ISSN 1075-7015, Geology of Ore Deposits, 2008, Vol. 50, No. 8, pp. 670–680. © Pleiades Publishing, Ltd., 2008. Original Russian Text © G.Yu. Ivanyuk, P.M. Goryainov, 2007, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2007, Pt CXXXVI, No. 6, pp. 1–17.
Structural and Compositional Zoning and Formation Conditions of the Greater Eastern Litsa BIF Occurrence, Kola Peninsula G. Yu. Ivanyuk and P. M. Goryainov Geological Institute, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, 184209 Russia Received September 11, 2006
Abstract—New geological and geochronological data on the Greater Eastern Litsa banded iron formation (BIF) occurrence demonstrate its similarity to the BIF of the Olenegorsk iron district in geology (lenticular orebodies with exponential distribution of their sizes), age (2.8 Ga), and typical structural and compositional zoning of orebodies. The temperature of ore formation (600–780°C) and BIF composition depend on the intensity of folding expressed in the fractal dimension D = 1.0–1.3 of a single layer. All BIF deposits of the Kola–Norwegian Megablock, including the Greater Eastern Litsa occurrence pertain to the one system. Variation m their composition is controlled by the size of orebodies (capacity of oxygen buffer) and the energy of metamorphic reactions, which strongly depend on the intensity of folding. DOI: 10.1134/S1075701508080023
INTRODUCTION In the late 1930s, in which a stratigraphic approach was applied to Precambrian geology, gneisses of the central and northwestern parts of the Kola Peninsula (Fig. 1) were called the Kola Group at the initiative by L.Ya. Kharitonov. Until now, no one has managed to find a stratotype, a definite area of occurrence, or a certain composition of this group, so that its single reproducible attribute remains banded iron formation (BIF) in the form of numerous lenticular bodies a few decimeters to a few hundred meters in thickness and a few meters to more than a kilometer in extent (Goryainov, 1990). More than 400 BIF occurrences and deposits, including the deposits of Finmarken, are known in the area occupied by Kola gneisses. This megablock is composed of oval and lenticular blocks of the so-called gray gneisses of amphibolite and granulite facies (migmatized tonalites and granodiorites) divided by a fractal network of BIF associated with mafic crystalline schists and amphibolites, biotite and muscovite–biotite (with sillimanite nodules of without them) gneisses, and leptites, magnetite–calcite–dolomite and magnetite–diopside rocks (Goryainov et al., 1997; Goryainov and Ivanyuk, 1998). The tonalitic blocks are 500– 1000 km2 in average area; their outlines vary from almost isometric to quite linear. The BIF and associated rocks are never crossed by contours of gray-gneiss blocks, which are always conformable. Two groups of BIF deposits are recognized. In the first case, they are hosted in amphibolites, and in the second case, in leucocratic gneisses, whereas amphiboCorresponding author: G.Yu. Ivanyuk. E-mail:
[email protected]
lites are driven away toward the contact with tonalitic gneisses. The BIF sections are referred to amphibolite and leptite types, respectively. All more or less large BIF deposits (Olenegorsk, Kirovogorsk, Professor Bauman, 15-letiya Oktyabrya, Komsomol’sky, Pecheguba, etc.) described in detail in the literature (Goryainov, 1976; Goryainov and Balabonin, 1988) pertain to the leptite-type BIF. The amphibolite type is exemplified by highly metamorphosed small occurrences in the northern Kola–Norwegian Megablock, including the Greater Eastern Litsa occurrence of the Uraguba group, which is considered in this paper. To explain the origin of such occurrences, Polkanov (1935) and his followers (Zhdanov and Malkova, 1974; Trusova, 1976) suggested metasomatic reworking of amphibolites with BIF formation, and some authors, e.g., Bogdanova and Dagelaisky (1965), transferred these occurrences from the Kola to the Tundra Group that supposedly overlies the rocks of the Kola Group. At the same time, no difference has been established in the geology and geochemistry of deposits belonging to the leptite and amphibolite types; as well, no indications are known in support of BIF formation by replacement of amphibolites and recognition of host “groups” differing in age. Instead of this, a single source and similar process of concentration of ore matter has been established for all—large and small—amphibolite and leptite deposits of the megablock (Goryainov and Ivanyuk, 2001). In the formation of structural and compositional zoning of orebodies, an important role is played by the mechanochemical differentiation of BIF (Bazai and Ivanyuk, 1996). The objective of this study is to furnish additional evidence for this statement.
