Located in the south of Sonora in the town of Alamos, Piedras Verdes is Mexico’s third most prolific copper operation. The deposit is comprised over an elongated copper porphyry vein which, according to new exploration results, spans over 5km from east to west and over 1km from north to south. The mineralization of Piedras Verdes is predominantly within Triassic-Jurassic metasedimentary units and various intrusive phases of Laramide granodiorite porphyry. The minerals that constitute the hypogene copper mineralization are chrysocolla, neotocite, azurite, native copper cuprite, and chalcocite, while the main types of copper ore are chalcocite, chalcopyrite, and bornite.
Piedras Verdes is owned and operated by Frontera Copper Corporation through its 100% interest in Cobre del Mayo, a Mexicanconstituted company. Frontera Copper initially purchased Phelps Dodge Corporation’s 70% interest in Cobre del Mayo in 2002. One year later the company acquired the remaining 30% interest to fully own the project. During 2008, operational deficiencies such as deposit modeling, grade control, and contract mining arrangements, combined with the global financial crisis, caused important increases in cash costs and negative cash flows for the company. Later that year, Invecture Group identified the Piedras Verdes unit as an asset that could render significant value if the mine processes were upgraded under the management of a new board of directors. In December of the same year, Invecture Group offered to purchase all the common shares of Frontera Copper. In May 2009, Invecture obtained full control of Frontera Copper’s 100% ownership on Cobre del Mayo.
According to Cobre del Mayo’s production reports, Piedras Verdes has mineral reserves of 1.13 million tonnes of copper with a mine life expectancy of 15 years. The process design for the CDM Piedras Verdes Leach SX/EW operation was originally based on a nominal production rate of 34,500 tonnes of LME Grade A copper cathode per year. This production primarily used conventional sulfuric acid/ ferric sulfate heap leaching and ROM ore. In order to efficiently process the ore at Piedras Verdes, the new management team implemented a crushing system that was specially engineered to reduce the extracted minerals to a particle size. This is so that the size of the minerals optimized the percolation of acidic solutions and allowed rocks to be screened, minimizing the clay content to ensure an adequate permeability and continued copper recovery. The new system has enabled Cobre de Mayo to further reduce its crushed particle size while enhancing circuit tolerance in the separation process. The minerals then undergo leaching and crushing with an acidified solution. Once the solution lixiviates through the heap, the copper is dissolved from the oxide and chalcocite ores.