0024-4902/01/3602- $25.00 © 2001

åÄIä “Nauka

/Interperiodica”0109

Lithology and Mineral Resources, Vol. 36, No. 2, 2001, pp. 109–115. Translated from Litologiya i Poleznye Iskopaemye, No. 2, 2001, pp. 126–133.Original Russian Text Copyright © 2001 by Evzerov.

INTRODUCTION

In the northeastern Baltic Shield, like in the wholeBaltic Shield, region, prospecting for placers was notcarried out for a long time owing to a belief about thesignificant role of glacier exaration. It was assumed thatsheet glaciers eroded from the surface not only differ-ent-age loose rocks but a great mass of crystalline rocksas well. The possibility of formation and conservationof placers of various metals in the region was theoreti-cally substantiated by Sidorenko (1958, 1960) and wassubsequently developed by A.D. Armand, M.K. Grave,V.Ya. Evzerov, G.S. Rubinraut, and S.A. Strelkov fromthe Geological Institute Apatity (Armand

et al.

, 1965;Strelkov

et al.

, 1976 and others). It was concluded thatthe abundance of ore and accessory minerals, many ofwhich are stable during both weathering and transpor-tation, in rocks of the Baltic Shield, as well as the pres-ervation of relics of old weathering crusts and synchro-nous continental and marine sediments, were favorablefor the placer formation in this region.

However, despite a considerable activation of pros-pecting in the northeastern Baltic Shield and neighbor-ing Finland, only two more or less significant placerswere revealed over the last 40 years: the proximalloparite placer in foothills of Lovozero Tundra and thecoastal–marine complex placer on the Tera coast of theKola Peninsula. We considered the prospect for findingnew placers on the Baltic Shield in a special work (Evz-erov and Goryainov, 1998) and concluded that they arevery limited in size. In this work, we tried to prove thatplacer deposits are extremely rare on the Baltic Shield.

MINERAGENIC, TECTONIC, AND PALEOGEOGRAPHIC PREREQUISITES

OF PLACER FORMATION

A detailed analysis of mineragenic, geodynamicand paleogeographic conditions of placer formation is

necessary for the adequate interpretation of materialson placers on the Baltic Shield and the assessment ofprospects of the discovery of new deposits in theregion.

Placer-forming minerals in crystalline rocks ofthe shield territory.

The Baltic Shield is a large geo-logical structure within the East European Platform. Itis composed of Archean–Proterozoic metamorphic andmagmatic rocks. The Paleozoic tectonic activation inthis region was marked by the formation of numerousdikes of alkaline rocks and basaltoids, as well as intru-sive massifs of alkaline–ultramafic rocks. The largestbodies are represented by the Khibiny massif with well-known apatite deposits and the Lovozero massif withrare metals deposits. As was mentioned, the abundanceof ore and accessory minerals, which are stable duringweathering and transportation, in crystalline rocks is aprerequisite of placer formation. However, the behaviorof placer minerals in the hypergenesis zone substan-tially depends on various parameters and, first of all, ontheir density. Materials on the minerageny of crystal-line rocks in the Baltic Shield were not analyzed fromthis viewpoint. In this work, the analysis of availabledata is based on Momdzhi and Blinov’s (1970) classifi-cation (table).

The list of minerals, which can be accumulated inplacers owing to favorable physicochemical properties,includes about 50 minerals (Patyk-Kara

et al.

, 1997).Crystalline rocks of the Baltic Shield contain signifi-cant amounts of loparite, ilmenite, titanomagnetite,magnetite, perovskite, apatite, garnets, and zircon. Allof them, except zircon, form native deposits in the Bal-tic Shield (

Mineral’nye…

, 1981; Isokangas, 1982;Grip, 1982; Bugge, 1982) and, according to the men-tioned classification, are assigned to the group of min-erals with low density and high abundance. These min-erals form placer deposits only as a result of the intensechemical weathering of parent rocks and subsequentredeposition of products from the upper zone of theweathering crust. As to the minerals with low andmedium abundance but high and medium density (plat-

Placer Deposits As Unique Formations in the Loose Cover of the Baltic Shield

1

V. Ya. Evzerov

Geological Institute, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, 184200 Russia

Received July 3, 2000

Abstract

—This works considers the mineragenic, tectonic, and paleogeographical factors of the formation ofplacers on the Baltic Shield. Main structural and compositional features of these placers prove that they areunique formations in the loose cover of this region.

