Mineralogy and Petrology (1996) 57:243-260 Mineralogy

p o%gy © Springer-Verlag 1996 Printed in Austria

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite

R. L. Romer ~, S.-A. Smeds 2, and P. (~ern~ a

1 Department of Applied Geology, Lulegt University, Lulefi, Sweden 2 Institute of Earth Sciences, Mineralogy-Petrology, Uppsala University, Uppsala, Sweden 3 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada

With 5 Figures

Received July 25, 1995; accepted November 16, 1995

Summary

Columbite-tantalite can accommodate considerable amounts of uranium and excludes lead almost completely. It is thus potentially suited for U-Pb dating. Discordias generally yield closely constrained upper intercept ages. Yet, in a few cases, the U-Pb system of columbite-tantalite gives poorly constrained ages due to excess scatter or inverse discordance. These anomalous features do not show any relation with gross mineral chemistry or pegmatite fractionation. Recrystallization or the presence of exsolutions and inclusions generally do not result in anomalous and scattered U-Pb systematics, unless the inclusions are uraninite or secondary Nb, Ta-bearing phases. Open-system behavior of uraninite, whose occurrence seems to be related to the local host-rock- influenced redox conditions during pegmatite crystallization, often results in inverse discordance. Secondary Nb, Ta-bearing phases that accept Pb and U derived from uraninite inclusions into their structure may inherit this inverse discordance.

Zusammenfassung

Kristallchemische und bildungsbedingte Einfliisse auf die U-Pb Systematik yon Columbit- Tantalit

Columbit-Tantalit kann bedeutende Mengen Uran aufnehmen and schlieBt Blei beinahe vollst~indig aus. Deshalb ist das Mineral potentiell zur U-Pb Datierung geeignet. Im allgemeinen ergeben Diskordias gut definierte obere Schnittalter. Doch in einigen FNlen ergibt das Kolumbit-Tantalit U-Pb System aufgrund yon gestreuten order invers diskor- danten Daten, welche keinen systematischen Zusammenhang mit der Mineralchemie oder dem Fraktionierungsgrad des Pegmatites zeigen, schlecht bestimmte Schnittalter.

244 R.L. Romer et al.

Rekristallisation oder Einschliisse bedingen im allgemeinen keine anomale oder gestreute U--Pb Systematik, ausser in FLillen wo die Einschliisse Uraninit oder sekundfire Na, Ta-Phasen sind. Uraninit, dessen Auftreten mit dem lokalen Nebengestein und den Redoxbedingungen wfihrend der Pegmatitkristallisation zusammenzuhiingen scheint, kann als offenes System eine inverse Diskordanz verursachen, welche yon sekund~iren Nb, Ta-Phasen, die Pb und U von Uraninit in ihre Kristallstruktur einbauen, geerbt wird.

Introduction

Complex Nb, Ta-oxide minerals of the columbite-tantalite (Fe, Mn)(Nb, Ta)206 group predominantly occur in highly evolved silicic, peraluminous to alkaline plutonic rocks and their pegmatites. They are also found in carbonatites and tin-bearing granites (Cern~ and Ercit, 1989).

Columbite-tantalite shows continuous substitutions between Nb and Ta and between Fe and Mn. Although the relative roles of external factors (such as temperature, pressure, co-crystallizing other Nb-Ta or Fe-Mn phases, and availabil- ity of complex-forming ions) controlling the composition of columbite- tantalite are not well understood, empirical trends are recognized for different pegmatite catego- ries. Within the same pegmatite body and commonly even within the same crystal, Mn generally increases with progressing crystallization. Usually, increased Mn- contents also are associated with increased Ta contents. In the Ta/(Ta + N b ) - Mn/(Mn + Fe) diagram (Fig. 1), thus, primitive columbite tends to plot in the left lower corner, whereas highly evolved tantalite generally falls closer to the upper

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Fig. ~. Ta/(Ta + Nb) - Mn/ (Mn + Fe) compositional space (atomic ratios) showing the chemical variability of the analyzed columbite-tantalite. Shaded area: Yellow muscovite reflecting reducing conditions of pegmatite crystallization. See Table 2 for representative com- positions

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 245

right corner (e.g., Cern~ and Ercit, 1985, 1989). The substitution trends are highly variable and may be very different even if genetically related pegmatites are compared (Cern¢ et al., 1986). Other elements that substitute into the columbite structure are Sn, W, U, Ti, and REEs. Uranium contents may reach several permilles and, therefore, columbite-tantalite has been potentially interesting for U-Pb dating since the early days of U-Pb geochronology (e.g., Aldrich et al., 1956). However, because of its tendency to behave as an open system and to yield variably normal and inverse discordant data, columbite-tantalite was never widely used. Recently, Romer and Wright (1992) demonstrated that leaching results in less discordant data and yields closely constrained ages that coincide with ages obtained from U-Pb zircon dating on cogenetic rocks. In some cases, however, leaching does not remove or diminish discordance and excess scatter about the discordia. These samples gen- erally yield unprecise ages.

