Geología, alteración hidrotermal y mineralogía del cerro Minas (Santa Ana, Costa Rica): Cuerpos subvolcánicos coetáneos con la sedimentación de la Formación Pacacua (Mioceno Inferior)

Cerro Minas, located on the NNW flank of the Escazú hills, corresponds to lava bodies with a fluidal structure (andesites to slightly porphyritic trachytes) associated with the Pacacua Formation. They are interpreted as cryptodomes and apophysis that intruded into the sediments during the Early Miocene. They have an intense silicification and calcification alteration. During the exploitation of the quarry (former Bretaña quarry), the largest geodes recorded so far nationwide (ca. 0.5 to 2 m diameter) with large decametric amethyst crystals were found; in addition, crystalline quartz, smoky quartz and citrines were reported, as well as an anhydrous micro to cryptocrystalline quartz (calcedony, agate, and jasper). Other minerals with very striking shapes and colors were pink, yellow to brown calcite/Mg-calcite and iron-bearing dolomite (ferroan dolomite) roses (a rare occurrence worldwide referring its aspect), associated with barite. The hydrothermal alteration can be classified as silicifying and carbonatic type in an alkaline system rich in Mg at a temperature of about 150 °C. Much of the fine-grained whitish matrix corresponds to calcite, montmorillonite, illite and quartz with the subordinate presence of zeolites (laumontite, mordenite). Among the rare occurrence metal ore minerals, such as specularite, bornite, chalcopyrite and, to a lesser extent, malachite, chalcosine and sphalerite appeared in small quantities.