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BA
Pe
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OG an M eg a PG
arz
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Fig. 1. Kola–Norwegian Megablock—area of BIF deposits and occurrences on the Kola Peninsula. (1) Alkaline plutons, (2) granitic plutons, (3) anorthosite and gabbroanorthosite plutons, (4) Proterozoic volcanic and sedimentary complexes, (5) Archean metamorphic complexes of the Kola Norwegian Megablock that hosts BIF deposits, (6) Archean metamorphic complexes of the Murmansk and Belomorian blocks. The asterisk indicates the Greater Eastern Litsa ore occurrence; (OG) Olenegorsk, (KG) Kirovogorsk, and (PG) Pecheguba deposits.
GEOLOGY AND PETROGRAPHY OF THE URAGUBA DEPOSIT The Uraguba deposit consists of predominant biotite and biotite–muscovite gneisses (with sillimanite nodules and without them) that host more than 20 conformable lenticular bodies of magnetite–grunerite,1 magnetite–grunerite–hornblende, and magnetite–ferroactinolite (± sulfides and almandine) BIFs closely associated with amphibolites (Fig. 2). Each of the amphibolite lenses encloses one or several orebodies. The amphibolite-hosted BIF lenses extend as a rather narrow tract, and the length and thickness of orebodies progressively decrease from north to south along with increase in their number, so that bulk percentage of BIF relative to other rocks remains unchanged. The lowangle dip of biotite gneiss (10°–30°) becomes steeper (40°–60°) in two-mica gneiss as the BIF tract is approached, and nearly vertical and even overturned within orebodies. Bands of hornblende–almandine–quartz skarnoid from a few centimeters to 1.5 m thick commonly occur at the contact of BIF and amphibolite, which contains auxiliary quartz and quartz interlayers parallel to the contact. The structural relationships of amphibolite1 All
minerals mentioned in this paper were identified with XRD and microprobe analyses and their nomenclature corresponds to IMA rules. GEOLOGY OF ORE DEPOSITS
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hosted BIF lenses with two-mica gneisses are peculiar. Their boundaries are conformable over almost the entire extent of a lens, but at the pinch-out, the banded structure of BIF and amphibolite clearly abuts on twomica gneiss, and it is as if it wrings out these rocks without necks of stretching characteristic of boudins (Fig. 3). As at other deposits in the Kola Peninsula, thin BIF lenses are characterized by rectilinear banding, whereas when the thickness of lenses exceeds ~10 m, plication appears in axial zones. The intrusive bodies of granitic microcline pegmatites are numerous and vary in thickness from 10 cm to a few tens of meters. At the same time, widespread dolerite dikes at the deposits of the Imandra district are almost completely absent at the Uraguba deposit. The large Litsa–Araguba granitic pluton that adjoins the deposit in the south (Fig. 2) is accompanied by intense migmatization and background microlinization of tonalite, amphibolite, and BIF. Biotite gneiss is a medium-grained massive or banded rock with distinct gneissic banding and granoblastic or lepidogranoblastic microstructure. The darkcolored minerals include phlogopite, magnesian hornblende, and almandine (listed in order of decreasing abundance); albite and quartz are felsic minerals. Pyrrhotite, magnetite, fluorapatite, titanite, and zircon are accessory minerals; microcline, muscovite, calcite, clinozoisite, and clinochlor are products of secondary
672
IVANYUK, GORYAINOV N 80
70
N
1 2 3
60
65 70
4
45 II
5
80 80
60
70
75 84
85
75 75
80
70 80
80 80
87 76
80 85 70
70
80 300 m
Fig. 2. Geological scheme of the Greater Eastern Litsa ore occurrence. (1) Biotite, muscovite–biotite, and nodular gneisses; (2) amphibolite; (3) banded iron formation; (4) granitic pegmatite; (5) Litsa–Araguba granitic pluton; I and II are the sites of detailed exploration.
alteration. The characteristic uniform-to-chaotic magnetite and calcite disseminations up to 2 mm in diameter appear near amphibolite-hosted BIF lenses, where their content reaches 5 vol %. The median2 chemical composition of biotite gneiss is given in Table 1. Two-mica (biotite–muscovite) and nodular gneisses are gray fine-grained phlogopite–quartz–albite rocks. Uniformly distributed muscovite flakes (up to 3 mm across), 0.3–0.8-cm quartz–muscovite–sillimanite nodules, and linearly oriented long-prismatic sillimanite crystals are incorporated into the granofelsic matrix. Sillimanite in both nodules and the matrix occurs as felted fibrous or radiate aggregates (fibrolite) consisting of acicular, often curvilinear crystals. Both muscovite flakes and sillimanite nodules may be distributed in rocks quite chaotically; however, more often they group in rather distinct, rhythmically alternating bands. Twomica gneiss frequently contains lenticular segregations, up to 3 m long, enriched in dravite (up to 7 vol %). Magnesian hornblende and almandine are auxiliary minerals; dravite, rutile, magnetite, fluorapatite, and zircon are accessory minerals. Secondary diopside and 2 The
median contents were chosen, because the oxide contents are, as a rule, characterized by lognormal or exponential distribution.