1

Report presented at XII International Conference on Geology ofPlacers and Weathering Crusts, Moscow, September 25–29, 2000.

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inum group minerals, Au, REE, Sn, Ta and Nb), whoseplacers are formed in a wider range of paleogeographi-cal environments, their significant deposits have notbeen as yet revealed in the study region. The same canbe said about native diamonds deposits, though pros-pects of their discovery are available (Kiselev

et al.

,1991; Polyakov and Kalinkin, 1993; Sorokhtin

et al.

,1996).

The value of erosional truncation.

Until recently,all researchers believed that several kilometers thickrock masses were washed out from the Baltic Shieldsurface during its existence. This conclusion was basedon pressure values determined with the help of geoba-rometers for magmatic and metamorphic processes.High pressures were regarded as derivatives of lithos-tatic load. Only in the 1980s, it was found that the fluidpressure can exceed the lithostatic one (

Petrografiya

,1976; Wain, 1983) and, therefore, is controlled only bythe roof strength rather than the rock sequence thick-ness. However, based on geological and geophysicalreconstructions of Paleozoic alkali–ultramafic intrusivemassifs on the Baltic Shield, researchers again sug-gested the hypothesis of a significant erosional trunca-tion that decreased from 8–11 km in the southeast to 5km or less in northwest (Arzamastsev

et al.

, 1998).This disputable argument was based on the constantrelation between the diameter of massifs and the depthof massif occurrence.

In our previous work (Evzerov and Goryainov,1998), we demonstrated that the available geologicalmaterial testifies to an insignificant erosion duringabout 2.5 Ga, but its value was not more preciselydetermined. Estimates of this value based on geologicalobservations are the most reliable ones. According toP.K. Skuf’in (personal communication), the denudationof subvolcanic edifices in the Pechenga–Varzugagreenstone belt (~1.9–2.3 Ga) ranges from 0.5 to1.5 km. Based on the preservation of neck facies of theYuovv–Oaiv subvolcanic granitoid complex (~1.8 Ga)established by M.I. Dubrovskii (1969), he estimatedthe erosional truncation at less than 1 km. Kirichenko(1970) studied Paleozoic sediments on the Kola Penin-sula and concluded that the erosional truncation duringthe Upper Devonian and Carboniferous differed little

from the recent value, because Devonian and Carbonif-erous basal conglomerates contain the same rocks thatare presently outcropped in the vicinity.

During the Paleozoic, the Baltic Shield was repeat-edly covered to a variable extent by waters of seabasins. This is evidenced by rare outliers of Cambrian–Silurian, Devonian, and Carboniferous metamorphosedsedimentary rocks. The information on their occur-rence and structure is systematized in (Kirichenko,1970). Continental conditions dominated on the BalticShield during the whole or major part of the Mesozoic(Afanas’ev, 1977; Evzerov and Nikolaeva, 1995). Atleast three (Paleogene, Upper Pleistocene, andHolocene) transgressions occurred within the region inthe Cenozoic (Evzerov and Nikolaeva, 1995).

Thus, a small volume of crystalline rock debris wasinvolved into the hypergenic migration cycle during thewhole history of the Baltic Shield. Most likely, theshield underwent vertical low-amplitude displacementsof different signs and partially subsided below the sea-level. However, on the whole, ascending movementsinsignificantly prevailed. This is a principle evolution-ary difference between the Baltic Shield and the Ukrai-nian Shield where a thick sedimentary cover (> 300 m)was accumulated owing to the prevalence of subsid-ence beginning from the Cretaceous. The cover is com-posed of the Upper Mesozoic and Cenozic intercalationof marine and continental sediments that host the raremetal, ilmenite, zircon–ilmenite, and other placers(

Metallogeniya…

, 1974).

Structure of the shield’s loose cover and exhuma-tion of place-forming minerals.

Crystalline rocks ofthe region are overlain by loose rocks that occupy 80–85% of the shield area. The thickness of the loose coveris generally a few meters but sometimes tens of metersand as much as 100 m or more in rare cases. Themoraine of sheet glaciations dominates in the cover.Polygenous, inter- late- and postglacial sediments aresubordinate: they are usually thin and fragmentary. Inaddition, the loose cover includes relics of the kaolinicand hydromicaceous weathering crusts.