We investigated columbite-tantalite samples from pegmatites in low-P meta- morphosed greywackes and acid gneisses using electron microprobe data and information from back-scatter electron images and leaching behavior. These colum- bite-tantalite samples have been dated earlier using U-Pb (Romer and Wright, 1992; Romer and Smeds, 1994, 1996). As not all columbite samples gave a closely constrained age, but showed excess scatter or inverse discordance, we investigated which factors influence the U-Pb systematics of columbite-tantalite.

Columbite-tantalite samples: provenance and heterogeneity

The investigated columbite-tantalite samples come from LCT-type (Li, Cs, Ta- enriched, peraluminous, mainly S-granite-derived) pegmatites in various grades of fractionation (Table 1). The only exception is the Skuleboda pegmatite, which represents a mixed NYF (Nb, Y, F-enriched, subaluminous, mainly A-granite- derived) type in the classification of Cerny: (1991). The pegmatites carry mainly beryl and columbite (Smeds, 1990, 1992; Romer and Smeds, 1994), but some of the more evolved pegmatites contain additionally spodumene, petalite, and locally pollucite (Orrvik, Stenbackberget, Nyk6pingsgruvan, Varutr~sk; Table 1).

Backscatter electron images and electron microprobe analyses were made on polished sections from the samples previously used for U-Pb dating (Romer and Wright, 1992; Romer and Smeds, 1994). The main interest was to establish whether chemical heterogeneities, inclusions, and traces of recrystallization and alteration of columbite-tantalite could be related to the behavior of the U-Pb systematics of columbite-tantalite. Of special interest were possible inclusions of uraninite as Romer and Wright (1992) suggested the presence of such inclusions based on the leaching behavior of columbite. Textural characteristics of the analyzed grains include oscillatory compositional zoning, veins, diffuse compositional heterogenei- ties, dendritic and skeletal intergrowths, and inclusions of other phases. Note that in continuation the term inclusion is used for all isolated grains enclosed in columbite- tantalite, disregarding whether they are true primary, early-crystallized inclusions senso stricto, or of late origin by exsolution or replacement. The term inclusion as used here, thus, does not have any genetic implications.

Unaltered magmatic columbite-tantalite can be chemically homogeneous to nearly homogeneous or distinctly zoned. Zoned samples generally show an oscilla-

246 R.L. Romer et aL

Table 1. Characterization of Swedish columbite-tantalite bearing pegmatites dated using the U-Pb method. References to original description of mineralogy and U-Pb geochronology

Locality Pegmatite category according to References Cern¢ (1991)

Norr6 LCT: beryl-columbite-phosphate

Rgm6 Nyk6pingsgruvan Lill-Gullberget,

Hoting Varutrfisk Vassijaure RSnegt

Orrvik

Stenbackberget

Dyngseiet

Perstorp Skuleboda

LCT: beryl-columbite-phosphate LCT: petalite/lepidolite LCT: beryl-columbite

LCT: petalite/lepidolite LCT?: columbite LCT: beryl-columbite

LCT: spodumene

LCT: beryl-columbite-phosphate/ spodumene

LCT: beryl-columbite-phosphate

LCT: beryl-columbite mixed NYF-LCT?

