REE-Y-Th-U-bearing accessory minerals and their ...

REE−Y−Th−U-bearing accessory minerals and their contribution to the lanthanide and actinide trace-element budget in an anorogenic granite SEE−Y−Th−U-führende akzessorische Minerale und ihr Beitrag zum Spurenelementhaushalt der Lanthaniden und Aktiniden in einem anorogenen Granit ROBERT B. TRUMBULL (Potsdam), HANS-JÜRGEN FÖRSTER (Potsdam) Keywords: Xenotime, monazite, zircon, microanalysis, U−Th−total Pb monazite age, trace elements, Erongo granite

Abstract Relatively high concentrations of the lanthanide and actinide trace elements are a characteristic feature of many anorogenic granites. This study presents quantitative mineral analyses of monazite-(Ce), xenotime-(Y) and zircon from the anorogenic Erongo granite in Namibia, and assesses the relative contribution of these minerals to the total REE, Y, Th and U budget in the granite. The results demonstrate that over 90% of the REE−Y and Th in whole-rock samples are hosted by monazite-(Ce) and xenotime-(Y). The role of zircon is negligible for Th and the LREE, whereas it contributes from 5 to 10% of the whole-rock budget for the HREE and Y. Whole-rock REE, Y and Th contents in the Erongo granite are well accounted for by the three magmatic accessory minerals in all samples studied. However, the U budget balances in only one of three samples, where the contributions of monazite(Ce), xenotime-(Y) and zircon are 25%, 60% and 10%, respectively. In the other two samples, these minerals account for only 30% of the U budget. The anomalous samples have distinctly higher wholerock U concentrations (28−33 ppm vs. 9 ppm) and non-chondritic U/Th ratios between 0.7 and 1. We conclude that the excess whole-rock uranium budget in these samples is dominantly secondary, related to usually small U-bearing species deposited in the interstitial network, in microcracks, or on mineral surfaces. This example shows that mineral mass balance can be useful in assessing the resource potential of uraniferous granites. The Th and U contents of monazite-(Ce) are sufficient in some samples to calculate radiometric ages from microprobe data. The weighted mean Th–U–total Pb age from 12 point-analyses on two

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samples of Erongo granite is 139 ± 12 Ma (2σ), which complements previous results of 130–136 Ma from conventional geochronology and confirms an Early Cretaceous age of emplacement.

Kurzfassung Charakteristisch für viele anorogene Granite sind erhöhte Gehalte an Spurenelementen der Lanthanidund Aktinidgruppen. In dieser Arbeit werden quantitative Mineralanalysen von Monazit-(Ce), Xenotim-(Y) sowie Zirkon aus dem anorogenen Erongo Granit von Namibia vorgestellt und deren Beitrag zum Gesamthaushalt der Seltenen Erden Elemente (SEE), Y, Th und U im Granit ermittelt. Das Ergebnis der Massenbilanzierung zeigt, dass über 90% der SEE, des Y sowie des Th allein in Monazit-(Ce) und Xenotim-(Y) enthalten sind. Der Zirkon spielt nur im Bezug auf die schweren SEE und Y eine grössere Rolle, indem er beispielsweise zwischen 5 und 10% der Gesamtgehalte an Y, Er und Yb führt. Die Gehalte der SEE, Y und Th im Gesamtgestein sind durch die drei Akzessorien in allen Granitproben vollständig bilanziert. Im Gegensatz dazu gelingt die Bilanzierung des U-Gehaltes nur in einer von drei untersuchten Proben. In dieser tragen Monazit-(Ce), Xenotim-(Y) und Zirkon zu 25%, 60% bzw. 10% zum Urangehalt des Gesamtgesteins bei. In den anderen zwei Proben erreicht der UGesamtbeitrag aller drei Minerale nur etwa 30%. Diese Proben zeigen zudem deutlich höhere UGehalte (28−30 ppm vs. 9 ppm) und erhöhte, nicht-chondritische U/Th-Verhältnisse (0.7−1). Diese Charakteristika lassen sich am besten durch eine sekundäre Anreicherung des Urans in den betroffenen Proben interpretieren, wobei die Hauptmenge des zugeführten Urans in U-haltigen Mineralphasen von geringer Grösse gebunden sein dürfte, die im Intergranularraum, auf Mikrorissen oder auf Korngrenzen kristalliserten. Dieses Beispiel zeigt, das eine mineralogische Massenbilanzierung für die Evaluierung des Ressourcenpotentials U-reicher Granite von Nutzen sein kann. Die gewichtete mittlere Th–U–Gesamtblei-Alter des Erongo Granits von 139 ± 12 Ma (2σ), berechnet aus 12 Einzelpunktanalysen der Th-reichsten Monazitkörner, stimmt gut mit bisher bestimmten Isotopenaltern (130–136 Ma) überein. Es ist ein weiteres Indiz für die Platznahme dieses Granits in der oberen Kreide.

