The Los Bronces deposit is part of the Miocene Pliocene Río Blanco – Los Bronces porphyry copper system with mineralisation extends 9km from North to South and 3km from East to West. The deposit is open at depth and laterally.

The main ore body is a complex assemblage of hypogene copper-molybdenum sulphide breccia hosted mineralisation that was subsequently overprinted by surface-related supergene processes. The hypogene sulphide mineralisation shows a close relationship with the intrusion of porphyry dykes and sills, development of large-scale alteration zones, vein development, and breccia emplacement. The ore body is related to at least seven hydrothermal breccia pipes forming a large elliptic body of 2km in length, 0.7km wide and 1.0km in depth. The shape of the breccia system is ‘funnel-like’, characterised by sharp contacts with the host rocks in the upper part of the column and transitional at depths.

The host rocks are the volcanic sequence of the Farellones Formation and the plutonic rocks of the San Francisco Batholith which intrude the Farellones Formation. Copper mineralisation is associated with the emplacement of a complex system of porphyry intrusions (quartz monzonites and quartz monzodiorites) and hydrothermal breccias. Early copper and molybdenum mineralisation is hosted in the porphyry stock, in high-grade breccia bodies and is disseminated throughout the bordering country rocks.

Breccia-hosted mineralisation is typically a blend of mineralisation inherited from the source rock and hosted in the breccia clasts and sulphides disseminated in the breccia matrix, or in the case of the matrix-poor Donoso breccia, rimming the clasts. Breccia mineralisation is dominated by pyrite- chalcopyrite.

The early copper and molybdenum mineralisation is overprinted by later structurally controlled and erratic Copper-Arsenic (enargite) mineralisation confined locally to parts of the deposit. On a regional scale the main breccia complex is oriented 10°N to 15°W, with a distance of 9km between the two largest known breccias bodies. Late porphyries and a diatreme complex postdate the mineralisation event.

Uplift and erosion of the Andean cordillera eventually exposed the deposit to surface, where meteoric waters played an important role in remobilising and enriching copper in the higher parts of the deposit. Supergene surface waters penetrated and opened the rock fractures and breccia matrix chemically altering and leaching minerals. Leached copper was redeposited as secondary copper sulphides, mainly chalcocite (Cu2S) with minor covellite (CuS) in the chemically reducing environment below the water table. This is the supergene (secondary) material that is extracted via leaching and SX/EW processes. This copper enrichment process was favoured by the porous nature of the breccia bodies, particularly that of Infiernillo to the south, where secondary sulphides are found at depths of 600m. However, the degree of enrichment decreases gradually giving way in depth to hypogene (primary) copper minerals (chalcopyrite – CuFeS2) which is recovered via flotation at Las Tórtolas.

The bulk of the copper and molybdenum rich Los Bronces ore, is crushed and milled by two plants, Los Bronces and Confluencia. The resultant slurry is then transported through a pipeline to the Las Tórtolas flotation plant located 35km north of Santiago where copper and molybdenum concentrates are produced. A proportion of lower grade ore is place on dump leach pads from which copper cathodes are produced via leaching, solvent extraction and electrowinning (SX/EW) processes at the San Francisco plant.

Bulk ore sorting pilot plants are already up and running or in the process of being commissioned at Los Bronces. This technology is helping to significantly reduce our energy and water consumption in line.

The primary benefit of the BOS system is in unlocking production capacity through early rejection of waste or unwanted material, thereby increasing the grade of the ore and reducing the volum that is transported to the processing plant or concentrator. BOS uses co ........