Smeds (1990), Romer and Smeds (1994)

Romer and Smeds (1994) Teertstra et al. (in prep.) Romer and Smeds (1994)

Teertstra et al. (in prep.)* Romer and Wright (1992) Smeds (1990), Romer and Smeds

(1994) Smeds (1990, 1992), Romer and

Smeds (1994) Smeds (1990, 1992), Romer and

Smeds (1994) Smeds (1990, 1992), Romer and

Smeds (1994) Smeds (1990) Smeds (1990), Romer and Smeds

(1996)

* Teertstra DK, Cern~ P, LanghofJ, Smeds S-A, Grensman F (in prep) Pollucite in Sweden: occurrences, crystal chemistry, petrology and subsolidus history

tory zoning or a quasi-lamellar texture (Fig. 2a) that originates from variable Nb/Ta. Late-crystallized parts tend to be richer in tantalum than the bulk of the grain. Because of the abundance of supercritical and hydrothermal fluids during late crystallization stages of pegmatites, columbite-tantalite is commonly affected by alterations, recrystallization, and fracturing. Thus, the primary magmatic texture is commonly overprinted by secondary features such as Ta-rich veinlets, quartz veins, irregular Ta-rich and Ta-Fe-rich blebs that cut across the primary zonation (Fig. 2b). Locally, secondary Ta, Nb-bearing phases, such as tapiolite, skeletal ixiolite, or wodginite, have formed at the expense of older columbite-tantalite (Fig. 2c). However, the origin of some of them is uncertain; primary inclusions also may be present. Columbite-tantalite from Norr6, Rfin6, Nyk6pingsgruvan, Hoting, Orrvik, Varutrfisk, Vassijaure, and Perstorp do not show any inclusions. In contrast, columbite-tantalite from Dyngselet, Stenbackberget, and Skuleboda have abundant inclusions that generally are smaller than 10 ~tm, but locally may be up to 30 ~tm in diameter. The dominant types of inclusion are uraninite and Bi-sulfides, whereas inclusions of cassiterite and native Bi are rare (Fig. 2d).

The chemical heterogeneity of columbite-tantalite is also documented by leach- ing with diluted HF, as Ta-rich phases seem to be less resistant to leaching than Nb-rich compositions (Figs. 2e and 2f). Generally, columbite-tantalite samples are

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 247

Fig. 2. a-d Backscatter electron images of columbite-tantalite, a Oscillatory zoning in an inclusion-free ferrocolumbite, crosscut by veinlets of a Ta-enriched columbite phase (white), approx. 0.5 gm thin (R~me~). b Near-homogeneous manganocolumbite (grey) with veinlets of a Ta-enriched manganocolumbite (white; Nyk6pingsgruvan). e Near-homogeneous tanta- lian ferrocolumbite (grey) with skeletal aggregates of ixiolite or wodginite (white; Orrvik). d "Ferromanganocolumbitetantalite" (grey; cf. Fig. 1 for composition), with faint complex zoning of slightly variable Nb and Ta content; Nb-enriched areas (darker) contain grains of uraninite (white), 8 gm or less in size (Stenbackberget-5). e - f HF-leached columbite-tantalite samples illustrating their mineralogical and textural heterogeneity, e Irregular, corroded Ta-enriched inclusion in apparently uncorroded columbite. Quartz-filled fractures appear as intersecting voids. They seem to occur predominantly in the inclusion (Timmerhault; Romer and Smeds, 1996). f Chemically heterogeneous columbite-tantalite sample. Heterogeneity is reflected in both porosity and intensity of grey tone. Fractures formerly filled with quartz appear as open fissures. Fractures filled with a secondary Nb, Ta-phase form a suite of parallel ridges that have been more resistent to leaching (Ruhembe; Romer and Lehmann, 1995)

248 R.L. Romer et al.

heterogeneous with many mineralogically different inclusions that originate from different stages of alteration, but may be in part primary. In a few cases they seem to represent products of exsolution or recrystallization. In other cases, they are related to the fracturing of columbite-tantalite and the healing of the fractures with externally derived material (Figs. 2e and 2f).

Host-rock lithologies

The sampled pegmatites occur in a variety of lithologies in various grades of metamorphism ranging from sillimanite-grade gneisses and migmatites (Rfin6, Norr6, Hoting, and Rgmegt) to andalusite-biotite-muscovite-bearing rocks at lower amphibolite grade (Varutrfisk, the Sidensj6 area pegmatites Orrvik, Dyngselet, and Stenbackberget, and Nyk6pingsgruvan on Ut6).

Wall and country rocks may locally have significant influence on the crystalliza- tion conditions of the pegmatites, as interactions between wallrock and pegmatite melts may result in contamination or buffering of the melts. For instance, at Nyk/Spingsgruvan on Ut6 Island, pegmatites are emplaced in an oxidic iron formation that is associated with marbles, skarns, and felsic metavolcanic rocks (Smeds and Cern¢, 1989). The pegmatite minerals are characterized by markedly enhanced lead contents, locally leading to amazonitic K-feldspar and Pb-enriched microlite (Smeds and Cerny:, 1989). The enhanced lead contents are interpreted to be due to contamination from the host-rocks that show abundant minor sulfide disseminations.

