Abstract

La Bodega and La Mascota are Plio-Pleistocene high- to intermediate-sulfidation epithermal gold deposits hosted in Proterozoic gneisses and Mesozoic granites of the Santander massif, eastern Cordillera of Colombia and are part of the California-Vetas mining district. Coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10.1 Ma are the only igneous rocks younger than Jurassic known from the vicinity of the epithermal deposits. As of 2010, La Bodega and La Mascota contained an inferred resource of 3.47 Moz Au, 19.2 Moz Ag, and 84.4 Mlbs Cu at a 2 g/t Au cut off. Mineralization exhibits NE-trending, NW-dipping structural control associated with the right lateral strike-slip La Baja fault. Mineralization at La Bodega is composed of vein networks and tectonic-hydrothermal breccias while at La Mascota it is largely contained in tabular, fault-controlled tectonic-hydrothermal breccia bodies.

Alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry-style phases and a late epithermal-style phase with up to four distinct stages. Porphyry phases comprise stages 1 and 2. Stage 1 is characterized by propylitic alteration with epidote, chlorite, calcite, specularite veins, minor pyrite, and chalcopyrite. Molybdenite from veins collected 0.7 km southwest of La Mascota yielded an Re-Os age of 10.14 Ma which constrains the age of stage 1. Stage 2 is related to a separate, younger magmatic-hydrothermal system and characterized by phyllic alteration (sericite-illite, quartz, pyrite) associated with quartz-pyrite veins and its age is constrained by 40Ar/39Ar geochronology on hydrothermal muscovite (i.e., sericite) to ~3.4 Ma. The epithermal phase comprises stages 3 through 6 and is related to multiphase hydro-thermal breccia development and quartz-alunite alteration. Stage 3 is defined by presence of copper sulfides such as bornite, covellite, and chalcopyrite, stage 4 by wolframite, stage 5 by enargite, and stage 6 by sphalerite. Pyrite, quartz, and alunite are common to all epithermal stages. Residual vuggy quartz is scarce. Gold-silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2. Alunite 40Ar/39Ar ages constrain epithermal mineralization to between ~2.6 and ~1.6 Ma.

Primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vapor-rich inclusions and, together with quartz vein textures, suggest that boiling was an important ore-precipitating mechanism. Homogenization temperatures of these range from ~143° and 238°C and salinities from 0.5 to 5.6 wt % NaCl equiv. Pyrite δ34S ranges from –16.9 to –11.3‰ at La Mascota and –8.3 to –6.1‰ at La Bodega. The strongly negative sulfur isotope signature can be explained by fluid boiling or, alternatively, by biogenic sulfur source from the Proterozoic paragneisses. Based on δ18O and δD data, alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water.

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