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J. C. 2, M. RODAS2, J. F. F. J. L INTRODUCTION Nontronite Is a 2:1 Swelling Phyllosilicate in Which Tetrahedral Sites Are Pred

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J. C. 2, M. RODAS2, J. F. F. J. L INTRODUCTION Nontronite Is a 2:1 Swelling Phyllosilicate in Which Tetrahedral Sites Are Pred View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by EPrints Complutense FORMATION OF NONTRONITE FROM OXIDATIVE DISSOLUTION OF PYRITE DISSEMINATED IN PRECAMBRIAN FELSIC METAVOLCANICS OF THE SOUTHERN IBERIAN MASSIF (SPAIN) 1 2 J. C. FERNANDEZ-CALIANI ,*, E. CRESP02, M. RODAS2, J. F. BARRENECHEA AND F. J. L UQUE2 1 Departamento de Geologia, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain 2 Departamento de Cristalografia y Mineralogia, Facultad de Geologia, Universidad Complutense de Madrid, 28040 Madrid, Spain Abstract-This paper describes a rare occurrence of nontronite associated with sulfide-bearing felsic metavolcanics, providing evidence of colloidal deposition in open spaces as result of a low-temperature water-rock interaction. Microbotryoidal masses of green nontronite with impurities of kaolinite, illite, barite, amorphous silica and iron oxyhydroxides are found as vein and cavity fillings in deeply kaolinized rhyolites and rhyolitic tuffs of Precambrian age, at Oliva de Merida in SW Spain. Clay mineral characterization has been carried out by X-ray diffraction, infrared spectroscopy, thermal analysis, analytical electron microscopy and stable isotope (oxygen and hydrogen) analysis. Nontronite was formed under low-temperature alteration conditions, from a continuous sequence of reactions and aqueous solution compositions, involving two basic processes that acted in concert: oxidative dissolution of pyrite and hydrolysis of K-feldspar. After acidity neutralization, dissolved silica released by incongruent dissolution of K-feldspar reacted with ferric sulfate derived from pyrite oxidation to form nontronite under oxidizing conditions, in the presence of relatively warm meteoric water. Key Words-Felsic Metavolcanics, K-feldspar Hydrolysis, Nontronite, Pyrite Oxidation, Smectite, Spain, Weathering. INTRODUCTION discharged from active sea-floor spreading centres Nontronite is a 2:1 swelling phyllosilicate in which (K eeling et al., 2000). For instance, it is a typical clay tetrahedral sites are predominantly occupied by Si mineral of deep-sea sediments from marine white cations, but some replacement of Si by Al and FeH smoker chimneys of the Red Sea, Galapagos rift, can occur, whereas most of the octahedral sites are filled Mariana trough, and Juan de Fuca ridge, among other by FeH (> 1.0 cation per half unit-cell according to hydrothermal systems (Cole and Shaw, 1983; Murnane Giiven, 1988), with minor amounts of AI. Thus, the and Clague, 1983; Singer et al., 1984; Singer and general structural formula for nontronite can be written Stoffers, 1987; Kohler et al., 1994). In this oceanic as follows: Mx y[Sis x yA lxFe�+][Fd+zAlz]0 o environment, nontronite occurs not only near hydro­ + 2 (OH)4·nH 0, where M refers to exchangeable interlayer thermal vent openings, but also on the sea-floor by 2 cations. N ontronite is regarded, therefore, as the ferric alteration of glassy balsaltic lava flows, as well as end-member of dioctahedral smectites. Details on its microbially-mediated low-temperature reaction of iron structure and crystal chemistry are given in Gfrven oxyhydroxides with silica (Juniper and Tebo, 1995; (1988) and references therein. Ueshima and Tazaki, 2001). Although nontronite has been studied extensively, it Under supergene conditions, nontronite is a common continues to attract a high level of research interest, weathering product of basalts and ultramafic rocks (e.g. especially for interplanetary life. Indeed, this ferric Bender-K och et a!., 1995). It is also found in highly smectite is relevant to the search for life on planets like weathered metamorphic rocks, such as schists, gneisses Mars, owing to its high Fe content and because it can be and amphibolites (K eeling et al., 2000). Moreover, it has a potential source of water (Frost et a!., 2002). been reported in felsic volcanics either as an alteration In terrestrial environments nontronite is not abun­ product of Fe-rich silicates (Shenk and Armbruster, dant. However, it is common in both oceanic and 1985), or around rootless fumaroles by leaching and continental environments and often occurs as an dissolution of rhyolitic ignimbrites (Reyes and Read, authigenic clay mineral in recent submarine sediments, 2002). formed by direct precipitation from hydrothermal fluids In this paper we describe a new occurrence of nontronite in Spain which is associated with sulfide­ bearing felsic volcano-sedimentary rocks. We suggest a 0« E-mail address of corresponding author: model of nontronite formation in these exotic parent [email protected] rocks based on