ISSN 10757015, Geology of Ore Deposits, 2011, Vol. 53, No. 6, pp. 455–473. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.A. Kremenetsky, N.A. Gromalova, E. Belousova, L.I. Veremeeva, 2011, published in Geologiya Rudnykh Mestorozhdenii, 2011, Vol. 53, No. 6, pp. 516–537.
Isotopic and Geochemical Features of Newly Formed Zircon Rims As a Criterion for Identification of Feeding Sources of Ti–Zr Placers A. A. Kremenetskya, N. A. Gromalovaa, E. Belousovab, and L. I. Veremeevaa a
Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, ul. Veresaeva 15, Moscow, 121357 Russia b Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia Received July 14, 2011
Abstract—Zircons were studied from the Ti–Zr placers of the Murray Basin (Mindarie and WIM150 depos its) and metamorphic rocks of the adjacent Kanmantoo Belt and the Ballarat Trough in Southeast Australia, and from Russian basins with Ti–Zr placers: the Cenomanian–Turonian and Poltavian basins in the East European Platform and the Sarmatian basin in the northern Caucasus, Stavropolye, and Kalmykia. The study of the primary source weathering mantle intermediate reservoir rock economic placer system includes (1) morphostructural and paleofacies reconstructions of the studied territories; (2) quantita tive analysis of ore mineral distribution in each element of this system; and (3) study of zircon typomorphism from the feeding source to the basin, where Ti–Zr placers have been deposited. In all elements of the system studied, zircons were examined using optical and cathodoluminescence microscopy, an electron microprobe, mass spectrometry (laser ablation and SHRIMP II), including U–Pb dating, Lu and Hf isotopes, distribution of trace elements (REE, Y, P), and comparative analysis of indicative ratios: Th/U, HfO2/ZrO2, Y2O3/(Y2O3 + REE2O3), (La + Sm)/(Gd + Yb), etc. Newly formed rims replacing detrital zircon grains and marking the time of late geological events were identified. The rims differ from the cores in distribution of trace elements. Genetic typification of diverse newly formed rims is based on discrimination of them by internal structure, isotopic and geochemical characteristics, which are used as a criterion of relationships between Ti–Zr placers and their inferred feeding sources. Based on these data, a prospecting model of the buried Ti–Zr placers with estimation of their resource potential has been produced. DOI: 10.1134/S1075701511060067
INTRODUCTION It is known that the total proved reserves of ilmenite–rutile–zircon placers of the superlarge Ti– Zr deposits of the East European Platform and Scyth ian Plate (East European and North Caucasus Ti–Zr provinces) and the South Australian Platform (Fig. 1) are estimated at 2635 and 2688 Mt, respectively. Taken together, they are 1.5 times greater than the total resources of other provinces in the world (South Africa, Mozambique, United States, Kazakhstan, etc). Although parts of these deposits are being actively mined as sources of 95%, 70% and 90% of the world’s zircon, ilmenite, and rutile, respectively, considerable volumes of placers are not yet utilized because of low quality of ore, or remain undiscovered, because the Ti–Zr placers were mostly formed under nearshore marine conditions and are now overlapped by loose sediments up to 50 m thick. This, in turn, complicates the establishment of their links with source regions and therefore decreases the efficiency of forecast, localization, and estimation of the resource potential of the buried placers. Corresponding
[email protected]
author:
A.A.
Kremenetsky.
Email:
Sources of ancient Ti–Zr placers are a subject of longterm debate. Most researchers deny the possibil ity to ascertain the relationships of these placers to specific primary sources taking into account a set of unfavorable circumstances: (1) remoteness of the bur ied nearshore marine placers from the inferred ancient primary sources; (2) the narrow stratigraphic interval of placer formation (Paleogene–Neogene, less frequently Cretaceous, and still less, Jurassic) against the heterogeneity and wide chronological range of their sources; (3) similar structure of orebod ies in placers (productive layers about 5 m in average thickness); and (4) similar mineral composition of placers (ilmenite > rutile ≥ zircon) and uniform grain size of ore minerals (predominance of grainsize class of 0.10–0.044 mm). As a consequence, in has not been possible to judge the effect of bedrock and overlapping weathering man tles on the early stage of placer formation. In other words, the role of feeding sources in the formation and localization of currently buried Ti–Zr placer is under estimated. A classic example of the uncertainty in identifica tion of feeding sources is the Ti–Zr placers of the Forecaucasus (Stavropol and Yergeni placer districts of the North Caucasus Ti–Zr province). According to
455
456
KREMENETSKY et al. (a)
M W
0
Perth
100 km
II
I
Sydney
III
Adelaide
IV
0 200 400 km
(b) Y Elista Stavropol S Moscow Pyatigorsk
VI 0
100 km
VII V VIII 0 200 400 km
1
2 6
3 7
4 8
5 9
Fig. 1. Ti–Zr placer provinces of (a) Australia and (b) Russia. (1) Placer province, (2) placer district or deposit, (3–5) metamor phic belts: (3) Kanmantoo, (4) Ballarat and Melbourn, (5) Wagga; (6) preAlpine basement of the Greater Caucasus; (7) fault; (8) boundary of placer provinces (numerals in map) in Australia: I, Western; II, Eukla; III, Murray; IV, Eastern) and Russia: V, North Caucasus; VI, Central; VII, Ural; VIII West Siberian; (9) boundary of placer deposits (letters in figure) in Australia: M, Mindarie and W, WIM150; placer districts in Russia: S, Stavropol and Y, Yergeni.
