Economic Geology Vol. 90, 1995, pp. 676-689

The Tomtor Alkaline Ultrabasic Massif and Related REE-Nb Deposits, Northern Siberia

S. M. KRAVCHENKO

Institute of Ore Deposit Geology, Petrography, Mineralogy and Geochemistry, Russian Academy of Science, Staromonetny per. 35, 109017 Moscow, Russia

AND t3.(;. POKROVSKY

Geological Institute of the Russian Academy of Sciences, Pyzhevski 7, 109017 Moscow, Russia

Abstract

Nb-REE ore deposits related to weathered carbonatite within the Tomtor massif, northern Siberia, contain the world's largest carbonatite-related concentrations of Nb and REE. Despite its remote location, economic exploitation of the deposit was made possible by the discovery of rich Sc-REE-Y-Nb ore.

The 300-km 2 Tomtor massif is a ring complex in which the Nb-REE deposits are situated within and above a central carbonatite stock. The deposits include: (1) pyrochlore-bearing carbonatite, (2) a lower ore horizon, and (3) an upper ore horizon. The average concentration of niobium increases from a few tenths of a percent in the carbonatite, to a few percent in the lower ore horizon, and up to 12 percent in the upper ore horizon. REE contents range from 0.5 to 0.7 percent in carbonatite, 4 to 6 percent in the lower ore horizon, and 11 to 30 percent in the upper ore horizon.

In a plot of 6•sO vs. 63C, the Tomtor carbonates plot along a trend from the Oka box for magmatic carbonates to the field of sedimentary carbonates. The Tomtor carbonatites have initial S7Sr/S6Sr ratios ranging from 0.70346 to 0.70374, a range consistent with mantle derivation for the magmas.

The lower ore horizon, which is up to 300 m thick, consists of niobium (Nb205 content 1-2%) and phosphorus ores that occur in a variety of original rock types (carbonatites, volcanics, picritic dikes, and country rocks) which have undergone several types of alteration including weathering, hydrothermal alteration, and low-temperature sideritization. S7Sr/86Sr ratios of the lower ore horizon range from 0.70421 to 0.70374 and suggest a dominantly mantle source for Sr with a very minor addition of crustal Sr.

The upper ore horizon is a placer deposit 3 to 25 m thick which has been partly affected by hydrothermal alteration. Rich ores of the upper ore horizon are composed mainly of pyrochlore, phosphates and alumophos- phates of REE, Sr and Ba (florencite, monazite, gorceixite, goyazite), rutfie, and pyrite.

Nb in the Tomtor deposits occurs principally in pyrochlore, columbite, rutfie, and ihnenorutile. REE and Y occur in fiorencite, monazite, rhabdophane, xenotime, and bastnasite, and Sc occurs in monazite, rhabdo- phane (0.55-0.95%), minerals of the crandallite group (0.017-0.030%), rutfie, and as a thin fihn of Sc(OH)3 on phosphates.

Introduction

THE Tomtor deposit, together with deposits such as Araxa (Brazil) and Mount Weld (Western Australia), contains one of the largest concentrations of Nb and REE in weathered carbonatite. The deposit occurs in the Tomtor massif, which has many similarities with the Khibina (Russia) and Lovozero (Russia) massifs. This family of giant massifs shows a regular combination of favorable features which led to the develop- ment of large deposits of phosphorus and rare elements dur- ing different stages of their evolution. These features include a kimberlitelike composition of mantle-derived parent melts (Kravchenko and Rass, 1985), an ability to extract phosphorus and rare elements from the mantle, very large heat capacities, and large magma chambers which allow extended crystalliza- tion-differentiation. The massifs have also been affected by intense postmagmatic alteration processes (Kravchenko et al., 1992b).

The large alkaline-ultramafic complexes are composed mainly of the end members of comagmatic igneous series such as syenites, and in the case of Tomtor, of carbonatites. A favorable factor in the formation of Tomtor ores was also

the large size of the carbonatite stock •vhich led to the devel-

opment of pyrochlore placers in local karst depressions (Ko- noplev et al., 1992).

Since the Tomtor carbonatite is remote from modern trans-

port routes and is almost completely covered by sedimentary rocks, it had been poorly investigated until recently. However, detailed studies of drill core have now shown that, as pre- viously suggested (Kravchenko, 1982), Tomtor is one of the world's largest Nb (more than Araxa) and REE (more REE than Mount Weld) deposits. The most important result of the recent work has probably been the discovery of a rich Sc-REE-Y-Nb ore (Kravchenko et al., 1990b) which made economic exploitation of the deposit possible in spite of its remote location.

Analytical Techniques

REE, Sc, Sr, Ba, and Th concentrations were obtained by instrumental neutron activation. Standard deviations for La, Sin, Eu, and Sc equal 5 to 8 percent, for Sr, Ba, Ce, Yb, and Lu, 10 to 15 percent, and for Th, 20 percent. Y, Zr, Nb, La, and Ce concentrations were determined by XRF. Standard deviations are 10 to 15 percent.

Sr isotope analyses were obtained on a MAT-260 mass

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TOMTOR REE-Nb DEPOSITS, N SIBERIA 677

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Fro. 1. A. Simplifed geologic scheme of the Tomtor massif. 1 = Permian coal-bearing deposits: conglomerate, sandstone, and argillite; 2 = maginatie and inetasomatie rocks of the central earbonatite stock; 3 = nepheline and nepheline-bearing syenites; 4 = altered jaeupirangite-urtite; 5 = mainly carbonate deposits and sandstone-siltstone of the Rhiphean Ulaehan- Kurung and Unguoehtaeh suites and the Vendian Tomtor suite. Inset from map (Orlova and Krasnoff, 1978): 6 = folded complex; 7 = orogenie-stage complex intra- and marginal depressions; 8 = crystalline rocks of the Anabar shield. B. Cross sections modified after Lapin and Tolstov (1993). 1 = Mesozoic-Cenozoic deposits; 2 = Permian coal-bearing deposits; 3 = upper ore horizon (after Lapin and Tolstov, 1993; "epigenetie altered products of wethering"); 4 = kaolinite-hydromiea weathering crust on silicate rocks; 5 = lower ore horizon: a = siderite rocks, b = limonitie weathering crust, e = limonite- francolite rocks; 6 = ijolite-melteigite rocks; 7 = metasomatie rocks: a = apatite-potassium feldspar-ehlorite rocks, b = calcite-potassium feldspar-phlogopite rocks; 8 = ore-bearing calcite-dolomite earbonatite; 9a = ankerite-ehamozite rocks; 9b = ore-bearing m•kerite earbonatite; 10 = early, unmineralized calcite-dolomite earbonatite.

spectrometer. Accuracy of the 87Sr/86Sr determination is bet- ter than 0.001. The isotopic compositions of O and C xvere measured on an MU-1201B mass spectrometer. The •SlsO results are given in per mil relative to SMOW and those for

•93C are in per mil relative the PDB standard. The results for •534S are in per rail relative to the Sikhote-Alin standard. Accuracy of the determinations is 0.2 per mil for •93C and •9s0 and 0.3 per mil for •5•4S.

678 KRAVCHENKO AND POKROVSKY

Geologic Features of the Tomtor Massif and Associated Ore Deposits

The Tomtor massif is located in northern Siberia between

the Anabar Shield to the west and the Olenyoc uplift to the east (Fig. 1). It occurs on the meridional Udgy palcorift (Shpunt et al., 1991) which is the boundary between base- ment crystalline rocks of different ages (Egoroy et al., 1985). The palcorift can be traced for 400 km using geophysical and morphostructural data. The depth of the Moho is 30 to 35 km in the palcorift zone. The Tomtor massif is located on the west shoulder of the rift. Riphean sedimentary rocks crop out in a meridional zone within the dominant Cambrian de-

posits. A present-day dome morphostructure 20 km in diame- ter corresponds to the Tomtor massif.

The Tomtor massif intrudes Riphean dolomite, siltstone, and sandstone of the Ulachan-Kurung suite, shale, siltstone, dolomite, and tuffaceous deposits of the Unguochtach suite, and Vendian siltstone and sandstone of the Tomtor suite. The

massif is overlain by a layer of Permian conglomerate and sandstone which contains interbedded coal in the upper part. The youngest units include Lower Jurassic argillo-arenaceous marine sediments and Neogene-Quaternary alluvial sands.