1. Introduction Anorogenic granites are commonly enriched in rare earth elements (REE) and high fieldstrength lithophile trace elements (HFSE; e.g., Nb, Ta, Zr, Th, U, Y) relative to orogenic granites at the same level of differentiation. This enrichment pattern is the basis for trace element discrimination schemes for the A-type or within-plate granites (e.g., PEARCE et al., 1984; EBY, 1990; FÖRSTER et al., 1997), and it probably relates to a higher temperature of 2

magma formation and the ability to melt or dissolve refractory minerals in the source rocks (KING et al., 2001; TRUMBULL et al., 2004). Where these trace elements end up after crystallization of the granitic magma, to what extent they are hosted by the rock-forming minerals or by accessory phases, or are lost to residual fluids, is rarely investigated quantitatively. However, the question is important because knowing the mineralogical hosts for the trace elements in an igneous rock is a prerequisite for geochemical modelling of magma evolution (BEA, 1996), for interpreting radiogenic isotope data (Nd, Pb, Hf) and for understanding element enrichment in ore-forming processes. The Early Cretaceous Erongo granite is an example of an anorogenic leucogranite with high trace element abundances and with minor Sn, U, W, and Be mineralization. The granite is thought to have formed by lower crustal melting during a regional episode of continental flood basalt magmatism related to the breakup of Gondwana (PIRAJNO, 1990; TRUMBULL et al., 2004a). The Erongo granite contains abundant and well-developed grains of magmatic xenotime-(Y), monazite-(Ce) and zircon, and this study was undertaken to determine the composition of these accessory phases and to asses their contribution to the overall REE and HFSE budget in the rock using mineralogic mass balance.

2. Regional Setting With a diameter of about 40 km, Erongo is the largest of more than 20 subvolcanic intrusive complexes in NW Namibia that were emplaced in the early Cretaceous (135 to 125 Ma) as part of the Paraná−Etendeka Large Igneous Province (MARTIN, 1960; HARRIS, 1995; PEATE, 1997; TRUMBULL et al., 2000; 2004b). The lithologic diversity of these intrusive complexes is remarkable, including carbonatites, felsic and mafic alkaline rocks, tholeiitic gabbros, and a suite of granitic or rhyolitic rocks with peraluminous as well as peralkaline members. The majority of the complexes, including the Erongo, intruded the Central Zone of the Damara Belt, which is made up of Neoproterozoic, amphibolite-grade metasedimentary rocks and synto post-tectonic granites of early Cambrian age (MILLER, 1983; MCDERMOTT et al., 1996; JUNG et al., 2003). Continental flood basalts and felsic volcanic units of the Etendeka province are exposed north of Erongo (Fig. 1), but the distribution of dolerite dike swarms indicate that they formerly occupied a much larger area, extending past Erongo (TRUMBULL ET AL., 2004b). Basaltic and andesite flows at the base of the Erongo massif are thought to represent erosional outliers of the flood basalt province (WIGAND et al., 2004).

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3. Geology of the Erongo Complex and previous work The Erongo igneous complex consists of three morpho-structural units: (1) a deeply-eroded, bowl-shaped central massif, which is composed mainly of rhyodacite tuffs overlying andesitic and basaltic basal lavas, and with a resurgent intrusion of granodiorite at its core; (2) several granite intrusions dispersed outside the central massif (Erongo granite); and 3) a semi-circular ring dike of tholeiitic dolerite (Fig. 1). Most of the volcanic and intrusive units in the Erongo complex are silicic in composition and they form two compositional series, each with an intrusive and a volcanic equivalent. The first and most voluminous series comprises rhyodacite lavas and tuffs of the central massif (Ombu tuffs) and the compositionally equivalent resurgent intrusion (Ombu granodiorite). The other silicic series consists of a highsilica rhyolite (Ekuta rhyolite), which occurs in scattered erosional remnants at the top of the central massif; and the Erongo granite, which intrudes the Ombu units as discordant dykes and forms a number of intrusive stocks outside the main massif. There are also late, lowvolume mafic intrusions in the complex (TRUMBULL et al., 2003), which comprise both tholeiitic (ring dyke and gabbro sill) and alkaline members (basanite−tephrite and lamprophyre plugs and dykes). The Erongo Complex was first mapped and described by CLOOS (1919). Later studies, complemented by geochemical analyses, include those of BLÜMEL et al. (1979), PIRAJNO (1990) and WIGAND (2004). The main silicic units Erongo granite and Ombu granodiorite, were included in regional petrogenetic studies of the intrusive complexes by HARRIS et al. (1995) and TRUMBULL et al. (2000, 2004a). The latter work employed O, Sr, and Nd isotope data and geochemical arguments to support a cogenetic origin for the Ombu granodiorite and the Erongo granite. The

