Abstract
Botija is one of seven Cu-Mo-Au porphyry deposits in the Cobre Panama district, which currently has a global resource of 4.2 Gt at a grade of 0.35 wt % Cu. Host rocks include a sequence of Eocene andesites that are intruded by equigranular and porphyritic intrusions of the 32 to 28 Ma Petaquilla batholith. The Botija deposit is an NW-elongated tabular body that dips 10° to 45° N, measures 2 by 1 km, and is 600 m thick. Ore is closely associated with intrusions of crowded granodiorite porphyry.
Cu-Fe sulfides (chalcopyrite > bornite) at Botija are largely disseminated but are also present in hydrothermal quartz veins associated with potassic alteration. These quartz veins are cut by pyrite ± quartz (D-type) veins with sericitic alteration selvages. A younger overprint of chlorite and zeolite has obscured much of the potassic alteration. Systematic vein measurements demonstrate that a quartz vein density of >0.5 vol % is coincident with >0.5 wt % Cu ore. Quartz veins have two orientation modes (azimuth of strike/right-hand dip) that average 233°/50° NW and 295°/45° NE, and D veins have 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 the Botija deposit suggests postmineralization tilting of 40° south-southeast. Geologic mapping of field exposures and drill core has identified a series of W- (268°-275°/50°-70° N), SW- (230°/65° NW), and NW-striking (314°/75° NE) normal and oblique-slip normal faults that cut the deposit and displace hydro-thermal alteration and Cu, Mo, and Au grade contours.
A three-dimensional geologic model of Botija was constructed from alteration, vein, assay, and structural data collected from field mapping and drill core logging. This model suggests that the Botija deposit has undergone postmineral northwest-southeast extension, which is responsible for its atypical elongate geometry. Restoration of the modeled Cu-Mo grade contours along identified faults removes 10° to 15° of tilting to the southeast and produces a single ore shell with an inverted cup-shaped geometry typical of many porphyry copper deposits. Restoring the ore shells to an upright geometry requires removal of an additional 25° to 30° of southeast tilting that we infer was produced by earlier generations of SW-striking and NW-dipping faults.
The seven porphyry deposits observed today at Cobre Panama were likely produced by normal faulting that dismembered and tilted two or more originally upright porphyry copper deposits. Additional parts of orebodies that are offset by faults and concealed likely remain undiscovered. We infer that published (U-Th)/He uplift and exhumation ages of 19.5 to 22.3 Ma record the period of normal faulting and extension that deformed the Cobre Panama district shortly after ore formation at 29 to 28 Ma.