The Tomtor massif is a typical ring complex (more 300 km 2) and has a relatively simple structure. Predominant biotite-, biotite-pyroxenemepheline-, and nepheline-bearing alkaline syenites (more than 60% of the area of the massif) compose a peripheral zone 2 to 6 km wide. The central stock crops out over a total area of 40 km 2 of which 12 km • is composed of carbonatite.

Much of the carbonatite stock is covered by rocks interpre- ted to be volcanics including melanephelenite, phonolite, tra- chyte, and the effusive equivalent of phoscorite (Entin et al., 1990). However, recognition of these rocks as volcanics is based mainly on structural features and the mineralogical features of intensely altered rocks and remains speculative (Entin et al., 1990).

The stock and the peripheral syenite ring zone are sepa- rated by a jacupirangite-urtite ring dike i to 3 km wide. The carbonatite stock is intruded by a dike complex of picrite, alnoite, nepheline syenite, melilitite, tinguaite, alkali trachyte, and dolerite. The jaeupirangite-urtite ring dike is intruded by ring dikes of magnetitite, apatitite, and other phoseorites.

The earbonatite stock has a rhomblike shape and includes intrusive rocks, possible metasomatie earbonatite, and in- tensely earbonatized ijolite. The oldest earbonatites are dolo- mite-calcite and calcite rocks which form meridionally elon- gated bodies. According to Lapin and Tolstov (1993) there are early, unmineralized calcite-dolomite earbonatites and ore- bearing calcite-dolomite and ankeritie earbonatites (Fig. lB). Dolomite-calcite and calcite earbonatites are medium to fine

grained, massive, and sporadically banded rocks with hypidio- morphie textures. Interstitial pyroehlore contains significant uranium and zireonium, a feature not found in other pyro- ehlore generations. The accessory minerals include apatite, phlogopite, magnetite, and pyrite.

Younger ankerite earbonatites are localized in the central part of the stock; they have granoblastie textures and contain accessory minerals including pyroehlore, apatite, magnetite, and richteritc. Carbonatization of ijolite has produced dolo-

TABLE 1. Ages in Magmatic and Metasomatic Rocks from Tomtor (Entin et al., 1990)

Ages Magmatie rocks Metasomatie rocks and ores (Ma)

Metasomatic K feldspar in 240? nepheline pegmatoid syenite

Lower ore horizon calcite-

chlorite-serpentine metasomatites

Upper ore horizon REE- <400 phosphate metasomatites

340-320

Nepheline syenite dike 410 Pierifle dikes and diatremes 430-470

Lower ore horizon K feldspar, 450-440 calcite, and chloritic metasomatites

Subalkaline gabbro 592-528 Nepheline syenite dike 614 Central stock carbonatite 660•510

Central stock phoscorite 660-650 and carbonatite

Nepheline syenite dike 675 Ring zone nepheline and 700-800

nepheline-bearing syenite Jacupirangite-urtite 700-800

mite-calcite-phlogopite rocks which are locally transitional into micaceous rocks. Apatite-feldspar-chlorite and calcite- francolite-chlorite carbonatites were also formed by carbona- tization.

The ore zones consist of: (1) pyrochlore-bearing carbona- tire, (2) a lower ore horizon up to 300 m thick containing ore lodes rich in niobium and phosphorus, and (3) a 3- to 25-m- thick upper ore horizon which is a buried placer deposit containing rich fiorencite-monazite-pyrochlore ore (Krav- chenko et al., 1990; Konoplev et al., 1992). The average con- centration of economic components increases from a few tenths of a percent in the carbonatite, to a few percent in the lower ore horizon, to a few tens of percent REE and to more than 12 percent Nb in the upper ore horizon.

The K/Ar and Rb/Sr ages of Erlich and Zagruzina (1981) and Entin et al. (1991) indicate two major events at about 700 Ma and 400 Ma, but Entin et al. (1991) recognize a number of stages (Table 1). The K/Ar age of potash feldspar may have been affected by argon loss. Lower ore horizon

Rocks and ores of the lower ore horizon are composed mainly of carbonates (calcite, dolomite, ankerite, siderite, rhodochrosite, magnesite, manganosiderite, etc.), phosphates (predominantly francolite, apatite, strontium apatite, mona- zite, florencite, crandallite, gorceixite, goyazite, etc.), oxides and hydroxides (goethite, hydrogoethite, magnetite, groutitc, manganite, pyrochlore, columbite, rutfie, anatase, etc.), and silicates (chlorite, hydromica, kaolinitc, montmorillonite, feldspar, garnet, harmotome, natrolite, etc.) (Fig. 2).

High-grade phosphate ores of the lower ore horizon (avg 12.5% P•O.•) occur essentially as lenses or beds up to 50 m thick. The lower ore horizon is composed of various original rock types including carbonatite, volcanics, picrite, and other dike rocks and possibly some country rocks as inferred from

TOMTOR REE-Nb DEPOSITS, N SIBERIA 679

Pc P Ca Fe Nb Ce

Fro. 2. Cross section of the lower ore horizon (drill hole 104) in the northern part of the central carbonatite stock. 1 = eruptive breeeia of pierite and other rocks, 2 = rocks with poq)hyritie textures (dikes, probably partly effusive), 3 = mainly geothitie, sideritie, limonitie, and other high iron rocks, 4 = mainly francolite rocks, 5 = metasomatie earbonatite, 6 = Permian conglomerate, 7 = quartz veins, 8 = rocks altered by sideritization, 9 = rocks altered by kaolinization. Pc = porosity.

their very light carbon isotope composition (Pokrovsky et al., 1990). These rocks have undergone several stages of alter- ation including weathering, hydrothermal alteration, and low- temperature sideritization. Some of the alteration has been dated, including calcite and chlorite alteration (450-440 Ma), serpentinization (340-320 Ma), and K feldspathization (240 ma) (Table 1).

Pyrochlore and pyrochlore-columbite Nb ores contain a variety of assemblages, including francolite-siderite, goethite- hydrogoethite-francolite-siderite, and chlorite-goethite-hy- drogoethite. The altered carbonatites are crosscut by numer- ous veins containing rhodochrosite, chicrite, quartz, albite, K feldspar, goethite, and magnetite.

The weathering crust of the lower ore horizon occurs as a blanket up to 10 m thick, and large bodies of weathered material, up to 300 m thick, are localized in circular structures up to 3.5 km in diameter in the southern part of the carbona- tire stock. These may represent cauldron subsidence struc- tures based on their rounded shape, large size, and the pres- ence of rocks interpreted as volcanics overlying the carbona- tire stock (Erlich, 1964; Entin et al., 1990). However, these circular features could also be formed by solution-collapse processes.

Some investigators (e.g., Lapin and Tolstov, 1991) have suggested that the lower ore horizon exhibits a regular se- quence of weathering zones above the carbonatite. This in- cludes francolite, goethite (rich in iron and manganese), sid- erite, and hydromica-kaolinite zones. The first two zones are thought to have been created by oxidation, with goethite representing a zone of leaching and francolite a zone of ce- menration. The formation of more highly dispersed zones of sideritization and hydromica-kaolinite may be related to a reduction stage. However, this sequence is seen only in the central part of the carbonatite stock. Elsewhere, some zones may be lacking, or the sequence may be inverted or repeated.

Upper ore horizon The upper ore horizon, which includes the higher grade

ore, lies on top of the altered carbonatites and beneath the mainly sandy, coal-bearing Permian strata. In plan the distri- bution of the upper ore horizon coincides with that of the Permian deposits (around 8 km 2, Fig. 1). The high-grade Sc- REE-Y-Nb ores are recognized as a buried placer deposit which formed in a lacustrine environment (Kravchenko et al., 1990). Interbeds of silt and fine-grained sand have also been identified within the upper ore horizon. The lacustrine-

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FIG. 3. Scanning electron micrographs of the surface of an REE phos- phate from the upper ore horizon. A and B have different magnifications.

A further ore type in the upper ore horizon contains fluida] and breccia textures, in which fragments of pyrochlore crys- tals are cemented by florencite. The pyrochlore is paler than that in bedded ores, whereas fiorencite appears isotropic in thin section.

