Geology & Mineralization
The geology at the El Roble mine and nearby volcanogenic massive sulfide (VMS) mineral occurrences on the concession block include basalt flows, black to grey chert and overlying pelagic sedimentary rocks and sandstone-shale turbidites (Ortiz et al., 1990). Whole rock (NaO, K2O, MgO, FeO) analyses reported by Ortiz et al. (1990) place the basalt flows in the tholeiitic field. These rock units belong to the Cretaceous Cañasgordas Group which can be traced for over 800 kilometres along the western cordillera of Colombia. Locally, mafic volcanic rocks including pillow basalts, tuffs, hyaloclastites and agglomerates are referred to as the Barroso Formation; pelagic sedimentary rocks including chert, siltstone and minor limestone belong to the Penderisco Formation. All of these rock units were deformed and metamorphosed during Late Cretaceous to Tertiary accretion to continental South America. Accretionary tectonics resulted in both low-angle and high-angle faulting of the stratigraphy.
The stratigraphic contact between volcanic rocks and black and grey cherts has been traced by Atico geologists for ten kilometres across the El Roble Project area. This contact has been determined to be an important control on VMS mineralization and will be the focus of the exploration program.
Massive sulfide mineralization is dominantly pyrite and chalcopyrite. The bulk of mineralization mined to date is fine-gained, massive sulfide with little internal structure or banding. Recently stockwork mineralization consisting of chalcopyrite with subordinate pyrite in massive veins and patches in a gangue of stockwork quartz and chlorite veins has been intersected in production exploration drill holes between the 2000 and 1980 meter levels.
Gangue minerals include quartz and chlorite along with lesser calcite, dolomite and minor hematite and magnetite. Euhedral, subhedral, crushed and colloform pyrite grains measure approximately 200 microns in diameter but vary from 0.04 to 0.01 millimetres. Chalcopyrite fills the space between pyrite grains along with minor pyrrhotite, sphalerite, magnetite, and electrum (as 10 to 100 micron irregular grains). No other sulfide minerals are reported.