Graphite- or sulfide-bearing host-rocks may keep the pegmatitic melt at rather reduced conditions, once they start to exchange and mix their volatile components, or the melt digests appreciable quantities of wall rock. In contrast, highly oxidized host-rocks may cause pegmatite crystallization under more oxidized conditions. The Rfin6, Norr6, Hoting, Varutr/isk, Rgme~, and the Sidensj6 (Dyngselet, Stenbackber- get, and Orrvik) pegmatites are all emplaced in locally turbiditic metasedimentary host-rocks that usually show minor sulfide- and graphite-bearing intercalations (e.g., Smeds, 1993). Most special is the Dyngselet pegmatite, which shows locally abundant graphite lumps that occur at the contacts and exceptionally in the dikes. These lumps may be up to one decimeter large (Smeds, unpubl, data). Several pegmatites also have variably abundant mafic host-rocks. For instance, the dikes Stenbackberget and Dyngselet occur in metasedimentary host-rocks that show minor intercalations of amphibolites, whereas the dikes Orrvik, Varutr~sk, and Vassijaure are entirely hosted by amphibolite intercalations in the metasediments. The kind of dominating host-rock is plotted in the Ta/(Ta + ( N b ) - Mn/(Mn + Fe) diagram (Fig. 3n) to

Fig. 3. Qualitative variation of trace-element contents in columbite, and of pegmatite properties, in relation to main element composition of columbite-tantalite. All eolumbite data from Fig. 1 are shown as small squares. For samples with small or large contents of the considered trace element, repectively, the square is replaced with a different symbol. Shaded area in Figs. 3b, 3g, 3i, 3j, 31, 3n, and 30 indicates the occurrence of yellow muscovite and reflects reducing crystallization conditions. Contents of U and common lead, as well as isotopic composition of lead are taken from Romer and Wright (1992) and Romer and Smeds (1994, 1996)

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 249

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illustrate the relation between host-rock characteristics and columbite-tantalite composition. Columbite-tantalite from variably metamorphosed greywackes and arenitic sediments seems to fall in two distinct fields depending on metamorphic grade. Columbite-tantalite from metagreywackes seems to have intermediate Ta/(Ta + Nb) and Mn/(Mn + Fe), whereas those from the stronger metamorphosed gneisses have low Ta/(Ta + Nb) and Mn/(Mn + Fe). Columbite-tantalite fi'om acid magmatic host-rocks seem to have high Mn/(Mn + Fe). However, these correlations probably are incidental to the regional distribution of different types of pegmatites, as the above ratios are primarily controlled by pegmatite fractionation, which tends to increase Mn and Ta contents ((2ern¢ et al., 1985). Fe precipitates early in pegmatites, and Fe, Mn-bearing minerals such as garnet, posphates, tourmaline, and columbite-tantalite generally become enriched in Mn with increasing fractiona- tion (Cern¢ et al., 1985). For Nb, Ta-bearing minerals a corresponding general trend goes towards Ta-rich members with increasing fractionation ((~ern~ et al., 1985).

The redox conditions become generally more oxidizing during pegmatite frac- tionation, and they are controlled by the host-rocks probably only in the marginal part of the vein. The above described pegmatites with locally reduced crystallization conditions deviate from the general pattern of pegmatite-melt fractionation due to the increased involvement of volatile phases derived from the host-rock. The redox conditions are in part monitored by the Fe 3 +/Fe 2+ of accompanying Fe-bearing minerals, which are not very common in pegmatites. A rough estimate of the redox conditions may be obtained from muscovite, whose color changes in dependence of Fe 3 +; white to yellow muscovite is Fetot- and Fe 3 +-poor, whereas reddish-brownish "ruby mica" has enhanced Feto t and Fe 3+ contents (Smeds, 1992). Among the investigated columbite samples, only those from Norr6, R~n6, Hoting, R~negt, and Varutrfisk are associated with ruby mica. Using the color of muscovite as an indicator for the redox-condition during the pegmatite and columbite crystalliza- tion, there seems to be no systematic subdivision of the cotumbite-tantalite composi- tional space in the Ta/(Ta + Nb) - Mn/(Mn + Fe) diagram (Figs. 1 and 3).

