The Aguila Project is located in the San Jose de Gracia Mining District in Oaxaca. Multiple volcanic domes of various scales, and probably non-vented intrusive domes, dominate the district geology. These volcanogenic features are imposed on a pre-volcanic basement of sedimentary rocks. Gold and silver mineralization in this district is related to the manifestations of this classic volcanogenic system and is considered epithermal in character.

The Aguila open pit mineralization is considered low sulfidation, epithermal mineralization primarily of gold with some silver and no base metals. The Arista underground mine is considered intermediate epithermal mineralization of gold, silver, copper, lead, and zinc. The host rock in the Arista vein system is primarily andesite.

As of December 31, 2016, proven and probable reserves had been established for the Mirador Underground Mine on Alta Gracia property. In July 2017, mine development reached the economic ore zone of the Mirador vein and mining began.

The sedimentary and volcanic units mapped at Alta Gracia are similar to those observed at the Aguila project. The district is dominated by Tertiary-age rhyolite flows and tuffs which are underlain by andesite flows and tuff. Granodiorite and felsic intrusives are observed to outcrop to north and east of the Mirador mine. Known vein occurrences at Alta Gracia are mainly hosted in andesite and rhyolite. The veins currently being mined at Alta Gracia are considered low sulfidation epithermal mineralization with economic values only for gold and silver.

The Aguila processing facility produces metal concentrates and doré from ore mined from both the Arista and Mirador Mines. The plant was built near the mine site and consists of both a differential flotation circuit and an agitated leach (oxide) circuit. Late in 2013, as part of the flotation plant expansion from 850 tpd to a nominal 1,500 tpd, two Knelson gravity concentrators were installed. In 2014, a Gekko Intensive Leach Reactor and Electrowinning Circuit was installed to upgrade the gravity concentrate to doré. In 2015, 40 percent of the gold production from the Aguila plant was produced as doré.

The products from the flotation circuit in the plant are four types of high-grade concentrates: copper concentrate with gold silver; lead concentrate with gold-silver; zinc concentrate with gold-silver and highgrade gravity concentrate which is subsequently smelted and poured into gold silver bullion (doré) bars. Concentrates are sold to two subsidiaries of the Trafigura Group Company: Consorcio Minero de Mexico Cormin Mex. S.A. de C.V. and Trafigura Beheer, B.V. of Lucerne Switzerland. Early in 2014, high-grade lead-gold-silver concentrates were sold to to LD Commodities Metals de Mexico SA de CV, a Louis Dreyfus Commodities Company. During the third quarter of 2014, GRC began to sell all of its doré to Johnson Matthey Gold & Silver Refining Inc.

Crushing and Milling
Crushing is a dry process which begins at the reception hopper, where ore from the mine is deposited. The maximum size of rock that can be fed to the crushers is a nominal 635 mm. The primary crusher is a 0.91 m X 0.51 m Pioneer jaw crusher. The crushed ore is transferred via conveyors to a 3-deck MCA vibrating screen with dimensions of 1.83- X 6.10-m per deck with the oversize from screening conveyed to a secondary 1.52-m Pioneer cone crusher, where the final product for milling is reduced to -95 mm. The maximum crushing rate for this plant is about 110 tonnes per hour. The fine ore is stockpiled before being fed into the crusher and ball mills.

The fine crushed ore is transported via conveyor belts to the flotation plant grinding circuit. The grinding circuit consists of an Allis-Chalmers 3.04- x 4.27-m ball mill, and a 15-inch cyclone classifier. The ball mills are filled with 38 % of their volume with 3-inch wrought steel balls used to further reduce (grind) the ore size. The product of the mill is pumped to the classification process comprised of hydro cyclones, where two products are generated; 1) a fine ore, which is expulsed thorough the top of the cyclone, and
2) a coarse ore that exits through the bottom and is recycled back into the mill for further grinding. The fine ore must comply with the metallurgical conditions for metal recovery, which indicates 55% of the product must be under the 200 mesh size (equivalent to 74 microns), before being sent to the flotation process.

GRC has also improved two areas of the crushing and milling. In 2013, a Knelson Concentrator gravity circuit was installed to help improve gold recovery. In addition, the mill was expanded to a nominal 1,500 dry tonnes per day employing the same process and generally the same equipment types as described above. The expansion included doubling the amount of flotation cells and thickener surge tanks and the addition of a second ball mill on the flotation circuit (Figure 8.4).

Differential Flotation
Most of the underground ore from theArista vein has consisted of very clean, primary sulfides, which have high recoveries in the differential flotation circuit. The principal economic components are gold and silver; however, the ores also contain economically significant amounts of lead, zinc, and copper. The flotation plant produces three concentrates for sale: a copper concentrate with gold-silver, a lead concentrate with gold-silver, and zinc concentrate with gold silver. Concentrates are sold to several buyers in Mexico.

The flotation process starts with ground pulp being pumped from the ball-mill sump to a 2.5- x 2.5 m general conditioner tank. From this tank, it is first pumped to a bank of four 5.10-m3 rougher flotation cells, from which a rougher copper concentrate is recovered. Underflow from the copper rougher circuit is pumped to a second bank of four 5.10-m3) rougher flotation cells for production of a lead rougher concentrate, and the underflow from the lead rougher cells is pumped to a third bank of 5.10-m3 rougher flotation cells to produce a rougher zinc concentrate. Copper rougher concentrates are passed through a bank of six 0.68-m3 copper cleaner cells to produce a copper product, which is pumped to a 5.0- X 1.7-m thickener tank for thickening before filtration into a saleable copper concentrate. The thickened copper concentrates are filtered in a 1.10-m pressure filter to produce a final saleable product.

The flotation product from the lead rougher stage are pumped to a bank of five 0.68-m3 lead cleaner cells as an initial cleaner stage for the lead concentrate. The product from the first lead cleaner stage is pumped to a second series of three 0.68-m3 cleaner cells to produce a lead concentrate that is also thickened and filtered for sale as a final product. The sizes and characteristics of both the lead concentrate thickeners and the press filter systems are identical to the copper thickener and filter.

Tailings from the lead flotation are pumped to two 2.5- x 2.5 m zinc conditioning tanks, and from these into a bank of four 5.10-m3 zinc rougher flotation cells. The product from the rougher cells passes to another set of four 5.10-m3 secondary flotation cells, and the product from these passes to a series of nine 0.68-m3 zinc cleaner flotation cells. The product from the cleaner cells is pumped to the zinc concentrate thickener; thickened concentrate is then filtered and stored for subsequent shipment to the concentrate buyer. Concentrates are bulk shipped on contracted tractor-trailer trucks.

Agitated Leaching
GRC has also installed a separate agitated leach plant with its own grinding circuit. It is expected that gold-rich or silver-rich deposits with little or no base metal components, or a combination of these, will be generated in the near future. Potential exists for such deposits and the agitated leach circuit was installed for milling and processing them. The agitated leach plant was commissioned in the fourth quarter of 2017 reaching 150 dry metric tonnes throughput per day.