Structural Setting of the Candelaria Fe Oxide Cu-Au Deposit, Chilean Andes (27 30' S)

Abstract

The Candelaria Fe oxide Cu-Au ore deposit lies within the thermal aureole of the Lower Cretaceous magmatic arc plutonic suite in the Candelaria-Punta del Cobre district, Atacama region, northern Chile (27°30′ S). Economic mineralization is present within a synplutonic ductile shear zone (Candelaria shear zone) containing foliated and potassically altered volcanic and volcaniclastic rocks of the Punta del Cobre Formation. In the Candelaria shear zone, there was a complex interplay in space and time between ductile and brittle deformation during mineralization. At high structural levels brittle, moderately to steeply dipping extensional faults curve downward into gently dipping ductilely deformed rocks in the shear zone. Textural and structural characteristics of the ore and the deformed rocks indicate that the dilation of the sulfide-bearing vein systems occurred at the same time as ductile deformation and implies that ductile-brittle cycling occurred in a cooling hydrothermal system in the thermal aureole of the arc plutonic rocks. The La Brea and the San Gregorio plutonic complexes are exposed in the plutonic arc immediately adjacent to the Candelaria orebody. The intrusions are interpreted to be flat-lying, tabular bodies emplaced by roof uplift-floor depression mechanisms during regional extensional deformation. An 40 Ar/ 39 Ar age of 119.6 ± 1.2 Ma and five K-Ar ages between 123 and 117 Ma were obtained for the La Brea pluton, and an 40 Ar/ 39 Ar age of 111.5 ± 0.4 Ma was obtained for the San Gregorio plutonic complex. 40 Ar/ 39 Ar ages of 111.0 ± 1.4 and 110.7 ± 1.6 Ma obtained from syntectonic biotite from the Candelaria shear zone are interpreted to be the age of the mineralization. These are indistinguishable from the age of the San Gregorio plutonic complex, and therefore heat from this pluton could have promoted ductile deformation during emplacement of the orebody. This result, together with known isotopic similarities between Candelaria ore and the plutonic rocks, implies that the San Gregorio plutonic complex was the most probable source of the mineralizing fluids at Candelaria, although the critical intrusive unit may not be exposed. The ore deposit is located where a roof-uplift fold in the side-wall of the San Gregorio plutonic complex provided a dilatational site which may have drawn hydrothermal fluids out of the adjacent fractionating magmas into a structural trap in the Punta del Cobre Formation below the carbonate rocks of the Chanarcillo Group.