TEXTURES and COMPOSITIONAL VARIABILITY in GERSDORFFITE from the CRESCENCIA Ni–(Co–U) SHOWING, CENTRAL PYRENEES, SPAIN: PRIMARY DEPOSITION OR RE-EQUILIBRATION?
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1513 The Canadian Mineralogist Vol. 44, pp. 1513-1528 (2006) TEXTURES AND COMPOSITIONAL VARIABILITY IN GERSDORFFITE FROM THE CRESCENCIA Ni–(Co–U) SHOWING, CENTRAL PYRENEES, SPAIN: PRIMARY DEPOSITION OR RE-EQUILIBRATION? ISABEL FANLO§ AND IGNACIO SUBÍAS Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/ Pedro Cerbuna 12, E–50009 Zaragoza, Spain FERNANDO GERVILLA Instituto Andaluz de Ciencias de la Tierra y Departamento de Mineralogía y Petrología, Facultad de Ciencias, C.S.I.C.–Universidad de Granada, E–18002 Granada, Spain JOSE MANUEL Cristalografía y Mineralogía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/ Pedro Cerbuna 12, E–50009 Zaragoza, Spain ABSTRACT At the Crescencia showing in the Pyrenees, Spain, three stages of mineral deposition can be distinguished: stage I: nickeline and pararammelsbergite, stage II: gersdorffi te, and stage III: uraninite. Gersdorffi te has been subdivided into seven groups on the basis of textural and compositional criteria. Some of these groups of gersdorffi te clearly show disequilibrium processes that may have been induced by secondary reactions associated with a re-equilibration of the system during cooling. Formation and subsequent growth of gersdorffi te nuclei on the nickeline (0001) surface, self-organization, or a coupled dissolution-and-repre- cipitation moving interface through pararammelsbergite or nickeline are some of the processes invoked during the re-equilibra- tion. Thus, the compositional variations found in the seven groups of gersdorffi te are likely due to intermediate steps during the bulk of the replacement and re-equilibration processes at low temperature, rather than a direct precipitation from the ore-forming fl uids under different conditions. Keywords: gersdorffi te, nickeline, pararammelsbergite, uraninite, mineral replacement, dissolution–precipitation, recrystallization front, reaction rims, self-organization, Pyrenees, Spain. SOMMAIRE A l’indice minéralisé de Crescencia, dans les Pyrénées espagnoles, nous distinguons trois stades de déposition du minerai: Stade I: nickeline et pararammelsbergite, stade II: gersdorffi te, et stade III: uraninite. La gersdorffi te est subdivisée en sept groupes selon des critères texturaux et compositionnels. Certains de ces groupes de gersdorffi te mettent clairement en évidence un déséquilibre, dû peut-être à des réactions secondaires associées à un ré-équilibrage du système au cours du refroidissement. La formation et la croissance de nucléi de gersdorffi te sur la surface (0001) de la nickeline, une auto-organisation, et un front de dissolution et reprécipitation couplées traversant la pararammelsbergite ou la nickeline seraient des processus possibles invoqués lors du ré-équilibrage. Les variations en composition que nous documentons parmi les sept groupes de gersdorffi te témoigne- raient de stades intermédiaires au cours des processus de remplacement et de ré-équilibrage à faible température, plutôt que de précipitation directe à partir des fl uides formateurs du minerai à de diverses conditions. (Traduit par la Rédaction) Mots-clés: gersdorffi te, nickeline, pararammelsbergite, uraninite, remplacement de minéraux, dissolution–reprécipitation, front de recristallisation, couronnes de réaction, auto-organisation, Pyrénées, Espagne. § E-mail address: [email protected] 1514 THE CANADIAN MINERALOGIST INTRODUCTION limestones have been strongly deformed during Alpine thrusting and locally possess a mylonitic fabric. In the foothills of the Pyrenees belt in northeastern The Crescencia ore is hosted by limestone of the Spain, we have found two groups of small-scale Ni–Co early Devonian sequence, which alternates with black deposits hosted by metasomatized Paleozoic lime- shale. This ore-bearing unit is extensively dolomitized. stones, depending on whether Co or Ni is dominant: The deposit consists of a horizontal east–west-striking cobalt is dominant in the San Juan de Plan deposit vein, 50 m long and 10 cm wide, located along the studied by Fanlo et al. (2004), whereas nickel is domi- contact between the limestone and black shale. An nant at the Crescencia showing, the subject of this irregular and discordant zone of disseminated mineral- communication. ization occurs along the vein. The Hercynian structure Deposits of Ni–Co ore minerals are widespread, and of both host rocks and ore is disrupted by an east–west they have been extensively studied in the literature. Alpine thrust fault verging to the south. The deposit is Recent investigations have focused on the composi- not obviously related to an intrusive body. Nickel was tional variations in sulfarsenides, arsenides and diar- extracted from