©2007 Society of Economic Geologists, Inc. Economic Geology, v. 102, pp. 257–276
Textural Features and Chemical Evolution in Tantalum Oxides: Magmatic Versus Hydrothermal Origins for Ta Mineralization in the Tanco Lower Pegmatite, Manitoba, Canada MARIEKE VAN LICHTERVELDE,†,* STEFANO SALVI, DIDIER BEZIAT, LMTG-Université de Toulouse-CNRS-IRD-OMP, 14 Avenue Edouard Belin, 31400 Toulouse, France AND
ROBERT L. LINNEN
Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Abstract Tantalum, a key element in the electronics industry, is produced mainly from rare element granitic pegmatites. Although their internal structure, mineralogy, and petrogenesis have been extensively investigated, the processes that control tantalum mineralization remain poorly understood, in particular the role of fluids in the crystallization of tantalum ore. One of the major problems arises from the difficulty in distinguishing primary magmatic from secondary, hydrothermal textures in such complex rocks. In the Tanco pegmatite, Manitoba, Canada, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Eight different families of tantalum oxides occur in various associations and compositions. The Tanco Lower pegmatite is an isolated body beneath the main pegmatite body that contains abundant tantalum associated with mica alteration. Tantalum mineralization in the Tanco Lower pegmatite occurs as three different styles. Facies 1 is hosted by the wall zone and hosts primary magmatic Ta oxides with simple textures (progressive and oscillatory zoning). Facies 2 is hosted by the lower and upper intermediate zones where most mineralization occurs with dendritic amblygonite. Facies 3 is hosted by the central zone, which is affected by mica alteration. In this latter facies, the oxides show particularly complex textures evident from X-ray mapping. By combining information obtained from textural observations and chemical analyses, we are able to determine the paragenesis for each Ta oxide-bearing mineral assemblage and hence to evaluate the relative contributions of magmatic versus hydrothermal processes. In complex associations, we observe relics of primary Ta phases that are replaced or overgrown by secondary Ta phases. We propose the paragenetic sequence: columbite group minerals + microlite (early primary magmatic) → columbite + wodginite group minerals + microlite (late primary magmatic) → wodginite group minerals + microlite (secondary magmatic) → ferrotapiolite (secondary magmatic). In addition, chemical variations were identified in the columbite and wodginite group minerals, both at the crystal scale and at the pegmatite scale. Columbite and wodginite group minerals show the typical Ta* = Ta/(Ta + Nb) and Mn* = Mn/(Mn + Fe) (atomic ratios) increase from earlier to later zones. At the crystal scale, the increase in Ta* and Mn* with fractionation in the columbite group is explained by the higher solubility of the Ta end member relative to the Nb end member and the crystallization of other minerals that shift the melt composition toward Mn* enrichment. The fractionation trend in the wodginite group shows Fe enrichment, which is consistent with experimental results that show a higher solubility of the Fe end member relative to the Mn end member in columbite group minerals. Considering the available experimental data, as well as the intimate association of Ta oxides with zircon observed in this study, we conclude that tantalum mineralization is a product of direct crystallization from the melt rather than hydrothermal in origin. Fluids are attributed an indirect role only, as they could have brought minor elements (Fe, Mn, or Ca) into the melt, which resulted in the crystallization of secondary Ta phases. Textural evidence shows that the late secondary phases were formed at the same time as mica alteration. We suggest that mica alteration was due to a late melt, rather than a hydrothermal fluid, that reacted with the blocky K-feldspar of the central zone. During the alteration event, tantalum was remobilized from primary phases and incorporated in new, secondary Ta oxide species.
Introduction THE DEMAND for tantalum in the current electronics industry has steadily increased with the advent of laptop computers and cellular phones (Cunningham, 2003; Fetherston, 2004). The most common host rocks for Ta mineralization (i.e., rare element granitic pegmatites) have been extensively studied, including their internal structure (e.g., Cˇern´y, 1991), their mineralogy (in particular Nb-Ta mineralogy: Cˇern´y and Ercit, † Corresponding author: e-mail,
[email protected] *A digital supplement to this paper is available at or, for members and subscribers, on the SEG website, .
