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.

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.

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.

Differential Flotation
Most of the undergroun ........