San Andrés Mine — NI 43-101 Technical Report

The San Andrés Mine processes open-pit oxide and mixed ore through a two-stage crushing circuit, agglomeration, and heap leaching with gold recovery via an ADR plant.

Report context

This technical report presents the San Andrés Mine process facilities based on a life-of-mine schedule generated from December 31, 2011 Mineral Reserves. The project has a 9-year life, with total run-of-mine production of 45.8 Mt containing approximately 784 k oz Au at an average grade of 0.53 g/t.

Processing route

Mining and ore delivery

Mining is by conventional open pit methods with 6 m high benches. Operating phases (push-backs) support mine production from the initial topography of December 31, 2010 to the final pit geometry. Ore is hauled to the jaw crusher using a contract haul fleet.

Crushing circuit

All ore passes through a two-stage crushing circuit before being transported on conveyors and stacked. The circuit was designed for a daily operating time of 18 hours and a throughput capacity of 1,100 t/h. Final product size is 80% passing 2.5 in.

The primary crusher is a 55 in x 66 in Telsmith Jaw crusher producing a 6 in product, which is transported by a 490 m long conveyor to an intermediate stockpile. Material is fed from the stockpile via a reclaim tunnel to a second series of conveyors totaling 510 m. A new primary crusher and conveyor system was constructed during 2009, with commissioning completed in the second quarter of 2010. Modifications to the stacking system were completed during the fourth quarter of 2010, allowing crushing and stacking of up to 1,100 t/h of rocky ore.

From the reclaim tunnel, ore passes through a Symons 7 ft Standard Cone crusher set at 2.5 in.

Agglomeration and cyanide addition

After crushing, cement is added at 2.5 kg/t to 4.0 kg/t before the ore reaches the agglomeration circuit. Lime is added for pH control, averaging 1.5 to 4.0 kg/t. In two 60-ft long x 12 ft diameter agglomerators, ore is retained and mixed for five minutes while intermediate solution is added to achieve optimum moisture of 18%. Before reaching the stacker, 350 ppm cyanide solution is sprinkled at three different points on the conveyor belts.

Heap leaching

Ore is stacked on the leach pad in 8 m lifts on previously leached ore that has been ripped and prepared. The ore is left for 72 hours to cure before any solution is applied. The ore is leached for an average of 60 days before the area is allowed to dry and prepared for the next lift. Solution for leaching comes from the ADR plant after cyanide concentration has been replenished.

The leach pad facility is a monolithic pad constructed in three phases. Phase I was completed during initial construction and start-up. Phase II-A was an expansion constructed in 2002. Phase II-B was another expansion completed in July 2005. Total capacity of the three phases is approximately 23.5 Mt. A vertical expansion followed by two horizontal expansions is currently being evaluated. The leach pad facility was designed by the consulting firm SRK, with engineering for stability, water balances, liner specifications, and earth work specifications completed. SRK was also involved during construction as the Quality Assurance inspector.

Carbon ADR plant

Gold recovery is achieved through the ADR plant. The plant has three trains of carbon columns, each capable of processing 350 m³/h. The trains each contain six carbon columns that can hold a total of 80 t of carbon. The plant has two 7-t pressure strip vessels.

The acid wash vessel processes approximately 10 t of carbon per day in 2.5-tonne batches using 30-33% hydrochloric acid. The volume ratio during wash is 1 to 4 of carbon to acid. This acid solution dissolves carbonates in the carbon micropores before the carbon is sent for regeneration in the carbon reactivation circuit. The carbon reactivation system consists of one 5-t carbon reactivation kiln, operating at approximately 10 tonnes per day.

The fine recovery system includes an electro-winning circuit with capacities of 70 gpm. The refinery has two parallel electro-winning cells with 16 cathodes and 14 anodes, running at a current of 1000 to 2500 A and a voltage of 15 V, and two parallel mercury retort systems.

The assay laboratory is located in the same complex as the ADR plant. Gold produced at the ADR plant is analyzed prior to shipment to Johnson Matthey Inc. for refining and sale. Comparison of ore recovered shipment shows gold ounces as measured at the ADR plant are no more than 1% above the Johnson Matthey measurements.

Energy, water, and process materials

A steady circulation of 18 to 20 tpd of solution is kept within the system at all times. The water balance is always positive from high local precipitation and the recycling water system. When required, water make-up is supplied by the booster pond into the heap leach operation.

Key reported parameters

Technical qualifications

This report presents a life-of-mine schedule based on December 31, 2011 Mineral Reserves within designed pit phases and has considered restrictions of the cemetery. Estimated metallurgical recoveries are 76% for oxide ore and 57% for mixed ore; annual gold production depends on average grade and the blend of oxide and mixed ores processed. An engineering study is currently underway to develop an optimum plan for leach pad expansions. The leach pad facility was designed by the consulting firm SRK. The process flow sheet for San Andrés is presented in the original report as Figure 17-1.

Source: San Andrés Mine , NI 43-101 Technical Report, relevant sections 17.0, 17.1.1, 17.2.1, 17.2.2, 17.2.3, and 17.2.4.