Boiko (2004), these placers are products of the erosion of northern massifs (Tokmokovo Rise) with the subse quent transport of sediments along the channel of the paleoDon river toward the Caucasus Range. Thus, the Ti–Zr placers of the Stavropol region are regarded as a distal periphery of the East European placer prov ince. In the opinion of Gurvich et al. (1968) and Ver
emeeva et al. (2004), the North Caucasus is a source for the Stavropol Ti–Zr placers. Our morphostruc tural and paleofacies reconstructions in combination with isotopic and geochemical features of zircons (Kremenetsky et al., 2006, 2007; Mikhailov et al., 2007) supported the second hypothesis and allowed us to state that the weathering mantles of the preAlpine GEOLOGY OF ORE DEPOSITS
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ISOTOPIC AND GEOCHEMICAL FEATURES
basement of the Greater Caucasus are the main source of Ti–Zr placers in the Stavropol krai. This concept served as the basis for substantial growth of the resource potential of zircon in products of erosion throughout the North Caucasus Ti–Zr province, and as a result, in the involvement of this province in the program “Centers of Economic Devel opment of Russia” (Mikhailov and Kimelman, 2010). The advantageous geographic position of this region, shallowseated placers, climatic conditions favorable for yearround mining, and developed infra structure are convincing arguments for the develop ment of deposits in the Stavropol placer district and creation of a domestic source of Ti and Zr mineral commodities in the south of Russia. The enormous ore potential of this province, like other Ti–Zr provinces of Russia, has yet to be realized, because in contrast to most recent nearshore marine placers of Australia and South Africa, the placers in Russia are buried and therefore require more complex mining conditions, and are economically less effi cient. As a rule, these placers require the removal of a large overburden or the use of hydromining, for which the technology is so far insufficiently elaborated. Sec ondly, the relatively high zircon grade of the Russian placers, in some cases comparable with the best for eign counterparts, is combined with lower ilmenite and rutile grades. Third, finegrained and clayey ore sands in buried placers are characterized by lower recovery of valuable minerals. Fourth, it is not easy to produce a conditioned concentrate at some of our deposits. The enhancement of investment appeals to these objects requires, together with application of new con centrating technologies (dry separation, etc.), the development of efficient prospecting criteria for prof itable Ti–Zr placers in the overlapped territories. The key solution of this problem should be based on the comprehensive geological and mineralogical study of the system comprising the primary source, weathering mantle, intermediate reservoir rock, and economic Ti–Zr placer. The study provides for (1) morphostruc tural and paleofacies reconstructions of the permissive territories; (2) quantitative analysis of the distribution of ore, accompanying, and auxiliary minerals in each element of this system; (3) study of typomorphic fea tures of zircon (±monazite), ilmenite, and rutile and their variations from the primary source to the area where Ti–Zr placers accumulate (Kremenetsky et al., 2010a). A special part is assigned to the optical and cathodoluminescence microscopy, electron micro probe and mass spectroscopy (laser ablation, SHRIMP II). The bedrocks studied and spatially associated Ti– Zr placers always contain a sufficient amount of detri tal zircons transformed at the late stages of metamor phism and dynamometamorphism. Such zircons are clearly identified in cathodoluminescent photomicro graphs by newly formed rims (5–30 μm) superim GEOLOGY OF ORE DEPOSITS
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posed on growth zones of premetamorphic detrital zir cons differing in age and indicative geochemical ratios (Zr/Hf, Th/U, Y/Ho, etc.). GENETIC TYPES OF ZIRCON RIMS Unique properties of zircon (resistance to weather ing, ability to recrystallize and regenerate, diverse appearance, habit, and internal structure, isotopic and geochemical inhomogeneities displaying genesis and staged evolution of host rocks) are well known and widely used in geochronology and reconstruction of geological processes. Recently, with the appearance of new research methods, the number of publications concerning zircon ages of geological complexes con tinuously increases (Tables 1, 2). Recent publications demonstrate a distinct departure from traditional sta tistical methods of age estimation based on monomin eralic fractions, because the estimates obtained thereby are actually artificial mixtures of different ages and habitually lead to erroneous inferences about the sequence of geological events. The study of the inter nal structure of zircon reflecting its crystallization and recrystallization conditions is currently predominant. A key role is assigned to the thin (3–100 μm) outer rims at the margins of zircon crystals. As a rule, these rims differ in composition from the central zones and correspond to the latest stage of the studied geological event. Allowing for mechanisms of crystallization and recrystallization of zircon, including experimental data on this subject (Feisler et al., 2007; Fraser et al., 1997; Vavra et al., 1996; Watson, 1996; Hormann et al., 1980; Pidgeon, 1992; Putnis, 2002; Putnis et al., 2005) and our new data, the magmatic, hydrothermal, metamorphic, and regenerated rims enveloping and overgrowing zircon grains are recognized. Magmatic rims of the late growth crystallize over zircon grains of early generations and xenogenic (detrital) grains captured by melts from country rocks. These rims develop as newly formed marginal growth zones (5–50 μm) typically conformable with oscilla tory growth zones of primary zircon. As in the latter, the magmatic rims contain melt inclusions and are characterized by Th/U ratio typical of this genetic group of zircons (Table 1). The rims of this type are azonal or thinbanded and characterized by lower U, Th, REE, Y, Hf, Ti, Sr, and Ba contents, Th/U and 1
Eu/Eu* relative to the central zones. The Th/U ratio in the late rims vary from 0.4 to 1.0; according to Rubatto (2002), this range is characteristic of mag matic zircons (Table 1). The age of magmatic rims marks the late magmatic stage of the igneous complex formation. A minimal difference in the age between the inner and the outer magmatic zones is 37–49 Ma (Table 1). Magmatic 1 Eu* is the content in chondrite
458
KREMENETSKY et al.