The kaolinic weathering crust, which perhaps occu-pied a larger area within the Baltic Shield, was formedin warm and humid climatic conditions during the LateTriasssic–initial Early Jurassic. During the Early andMiddle Jurassic, its upper horizons were the mainsource of material delivered to the South Barents Seadepression. By the beginning of the Late Jurassic, onlyroots of the kaolinic weathering crust retained, becausea substantially less weathered material was washed outfrom the shield in the Late Jurassic and Early Creta-ceous, relative to the preceding period. These rootsplayed no part in placer formation on the continent.

During the kaolinic weathering, all unstable andintermediate (with respect to chemical weathering)minerals, such as biotite, amphiboles, pyroxenes feld-spars and others, were destructed and mainly kaolinitewas synthesized. Quartz, loparite, zircon, ilmenite, leu-

Classification of placer minerals by their behavior in mecha-nogenesis

Mineraldensity, g/cm

3

Occurrence frequency

low and medium high

High (>12) Noble metals –

Medium(5.5–12)

Ore minerals (cassite-rite, tantalite, and others)

–

Low(3.5–5.5)

(a) Rare earth minerals(b) Diamonds

Minerals of Ti and Zr

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PLACER DEPOSITS AS UNIQUE FORMATIONS 111

coxene, and several other stable minerals were notpractically altered.

During the hydromicaceous weathering in theMiocene–Pliocene under moderate climatic conditions,major minerals (feldspars, amphiboles, and pyroxenes)were only partially disintegrated. Therefore, ore andaccessory minerals were less completely exhumed, rel-ative to exhumation during the kaolinic weathering. Inthe terminal Neogene, redeposition of weathering prod-ucts resulted in the concentration of some low-densityminerals, almost up to the level of their concentration inthe ore horizons of crystalline rocks (Evzerov, 1998).However, finds of Neogene concentrates are extremelyrare, which is explained both by washout of the weath-ering crust due to the region uplift in the terminalPliocene and by assimilation of Neogene weatheringproducts by sheet glaciers.

Under the action of hypergene agents in the Quater-nary with a severe climate, crystalline rocks werealtered to a less degree, relative to the Neogene trans-formation, since the frost weathering, which replacedthe thermal weathering, produced a substantiallycoarser clastic material (Chernyakhovskii, 1980).Besides, the intensity of chemical weathering decreaseswith cooling. Accordingly, placer-forming mineralswere less completely exhumed in the Quaternary, ascompared to the preceding Mesozoic and Cenozoicperiods of the Baltic Shield evolution.

Influence of sheet glaciations on the formationand preservation of placers.

It was noted above thatthe shield’s loose cover is mainly represented bymoraines. Sheet glaciers, which left the moraines,interrupted the process of placer formation by screen-ing the supply of placer-forming minerals with a sheet,contaminated disintegration products of the placer sub-strate with alien materials, destroyed placers formedduring interglacial epochs, and, finally, supplied thecoastal regions of large water basins with a greatamount of clastic material during late glaciations. Theclastic material was so abundant that it could not bereworked by the wave activity. The interruption ofplacer formation during the screening of source rockswith moraine is obvious. Arguments in support of otherstatements are concisely discussed below.

According to Momdzhi (Momdzhi, Blinov, 1970),the mineral content in placer C

p

= aK

l

C

r

, where a is thecoefficient of concentration, K

l

is the coefficient oflocalization and C

r

is the average mineral content in thesource rock. The coefficient of concentration dependson the mineral density. The concrete character of thisdependence is determined by the sedimentation regime,which is independent of glacier. The coefficient oflocalization for alluvial placers is the ratio of nativesource area to the whole placer source area. Within thesheet glacier area, the alien material is supplied to theplacer not only from provenance rocks but also fromalmost ubiquitous glacial deposits, which often can bethe sole supplier of this material into the placer. There-

fore, the K

l

value on the Baltic Shield is an index of theportion of alien fragments, derived primarily from gla-cial deposits, in the placer. The value will be equal tounity, if the alien material is absent, and less than unity,if the admixture content increases.

