Download Structural controls on the emplacement of porphyry systems: the

0393-000037
Structural controls on the emplacement of porphyry systems: the case of the Rio BlancoLos Bronces cluster, Andes of Central Chile*
Corresponding author: Jose Piquer, CODES, University of Tasmania,
[email protected]
Co-authors:
Jorge Skarmeta, Gerencia de Exploraciones, CODELCO-Chile, [email protected]
David R Cooke, CODES, University of Tasmania, [email protected]
In several porphyry Cu-(Mo-Au) clusters, individual deposits are aligned following major crustal
weakness zones, very commonly oriented oblique to the axis of the respective magmatic arc. The
characterization of such weakness zones in the field, however, is difficult as they have been
obscured by magmatic and hydrothermal events and sometimes by younger and more evident arcparallel fault systems. Here we present a new structural interpretation for the district around the
strongly elongated Rio Blanco-Los Bronces porphyry Cu-Mo cluster, which contains the world’s
largest exploitable concentrations of both Cu and Mo. This interpretation is based on recently
completed district-scale geological mapping, cross-section interpretation, and U-Pb
geochronology.
The Rio Blanco-Los Bronces district is part of a Tertiary magmatic arc which is characterized by
the opening and subsequent inversion of an intra-arc volcano-tectonic basin, the Abanico Basin.
In the district the transition between syn-extension and syn-inversion volcanic rocks is marked by
progressive unconformities formed around the Oligocene-Miocene boundary. Intrusive activity
began in the Early Miocene, immediately after the change in tectonic regime from extension to
compression, and culminated in the Early Pliocene. We have identified NW- and NE-oriented fault
systems which can be followed throughout the district, oblique to the N-oriented margins of the
Abanico Basin. They can be correlated with Mesozoic and Palaeozoic faults which crop out further
to the west, suggesting that they correspond to pre-Tertiary structures present in the Andean
basement. These oblique faults control violent changes in facies and thicknesses of Tertiary
volcanic rocks, with stratigraphic correlations indicating normal movements of hundreds of
meters. Kinematic indicators given by syn-tectonic hydrothermal minerals, however, show that
these structures were later reactivated under E- directed compression during tectonic inversion,
with dextral movements in the NE faults and sinistral displacement in the NW structures, both
with varying degrees of reverse component. The Rio Blanco-Los Bronces cluster was emplaced at
the intersection of two regional-scale, NW- and NE-oriented faults. Major mineralized centres are
aligned following the NW structure, but several generations of dikes and veins were emplaced
along both fault systems. Space for the emplacement of NW-oriented hydrothermal breccias,
which are ubiquitous in this cluster, could have been created by the interplay between these two
strike-slip fault systems.
The geometry and position of several of the world’s major porphyry clusters appears to be
controlled by similar arc-oblique fault systems, sometimes interacting with arc-parallel faults.
Well-documented examples include the Chuquicamata porphyry Cu-Mo cluster in northern Chile,
emplaced along the NNE-trending Mesabi fault and equivalent structures at its intersection with
the Calama-Olacapato-El Toro NW-oriented fault zone and the arc-parallel, N-oriented Domeyko
fault system; the Oyu Tolgoi district in Mongolia, emplaced along a NNE-trending, arc-oblique
structural corridor; and the porphyry Cu-Au systems of the Cadia district in New South Wales,
Australia, emplaced along the strongly oblique, WNW-trending Lachlan transverse zone. A proper
recognition of these long-lived, regional or continental scale oblique fault systems could be of
crucial importance for exploration success under pre- and/or post-mineral cover in porphyry belts.