Table 1. The UPb age of magmatic rims in zircon grains Zircon Locality
Source central zone, Ma
Moore porphyry dike, Slightly zonal magmatic Australia xenocryst, 2765 ± 17
Granite gneiss of Cen Zonal detrital xenoc tral Tien Shan, China ryst, 969 ± 11 Eclogite of the Salma Dark to black zonal pluton, Belomorye magmatic core, 2868 ± 35
Th/U
–
rim, Ma
Th/U
Thin banded magmatic 0.44–0.74 Baggott et al. (2005) with unconformable growth, 2765 ± 7
0.17–0.25 Light and thin anatectic 0.14–0.44 Yang et al. (2008) 453 ± 11 0.8–1.5
Light gray metamor phic, recrystallized at eclogitization, 1892– 1822
0.0
Skublov et al. (2010); Mints et al. (2010)
Mafic and ultramafic rocks of the South Kovdor area, Kola Peninsula, Russia
Zonal crustal xenocryst, 0.66–1.77 Light gray magmatic, 2754 ± 13 2408 ± 8.0
0.46
Krivolutskaya et al. (2010)
Granitic rocks of the Strel'tsovka uranium district, the Argun Bureya Median Mas sif, Russia
Zonal magmatic core with melt inclusions and uraninite, 282 ± 1
–
Dark magmatic with melt inclusions and ura ninite, 245 ± 2
–
Golubev et al. (2010)
Nepheline syenite of Rhythmically zoned the Sakharjok pluton, magmatic core, Kola Peninsula, Rus 2613 ± 35 sia
–
Porous hydrothermal, 1810–1680
–
Lyalina et al. (2010)
Granitic rocks of the Zonal detrital core, Tyn'yar area, West Si 2063 ± 29 berian Plate, Russia
0.6
Dark gray and homoge neous magmatic, 1542 ± 22
0.08
Ivanov and Erokhin (2011)
Amphibolite of the Karelian Craton, Russia
Zonal xenocryst, 2816 ± 22
Granulite of the Azonal magmatic core, Khanka massif, Russia 757.4 ± 4.4 Tonalite, the Vyg River, Zonal magmatic core southeastern Karelia with melt inclusions, 3122 ± 8
0.70–0.84 Light and homogeneous 0.42–0.87 Zlobin et al. (2010) magmatic, 2125 ± 15 –
Zonal metamorphic, 506.9 ± 2.6
–
Khanchuk et al. (2010)
0.13–0.9 Finezonal metasomat 0.006–0.6 Sergeev et al. (2008) ic with fluid inclusions, 3073 ± 8 GEOLOGY OF ORE DEPOSITS
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ISOTOPIC AND GEOCHEMICAL FEATURES
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Table 2. The UPb age of metamorphic rims in zircon grains Zircon Locality
Source central zone, Ma
Paragranulite, southern Spain
Diffuse and zonal detrital core, 950–480 Metamorphic rocks of the Zonal primary Chupa Sequence, north magmatic core, ern Karelia, Russia 2900–2800 Goldsulfidequartz ore from the Sukhoi Log de posit, the Bodaibo Syn clinorium, Russia Tonalite of the Orekhovo Pavlograd Zone, the Ukrainian Shield, Ukraine Eclogite of the South Tien Shan, northwestern China
Heterophase detri tal core, 2600–500 Zonal magmatic core, 3500 ± 13 Detrital zonal core, 2450 ± 31
Tonalite of the Boras plu Zonal magmatic ton, Sweden core, 1680 Granulite of the south Zonal primary western Norway magmatic core, 1680–1570 Granulite of the Khanka Azonal magmatic pluton, Russia core, 757.4 ± 4.4 Eclogite of the Sesia Detrital core, Lanzo Zone, the Alps 264–390
Th/U
0.59–2.07 Light homogeneous metamorphic, 21.3 ± 0.3 0.34–0.47 Homogeneous meta morphic, 1.2747 ± 6 2.1894 ± 17 0.60–1.55 Porous metamorphic hydrothermal, 447 ± 6 0.4–0.6
Š0.92
–
0.1–0.7
– 0.03–0.6
rims are characterized by concordant U–Pb ages, which are close to or younger than those of the central zones. An inverse relationship established occasionally is caused by partial loss of radiogenic lead from cores of zircon crystals, for instance, due to its metamictiza tion. Hydrothermal rims develop on both magmatic and detrital zircons owing to the mechanism of dissolu tion–redeposition under the effect of highto low temperature solutions (including metalliferous fluids). Rims of this type make up conformable or uncon formable envelopes (3–40 μm) enclosing replaced zir cons. As a rule, they contain fluid inclusions, are thin banded or homogeneous, characterized by redistribu tion of trace elements, and differ in Th/U, REE/REE*, and other indicative ratios depending on composition of hydrothermal solution and replaced zircon (Tables 1, 2). A zone of enrichment in trace ele ments is displayed at the contact between hydrother mal rim and replaced zircon in CL images. The con centration of these elements in the rim itself is always lower. The partial or complete dissolution of marginal zones results in opening of the U–Pb isotopic system. GEOLOGY OF ORE DEPOSITS
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rim, Ma
No. 6
2011
Homogeneous meta morphic, 2863 ± 22 2105 ± 40 Homogeneous meta morphic, 319.5 ± 2.9 Metamorphic, 940 Homogeneous meta morphic, 1560–1555 Zonal metamorphic, 506.9 ± 2.6 Metamorphic, 65 ± 3
Th/U 0.08–0.39
Whitehouse and Platt (2003)
0.53–0.66
Krylov et al. (2011)
0.48–1.15
Yudovskaya et al. (2011)
0.01–0.13
LobachZhuchenko et al. (2010)
