Elsevier

Geochimica et Cosmochimica Acta

Volume 289, 15 November 2020, Pages 130-157
Geochimica et Cosmochimica Acta

Magmatic fractionation and the magmatic-hydrothermal transition in rare metal granites: Evidence from Argemela (Central Portugal)

https://doi.org/10.1016/j.gca.2020.08.022Get rights and content

Abstract

Rare Metal Granites (RMGs) are highly evolved intrusions with specific geochemical signatures that are the result of combined magmatic and hydrothermal processes. In most examples, the early evolution is overprinted by late subsolidus transformations and the role of magmatic and magmatic-hydrothermal fractionation processes remains unclear. The origin of fluids involved in magmatic-hydrothermal systems is also an open question. The Argemela RMG (Portugal) is a small subvolcanic intrusion, which exposes in remarkable continuity a sequence of magmatic, magmatic-hydrothermal and early hydrothermal processes. The intrusion comprises a main granitic facies and a border unit made of a complex alternation of aplitic and pegmatitic facies plus three generations of intragranitic veins. The granitic mineral assemblage includes quartz, K-feldspar, albite, muscovite and accessory montebrasite, cassiterite and columbite-tantalite. The whole-rock chemistry is strongly peraluminous, with very high P2O5, Li and F enrichments, elevated rare metal (Sn, Nb, Ta, W) concentrations and strong depletions in Ca, Fe, Ti, Mg. Trace element zonation in minerals, mainly muscovite and quartz, record crystallization/fractionation processes along the magmatic-hydrothermal transition. Three main stages are recognized, magmatic (crystal fractionation and accessory/minor mineral saturation), magmatic-hydrothermal (fluid/melt element partitioning) and early hydrothermal, corresponding to the segregation and collection of magmatic fluids in the intragranitic vein system. Those are followed by limited metasomatic transformations and a late sulphide stage. Stable isotope (O, H) data demonstrate a magmatic origin for the early fluids and a granite-buffered signature for the vein-forming fluids. The fractionation of rare elements (Sn, Nb, Ta, Li, W, Mn, B, Rb, Cs) from the magmatic to the early hydrothermal stage is quantitatively modelled from the combination of three mechanisms, crystal fractionation, magmatic crystallization of accessory/minor phase (MCMA) and fluid/melt partitioning. Sn and W show a markedly contrasted behaviour with Sn precipitated as disseminated mineralization within the intrusion and W partitioned toward the magmatic fluid and extracted from the intrusion. Geochemical trends along the magmatic-hydrothermal transition do not require phases with unusual properties, such as immiscible hydrous melts to be explained and they do not necessarily result in an amplification of initial rare metal melt enrichments. The Argemela RMG stresses the importance of MCMA to generate magmatic rare element pre-concentrations, which later can be hydrothermally redistributed to form large economic deposits.

Introduction

Rare Metal Granites (RMGs), also designated as tin, topaz, zinnwaldite or Li-F granites, are highly evolved granites, characterized by a specific geochemical signature - i.e., elevated concentrations in fluxing elements (F, B, P, Li), significant depletions in REE, Y, Ca, Fe, Ba and Sr and marked enrichments in rare lithophile elements and metals (Sn, Nb, Ta, Be, Cs, Rb and W; see Černý et al., 2005, Linnen and Cuney, 2005) - which strongly differs from most other granites. RMGs occur worldwide, such as in China (Belkasmi et al., 2000), Finland (Haapala and Lukkari, 2005), Nigeria (Kinnaird et al., 1985), Egypt (Zoheir et al., 2020), Canada (Kontak, 1990), Russia (Raimbault et al., 1995) and Western Europe (Charoy and Noronha, 1996, Breiter et al., 2017, Manning and Hill, 1990). The source of rare elements in peraluminous RMGs and the origin of their geochemical specificities have been strongly controversial topics since the 1960s. There is now general agreement that both magmatic (melt-driven) and hydrothermal (fluid-driven) processes are involved in the genesis of RMGs (Beus et al., 1962, Beus and Zalashkova, 1964, Zhalashkova and Stinin, 1967, Kovalenko, 1976, Hu et al., 1984, Pichavant et al., 1988a, Pichavant et al., 1988b, Raimbault and Burnol, 1998). However, evaluating the respective importance of each group of processes continues to be an area of active research.

In most natural RMGs, the early evolution is obliterated by late subsolidus metasomatic processes, such as greisenization (e.g., Pollard et al., 1987, Štemprok, 1987, Breiter et al., 2017) and kaolinization (Fouillac and Rossi, 1991, Cuney et al., 1992, Raimbault et al., 1995). Consequently, the roles of magmatic (partial melting, mineral saturation and fractionation) and magmatic-hydrothermal (fluid/melt partitioning) processes on RMGs geochemical signatures are still unclear. Several models of the magmatic-hydrothermal transition (MHT, defined here as the transition from a melt-driven to a fluid-driven crystallization/fractionation regime) have been proposed recently from studies of rare element pegmatites (Kontak and Kyser, 2009, Hulsbosch et al., 2014, Kaeter et al., 2018, Hulsbosch, 2019). Immiscible hydrous melts are postulated (e.g., Thomas and Davidson, 2012, Thomas and Davidson, 2016) and fractionation assumed to follow element partitioning between three phases, silicic melt, aqueous melt and hydrothermal fluid (e.g., Kaeter et al., 2018). However, the concept of immiscible hydrous melts has so far not received confirmation (London, 2015, Thomas and Davidson, 2016), which stresses the need for alternative fractionation models in rare metal magmas. Another critical issue in magmatic-hydrothermal systems concerns the origin of fluids. Crystallization of anhydrous magmatic phases builds up the H2O concentration in the residual melt (e.g., Jahns and Burnham, 1969), ultimately leading to the exsolution of a magmatic fluid (e.g., Smith et al., 1996, Harlaux et al., 2018, Korges et al., 2018). However, some magmas of crustal origin are fluid-saturated very early on during their evolution (e.g., Le Fort et al., 1987). Non-magmatic (metamorphic and/or meteoric) fluids can be present in and around granitic plutons (Kelly and Rye, 1979, Fouillac and Rossi, 1991, Polya et al., 2000). In some systems, hydrothermal fluids from various sources can superimpose (e.g., Sheppard, 1977, Vallance et al., 2001).

