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 (~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 ~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 ~200°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 characte...
The San Dimas district, Durango, Mexico, was investigated to define the structural control of epithermal AgAu mineralization and to unravel the tectonic history of the area. Three distinct deformational events have been identified. Deformational event D1 is characterized by the formation of subvertical east-west-to east-northeast-west-southwest-trending tension gashes and hybrid extensional shear fractures that were caused by the ongoing but weakened compression due to subduction west of Mexico during the Early Oligocene. Changing stress conditions during deformational event Ds developed north-south-trending right-lateral strike-slip faults with accompanying secondary structures, forming a complex pattern of a transtensional corridor. The structures of both D1 and D2 are hosted by the Lower Volcanic Group and the underlying Piaxtla intrusive rocks, and they carry low-sulfidation epithermal silver-gold mineralization. Subsequent extension on reactivated north-south-trending strike-slip faults and northwest-southeast-trending normal faults established Basin and Range-type tectonics through both the Oligocene and Miocene. The transition from compression to extension is documented by this strike-slip corridor which was established during the Late Oligocene.
Digital data acquisition, data management and 3D modelling techniques are common techniques in the mining industry. On the other hand, civil engineering projects still lag behind in applying advanced technologies during geological reconnaissance and investigation. The La Colosa gold mining project (Colombia) is presented as an example, where sophisticated digital mapping techniques and 3D geological modelling is not only used for mining related issues, but is also successfully applied for the geological, geotechnical and hydrogeological investigations of adjacent civil engineering sites of the associated mine infrastructure.
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