0.002–0.004 Su et al. (2010)
–
Cornell et al. (1999)
0.05
Hoskin and Hanchar (2003), Hoskin (2005) Khanchuk et al. (2010) Rubatto and Her mann (2003)
– >0.01
The subsequent crystallization of the newly formed rim closes this system, thus retaining a concordant U– Pb age of hydrothermal process superimposed on magmatic or detrital zircons Metamorphic rims develop on magmatic and detri tal zircons under effect of highpressure (eclogite facies) or high, medium, or lowtemperature meta morphism of granulite, amphibolite, or greenschist facies. Since eclogite and granulitefacies metamor phism is accompanied by partial melting of host rocks, the corresponding Itype metamorphic rims resemble magmatic rims. Amphibolite and greenschistfacies metamorphism is accompanied by solidphase reac tions in the presence of aqueous fluid and character ized by development of metamorphic rims of Htype resembling hydrothermal rims. Metamorphic rims (3–100 μm) are homogeneous or thinbanded and contain melt (Itype) or fluid (Htype) inclusions. They are distinguished by sharply lowered Th/U ratio (0.002–0.08, less frequently 0.10–0.66) (Table 2). According to Rubatto (2002), the composition of newly formed metamorphic rims depends on P–T conditions of metamorphism. For
460
KREMENETSKY et al.
example, the composition of metamorphic rims on zircon from granulites is close to magmatic zircons (high U, Y, Hf, and P contents; enrichment in HREE, positive Ce anomaly, and negative Eu anomaly with Eu/Eu* = 0.08–0.41). At the same time, the Th/U ratio is low (0.14 mm). This discrepancy cannot be admitted as a significant distinguishing criterion; therefore, attention was mainly paid to the internal structure of zircon from the compared samplings and their isotopic and geochem ical characteristics. The images of the studied zircons in reflected and transmitted light, as well as CL images, show that despite their different habit and degree of roundness,
464
KREMENETSKY et al. (a) P, % 100
(b)
WIM150 Mindarie
1400
Ma
1.3 1200
1.1
80 1000 60 Kanmantoo Belt
40
Ballarat Trough
810 0.6 600 0.4 400
20 200 0
0 0.14
0.25 mm
Metamorphic rocks of the Kanmantoo Belt
Mindarie deposit
WIM150 deposit
Metamorphic rocks of the Ballarat Trough
Murray Basin Fig. 5. Abundance frequency (P) of (a) grain size and (b) U–Pb age of central zone (upper lines) and regeneration rims (lower lines) of zircon grains from feeding sources of Ti–Zr placers in the Murray Basin, Australia.
most grains reveal three zones: the central zone with oscillatory zoning typical of magmatic zircons (Fig. 3a), the metamorphic rim (5–50 μm), which conformably envelops the central zone or less frequently overgrows it unconformably (Fig. 3b), and the regeneration rim (3–30 μm) developing at grain margins discordantly with respect of preceding zoning (Fig. 3c). The U–Pb (SHRIMP II) dating of each of these zones gave the following age ranges: 3600–1050 Ma for the central zone; 600–500 Ma for the metamor phic rim; and 500–450 Ma for the regeneration rim (Figs. 3d, 3e). The U–Pb ages of the central zones and the outer rims in zircons from metamorphic rocks of the Kanmantoo Belt and Ti–Zr placers of the Minda rie deposit are given in Table 3; the same for the rocks of the Ballarat Trough and Ti–Zr placers of the WIM150 are given in Table 4. The diagrams with concordia drawn on the basis of these data show that the central zones of zircons from metamorphic rocks of the Kan mantoo Belt and from the placers of the Mindarie deposit are close in concordant age—mean values are 1000 and 1190 Ma, respectively (Fig. 4a, 4c), whereas the outer rims are dated at 580–600 Ma (Figs. 4b, 4d). Zircons from the Ballarat Trough and the WIM150 deposit display the same mean concordant age of cen tral zones (1100–1000 Ma), whereas the outer rims are somewhat younger (425–450 Ma). The above data lead to the conclusion that despite widely scattered attributes of detrital zircons, their metamorphic and regeneration rims mark the age of the last geological event in the feeding source and retain it in all other elements of the placer formation
system: weathering mantle–intermediate reservoir rocks, and Ti–Zr placer. These relationships allowed us to trace direct genetic links of the Ti–Zr placers localized therein with different feeding sources. The Mindarie deposit is a product of erosion of metamorphic rocks of the Kan mantoo Belt (age of regeneration of zircon rims is 580–600 Ma and relatively large (>0.14 mm) zircon grains). The WIM150 deposit is a product of erosion of metamorphic rocks of the Ballarat Trough (age of regeneration of zircon rims is 425–450 Ma and rela tively small (0.25 mm) with abun dant fractures and various degree of metamictization.
