STRUCTURAL EVOLUTION, VEIN ORIENTATION, AND PARAGENESIS OF THE BOTIJA PORPHYRY CU-MO-(AU) DEPOSIT, COLON, PANAMA

The Botija Cu-Mo-Au porphyry deposit is one of seven deposits located in the Cobre Panamá mining district which contains a global resource of 14.8 MT of Cu. These deposits are associated with the 26-33 Ma Petaquilla batholith. The Botija deposit is an elongate tabular body striking NW, dipping 20-40° N, and measures 2 km x 1 km x 600 m in size.

Ore is characterized by disseminated Cu-Fe sulfides, but also occurs in magmatic-hydrothermal quartz veins. Vein measurements at Botija suggest that copper is spatially associated with a quartz vein density >0.5 vol.%. The veins have two modes of strike/dip of 235/50°NW and 295/45°NE. Most early quartz veins in porphyry deposits are emplaced along near-vertical hydrofractures. The dip of quartz veins at Botija suggest the deposit has been tilted about 40°S-SW after emplacement. Typical potassic and sericitic alteration zones are present in Botija, however, a late overprint of chlorite and zeolite has obscured most of the original magmatic hydrothermal alteration.

Although the Botija deposit exhibits many of the common porphyry copper features, its geometry is atypical with abrupt transitions (<10 m) in alteration and grades (Cu & Mo) whereas most porphyry copper deposits typically exhibit gradual transitions over 100s of meters. These sharp breaks correspond with mapped faults from field exposures and drill core. Restoration of ore shells indicate several stages of post-mineral faulting including ~500 m left-oblique normal offset on the Botija fault (267/50°N) and Santa Fe fault (268/70°N), >250 m of normal displacement on the NW striking Oeste fault (314/75°NE), and >500 m offset on other S50°W-striking Strike Central faults (230/65°N). Restoration of these faults bring the ore shells to an appropriate inverted cup-shape geometry typical of porphyry copper deposits. Restoration of the 50°N-dipping quartz veins to vertical restores the tilted ore shells to a classical shape for a porphyry copper deposit. The tilting is likely accommodated by progressive deformation initiated by presently gently dipping, southwest striking normal faults (240/35°NW) faults identified from ZTEM geophysics. This interpretation is consistent with (U-Th)/He ages by Farris et al. (2011) which constrain the exhumation of Botija to a period of localized extension in Panama between 23-25 Ma.