Atacamite inclusions in rock-forming feldspars and copper-bearing smectites from the Radomiro Tomic mine, Chile: Copper-insoluble mineral occurrences

Recovery of copper from the Radomiro Tomic deposit in Chile employs innovat ive large-scale hydrometallurgical extraction methods, with sulfuric acid u sed for heap leaching. Here, we report on improvements in un derstanding mi neral components of copper oxide ores, which are insoluble in acid leach so lutions, in order to advance leaching technology. Using optical, electron m icroprobe, X-ray diffraction, energy dispersive spectroscopy and least squa res inversion methods, two different occluded mineral occurrences were disc overed in oxide ores, which present two distinct metallurgical challenges f or heap leaching but offer important new opportunities in process mineralog y. First, in a high chloride to sulfate ratio, feldspar-stable portion of t he ore body at Radomiro Tomic, disseminated atacamite (Cu4Cl2(OH)(6)) inclu sions, 1 to 8 mum in diameter, occur in microfractured feldspars and biotit es. These account for tile acid-insoluble fraction, often as high as 30 per cent of the total contained copper. About 70 percent of the total copper oc curs generally as atacamite in cracks, which upon crushing is exposed along surfaces of the rock fragments and is hence soluble. In contrast, in more intensely hydrothermally altered, feldspar destructive, weak argillic alter ation zones, insoluble copper exists more probably within the crystal struc ture of well-crystallized saponitic smectite clays in nonexchangeable, octa hedral crystallographic sites. The two different forms of applied acid-inso luble components are products of contrasting microchemical environments, on e in a reactive potassic alteration gangue and the second in a nonreactive gangue. In both cases, however, the nature of hydrothermal alteration, thou gh pervasive, was relatively weak and left intact rock mineral buffers capa ble of neutralizing acidic and reducing fluids. Hence, the insoluble compon ents with respect to tile applied sulfuric acid reflect strong wall-rock mi neral assemblage control on tile behavior of copper on all scales: tile mac rofield zonal scale, the microscale of individual crystals in cracks, and t he atomic scale of octahedral sites in the clay structure. The metallurgical implications of these atacamite microinclusions in rock-f orming minerals and structurally bound copper in smectites are different. I n the former case, exposing ore inclusions on fragment surfaces would requi re extremely costly grinding to a grain size well below 400 mesh. Secondly, chemical extraction of microinclusion copper from feldspar host phases wou ld entail high acid consumption, as hydrolysis of feldspars Ly chemical rea ction with applied acid would occur causing neutralization. In the case of Cu smectites, since the cooper occurs in structurally bound, nonexchangeabl e sites, processes to remove the copper via ion exchange or acidic leach ca nnot work. Alternatives to ion exchange could involve destroying the smecti te frost mineral, perhaps by its conversion to kaolinite that may only inco rporate copper or other divalent cations in negligible amounts.