468 3.0
KREMENETSKY et al. Ma
(a)
(b)
(c) 2.9
2.8
2.7 2.5
2.5
2.5 2.1
1.9
2.0
2.0
2.0 1.7
1.7
1.7 1.5
1.7
1.7
1.6 1.4 1.3 1.2
1.0 0.56 0.46 0.35
0.5 0.3
0.3
Pereval’ny
Main Range
0.47
0.43 0.31
Bechasyn
0.32
Stavropolye
Zones of the Greater Caucasus
Kalmykia
Central deposit
Ti–Zr placers
1
2
Granitic rocks of basin
VCM
3
Fig. 9. Age of (1) metamorphic and (2) regeneration rims on (3) detrital zircons from Ti–Zr placer of the North Caucasus prov ince (b) and in their alternative feeding sources: (a) Greater Caucasus and (c) Voronezh Crystalline Massif (VCM).
Such zircons, moving away from a source, are com monly broken and dispersed and thus do not reach a region of accumulation of nearshore marine placers. The metamorphic rims of zircons from (a) bed rocks of the VCM and (b) newly formed rims of zir cons from the North Caucasus (b) are shown in Fig. 7. The former are slightly zonal and often unconform ably superimposed on the oscillatory magmatic zoning in the grain cores, whereas the latter are azonal and thinner (3–30 μm). The following features have been revealed from the data on U–Pb (SHRIMP II) age of the aforemen tioned zircons. First, the ages of central zones and metamorphic rims of the compared zircons range from 2800 to 1300 Ma (Figs. 7c, 7d), so it is impossible to identify the sources using statistical methods. Sec ond, the occurrence of regeneration rims with an aver age concordant age of 318 ± 2.5 Ma for zircons from rocks of the preAlpine basement of the Greater Cau casus (Fig. 8) and from the Ti–Zr placers of the Stavropol and Yergeni ore districts (Fig. 9) clearly indicates that precisely these rocks fed the placers. Third, in the basement rocks of the VCM as an alter
native source, young newly formed rings do not occur at all. Only ancient metamorphic shells dated at 1800–1600 Ma enveloping older cores (2.8–2.0 Ga) are widespread in both bedrocks and placers (Fig. 9). Thus, the paleoDon hardly can be regarded as a channel feeding the placers in the North Caucasus province. The fourth feature is concerned with the specific evolution of the gneiss–migmatite complex of the Greater Caucasus. As is known, this complex is traditionally dated as Proterozoic (Somin et al., 2006). The occurrence of zircon grains with the outer rims dated at 318 Ma enveloping the older detrital cores (1500–2000 Ma), as well as findings of metamorphic zircons having an age of 310–320 Ma in orthogneisses and migmatized paragneisses allowed Somin to state that the gneiss–migmatite core of the Elbrus Subzone of the Central Caucasus was formed during the Variscan tectonic epoch due to reworking of the Prot erozoic and Paleozoic rocks. Evidence that allowed us to regard the preAlpine rocks of the Greater Caucasus, primarily, of the Bechasyn Zone (Fig. 9), as a source that fed the sedi ments of the Miocene sea in the Forecaucasus with GEOLOGY OF ORE DEPOSITS
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GEOLOGY OF ORE DEPOSITS
Vol. 53
0.05
AR17.6.2
No. 6
0.33
0.00
1.48
0.00
0.46
0.35
0.45
1.14
0.00
0.30
0.00
0.08
0.49
0.00
AR02.4.3
AR02.12.2
AR02.21.1
AR08.7.2
AR09.7.1
AR16.6.1
AR17.6.1
MD1.1.2
MD1.3.1
MD1.4.1
MD1.7.2
MD1.3.3
MD2.2.2
MD2.3.2
0.23
0.04
AR16.6.2
AR02.4.1
0.18
0.00
0.09
AR02.12.2
AR09.7.2
0.60
AR02.12.1
AR08.7.1
0.04
AR02.4.2
Analytical point
2011
318
491
376
879
168
434
229
821
1054
1108
2836
13628
706
174
188
2467
1262
1798
1459
3411
2414
417
206Pb , c
220
146
301
1097
130
305
152
38
88
606
27
3446
30
66
70
541
827
1121
674
1217
618
123
0.71
0.31
0.83
1.29
0.80
0.73
0.69
