Journal of Geosciences, 56 (2011), 81–104 DOI: 10.3190/jgeosci.090 Original paper Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán – exploration geochemistry and genesis of gold deposits PETR MIXA1*, PETR DOBEŠ1, VLADIMír ŽÁČEK1, PETR LUKEŠ1, ENRIQUE M. QUINTANILLA2 1 Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic;
[email protected] 2 MINAET, Dirección de Geología y Minas, Apdo. 10104, San José, Costa Rica * Corresponding author Epithermal gold mineralization in quartz veins forms part of a large ore belt extending in the NW–SE direction paral- lel to the Cordillera de Tilarán, Costa Rica. It is confined to Miocene–Pliocene andesites and basalts of the Aguacate Group volcanic arc. Gold-bearing quartz veins are related to faults and fractures of steep inclinations, accompanied by pronounced hydrothermal alteration. The key tectonic zones strike NW–SE but the majority of the ore veins are control- led by local extensional structures of Riedel shear type in the NE–SW, N–S to NNW–SSE directions. The brecciation, mylonitization and healing of deformed structures suggest that three main pulses of mineralization took place during the hydrothermal process. The gold is present as electrum (30 and 42 wt. % Ag) tiny inclusions up to 25 µm in size enclosed in quartz, pyrite and arsenopyrite. The other ore minerals are chalcopyrite, galena, sphalerite and marcasite and less abundant to scarce acanthite, pyrargyrite, greenockite, covellite, bornite and cassiterite. The principal elements exhibit- ing significant positive correlations with Au are Ag, Sb, As, Pb and Hg. Fluid inclusions of the H2O type were found in quartz and sphalerite from several Au-bearing occurrences.