Structural geology of the Andes of central Chile : controls on magmatism and the emplacement of giant ore deposits

The Andes of central Chile contain the world’s largest concentrations of Cu and Mo in two porphyry-type deposits of late Miocene to early Pliocene age: Rio Blanco-Los Bronces and El Teniente. The mineral deposits are hosted by a belt of Tertiary volcanic and intrusive rocks. The geologic evolution of this late Eocene to early Pliocene magmatic arc is characterized by the opening and subsequent inversion of the intra-arc volcano-tectonic Abanico Basin; mineralisation was coeval with the last pulses of tectonic inversion. Individual segments of the inverted basin-margin faults have been documented by previous works, but the internal architecture of the Abanico Basin remains poorly understood. Most of the syn-inversion plutonic bodies and in particular the mineral deposits and their associated sub-volcanic complexes were emplaced in the central part of the inverted basin; consequently, relationships between magmatism, hydrothermal activity and upper-crustal structures in this porphyry Cu-Mo belt have remained obscure. Based on a combination of new structural and stratigraphic mapping, cross-section interpretation, dynamic and kinematic analysis of fault-slip data, regional geophysical datasets, U-Pb and 40Ar/39Ar geochronology, (U-Th)/He thermochronology and whole-rock geochemistry, this study describes the structural architecture and evolution of the Abanico Basin and its relationship with magmatism and the emplacement of porphyry Cu-Mo deposits. The evolution of the Andes of central Chile, and in particular the emplacement of Miocene plutonic bodies and late Miocene-early Pliocene porphyry Cu-Mo deposits, was strongly controlled by NW- and NE-striking fault systems, oblique to the N-striking basin-margin faults and to the general trend of the magmatic arc and the Andean orogen. They correspond to deep, long-lived, translithospheric weakness zones which have been recognized in the basement of the Andes to the east and west of the study area. These structures remain active under the current stress regime, as shown by the distribution of upper-crustal earthquakes which form clusters and alignments along them. Arc-oblique fault systems were active as normal ± strike-slip faults during the opening of the Abanico Basin in the late Eocene-Oligocene, as shown by stratigraphic correlations and syn-extensional pyroclastic deposits. These faults controlled the compartmentalization of the basin into individual sub-basins with characteristic volcano-sedimentary facies and thicknesses. Fault plane kinematics and 40Ar/39Ar dating of syn-tectonic hydrothermal minerals, in turn, demonstrates that arc-oblique faults were reactivated during Mio-Pliocene tectonic inversion as strike-slip ± reverse faults. Syn-inversion magmatic and hydrothermal activity was channelled and focused by both sets of pre-existing arc-oblique structures, and in turn fault rupture was promoted by high fluid pressures. The kinematic and dynamic analysis of fault-slip data shows that structural reactivation during tectonic inversion was concentrated in the mining districts and around major plutons. This suggests feedback between magmatic and hydrothermal activity, fluid pressure, and the reactivation under compression of the structural architecture inherited from the extensional period. In the Rio Blanco-Los Bronces segment, fault-slip kinematics is consistent with fault reactivation under E- directed compression, while in the El Teniente segment the compression direction was ENE. In the inverted margins of the Abanico Basin, compression was accommodated by reverse faulting (sub-vertical σ3), while in the central part of the basin, where the rock column is considerably thicker and the topography higher, a strike-slip regime (sub-vertical σ2) was predominant during Miocene-early Pliocene tectonic inversion. Based on the results of geological mapping, U-Pb and 40Ar/39Ar geochronology and whole-rock geochemistry, the Abanico Basin can be divided into two main segments, separated by the NW-trending Piuquencillo fault and conjugate, NE-trending structures. The northern segment contains the Rio Blanco-Los Bronces cluster, while El Teniente is located in the southern segment. Volcanic and sedimentary rocks of the northern segment can be grouped into the syn-extensional Abanico Formation (late Eocene-Oligocene) and the syn-inversion Farellones Formation (early to middle Miocene). In the southern segment, syn-extensional rocks are grouped into the Coya-Machali Formation (late Oligocene-middle Miocene) which is overlain by the syn-inversion Teniente Volcanic Complex (middle to late Miocene). Stratigraphic units defined in the northern segment cannot be correlated with the rocks of the southern segment in terms of lithofacies, depositional ages and geochemistry. The northern and southern segments show temporal differences in their tectonic evolution. An early deformation event beginning at ~22 Ma affected only the northern segment and is associated with the formation of progressive unconformities between the Abanico and Farellones formations and with crustal thickening, reflected in the geochemistry of the Farellones Formation. A second stage of crustal thickening and exhumation began at ~12 Ma, resulting in a sharp increase of the La/Yb ratios in the northern segment, whereas in the southern segment it is reflected by a moderate increase of La/Yb ratios and the transition from the strongly folded Coya-Machali Formation to the flat-lying to gently folded Teniente Volcanic Complex. Finally, a third stage of exhumation and crustal thickening beginning at ~7 Ma affected both the northern and southern segments. According to (U-Th)/He thermochronology in zircons and apatites, this last stage was the main exhumation event affecting the rocks of the Andes of central Chile. It is recorded in the geochemistry of igneous rocks by a sharp increase in the La/Yb ratios in the southern segment. As the crust thickened and the tectonic regime changed from extensional to transpressional, the rates of volcanic output diminished and magmas began to pool in the upper crust. This is reflected by the crystallization ages of Tertiary plutons, which are coeval with tectonic inversion of the Abanico Basin. The oldest Tertiary plutons were emplaced during the early Miocene. They occur only in the northern segment and are coeval with the transition between the Abanico and Farellones formations. The oldest plutonic rocks of the southern segment were emplaced in the middle Miocene, coeval with the transition between the Coya-Machali Formation and the Teniente Volcanic Complex. Intersections of conjugate, arc-oblique strike-slip faults are a favourable geometric location for the emplacement of magmatic bodies and hydrothermal breccias related with porphyry Cu-Mo systems. Both porphyry Cu-Mo deposits (Rio Blanco-Los Bronces and El Teniente) are located at the intersection of regional-scale, conjugate strike-slip fault systems. Favourable areas for exploration can be defined based on the criteria that they must be located at the intersections of major arc-oblique conjugate fault systems and that the area contains evidence of Miocene-early Pliocene intrusive activity. The new knowledge generated by this work about the structural architecture of the Andes of central Chile can also be applied to problems such as the location of active hydrothermal systems (economically important as geothermal energy reservoirs), the position and alignment of active volcanic centres, and the geological hazards associated with active, upper-crustal faults.