0.05
0.09
0.57
0.01
0.26
0.04
0.39
0.39
0.23
0.68
0.64
0.48
0.37
0.26
0.30
% U, ppm Th, ppm
57.7
75.8
57.0
341
26.0
1236
1060
1048
2404
1064
1153
919
30.5 73.0
642
577
579
585
574.4
692
534
529
1043
894
1044
1450
938
960
947
Pb*, ppm
206
74.2
85.2
89.9
231
1110
68.7
12.9
13.8
372
161
272
317
459
335
56.7
232Th/238U
(1)
18
17
±22
26
19
16
15
Core
TiZr placers
±13
±11
±12
±12
±9
±16
±14
±13
Outer rim
±17
±16
±18
±24
±15
±17
±22
Core
Feeding source
age, Ma
1073
1061
1009
2377
992
1165
960
695
547
585
591
605
703
520
550
1054
910
1044
1544
916
962
969
age, Ma
206Pb/238U, 207Pb/206Pb,
(1)
42
79
±36
11
78
51
130
±100
±98
±100
±66
±72
±98
±96
±55
±30
±43
±32
±29
±33
±61
±22
207Pb*/235U
(1)
2.19
1.84
1.77
9.52
1.79
2.12
1.50
0.90
0.76
0.77
0.78
0.77
0.98
0.69
0.69
1.80
1.42
1.80
3.33
1.50
1.58
1.56
±%
2.60
4.30
2.90
1.50
4.30
3.00
6.70
5.20
5.00
5.30
3.70
3.80
5.20
5.10
3.60
2.30
2.80
2.50
2.40
2.40
3.60
2.70
206Pb*/238U
(1)
0.21
0.18
0.18
0.45
0.18
0.20
0.15
0.10
0.09
0.09
0.10
0.09
0.11
0.09
0.09
0.18
0.15
0.18
0.25
0.16
0.16
0.16
±%
Table 3. Results of U–Pb isotopic study of zicons from feeding source and Ti–Zr placers of the Mindarie deposit, the Murray Basin, Australia
1.60
1.70
2.30
1.30
1.90
1.50
1.70
2.10
2.10
2.20
2.10
1.70
2.40
2.60
2.50
1.80
1.90
1.90
1.80
1.80
1.90
2.50
Rho
0.60
0.40
0.80
0.89
0.45
0.50
0.26
0.40
0.42
0.42
0.57
0.46
0.47
0.52
0.71
0.77
0.67
0.77
0.76
0.75
0.54
0.91
ISOTOPIC AND GEOCHEMICAL FEATURES 469
GEOLOGY OF ORE DEPOSITS
Vol. 53
0.00
1.23
0.55
0.74
0.04
0.00
0.22
0.00
0.00
1.34
0.52
4.16
1.98
2.78
0.21
0.45
8.41
0.00
0.00
MD2.11.2
MD2.4.3
MD2.4.4
MD2.3.3
MD2.3.4
MD2.11.3
MD2.11.4
MD1.1.1
MD1.3.2
MD1.4.2
MD1.7.1
MD1.7.4
MD1.3.4
MD2.3.1
MD2.4.1
MD2.8.1
MD2.9.1
MD2.10.1
MD2.11.1
No. 6
232
213
612
245
1740
659
399
1423
437
463
1336
180
226
113
223
285
331
211
285
167
334
169
400
159
358
80
59
34
110
1498
161
151
803
729
453
114
32.0
12.0
79.0
88.0
156
214
134
187
73
212
117
186
65
303
0.36
0.29
0.06
0.46
0.89
0.25
0.39
0.58
1.72
1.01
0.09
0.18
0.06
0.73
0.41
0.56
0.67
0.65
0.68
0.45
0.66
0.71
0.48
0.42
0.88
Pbc, % U, ppm Th, ppm
206
25
22.6
55.4
17.6
84.6
57.3
31.8
97.5
43
46.8
83.8
16.0
19.6
10.1
36.2
51.7
50.0
34.7
39.8
20.0
59.8
32.3
51.2
23.7
58.7
Th/238U
232
752
747
645
518
325
619
571.5
480.4
685
688
452.1
627
619
640
1116
1236
1044
1122
966
832
1219
1285
893
1029
1126
ppm
206Pb*,
±17
±17
8.8
8.4
4.5
8.3
8.6
7.2
±15
±15
6.1
12
10
13
Outer rim
±25
±27
±23
±24
±21
±19
20
21
12
19
15
(1) Pb/238U, age, Ma
206
706
893
672
506
490
574
508
962
811
902
474
606
678
591
1068
1101
1083
988
1034
1000
1165
1225
1332
1028
1144
(1) Pb/206Pb, age, Ma
207
(1)
±96
±93
43
78
260
75
74
130
±99
±110
75
220
70
100
±43
±30
±27
±68
±46
±98
41
80
48
65
34
207Pb*/235U
1.07
1.16
0.90
0.66
0.41
0.82
0.73
0.76
1.02
1.07
0.57
0.84
0.86
0.86
1.95
2.22
1.83
1.88
1.64
1.38
2.26
2.47
1.75
1.75
2.05
±%
(1)
204Pb;
5.10
5.10
2.50
3.90
12.0
3.70
3.70
6.60
5.30
6.0
3.70
10.0
3.70
5.30
3.20
2.80
2.80
4.10
3.30
5.40
2.70
4.40
2.80
3.80
2.20
0.12
0.12
0.11
0.08
0.05
0.10
0.09
0.08
0.11
0.11
0.07
0.10
0.10
0.10
0.19
0.21
0.18
0.19
0.16
0.14
0.21
0.22
0.15
0.17
0.19
±%
2.40
2.40
1.40
1.70
1.40
1.40
1.60
1.60
2.40
2.40
1.40
2.0
1.70
2.10
2.40
2.40
2.40
2.40
2.40
2.50
1.80
1.80
1.40
2.0
1.50
Rho, correlation of
206Pb*/238U
Notes: Here and in Table 4, Pbc and Pb* are common and radiogenic lead, respectively; (1) correction for common lead from the measured and 206Pb*/238U uncertainties.
1.18
0.53
MD2.10.2
MD2.10.4
0.35
MD2.9.2
0.90
0.00
MD2.8.2
MD2.10.3
–
MD2.4.2
Analytical point
Table 3. (Contd.)
