Bioleaching is a mature technology, which is widely employed commercially in the leaching of low-grade secondary copper sulphide ores. In this work, the bioleaching potential of a low-grade (marginal) ore with a significant content of fluoride was assessed. Small columns were inoculated with a strain of At. ferrooxidans, at 30 degrees C, and resulted in copper extractions above 89%, in 255 days. The dissolution of fluorite from the gangue minerals accounted for up to 2.5 g/L fluoride in solution, which affected bioleaching shortly after column inoculation. However, both the released of aluminium from the ore and ferric iron production by the bacteria reduced fluoride toxicity. A fluoride-toxicity parameter (eta) was proposed to represent the mass ratio between total fluoride, total aluminium and total ferric iron concentrations in the system. Low values of the n parameter resulted from high ferric iron and aluminium concentrations and implied in large bacterial population in the systems. Thus, the presence of fluoride-bearing minerals in the ore may be an important issue, but the content of both cations should be also considered. Despite the absence of ferric iron and a high fluoride concentration prior to first solution bleeding, the eta parameter was lower than 0.3 in the column containing an external aluminium source. In this condition, as the aluminium content surpassed that of fluoride the bacterial population reached 10(7) cells/mL and enabled copper bioleaching. (c) 2015 Elsevier B.V. All rights reserved.