Relation of columbite-tantalite composition to U-Pb systematics

The data originate from several different pegmatite fields. Most columbite data originate from variably evolved LCT-type pegmatites emplaced in low-pressure metamorphosed greywackes. The four main metals Ta, Nb, Fe, and Mn entering columbite-tantalite minerals show paired substitutions, Nb-Ta and Fe-Mn. Thus, the whole compositional range of minerals such as columbite-tantalite, wodginite, ixiolite, and tapiolite is most easily displayed in a Ta/(Ta + Nb) - Mn/(Mn + Fe) diagram (Fig. 1). The investigated columbite-tantalite samples define coherent groups in this quadrilateral from low Mn/(Mn + Fe) for geochemically primitive pegmatite dikes to increasingly higher Mn/(Mn + Fe) in more evolved dikes (Fig. l; see Table 2 for selected representative compositions). Similarly, Ta/(Ta + Nb) varies from low values in primitive pegmatites to intermediate values (as high as 0.6) in intermediately to highly evolved ones (Fig. 1). This is in agreement with general evolution trends of columbite-tantalite as observed on different scales of single grains, single pegmatites, and cogenetic pegmatite swarms. Among the analyzed columbite-tantalite samples, only the one from Varutr/isk does deviate from the

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 251

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Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 253

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general distribution of the other samples (Fig. 1). This columbite sample originates probably from a less evolved outer zone of this otherwise extremely fractionated rare-element pegmatite.

The variation of trace components in relation to the variation of the main components of columbite-tantalite is shown in Fig. 3. Trace-element contents, superposed on the Ta/(Ta + Nb) - Mn/(Mn + Fe) diagram, are shown here quali- tatively at three levels that differ by one order of magnitude. Most prominent in these variation diagrams is the positive correlation among the contents of Ti, W, and Mg. These elements reach high contents in columbite-tantalite samples from poorly and moderately evolved pegmatites, whereas their content is very low in columbite- tantalite samples from highly evolved pegmatites (Figs. 3b, 3c, 3d, 4). Sn does not correlate with the other trace elements (Figs. 3a-3d).

Many samples with high contents of Ti, W, and Mg (Figs. 3b, 3c, 3d) also have rather high contents of U (Fig. 3g). These samples do not show any inclusions of uraninite (Fig. 31), i.e., the high U contents seem to be due to U in the columbite structure. High contents of W and Mg in the columbite structure may possibly generate structural sites and local charge balancing favorable for the incorporation of uranium (Fig. 4). Uranium occurs in a few cases in a phase of its own, i.e., uraninite (Fig. 31). Samples with uraninite inclusions seem to have always low contents of W and Mg (Figs. 3b-d, 31, 4). We suggest that this apparently mutually exclusive occurrence of uraninite inclusions and high contents of W and Mg could be controlled to some extent by the redox conditions during columbite crystallization. Under reduced conditions, uranium occurs as U 4+ and tungsten probably is not available as W 6 + whereas under more dxidized conditions uranium occurs as U 6 ÷ and tungsten is present in the melt or supercritical solution. U 4 ÷ probably does not substitute into the columbite structure, whereas U 6 ÷ and W 6 ÷ do. Thus, redox- conditions would control the availability of the element and its compatibility with the columbite structure. Ionic radii of U 6 ÷ and U 4 ÷ in sixfold coordination with

254 R.L. Romer et al.

oxygen are 0.73 and 0.89,~, respectively, whereas corresponding radii for Mg 2+, Fe 2 ÷, and Mn 2 + are 0.72 ~, 0.78 A, and 0.83 A, respectively (Shannon, 1976). Thus, U 4 ÷ is too large to substitute in the columbite structure. This reasoning corrobor- ates with the qualitative estimation of the local redox-conditions during the colum- bite crystallization using the color of muscovite. Columbite with uraninite inclusions (detectable on the microprobe) is confined to samples associated with yellow mica (Fig. 3l).

Arbitrarily defining all U-Pb columbite upper intercept ages with a 2o- uncertain- ty larger than 10 Ma to be poorly constrained ages (see Appendix), all columbite data are plotted in the composition diagram for columbite-tantalite (Fig. 3i). There seems to be no systematic relation between the quality of the columbite-tantalite U-Pb age and the chemical composition of the mineral with respect to its main constituents as well as its trace-element contents. As the variation of Nb-Ta and Fe-Mn strongly reflects the extent of fractionation of the columbite-tantalite-bearing pegmatite, there is apparently no relation between pegmatite category and age quality, i.e., columbite-tantalite samples may render closely constrained ages independent of the overall type or internal zone of the parent pegmatite.