0.48
0.48
0.58
0.43
0.12
0.38
0.42
0.24
0.45
0.39
0.38
0.19
0.47
0.41
0.75
0.85
0.87
0.58
0.72
0.46
0.66
0.40
0.50
0.52
0.65
207Pb*/235U
Rho
470 KREMENETSKY et al.
2011
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No. 6
391 217
682 430
WmII.8.1 0.01 WmII.11.1 0.13
WmII.8.2 3.43 WmII.11.2 0.03
148 337 212 100 79 192 146 626 237 320 56 463 511
530 2043 142 345 716 900 93 320 73 263 320 269 416
0.53 0.09 0.48 0.71 0.00 0.10 0.43 0.39 0.00 0.31 0.00 1.12 0.25
206Pb , c
H2.3.2 11.33 H6.1.2 15.20 H8.3.2 4.16 H10_3.2 1.27 H3_1.2 3.79 H3_3.2 0.25 H3_2.3 0.76 H5_3.2 0.11 H7_4.2 2.01 H7_3.2 2.19 H7_3.1 0.31 H9.2.2 2.38 H11.4.2 0.71
H2.3.1 H3_2.2 H3_1.1 H3_3.1 H3_2.1 H5_3.1 H10_3.1 H8.3.1 H6.1.1 H7_3.1 H7_4.1 H9.2.1 H11.4.1
Analytical point
2011
545 159
257 201
714 426 81 269 154 83 10 13 23 27 117 309 5
57 19 217 49 77 182 64 201 83 117 22 649 467
% U, ppm
0.83 0.38
0.68 0.96
1.39 0.22 0.59 0.81 0.22 0.10 0.11 0.04 0.33 0.10 0.38 1.19 0.01
0.40 0.06 1.06 0.50 1.00 0.98 0.46 0.33 0.36 0.38 0.41 1.45 0.94
Th, ppm
76.4 24.8
61.1 41.3
48.5 120.0 12.8 13.0 44.0 70.1 8.54 24.2 6.02 22.2 39.1 15.9 35.5
25.3 45.7 34.6 17.2 24.4 29.5 16.1 75.2 84.4 39.1 8.95 61.5 74
232Th/238U
764 419.1
1077 1291
578 361 615 272.4 445 559 653 544 590 606 856 418.8 605
1165 945 1123 1181 1971 1059 778 841 2236 856 1102 916.0 1000
ppm
206Pb*,
(1) age, Ma
Feeding source Core ±13 ±10 978 ±13 1138 ±18 1349 ±26 1990 ±14 985 ±12 777 ±7.0 ±18 2729 ±12 899 ±22 1255 ±8.8 ±9 Outer rim ±8.9 ±4 ±17 ± 4.8 264 ±13 441 ±15 593 ±15 731 ±7 535 ±12 780 ±8 1036 ±12 899 ±6.0 ±7 TiZr placers Core ±23 1151 ±28 1593 Outer rim ±17 1165 ± 9.4 485
age, Ma
206Pb/238U, 207Pb/206Pb,
(1)
±160 ±48
±22 ±28
±240 ±262 ±32 ±107 ±50 ±282 ±107 ±57
±12 ±57 ±103
±19 ±38 ±90 ±18 ±40 ± 85
(1) Pb*/235U
207
1.37 0.526
1.959 3.005
0.71 0.88 1.06 0.307 0.55 0.75 0.94 0.71 0.86 1.00 1.35 0.494 0.784
2.182 1.56 2.04 2.40 6.03 1.77 1.151 1.316 10.78 1.35 2.12 1.497 1.655
±%
8.3 3.2
2.6 2.9
18 5.6 21 11 12.2 3.1 5.6 2.7 13.6 5.5 3.1 11 5
4.2 1.5 2.3 4.9 1.8 2.4 4.4 2.1 1.2 3.1 5.7 3.2 2.7
(1) Pb*/238U
206
0.12 0.07
0.18 0.22
0.09 0.06 0.10 0.04 0.07 0.09 0.11 0.09 0.10 0.10 0.14 0.07 0.10
0.20 0.16 0.19 0.20 0.36 0.18 0.12 0.14 0.41 0.14 0.19 0.15 0.17
±%
Table4. Results of UPb isotopic study of zircons from feeding source and TiZr placers of the WIM150 deposit, the Murray Basin, Australia
2.4 2.3
2.3 2.4
1.6 1.2 2.9 1.8 3.1 2.7 2.4 1.3 2.1 1.4 1.5 1.5 1.2
1.2 1.2 1.3 1.6 1.5 1.5 1.7 0.9 0.9 1.5 2.2 1.0 1
Rho
0.29 0.73
0.91 0.85
0.09 0.21 0.13 0.17 0.26 0.88 0.42 0.50 0.16 0.26 0.47 0.13 0.24
0.29 0.78 0.56 0.33 0.83 0.60 0.38 0.42 0.80 0.47 0.39 0.33 0.37
ISOTOPIC AND GEOCHEMICAL FEATURES 471
472
KREMENETSKY et al.
