Evidence that allowed us to regard the pre�Alpinerocks of the Greater Caucasus, primarily, of theBechasyn Zone (Fig. 9), as a source that fed the sedi�ments of the Miocene sea in the Forecaucasus with

3.0

2.5

2.0

1.5

1.0

0.5

(a) (b) (c)Ma

Pereval’ny Main Range Bechasyn Stavropolye Kalmykia Central deposit Graniticrocks of basin

VCMZones of the Greater Caucasus Ti–Zr placers

321

2.1

1.7

1.9

1.4

1.7

2.5

0.3 0.3

0.560.460.35

2.72.5

1.7

1.6

1.21.3

0.43

0.31

0.47

0.32

2.92.8

2.0 2.0

1.7 1.7

Fig. 9. Age of (1) metamorphic and (2) regeneration rims on (3) detrital zircons from Ti–Zr placer of the North Caucasus prov�ince (b) and in their alternative feeding sources: (a) Greater Caucasus and (c) Voronezh Crystalline Massif (VCM).

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

ISOTOPIC AND GEOCHEMICAL FEATURES 469

Tabl

e 3.

Res

ults

of U

–P

b is

otop

ic s

tudy

of z

icon

s fr

om fe

edin

g so

urce

an

d T

i–Z

r pl

acer

s of

th

e M

inda

rie

depo

sit,

th

e M

urra

y B

asin

, A

ustr

alia

An

alyt

ical

poi

nt

206 P

b c,

%U

, pp

mT

h, p

pm23

2 Th

/238 U

206 P

b*,

ppm

(1)

206 P

b/23

8 U,

age,

Ma

(1)

207 P

b/20

6 Pb,

ag

e, M

a

(1)

207 P

b*/23

5 U±

%(1

)20

6 Pb*

/238 U

±%

Rh

o

Fee

ding

sou

rce

Cor

e

AR

02.4

.2

0.04

417

123

0.30

56.7

947

±22

969

±22

1.56

2.70

0.16

2.50

0.91

AR

02.1

2.1

0.60

2414

618

0.26

335

960

±17

962

±61

1.58

3.60

0.16

1.90

0.54

AR

02.1

2.2

0.09

3411

1217

0.37

459

938

±15

916

±33

1.50

2.40

0.16

1.80

0.75

AR

08.7

.1

0.18

1459

674

0.48

317

1450

±24

1544

±29

3.33

2.40

0.25

1.80

0.76

AR

09.7

.2

0.00

1798

1121

0.64

272

1044

±18

1044

±32

1.80

2.50

0.18

1.90

0.77

AR

16.6

.2

0.04

1262

827

0.68

161

894

±16

910

±43

1.42

2.80

0.15

1.90

0.67

AR

17.6

.2

0.05

2467

541

0.23

372

1043

±17

1054

±30

1.80

2.30

0.18

1.80

0.77

Out

er r

im

AR

02.4

.1

0.23

188

700.

3913

.852

9±

1355

0±

550.

693.

600.

092.

500.

71

AR

02.4

.3

0.33

174

660.

3912

.953

4±

1452

0±

960.

695.

100.

092.

600.

52

AR

02.1

2.2

0.00

706

300.

0468

.769

2 ±

1670

3±

980.

985.

200.

112.

400.

47

AR

02.2

1.1

1.48

1362

834

460.

2611

10

574.

4±

960

5±

720.

773.

800.

091.

700.

46

AR

08.7

.2

0.00

2836

270.

0123

158

5±

1259

1±

660.

783.

700.

102.

100.

57

AR

09.7

.1

0.46

1108

606

0.57

89.9

579

±12

585

±10

00.

775.

300.

092.

200.

42

AR

16.6

.1

0.35

1054

880.

0985

.257

7±

1154

7±

980.

765.

000.

092.

100.

42

AR

17.6

.1

0.45

821

380.

0574

.264

2±

1369

5±

100

0.90

5.20

0.10

2.10

0.40

Ti�

Zr

plac

ers

Cor

e

MD

1.1.

2 1.

1422

915

20.

6930

.591

915

960

130

1.50

6.70

0.15

1.70

0.26

MD

1.3.

1 0.

0043

430

50.

7373

.0

1153

1611

6551

2.12

3.00

0.20

1.50

0.50

MD

1.4.

1 0.

3016

813

00.

8026

.0

1064

1999

278

1.79

4.30

0.18

1.90

0.45

MD

1.7.

2 0.

0087

910

971.

2934

1 24

0426

2377

119.

521.

500.

451.

300.

89

MD

1.3.

3 0.

0837

630

10.