1 2 3 4 5
65
70 80
1m
Fig. 3. Relationships between (1) BIF, (2) amphibolite, and (3) muscovite–biotite gneiss in site I (Fig. 2), (4) granitic pegmatite and migmatite, (5) quartz vein.
clinochlore replace biotite, while sillimanite (fibrolite), microcline, and calcite replace muscovite. The median chemical composition of two-mica gneiss is presented in Table 1. The major minerals of amphibolite are an amphibole of the actinolite–magnesian hornblende series (Na0.08–0.24K0.08–0.09)0.17–0.32(Ca1.83–2.00Na0.00–0.17)2.00 2+
(Mg2.67–3.22 Fe 1.47–1.71 Al0.21–0.46Mn0.04Ti0.04–0.05Cr0.01–0.02 3+ Fe 0.00–0.07 )5.0[(Si7.37–7.57Al0.43–0.63)8.00O22] (OH)2 and albite (Na0.86Ca0.22K0.01)1.09[Si2.86Al1.09O8]; phlogopite and magnetite are auxiliary minerals; ilmenite, rutile, titanite, and fluorapatite are accessory minerals; and quartz, microcline, muscovite, clinozoisite, clinochlore, titanite and calcite are secondary minerals. The structure of rocks may be massive and banded; banded amphibolite is commonly slightly enriched in quartz, phlogopite, and magnetite. Some amphibolite varieties have nodular structure with an occurrence of small (up to 1 cm across) oval albite segregations (±quartz and titanite). The median chemical composition of amphibolite is shown in Table 1. The BIF is more or less contrastingly banded rock composed of rhythmically alternating bands of different grain size and quantitative proportions of minerals GEOLOGY OF ORE DEPOSITS
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and their properties. Depending on mineral composition, the color of rocks may be black and white, gray, greenish gray, pinkish gray, and approaching black. The mineral composition of the Uraguba BIF is rather primitive. The major minerals are quartz, magnetite 3+ 3+ 2+ Fe 1.00 ( Fe 0.94–0.98 Fe 0.97–0.99 Ti0.00–0.04Al0.00–0.02Mn0.00–0.01 2+
Mg0.00–0.01Si0.00–0.01)1.98O4, grunerite ( Fe 4.52–4.89 Mg1.52–1.95 3+
Ti0.00–0.02Mn0.01–0.10Al0.01–0.24 Fe 0.00–0.15 Ca0.07–0.23Na0.00–0.08 K0.00–0.08Cr0.00–0.01)6.60–7.15[(Si7.94-8.02Al0.01–0.06)8.00–8.02O22] (OH)2, and amphibole of the ferroactinolite–Fe-hornblende series (Na0.00–0.16K0.03–0.20)0.03—0.35(Ca1.68–1.94 2+
3+
Na0.00–0.28)1.75–2.00( Fe 1.73–3.18 Mg0.80–1.58 Fe 0.00–1.63 Al0.12–0.87 Ti0.00–0.03Mn0.00–0.05)5.00[(Si6.57–7.86Al0.17–1.43)8.00O22](OH)2; 2+
auxiliary minerals are almandine ( Fe 2.14–2.27 Mn0.04–0.17 Mg0.10–0.14Ca0.48–0.54)2.92–3.01Al1.94–1.98[Si3.00–3.05O12] and 2+
annite K0.94( Fe 2.03 Mg0.83Ti0.07Al0.12)3.05[(Si2.07Al1.30)4.00 O10](OH)2; accessory minerals are zircon, fluorapatite, pyrrhotite, and titanite; pyrite, microcline, and goethite are secondary minerals. The texture of BIF varies from massive to gneissic and contrasting banding, which may be rectilinear, lenticular or finely plicated. The median chemical composition of BIF is presented in Table 1. TIME AND TEMPERATURE OF ORE FORMATION To determine the age of BIF, actinolite–hornblende amphibolite (Fig. 4, sampling points (SP) 17 and 18) have been chosen. Amphibolite occurs as separate lenses within the BIF of the largest orebody (Fig. 2, site II). The rock consists of amphibole pertaining to the actinolite–Mg-hornblende series, albite (up to 50 vol %), and phlogopite (up to 5 vol %); titanite (up to 3 vol %), quartz, allanite-(Ce), zircon, and magnetite are accessory minerals. Exsolution zircon (Zr0.98Hf0.02)1.00[Si1.00O4] makes up the chains of yellowish brown polyzonal dipyramid–prismatic crystals (up to 0.5 mm across) along cleavage planes in phlogopite 2+ (K0.9Na0.09)1.00(Mg1.3 Fe 1.25 Al0.19Ti0.13)2.90[(Si2.70Al1.30)4.00 O10](OH)2 and is readily identified owing to characteristic pleochroic halos. Titanite (Ca0.98Na0.01–0.02 3+