clastic material made it possible to calculate a balance between the mass of terrigenous material removed from the Caucasus (Jurassic–Paleogene) and the mass of the deposited in Neogene (Chokrakian and Sarma tian stages) sands prospective for Ti–Zr placers and to estimate hypothetical resources of highgrade Ti–Zr placers in the Stavropol region supplementing the already known economic placers. According to our estimates, 51 Mt of zircon can be expected at pros pects and 18 Mt at highgrade sites. This implies that at least one deposit comparable in resources with the Beshpagir deposit could be discovered. This forecast was one of the grounds for conducting revision pros pecting in this region, which was completed with the discovery of the Kambulat prospect. CONCLUSIONS (1) Four genetic types of newly formed rims (mag matic, hydrothermal, metamorphic, and regenera tion) enveloping zircon grains derived from the rocks—potential sources of Ti–Zr placers—were defined and characterized. (2) The primary source–weathering mantle–inter mediate reservoir rock– Ti–Zr placer system has been studied for placers of the Murray Basin in Southeast Australia and the North Caucasus placer province in Southwest Russia. Each element of this system is char acterized by main physical properties of zircon (habit, outer appearance, degree of roundness, inclusions, etc.), specific attributes of internal structure and isoto pic and chemical compositions determined by optical and cathodoluminesce microscopy, laser ablation, and mass spectroscopy (SHRIMP II). (3) Three zones are recognized in the detrital zir cons studied: (i) the central zone with oscillatory mag matic zoning; (ii) metamorphic rim (5–50 μm) con formably or unconformably overgrowing the central zone; and (iii) regeneration zone (3–30 μm) com monly discordant with respect to the preceding zones. With allowance for U–Pb ages, the latter two zones can be used as criteria for ascertaining relationships between placers and their feeding sources. To distin guish metamorphic and regeneration rims, the differ ence in trends of trace element distribution between central zones of detrital zircons and newly formed rims should be used. (4) The elaborated prospecting model of buried Ti–Zr placers comprises three main stages: the premetamorphic stage—accumulation of detrital zircons, including rounded grains with cores and growth zones derived from the Precambrian supracrustal complexes 3.0–1.0 Ga in age; the synmetamorphic stage—metamorphism and deformation under conditions of high and medium temperature facies with newly formed metamorphic or regeneration rims, respectively; and the postmetamor phic stage—weathering and erosion of ancient com plexes with separation of zircon grains having newly
formed outer rims; transport and accumulation of these grains in the Mesozoic and Cenozoic nearshore marine placers of proximal and/or distal removal. (5) Correspondence of the peak ages of newly formed rims over zircon grains from the Ti–Zr placers to the age of rims of zircon from certain ancient com plexes or to the age of their synmetamorphic deforma tion indicates that these complexes served as feeding sources for the placers in the paleobasin studied. These new data are offered as the scientific basis of the inno vative techniques for reconstructing localization con ditions of Ti–Zr placers within overlapped paleobasins and for estimating their resource potential. ACKNOWLEDGMENTS We thank M.A. Anosova, A.V. Antonov, N.G. Berezh naya, Yu.A. Kostitsyn, I.M. Kulikova, A. N. Larionov, E.N. Lepikhina, I.P. Paderin, S.L. Presnyakov, and N.V. Rodionov for their assistance in the isotopic, geochemical, and microprobe study of zircons. REFERENCES Baggott, M.S., Vielreicher, N.M., Groves, D.I., et al., Zir cons, Dikes, and Gold Mineralization at JundeeNimary: Post Ca. 2.66 Ga Archean Lode Gold in the Yandal Belt, Western Australia, Econ. Geol., 2005, vol. 100, pp. 1389– 1405. Boiko, N.I., TitaniumZirconium Placers of the Stavropol Region, Lithol. Minneral Res., 2004, vol. 39, no. 6, pp. 523– 529. Cornell, D.H., Schersten, A., Hoskin, P., et al., Application of ChargeRadius Concepts to Trace Elements in Igneous and Metamorphic Zircons, Goldschmidt conf. LPI Contribu tion, no. 971, Houston, Lunar and Planetary Inst., 1999, pp. 61–62. Fisher, F.N. and Warren, R.G, Outline of the Geological and Tectonic Evolution of Australia and Papua New Guinea, in Economic Geology of Australia and Papua New Guinea, Parkville: Australasian Inst. Mining Metallurgy, 1975, p. 27–40; Moscow: Mir, 1980, pp. 13–35. Fraser, G., Ellis, D., and Eggins, S., Zirconium Abundance in GranuliteFacies Minerals, with Implications for Zircon Geochronology in HighGrade Rocks, Geology, 1997, vol. 25, pp. 607–610. Geisler, T., Schaltegger, U., and Tomaschek, F., ReEquili bration of Zircon in Aqueous Fluids and Melts, Elements, 2007, vol. 3, pp. 43–50. Golubev, V.N., Chernyshev, I.V., Kotov, A.B., et al., The Strel’tsovka Uranium District: Isotopic Geochronological (U–Pb, Rb–Sr, Sm–Nd) Characterization of Granitoids and Their Place in the Formation History of Uranium Deposits, Geol. Ore Deposits, 2010, vol. 52, no. 6, pp. 496– 513. Gurvich, S.I. and Bolotov, A.M., Titanotsirkonievye rossypi Russkoi platformy i voprosy poiskov (Ti–Zr Placers in the Russian Platform and Their Prospecting), Moscow: Nedra, 1968. Hörmann, P.K., Raith, M., Raase, P., et al., The Granulite Complex of Finnish Lapland: Petrology and Metamorphic Conditions in the Ivalojoki–Inarijrvi Area, Geol. Surv. Finl. Bull, 1980, no. 308, p. 1–95. GEOLOGY OF ORE DEPOSITS
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