Development of exploration tools for porphyry-breccia-hosted mineral deposits

Abstract

Porphyry Cu systems often host tourmaline breccias, some, or a part of which, may be economically mineralised. Assessing which are significantly endowed, and at what depth, is problematic as there are relatively few models and exploration tools available for these systems. To address this, breccias from El Teniente in central Chile, one of the world’s largest Cu-Mo deposits, were studied. Three mechanisms have previously been suggested for the formation of the breccias at El Teniente: 1) by replacement of rock flour breccias; 2) as magmatic-hydrothermal intrusion-collapse breccias; and 3) diatreme breccias. All tourmaline breccia types at Teniente show cyclical dissolution and precipitation textures indicating interaction with multiple fluids, and contain at least two generations of tourmaline in the cement. The generation of permeability and porosity through re- brecciation and/or widespread dissolution of the cement produced favourable conditions for high-grade Cu-Mo mineralisation, frequently around breccia margins. Significant fluid-wall-rock interaction caused substantial fluid contamination resulting in vertical and horizontal changes in tourmaline composition within the breccia pipes. Explosive devolatilisation and fragmentation of the dacite porphyry underlying the Braden Diatreme generated semi-solidified vermiform clasts that were entrained into the upwardly rising fluidised breccia column. On crystallisation, these exsolved fluids which precipitated molybdenite-quartz (±Cu-Pb-Zn) around the clasts. Magmatic quartz within the dacite clasts, rock flour matrix and the underlying dacite all contain similar internal grain characteristics, CL emission and Ti concentrations, suggesting that the matrix is a highly fragmented derivative of the dacite. Despite the composition of tourmaline being strongly influenced by its mode of formation (clast- vs. cement-hosted) and proximity to wall rocks, it could not be used to discriminate well- from poorly-endowed breccias, which limits its use as an exploration indicator.