Abstract

RICHTER, Stefan; GOTZE, Jens; NIEMEYER, Hans  and  MOCKEL, Robert. Mineralogical investigations of agates from Cordon de Lila, Chile. AndGeo [online]. 2015, vol.42, n.3, pp.386-396. ISSN 0718-7106.  http://dx.doi.org/10.5027/andgeoV42n3-a06.

In the present study a first found of agates in Chile is reported from the Cordón de Lila region in northern Chile. The agates occur as veins and lenses in altered Permian volcanic rocks. The rock composition is rhyodacitic/dacitic consisting of a fi ne-grained K-feldspar-quartz groundmass with phenocrysts of plagioclase (An_50-60) and pyroxene (augite). The volcanic host rocks show strong features of alteration and brecciation. Results of XRD, polarizing microscopy and cathodoluminescence (CL) microscopy and spectroscopy point to a late- to postvolcanic formation of the agates by hydrothermal fluids and SiO₂ which was released during the alteration of unstable minerals and volcanic glass. Strongly varying agate micro-textures and the appearance of euhedral quartz crystals with pm-sized growth lamellae (so called Bambauer quartz) indicate fluctuations in the physico-chemical conditions (SiO₂ concentration, pH) during alteration and agate formation. Another indication for the alteration processes is the occurrence of secondary calcite in the agate-bearing rocks.

Keywords : Cordón de Lila; Volcanic rocks; Mineralogy; Geochemistry; Agate.

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