Gold-silver mineralization at Cerro Moro is associated with epithermal veins. Geological mapping and ArAr age dating on vein adularia have defined at least three episodes of veining, spread over 9 million years from 180 to 171 Ma. The different ages of veining tend to have different orientations and structural controls on highgrade shoots. The earlier pulses of veining (Michelle vein at 180 Ma, Esperanza at 175 Ma, and Gabriela at 178 Ma) are characterized by banded crystalline quartz veins with local adularia and low sulphide content.

These veins are generally poorly mineralized although they locally contain significant ore shoots. Grades are lower than in the younger pulse of mineralization and ore shoots terminate at shallow depths, suggesting significant erosion of the vein systems has taken place.

A second later pulse (171 Ma) consisting of black silica, is rich in base metal and silver sulphides and hosts high-grade mineralization, mainly in the Escondida-Zoe vein system. These high-grade veins consist of banded veins with white quartz, fine-grained black silica, and coarse sulphides including pyrite, pale-coloured sphalerite, galena, and acanthite as well as local electrum. The black silica is characterized by highly anomalous molybdenum.

Veining at Cerro Moro is complex and widespread. Veining varies from simple single veins to complex vein systems. Veins are typically steeply dipping to sub-vertical. Outcropping veins locally reach widths up to 4 m, whilst associated zones of quartz stringers and stockwork may reach widths in the order of 10 to 15 m. The strike length of individual veins is variable and ranges generally between 200 metres and 1 kilometre. Alteration has been identified by Terraspec using spectrometry and is typical of the low-sulphidation model, with broad haloes of white mica and less common kaolinite alteration around the mineralized veins.

Structural controls on veining at Cerro Moro vary with the age of the veins. The oldest veins at Cerro Moro strike north to northeast and mineralization is preferentially hosted in northeast-striking segments, especially in areas close to intersections with northwest or east-west structures, suggesting possible reactivation with emplacement of younger mineralization. A second episode of white quartz-adularia veining was emplaced along northwest-striking structures. These veins are widespread in the main mine area and host lower-grade but significant mineralization in the Gabriela and Esperanza-Nini areas. The mineralization in these veins extends to relatively shallow depths below the current surface and probably represent the roots of deeply eroded veins.

The third high-grade episode of sulphide-rich mineralization is also hosted along northwest-striking faults. The main Escondida fault is a large displacement south side-down fault. Mineralization is localized around east-west trending segments as well as in small east-west splays off the main structure. These observations, along with the stratigraphic displacement observed above, suggest a strong sinistral-normal oblique movement vector that controls mineralization.

Cerro Moro consists of several open pit and underground mines which feed a single processing plant with a throughput capacity of approximately 1,000 tpd.

Production from mines located close to the Run of mine (ROM) pad is hauled directly from the mine. For mines located at greater distances, ore is hauled to a stockpile located close to the portal or pit and then hauled to the ROM pad in hauling campaigns.

Open pit operations are currently carried out by a contractor. The average production rate is of approximately 600 tpd of ore and 16,000 tpd waste. However, production from the open pits will gradually reduce as Cerro Moro transitions to increased production from the underground mines. There are typically two to four pits in operation at any one time. The open pit mining sequence consists of first pre-splitting both sides of the vein with holes spaced every 1 meter apart. Then, from the ramp access, waste polygons on the hangingwall side of the vein are mined to create a free face for the vein. Once the vein is fully exposed, the vein is blasted and mined separately to minimize dilution. Once the vein is completely extracted, the remaining waste polygons on the footwall are extracted.

Underground mining at Cerro Moro is carried out using longitidinal long-hole stoping methods. Two variations of long-hole stoping will be employed; bench-and-fill (at Escondida, Martina, and Zoe), and uphole retreat (at Gabriela). Both methods involve ore development at regular level intervals. Stopes are formed by drilling blast holes between levels. After blasting, the broken ore is extracted from the lower level using conventional and remotely operated load-haul-dumps (LHDs).

Bench-and-fill is a bottom-up method, in which mining takes place on top of and adjacent to previously mined and filled stope voids. Once the maximum allowed stope span is reached, and after completion of ore extraction from the blasted stope, stopes are filled with loose rockfill with selective use of cemented rock fill. Uphole retreat is a top-down method, where the stope voids are left open and rock pillars are left between stopes to provide ground support.

The LOM consists in an integrated operation from open pits and underground stopes, in order to feed the 1,000 tpd mill. The LOM indicates mining for a total period of five years, with lower production in the last year.

The grinding circuit consists of a single-stage overflow ball mill operated in closed circuit with hydrocyclones, and a flash flotation cell (on cyclone underflow) to produce a cyclone overflow product with a grind of 80% passing 75 µm. A portion of the mill discharge stream is treated in a gravity circuit for removal of free gold and electrum, with the concentrate going to the refinery for further concentrating and smelting and the tails going back to the cyclones. The gravity circuit consists of a single high-capacity continuous centrifugal concentrator and a concentrating table.

The processing plant at Cerro Moro is currently designed for a throughput of 1,000 tpd or 365,000 tpy on an operating basis of 92 percent availability. The design metal recoveries are 95% for gold and 93% for silver.

The principle processing stages are: crushing, milling, gravity concentration, flotation, leaching by agitation, countercurrent decant system to wash the pulp (CCD), precipitation with metallic zinc (Merrill-Crowe process), detoxification of the pulp to destroy the cyanide, refining, and tailings disposal. Ancillary processes are reagent preparation, water supply treated through a reverse osmosis plant, and reclaim water from the tailings dam.

There is a bulk rougher flotation with a single stage of cleaning. Concentrate thickening of combined flash flotation and conventional cleaner concentrate and regrinding produce a concentrate leach feed with P80 of 30 µm. The re-ground product of the concentrated thickener is sent to an intensive leach tank to ........