mineralogical and geochemical data, together with field and petrographic observations. GEOLOGICAL SETTING and form part of the Tentudfa-Montemolin series (Eguiluz Nontronite occurrences are located near Oliva de et al., 1984), belonging to the Alange Unit of the Obejo­ Merida, -20 km to the SE from the roman city of Valsequillo-Puebla de la Reina domain. All these rocks Merida, in the province of Badajoz (SW Spain). From a were affected by Cadomian tectonothermal events geological standpoint, the mineralization is found in (Quesada, 1990) and later they were deformed and association with a volcano-sedimentary sequence of metamorphosed during the Variscan orogeny. Middle-Upper Riphean age (Figure 1), at the Precambrian basement of the Ossa-Morena Zone, in MATERIALS AND ANALYTICAL METHODS the SW margin of the Iberian Massif (Gibbons and Moreno, 2002). A cm-scale grid was used to collect clayey samples The volcano-sedimentary sequence is a succession of from the outcrop examined in the field (section A-B in predominantly felsic metavolcanics (volumetrically, Figure I). In addition, a series of representative samples rhyolites are the most abundant) and volcano clastic of the host rocks was collected for petrographic analysis. rocks (mainly rhyolitic tuffs and tuffaceous shales) Mineralogical characterization was performed using a interbedded with metasediments (schists, quartzites and combination of analytical methods including trans­ pelitic gneisses), mafic metavolcanics and amphibolites. mitted-light optical microscopy, X-ray diffraction Volcanic-hosted disseminated pyrite appears within the (XRD), Fourier transform infrared spectroscopy volcano-sedimentary package, in close spatial relation­ (FTIR), differential thermal and thermogravimetric ship to the felsic rocks. Supergene weathering oxidized analysis (DTA-TGA), scanning electron microscopy the sulfides forming numerous goethite concretions and (SEM), energy dispersive X-ray spectroscopy (EDS), abundant fenuginous surface coatings. and stable isotope (oxygen and hydrogen) analysis. According to IGME (1988), the Precambrian materials X-ray diffraction was used to determine the bulk constitute the core of the Oliva de Merida anticlinorium, composition of the samples, after gentle grinding and Holocene sediments D Clays. sands and gravels Precambrian rocks :�l�:iCS and Ilmmj!!j!1� i c:J Felsic volcanics, gneisses l::::::::::J and schists Oliva de Merida 500m Section A-B N S I"VVl� � � I7!I7l /;J\ Nontronite Rhyolites Rhyolitic tuffs � Schists and pelitic gneisses � occurrences Figure I. Generalized geological map of the Oliva de Merida area depicting a section with the nontronite occurrences within a volcano-sedimentary sequence of Middle-Upper Riphean age. homogenization to <53 Ilm. For clay mineral identifica­ fissures (Figure 2b). Most of the mineralized fractures tion, carefully hand-picked samples were disaggregated are oriented subvertically following the regional fractur­ in an ultrasonic bath. From these samples, oriented ing trend (mainly N500E and N135°E). Furthermore, aggregates of the <2 Ilm size fraction were obtained by small masses of green clay, up to several dm thick, are sedimentation from an aqueous suspension, pipetted onto found as veins, patches and cavity fillings in deeply glass slides and dried. The oriented films were solvated kaolinized rhyolitic tuffs (Figure 2c). These felsic rocks with ethylene glycol and subjected to thermal treatment are the main host to the mineralization, although the at 550°C for 2 h (Moore and Reynolds, 1997). The XRD distribution of minerals is controlled by fracturing. patterns of random powders and oriented aggregates were collected using Ni-filtered CuKct radiation with a GENERAL CHARACTERIZATION Siemens D500 powder diffractometer equipped with a XRD graphite monochromator. The XRD patterns were recorded with a microcomputer using the Siemens The green clays are composed of virtually pure SOCABIM Diffract AT3.3 software. smectite (Figure 4). The patterns of the oriented The IR absorbance spectra were recorded in the middle IR region (spectral range between 400 and 4000 cm 1) on a Nicolet, 20SXC FTIR spectrometer, using pressed pellets containing 3 mg of sample and 300 mg KEr. The DTA-TGA curves were obtained with a Seiko 320U instrument operating at a heating rate constant of, 20°Clmin, from ambient temperature (25°C) to l000°C, in a static air atmosphere. Fragments of each sample were mounted on stubs, coated with carbon, and examined by SEM in secondary electron mode in a Jeol JSM-5410 microscope, operated at an accelerating voltage of 15 kV. Quantitative chemical analyses were obtained by means of an Oxford Link system energy dispersive X -ray (EDS) analyzer coupled to the microscope. The isotopic analyses were performed at the Stable Isotope Laboratory of Salamanca University, using a SIRA series IT spectrometer from VG Isotech.
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