Metalogênese da jazida aurífera “El Pescado”, Andes do Norte, Colômbia

Abstract

In Colombia, gold deposit exploration and exploitation represent one of the most important sources for local economy. Although the significant advances in Colombian mining, the characterization of gold deposits is still scarce and signify a challenge for geologists. The geodynamic history of Colombian Andes resulted in the occurrence of Jurassic calcium-alkaline magmatism related to accretionary suture process of terrains, also origined important gold deposits such as “El Pescado”, located in the northern side of Andean central mountain range, at the eastern flank of the Otú-Pericos faults system, nested in the Segovia Batholith. This batholith corresponds to quartz monzodiorite that vary locally to quartz-syenite, granodiorite and host porphyritic diorite dikes. The quartz monzodiorite present crystallization conditions with temperatures intervals of 676°C-754°C and pressures of 2.1-2.6 kbar, for diorite dikes, the intervals occur in 791°C-892°C and 3,6-3,7 kbar. Quartz with sulphides and gold form veins host mainly in the diorite dikes. Hydrothermal fluids were enriched in K, Ca, Mg, Fe, S, CO2, and SiO2, and formed chlorite, epidote, calcite, sericite, Fe-Ti oxides, massive amounts of silica and sulfide of Fe, Cu, Zn, Pb and telluride of Au e Ag. This mineralization mainly occurs in quartz veins and is related to propylitic alteration in the host rocks, development at 300°C-332°C (chlorite geothermometer). The activation of trans tensive shear zones with N-S and NNW-SSE trends and dextral kinematic (related to the El Bagre -Otú fault system) enabled the circulation of hydrothermal fluids. The ore mineral paragenetic sequence was related two hydrothermal fluid pulses; first one is characterized by quartz, pyrite-I, sphalerite-I, galena-I and low Au, at a temperature between 175°C-225°C, second one consist in ore mineral association including sericite, calcite, barite and pyrite-II, sphalerite-II, galena-II, chalcopyrite, silver-gold tellurides (calaverite (AuTe2), sylvanite [(Au,Ag)2Te4]) and Au. This last hydrothermal fluid pulse had temperatures between 170°C-300°C. The results of this research suggest that the mineralizing hydrothermal fluids in the "El Pescado" deposit in the Segovia Batholith, were related to magmatism of the diorite dykes. Magmatic and hydrothermal fluids were displayed through shear zones. These structures are related to Otú-Palestina Faults System and were activated in a transtenssive fragile regime during the emplacement of quartz veins. The Au and metallic ions associated were transported by thio-complexes (HS– and H2S) in acidic to alkaline hydrothermal fluids pulses, with intermediate to low salinity and low to moderate temperatures.