Regularities of the composition of moraine depositsin the glacial sheet were studied by H. Hirvas in theFinnish Lapland and V.Ya. Evzerov on the Kola Penin-sula. It was revealed that the content of clastic materialfrom underlying rocks in the moraine is directly corre-lated with distance from the contact of two heteroge-neous strata of crystalline rocks in the ice movementdirection. This content is a few percents near the con-tact and amounts to 70–80% at a distance of 5 km from.In other words, K

l

approaches unity, if the cross-sectionof the sourch rock mass exceeds 5 km in the ice move-ment direction (Evzerov, 1998).

We assume that small coastal-marine placers can beformed during ice-free periods, just as it takes place inthe Holocene (Evzerov, 1980). However, subsequentlydeveloped glaciers are likely to assimilate the newlyformed placers and significantly destroy them in theprocess of displacement. That is the reason why theplacers of ice-free periods, older than the Holocene,have not been yet revealed, despite a comprehensivestudy of interglacial sediments in boreholes and out-crops. It should be noted that the glacial exaration wasonly insignificant in inner continental regions, namely,near icesheds and in areas located within the shades oflarge positive morphologies.

The territory released from ice was intensivelyuplifted during late glaciations. At the same time, sig-nificant masses of the pebble–sand and silt–clay debriswere delivered by glacier meltwater flows and turbidityflows to water basins. Naturally, such paleogeographi-cal and tectonic conditions were unfavorable for theseparation of heavy minerals. The heavy minerals wereconcentrated during the deceleration of uplifting andthe reduction of clastic material supply, i.e., duringinterglacial and postglacial (Holocene) stages of theBaltic Shield evolution. Therefore, at present, onlyHolocene placers are known in the region.

PLACER DEPOSITS ON THE BALTIC SHIELD

In the light of above-described features of placerformation, it will be interesting to examine availablematerials related to specific placers on the BalticShield. In the eastern half of the shield, only the Lovoz-ero loparite placer can be confidently assigned to thecategory of deposits. This area also incorporates a com-plex coastal-marine placer, whose practical signifi-cance remains to be proved. Perspectives of the rest ofthe Baltic Shield region are also uncertain. For exam-ple, only insignificant gold placers developed by pros-pectors are known in Finland (Isokangas, 1982;Saarnisto, Tamminen, 1987).

112

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EVZEROV

333

336

340

3.34 3.

62 4.39 4.84

7.3

7.3

ββ

7.3

β

β

β

4.87

4.39

3.61

3.35

3.24

2.29

2.44

2.39

2.17

4.41

4.18

3.63

3.25

3.00

2.56

2.38

343

344

346

4.84

ββ

ββ

β

β

β

7.3(

001)

Mh

4.84

Hg

4.42

4.16

G7.

61(0

02)M

h

2.45

8(00

3)M

h2.

381

Hg

10.1

4.86

4.41

4.193.

583.

35

2.54

72.

424

7.14

(001

)K10

.0(0

01)I

β

4.86

4.424.

16

3.58

(002

)ä

3.35

(003

)I

2.38

1(00

3)ä

2.00

7(00

5)I

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

2m 4 6 8 10 12BÄ

C

arb. units

333

336

340343344

3461 2 3

5 6 74

9 108

b

a

3

Fig. 1.

(A) Borehole section of the Lovozero loparite placer near the Lovozero massif contact, (B) loparite content, and (C) diffrac-tograms of clay minerals. (1) Boulders; (2) pebble; (3) gravel; (4) sand; (5) sandy loam; (6) clay of predominantly (

a

) metahalloysiteor (

b

) kaolinite compositions; (7) clay fraction sampling sites and their numbers; (8) moraine of the Valdai sheet glaciations; (9) mo-raine of mountain glaciation;(10) productive unit. (I) Illite, (K) kaolinite, (Mh) metahalloysite, (Hg) hydrargillite, (G) goethite.

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PLACER DEPOSITS AS UNIQUE FORMATIONS 113

Proximal placers.

The loparite placer in Lovozeroand gold placers in Finland are typical proximal plac-ers. Materials on their structure and composition aregiven in (Evzerov, 1980; Likhachev, 1987; Isokangas,1982; Saarnisto, Tamminen, 1987; Gavrilenko

et al

.,1996). Therefore, we shall discuss here only some prin-cipal aspects of the problem considered in this work,such as the high average loparite content in nephelinesyenites outcropped on northern slopes of the Lovozeromassif, the presence of outliers of the Neogene weath-ering crust, and the existence of favorable geomorpho-logical and paleoglaciological conditions in northernfoothills for the formation of placer and its preservationfrom glacial exaration.

