Research paperNew insights on mineralogy and genesis of kaolin deposits: The Burela kaolin deposit (Northwestern Spain)
Introduction
The origin of primary kaolin deposits is usually a matter of controversy because kaolinite can be formed in situ by weathering reactions (supergene kaolins), hydrothermal alteration during the late stages of magma cooling (hydrothermal kaolins), or by a combination of both processes (Murray, 1988). Kaolinite can be formed also within sedimentary continental basins by diagenetic processes (Galán and Ferrell, 2013). Various criteria and indicators have been proposed to distinguish supergene from hypogene kaolins (e.g. Dill et al., 1997, Gilg et al., 1999), although some of them are considered as ambiguous. Unravelling the genesis of kaolinite in deeply weathered magmatic rocks is one of the most important challenges faced by clay geologists.
In the Variscan belt of Galicia (NW Spain) kaolinization of crystalline rocks is widespread. Large high-grade deposits of kaolin (e.g. Vimianzo, Nuevo Montecastelo) and other minor occurrences are found in association with weathered granites. The combined oxygen and hydrogen isotope composition of kaolinite from such deposits is consistent with data of supergene formation (Clauer et al., 2010, Clauer et al., 2015, Fernández-Caliani et al., 2010). However, some deposits associated with felsite dykes or sills and quartz vein networks could have been formed in situ by complex fluid/rock hydrothermal and supergene interactions.
The Burela kaolin deposit (Northern Galicia) is a volcanic-hosted deposit, which is of particular interest in addressing the origin and timing of kaolinization. Volcanic-hosted kaolin occurrences of no current economic interest are also found in other areas of Spain, such as the Canary Islands and within the volcano-sedimentary complex of the Iberian Pyrite Belt. However it is in the Burela area where kaolinization is best represented. The deposit is geologically located (Fig. 1) in the West Asturian-Leonese Zone (WALZ), one of the major tectono-stratigraphic terranes into which the Iberian Massif is classically subdivided (Julivert et al., 1972). The WALZ exposes Precambrian to Devonian metasediments that experienced the effects of the Variscan orogeny between Late Devonian and Late Carboniferous times (Martínez-Catalán et al., 1997, Pérez-Estaún and Bea, 2004).
The main Burela quarries are San Andrés and Ramón Fazouro that are located near the western edge of Fazouro village (Fig. 1). The kaolin is spatially and genetically related to felsites and a swarm of quartz-porphyry dykes that intruded Lower Cambrian metasediments. Kaolin is dominantly associated with Lower Cambrian felsites, interbedded with quartzites and sandstones, and metapelites (Cándana Series), which were strongly folded during the Variscan orogeny. Kaolin-rich layers became a ductile and incompetent material interleaved among the more competent ones, producing many slides with a diastrophic appearance. Consequently, the kaolin outcrops are morphologically very variable (i.e. pockets) interlayered between other rocks, resulting in difficult prospection and mining.
This world-class kaolin deposit has been mined by ECESA for more than half a century (1957–2014), with the kaolin being mainly used as raw material in the manufacture of high quality ceramics, especially porcelain-ware. The kaolin material consists of a mixture of kaolinite and halloysite with minor allophane (Galán et al., 2013). The origin of this important economic deposit is still an ongoing matter of debate. It is unclear if extensive kaolinization resulted from hydrothermal (by meteoric fluids) alteration of felsitic rocks, or if weathering affected the materials that were already subjected to a first stage of low grade metamorphism, or if a combination of the two processes induced the deposit (Galán and Martín Vivaldi, 1972). The aim of this paper is to revisit the arguments first presented, (i.e. hydrothermal alteration by meteoric fluids), on the basis of new insights from mineralogy, trace-element geochemistry and stable isotope data. Such understanding will be helpful for future exploration of new deposits in the region and to the rationalization of mining operations.
Section snippets
Sampling and analytical methods
A representative sampling of the Burela kaolin deposit was undertaken in the Ramón Fazouro quarry (Fig. 2). A total of fourteen samples of different kaolin occurrences and their associated rocks were sampled. Three size fractions (< 45, < 2 and < 1 μm) were extracted from each whole rock by wet sieving, sedimentation in distilled water and centrifugation. The size fractions were mineralogically analysed by X-ray diffraction (XRD) on a Bruker D8 Advance instrument with a scanning speed of
Precursor host rocks
The Burela kaolin deposit is hosted in felsic porphyritic volcanic rocks interbedded with metasedimentary rocks, notably Lower Cambrian metapelites and quartzites (Cándana Quartzite). Representative mineralogical and chemical analyses of the precursor rocks of the kaolin deposit are given in Table 1, Table 2.
In thin section, the felsic volcanic rocks display a holocrystalline, porphyritic texture (Fig. 3a), with phenocrysts of feldspar and partially reabsorbed quartz and set in a groundmass
Conclusions
The Burela deposit is characterized by a high concentration of kaolin minerals (kaolinite and halloysite). In the coarse (< 45 µm) fraction the most abundant kaolin mineral is kaolinite of perfect pseudo-hexagonal morphology. Tubular halloysite is concentrated in the finer (< 2 µm and < 1 µm) fractions.
The geochemical study of the trace and REE showed a close relationship between kaolin and associated rocks. From the geochemical point of view two kaolin types can be differentiated: associated to
Acknowledgements
This work has been partially supported by the Spanish Ministry of Education and Science (Project BET2001-2415) and the Government of Andalusia through the Research Group Applied Mineralogy (RNM135). Authors are indebted to Dr. Collin Harvey and an anonymous reviewer for constructive and improving comments. Authors are grateful to Haydn H. Murray for his advice and kind collaboration during the field research (Fig. 10).
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