Causes for scattered and inverse discordant U-Pb systematics in columbite-tantalite

Fractionation of uranium and lead in the dated mineral by geological and labora- tory processes results in disturbed geochronologic systems. Samples that suffered one single disturbance define a linear array that intercepts the concordia at ages corresponding to the time when the system was established and disturbed, respect- ively. Laboratory-induced fractionation yields lower intercepts at zero age. Several fractionation events result in scattered data, and discordia lines fitted to these data may yield fortuitous intercept ages.

Romer and Wright (1992) argued that laboratory-induced U-Pb fractionation due to HF-leaching is not relevant because stronger leaching resulted in more concordant data (Fig. 5a). This was later confirmed by Romer and Smeds (1996) who obtained concordant and close to concordant U-Pb systematics in strongly leached and partially dissolved columbite-tantalite samples. For instance, the strongly corroded and partially dissolved columbite sample from Timmerhault (Fig. 2e) yields five concordant fractions (Fig. 5b). This demonstrates that leaching does not selectively remove uranium or lead from columbite-tantalite samples.

Leaching seems to be more efficient for fine-grained samples than for coarse- grained samples. Even mineralogically highly heterogeneous columbite-tantalite samples involving several chemically different columbite-tantalite phases (see Fig. 2) can result in concordant systems if sufficiently severely leached. Concordant U-Pb systematics in mineralogically heterogeneous columbite-tantalite samples imply that (1) there had been several phases present already when columbite-tantalite formed, (2) the exsolutions formed very soon after the crystallization of columbite- tantalite and are therefore not resolvable in the geochronologic system, or (3) exsolution and recrystallization did not result in a fractionation and redistribution of U and Pb. Independent on which explanation may apply, the presence of several phases is not sufficient reason to yield highly scattered or inverse discordant systems.

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 255

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• ~1:22o (MSWD= 5.8) 700 f / VTdO r"X', . . . . i ~ I , I ,

5 6 7 2OTpb/Z3s U

Fig. 5. Concordia diagram for Swedish columbite-tantalite samples illustrating different kinds of U-Pb systematics (original data reprinted in Table A2). a single stage lead loss; data do not show excess scatter about the discordia; more strongly HF-leached samples (5, 10, and 20) are increasingly more concordant (Romer and Smeds, 1994). b Five severely HF-leached samples. Columbite is strongly corroded (Fig. 2e). The concordant nature of the sample demonstrates that uranium and lead do not fractionate during the leaching process (Romer and Smeds, 1996). e Five variably leached samples displaying lead loss. The excess scatter (large MSWD) demonstrates that there were several events of lead loss (Romer and Smeds, 1994). d Five variably HF-leached samples show decreased discordance at higher degree of leaching. The initial inverse discordance and the non-zero lower intercept ages indicate uranium loss in the past. The data set shows excess scatter (Romer and Wright, 1992)

The isotopic systematics of mineralogically distinct phases in columbite-tantalite that have formed during a later event may have been completely or partially reset. Exsolutions may inherit non-ideal U-Pb systems if the isotopic systematics of the parent mineral was disturbed. If the new phase shows similar leaching behavior as the hosting columbite-tantalite, i.e., it is not preferentially dissolved, such an inherited non-ideality will persist and the sample will never yield a concordant system. Instead, these systems show normal or inverse discordance at any extent of leaching. If there was only one fractionation event between uranium and lead, then the samples will fall on a straight line that intersects the concordia at ages corresponding to the time when the system was established and disturbed, respect- ively. However, if there were several disturbances of the uranium-lead system, the data will show excess scatter about the discordia (Fig. 5c).

Inverse discordant U-Pb systematics are very common in uranium ores as the solubility of uranium is very different from that of lead; uranium occurs as oxide or

256 R.L. Romer et al.

hydroxide, and uranium may be mobilized by simple oxidation reactions (Langmuir, 1978; Bell, 1985). As all investigated columbite-tantalite samples with inverse discordant U-Pb systematics have inclusions of uraninite (Figs. 3h and 31), the anomalous behavior of the U-Pb system is interpreted to be due to uranium loss (Fig. 5d and Appendix). Lead gain, which also results in inverse discordant U-Pb systems, is considered less likely, as columbite-tantalite has a very low acceptance of lead in its crystal structure.

