El Soldado, located in Central Chile (32°38’S; 71°04W) 120 km northwest of Santiago, is the largest (>200 Mt @ 1.4 % Cu) of the known copper manto-type deposits in central Chile. Although structurally controlled and discordant at the scale of the mine, it is regionally strata-bound within shallow marine, bimodal calc-alkaline basalt - rhyodacite (extrusive- intrusive) rocks of the Lower Cretaceous Lo Prado Formation, which also contains marine carbonaceous shale and volcaniclastic sandstones. It is generally acknowledged that hydrothermal sulphide base metal deposits in the Andes obtained their metals and sulfur from calc-alkaline magmatic systems.

Several structurally-controlled orebody clusters occur within an area of ca. 2000 x 800 m, 600 m vertical, in a dilational fault jog within a large NS orogen parallel (sinistral) strikeslip shear fault system that was active in the Cretaceous. The orebodies are zoned with pyrite in outer halos or envelopes, followed inwards by zones with abundant pyrite-chalcopyrite, chalcopyrite-bornite and a bornite-chalcocite core with minor hematite in the higher levels and the chalcocite cores; pyrite is relatively more abundant at depth and in the northern half of the deposit.

Samples of massive pyrite and associated chalcopyrite and bornite, devoid of visible bitumen, were selected from the Valdivia Sur, Arauco Norte, Catedral and Morro orebody clusters, most from the pyrite-chalcopyrite halos such as those shown in, or veins. Under close inspection under the reflected microscope and the microprobe, even the most massive pyrite assemblages show remnants of colloidal or framboidal textures. As temperature rose, framboidal structures recrystallized as cubes, yet (at high magnification) their cores preserve the diagenetic framboidal structures. Partial or total replacement of pyrite by chalcopyrite and bornite did not totally destroy the original diagenetic textures.

El Soldado is a tightly integrated operation consisting of an underground and open pit copper mine, a concentrator and an oxide plant.

Mining at El Soldado started in 1842. Since 1978, when Exxon Minerals acquired the operation, about 70 million t of ore containing 1.8% copper have been mined by the underground sublevel open stoping method. In 1989, the El Morro open pit commenced production to increase output.

Mine stability is a matter of prime importance in the planning process, particularly as the El Morro open pit is situated immediately above the underground mine. An integral mine plan is therefore required, in which the sequence of extraction, both in the open pit and underground, needs to satisfy safety and efficiency criteria.

Production block access is provided by developing sublevels, with a pattern of 5.0 m x 3.7 m LHD drawpoints at the base of the stope. Block undercutting is accomplished with a fan pattern of 60 to 75 mm-diameter holes up to 25 m-long loaded with ANFO and HE boosters. Slots are made by enlarging a 2.5 x 2.5 m blast hole slot raise, at one end, or in the middle, of the stope. Blast holes of 165 mmdiameter and up to 80 m-long, are drilled with an underhand pattern.

Blast size and blasting sequence is defined for each stope, according to major structural features and the proximity of existing cavities. Dilution control is improved, and blasthole losses avoided, by carefully considering the particular geometries created by the intersection of major discontinuities and the free faces of the planned excavation.

El Soldado – engineering advances in processing technology.

During our ‘Concentrating the Mine’ workshops held in 2015 and 2016, we identified coarse particle recovery (CPR) as a key enabling technology. CPR is one of many significant breakthrough technology initiatives that has the potential to increase throughput and productivity, while simultaneously reducing our environmental footprint, through rejection of coarse gangue (near-worthless waste material), dry stacking of sand waste, minimising the production of traditional tailings and reducing overall water consumption.

Following successful testing in the laboratory and at small pilot scale at Los Bronces, we are now constructing a demonstration plant at El Soldado copper mine that uses a CPR technology called Hydrofloat™ in a new way. Here, a single, five metres in diameter Hydrofloat™ cell, the largest in the world, will treat 100% of mill throughput, with the objective of proving the waste rejection process ........