Summary:
Deposit Type
Deposits found at El Bagre and Nechí are best described as structurally controlled mesozonal gold-silver sulphide-bearing quartz vein lode deposits, with veining emplaced in shear zones within competent intrusive host rocks.
Mesozonal veins occur in rock assemblages of Archean, Proterozoic, and Phanerozoic age, commonly hosted in metamorphosed mafic volcanic flows (greenstone-hosted type) and sedimentary rocks (slatebelt or turbidite-hosted type). In other cases, the veins are located within the contact aureole of granitic intrusions in various host rocks, as is the case at the Properties.
Geology
The Soma underground mining concessions lie within the Central Cordillera, one of the three physiographic subdivisions (Central, Western, and Eastern Cordillera) of the Andes Mountains in northern South America. The area is part of the Bagre-Nechí mining district that includes gold mineralization within the Bagre, Zaragoza, and Nechí regions. The vast majority of historical gold production comes from alluvial deposits within the river basins, including the Nechí River. In addition to alluvial gold, vein-type gold mineralization is present and can be followed for up to three kilometres. Mineralization is hosted within intrusive rocks observed near Nechí, El Bagre, and Zaragoza.
Cordilleran rocks in the area have had a complex geological and tectonic evolution, spanning from the Precambrian era to the Quaternary age.
At El Bagre, the veins of La Ye, Mangos, and Cordero are hosted in shear zones with brittle-ductile deformation within carboniferous granite rocks of the El Carmen stock. At the regional level, the main structure to the west is the sinistral Otú fault system which trends north-south to north-northwest near the municipality of Zaragoza. This fault which brings into contact carboniferous plutonic rocks from the El Carmen stock with metamorphic Permo-Triassic rocks grouped regionally as the Cajamarca Complex (Londoño et al., 2009).
Mineralization
Auriferous quartz-sulphide veins are hosted within the El Carmen – El Cordero stock (Leal-Mejía, 2011). The numerous veins trend north-northwest to north-northeast, the most important of which include the El Carmen and La Ye systems, which can be traced over five kilometres. Mineralization is hosted in the structurally controlled quartz veins and is often associated with late, brittle reactivation of the fault zones.
The veins consist of a milky white quartz containing native gold and up to 20% mixed sulphides, dominated by pyrite with occasional galena, and chalcopyrite. Sulphide and native gold distribution within the veins is patchy. The veins average approximately one metre in thickness and range from half a metre to four metres in length. Related wall rock alteration includes haloes of moderate to pervasive sericite ± chlorite and carbonate replacing feldspar within the host intrusive.
The Cordero Deposit is hosted in the El Carmen Stock, an early Carboniferous-age tonalite to diorite intrusive that occurs along the western boundary of the Segovia Batholith. Gold mineralization in the Carmen Stock is hosted in shear-zone-controlled quartz veins that strike north-northwest and dip moderately to the northeast in the Cordero and Los Mangos mines. In contrast to the eastern deposits, quartz veins in the La Ye Mine notably have a northerly strike and dip steeply to the west. Gold mineralization in the quartz-carbonate veins is associated with pyrite, galena, sphalerite and tellurides, and locally free gold.