Inclusions alone do not necessarily result in disturbed systems. However, if they formed much after columbite-tantalite had crystallized, they may give rise to disturbed U-Pb systems. For instance, the formation of tapiolite, wodginite, or ixiolite in columbite-tantalite during the same process that opens the U-Pb system of uraninite may result in fractionation of the U-Pb system of the whole mineral. Secondary Ta, Nb-bearing minerals may in part inherit an inverse discordant U-Pb system. As tapiolite, wodginite, and ixiolite may have a highly different leaching behavior than uraninite, the laboratory treatment would resolve the inverse discord- ance originating from the partially opened uraninite, yet it would fail to remove the inherited inverse discordance bound to the secondary Ta-Pb-phases.

Summary Columbite-tantalite may yield precise U-Pb ages. Yet in some cases, the age is poorly constrained or the data plot inversely discordant in the concordia diagram. There seems to be no relation between the U-Pb systematics of columbite-tantalite samples and their chemical composition, extent of pegmatite evolution, or kind of host-rock. In contrast, the local redox-conditions during columbite-tantalite crystallization seem to have major influence whether inverse discordant systems develop. If columbite-tantalite formed under reduced conditions, uranium was present as U 4 ÷ and precipitated mainly as uraninite inclusions, whereas crystallization under oxidized conditions caused uranium to be incorporated as U 6÷ in the crystal structure of columbite-tantalite.

Veining, oscillatory grain growth, recrystallization, replacement, and inclusion of other phases seem to have no effect on the U-Pb systematics of columbite- tantalite, unless the inclusions are uraninite. Apparently unaffected U-Pb systems may reflect that most secondary textural features formed during the late-magmatic and hydrothermal alteration of the pegmatites, that are geochronologically not resolvable from the crystallization of the primary columbite-tantalite, or represents closed-system redistributions. In contrast, recrystallization involving disturbed uraninites will result in discordant and/or scattered systems that can not be improved by leaching.

Acknowledgment We thank E. Schiirli (Z/irich) for careful preparation of the polished section and R. Chapman (Manitoba) for the electron microprobe work, which was supported by NSERC of Canada Research and Major Installation grants to P. Cern¢. We are grateful to U. Sturesson (Uppsala) for the backscatter electron images of the leached columbite samples. Constructive journal reviews by U. I-Iaack and an anonymous reviewer are greatly appreciated.

Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 257

Appendix Figure 3 includes in part, information published in earlier geochronol0gic studies. The results of these earlier geochronologic studies are summarized in Table A1, to the extent they are relevant for the present work. The distinct increase in radiogeneity of the lead isotopic composition in samples that were leached with diluted HF (e.g., Table 2A) indicates that the common lead component is easier removed by leaching than the radiogenic lead component (see Romer and Wright, 1992; Romer and Smeds, 1994). Figure 5 is based on the data given in Table A2.

Table A1. Compilation of earlier published geochronologic information used in Fig. 3h to 31. Data sources given in the text

Sample

Pbeommon U a unleached/ 2°6pb/2°4pb unleached leached b unleached/ age c [ppm] [ppm] leached b [Ma]