This paper presents geochemical data (major and trace element data on whole-rocks and minerals, O and H stable isotope data) on the Argemela rare metal granite (Central Portugal), a small subvolcanic intrusion in the Portuguese part of the Central Iberian Zone (Charoy and Noronha, 1996, Michaud, 2019). The intrusion is mostly free of subsolidus processes, such as greisenization and kaolinization and it allows the early stages of fractionation to be documented. A sequence of magmatic, magmatic-hydrothermal and early hydrothermal processes is exposed in remarkable continuity for a small intrusion. Petrographical, mineralogical and textural studies allow the successive stages of RMG crystallization along the MHT to be described. Three main fractionation mechanisms are identified from the quantitative modelling of trace element zonation in muscovite, using mineral/melt, fluid/melt partition coefficients and accessory mineral solubility data. The magmatic origin of early and vein-forming hydrothermal fluids is demonstrated from the stable isotope data. Results are incorporated in a model of magmatic and magmatic-hydrothermal evolution and implications for fractionation mechanisms and element transport and deposition in rare metal magmas are discussed.

Section snippets

Geological setting

The Argemela granite (Fig. 1) is located in the southern part of the Central Iberian Zone (CIZ), mainly made of autochthonous Gondwanan terranes corresponding to low grade external metamorphic zones. Occurring ∼13 km east of the world class Panasqueira W mine and ∼1.5 km west of the Fundão composite granite, the Argemela granite forms a small almost elliptical body (i.e., 250 × 125 m) in map view and is characterized by a pipe-like geometry in cross-section extending to a maximum depth of 1 km (

Strategy and sampling

In this study, whole-rock data on representative samples of the Argemela granite and veins are combined with textural and chemical analyses (electron microprobe, LA ICP-MS) of mineral phases and stable isotope (O, H) data. Focus is placed on the characterization of in particular micas and quartz but also of K-feldspar and Li-phosphate phases, which serve as monitors of fractionation processes along the MHT.

Representative rock samples were collected from in-situ outcrops, including samples of

Granitic facies and border unit

The main petrographical, mineralogical and textural features of these two units are summarized in Table 1. The granitic facies is made of quartz and micas phenocrysts embedded in a fine-grained matrix composed mainly of albite and scarce K-feldspars. This mineralogical assemblage forms the skeleton of the granite and accessory minerals include montebrasite, cassiterite and columbite-tantalite. In the border unit, the mineralogical assemblage varies with the specific zone and the related texture.

Bulk rock compositions

The 7 new analyses of the granitic facies yield quite homogeneous compositions (Table A.2). Major elements show relatively small variations (68–70 wt% SiO2, 17.5–18.5 wt% Al2O3, 4.5–6 wt% Na2O, 2.5–4 wt% K2O, Fig. 4a; b). Granite analyses at vein margins do not differ from away from the veins but, for the border units, data are less grouped (Table A.2). Al2O3 extend to higher and SiO2 to lower concentrations than in the granitic facies; Na2O (and K2O) are quite dispersed (Fig. 4a, b). This

Stable isotope results

O and H isotopic compositions have been determined for quartz and a few analyses are available for muscovite. The analysed quartz include QtzI from the granitic facies, QtzI-III from the micaceous facies and QtzIV-V-VI from each generation of intragranitic veins. The analysed muscovites are MsI-III from the granitic and the micaceous facies (see Table 1 for textural details).

Crystallization of the Argemela RMG along the magmatic-hydrothermal transition

Several lines of evidence demonstrate that the Argemela granite results from the crystallization of a highly evolved rare-metal magma (see also Charoy and Noronha, 1996). The subvolcanic texture (Fig. 2a) exhibited by the granitic facies is a typical feature of RMGs and indicates crystallization at shallow levels (e.g., Müller and Seltmann, 1999). The presence of a two-feldspars, muscovite plus quartz mineral assemblage calls for a parental granitic melt and the bulk granite composition

Implications for rare element fractionation, transport and deposition

Recently, fractionation in rare metal magmas has been discussed in a number of studies, several on pegmatites (Kontak and Kyser, 2009, Hulsbosch et al., 2014, Hulsbosch et al., 2016, Kaeter et al., 2018). As melts parental to PHP RMGs share many similarities with LCT pegmatite melts (granitic, peraluminous and felsic chemistries, high concentrations of fluxing components and rare metals, Černý et al., 2005, Linnen and Cuney, 2005, London, 2015, Villaros and Pichavant, 2019), fractionation in

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge Dr. A. Lima, Pr. F. Noronha, F. Pinto and R. Ribeiro for their support in the field and for discussions; P. Moutela and P. Ferraz for giving access to the Argemela quarry and the mine. Dr. I. Di Carlo (ISTO, Orléans, France) is acknowledged for assistance with the electron microprobe analyses and with CL imaging. The authors also acknowledge Dr. T. Rigaudier and Dr. C. France-Lanord (CRPG, Nancy, France) for their help with the stable isotope study, Dr. J-L Devidal (LMV,

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