Important Mineralogical Features of the Tomtor Complex

The formation of the weathered zone occurred predomi- nantly before the Carboniferous (more than 345 Ma ago) but also included a weathering period during the Jurassic. Several alteration events have also been dated (Table 1): francolitiza- tion and formation of potash feldspar (450-440 Ma), carbona- tization (439-332 Ma), metasomatic alteration of the upper ore horizon (less than 400 Ma), formation of low-temperature calcite-chlorite-serpentine metasomatic rocks (340-320 Ma), and formation of potash feldspar in a dyke of nepheline sy- enite (240 Ma). This complicated history of formation is re- flected in the features and relations of mineral and element

ratios from different parts of the complex. In this paper we discuss only the composition of pyrochlore and the mineral associations of the lower and the upper ore horizons.

Pyrochlore Pyrochlore occurring in carbonatite, the lower ore horizon,

and sometimes in the upper ore horizon can be divided into five main groups (Table 2; Belyakov et al., 1989, 1990): (1) transparent, black, brown, red, orange, and yellow octahedral and rarely cubic-octahedral, stoichiometric; (2) nontranspar- ent, pale yellow in the lower ore horizon and yellow-gray in carbonatite, Sr- and Ba-rich with some cation deficiency in the A site; (3) transparent gray in carbonatite, the highest Ba content, with a larger cation deficiency in the A site; (4) nontransparent, octahedral in the lower ore horizon and car- bonatite with the highest Sr content and largest cation deft- ciency in the A site; and (5) white and beige, rounded, shelly, strongly altered in the lower and upper ore horizon.

The first four groups of pyrochlore were formed as a result

type ore is clearly stratified, with gray, light gray, and yellow, fine-grained layers 0.5 to 1.5 mm thick. In the richest sector, leucocratic and melanocratic bands are seen to alternate. The

leucocratic bands consist of finely dispersed, earthy monazite (rhabdophanite) with florencite, siderite, dolomite, quartz, and feldspar. The melanocratic bands consist of pyrite, ana- tase, and oxides and hydroxides of iron (Figs. 3 and 4). Within the individual leucocratic bands, there are euhedral and octa- hedral crystals of pyrochlore which sometimes occur in cavi- ties. The layers often form small folds, which are apparently of secondary origin.

In addition, talus ores (2-15 m thick) and brow ores (2- 15 m thick) have been distinguished by Konoplev et al. (1992). These are coarser grained than the lacustrine-type ore and are composed predominantly of earthy phosphates. They generally contain pyrochlore only in discrete horizons. The formation of REE phosphate in the upper ore horizon has been ascribed to the infiltration of ground water by Ko- noplev et al. (1992).

FIG. 4. Photomicrograph of high-grade Sc-REE-Y-Nb ore. Transmitted light. P = pyrochlore crystal.

TOMTOR REE-Nb DEPOSITS, N SIBERIA 681

TABLE 2. Composition of Pyrochlore from the Tomtor Phosphorus-Rare Element Deposits

Wt % Nb2Os TiO2 Ta2s Fe2Oa CaO Na20 K2 BaO SrO Ce2Oa

Carbonatite Black 67.68 1.88 1.15 0.10 15.81 5.77 0.02 0.10 0.45 0.41 Brown 66.92 4.15 2.64 0.72 14.63 4.43 0.03 0.49 1.86 1.47 Red 65.67 2.15 1.42 0.91 15.44 5.04 0.04 0.11 1.24 1.62

Orange 63.65 3.33 0.02 0.40 16.00 5.3 0.03 0.02 0.00 1.70 Yellow 68.64 3.76 0.20 0.04 14.77 6.10 0.11 0.75 3.20 0.78

Yellow-gray 68.87 3.76 2.15 1.55 9.35 2.28 0.01 7.43 2.23 0.72 White 67.41 3.47 0.20 1.35 1.79 1.33 0.02 9.38 10.59 1.27

Lower ore horizon Black 63.50 1.26 0.13 1.01 16.35 6.06 0.16 0.06 0.89 2.01 Brown 65.81 1.39 0.56 1.13 17.7 16.79 0.03 0.00 0.57 0.90 Red 63.38 1.39 0.14 0.99 15.77 5.87 0.18 0.17 0.57 3.18

Orange 64.33 2.82 0.34 0.70 14.90 7.93 0.17 0.00 1.74 0.59 Yellow 64.33 2.82 0.34 0.70 14.90 7.93 0.17 0.00 1.74 0.59

Pale-yellow 57.18 3.06 0.20 0.59 8.00 2.10 0.02 8.86 3.60 3.90 Beige 61.20 2.70 0.07 0.70 4.00 3.60 0.03 5.30 14.50 0.75 White 57.03 3.29 trace 0.82 0.99 1.71 0.01 5.52 15.2 3.93

Microprobe analyses by O.M. Georgievskaia.

of hydrothermal-metasomatic processes and as a product of eation exchange during low-temperature processes. The rounded, shelly variety was probably formed during later ear- bonatization, evidence of which may be seen in thin sections. The activities of BEE, Ba, Sr, and Pb substituting for Ca and Na in group A pyroehlore, increased progressively during low- and middle-temperature processes. PbO contents in Tomtor pyroehlore range up to 36 percent (Entin et al., 1991). The diminishing color intensity of pyroehlore corresponds to a diminishing content of iron. Almost all varieties of pyroehlore are present in all samples but in different ratios. The rounded, shelly pyroehlore is the predominant type in the upper ore horizon. The presence at Tomtor of Ce-Ba-Sr pyroehlore and Pb pyroehlore appears to result from the spatial superposition of different processes that led to the formation of the five varieties of pyroehlore. The ratio of •sO ealeite-pyroehlore in the lower ore horizon probably reflects hydrothermal tem- peratures (Pokrovsky et al., 1990).

Mineral composition of the lower ore horizon The mineral composition of the lower ore horizon has been

investigated in the northern part of the carbonatite stock (Fig. 2; Table 3). The main minerals are francolite (19.45-64.56%) and siderite (10.86-60.38%) to a depth of 200 m, where car- bonarite is encountered. These two minerals were formed at different times as a result of different P-T-Eh conditions.

The formation of siderite is interpreted to have occurred under reducing conditions (partly by percolation of rain water that had been reduced by interaction with coal), whereas the formation of francolite is interpreted to have taken place under oxidizing conditions (Lapin and Tolstov, 1991) and at least partly under hydrothermal conditions (Aplonov and Erlich, 1980). The mineral composition of the lower ore hori- zon in this part of the complex was created by the spatial superposition of at least three processes mentioned above: weathering, hydrothermal metasomatic processes as estab- lished by isotope geothermometry (Pokrovsky et al.,1990), and low-temperature sideritization.

Lower ore horizon rocks which contain pyrochlore and

pyrochlore-columbite in thin veins are strongly phosphatic (Table 3B), and sometimes reach ore grade. The Fe203/FeO ratio (6.2-0.4) correlates with the ratio of siderite to Fe oxides and hydroxides.

The Tomtor complex is rich in manganese like the Oka massif in Canada (MeMahon and Haggarry, 1979). The con- tent of manganese oxides in the lower ore horizon is up to 30 percent. We have investigated a zone rich in Mn in the northern part of the earbonatite stock. A new variety of ser- pentinelike mineral named A1 eariopelite was found at a depth of 100 to 150 m in an assemblage with brownire, hausmannite, Mn goethite, monazite, rhodoehrosite, BEE svanbergite, bar- ite, pyrolusite, and BEE goyazite. The composition of one structural hexagon is (Mn46 Mg2)4s (8i25.5 Ai14.5)(OH48,O8)56 (Organova et al., 1993).

Mineral composition of upper ore horizon

The upper ore horizon is essentially a buried fioreneite- monazite-pyroehlore placer which was formed by redeposi- tion of weathered earbonatite (Fig. 4). It is considered by Konoplev et al. (1992) that BEE, Ba, and Sr phosphates belonging to the erandallite group formed after placer forma- tion. The main minerals, besides pyroehlore (up to 20%) include fioreneite, rhabdophanite, goreeixite, and goyazite (Figs. 3 and 4; Table 4A and B). All phosphates and alumo- phosphates are Se and Y bearing and very fine grained.

Three types of pyroehlore are distinguished in the upper ore horizon (Table 4): predominantly yellow, transparent; beige, rounded, and shelly, and intensely altered by earbonati- zation; and eolloform, observed as thin veins in carbonate.