discordia d inclusions e

Vassijaure 600 1.2-1.5 4000 1795 + 5 d,i - 0.1-0.7 41000

Rgme5 400 3-6 1000 1765 ± 14 d - 0.1-0.8 75000

Varutr~isk 450 18 480 1775 ± 11 i,c - < 1 50000

Hoting 460 100 1100 1803 ± 27 i,d - < 1 29000

Stenbackberget-5 600 6 1700 1797 ± 7 i, c u, i/w 0.3 21000

Stenbackberget-9 550 6-12 460 1760± 10 d u 2-4 1750

Orrvik 1000 1 13000 1795 ± 6 i, c i/w O.O3 96OO0

Dyngselet 180 1 2700 1799 _ 11 d u, Bi, t, i/w 0.03-0.3 65000

NykSpingsgruvan 3-20 40 19.5 1821 + 16 i ,c,d - 4-6 123

R~n6 400 > 100 850 1819 ± 4 d - 0.5-6 8000

Norr6 490 1 8300 1819 _+ 4 d - 0.06 120000

Perstorp 2400 0.7 4000 1750 ± 37 d - 0.1 1O0OO

Skuleboda 5700 40 1040 984 ± 6 d, c, i Bi, c, BPU 5 3200

aAverage value of 2 to 4 unleached samples, bSamples leached with 5 to 20% HF. Minimum value of unleached samples, maximum value for leached samples. CIntercept age and 2a uncertainties, dd discordant, i inverse discordant, c concordant, eInclusions identifiable on BSE (back-scatter electron) images; t tapiolite, w wodginite, i ixiolite, c cassiterite, u uraninite, Bi native bismuth or bismuth sulfide, BPU unidentified mineral with Bi, Pb, U, possibly also Ta and Nb. For mineral textures (fractures, veins, and zoning) see text

258 R.L. Romer et al.

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Crystal-chemical and genetic controls of U-Pb systematics of columbite-tantalite 259

Vassijaure. Normal and inverse discordance in unleached samples; normal discordance in leached samples (Romer and Wright, 1992). No inclusions identified.

Rgme& Five discordant samples; at higher degree of leaching they are increasingly less discordant; large excess scatter of the discordia (Romer and Wright, 1992). Strongly oscilla- tory zoning, but no inclusions.

Varutriisk. Untreated samples are inversely discordant (Fig. 5d); HF-leached samples are concordant, but show excess scatter (Romer and Wright, 1992). Homogeneous, inclusion-free sample in part with minor veinlets of Ta-enriched columbite.

Hoting. Normal and inversely discordant samples (Fig. 5c); degree of discordance by large correlates negatively with the extent of HF-leaching; large excess scatter of the discordia (Romer and Smeds, 1994). Highly heterogeneous granular aggregates with oscilla- tory zoning. In part younger overgrowths of Ta-rich irregular blebs.

Stenbackberget-5. Inverse discordant samples; most strongly leached samples are con- cordant (Romer and Smeds, 1994). Homogeneous; local Nb-rich domains contain rare inclusions of uraninite (< 10 gin) and ixiolite or wodginite (up to 30 gm).

Stenbackberget-9. Discordant samples, increasingly less discordant at higher degree of HF-leaching (Romer and Smeds, 1994). Rare uraninite inclusions ( < 10 gm) in heterogeneous to patchy ferrocolumbite to ferrotantalite.

Orrvik. One leached and two unleached samples are slightly inversely discordant, whereas two strongly leached samples are concordant (Romer and Smeds, 1994). Late aggregates of dendritic aggregates of stannian ixiolite or wodginite; no inclusions.

Dyngselet. All samples are slightly discordant, but untreated samples are more discordant (Romer and Smeds, 1994). Patchy aggregate with inclusions of tapiolite, vermicular ixiolite or wodginite, and rare uraninite (< 10 gm) and bismuth.

Nyk@ingsgruvan. Inverse discordance but in one leached sample. Distinct from other columbite-tantalite samples by its exceedingly low contents of uranium (3 to 23 ppm; Romer and Smeds, 1994). No inclusions, gut irregularly distributed Nb and Ta.

Rgm& Variable discordance; no systematic relation between degree of discordance and HF-leaching. Quasi-lamellar texture of columbite with variable Nb/Ta. Ta-rich spots, but no other inclusions (Romer and Smeds, 1994).

Norr6. Inverse discordance that at higher degree of leaching decreases (Romer and Smeds, 1994). Nearly homogeneous grains with rare minor Ta-enriched veinlets; no inclusions.

Perstorp. Decreasing normal discordance at higher degree of leaching; strong excess scatter, 1750 +__ 37 Ma age is anomalously low in comparison with the age on an undisturbed sample from the same pegmatite swarm (Romer and Smeds, unpubl.). Homogeneous to slightly zoned, rare veinlets, no inclusions.

Skuleboda. Decreasing normal discordance at higher degree of leaching; most strongly leached sample is slightly inversely discordant (Romer and Srneds, 1996). Rare inclusions of cassiterite, native Bi or Bi-sulfide, and a Bi, Pb >> U-bearing phase.

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Authors' addresses: R. L. Romer, Department of Applied Geology, Lule& University, S-97187 Lnlefi, Sweden; S.-A. Smeds, Institute of Earth Sciences, Mineralogy-Petrology, Uppsala University, Norbyv. 18B, S-75236 Uppsala, Sweden; P. Cern¢, Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2