Interpreting and validating trans-lithospheric faults in the CentralAndes to investigate their control on the localisation of giantporphyry copper deposits

The central Andes (between latitudes of 14°S and 35°S) accounts for approximately 40% of the world’s annual copper production and is the most important copper province on the planet. However, since 1998, just one giant porphyry copper greenfield discovery has been made in the central Andes. Post-mineralisation cover consisting of transported gravels and young volcanics make up at least 50% of the surficial outcrop of the central Andes and of the 60 or so known giant Cu ± Au ± Mo deposits, only three giant ore deposits have been discovered beneath these surficial materials in the greenfields domain. Therefore it is likely that many concealed, undiscovered giant porphyry copper deposits are waiting to be discovered. However, no proven effective exploration process exists that enables explorers to consistently achieve economic greenfield discoveries through cover.

Giant porphyry copper deposits tend to cluster in discrete geographic “camps” of a similar age. This indicates that exceptional transient geologic processes have affected localised regions of the crust prior to and during the age of mineralisation and that the formation of giant porphyry deposits is non-random. Key predictive geological features of giant porphyry Cu deposits are the structural pathways (basement faults) that focus fluid and magma flow from the mantle to upper crust. Nevertheless, these so called trans-lithospheric faults (TLFs) are notoriously difficult to identify in the field due to their subtle surficial characteristics,complex multi-stage reactivation history and continental-scale. As a result, the notion of TLFs has, until recently, been treated with scepticism by many in the geologic community.

This research focuses on identifying and mapping the continental-scale trans-lithospheric structural architecture of the central Andes through the integration and interpretation of multiple geoscience datasets supported by field observations. Datasets used in this analysis include geophysical inputs such as airborne magnetics, regional gravity, magneto-tellurics and seismic epicentres as well as geologic reconstructions through time from the Proterozoic to present, which map out inherited basement architecture as well as regions of rapid crustal thickening or thinning. Fieldwork undertaken in the regions of the interpreted TLFs demonstrates that on the surface they are expressed as linear zones of brittle faulting, tens of kilometres wide and hundreds of kilometres long,consisting of thousands of individual fault planes. This is interpreted to reflect the upper crustal propagation of the underlying zone of basement weakness through younger sequences in the geologically active convergent plate margin.

A map of the TLFs across the central Andes shows that the TLFs have a fractal distribution with N, NW and NE strikes. A prominent relationship exists with the location of known giant porphyry deposit camps occurring where two or more TLFs meet.Such regions are inferred to have been loci of deep-seated strain-anomalies which have localised dilation and increased permeability, during transient changes to the regional stress field. Regions adjacent to the intersection of two or more TLFs that overlap with the magmatic arc during metallogenic epochs (themselves transient geodynamic anomalies) are deemed to represent valid exploration targets in this model.