artisanal workings toward the end of the senides, with studies of natural compositional trends, 19th century and the beginning of the 20th century. bonding models and schemes of coupled substitution, and descriptions of the phase relations involving the MINERALOGY AND PETROGRAPHY solid solutions at high temperatures (Yund 1962, Klemm 1965, Petruk et al. 1971, Maurel & Picot 1974, Ixer The mineral assemblage at the Crescencia showing et al. 1979, Fukuoka & Hirowatari 1980, Oen et al. is characterized by Ni sulfarsenides, arsenides and 1984, Béziat et al. 1996, Gervilla et al. 1996, Hem et diarsenides, together with late uraninite. The gangue al. 2001, Wagner & Lorenz 2002, Hem & Makovicky minerals consist of ankerite and dolomite. 2004, Fanlo et al. 2004). Three stages of mineral deposition can be distin- In general, the infl uence of mineral replacement guished on the basis of mineralogical and textural as a consequence of re-equilibration processes is studies: Stage I: nickeline (NiAs, Nc) and pararam- not considered as a major process in explaining the melsbergite (NiAs2, Prr); Stage II: gersdorffi te (ideally composition of minerals. In this paper, we focus on NiAsS, Gdf), and Stage III: uraninite (UO2, Urn). The textures and compositional variations of gersdorffi te at black shales adjacent to the vein contain a dissemina- the Crescencia showing. These features may provide tion of pyrite (Py) rimmed by gersdorffi te, although a evidence for a series of re-equilibration processes at relation between these minerals in the shale and in the low temperature, rather than a direct precipitation from vein assemblage is not evident. the ore-forming fl uids of different composition under Stage I comprises masses of allotriomorphic- distinct conditions of temperature. granular aggregates of nickeline, showing a random orientation and variable grain-size. The nickeline GEOLOGY OF THE DEPOSIT shows a distinct (0001) cleavage and, occasionally, an incipient lamellar twinning. In some crystals, this The Crescencia showing is located in the central part lamellar twinning is made more prominent by alteration of the Pyrenees (longitude 42°31’53.8” N, and latitude or destabilization of the grains (Fig. 1A). Pararammels- 00°34’17.5” E), a doubly verging collisional mountain bergite occurs as roundish, poorly developed crystals belt that resulted from the interaction between the Ibero- included in nickeline or along the contact between African and European plates in Mesozoic and Cenozoic nickeline and gersdorffite. The pararammelsbergite times. Even now, the Ibero-African plate margin is does not show twinning and exhibits rotation tints rich converging at a rate of 4 mm/year. in various shades of brown. Both nickeline and para- The oldest rocks in the area comprise a monotonous rammelsbergite are found as remnants of resorption in sequence of sedimentary rocks of Cambrian–Ordovician gersdorffi te. age. Pre-Variscan augen gneisses and granitic gneisses Stage II is dominated by gersdorffi te, which over- occur as elongate east–west domes surrounded by grows or replaces grains of nickeline and fi lls small Variscan aureoles. An overlying Devonian sequence cracks in them. We recognize seven groups of gersdorf- consists of grey limestone and shale, locally calcareous. fi te on the basis of mode of occurrence and composi- These rocks are conformably overlain by Carboniferous tional variability. The groups are named gersdorffi te A, chert, nodular limestone and, unconformably, by dark B, C, D, E and F throughout the text, tables and fi gures. shale and fl ysch sequences. Batholiths of granodiorite The black-shale-hosted gersdorffi te overgrowing pyrite were emplaced in the Paleozoic rocks. Triassic redbeds crystals is described as gersdorffi te G. lie unconformably on the Hercynian basement, with a Euhedral grains of gersdorffi te A, generally of small basal conglomerate horizon fi lling an erosional surface. size (<20 m), are scattered in the masses of nickeline Limestones of Cretaceous age rest unconformably on (Fig. 1B), or aligned among the grains of nickeline the Triassic redbeds or directly on the basement. These (Figs. 1C, D). Along the contact between the relics of VARIABILITY IN GERSDORFFITE FROM THE CRESCENCIA Ni–(Co–U) SHOWING, SPAIN 1515 nickeline and the euhedral gersdorffi te, resorbed remains 20 kV and a beam current of 20 nA. The counting times of pararammelsbergite are observed. As the euhedral were 20 s on TAP/PET and 30 s on LiF crystals. ZAF crystals replaced nickeline and, locally, pararammels- corrections were performed using the program supplied bergite, they coalesced, growing outward and leading by CAMECA. Pyrite, GaAs, NiO, as well as pure Co to irregular and inhomogeneous masses of gersdorffi te metal were used as primary standards. Chemical compo- enclosing the relics of nickeline and pararammelsbergite