Mg0.00–0.01)0.99–1.01(Ti0.96–0.98Al0.02–0.04 Fe 0.01–0.02 )1.02[(Si0.99 Al0.01)1.00O5] occurs as wedge-shaped crystals (up to 0.5 mm) uniformly disseminated along amphibole–plagioclase boundaries. However, the main amount of this mineral concentrates in late quartz–titanite–plagioclase segregations characteristic of all BIF deposits in the Kola Peninsula. The segregations reach 1 × 3 cm in size and contain up to 15 vol % titanite. The U and Pb isotopic compositions and contents in zircon and titanite were determined by T.B. Bayanova GEOLOGY OF ORE DEPOSITS
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Table 1. Median chemical composition of BIF and host rocks at the Uraguba deposit, wt % Component
Biotite gneiss
SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S Number of analyses
64.87 0.62 14.61 1.04 5.01 0.05 2.28 1.65 3.45 2.22 0.07 0.00 0.10 4
Two-mica Amphibogneiss lite 71.09 0.58 14.08 1.00 3.64 0.05 1.98 1.47 2.95 2.02 0.04 0.11 0.09 12
51.00 1.33 14.29 3.33 10.79 0.20 6.55 8.07 2.88 0.98 0.09 0.14 0.04 3
BIF 49.59 0.03 2.93 19.25 19.68 0.13 2.43 2.89 0.20 0.18 0.10 0.04 0.07 21
on an MI 1201-T mass spectrometer at the Laboratory of Geochronology of the Geological Institute, Kola Scientific Center, Russian Academy of Sciences (Table 2; Fig. 5). The established age of 2813 ± 48 Ma is the oldest date ever obtained previously for BIF and tonalites of the Kola–Norwegian Megablock. Nevertheless, this estimate is rather close to the age of other deposits in the Imandra district (2790–2760 Ma) (Bayanova et al., 2002). The age of titanite (2560 Ma) may correlate with the age of hydrothermally altered rocks from other BIF deposits in the Kola Peninsula, in particular, with U−Th–Pb age of 2630–2530 Ma determined for titanite from the meionite–quartz–plagioclase vein in amphibolite at the Olenegorsk deposit (Zhirov et al., 1979). Thus, the results indicate that the age interval of the BIF in the Kola–Norwegian Megablock (2800–2550 Ma) is valid for the deposits of the northern group and allow us once and for all to deny the hypothesis of a younger BIF related to the Tundra Group. The thermodynamic parameters of the formation of the orebody from site II shown in Fig. 4 were determined in collaboration with P.Ya. Azimov from the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, using the program TWQ (Berman, 1991), a dataset compiled by Mader and Berman (1982), and the results of microprobe analyses of rock-forming minerals of the BIF from SPs 1, 7, and 16 (Table 3). Microscopic examination has shown that these minerals do not reveal the relationships of the reaction exhibited in the replacement of almandine by grunerite, grunerite by hornblende, and so on (Ivanyuk et al., 1999). It turned out that in all cases equilibrium lines are arranged almost parallel to one another at a
674
IVANYUK, GORYAINOV 13 10 16 7
1
8
2
9
3
14
5 17
11
6 7
75
70
75
12 17
5 6
80 75
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80 1
18 15
80
3
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Fig. 4. Relationships between (1) BIF, (2) quartz–almandine–grunerite rocks, and (3) actinolite–hornblende amphibolite in site II (see Fig. 2); (5) granitic pegmatite; (6) quartz vein; (7) sampling points.
small angle to the ordinate axis, so that only temperature interval of mineral formation can be estimated (Fig. 6). From the marginal zone of the orebody (SP 1)
toward its central portion (SP 7 and, farther, SP 16), the temperature varies as follows: 597 ± 25–710 ± 46– 780 ± 30°C at 6–8 kbar typical of the BIF in this region.