8357

.0

1048

±22

1009

±36

1.77

2.90

0.18

2.30

0.80

MD

2.2.

2 0.

4949

114

60.

3175

.810

6017

1061

791.

844.

300.

181.

700.

40

MD

2.3.

2 0.

0031

822

00.

7157

.712

3618

1073

422.

192.

600.

211.

600.

60

470

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

KREMENETSKY et al.

Tabl

e 3.

(Con

td.)

An

alyt

ical

poi

nt

206 P

b c,

%U

, pp

mT

h, p

pm23

2 Th

/238 U

206 P

b*,

ppm

(1)

206 P

b/23

8 U,

age,

Ma

(1)

207 P

b/20

6 Pb,

ag

e, M

a

(1)

207 P

b*/23

5 U±

%(1

)20

6 Pb*

/238 U

±%

Rh

o

MD

2.4.

2 –

35

830

30.

8858

.711

2615

1144

342.

052.

200.

191.

500.

65

MD

2.8.

2 0.

0015

965

0.42

23.7

1029

1910

2865

1.75

3.80

0.17

2.0

0.52

MD

2.9.

2 0.

3540

018

60.

4851

.289

312

1332

481.

752.

800.

151.

400.

50

MD

2.10

.2

0.53

169

117

0.71

32.3

1285

2112

2580

2.47

4.40

0.22

1.80

0.40

MD

2.11

.2

0.00

334

212

0.66

59.8

1219

2011

6541

2.26

2.70

0.21

1.80

0.66

MD

2.4.

3 1.

2316

773

0.45

20.0

832

±19

1000

±98

1.38

5.40

0.14

2.50

0.46

MD

2.4.

4 0.

5528

518

70.

6839

.896

6±

2110

34±

461.

643.

300.

162.

400.

72

MD

2.3.

3 0.

7421

113

40.

6534

.711

22±

2498

8±

681.

884.

100.

192.

400.

58

MD

2.3.

4 0.

0433

121

40.

6750

.010

44±

2310

83±

271.

832.

800.

182.

400.

87

MD

2.11

.3

0.00

285

156

0.56

51.7

1236

±27

1101

±30

2.22

2.80

0.21

2.40

0.85

MD

2.11

.4

0.22

223

88.0

0.41

36.2

1116

±25

1068

±43

1.95

3.20

0.19

2.40

0.75

Out

er r

im

MD

1.1.

1 0.

0011

379

.00.

7310

.164

013

591

100

0.86

5.30

0.10

2.10

0.41

MD

1.3.

2 0.

0022

612

.00.

0619

.661

910

678

700.

863.

700.

101.

700.

47

MD

1.4.

2 1.

3418

032

.00.

1816

.062

712

606

220

0.84

10.0

0.10

2.0

0.19

MD

1.7.

1 0.

5213

3611

40.

0983

.845

2.1

6.1

474

750.

573.

700.

071.

400.

38

MD

1.7.

4 4.

1646

345

31.

0146

.868

8±

1590

2±

110

1.07

6.0

0.11

2.40

0.39

MD

1.3.

4 1.

9843

772

91.

7243

685

±15

811

±99

1.02

5.30

0.11

2.40

0.45

MD

2.3.

1 2.

7814

2380

30.

5897

.548

0.4

7.2

962

130

0.76

6.60

0.08

1.60

0.24

MD

2.4.

1 0.

2139

915

10.

3931

.857

1.5

8.6

508

740.

733.

700.

091.

600.

42

MD

2.8.

1 0.

4565

916

10.

2557

.361

98.

357

475

0.82

3.70

0.10

1.40

0.38

MD

2.9.

1 8.

4117

4014

980.

8984

.632

54.

549

026

00.

4112

.00.

051.

400.

12

MD

2.10

.1

0.00

245

110

0.46

17.6

518

8.4

506

780.

663.

900.

081.

700.

43

MD

2.11

.1

0.00

612

340.

0655

.464

58.

867

243

0.90

2.50

0.11

1.40

0.58

MD

2.10

.3

0.90

213

590.

2922

.674

7±

1789

3±

931.

165.

100.

122.

400.

48

MD

2.10

.4

1.18

232

800.

3625

752

±17

706

±96

1.07

5.10

0.12

2.40

0.48

Not

es:

Her

e an

d in

Tab

le 4

, P

b c a

nd

Pb*

are

com

mon

an

d ra

diog

enic

lead

, re

spec

tive

ly;

(1)

corr

ecti

on f

or c

omm

on le

ad f

rom

th

e m

easu

red

204 P

b; R

ho,

cor

rela

tion

of

207 P

b*/23

5 Uan

d 20

6 Pb*

/238 U

un

cert

ain

ties

.