Geochemical Features of the Carbonatite

and Nb-REE Deposits

Isotopic geochemistry

The Sr isotope compositions of carbonate, pyrochlore, and apatite (Table 5) are useful genetic indicators because of the minerals' very low Rb/Sr ratios (less 0.01). Initial S7Sr/S6Sr ratios for these minerals in Tomtor carbonatites range from

682 KRAVCHENKO AND POKROVSKY

TABLE 3. A. Mineralogical Composition (mass %) of Rocks and Ores from the Lower Ore Horizon (drill hole 104)

Interval (m) 92-113 118-128 177-197 199-206 237-249

Wt percent of 0.05 mm fraction 66 83 76 69

F apatite 3.80 10.72 tr 0.48 Francolite 64.56 37.94 19.45 36.19 27.07

Svanbergite tr Monazite 0.06 tr tr

Pyrochlore 0.76 0.08 2.55 0.08 0.10 Columbite 0.44 0.87 tr 1.04 tr Ilmenorutile 0.06 0.04 0.23 Rutile 0.04 0.01 Brucite 0.23

Anatase ,0.47 0.01 Priderite 0.08 Fe oxides and

hydroxides 0.91 tr 28.83 Hematite, martRe 0.82 4.58 0.52 0.05 Pyrite 0.12 0.47 6.00 6.24 0.10 Sphalerite 0.01 tr tr 0.01 Galena 0.03 0.01 0.01 0.01 tr Mica tr 1.31

Clay aggregate with limonite 19.66

Calcite 0.36 0.04 0.02 0.08 0.03

Siderite, rhodochrosite 12.67 44.45 60.38 35.91 10.86 Quartz + feldspar 14.88 5.37 4.70 18.38 12.86

Tr = trace minerals including sphene, zircon, garnet, barite, thaumasite, fluorite, moissonite, cinnabar, ilmenite, chalcopyrite, pyrrhotite, hydromica, chlorite, halloysite, Sr apatite, braunate, hausmannite, Mn goethite, pyrolu- site, and kaolinitc

0.70346 to 0.70374. This interval corresponds to the modal mean of a large selection (n = 118) of carbonatites compiled by Samoilov (1984) and is consistent with a mantle origin for the magmas. However, the range is higher than that for carbonatites from the Guli massif located west of the Anabar

Shield. The observed range is larger than the analytical uncer- tainty.

Carbonates from the lower ore horizon (calcite + dolomite) are noticeably richer in SVSr and have a wider range of S7Sr/ S6Sr initial r•tios (0.70374-0.70421) than is observed in the carbonatites. However, pyroehlore from the lower ore hori- zon has an Sr isotope composition similar to that of pyroehlore from the the carbonatites.

One sample from the outer contact of Ripbean marble (dolomitic limestone) has an S7Srff6Sr ratio of 0.70667. As a whole the Anabar Ripbean sedimentary deposits are charac- terized by relatively low S7Sr/S6Sr ratios (0.7048-0.7064, Po- krovsky and Vinogradov, 1991).

The Sr isotope inhomogeneity in the Tomfor rocks and ores suggests that at least two Sr reservoirs were involved in their formation. One reservoir was depleted mantle repre- sented by the carbonatites. The correlation of S7Sr/86Sr' and 8•sO shows that sedimentary country rocks were probably the second source of Sr. However, Sr concentrations are very high in the carbonatites and the weathered rocks, whereas those in the sedimentary limestone are an order of magnitude lower. Mixing of mantle and crustal Sr was probably carried out by high Sr brines which are a common feature of sedimen- tary basins.

The 8•sO values in carbonates from the carbonatites range from 8.9 to 14.0 per mil and the 8•aC values range from -4.6 to -3.0 per mil (Table 6). In 8•sO-8•aC coordinates the

TABLE 3. B. Chemical Composition (mass %) of Rocks and Ores from the Lower Ore Horizon (drill hole 104), Together with Average Data from Lapin and Tolstov (1991)

Interval (m) Averages

92-113 117-128 177-197 199-206 237-249 i 2 3 4 5

SiO2 12.06 5.67 5.76 13.59 14.43 5.04 2.17 5.90 7.84 0.98 TiO2 1.64 1.28 1.96 0.74 0.48 0.14 0.70 0.25 8.86 1.18 A1203 1.96 1.00 0.92 2.96 0.60 0.10 0.35 0.10 11.79 3.58 Fe203 10.40 4.33 22.38 7.03 33.03 4.72 59.02 19.08 6.15 5.96 FeO 3.59 12.93 17.96 16.8 5.32 2.10 2.37 1.15 1.62 35.78 MnO 1.56 3.53 3.92 4.39 0.74 0.90 6.00 3.92 0.08 8.00 CaO 30.70 26.45 14.87 16.56 17.48 45.50 3.74 34.75 2.79 3.43

MgO 0.50 0.20 0.50 1.70 0.35 1.06 0.40 0.26 0.09 0.68 NaaO 0.30 0.20 0.20 0.10 0.15 0.12 0.12 0.13 0.22 0.07 K20 0.70 0.15 0.30 2.30 0.10 0.11 0.04 0.05 0.10 0.07 P•O5 20.40 19.44 12.01 10.67 11.42 3.16 4.26 25.02 16.73 3.62 COa 3.52 10.12 11.66 13.86 3.74 33.59 4.74 4.00 0.31 25.62 H•O + 0.38 0.24 0.16 0.26 0.54 HaO- 2.24 1.30 1.38 1.54 1.45 F 2.10 2.00 1.58 1.54 1.45 S 0.17 0.41 2.31 2.08 0.21 0.27 0.25 0.25 0.32 1.62

O•F2 0.88 0.84 0.67 0.65 0.61

TotM 91.32 88.41 97.20 94.8 94.83

Analyst K. Poljacova REE, Sr, and Ba not determined. Samples: 1 = calcitic carbonatite (n = 2), 2 = goethitic ocher (n = 2), 3 = goethite-francolite rocks (n = 2), 4 = kaolinite-crandallite rocks (n = 3), 5

= sideritic rocks (n = 2).

TOMTOR REE-Nb DEPOSITS, N SIBERIA 683

TABLE 4. A. Mineralogical Composition (mass %) of the Upper Ore Horizon (sample 5649-83, 0.05-ram fraction)

TABLE 5. Strontium Isotope Composition of Tomtor Carbonate and Pyrochlore

F apatite 0.20 Fe oxides and hydroxides 0.24 Francolite 0.32

Pyrite 19.60 Monazite, fiorencite, gorceixite, goyzite 27.83 Galena 0.03

Svanbergite 1.27 Calcite 0.28

Pyrochlore 7.26 Siderite, rhodochrosite, rutfie 16.13 Dolomite 10.08 Anatase 6.05

Quartz and feldspar 9.27

Tomtor carbonatites define a trend linking normal carbona- tites such as Oka (Convey and Taylor, 1969) or Guli (Pokrov- sky and Vinogradov, 1987) and sedimentary carbonate rocks (Fig. 5). This trend can be attributed to fraetionation between CO2-rieh fluid and earbonatite melt as well as to mixing of mantle and crustal material. The correlation between •9sO

and 87Sr/86Sr supports mixing rather than differentiation, but we cannot exclude a combination of crystallization differentia- tion of the earbonatite melt and interaction with crustal mate-

rial during formation of the metasomatie earbonatites. Carbonates of the lower ore horizon have •51sO values rang-

ing from 11.0 to 23.1 per rail and •93C values ranging from

TABLE 4. B. Chemical Composition (mass %) of Minerals from the Upper Ore Horizon (sample 5649-83)

1 2 3 4 5 6

(7) (3) (4)

SiO2 0.05 0.57 0.08 0.5 1.2 0.3 TiO2 4.21 2.97 ZrO2 0.21 NbzO5 63.55 54.31 LazO3 13.18 0.32 0.9 0.8 3.8 Ce•Oa 33.79 0.68 1.76 2.2 1.3 5.65 Pr2Oa 0.67 0.3 0.4 Nd2Oa 11.59 0.04 0.18 0.6 0.4 1.45 Sm•Oa 1.12 Gd203 0.43 Sc•O,• 0.65 UOz 0.07 ThO,2 0.00 Fe2Oa 0.00 0.18 0.90 FeO 0.8 2.6 0.8 AlcOa 0.00 32.0 28.3 26.4 SrO 2.46 15.92 10.6 0.0 4.8 BaO 5.59 6.5 11.0 4.6 CaO 1.18 15.10 0.50 2.7 3.1 0.9 Na•O 7.42 1.58 K20 0.07 P205 32.46 26.1 26.4 28.3 F 0.86 4.86 1.08 Sum 99.68 92.74 O•---F 2.04 0.45 Total 99.05 97.64 92.29