The native source of loparite placer is representedby the large Lovozero massif of nepheline syenitesabout 650 km

2

in area. This massif forms a low-reliefisland massif (Lovozero Tundra) with plateau-shapedsurface and has an elevation of 500–950 m. On itsslopes, rocks of the differentiated (or loparite-bearing)and eudialyte lujavrite complexes with a relatively highaverage loparite content are exposed practically alongthe whole perimeter. However, the placer loparitedeposit is only found near northern foothills of the mas-sif, because remnants of the Miocene–Pliocene weath-ering crust of loparite-rich nepheline syenites wereretained on the northern, most rugged slopes. The sig-nificant extension (about 10 km) and ruggedness of thenorthern slopes of the massif provided a great quantityof clastic material, including that from the Neogene

weathering crust, for the Lovozero placer. Continentalplacers, which were formed only at the expense ofproducts the mechanic disintegration of the aforemen-tioned rock complexes, were established in intramon-tane depression near the eastern and southern slopes ofthe Lovozero Tundra. They are of no practical impor-tance, because the average loparite content is at leasttwo times less than the content at the Lovozero placerdeposit.

Products of the Neogene weathering crust are com-posed of Neogene deluvial–proluvial sediments thatare rare at the base of the placer section. They are dis-tinguished by a very high loparite content, which is 6–8 times more than the average content in the whole pro-ductive unit mostly represented by Quaternary sedi-ments. In addition, the observed loparite contentdecrease upward the placer section is explained by thesimultaneous decrease in Neogene weathering productsof nepheline syenites. Figure 1 presents the representa-tive section of loose sediments in a borehole locatednear the contact between Lovozero nepheline syenitesand host gneisses and granite gneisses. Here, the pro-ductive unit, which is overlain by moraines of sheet andmountain glaciations, is composed of Neogene andQuaternary deluvial–proluvial sediments. Their sandyand clayey fractions include metahalloysite, hydrargil-lite, and goethite from the Neogene weathering crust ofnepheline syenites. The coefficient of correlationbetween the loparite content, which increases towardthe placer bed, and the content of neogenic minerals

100 m

1 2 3 4 5 6 7 8 9 10 11

290

270

250

230

Em masl

W

Fig. 2.

Fragment of the cross-section across the central part of the Lovozero placer. (1) Boulders; (2) pebble; (3) rock debris;(4) gravel; (5) sand; (6) silty sand; (7) clayey sand; (8) gneisses and granite gneisses hosting the Lovozero massif; (9) core drillingholes; (10) moraine of the Valdai sheet glaciation; (11) deluvium consisting of gneiss and granite gneiss weathering products. Theproductive unit of the Lovozero placer is located between the moraine cover and deluvium.

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EVZEROV

(mostly metahalloysite), which was identified in theproductive unit of the section, is equal to +0.85 at aprobability of 99.9% (Evzerov, 1980).

The productive unit was formed in two stages. Dur-ing the Neogene stage, products of the nepheline syen-ite weathering were redeposited by sheetwash andephemeral streams. During the second stage, includingthe late glaciation of the Moscow Glaciation, Mikulin-ian Interglaciation, and probably an episode of the Val-dai Glaciation, water flows, mountain glaciers, andtheir meltwaters, coupled with the aforementionedagents, participated in the redeposition of weatheringproducts. The polygenous productive unit lies under theValdai Glaciation sequence, which is usually 1 to 5 mthick (rarely 10–12 m). The sequence is characterizedby an alternation of horizons of slightly sorted, more orless washed boulder–pebble sands, boulder–pebbleclayey sands, and less common sandy loam. Thin inter-layers of well-sorted sands are rare. On the whole, theproductive unit is slightly differentiated by size (Fig. 2),which is not characteristic for placers of low-densityminerals that are usually well sorted.