Table 2. Isotopic U–Pb data on zircon (1–3) and titanite (4) from actinolite–hornblende amphibolite at the Greater Eastern Litsa ore occurrence Content, ppm
Pb isotopic composition*
Sample
Weight, mg
Pb
U
Pb ----------204 Pb
1 2 3 4***
0.20 0.20 0.25 3.00
11.7 4.1 4.0 46.1
22.3 8.8 8.8 76.6
484 411 145 1100
206
206
Pb ----------207 Pb 4.563 4.421 3.713 5.514
206
Pb ----------208 Pb 6.195 5.627 2.995 3.356
Isotope ratios and age, Ma** 207
Pb ----------235 U
Pb ----------238 U
207
%
206
%
Pb ----------206 Pb
Rho
11.473 9.826 7.634 10.922
3 8 8 1
0.4282 0.3652 0.2985 0.4657
2 7 6 1
2779 2802 2703 2559
0.89 0.90 0.85 0.90
Note: * All ratios are corrected for blank contamination: 0.08 ng Pb and 0.04 ng U and mass discrimination 0.17 ± 0.05%. ** Correction for isotopic composition of plagioclase is introduced: 206Pb/ 204Pb = 14.63, 207Pb/ 204Pb = 14.86; 208Pb/ 204Pb = 34.05. *** Correction for admixture of common lead is calculated for age according to the Stacey and Kramers (1975) model. GEOLOGY OF ORE DEPOSITS
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To ascertain the cause of such variation m temperature, the structural and compositional zoning of orebodies should be considered. STRUCTURAL AND COMPOSITIONAL ZONING OF OREBODIES As was shown by Bazai and Ivanyuk (1996), Egorov and Ivanyuk (1996), Ivanyuk (1997), Goryainov et al. (1997), and Goryainov and Ivanyuk (1998, 2001), the fractal dimensions of banding, folds, and intergranular boundaries determined by different methods are convenient parameters for characterization of BIF structure. In particular, the Minkowski dimension (cellular dimension, embedding dimension), see Cronover (2000) can be determined, covering an image with square cells having a side r and counting a number N of required cells. The slope of bilogarithmic relationship log N = log c – D(logr) corresponds to the Minkowski dimension DM of an analyzed object. An isoclinal fold in the grunerite–magnetite BIF from SP 13 (Fig. 4) in II (Figs. 2) is shown in Fig. 7a. The fold is multiorder: a large fold is complicated by smaller folds down to microplication discernible only
675
206Pb/ 238U
0.6
2813 ± 48 Ma 2800 2560 Ma
0.5
4
2400
1
0.4 2000
2
0.3
3
1600
0.2
4
6
8
10 12 14
207Pb/ 235U
Fig. 5. U–Pb diagram with concordia: (1–3) zircon and (4) titanite from intramineral amphibolite in site II (SPs 17 and 18), after Bayanova et al. (2002).
at large magnification. The Minkowski dimension 2Df of the lower quartz lamina in this section is 1.30 ± 0.03 (Fig. 7b), although commonly 2Df does not exceed 1.15
Table 3. Chemical compositions (microprobe analyses) of rock-forming minerals (microprobe analyses) from BIF used in thermodynamic calculations Component SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Total cations
Gru
Hbl-Act
Alm
Mgt
1
16
1
7
16
1
7
16
1
7
16
51.93 0.03 1.31 – 36.36 0.28 7.41 0.77 – – 98.09 8.03 – 0.24 – 4.70 0.04 1.71 0.13 – – 14.85
50.80 0.05 0.51 0.02 36.37 0.23 8.29 0.55 – 0.02 96.84 7.99 0.01 0.10 – 4.79 0.03 1.95 0.09 – 0.01 14.97
44.53 0.19 9.03 – 27.06 0.09 4.75 11.87 0.64 0.63 98.79 6.80 0.02 1.63 0.53 2.93 0.01 1.08 1.94 0.19 0.12 15.25
50.87 0.06 5.57 – 25.23 0.35 5.43 10.78 0.44 0.27 99.00 7.60 0.01 0.98 0.17 2.99 0.05 1.21 1.73 0.13 0.05 14.92
44.61 0.17 11.77 0.02 26.2 0.09 3.90 10.85 1.42 0.91 99.94 6.71 0.02 2.09 0.37 2.93 0.01 0.87 1.75 0.41 0.18 15.34
37.61 – 20.43 0.05 33.46 0.92 0.90 6.14 – – 99.51 3.05 – 1.96 – 2.28 0.06 0.11 0.54 – – 8.00
38.07 0.05 20.98 – 31.97 2.17 0.97 5.87 – – 100.08 3.07 – 1.99 – 2.16 0.15 0.12 0.51 – – 8.00
37.68 0.03 20.34 – 34.72 0.62 1.14 5.56 – – 100.09 3.05 – 1.94 – 2.35 0.04 0.14 0.48 – – 8.00
– 1.45 0.41 0.06 93.16 0.30 – – – – 95.38 – 0.04 0.02 1.94 0.97 0.01 – – – – 2.98
– 0.24 0.29 0.04 93.67 0.30 – – – – 94.54 – 0.01 0.01 1.98 0.99 0.01 – – – – 3.00
– 0.35 0.27 0.05 93.96 0.22 – – – – 94.85 – 0.01 0.01 1.97 1.00 – 0.01 – – – 3.00
Note: (Gru) grunerite, (Hbl) Fe-hornblende, (Act) ferroactinolite, (Alm) almandine, (Mgt) magnetite. Analyses were performed at the Geological Institute, Kola Scientific Center, Russian Academy of Sciences on a MS-46 Cameca microprobe, analyst S.A. Rezhenova. Oxide contents are given in wt % and element contents, in apfu. Dash denotes a content below the detection limit. Formula coefficients were calculated with Minal program elaborated by D.V. Dolivo-Dobrovol'sky (on the basis of O = 23 and Fetot = Fe2+ for grunerite and O = 4 for magnetite); with 13eCNK method for Ca-amphibole; and the method proposed by Droop (1987) for garnet. GEOLOGY OF ORE DEPOSITS