(1) monazite, (2) yellow pyrochlore, (3) beige pyroehlore, (4) goyazite, (5) gorceixite, (6) florencite; 1-3, microprobe analyses (analyst O.M. Georgiev- skaia); 4-6, averages from Konoplev et al. (1992): number of samples in parentheses

Sample no. Mineral, rock SVSr/S•*Sr Sr (ppm)

Carbonatite

103/80 Calcite + dolomite 0.70362 3,400 103/113 Calcite + dolomite 0.70347 4,700 103/202 Calcite + dolomite 0.70346

103/213 Calcite + dolomite 0.70355 2,900 103/185 Pyrochlore 0.70374 3,500

Lower ore horizon

107/172 Calcite + dolomite 0.70374

107/202 Calcite + dolomite 0.70374 4,700 107/228 Calcite + dolomite 0.70421 4,300 107/292 Calcite + dolomite 0.70395 2,100 111/58 Pyrochlore 0.70371 7,000

Riphean sediments

F-2A/86 Marble 0.70667 170

-3.4 to -15.8 per mil (-59.9%0 in one specimen). Carbon- ates from the upper ore horizon have •9So (10.5-21.6%o) and •93C (-3.2 to -19.9) values similar to those from the lower ore horizon. On the whole, carbonates from the upper and lower ore horizons are enriched in •so and depleted in •3C compared with the carbonatites. The low •93C values indicate that the rocks were formed not only from mantle CO2 and sedimentary carbon but also with an important contribution of CO., derived from the oxidation of organic matter and methane. Both carbon and oxygen isotope fractionation val- ues between dolomite and calcite vary widely (Fig. 5). In the majority of specimens these minerals are not in isotopic equilibrium.

The range in the oxygen isotope composition of quartz from carbonatites (•9so = 11.3-17.2%o) and the lower ore horizon (•51so = 10.2 -16.3%o) is similar. These values are unusually high for plutonic rocks and suggest formation under hydrothermal conditions. Magnetite and pyrochlore from car- bonatite and the lower ore horizon have very low •51SO values, from -9.4 to 0.0 per mil, also indicating a hydrothermal origin.

If it is assumed that calcite and magnetite are in isotopic equilibrium, the oxygen isotope fraetionation between these minerals can be used to determine the temperatures of ore formation. The temperatures calculated for the lower ore horizon using the equation of Convey and Taylor (1969) range from 131 ø to 267øC (Fig. 6).

The •534S values in sulfide minerals (Table 7, Fig. 7) show a very wide range from -33.9 to 28.8 per mil. The sulfides in the lower ore horizon are therefore not derived from mantle

magmatie sulfur. It is also very unlikely that sulfur in pyrite ((53zfS = -5.4 to +2.8%0) from the carbonatites is of magmatic origin. A mantle source for sulfur in galena and pyrite from the carbonatites can also be ruled out and they are very much out of equilibrium with the pyrite. Sulfate reduction, particularly at low temperatures, is accompanied by a very large fractionation of sulfur isotopes. Nevertheless, even ex- treme evolution of a single solution cannot explain the heavi-

684 KRAVCHENKO AND POKROVSKY

TABLE 6. Oxygen and Carbon Isotope Composition of Tomtor Calcite and Dolomite

Dolomite

Calcite (_+MnCO3 + FeCO3)

Sample 6'3C 6180 613C no. (%o) (%o) (%o)

Upper ore horizon

4825/77 - 14.0 21.6 - 16.0 4075/45 -5.6

4068/128 - 10.6 13.4 -9.6

4825/153 - 17.5 15.4 - 19.9 4829/135 -3.2 4071/149 - 12.8 16.2 - 18.4 110/47 -4.5 5650/95 - 11.3 16.6 - 12.9 5648/69 - 12.9 15.2 - 12.9

4009/49 -9.8

5650/81 -9.9 5651/64 -7.7 18.4 -7.8 4825/83 - 13.8 18.1 - 15.3

Lower ore horizon

104/118 -7.5 15.0 107/118 -4.8 16.5

107/172a -6.4 17.5 107/172b -5.2 12.9 -4.5 106/202 -4.5 14.8 107/228 -5.8 15.9 107/247 -4.3 17.3 107/292 -4.0 12.0 -3.4 111/159 -5.0 19.7 111/215 -3.3 22.4 -3.3

113/145 -15.8 19.4 113/185 -59.9

Carbonatite

104/118 -4.2 10.5 103/116 -4.3 10.0

103/133 -4.6

103/172 -4.3 10.2 103/185 -3.0 14.0 103/202 -3.8 11.6

103/213 -3.8 13.8

ilSø ß m ßm ß <%o) -

-- ß ß 19.9 • ß 10.5 -•0 Lb, --

13.1 15.1 14.5 21.5

12.5 16.9

10.9

12.6

12.8 20.8 • ,

12.4

-3.9

FIc. 5. 6•3C-6•sO relationships of Tomtor carbonates. 1 = carbonatite, 2 = lower ore horizon, 3 = upper ore horizon, 4 = Guly carbonatite (Pokrov- sky and Vinogradov, 1987), 5 = sedimentary rocks. Open symbols = dolo- mite, filled symbols = calcite.

11.9 On the whole, sulfur and oxygen isotope temperatures are in good agreement. They show that isotopic systems of the

23.1 lower ore horizon were formed partly under hydrothermal 11.0

9.2

8.9

Riphean marble F-2A/86 -3.4 20.8 -2.1 22.4 F-2B/86 -2.2 23.3 -2.0 23.0

est and lightest sulfide values for the Tomtor rocks. At least three sources of sulfur may be suggested for Tomtor sulfides: heavy seawater sulfate, 6•4S values more than 20 per mil; light diagenetic H2S, 6•4S values less than -20 per mil; and magmatic S, 634S near 0.

The 634S fractionation between sphalerite and galena in the lower ore horizon ranges from 3.0 to 4.0 per mil (Fig. 7) and corresponds to temperatures of 135 ø to 245øC using the equations collected by Friedman and O'Neil (1977). Two pyrite-galena pairs give higher temperatures (272ø-313øC), whereas two other pairs appear not to be in equilibrium. One sphalerite-galena pair from carbonatite gives an improbably low temperature (50ø-60øC), suggesting a lack of equilibrium between the two minerals.

•,mO, % Oka

20

16

12

[ 0-1 $-2

Tomtor

FIc. 6. 6180 fractionation between equilibrium minerals. 1 = calcite, 2 = magnetite, 3 = pyrochlore. Temperatures calculated after the equation of Pokrovsky et al. (1991).

TOMTOR REE-Nb DEPOSITS, N SIBERIA 685

TABLE 7. Isotopic Composition of Sulfur in Tomtor Sulfides

Sample no.

6a4S (%)

Pyrite Galena Sphalerite

Lower ore horizon

107/172 19.4 -0.9 3.0 107/202 -7.7 - 11.4 107/228 28.8 - 17.3 - 13.25 107/277 - 7.3 - 10.5 - 7.5

103/277 0.3

103/80 0.0 103/101 2.8 103/213 -0.8 103/133 -5.4 103/172 -3.8

Carbonafire

-33.9 -26.5

conditions at mean temperatures of 200 ø to 250øC. A single measurement of the hydrogen isotope composition of chlorite from the lower ore horizon (tD = -135%o) suggests surface water as the main source of the solutions.

