The Mulatos mine is an open pit gold mine located in the state of Sonora, Mexico, approximately 140 miles east of Hermosillo city. Owned and operated by Alamos Gold, the mine includes four pits designated the Mulatos pit, Victor pit, La Yaqui pit and the Cerro Pelon pit. It forms part of the Salamandra property that also hosts the El Halcon, Los Bajios, El Jaspe, La Dura, and the El Carricito gold systems.
The mine is currently undergoing an advanced exploration program aimed at identifying additional resources within the expanded Mulatos pit. The mine produced 80,000oz of gold in the first half of 2010 and is on track to meet the production forecast of 160,000oz-175,000oz of gold for the whole year.
The mine hosts approximately 10.3Mt of proven reserves and 51.2Mt of probable reserves graded at 1.63g/t Au and 1.12g/t Au respectively. Measured and indicated resources at a cut off grade of 0.5g/t Au have been estimated to be 60Mt grading at 0.97g/t Au. Resources in the inferred category amount to 25.8Mt at a grade of 1.12g/t Au.
Geology and mineralisation
The mine is a disseminated, epithermal and high-sulphidation gold system hosted within an Oligocene aged rhyodacite flow dome and breccia sequence. The deposit is related to a vast hydrothermal alteration zone that extends over 10km². Gold mineralisation is found predominantly within large pervasive zones of silica and advanced argillic alteration hosted in the overlying volcaniclastic rocks and dacite flows and proximal to the upper contact of a rhyodacite porphyry. Approximately 80% of the gold in the deposit is found within the silicified rocks. Zones of quartz veins and quartz stockwork are rare.
Gold occurs within different types of ore including oxide, sulphide and oxide-sulphide combinations. The oxide rocks that occur largely near surface in areas of leaching constitute a small portion of the deposit. Deeper areas of oxidised material are found in immensely fractured zones where permeability improves along major structural zones.
The sulfide zone constitutes the deepest part of the deposit and underlies a layer of mixed zone.
Mining and processing
Mining at Mulatos is by conventional open-pit methods. Front end loaders of 12.3m³ capacity and 14 haul trucks ranging in size between 90t and 100t are used. The processing plant includes a 135,000t crushing plant, a 20,000tpd conveyor, a heap leaching facility, a carbon-in-column circuit, and an electrowinning and refining facility.
Crushing is completed in three phases. Ore is passed through a 15cm screen in a primary jaw crusher where it is crushed to 80%. It is then transported to a coarse ore stockpile from where apron feeders feed the ore to the secondary crusher through a conveyor. The ore is further crushed to less than 3/8in in the tertiary crushers.
Approximately 95% of the ore is sent through the 3/8in screen for pad delivery as the oversized material is re-circulated via the crushing process following the closure of the crushing circuit in early 2010. Better crushed product is expected to boost recoveries, positively impacting the production and cost of gold.
Upon completion of crushing, the ore is delivered to the leach pad for stacking and leaching. An impermeable layer of plastic overlies a compacted clay layer that covers the heaps.
Using a low-pressure irrigation sprinkler system, cyanide solution of low concentration is applied to the ore on the leach pad for gold extraction. Inter-lift liners are periodically installed on the pad to reduce percolation time and the time required to extract and produce gold from each lift of the heap.
The gold-bearing solution is directed to the pregnant solution pond and pumped to the carbon-in-column circuit for gold recovery. The solution that is barren is added with cyanide and re-circulated to the heap. Dore bars produced on site are sent to third party refineries for final gold recovery.