The Mulatos gold mine in Sonora, Mexico is being developed by Alamos Gold, a gold producer based in Canada.
First gold was poured in 2005 and commercial production was achieved in April 2006.
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The mine includes two main pits, Mulatos and La Yaqui Grande, that are currently in operation. Past producing pits include the Victor, Cerro Pelon and San Carlos open pit mines in addition to the Escondida Deep and San Carlos underground mines.
A new higher-grade underground deposit Puerto Del Aire (PDA) located adjacent to the main Mulatos pit was found. The development plan for the pit is expected to be announced in the first quarter of 2024.
Project location
The Mulatos mine encompasses a total area of 34,682 hectares within the Mulatos District in the east-central part of Sonora. The site lies within the Sierra Madre Occidental mountain range.
Geology and mineralisation
The mine is a disseminated, epithermal and high-sulphidation gold system hosted within an Oligocene 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 the 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.
The PDA deposit comprises four zones: PDA, PDA Extension, GAP-Victor and Estrella. The PDA zone is the most substantial among these, housing 78% of the current mineral resource.
Reserves
The proven and probable reserves of the Mulatos mine are estimated at 16.9Mt grading 2.97g/t Au, as of December 2023.
Mining method
The Mulatos mine employs conventional open-pit mining methods utilising standard drill, blast, load and haul processes. In the main pit, the mining operations are carried out with 9m bench heights.
The mining fleet consists of front-end loaders of 12.3m³ capacity, 14 haul trucks ranging in size between 90t and 100t, three CAT 992 front-end loaders, 14 CAT 777 haul trucks, three drills and various pieces of support equipment.
The PDA underground deposit will be accessed through two portals situated in the Estrella section of the main Mulatos pit. The mining operations will be entirely mechanised and conducted by a regional underground mining contractor. The principal mining techniques envisaged are longhole stoping and drift and fill stoping.
Ore processing
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. The crushed product boosts 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, a 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 barren solution 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.
Site infrastructure
Access from Hermosillo to the Mulatos site is via routes through Mazatan, Sahuaripa, Arivechi, Tarachi, and Matarachi, or through Tecoripa and Yecora on the Chihuahua-Hermosillo highway.
The power supply for the project relies on on-site diesel generators, with a plan to shift to grid power in the future.
The project’s water needs are met through a system consisting of two pump stations: one located at the well site and another serving as a booster pump station.
Contractors involved
The 2023 technical report was prepared by Alamos with inputs from Ginto Consulting, Golder Associates, Knight Piesold, Independent Mining Consultants, M3 Engineering, Practical Mining, SND Consulting and Spire Exploration.
The open pit mining operations at the Mulatos and La Yaqui Grande pits are undertaken by construction services providers Grupo Construcciones Planificadas and Grupo Desarrollo Infraestructura respectively.