Geochemistry of major and trace elements The composition of the carbonatite varies from dolomite-

calcite and calcitic to ankeritic. As can be seen in Figure 8, the composition of the ankerite carbonatite is very similar to that of ankerite carbonatite from Gudini (Africa). The compo- sitional trend for carbonatites in the CaO-MgO-FeO diagram shows an intermediate position between the calcite-dolomite carbonatites of the Chilwa massif (Africa) and the calcite-

•s carbonatites L OH

o

o • •

o

I o

i I o 50 Mg 0

CaO

Cl

FIG. 8. Geochemistry of carbonatites in the CaO-MgO-FeO diagram (mass percent). The lines approximate chemical trends for Tomtor and other carbonatites. C = Chilva, F = Fen, K = Kangankunde, T = Tundulu; circles = single carbonatite compositions from Nelson et al. (1988). i = Lokupoi, calcite; 2 = Suculu, dolomite; 3 = Nachendaz, calcite; 4 = Goudini, ankerite- dolomite; 5 = Goudini, ankerite-dolomite; 6 = Wallouway, calcite; 7 = Mudtank, dolomite; 8 = Mount Weld, calcite; 9 = Mount Weld, calcite; 10 = Jacupiranga, calcite; 11 = Jacupiranga, calcite; 12 = Magnet Cove, calcite; 13 = Kaiserstuhl, calcite.

siderite carbonatites of the Tundulu massif (Africa). There is a trend toward increasing Mg in the middle stage, and in- creasing Fe in the late evolution of the carbonatite melt. The same Mg trend is a feature of the carbonatite series of Kangancunde (Africa), Fen (Norway), and Tundulu (Africa).

The REE patterns for carbonatites are similar to those of other Tomtor rocks and ores (Fig. 9) and show an enrichment in light lanthanoids and an absence of Eu anomalies. The relationships between Nb and Ce in carbonatites containing pyrochlore and phosphates have been investigated in Tomtor samples (n = 88). Nb and Ce have a high correlation coeffi- cient of 0.56 (Table 8), but Sr and Ba, which are dispersed in carbonatites, are poorly correlated with Nb, Ce, and Y.

The Tomtor ores typically have a high effective porosity, up to 47 5 percent in high- rade sectors Their s ecific ravi ß 3 g ' P g ty is 2.24 g/cm compared with 4.18 g/cm 3 for the same material without pores. The porosity of the lower ore horizon has been

I0 s

I0 a

.-3 -_

I • I i l I I I

La Ce Nd SmEu Tb Yb tu

FIG. 7. Sulfur isotope compositions of sulfides from carbonatite and the lower ore horizon. Minerals in equilibrium are connected with solid lines, nonequilibrrum minerals with dotted lines. i = pyrite, 2 = sphalerite, 3 = galena.

FIG. 9. Chondrite-normalized pattern of lanthanides (Evensen et al., 1978). i = carbonatite, 2 = rocks and ores of the lower ore horizon, 3 = ores of the upper ore horizon, 4 = Middle Permian sedimentary rocks above the upper ore horizon.

686 KRAVCHENKO AND POKROVSK•

TABLE 8. Correlation Coefficients (X100) of Concentrations of Rare Elements in Carbonatite (n = 88)

Nb Ce

Y 57 65 Nb 56

examined in detail in the north-central part of the massif (drill hole 104, Table 9A). The effective porosity correlates positively only with the Ba content and was probably formed after generation of pyroehlore and francolite, because of the negative correlation between porosity and Nb and P (Table 9A). As can be seen from the type of correlations between effective porosity and the contents of minor and trace ele- ments, they are different from typical weathering crusts in which, as a rule, an increase in porosity increases the contents of the inert components. The correlation of elements in sam- ples collected from different parts of the lower ore horizon (Table 9B) is similar to that for samples from the one vertical section. The good correlation between Nb and Ce(Y) proba- bly means that in rocks and ores containing low REE and P, the REE are contained mainly in pyroehlore. The REE pat- tern for the lower ore horizon is similar to that of other rocks

of the massif and shows an enrichment in light lanthanides and lack of an Eu anomaly.

Differentiation of lanthanides during formation of the Tomtor massif (Fig. 9) was not as intense as during the forma- tion of weathered earbonatites at Mount Weld in Australia, where ehurehite has been observed (Lottermoser, 1990). The ratio of REEce/REE¾ in weathered material at Mount Weld ranges up to 40, whereas that at Tomtor ranges up to 20. REE contents in Se-REE-Y-Nb ores at Tomtor (Table 10) are very similar to those in Lovozero loparite in which the contents of Nb and REE range from 4.24 to 8.25 and 25.58 to 30.81 percent, respectively. The Tomtor ores are a natural concentrate of pyroehlore and REE (Sr, Ba) phosphates and are three times richer in pyroehlore than ores from Araxa in Brazil, and an order of magnitude richer in rare-earth phosphates (Table 4A and B). The REE pattern is similar to that in Khibina apatite, but the REEcdREEy ratio is twice as high at Tomtor and the REE content an order of magni- tude higher (Table 11). Khibina apatite also has a small nega- tive Eu anomaly.

REE concentrations at Tomtor are highest in the upper ore horizon and the REE pattern is very similar to that in earbonatite and ijolite. The correlations between elements in the upper ore horizon are different from those in the lower ore horizon (Tables 9 and 12). This results from the high content of REE-(Sr-Ba)-Se-Y-bearing phosphates: Ba in rich ores correlates with Y, Se, and Sr. The Nb/Ce ratio was nearly constant (0.6-0.7) during formation of the Tomtor massif (Fig. 10, Table 13). Only the Permian sedimentary rocks have higher Nb/Ce ratios and these probably result from redeposi- tion of ore minerals from the upper ore horizon since frag- ments of crystals of pyroehlore have been observed in thin sections of the Permian rocks. Sr/Ba ratios decrease from

0.072 in ijolite to 0.013 and 0.010 in lower and upper ore horizons, respectively. The maximum Y/Ce values are found in earbonatite and ijolite (0.12), with lower values in the lower

and upper ore horizons (0.06-0.07). The nearly constant Ce/ Nb ratio in different Tomtor rocks and ores is probably a consequence of buffering by the mineral association.

Mineralogieal data show that Nb is contained in pyroehlore, eolumbite, rutfie, and ilmenorutile, and REE and Y in fioren- cite, monazite, rhabdophane, bastn•isite, xenotime, etc., Se in monazite, rhabdophane (0.55-0.95%), minerals of the eran- dallite group (0.017-0.030%), ruffle, and films of Se(OH)a on phosphates (discovered by Y. P. Diekov using the X-ray photo-electron method).

Discussion and Conclusions

Tomtor represents a voleano-plutonie complex with an ex- tensive weathering cap developed on the earbonatites and is similar to deposits such as Araxa and Seis Lagos in Brazil and Mount Weld in Australia, which are important deposits of Nb and REE (including Se and Y). In the Tomtor ore depos- its, several different processes were superimposed in space during their geologic history. This led to the development of many generations of mineral development, particularly for pyroehlore (Belyaeov et al., 1989, 1990). Several types of ores were developed including Se-REE-Y-Nb and Nb (Krav- ehenko et al., 1990, 1992b; Konoplev et al., 1992), P and Fe (Egorov et al., 1985), and manganese (Entin et al., 1990). These deposits often have very high concentrations of eco- nomic components.

The Tomtor earbonatites contain magmatie calcite and do- lomite, as well as metasomatie carbonates, whose range of Fe content is very wide. The initial 87Sr/S6Sr provides evidence of a mantle source for the magmas, with a minor addition of radiogenic Sr from the host sedimentary rocks. However, assimilation of the host sedimentary rocks appears unlikely because of their very low Sr content. The addition of Sr is probably due to the effect of brines with a high Sr content.

The lower ore horizon developed on earbonatite, rocks interpreted as voleanies, and probably also in sedimentary

TABLE 9. A. Correlation Coefficients (X100) of Porosity and Rare Elements in a Section of the Lower Ore Horizon (drill hole 104, n = 12)

PsOs Nb Sr La Ce Ba Y

Porosity PsOs Nb Sr

La

Ce

Ba

-35 -35 -31 -28 -25 54 63 44 57 59

34 95 96

98

56

62

71 80

37

TABLE 9. B. Correlation Coefficients (X100) of Rare Element Concentrations in the Lower Ore Horizon (n = 27)

Sr Y Nb Ba Ce Th

Sc

Sr

Y

Nb Ba

Ce

24 47 52 52 73 25 52 45

83 73 76 43 40

66

TOMTOR REE-Nb DEPOSITS, N SIBERIA 687

TABLE 10. Concentrations (mass %) of Economic and Other Co•nponents in Ores of the Upper Ore Horizon

Sc Y RE E Nb V U Th P Reference

Mean concentration

0.039 0.62 11.00 4.20 0.039 0.79 13.82 5.40 0.038 0.30 8.34 3.01

Sample 5649/83

0.085 0.70 31.49 8.72

0.61 0.01 0.11 4.97 (Konoplev et al., 1992) 0.68 (Lapin and Tolstoy, 1993)

0.12 This paper

6.98 (Kravchenko et al., 1990)

rocks due to a combination of hydrothermal alteration, low- temperature sideritization, and weathering.