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1985; Ercit, 1986), and their petrogenesis (Jahns and Burnham, 1969; London, 1990; Cˇern´y et al., 2005). However, the processes that control tantalum mineralization remain poorly understood. Only a few studies have examined the role of aqueous or aqueous-carbonic fluids in tantalum ore formation (Thomas et al., 1988; Linnen and Williams-Jones, 1993; Beurlen et al., 2001). To our knowledge, very few studies have provided insight into the relative importance of direct crystallization from the melt versus hydrothermal processes in tantalum mineralization (e.g., Lumpkin, 1998; Novàk and Cˇern´y, 1998; Marignac et al., 2001; Kontak, 2006). The reason for this is the difficulty in distinguishing primary magmatic
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Geologic Setting The Tanco pegmatite is located 180 km northeast of Winnipeg, Manitoba. It is a member of the Bernic Lake pegmatite group and occurs in the Bird River greenstone belt that flanks the exposed part of the Bird River subprovince of the Archean Superior province, in the southwestern part of the Canadian Shield (Fig. 1). It is a highly fractionated pegmatite of the rare element class, petalite subtype (LCT family of Cˇern´y, 1991). It is one of the world’s largest pegmatites, measuring some 1,990 m in length, 1,060 m in width, and up to 100 m thick (Stilling et al., 2006). During the Kenoran orogeny (2.75–2.55 Ga), the Bird River greenstone belt was affected by greenschist facies regional metamorphism and east-west faulting (Cˇern´y et al., 1981). Late to post-tectonic leucogranites were intruded along the east-west–trending faults and are believed to have been the source of a swarm of rare element pegmatites, of which the Tanco pegmatite is the largest and economically the most important. It was emplaced at the end of the orogeny, and its age is estimated at 2640 ± 7 Ma (U-Pb on tantalite: Baadsgaard and Cˇern´y, 1993). It occupies a subhorizontal position within a subvertically foliated metagabbro. The contacts of the pegmatite with the metagabbroic wall rock are knife-sharp and do not show any assimilation of the host rock. The emplacement of the pegmatite was influenced by gently east- and west-dipping joints and fractures (Cˇern´y et al., 1998). Its form is best described as a bilobate, shallowly north-dipping and doubly (E and W) plunging 0361-0128/98/000/000-00 $6.00
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crystallization from secondary, hydrothermal textures in texturally complex pegmatitic rocks. In many rare element pegmatites at least a portion of the tantalum ore is associated with mica alteration, such as Greenbushes, Australia (Partington et al., 1995), Wodgina, Australia (Sweetapple and Collins, 2002), and the McAllister pegmatite, United States (Foord and Cook, 1989). At the Tanco deposit, tantalum is particularly abundant in the central intermediate zone of the pegmatite (Cˇern´y et al., 1998; Stilling et al., 2006), which consists of mica-quartz alteration after microcline (referred to as MQM alteration by Tanco geologists). In this zone, Ta occurs as eight distinct chemical groups in various associations. Such mineralogical diversity reflects the complex petrogenesis of the Tanco pegmatite and provides a unique opportunity to investigate Ta mineralization in both primary magmatic zones and late replacement zones. In a previous study (Van Lichtervelde et al., 2006), we constrained the chemical evolution of columbite group minerals associated with an aplite-hosted style of mineralization, which was interpreted as being magmatic in origin. For the present study, we concentrated our efforts on the Tanco Lower pegmatite, a separate body that occurs beneath the main body, west and south of the Tanco pegmatite, which contains particularly abundant “mica-quartz alteration-style” Ta mineralization. The Tanco Lower pegmatite was discovered in the early 1980s and has not been previously studied. In the present study the complex textures of the tantalum mineralization in the Tanco Lower pegmatite are examined in detail. These textures are then considered in conjunction with the available experimental data to evaluate whether the mineralization was a result of direct crystallization from a melt or hydrothermal processes.
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FIG. 1. Location of the Tanco mine, and east-west and south-north cross sections through the pegmatite, showing the mine openings during the summer of 2004 and the location of the Lower pegmatite. The limits of the Tanco pegmatite are extrapolated from drill hole information. Modified after unpublished Tanco geologic maps.
body, which fingers out into swarms of parallel dikes along most of its margins. The Tanco Lower pegmatite is a smaller, separate body located beneath the southwestern limb of the main body (Fig. 1). It is parallel with, and closely underlies, the main Tanco deposit, separated by ~50 m of metagabbro. Drilling is limited at this depth and there is no evidence of the Tanco Lower pegmatite being connected to the main Tanco body. Although it is probably part of the same intrusion, we will consider the Tanco Lower pegmatite as an independent pegmatite body. Methodology The site was mapped in detail using both diamond drill cores and underground observations. A three-dimensional model based on more than 30 drill holes crosscutting the pegmatite was constructed in order to understand the complex geology of the Tanco Lower pegmatite. Figure 2 shows the location of the four drill holes studied and the general petrology of the body. Fifty mineralized samples were studied in detail (46 samples from four drill holes and four samples from mine walls). Polished sections were studied under transmitted and reflected light in order to determine mineral associations and parageneses. Backscattered electron images were collected with the scanning electron microscope-energy
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FIG. 2. East-west section showing the general geology of the Tanco Lower pegmatite and the locations of the four studied drill holes and of the core samples (small circles, with the corresponding sample numbers adjacent to the drill holes). Note that the locations of the drill holes are projected onto the section, and the holes are not entirely within the east-west plane. Hand samples were collected at level 650 (depth of ~160 m). CG = coarse grained, qz = quartz, Kfd = K-feldspar, MQM = mica-quartz alteration, SQUI = spodumene-quartz intergrowths.
