The San Andrés deposit is classified as an epithermal gold deposit associated with extension structures within tectonic rift settings. These deposits commonly contain gold and silver mineralization, which is associated with banded quartz veins. At the Mine, however, silver does not occur in significant economic quantities. Gold occurs in quartz veins predominantly comprised of colloform banded quartz (generally chalcedony with lesser amounts of fine comb quartz, adularia, dark carbonate, and sulphide material). The gold mineralization is deposited as a result of the cooling and interaction of hydrothermal fluids with groundwater and the host rocks. The hydrothermal fluids may have migrated some distance from the source; however, there is no clear evidence at the Mine that the fluids or portions of the fluids have been derived from magmatic intrusions.

The rocks hosting the San Andrés deposit have been oxidized near surface as a result of weathering. The zone of oxidation varies in depth from 10 m to more than 100 m. The zone of oxidation is generally thicker in the East Ledge deposit compared to the Twin Hills deposit.

In the oxide zone, the pyrite has been altered to an iron oxide such as hematite, goethite, or jarosite. The oxide zone generally overlies a zone of partial oxidation, called the mixed zone, which consists of both oxidized and sulphide material. The mixed zone may not occur continuously, but where it is present, it reaches thicknesses of over 50 m. below the zone of oxidation; the gold is commonly associated with sulphide minerals such as pyrite. The sulphide, or “fresh”, zone lies below the mixed zone.

The gold contained in the oxide zone is amenable to extraction by heap leaching using a weak cyanide solution. The gold recovery is reduced in the mixed zone as a result of the presence of sulphide minerals and the gold cannot currently be recovered economically from the sulphide zone by heap leaching.

High clay content in the ore, resulting from alteration, is detrimental to the heap leaching process because of reduced through-put rates in the crushing plant and reduced permeability in the heap leach operation. This poor leaching situation is resolved by agglomerating the crushed ore by adding cement to increase the permeability of the heap prior to leaching.

Based on metallurgical studies, the gold is primarily contained in electrum as fine-grained particles. The particle size of the electrum grains varied from 1 micron (“µ”) x 1 µ up to 10 µ x 133 µ. One native gold grain was noted. The silver generally occurs at about the same grade as gold and the correlation between silver and gold is low at 0.24. Silver is not considered important because of the lower price for silver compared to gold and the lower metal recovery of silver.

Mining at San Andrés is by conventional open pit methods. Historical production rates for the years 2009 to 2013 averaged approximately 13,000 t of ore and 10,000 t of waste produced daily with generally continuous mining 24 hours a day for 360 days per year.

The San Andrés Mine is anticipating a material expansion in ore throughput from approximately 5Mtpa to 7Mtpa. This expansion was justified by the improved incremental economics with modest capital investment.

Mine production utilizes conventional drill and blasting methods with excavation on 6m high benches. Blasted material is then loaded via shovels and excavators onto haul trucks and is hauled to one of two jaw crushers utilizing a contract haul fleet.

All of the ore is processed through a two stage crushing circuit and transported on conveyors before being stacked as the final product sized at 80% passing 2.5 inches. The crushing and conveying circuit is designed for a nominal capacity of 1,100 t/h, which is adequate for the expanded production rate if operating at approximately 74% overall utilisation rate. For the expansion, most of the capital investment is applied to improve the secondary screening and crushing plant in order to consistently achieve or exceed 74% utilisation factor.

After the ore has been crushed it is treated with 2.5 to 4.0 kg/t of cement and 1.5 to 3.5 kg/t of lime before reaching the agglomerators where the ore is retained and mixed while adding an intermediate process solution to achieve the optimum moisture of 18%. The process solution contains up to 400 ppm cyanide solution.

The ore is stacked on the leach pad in 8 m lifts on previously leached ore that has been ripped and prepared. The ore is leached for an average of 120 days before the area is allowed to dry and prepared for the next lift. The solution used for leaching comes from the ADR plant after the cyanide concentration has been replenished.

Gold is recovered through the ADR plant, which has 12 carbon columns that can be configured in a two or three train configuration with a nominal capacity of 500 m³/h per train. The assay lab which processes both Mine grade control samples and process plant samples is located in the same complex as the ADR plant. The gold produced at the ADR plant is analyzed prior to shipment for refining and sale. The ADR plant is being upgraded to couple with expanded capacity. Upgrades include improvements to the carbon handling and elution circuits and the addition of a number of cathodes and anodes to the existing electrowinning cells in the refining portion of the plant.