The Boleo deposits occur within a Miocene rift zone basin and consist of a succession of fine to coarse clastic sedimentary rocks characterized by a number of coarsening upward cycles of deltaic deposited sediments. Faulting is common throughout the district.

The dominant faults are steeply dipping normal faults striking northwest and downthrown to the west by up to 200 meters (m). These offsets, combined with the easterly dip of the mantos, yield a stepwise configuration of the mineralized beds. Major faults are typically separated horizontally by several hundreds of meters, although lesser faults are common and are more closely spaced. The extensive faulting, with vertical offsets ranging from less than a meter to tens of meters, divides the project area into relatively small, irregularly-shaped blocks. The ground immediately adjacent
to fault zones is generally observed to be degraded, with little or no structural strength. The surface topography is highly variable and= consists of flat-topped mesa structures that are deeply incised by rugged, steep-sided arroyos. The depth of cover varies from zero to several hundred meters.

Copper-cobalt-zinc mineralization occurs throughout the Boleo District within clay-rich horizons or beds known as “mantos.” The mantos correspond to the lower units of similar stratigraphic sequences occurring in a repeated series. Each sequence consists of a general coarsening of the sediments from the base to the top. The ore-bearing mantos are the lowest beds in each sequence and consist of a 1-m-thick basal laminated mud horizon overlain by mineralized slump breccias up to 20 m thick. The ore zones are overlain by progressively coarser material of tuffaceous claystone, siltstone, sandstone, pebbly sandstone, and eventually cobble- to bouldersized conglomerates.

The contact between the mantos and floor rocks is sharp; however, the contact with the roof rocks is more traditional. Mineralization is finely disseminated in the breccias with the richest material occurring in the laminated basal section of the mantos. The Boleo District has been extensively mined by underground methods for nearly a century, and much of this early mining extracted only the very high-grade material within the basal part of the manto and backfilled the stopes with the lower grade breccias, containing copper grades of 1% to 2%, or more.

The project is an underground and surface mining operation to provide run-of-mine ore to a fully integrated hydrometallurgical facility. Integrated underground mining which comprise short wall and room and pillar method using continuous miners along with small cuts open mining techniques are to be employed. The mine plan will be continuously optimized on the basis of resource update by additional drilling and operational condition with the aim of maximizing economics.

The Boleo ores are clayey and generally fine grained and easily broken. The milling circuit design takes material from the ROM pad into a static grizzly with the oversize to a primary crusher. The combined product is then scrubbed in copper solvent extraction raffinate. The scrubber product is screened with the oversize sent to a secondary crusher. The combined product goes to the ball mill sump before cycloning. The coarse cyclone product is directed to the ball mill. Milling is conducted in raffinate.

The process developed involves two-stage oxidation–reduction leaching, followed by several stages of solvent extraction to recover Cu, Co, and Zn. The multi-stages of the solvent extraction were based on conventional copper solvent extraction and electrowinning, and the patented DSX® (Direct Solvent Extraction) process developed by Australian CSIRO (Commonwealth Scientific and Industrial Research Organisation, Australia) to selectively recover the target metal values. Mn is not recovered from the proposed process and current plant operation at the Boleo project.

The milled product is heated to a target temperature of 80ºC for atmospheric leaching. Leaching proceeds in two steps: acid, oxidative leaching with manganese dioxide oxidation for leaching of Cu, Zn and Co sulphides and: acid reductive leaching with addition of sulphur dioxide gas to reduce residual manganese dioxides. The reductive leaching of surplus manganese dioxide is essential for maximizing the extraction of ........