dispersive system (SEM-EDS) at the Laboratoire des Mécanismes de Transfert en Géologie of Toulouse, and X-ray element distribution maps were produced for the most complex grains. Using these images for guidance, mineral compositions were determined on a Camebax SX 50 electron microprobe at the University Paul Sabatier of Toulouse; operating conditions were a voltage of 15 kV, a beam current of 20 nA, and a 3-µm beam diameter for all elements. Standards were Ta, Nb, and W metals (for Ta, Nb, and W), cassiterite (for Sn), MnTiO3 (for Mn and Ti), zircon (for Zr), UO2 (for U), ScPO4 (for Sc), hematite (for Fe), antimony (for Sb), Pb glass (for Pb), ThO2 (for Th), periclase (for Mg), wollastonite (for Ca), albite (for Na), and topaz (for F). Structural formulas were calculated on the basis of six oxygens for ferrotapiolite and columbite group minerals, and 32 oxygens for wodginite 0361-0128/98/000/000-00 $6.00
group minerals, using the calculation programs of Tindle et al. (2002). Some zircon crystals associated with Ta mineralization were analyzed as well, using the same operating conditions. The Tanco Lower Pegmatite Internal structure The Tanco Lower pegmatite orebody extends over 80 m in length, 50 m in width, and 30 m in thickness. Its internal structure is similar to that of the main Tanco body (see Cˇern´y et al., 1998). It consists of nine, roughly concentric, although commonly discontinuous, zones (Fig. 2): the border zone (10), the wall zone (20), the aplitic albite zone (30), the lower intermediate zone (40), the upper intermediate zone (50), the central intermediate zone (60), the quartz zone (70), the
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pollucite zone (80), and the lepidolite zone (90). The zone numbers correspond to the terminology used by Tanco geologists. The border zone (10) is mostly absent in the Tanco Lower pegmatite. The wall zone (20) forms an envelope around the interior mineralized zones. Figure 2 shows that zone 20 is variable in thickness, especially in the upper parts where it disappears locally. The lower and upper intermediate zones (40) and (50) share a gradational boundary and can also be considered as a single shell with a gradual upward increase in lithium content. Like zone 20, zone 40 is quite continuous in the lower parts, but it is discontinuous in the upper parts. As opposed to the main Tanco body, where it forms a major constituent, zone 30 aplitic albite is mostly absent in the Tanco Lower pegmatite. In contrast, zone 60 occupies 75 vol percent of the orebody. It is concentrated in the central parts of the pegmatite and can also form pockets with diffuse contacts in zone 40. Zone 90 (lepidolite) is also present throughout zone 40, as pockets and bands. Zone 80 (pollucite) forms large pockets, tens of meters in size, in the upper parts of the orebody. The most significant host lithologic units of tantalum mineralization are the central intermediate zone (60), the lower intermediate zone (40), and the lepidolite zone (90). Units 20 and 50 are only weakly mineralized. The orebody is capped by zone 70 (massive quartz) and zone 50, or a mixture of the two. Zones 70 and 80 do not host any appreciable Ta mineralization. It is well established that the bulk composition of the Tanco pegmatite is granitic and that the pegmatite was produced primarily by crystallization of a volatile-rich silicate melt (Stilling et al., 2006). It is also commonly accepted that the concentric organization of pegmatites in general reflects crystallization from the borders to the center of the body (Jahns, 1982; London, 2005). The internal structure of the Tanco Lower pegmatite is similar to that of the Tanco main body, so it undoubtedly crystallized by the same processes. Petrography The petrography of the Lower pegmatite units is very similar to that of the main Tanco body (see Stilling et al., 2006, and Van Lichtervelde et al., 2006, for detailed descriptions). Zone 20 (wall zone) consists of coarse-grained (a few centimeters) quartz + K-feldspar (leopard rock) with minor tourmaline (mainly schorl-type) + mica + beryl and rare phosphates. A fine-grained interstitial aplite (millimeter-size quartz + albite + minor tourmaline + rare apatite or mica, termed footwall aplite and different from zone 30 aplitic albite) can host rare disseminated Ta oxides. Zone 40 (lower intermediate zone) consists of coarse-grained K-feldspar + minor spodumenequartz intergrowths + quartz. Coarse-grained phosphates (mainly amblygonite-montebrasite) and yellow curvilamellar mica are present, as well as minor fine-grained beryl. A light-colored aplite with fine-grained, colored tourmaline (blue-green to pink) and blue apatite is rarely observed. A characteristic feature in zone 40 of the Tanco Lower pegmatite is the presence of coarse-grained dendritic amblygonite (Fig. 3A, B) with interstitial mica + Ta oxides (1) from tantalite (Nb/Ta atomic ratio