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IVANYUK, GORYAINOV P, kbar 12 1 10
1.0 in the segments with rectilinear banding (SPs 1, 6, 12) to 1.3 in plicated segments (SPs 3, 13, 15, 16), increasing from the contact with amphibolite to the axial zone of orebody, as has been established previously for the BIF at the Pecheguba deposit (Egorov and Ivanyuk, 1996; Goryainov and Ivanyuk, 2001). Because the BIF is not fractal in the direction of lineation, the total dimensions of a single layer will be greater by unity: 3Df = 2.0–2.3. At the same time, the alternation of laminas also obeys the laws of fractal geometry, because the unidimensional Minkowski dimension of banding (aggregated of magnetite laminas) 1Db ranges from 0.30 to 0.50, decreasing with an increase in the fractal dimension of folds. This is a manifestation of the general tendency toward increasing contrast of BIFs passing from rectilinear to plicated varieties, when only monomineralic quartz and iron oxide laminas are left in the rock. This transition is expressed in the chemical composition as an increase in Fe content at the expense of depletion in other components, Si, Al, Mg, Mn, Ca, K, and Na above all else (Table 4; Fig. 8). Similar relationships were established at the Olenegorsk and Kirovogorsk deposits, whereas at the Pecheguba, Iron Varaka, and some other small deposits of the Imandra district, the removal of auxiliary components is compensated by a primary gain m silica (Goryainov and Ivanyuk, 2001). In the process of folding, the Fe2+ content in the BIF first decreases owing to its oxidation to Fe3+ during magnetite formation as a product of the reaction 7(Fe6Mg1)Si8O22(OH)22 + 8H2O
1 32
8 6 4 2 12 7
1
3 2
10 8 6 4 2 12 1 32
16 10 8
Mg7Si8O22(OH)2 + 14Fe3O4 + 48SiO2 + 14H2,
6 4 2 400
500
600
700 T, °C
800
900
1000
1. Alm + Tsc = FeTsc + Prp 2. 3Trm + 5Alm = 5Prp + 3FeTrm 3. 3Pgs + 4Alm = 4Prp + 3FePgs
Fig. 6. PT diagrams for BIF in site II (Fig. 4, SPs 1, 7, and 16) constructed with the program TWQ (Berman, 1991; dataset compiled by Mader and Berman (1982)).
(Kulik and Chernovsky, 1990; Bazai and Ivanyuk, 1996). As follows from comparison of fractal dimensions estimated from a photograph and a field observation,3 the segment where this fold is fractal extends at least from 0.1 to 1000 mm. The 2D fractal dimension 2D of the folds in the BIF shown in Fig. 4 varies from f 3
In this case, fractal dimension D is determined from the relationship L(r) = cr1 – D, where L is the fold length and r is the spread of a pair of compasses used for measuring this length (Goryainov and Ivanyuk, 2001).
and then increases due to the concentration of magnetite in a complexly folded BIF. For the same reason, the Fe content, directly related to magnetite (FeMgt, Fe3+), grows linearly with increasing fractal dimension of folds, 2Df (Fig. 8). Apparently this increase is caused by activation of metamorphic reactions by folding, which is accompanied by an increase in shearing (Ivanyuk, 1991; Bazai and Ivanyuk, 1997; Goryainov and Ivanyuk, 2001) and temperature, respectively. In addition, it should be kept in mind that metamorphic reactions could have been activated by mechanochemical phenomena, and the estimated temperature is an integral parameter that characterizes the energetic contribution of the process. DISCUSSION As was shown by Goryainov and Ivanyuk (1998, 2001) and Golikov et al. (1999), a fully differentiated lens in the BIF of the Kola Peninsula consists of a hematite–magnetite–quartz core with talc, antophyllite, and tremolite; and intermediate zone of magnetite jaspilite with Mg–hornblende, actinolite, and biotite; a zone of sulfide–magnetite jaspilite with hedenbergite, grunerite, and Fe–hornblebde; a zone of magnetite– GEOLOGY OF ORE DEPOSITS
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677
(a) logN 3.8
(b)
3.4
DM = 1.30 ± 0.03
3.0 2.6 2.2 1.8 1.4 1.0 0.1
0.5 1
5 10 r, mm
Fig. 7. (a) Fold in BIF in site II (Fig. 4, SP 13), the width of photograph is 15 cm and (b) bilogarithmic relationship between the number of asquare with a side r required for complete coverage of the contact line between substantially quartz (light) and magnetite–grunerite (dark) bands and r. The slope of the straight line determines thefractal dimension of the band 2D = 1.30.
