Contrasting fluid behavior during two styles of greisen alteration leading to distinct wolframite mineralizations: The Echassières district (Massif Central, France) Loïs Monnier, Stefano Salvi, Victor Jourdan, Souleymane Sall, Laurent Bailly, Jérémie Melleton, Didier Béziat To cite this version: Loïs Monnier, Stefano Salvi, Victor Jourdan, Souleymane Sall, Laurent Bailly, et al.. Contrast- ing fluid behavior during two styles of greisen alteration leading to distinct wolframite mineraliza- tions: The Echassières district (Massif Central, France). Ore Geology Reviews, Elsevier, 2020, 124, 10.1016/j.oregeorev.2020.103648. hal-02989819 HAL Id: hal-02989819 https://hal.archives-ouvertes.fr/hal-02989819 Submitted on 5 Nov 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 Contrasting fluid behavior during two styles of greisen 2 alteration leading to distinct wolframite mineralizations: the 3 Echassières district (Massif Central, France) 4 Loïs Monniera, Stefano Salvia, Victor Jourdana, Souleymane Salla, Laurent Baillyb, Jérémie 5 Melletonb, Didier Béziata 6 a Géosciences Environnement Toulouse (GET), CNRS, IRD, UPS, CNES. Université de Toulouse, 14 7 avenue Edouard Belin, 31400 Toulouse, France 8 b Bureau de Recherches Géologiques et Minières (BRGM), 3 Avenue Claude Guillemin, 45000 Orléans, 9 France 10 Corresponding author: [email protected] 11 Highlights 12 Fluid cooling, without evidence of sharp pressure variations, is the main cause for one 13 wolframite mineralization episode. 14 Fluid flashing (vaporization), triggered by fracture-induced pressure drops, initiated 15 crystallization of a second type of wolframite mineralization. 16 Greisenization of the Beauvoir granite is a continuous process (ca. 400°C down to 190°C), 17 occurring at high temperature as pervasive alteration of the granite body, while at lower 18 temperature it is localized to a vein system. 19 Globally, greisen-forming fluids have a strong potential for transporting and precipitating W. 1 20 In greisen systems worldwide, regional veins distal to the altered causative magmatic body 21 can be mineralized by greisen-forming fluids. 22 Abstract 23 The Echassières district of central France hosts diverse magmatic and magmatic- hydrothermal 24 deposits of rare metals, mostly related to the well-known Beauvoir granite. Tungsten mineralization 25 crops out at three distinct wolframite occurrences, the two most important of which are related to 26 two distinct magmatic bodies, emplaced ca. 335 and 310 Ma (Monnier et al., 2019). The 27 mineralization occurred at 335 Ma formed during a hydrothermal episode marked by precipitation of 28 topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist. Fluid 29 inclusions in topaz and quartz display similar features, i.e., all have low salinity, contain significant 30 amount of LiCl, display constant liquid/vapor ratios, and homogenized within a narrow temperature 31 range (Th ≈ 380°C). No evidence for fluid pressure variations was observed, and temperature 32 decrease is considered to be the main cause for wolframite deposition. The younger W 33 mineralization is related to greisenizing fluids that altered the Beauvoir granite and generated several 34 quartz (± topaz and apatite) veins. All greisen-related fluid inclusions display low salinity, however, Th 35 are spread from ca. 190 to 400°C, and several populations exhibit heterogeneous liquid/vapor ratios 36 while others consist of only vapor-rich fluid inclusions. Respectively, these populations are 37 interpreted to have been trapped during boiling or flashing (vaporization) of the fluid. In contrast 38 with the other regional veins, flashing was particularly intense in the Mazet veins, which host the 39 bulk of the last wolframite generation. Consequently, it is proposed that flashing is the key factor 40 that triggered W precipitation. 41 This work highlights the role of two physical parameters, pressure and temperature, whose 42 variations played a preponderant role on wolframite mineralization. It documents, in depth, an 2 43 example of greisen fluid evolution, providing critical information on W behavior in orthomagmatic 44 fluids, and on greisen-related rare-metal deposits. 