STRUCTURAL GEOLOGIC EVOLUTION OF THE BOTIJA PORPHYRY CU – MO – (AU) DEPOSIT, PANAMA

Botija is one of seven Cu – Mo – Au porphyry deposits in the Cobre Panamá mining district that are associated with porphyritic intrusions of the 26-33 Ma Cerro Petaquilla batholith, and contains a global resource of 16.4 MT of Cu. Botija is a northwest – elongate tabular body that dips 10 – 45° north and measures 2 km by 1 km and is 600 m thick.

Systematic vein measurements in oriented drill core and outcrop demonstrate that a quartz vein density >0.5 vol.% is coincident with >0.5 wt.% Cu ore. These quartz veins are stable in K-silicate alteration and cut by D-type pyrite ± quartz veins with sericitic selvages. Quartz veins have two dominant orientation modes (azimuth of strike/right – hand dip) that average 233°/50° NW and 295°/45° NE, and D veins have dominant orientation modes of 295°/73° NE, 260°/69° NW, and 228°/48° NW. Quartz and D veins in numerous porphyry deposits globally occupy near – vertical hydrofractures. Accordingly, the dip of veins at Botija deposit suggests moderate tilting of 40° south – southeast after mineralization. A series of west (367°/50°–70ºN), southwest (230/65º NW), and northeast (314/75º NE) striking normal and oblique – slip normal faults cut the deposit and displace hydrothermal alteration and Cu, Mo, and Au ore zones.

A new three – dimensional geological model of Botija was constructed and is used to illustrate that post-mineral northwest – southeast extension is responsible for the present atypical geometry. Restoration of the modeled Cu – Mo grade contours along faults removes 10-15° of tilting to the southeast and produces a single ore shell with an inverted cup – shaped geometry typical of porphyry copper deposits. In order to restore the 50° N – dipping quartz veins to vertical and the ore shells to an upright geometry an additional 25 – 30° of southeast tilting must be restored.

We infer that this southeast tilting was produced by district – scale southwest (240º) and northeast (050º) striking faults which dip 35º NW and 70º SE, respectively, and can be identified on ZTEM geophysical maps and Lidar imagery. The seven porphyry deposits observed today at Cobre Panamá were likely produced by normal faulting that dismembered and tilted two or more originally upright porphyry copper deposits. Additional fault offset and concealed parts of ore bodies likely remain undiscovered.