diopside or almandine–biotite skarnoids; and finally, the outermost zone of magnetite–calcite–dolomite rocks at the contact with host gneisses and amphibolites. In other words, the assemblage of ore minerals controls not only the mineral paragenesis but also the chemical composition of coexisting silicates. The formation of such zoning may be caused by the effect of reduced endogenetic fluid enriched in SO2, H2S, CO2,
CO, CH4, and other sulfur and carbon compounds on a hematite–quartz protolith according to following reactions: 3Fe2O3 + H2
2Fe3O4 + H2O,
7Fe2O3 + 24SiO2 +7H2
3Fe7Si8O22(OH)2 + 4H2O,
3FeS2 + Fe3O4 + 4H2
6FeS = 4H2O.
Table 4. Chemical composition of BIF, wt % (analyses 1–16) and fractal dimension of folds (Df) in site II Component
1
3
5
6
7
8
9
10
11
12
13
14
15
16
1.0
1.3
1.2
1.0
1.1
1.1
1.2
1.1
1.2
1.0
1.3
1.1
1.3
1.3
SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 CO2 S
59.19 – 5.20 6.69 20.76 0.13 2.56 2.80 0.14 0.27 0.11 – 0.06
43.68 – 1.99 24.96 20.95 0.13 2.11 3.48 0.18 0.15 0.10 – 0.04
49.62 – 2.93 19.66 18.69 0.12 2.33 3.79 0.28 0.13 0.07 0.08 –
50.35 – 3.00 16.19 20.89 0.17 2.94 3.11 0.28 0.28 0.09 – 0.10
49.56 – 3.58 18.84 19.84 0.15 2.29 2.94 0.23 0.18 0.05 0.35 0.13
48.78 – 2.71 21.08 19.44 0.14 2.34 2.89 0.20 0.17 0.08 0.04 0.06
52.25 – 4.19 15.34 19.96 0.18 2.44 2.16 0.20 0.56 0.10 0.25 0.14
58.86 – 4.50 8.64 19.52 0.15 2.41 2.30 0.20 0.78 0.09 – 0.12
50.05 – 2.24 21.17 19.03 0.11 2.00 2.35 0.18 0.14 0.09 – 0.13
52.40 – 4.01 13.73 20.29 0.19 2.67 3.25 0.29 0.49 0.11 0.22 0.10
49.33 0.16 2.86 16.38 22.74 0.17 2.80 1.97 0.18 0.49 0.12 0.09 0.09
58.70 0.06 4.05 9.79 17.26 0.17 2.43 5.09 0.32 0.37 0.08 – 0.14
44.84 0.11 1.56 26.75 20.35 0.11 1.93 1.93 0.15 0.11 0.12 – 0.05
48.61 0.23 3.44 17.60 21.36 0.11 2.48 3.08 0.18 0.35 0.08 – –
Note: Sample numbers corresponds to sampling points in Fig. 4. The dash denotes a content below the detection limit. GEOLOGY OF ORE DEPOSITS
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IVANYUK, GORYAINOV
1
14
10
9 11
12 7
6
8
16 15
y = 57.51–0.18x13.52 r2 = 0.393
3
3 15
32 FeMgt (at.%)
13
5
Fe2+ (at.%)
64 62 60 58 56 54 52 50 48 46
28
11
8 7
24
16
5
6
9
13
12
20
10
16
1
12 0.95
y = 14.93 + 3.63x 4.80 14 r2 = 0.465
1.05
1.15 Df
1.25
1.35
Al + Ti + Mg + Ca + Mn + Na + K + P + S (at.%)
Si (at.%)
678
22 y = 133.53–116.60{1 + 21 [(x–1.12)/1.16]2} 13 2 20 r = 0.691 3 16 19 61 15 7 9 18 12 8 10 11 17 5 16 15 14 14 17 6 14 16 12 15 1 10 9 14 16 7 5 13 13 8 12 3 11 11 10 y = 28.65–13.46x 15 9 r2 = 0.489 8 0.95 1.05 1.15 1.25 1.35 Df
Fig. 8. Composition of BIF versus fractal dimension of folds. (1–16) are sampling points (see Fig. 4).