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

ISOTOPIC AND GEOCHEMICAL FEATURES 471

Tabl

e4.

Res

ults

of U

�Pb

isot

opic

stu

dy o

f zir

con

s fr

om fe

edin

g so

urce

an

d T

i�Z

r pl

acer

s of

th

e W

IM�1

50 d

epos

it,

the

Mur

ray

Bas

in,

Aus

tral

ia

An

alyt

ical

poi

nt

206 P

b c,

%U

, pp

mT

h,

ppm

232 T

h/23

8 U20

6 Pb*

, pp

m

(1)

206 P

b/23

8 U,

age,

Ma

(1)

207 P

b/20

6 Pb,

ag

e, M

a

(1)

207 P

b*/23

5 U±

%(1

)20

6 Pb*

/238 U

±%

Rh

o

Fee

ding

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472

GEOLOGY OF ORE DEPOSITS Vol. 53 No. 6 2011

KREMENETSKY et al.

clastic material made it possible to calculate a balancebetween the mass of terrigenous material removedfrom the Caucasus (Jurassic–Paleogene) and the massof the deposited in Neogene (Chokrakian and Sarma�tian stages) sands prospective for Ti–Zr placers and toestimate hypothetical resources of high�grade Ti–Zrplacers in the Stavropol region supplementing thealready known economic placers. According to ourestimates, 51 Mt of zircon can be expected at pros�pects and 18 Mt at high�grade sites. This implies thatat least one deposit comparable in resources with theBeshpagir deposit could be discovered. This forecastwas one of the grounds for conducting revision pros�pecting in this region, which was completed with thediscovery of the Kambulat prospect.

CONCLUSIONS

(1) Four genetic types of newly formed rims (mag�matic, hydrothermal, metamorphic, and regenera�tion) enveloping zircon grains derived from therocks—potential sources of Ti–Zr placers—weredefined and characterized.

(2) The primary source–weathering mantle–inter�mediate reservoir rock– Ti–Zr placer system has beenstudied for placers of the Murray Basin in SoutheastAustralia and the North Caucasus placer province inSouthwest Russia. Each element of this system is char�acterized by main physical properties of zircon (habit,outer appearance, degree of roundness, inclusions,etc.), specific attributes of internal structure and isoto�pic and chemical compositions determined by opticaland cathodoluminesce microscopy, laser ablation, andmass spectroscopy (SHRIMP II).

(3) Three zones are recognized in the detrital zir�cons studied: (i) the central zone with oscillatory mag�matic zoning; (ii) metamorphic rim (5–50 μm) con�formably or unconformably overgrowing the centralzone; and (iii) regeneration zone (3–30 μm) com�monly discordant with respect to the preceding zones.With allowance for U–Pb ages, the latter two zonescan be used as criteria for ascertaining relationshipsbetween placers and their feeding sources. To distin�guish metamorphic and regeneration rims, the differ�ence in trends of trace element distribution betweencentral zones of detrital zircons and newly formedrims should be used.

(4) The elaborated prospecting model of buriedTi–Zr placers comprises three main stages:

the premetamorphic stage—accumulation ofdetrital zircons, including rounded grains with coresand growth zones derived from the Precambriansupracrustal complexes 3.0–1.0 Ga in age;

the synmetamorphic stage—metamorphism anddeformation under conditions of high� and medium�temperature facies with newly formed metamorphic orregeneration rims, respectively; and the postmetamor�phic stage—weathering and erosion of ancient com�plexes with separation of zircon grains having newly

formed outer rims; transport and accumulation ofthese grains in the Mesozoic and Cenozoic near�shoremarine placers of proximal and/or distal removal.

(5) Correspondence of the peak ages of newlyformed rims over zircon grains from the Ti–Zr placersto the age of rims of zircon from certain ancient com�plexes or to the age of their synmetamorphic deforma�tion indicates that these complexes served as feedingsources for the placers in the paleobasin studied. Thesenew data are offered as the scientific basis of the inno�vative techniques for reconstructing localization con�ditions of Ti–Zr placers within overlapped paleobasinsand for estimating their resource potential.

ACKNOWLEDGMENTS

We thank M.A. Anosova, A.V. Antonov, N.G. Berezh�naya, Yu.A. Kostitsyn, I.M. Kulikova, A. N. Larionov,E.N. Lepikhina, I.P. Paderin, S.L. Presnyakov, andN.V. Rodionov for their assistance in the isotopic,geochemical, and microprobe study of zircons.

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