Structural Evolution, Vein Orientation, and Paragenesis of the Botija Porphyry Cu-Mo-(Au) Deposit, Colón, Panamá Public Deposited


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  • The Botija Cu-Mo-Au porphyry deposit is located in the Cobre Panamá mining district, which contains several deposits with a global measure and indicated and inferred resource of 14.8 MT of Cu. These deposits are associated with the Cerro Petaquilla batholith, which has U/Pb zircon ages of 26-33 Ma (Whattam et al., 2012; Baker et al., 2016). The Botija deposit is an elongate tabular body striking northeast and dipping 20-40° N and measures 2 km (length) x 1 km (width) x 600 m (height) in size. The deposit is hosted in andesite, the equigranular Petaquilla granodiorite, and a younger crowded porphyritic granodiorite with 15-50 vol.% groundmass of quartz and K-feldspar. Phenocrysts include plagioclase, hornblende ± K-feldspar and occasionally 5-15 vol.% rounded quartz eyes. Ore is characterized by Cu-Fe sulfides (chalcopyrite > bornite) that is dominantly disseminated, but also present in magmatic-hydrothermal quartz veins. Systematic vein measurements at Botija demonstrate that copper ores are spatially associated with a quartz vein density >0.5 vol.%; these veins have two modes of structural orientation (azimuth of strike/right-hand dip) of 233/50°NW and 295/45°NE. Most similar early quartz veins are emplaced along near-vertical hydrofractures in numerous porphyry deposits globally. The present dip of these quartz veins suggest that the veins and the Botija deposit have been moderately tilted about 40° south-southeast after mineralization. Typical potassic and sericitic alteration and zonation is present in Botija, however, a late overprint of chlorite and zeolite has obscured much of the original magmatic hydrothermal alteration footprint. Although the Botija deposit exhibits many of the common porphyry copper features, its geometry is atypical with abrupt spatial transitions (<10 m) in alteration and grades (Cu, Mo & Au) whereas most porphyry copper deposits typically exhibit gradual transitions in over hundreds of meters. These sharp breaks in the geology in some cases correspond to observed faults, and together with other faults mapped in field exposures and drill core allow identification of three main fault sets that offset the deposit. A restoration of the Cu and Mo grade shells indicates a multistage post-mineral deformation history with a first stage of ~500 m left-oblique normal offset on the Botija Fault (267/50°N) and Santa Fe fault (268/70°N), normal displacement in excess of 250 m on the NW striking Oeste fault (314/75°NE) and a final stage of normal displacement in excess of 500 m offset on S50°W-striking Strike Central faults (230/65°N). Restoration of these fault offsets brings the ore shells to the appropriate inverted cup shaped geometry typical of porphyry copper deposits. The faults identified at Botija likely accommodated a modest amount (~10-15°) of tilting to the southeast. An additional restoration of the 50°N-dipping quartz veins to vertical restores the ore shells to geometry in line with the classical shape for a porphyry copper deposit. This restoration of quartz veins can be explained by progressive deformation initiated by presently shallowly dipping, southwest striking normal faults (240/35°NW) identified from ZTEM™ geophysics which bound the deposits of Cobre Panamá. This reconstruction suggests the deposits of Cobre Panamá were likely dismembered by normal faulting from two or more original upright and intact porphyry copper deposits, generating the seven segmented and tilted deposits present today. This interpretation of normal displacement of the deposit is in lines with (U-Th)/He age by Farris et al. (2011), which constrain the exhumation of the deposit during a period of localized extension in Panamá between 19.5 – 22.3 Ma shortly after emplacement.
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