Number of orebodies, N (‡) 21
logN = 3.655–0.779 logS r 2 = 0.9927
13 9 7 5 3 2
Area of orebody, S, m2 Number of orebodies, N 20 (b) 10 8 6
>50000
>14000
>5000
>2000 >3000
>1000
>500
1
logN = 2.70–0.97 logS r 2 = 0.9996
4
1 40
200 Tonnage, S, Mt
400
600 800
Fig. 9. (a) Bilogarithmic relationship between number of orebodies at the Greater Eastern Litsa occurrence and their size and (b) number of BIF deposits and ore occurrences in the Kola–Norwegian Megablock and their tonnage.
These reactions give rise to the gradual inward propagation of magnetite and (carbonate-)sulfide–magnetite shells, so that magnetite is retained only in apical portions of the axial zone in the largest orebodies formed under conditions of greenschist and amphibolite metamorphic facies. Ore lenses at the Olenegorsk, Kirovogors, and Professor Bauman deposits serve as examples. For the same reason, hematite is found only in high-grade ore (Goryainov and Balabonin, 1988), whose buffer capacity allowed this mineral to escape replacement in the reducing fluid flow. In small and especially highly metamorphosed orebodies, including those at the Greater Eastern Litsa occurrence, hematite zones disappeared altogether. This compels us once again to call attention to the fact that the quality of the Kola BIF deposits directly correlates to the size of the orebody. This correlation, in turn, raises a question on the expediency of mining medium- and small-size deposits in view of the exhaustion of the upper levels at the Olenegorsk and Kirovogorsk deposits and disappointing results of geological exploration beyond the Imandra district. Unfortunately, the limitations related to the Kola BIF belonging to a system with self-organized criticality (Bak, 1997; Bak et al., 1987) come into play. In full measure, this concerns the Uraguba deposit, where the distribution of orebodies by their size (area at the surface) obeys an exponential law with an exponent of 0.8 (Fig. 9a). This law is characteristic of various mineral deposits related to a single source of ore matter (Goryainov and Ivanyuk, 2001). Judging from the graph, only small lenses no larger than 100 m2 in area at the surface have remained undiscovered to date. The number of such GEOLOGY OF ORE DEPOSITS
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small lenses can be rather great. For example, at the Lake Livlinsky occurrence located in the same zone, a tract of ore-bearing amphibolite about 1.8 km in length hosts about 60 shoestring BIF interlayers, each less than 10 cm in thickness; no larger orebodies are known. According to the theory of self-organized criticality, the exponential distribution function indicates that large-scale events (catastrophic earthquake or formation of a giant deposit) happen extremely rarely, but there is no need to search for a special mechanism for these cases, because this mechanism is the same as for ordinary events. Therefore, the relationship of the number of deposits of a certain type versus their size is exponential with a negative exponent. Within a particular ore cluster, such a relationship not only limits the possible number of deposits of specific size but also makes it possible to suggest several types of ore matter sources (Goryainov and Ivanyuk, 2001). The bilogarithmic relationships of the number of BIF deposits in the Kola Peninsula with tonnage that exceed a value S versus S based on official inventory data is approximated by a straight line with a correlation coefficient close to unity (Fig. 9b). This implies that all BIF deposits of the Kola–Norwegian Geoblock, including those of the Uraguba Group, pertain to one BIF system, and the deposits of all possible sizes are formed in this system. There are no grounds to suggest that small deposits and occurrences related to highgrade metamorphic rocks are derivatives of another process than the deposits located in the Imandra district. The difference of BIF in composition is caused by size of orebodies (capacity of oxygen buffer) and energy of metamorphic reactions. Folding makes a substantial contribution to this energy. ACKNOWLEDGMENTS We thank S.A. Rezhenova for microprobe analysis of minerals, Yu.P. Men’shikov for XRD analysis, T.B. Bayanova for dating zircon and titanite with U–Pb method, and P.Ya. Azimov for his assistance in thermodynamic calculations. This study was supported by the Ministry of Natural Resources of the Russian Federation (project nos. 5120023/1 and 4-26/598, the Geokart Interregional Center and OOO Minerals of Laplandia. REFERENCES 1. P. Bak, How Nature Works. A Science of Self-Organized Criticality (Univ. Press, Oxford, 1997). 2. P. Bak, C. Tang, and K. Wiesenfeld, “Self-Organized Criticality: An Explanation of 1/f Noise,” Phys. Rev. Lett. 59, 381–384 (1987). 3. T. B. Bayanova, V. I. Pozhilenko, V. F. Smol’kin, et al., Catalogue of Geochronological Data on the Northeastern Baltic Shield (Kola Sci. Center, Russian Acad. Sci., Apatity, 2002) [in Russian]. GEOLOGY OF ORE DEPOSITS
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