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San Francisco mine, Mexico

7th December 2018

By: Sheila Barradas

Creamer Media Research Coordinator & Senior Deputy Editor

     

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Name: San Francisco mine.

Location: The project is located in Sonora, Mexico.

Controlling Company: Alio Gold (previously Timmins Gold Corp).

Brief Description: The San Francisco project comprises two previously mined openpits – San Francisco and La Chicharra – together with heap-leach processing facilities and associated infrastructure located close to the San Francisco pit.

Brief History: Compania Fresnillo undertook exploration on the project between 1983 and 1992, and sold the property in 1992 to Geomaque Explorations.

After conducting further exploration, Geomaque decided to bring the project into production in 1995. In November 2000, mining operations ceased at the mine, owing to poor economic conditions, and the operation entered into the leach-only mode. In May 2002, the mine poured its last gold, the plant was mothballed and clean-up activities at the mine site began.

In 2003, Geomaque sought and received shareholder approval to amalgamate the corporation under a new Canadian company, Defiance Mining Corporation. Defiance sold its Mexican subsidiaries (Geomaque de Mexico and Mina San Francisco), which held the San Francisco gold mine in November 2003, to the Astiazaran family of Sonora and their private company.

Alio Gold acquired an option to earn an interest in the property in early 2005. That option agreement was subsequently superseded by an acquisition agreement. Initially, Alio had the option to earn a 50% interest in the exploitation concessions by spending $ 2.5-million on exploration and development over a two-year period and, after Alio had earned its interest, the property would be operated as a joint venture with Alio as the operator.

In March 2007, Alio announced its intention to increase its interest in the mine from 50% and had entered into an agreement to acquire a 100% interest in Molimentales, a company specifically formed to own 100% of the previously producing San Francisco mine.

In October 2007, Alio completed the acquisition of the San Francisco mine.

Products: Gold.

Geology/Mineralisation: The San Francisco mine is a gold occurrence with trace to small amounts of other metallic minerals. The gold occurs in granitic gneiss and the deposit contains principally free gold, and occasionally, electrum. The mineralogy, the possibility of associated tourmaline, the style of mineralisation and fluid inclusion studies suggest that the San Francisco deposits may be of mesothermal origin.

The deposits are roughly tabular with multiple phases of gold mineralisation. The deposits strike 60 ˚ to 65 ˚ west, dip to the north-east, range in thickness from 4 m to 50 m, extend more than 1 500 m along strike and are open-ended. Another deposit, the La Chicharra zone, was mined by Geomaque as a separate pit.

Reserves: Total proven and probable reserves as at July 1, 2018, were estimated at 48.28-million tonnes grading 0.512 g/t gold.

Resources: Total measured and indicated resources as at July 1, 2018, were estimated at 84.22-million tonnes grading 0.55 g/t gold. Inferred resources were estimated at 1.95-million tonnes grading 0.52 g/t gold.

Mining Method: Mining is conducted by a contractor, using openpit mining methods.

Major Infrastructure and Equipment: Ore extracted from the openpit is transported in 100 t capacity haulage trucks, which feed directly into the 900 t/h gyratory primary crusher.

The crushed product is transported on conveyor belts to a 6 000 t stockpile.

Two feeders beneath the stockpile deliver the ore onto a conveyor belt, which feeds the secondary crushing circuit. The ore is screened and the undersized ore (­–0.5 inch) is added to the final product, while screen oversize is fed to two parallel secondary crushers.

Product from the secondary crushers is transported on conveyor belts to the tertiary crushing circuit, which comprises three tertiary crushers in parallel operating in closed circuit with screens. The –0.5 inch undersize from the screens is delivered to the leach pad.

Product from the crushing plant is transported to the leach pad on overland conveyors, and deposited on the pad with a stacker, forming lifts between 8 m and 12 m high. A bulldozer is used to level the surface of each lift. The irrigation pipelines are then installed to distribute the leach solution over the entire surface of the lift.

A sodium cyanide leach solution flows downward through the crushed ore, dissolving the precious metals. The solution percolates to the bottom of the lift and is collected in the channel that carries the pregnant solution to a storage pond, from which it is pumped to the gold recovery plant.

The gold recovery operation comprises two adsorption/desorption/recovery (ADR) plants with three parallel sets of carbon columns comprising a total feed capacity of 1 475 m3/h of pregnant solution.

Barren solution exiting the ADR plant flows to a second storage pond where fresh water and sodium cyanide are added before the solution is pumped back to the leach pad.

A new stripping circuit with a capacity of 5.5 t of carbon, and a carbon tank with a capacity of 6 t of activated carbon have been added to the process.

Prospects: Alio continues to focus on cost efficiencies and improving overall productivity at the San Francisco mine.

Contact Details:
Alio Gold
Tel +1 604 682 4002
Fax +1 604 682 4003
Email info@aliogold.com

 

Edited by Creamer Media Reporter

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