There is agreement that the formation of the lower and upper ore horizons was a complex process. Lapin and Tolstoy (1991, 1993) have described two processes: upper ore horizon enrichment due to removal of mobile components, and to accumulation of inert constituents during oxidation and re- duction. Entin et al. (1991) considers only the addition of material during development of the upper ore horizon by metasomatism of rocks interpreted to have been phosphatic lavas. Kravchenko et al. (1990) and Konoplev et al. (1992) have described the redeposition of weathered material and enrichment in heavy minerals (pyrochlore, monazite, etc.) during erosion of the ore-bearing sequence and deposition

TABLE 11. Concentrations of Major Elements and REE (mass %) and REE in Apatite froIn the Khibina Massif

Upper ore horizon Khibina massif apatite (Sample 5649/83) (n = 5)

SiO2 3.11 TiO2 5.54 Nb205 12.47 Fe203 7.40 A1203 1.14 PeO 1.61 MnO 0.02

MgO 1.5 CaO 2.63

Na,20 0.15 K•O 0.1 P205 18.24 CO• 0.88 F 0.62

S 1.0

H,20 1.12 La20• 9.75 Ce2Oa 17.81 Pr•O3 1.64 Nd203 5.69 Sm203 0.61 Eu20• 0.14 Gd,20• 0.40 Tb203 0.07 Dy203 0.21 Ho•O3 0.04 ErgO3 0.09 Tm203 0.03 Yb203 0.007 Lu.203 0.002 Y•O3 1.36 Sc•O3 0.13

0.28

0.43 0.04 0.14

0.02 0.005 0.0015

0.002 0.083 0.0015

0.0034 0.0009

0.0016

0.0005

of heavy minerals in an aqueous environment. In addition Konoplev et al. (1992) attribute the development of REE phosphate minerals to ground-water infiltration.

In the upper ore horizon, several periods of weathering and hydrothermal alteration (including sideritization) proba- bly occurred following deposition of early, weathered material in an aqueous environment. The possible role of bacterial growth and decay in changing water pH and promoting the precipitation of phosphates in the upper ore horizon has been discussed by Zhmur et al. (1994).

The Tomtor massif is composed mainly of late differenti- ates including nepheline and nepheline-bearing syenites and earbonatites. This possibly indicates a substantial role for crys- tallization differentiation as a precursor to mineralization. A correlation between Nb resources and the area of syenite and earbonatite in alkaline-ultrabasie massifs has been noted by Kravehenko (1982).

Francolite zones are usual in weathering crusts formed on earbonatites. Homogenization temperatures of vapor-liquid inclusions (Aplonov and Erlieh, 1980) suggest that at least some of the francolite may be hydrothermal in origin. Ac- cording to deerepitation temperatures, Aplonov and Erlieh (1980) propose a stage of hydrothermal alteration at 500 ø to 300øC based on quartz, hematite, and francolite data. Most francolite, which forms a net of veins with associated pyrite and hematite, may have formed in the temperature interval between 310 ø and 350øC (main deerepitation peak).

Zones of sideritization are usually developed via low-tem- perature fluid interaction in a reducing environment. The H isotope composition in sample 111/125 from the lower ore horizon (tSD = -135%o; Pokrovsky et al., 1990) may give an indication of the palcoclimate. Fractionation in the system chlorite-water at 200 ø to 250øC is 40 per mil and the palco- water in equilibrium with chlorite had tSD = -90 per mil, similar to that of modern waters in temperate climatic regions (Maruto et al., 1980).

TABLE 12. Correlation Coefficients (X100) of Upper Ore Horizon Rare Element Concentrations (n = 90)

Sr Y Nb Ba Ce Th

Sc 24 71 49 33 49 74 Sr 16 62 42 22 28 Y 33 18 68 53

Nb 24 34 Ba 16 Ce 52

688 KRAVCHENKO AND POKROVSKY

TABLE 13. Range (in parentheses) and Average Concentration (ppm) of Rare Elements in Rocks and Ores from the Tomtor Massif

Ijolite Carbonatite Lower ore horizon Upper ore horizon Permian sediments n 11 86 26 92 27

La 536 (115-3,140) 1,396 (20-12,610) 4,043 (358-15,928) 21,851 (874-126,744) 5,264 (323-40,130) Ce 1,027 (254-5,295) 2,606 (260-12,610) 7,561 (280-38,158) 41,150 (2,123-210,584) 10,997 (182-72,531) Sm 60 (6-313) 377 (21-1,582) 2,304 (40-10,690) 664 (6-7,571) Eu 23 (3-127) 156 (19-797) 702 (33-7,319) 257 (5-3,252) Yb 75 (6-497) 283 (15-1,876) Lu 9 (1-80) 35 (0.5.260) 9 (1.24) Se 74 (13-332) 89 (16-358) 390 (39-2,214) 204 (4-1,918) Y 75 (2-556) 285 (20-2,300) 528 (2-3,680) 2,969 (58-25,288) 1,520 (2-25,288) Sr 4,786 (1,273-12,372) 4,018 (1,260-28,000) 6,367 (610-20,007) 19,521 (907-62,748) 10,948 (1,009-51,117) Ba 3,003 (508-16,695) 3,429 (150-17,000) 7,542 (1,066-20,604) 20,119 (518-141,291) 10,739 (1,176-30,295) Zr 670 (2-3,393) 41 (1-600) 338 (2-2,330) 121 (2-3,213) 769 (2-2,186) Nb 743 (65-5,199) 1,500 (20-8,540) 4,435 (60-40,749) 30,102 (1,190-144,578) 12,573 (2-91,470) Th 90 (6-664) 248 (14-1,495) 1,155 (14-9,209) 571 (6-3,467)

Average concentrations are lower than ore grades because they include low-grade rocks; analysts: V.I. Kaluzny, J.V. Terechov, and B.A. Ziskin

The (•180 values in Tomtor carbonatites are very high and suggest the existence of at least two sources of oxygen (Fig. 5). In rocks and ores of the upper and lower ore horizons, •980 values are higher still and overlap the range for sedimen- tary carbonates. The •5•3C values in carbonatites are high and are very low in rocks and ores of the lower ore horizon, and especially the upper ore horizon. The latter possibly reflects the influence of organic matter during the Permian.

A good correlation between Nb and REE in carbonatites and the lower ore horizon may indicate that these elements are contained mainly in pyrochlore. Correlation of lower ore horizon porosity only with Ba is not characteristic of typical weathering crusts. As noted above, a small change in the Nb/ Ce ratio in different rocks and ores during superposition in space of several processes (Fig. 9) may be due to buffering by the mineral association. These process include develop-

-- ' I I I I I I III • --

- LI

I t• :0,47

Fro. 10. Average concentration of Nb vs. Ce. 1, Tomfor massif.' J = ijolite-urtite, K = carbonatite, LOH = lower ore horizon, NS = nepheline syenite, P = Permian rocks, UOH = upper ore horizon; 2, Kh = Khibiny, L1-3 = first, second, and third phases of Lovozero; 3, K = kimberlites, N = nephelenites, after Wedepohl and Muramatsu (1979).

ment of the large carbonatite stock as a result of crystallization differentiation and metasomatism, weathering and hydrother- mal alteration of the lower ore horizon, erosion and redeposi- tion of pyrochlore and monazite. The development of REE phosphates in the upper ore horizon may be a consequence of percolating ground water and at last partly due to hydro- thermal processes and low-temperature, possibly repeated, sideritization.

The development at Tomtor of large P, REE, and Nb ore deposits was not accidental but, as with other giant massifs (including Khibina and Lovozero), was the result of a regular combination of features favorable for development of the largest ore deposits of P, Nb(-Ta), REE, Sc, and Y during their evolution (Kravchenko et al., 1992b).

Acknowledgments We thank Economic Geology reviewers for help with the

manuscript, I.N. Kigai for help in translation, and A.V. Zhari- kov for help in transferring the manuscript to disk. We also thank the manager of geology of the Tomtor deposit, A.I. Kubishev, geologist A.V. Tolstov, and the manager of geology of the Chernishevsky Expedition of the Yacutian Geological Survey, A.T. Vasilyev, for help during field investigations.

