Mineralogical signature of nonsulfide zinc ores at Accha (Peru): A key for recovery

Abstract

The Accha deposit in Southern Peru is the first case in which an integrated study between mineralogy, petrography and metallurgical characteristics of a nonsulfide Zn orebody has been carried out. The Accha–Yanque Zinc Belt is located in a major zinc-rich metallogenic province hosting a number of economic mineral deposits, which extends in a zone peripheral to the edge of the Oligocene–Miocene Yauri–Apurímac granite batholith. This belt is comprised of a number of weathering-derived nonsulfides occurrences, showings and mineral deposits. A thorough mineralogical and petrographic study has been carried out on the MET 1, MET 2, MET 3 and MET 4 cores, drilled between 2006 and 2007 for metallurgical purposes. The nonsulfides mineral association in the deposit consists mainly of smithsonite and hemimorphite replacing both primary ore minerals and carbonate host rocks. Smithsonite occurs in zoned concretions with goethite, Mn(hydr)oxides and Zn-clays. One of the most peculiar nonsulfide Zn mineral at Accha is a sauconite-like Zn-smectite, which can be locally very abundant (up to 30%). Differences in mineralogy of the ore and gangue minerals, as well as the composition of the host rock can have a profound impact on zinc recovery and reagent consumption in any extraction process. The Accha nonsulfide ore minerals are intimately intergrown with the limestone host and therefore difficult to separate with traditional methods, this rendering the economic recovery of the metal through the acid leaching process difficult. This is further complicated by the abundance of sauconite in the deposit. It was therefore necessary to envisage a flowsheet development, derived from a thorough metallurgical test work program on various ore types, which could result in an economically viable treatment route in the light of both the carbonate host rock and the peculiar mineralogical association. Several careful processing steps comprised of comminution, dense media separation and flotation before acid leaching, and including precise acid leaching conditions to prevent the formation of deleterious ‘gels’ that would interfere with subsequent solid–liquid separation processes, have been tested for Accha as part of the development of an economic processing flowsheet.