The Boleo project is an advanced stage copper/cobalt/zinc/manganese development located in Baja California Sur, Mexico. Discovered in the late 1800s, the property consists of 25 mineral concessions covering 20,490.9ha, with 24 being contiguous.
The property was staked by Baja Mining in 1992. In the mid-1990s, it was optioned to International Curator. Baja Mining acquired it in 2001.
Construction of the project started in November 2010 and mining operations began in the first half of 2014. First copper from the project was produced on 17 January 2015. The cobalt-zinc circuits are expected to be commissioned by March 2015, and will achieve production ramp-up by October 2015. The mine has an estimated life of 22 years.
The deposit lies within the late Miocene aged El Boleo Formation within fine to coarse clastic sedimentary rocks. The rocks lie unconformably over Comondú Volcanics, andesitic rocks belonging to the early or middle Miocene age. The andesitic rocks are underlain by Cretaceous aged granodiorite.
A cyclic succession of up to 270m thick clastic beds forms the next stratigraphic sequence. The cycles range between 20m to 140m in thickness with the earliest cycle being the thickest. It includes basal mud and fine volcanic ash horizon that hosts the mantos. The mantos are covered by progressively coarser material of maroon coloured, tuffaceous claystone, siltstone, feldspathic sandstone, pebbly sandstone and eventually cobble to boulder orthoconglomerates.
The upper, regionally eroded section of the formation is unconformably overlain by barren and fossil-rich similar sedimentary sequences of Pliocene and Pleistocene aged delta and beach deposits namely Gloria, Infierno and Santa Rosalía Formations. The Boleo basin is hosted within the sub-basins of the Santa Rosalía basin. The El Boleo basin comprises the El Boleo and the overlying formations.
Mining and processing
The project employs both underground and open-pit mining methods. Approximately 90% of the material is anticipated to come from underground operations and the remainder from surface operations. Room-and-pillar mining methods using continuous miners are used.
The processing plant operates at a rate of 3.1Mt/y. It crushes and mills the ore in seawater. This is followed by atmospheric leaching of the whole ore stream at elevated temperatures. Oxidative and a reductive leach are included in the leach circuit to ensure the recovery of oxide and sulphide minerals. From the leach slurry, the metals are separated in a counter current decantation washing circuit.
The dissolved metals are recovered at high-efficiency from the wash solution and concentrated in four independent solvent extraction circuits, two electrowinning circuits and a fluid bed drying operation to produce high-quality copper and cobalt metal cathode and zinc sulphate monohydrate crystals.
Direct solvent extraction technology is used in the second solvent extraction circuit to separate Mn from Co and Zn in the process solution.