New insights on mineralogy and genesis of kaolin deposits: The Burela kaolin deposit (Northwestern Spain)

Abstract

Listen

Translate article

The Burela deposit is the largest kaolin deposit in Spain, mined for more than 50years, the product being mainly used for porcelain. Kaolin is dominantly associated with Lower Cambrian felsites, interbedded with quartzites, micaschists and metapelites (Cándana Series), and was strongly folded during the Variscan orogeny. Kaolin layers were ductile and incompetent materials among more competent ones, producing many slides with a diastrophic appearance. Consequently, kaolin outcrops are morphologically very variable– i.e. pockets – and interlayered between metapelites and/or quartzites, resulting in complication for prospection and mining.The kaolin consists mainly of kaolinite, tubular halloysite, and spherical allophane along with quartz and minor illite. The content of kaolin minerals reaches up to 90% in the finer fractions (<2μm and <1μm).Geochemical analyses of trace and REE show a close relationship between kaolin and associated rocks. Two kaolin types can be differentiated: (i) massive, associated to felsite; and (ii) related to metapelite. A temperature range from 15 to 35°C, with an average of approximately 28°C was calculated on the basis of the isotopic signatures (δ18O, δD) for the kaolin materials. This scatter suggests that if continental weathering was involved in the kaolin formation on the lower side of the temperatures, it was not the only process, especially for kaolin associate with felsites and metapelites. The higher temperatures are indicative of a hydrothermal auto-metamorphic alteration, followed by a folding of the series that induced an apparently chaotic kaolin distribution with a combined continental weathering superimposed on the previous low-temperature hydrothermal felsite transformation.