45 Introduction 46 An important proportion of felsic igneous bodies emplaced in the upper part of the crust, particularly 47 the more evolved, rare-metal enriched, and/or peraluminous varieties, exhibits greisen alteration to 48 some extent. Greisenization is characterized by replacement of igneous minerals by various 49 proportions of muscovite, quartz and topaz (Štemprok, 1987), ± tourmaline, apatite, HFSE-bearing 50 minerals, to mention the most common (Pirajno, 2009). It is well established that greisen alteration is 51 caused by magmatic-related, acidic fluids that trapped fluid inclusions (FI) showing a wide range of 52 homogenization temperatures (Th; 200 to 450°C; e.g., Burt, 1981; Štemprok, 1987; Halter et al., 53 1998; Cui et al., 2019), and, in the case of rare metal granites, low salinity (<10 wt.% NaCl eq.; 54 Charoy, 1981 ; Cuney et al., 1992; Dobeš et al., 2005; Breiter et al., 2017a). Similarly, FI associated 55 with W-Sn ore deposits display for the most part low salinity and range in homogenization 56 temperature from 200 to 400°C (e.g., Naumov et al., 2011). Based on this evidence, as well as on 57 structural, petrographic, geochemical and geochronological data, numerous models propose that the 58 same fluids (with possible local evolution or mixing etc.) that form greisen alteration are also 59 responsible for the formation of the surrounding wolframite (± cassiterite)-bearing stockwork (e.g., 60 Štemprok, 1987; Pirajno, 2009; Halter et al., 1998; Williamson et al., 1997; Yokart et al., 2003; Mao et 61 al., 2013; Zhao et al., 2017; Korges et al., 2018; Monnier et al., 2018; 2019; Zheng et al. 2018; Chen et 62 al., 2019). A remaining challenge to the understanding of greisen-related wolframite mineralization 63 are the different factors controlling wolframite precipitation, particularly its location, i.e., in quartz 64 veins that are quasi systematically in the host rock and not in the greisenized granite. 65 To provide an answer to this question, numerous studies have nourished a recent debate on W 66 transport/deposition mechanisms. Lecumberri-Sanchez et al. (2017) suggest alteration of the host 67 rock as a key factor for providing the Fe and Mn necessary for wolframite ((Fe,Mn)WO4) 3 68 precipitation. However, Heinrich (1990) and Yang et al. (2019) argue that orthomagmatic fluids 69 contain sufficient Fe and Mn to permit wolframite precipitation. This affirmation is verified by LA-ICP- 70 MS analyses of magmatic-related FI, which record high amounts of Fe and Mn, in addition to W 71 (Audétat et al., 1998; Harlaux et al., 2017; Yang et al., 2019). Yokart et al. (2003), Legros et al. (2019) 72 and Liu et al. (2018) propose mixing between orthomagmatic and meteoric fluids as the cause for 73 wolframite deposition, while Pan et al. (2019) suggest an input of sedimentary fluids as instrumental 74 in precipitating wolframite. Based on microthermometric data, Korges et al. (2018) and Jiang et al. 75 (2019) propose that depressurization, triggering boiling of the fluid, is the main factor for formation 76 of greisen alteration and simultaneous wolframite-bearing veins. On the other hand, Ni et al. 77 (2015a), Li et al. (2018) and Chen et al. (2018) suggest that simple cooling during fluid transport is 78 sufficient to induce wolframite precipitation. The experimental data of Wood and Samson (2000) 79 confirm that, given a sufficient W concentration in the fluid, cooling and depressurization can control 80 wolframite precipitation, as also proposed by Yang et al. (2019) to explain wolframite precipitation as 81 an infill along fractures. Liu et al. (2018), based on numerical modelling, confirm the important role of 82 repeated depressurization episodes, caused by cyclic fracturing. Concerning the fluid chemistry, 83 Wang et al. (2019a) and Wang et al. (2020) show, respectively, that salinity and CO2 have only a 84 minor impact on W mobility, whereas the role of pH seems critical (see also Wood and Samson, 85 2000), notably the pH increase during interaction with graphite-rich schist host rock (O’Reilly et al., 86 1997). 87 The Echassières district in central France is well suited to investigate the role that different 88 parameters might play on the origin of W mineralization. In this area, most of the W is in the form of 89 three wolframite generations related to three distinct hydrothermal events, separated in time 90 (Monnier et al., 2019). The earliest W episode occurred in the form of wolframite a, outcropping in a 91 stockwork vein system. The bulk of the W stock consists of a wolframite generation that precipitated 92 during a topazification event (wolframite b), attributed to percolation of a F-rich greisenizing fluid. A 93 last major mineralization event (wolframite c) took place during OH-rich greisen alteration following 4 94 emplacement