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Ar−39Ar and U−Pb zircon dating by PIRAJNO et al. (2000) and

WIGAND et al. (2004) determined ages of most units of the complex in the range of about 130−136 Ma. The U-Pb geochronology of the silicic units is complicated by the abundance of inherited zircons, as discussed below. 4. The Erongo granitoids: field and petrographic description The older of the two granitoids is the Ombu granodiorite, which crops out in the center of the complex (Fig. 1). The granodiorite is a medium-grained, equigranular rock containing plagioclase, quartz, K-feldspar, biotite, and othopyroxene as the major phases, with minor FeTi oxides, garnet, and cordierite. Characteristic of the granodiorite are abundant, subangular xenoliths of millimeter to tens of centimeters in size derived from the local basement 4

(megacrystic granite, pegmatite, biotite schist; see Fig. 2a). The Ombu granodiorite has a hypabyssal facies which occurs in dikes and in the upper levels of the intrusion at the gradational contact with the overlying, chemically equivalent Ombu tuffs. The Erongo granite forms small stocks, dikes, and sills located mostly outside the central massif (Fig. 1). Most exposures of Erongo granite consist of equigranular, coarse-grained, tourmaline−biotite monzogranite, with local patches of two-mica leucogranite. A fine-grained facies occurs as irregular patches and aplitic dikes, and there are also rare lenses of pegmatitic texture. The average modal composition of the coarse equigranular granite is 36% quartz, 33% perthitic orthoclase, 25% albite, 4.5% biotite, and 1.5% accessory minerals (BLÜMEL et al., 1979). The latter include tourmaline, zircon, monazite, xenotime, fluorite, apatite, and topaz. A distinctive and characteristic feature of the Erongo granite is the presence of round, tourmaline−quartz orbicules up to 30 cm in diameter (Fig. 2b), which occur in all exposures of the granite including its fine-grained facies. The segregations consist of tourmaline and quartz for the most part, with minor K-feldspar, plagioclase, biotite, and accessory fluorite, apatite, topaz, and cassiterite (TRUMBULL et al., 2008). Like many chemically evolved granites emplaced at a shallow level, the Erongo granite is associated with secondary mineralization. Boron metasomatism (tourmalinization) is widespread at the country rock contacts, along with local greisen-type W, Sn, F, and Be mineralization (PIRAJNO, 1990). The only economic mineralization is in W (ferberite)-bearing quartz veins and greisen in country rocks just NE of the central massif, which were mined at Krantzberg until 1979 (PIRAJNO

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SCHLÖGL, 1987). Of special interest for this study are

local showings of secondary U mineralization, which were described by PIRAJNO (1990) but not yet studied in detail. An important result of our mass-balance calculations is that the whole-rock U contents in the Erongo granite are poorly accounted for by the rock-forming and accessory minerals, and up to 70% of the U may be hosted by secondary phases dispersed along the interstitial network.

5. Whole-rock geochemistry Geochemical analyses of the Erongo units have been published before (BLÜMEL et al., 1979; PIRAJNO, 1990; TRUMBULL et al., 2000; 2004a); however, a set of complete, high-quality trace-element data as required for mineralogical mass-balance calculations was lacking. For this study, therefore, we re-analyzed a suite of ten granite samples by ICP-MS, based on the

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methods described by DULSKI (2001). The sample coverage is illustrated in Figure 1 and results are reported in Table 1 and on selected variation diagrams in Figure 3. The Erongo granite is quite variable in composition, and to assess if this variability relates to the location of samples around the massif we distinguish the samples by symbol in Figure 4 according to their location in the northeast (area A), northwest (B), and south (C) of the central massif (Fig. 1). In terms of their major-element compositions, all granite samples can be described as one group, with (in weight percent) 74 to 78 SiO2, 1.3 to 1.9 total Fe as Fe2O3,