As a result of the polygenous and long-term forma-tion and continuing rise of the Lovozero massif (rela-tive to the surrounding plain), the Lovozero placer issimilar to placers of tectonic scarp zones, which formthe most important type of large and unique deposits ofminerals with high and medium density (gold and cas-siterite). Evidently, the formation of loparite (low-den-sity mineral) placer in conditions favorable for the for-mation of high- and medium-density minerals can onlybe explained by the abundance of native source. Theloparite placer preservation from exaration is providedby its location in the iceshed area, where the impact ofsheet glaciers upon bedrocks is minimal.

The gold placer in Finnish Lapland, like the Lovoz-ero placer, was formed during the redeposition of prod-ucts of the preglacial (most likely Neogene) weatheringof veins with a poor gold mineralization. The gold-bearing area is located near the iceshed. Therefore, theglacial exaration was insignificant, and the placer waspreserved (Isokangas, 1982).

Coastal-marine placer.

The coastal-marine com-plex (zircon–ilmenite–magnetite–garnet) placer, whichwas found on the southern coast of the Kola Peninsulaby the prospecting team supervised by V.Ya. Prozorov(Northwestern Geological Administration, Ministry ofGeology, USSR), is only regarded as a potentialdeposit. Like the Lovozero placer, this placer is some-what unique, since it is not related to products of theintense chemical weathering of rocks and infrequentconditions of relative equilibrium between the amountof clastic material delivered to the coastal zone and thepossibility of its reworking by waves (Evzerov, 1980).At the same time, the small size of the coastal-marineplacer may be regarded as an evidence of unfavorableconditions for the formation of similar placers on theBaltic Shield.

CONCLUSIONS

The rare occurrence of placers on the Baltic Shieldcan be explained by the comprehensive analysis ofmain (placer-forming) mineragenic, geodynamic andpaleogeographic factors. The single placer deposit inthe study region was formed as a result of the uniquecombination of favorable conditions. This statement isalso valid for other placers on the Baltic Shield, which,however, are not assigned to the category of deposits.

Perspectives for the discovery of new placers on theBaltic Shield are low because of several reasons:

(1) Mineragenic feature of the region. Primarydeposits of high- and medium-density minerals whichcan produce placers within a wide range of paleogeo-graphic conditions, are practically absent.

(2) The sedimentary cover, which usually gives riseto placers, e.g., for example, on the Ukrainian Shield, issignificantly reduced.

(3) The paleogeographic environment wasextremely unfavorable for the formation and preserva-tion of placers from glacial exaration during almost thewhole Quaternary.

This conclusion is also valid for diamond placers,whose primary deposits have not been found in theregion. However, small eluvial placers can be assignedto the largest primary deposits, if they will be discov-ered at all, like on the Siberian Platform (Patyk-Kara

et al.

, 1997) where the cryogenic-type lithogenesis(similar to the glacial-type lithogenesis on the BalticShield) was set in the second half of the Cenozoic.Therefore, it is worthwhile to concentrate forces in theprospecting for, first of all, native diamonds depositswithin the Baltic Shield.

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Fanerozoiskie kory vyvetrivaniya Balti-iskogo shchita i svyazannye s nimi poleznye iskopaemye

(Phanerozoic Weathering Crusts on the Baltic Shield), Len-ingrad: Nauka, 1977.Armand, A.D., Grave, M.K., and Evzerov, V.Ya., SpecificFeatures of Placer Formation on the Central Kola Peninsula,

Geologiya rossypei

(Geology of Placers), Moscow: Nauka,1965, pp. 250–255.Arzamastsev, A.A., Arzamastseva, L.V., Glaznev, V.N., andRaevskii, A.B., Structure of Paleozoic Alkaline–UltramaficIntrusions on the Kola Peninsula Based on Gravimetric Data,

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Protsessy kontinental’nogo litogeneza

(Processes ofthe Continental Lithogenesis), Moscow: Nauka, 1980,pp. 28–59.Dubrovskii, M.I., Petrographical and Mineralogical–Geochemical Features of the Yuovv-Oaiv Granitoid Com-

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Materialy po mineralogii Kol’skogo poluostrova

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Mineral Deposits of Europe,

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et al.

, Eds., London: Inst. Mining Metallurgy, 1978. Trans-lated under the title

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, Moscow: Mir, 1982, vol. 1,pp. 156–329.Isokangas, P., Finland, in

Mineral Deposits of Europe

,Bowie, S.

et al.

, Eds., London: Inst. Mining Metallurgy,1978. Translated under the title

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