REFERENCES

Aplonov, V.S., and Erlich, E.N., 1980, The temperatures of the Tomtor miner'ds, rocks and ores generation, in Alkaline magmatism and apatite- bearing of northern Siberia: Leningrad, Arcticgeology Institute, p. 112- 123 (in Russian).

Belyakov, A.Y., Georgievskaj, O.M., and Grjaznova, J.A., 1989, Chemistry of Tomfor massif pyrochlore: VUZ'ov Izvestia Seriia Geologicheskia Razvedca Rudnich Mestorogdenii, no. 8, p. 33-39 (in Russian).

Convey, C.M., and Taylor, H.P., 1969, •SO/•60 and •3C/•2C ratio of coexisting minerals in the Oka and Magnet Cove carbonafire bodies: Journal of Geology, v. 77, p. 618-626.

Egorov, L.S., Surina, L.S., and Porshnev, G.J., 1985, Ore magmatic ultrabasic alkaline rocks and carbonatites complex of Udgy, in Ore-magmatic com- plexes in north-west Siberia platform and Taymyr: Leningrad, Nedra, p. 138-154 (in Russian).

Entin, A.R., Zaitzev, A.J., and Nenashev, N.J., 1990, About the range of the geological events related to the ultramafic alkaline rocks intrusion of the Tomtor massif (NM Yakutija): Geology and Geophysics, v. 12, p. 42.51 (in Russian).

Entin, A.R., Eremenko, G.K., and Tjan, O.A., 1991, About alteration ranges of the parent pyrochlores: Akademiia Nauk SSSR Doklady, v. 319, p. 1218-1221 (in Russian).

TOMTOR REE-Nb DEPOSITS, N SIBERIA 689

Erlich, E.N., 1964, The new province of alkali rocks on the north of Siberian platform and its geological aspects: Zapisky Vsesoiuznogo Mineralo- gicheskogo Obshestva, v. 93, p. 6824393 (in Russian).

Erlich, E.N., and Zagrusina, J.A., 1981, Geochronology of the north part of the Siberia platform and its geological aspects: Akademiia Nauk SSSR Izvestiya, Seriia Geologicheskia, no. 9, p. 3-13 (in Russian).

Evensen, N.M., Hamilton, P.J., and O'Nions, R.K., 1978, Rare-earth abun- dances in chondritic meteorites: Geochimica et Cosmochimica Acta, v. 42, p. 1199-1212.

Friedman, J., and O'Neil, J.R., 1977, Compilation of stable isotope fraction- ation factors of geochemical interest, in Fleischer, M., ed., Data of geo- chemistry, 6th ed.: Washington, D.C., U.S. Printing Office, 116 p.

Konoplev, A.D., Kuzmin, V.I., and Epshtein, E.M., 1992, Geological and geochemical peculiarities talus-lake placer above the weathering crust of the rare-elements-bearing carbonatites, in Mineralogy and geochemistry of placers: Moscow, Nauka, p. 111-123 (in Russian).

Kravchenko, S.M., 1982, Correlation of the Nb resources of the carbonatitic complexes with their geological peculiarities: Akademiia Nauk SSSR Doklady, v. 263, p. 168-71 (in Russian).

Kravchenko, S.M., and Rass, I.T., 1985, The alkaline-ultramafic association-- the paragenesis of two comagmatic series: Akademiia Nauk SSSR Doklady, v. 283, p. 973-978 (in Russian).

Kravchenko, S.M., Belyakov, A.Y., Tolstov, A.V., and Kubishev, A.I., 1990, Sc-REE-Y-Nb ores a new type of rare element economic resource: Inter- national Geology Reviews, v. 33, p. 280-284.

Kravchenko, S.M., Belyakov, A.Y., Babushkin, P.N., et al., 1992a, Sodium nepheline syenites of Khibina massif are the possible derivates from high calcium alkaline-ultrabasic magma: Geochemistry, no. 5, p. 6854396 (in Russian).

Kravchenko, S.M., Belyakov, A.Y., and Pokrovsky, B.G., 1992b Geochemistry and genesis of Tomtor massif (north of Siberian platform): Geochemistry, no. 8, p: 1094-1110 (in Russian).

Lapin, A.V., and Tolstov, A.V., 1991, Oxidizing and reduclfon stages forma- tion of zone carbonatites epygenesis and their ore-content: Geology of Ore Deposits, v. 33, no. 4, p. 81-92 (in Russian).

-- 1993, A new unique rare metals ore deposit in the carbonatitic crust weathering: Razvedca i Ochrana Nedr, no. 3, p. 7-11 (in Russian).

Lottermoser, B.G., 1990, Rare-earth element mineralization within the Mr. Weld carbonatite laterite, Western Australia: Lithos, v. 24, p. 151-167.

Maruto, K., Nagasawa, K., and Kuroda, Y., 1980, Mineralogy and hydrogen isotope geochemistry of clay minerals in the Ohnuma geothermal area, northeastern Japan: Earth and Planetary Science Letters, v. 47, p. 25- 262.

McMahon, B.M., and Haggerty, S.E., 1979, The Oka carbonatate complex: Magnetite composition and related role of the titanium in pyrochlore: International Kimbedite Conference, 2nd, Proceedings, v. 1, p. 382-392.

Nelson, D.R., Chivas, A.R., Chappell, B.W., and McCulloch, M.T., 1988, Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources: Geochimica et Cosmochimica Acta, v. 52, p. 1-17.

Organova, N.I., Gorshkov, A.I., Zhuchlistov, A.P., Kuzmina, O.K., Sivzov, A.V., and Kravchenko, S.M., 1993, Al-cariopelite from the weathered car- bonatites of the Tomfor (north Siberian platform): Akademiia Nauk RAN Doklady, v. 328, p. 234-238 (in Russian).

Orlova, M.P., and Krasnov, V.I., 1978, Map of alkaline magmatic association and their ore specialization's of USSR: Leningrad, VSEGEI, scale 1:10,000,000 (in Russian).

Plusnin, G.S., Sa•noilov, V.S., and Glishev, s.J., 1980, Isotopic pairs (5•C- (SXSO method and temperatures carbonatite facies: Akademiia Nauk SSSR Doklady, v. 254, p. 1241-1245 (in Russian).

Pokrovsky, B.G., and Vinogradov, V.I., 1987, Some elements isotope compo- sitions of ultrabasic-alkaline rocks, Maimecha-Kotuy province: Sovetskaja geologija, no. 5, p. 81-90 (in Russian).

-- 1991, Strontium, oxygen and carbon isotope composition in upper Precambrian carbonates of Ahabarian rise: Akademiia Nauk SSSR

Doklady, v. 320, p. 1245-1250 (in Russian). Pokrovsky, B.G., Belyakov, A.Y., Kravchenko, S.M., and Gryaznova, Y.A.,

1990, Isotope data on the origin of carbonatites and mineraized strata in the Tomtor intrusion, NW Yakutia: Geochemistry International, v. 33, p. 93-101.

Samoilov, V.S., 1984, Geochemistry of carbonatites: Moscow, Nauka, 190 p. (in Russian).

Shpunt, B.R., Shamshina, E.A., Brachfogel, F.F., and Filipov, N.D., 1991, Composition and petrochemical peculiarities of the alkaline-utrabasic rock from Udzha uplift (north of Siberia platform): Akademiia Nauk SSSR Izvestija, Seriia Geologicheskia, no. 8, p. 68-80 (in Russian).

Taylor, H.P., Fuchen, J., and Degens, E.T., 1967, Oxygen and carbon isotope studies of carbonatites from Laacher See district, West Germany and Alno district, Sweden: Geochimica et Cosmochimica Acta, v. 31, p. 407•430.

Wedepohl, K.H., and Muramatsu, J., 1979, The chemical composition of kimberlites compared with the average composition of three basaltic magma types: International Kimberlite Conference, 2nd, Proceedings, v. 1, p. 300-312.

Zhmur, S.Y., Kravchenko, S.M., Rosanov, A.Y., and Zhegallo, E.A, 1994, On the rich ores origin from Tomtor massif (north of Siberia platform): Rossiiskia Akademiia Nauk Doklady, v. 336, p. 372-375 (in Russian).