The Andes mountains of Colombia consist of 3 ranges (Cordillera), each underlain by a series of former magmatic arcs which were emplaced along the western margin of South America from Jurassic to upper Miocene. The Andes project lies on the eastern flank of the western cordillera.
Most of the project area overlies former oceanic crust that was accreted onto the South American plate during the Neogene (formerly Late Tertiary). These sequences of oceanic crust, mafic volcanics and related sediments, intruded by calc-alkaline intermediate intrusions provide a somewhat different geological setting to much of the Andes intrusive belts in countries to the south.
In addition to the accreted oceanic crust, the Colombian Andes also differ from classical Andes geology developed in Chile and Peru in having had a highly oblique plate collision and low-angle and limited subduction.
Precious and base metal mineralisation has developed in the Colombian Andes in a wide range of ages and geological settings. The Farallones Batholith (11-12 Ma) in the Andes Project area, host significant precious metal bearing vein mineralization both in the intrusive rock and adjacent hornfelsed meta-sediments. An eastern belt of 6-8 Ma porphyries hosts several types of precious metal mineralization including sheeted veins at Marmato, porphyry Au-Cu at Titiribi and La Colosa, breccia and porphyry mineralization at Miraflores, and the carbonate base metal vein system at Buriticá.
Most of the Andes Project tenure lies in or near the contact zone of the granodioritic Farallones Batholith which has intruded into flysch sediments. A major terrane boundary fault system, separates the easternmost tenements from the bulk of the tenements.
Within the Andes Project area some 150 former and current artisanal mines have been recorded with vein widths varying from a few cm to over 10 metres and mined vertically up to 700m.
The majority of veins directions trend broadly in one of 2 directions, southeast or northeast and steeply dipping, generally sub-vertical.
Further east, this intrusion is associated with epithermal gold deposits loosely associated with porphyrytic intrusions, and tentatively with Porphyry Copper Deposit magmatic-hydrothermal systems. At Andes, the granodioritic intrusions found at Gibraltar could be part of the Farallones magmatism.
Mineralization at El Columpio and San Agustin consist of 0.5 to 3 m thick sub-vertical and tabular sericite-pyrite altered zones, with or without quartz veins. Both the El Columpio and San Agustin, sub-parallel sericite-pyrite±quartz structures have tens of meters of exposed horizontal and vertical extent and are spaced every 20 to 50 m. The lateral extent of the area with mineralized structures is not constrained and remains open in all directions. Previously reported Au-concentrations of the sericite-pyrite-rich zones are promising and if low Au-grade exists between the main zones, the prospect could be amenable for bulk underground mining.
The veins observed are typically mid-sulfidation consisting of disseminated to semi-massive pyrite and pyrrhotite with variable amounts of arsenopyrite, galena, sphalerite and rarely chalcopyrite and other copper sulphides. The veins also contain variable amounts of quartz and generally lesser quantities of carbonate. The veins are commonly brecciated.
The host rocks observed in the area are generally fine grained metasediments, probably turbiditic, with weakly developed slaty cleavage. These are variably hornfelsed and in the vicinity of some veins silicified over widths of many metres.