Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Petr Mixa, Petr Dobeš, Vladimír Žáček, Petr Lukeš, Enrique M. Quintanilla

Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán — exploration geochemistry and genesis of gold deposits

Journal of Geosciences, volume 56 (2011), issue 1, 81 - 104

DOI: http://doi.org/10.3190/jgeosci.090

Epithermal gold mineralization in quartz veins forms part of a large ore belt extending in the NW—SE direction parallel 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 fault and fracture of steep inclinations, accompanied by pronounced hydrothermal alteration. The key tectonic zones strike NW—SE but the majority of the ore veins are controlled 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 exhibiting 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. Temperatures of homogenization of fluid inclusions from several quartz generations and sphalerite vary generally between 150 and 290 °C; the salinity of the aqueous solution was very low, not exceeding 5 wt. % NaCl equiv. The age of the mineralization is estimated in the period between the intrusion of the Guacimal Pluton and effusions of the discordant volcanic Monteverde Formation, which is devoid of mineralization (i.e. between c. 6.0 and 2.1 Ma). Geochemical study indicated altogether 14 promising gold-bearing areas in the Montes del Aguacate and Cordillera de Tilarán, of which four can be recommended for further exploration.