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Allophane-like materials in the weathered zones of Silurian phosphate-rich veins from Santa Creu d’Olorda (Barcelona, Spain)

Departamento de Geología de la Universidad de Salamanca. 37008 Salamanca, Spain

^* E-mail: cembranos{at}gugu.usal.es

(Received 12 February 1999; revised 30 July 1999)

Allophane-like materials occur in the weathered zones of the phosphate-rich veins hosted in Silurian metasediments of the Catalonian Coastal Ranges. These metasediments also host sulphide and phosphate sedimentary mineralizations. Mineralogical and geochemical investigations of the allophanic samples indicate that they comprise Si-rich allophane, with a molar SiO₂/Al₂O₃ ratio ranging between 1.19 and 2.23, with amorphous Al-(Ca) phosphate and hydroxylapatite as major minerals, and minor goethite and quartz. It is assumed that allophane, amorphous Al-(Ca) phosphate and hydroxylapatite come from the reaction of acid solutions, released during the weathering of sulphide interbedded in black shales, with phosphate-rich veins and volcano-sedimentary host rocks. Silica-alumina gels were deposited in fissures and cavities left by a previous dissolution of the phosphate-rich veins. Later the phosphate minerals filled the conchoidal microfractures and shrinkage microcracks of the allophane.

KEYWORDS: allophane, hydroxylapatite, phosphate-rich veins, Spain

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