La cantera de “puzolana” de Llano Grande, flanco SW del volcán Irazú, (Costa Rica): caracterización geológica y mineralógica

The Llano Grande quarry, located on the southwestern flank of the Irazú volcano (Costa Rica), is used for the extraction of pozzolan for cement manufacturing by the company Holcim. The protolithic volcanic rocks (coherent lavas, pyroclastic rocks, and lahars) exhibit advanced to complete obliteration of their original textures, resulting from pervasive alteration characterized by the replacement of most of the original rock components by silica (primarily alpha-quartz, alpha- and beta-tridymite, and cristobalite). This transformation has led to the formation of vuggy silica and pseudo-perlitic-textured rocks. Iron oxide and hydroxide minerals are present in fractures (goethite and minor magnetite) and in hydrothermal breccia chimneys, often associated with gypsum, or occurring as disseminated phases, predominantly in the middle to upper parts of the quarry. In these areas, the rocks also contain cavities filled with vacuolar native sulfur. The abundance of quartz and quartz-pyrite-marcasite assemblage indicates a silicic alteration transitioning to an argillic type, with the presence of clay minerals (kaolinite and calcic montmorillonite) due to intense H⁺ metasomatism with acidic leaching, estimated to have occurred in a vapor-heated environment at approximately 100°C. In addition, sanidine facies, a type of metamorphic contact, were also observed. The tectonic context of the area, characterized by its proximity to the continental divide, its relationship with extinct volcanic centers, and its emplacement within a pull-apart system, suggests that hydrothermal alteration took place near an ancient eruptive center, in proximal volcanic facies and under hyperacidic conditions, possibly during the Middle Pleistocene. A porphyritic basaltic andesite dike intrudes the altered volcanic sequence, evidencing a late-stage phase within the hydrothermal system.