Summary:
The Cerro Bayo and Laguna Verde Mine Complexes together comprise the overall Cerro Bayo Project.
Local Geology
The rocks exposed in the Cerro Bayo District are part of a thick pile of silicic volcanic rocks assigned to the Ibañez Formation and are believed to represent a more or less continuous record of Jurassic to Cretaceous volcanism. The Palaeozoic basement that regionally underlies the Mesozoic volcanic rocks is not exposed in the district, at least to a depth of 370 m from the surface, although fragments of the metamorphic rocks are contained in the overlying volcanic rocks.
The Mesozoic units are overlain by Tertiary basalts and minor volcaniclastic rocks. The volcanic sequence that hosts the precious metal mineralization is interpreted to be related to a system of large volcanic centres and rhyolitic domes, the most prominent being the Cerro Bayo-Mallines dome complex, which is hosted in a regional north-south trending fracture zone. The volcanic sequence consists of alternating units of pyroclastic rocks of rhyolitic to dacitic composition, including basal surge deposits, interbedded with ash fall tuffs, lavas, and sediments. The older units are exposed at Laguna Verde, while the youngest are exposed at Brillantes. The thickness of individual flows varies from tens to over a hundred meters, however, an estimate of the total thickness of the sequence is difficult to determine due to block faulting of the units. Several volcanic cooling units displaying varying degrees of welding are recognized in the area. Radiometric dates ranging from 130 Ma to 159 Ma have been produced in volcanic and intrusive rocks from the Ibañez Formation south of the Cerro Bayo District using K/Ar and Ar/Ar methods.
Extensive basaltic flows (Meseta Lago Buenos Aires Formation) top the sequence to the south of the district, representing the westernmost outcrops of a large basaltic province. A 300m thick sequence of olivine tholeiites to alkaline basalts is exposed, consisting of 5m to 25m thick flows, locally separated by interflow detrital or tuffaceous horizons. Radiometric dating indicates two main cycles of effusion, during the Eocene and Miocene to Pliocene respectively.
Intrusive rocks are sparsely exposed at Cerro Bayo, and are restricted to Mesozoic and Tertiary subvolcanic domes, plugs, and dyke swarms of varying compositions. Mesozoic intrusions are generally sub-circular, flow-banded rhyolitic and dacitic domes characterized by columnar jointing and sharp walls that rise 100m to 250m above the surface. The most prominent of these is the Cerro Bayo Dome. Their emplacement is controlled by deep regional north-south fractures and/or intersections of major faults. The domes and a set of dacitic flow banded dykes are estimated to be cogenetic with the felsic tuffs, predating and postdating the main mineralization events. Andesitic dyke swarms controlled by east-northeast fractures are common across the district, postdating mineralization. Tertiary necks and plugs of massive basalts and dioritic-gabbro porphyries are exposed in the southern highlands of the district, generating local argillization of the host rocks. These bodies are cogenetic with the Tertiary basaltic volcanism.
Mineralisation – Cerro Bayo Mine Complex
In the Cerro Bayo Mine Complex (“CBMC”), gold and silver mineralization occurs in veins, vein systems and veinlets hosted in a moderately welded sequence of dacitic and rhyolitic tuffs. The volcanic sequence is intruded by the Cerro Bayo and other dacitic domes, considered to be post-mineral events.
The main vein systems have a 325° to 345° orientation, vary in dip from vertical to 75° northwest and southeast, with strike lengths between 50m and 1,200m. Additional extensions in excess of 1,000m have been disrupted by major faults trending east to northeast. Widths are highly variable between the different vein systems and within individual veins along strike and down dip, varying from centimetres up to 5m.
Mineralization is typically low sulphidation, epithermal, and representative of the main mineralization stage in the district. Veins are normally brecciated with local banded textures. They consist mainly of fine-grained quartz and chalcedonic silica, adularia, and fluorite, with minor amounts barite and carbonates. Sulphide content is less than 5% and consists mainly of pyrite and silver sulphosalts as disseminations and bands. Moderate argillic alteration is common in the area, with strong silicification and silico-argillic alteration occurring as haloes along the veins
Mineralisation – Laguna Verde Mine Complex
Gold and silver mineralization occur in veins, hydrothermal and tectonic breccias, stockworks (sheeted veins) and veinlets hosted in a moderately to strongly welded sequence of rhyolitic and dacitic tuffs. Post-mineralization flow-banded dacitic domes intrude the volcanic sequence at Coigues Hill.
Two main vein systems are recognized at the Laguna Verde Mine Complex (“LVMC”):
• North-south to north-northeast trending brecciated veins and breccias varying in dip from vertical to 45° E.
• 315° to 345° oriented veins varying in dip between vertical and 75° northwest and southeast.
Strike lengths up to 800m have been recognized in some of the vein systems. Widths are highly variable between the different vein systems and within individual veins along strike and down dip, varying from centimetres up to 50m in breccias and stockworks (sheeted zones).
Two different mineralization events can be recognized at Laguna Verde.
• A mesothermal early-stage Ag-Mo-Zn-Pb event with lesser gold.
• A late-stage epithermal low sulphidized gold-silver rich system, representative of the main mineralization stage of the district (Delia, Coigues Este, and Tranque vein systems).
Brecciated veins and tectonic breccias are the typical structures of the early-stage mineralization, whereas the late-stage epithermal mineralization is represented by banded veins that are locally brecciated. The veins consist mainly of fine-grained quartz and chalcedonic silica, adularia, and fluorite, with minor amounts of barite and carbonates. The overall sulphide content is less than 5%, although higher in the early-stage event. Sulphides are mainly pyrite, silver sulphosalts, and local sphalerite disseminations, clusters, and bands.