La Colosa Au Porphyry Deposit, Colombia: Mineralization Styles, Structural Controls, and Age Constraints

The La Colosa porphyry Au deposit is located on the eastern flank of the Central Cordillera of Colombia, within the Middle Cauca metallogenic belt. The deposit contains more than 800 t (22.37 Moz) Au at grades up to 0.8 g/t and is hosted by a composite porphyry stock of dioritic to tonalitic composition, which was emplaced into Triassic-Cretaceous schists of the Cajamarca Complex in the late Miocene (similar to 8 Ma). The country rocks underwent two ductile deformation events, including development of shear zones, folds, and penetrative foliation, prior to emplacement of the stock. Subsequent brittle deformation reactivated preexisting N- and NNE-trending structures and formed secondary faults due to a change from right- to left-lateral shear sense on regional faults. This switch in stress orientations is attributed to a new plate configuration in the mid-Miocene. The left-lateral movement along regional faults favored emplacement of intrusive centers in dilational pull-apart zones, including the La Colosa stock within the regional Palestina fault zone. The La Colosa porphyry stock was intruded in three stages, termed early, intermineral, and late, all within a relatively short time interval of similar to 1.1 m.y. The early and intermineral stages are diorite porphyries and related intrusion breccias, whereas the late stage consists of quartz diorite and tonalite porphyries. The intrusions caused contact metamorphism and hydrothermal alteration that partially obliterated the original texture and composition of the schistose country rocks. The early and intermineral stages are dominated by potassic alteration, with local chloritic alteration in the core of the intermineral stage, sodic-calcic alteration in the deeper parts of the stock, and propylitic alteration confined to the late stage. Three gold mineralization events are recognized at La Colosa. The first was of porphyry style, during which hypersaline fluids (40-50 wt % NaCl equiv) formed predominantly A- and S-type veinlets and caused multistage wall-rock silicification accompanied by potassic and sodic-calcic alteration. The early-stage intrusions contain the highest gold grades varying from 0.75 to 1 g/t Au, associated with pyrite and minor chalcopyrite, molybdenite, and magnetite in the porphyries and with pyrrhotite-pyrite-melnikovite in the country rocks. In the intermineral-stage intrusions the gold grades drop to 0.5 to 0.75 g/t Au, and pyrrhotite and pyrite are the major sulfides. Gold grades reach low values of <0.3 g/t Au in the late-stage porphyries. The second gold-precipitating event formed sheeted veinlets of drusy quartz and pyrite with centimeter-wide halos of albite-sericite-pyrite overprinting all other alteration types at the deposit. A similar to 200 degrees C hydrothermal brine (21-28 wt % NaCl equiv) deposited gold at high grades (>1.5 g/t Au over >10 m drill core intervals) within N-striking normal faults that developed during and after emplacement of the porphyry stock. The third mineralization event was supergene, with Au enrichment confined to late porphyries and characterized by sulfide boxworks, resulting in gold grade increases from 0.3 to 1.2 g/t Au.