At the San Francisco Project, Alio is targeting large volume, low grade disseminated gold deposits contained within leucocratic granite, granite-gneiss and gneiss and schist horizons. Leucocratic granite and gneiss are the main rocks hosting the gold mineralization.

The gold mineralization occurs in a series of west-northwest to east-northeast trending quartztourmaline veins and veinlets that lie sub-parallel to the local lithology and foliation trends, dipping to the southwest, within the more brittle rocks such as the leucocratic granite and more felsic lithologies within the Precambrian sequence. Extensive studies of the veins and alteration describe the mineralization as mesothermal/orogenic in style, but with a potential link to magmatic fluids and an intrusive source (Calmus et al., 1992; Luna and Gastelum, 1992; Perez Segura, 1992; Perezsegura et al., 1996; Perez Segura, 2008; Albinson, 1997; Poulsen and Mortensen, 2008).

At the San Francisco Project, gold occurs principally as free gold and as electrum occasionally. Gold is found, in decreasing abundance, with goethite after pyrite, with pyrite and, to a much lesser extent, with quartz, galena and petzite (Ag3AuTe2). Although it is clear that the gold was deposited at the same time as the sulphides, the paragenetic relationships are not well understood. There is the possibility that some secondary remobilization may have occurred as evidenced by minor amounts of gold occurring in irregular forms along with or on top of drusy quartz (Prenn, 1995).

The gold occurs in a granitic gneiss and the presence of pyrite (or goethite after pyrite) may be an indication of gold. Stockwork quartz veinlets, some with tourmaline, also exist in the mineralized zone. However, the presence of quartz, even with tourmaline, is not necessarily an indication of the presence of gold. Quartz veinlets with tourmaline but without gold mineralization were found hundreds of metres away from the San Francisco deposit. Alvarez (in Prenn, 1995) suggested that some tourmaline was part of the mineralizing system, but could be distinguished from the tourmaline found elsewhere.

The relationship between the quartz and tourmaline at the Project is not well understood, though at least one event is closely related to the gold mineralization. Calmus (1992) and Perez (1992) described the gold as being in quartz, acicular tourmaline, and albite veins and breccias. It was noted (Perez, 1992) that two types of tourmaline exist: schorl and dravite, but these are difficult to distinguish. There is some suggestion that a more greenish tourmaline is associated with the San Francisco zone while the black tourmaline (schorl) is generally barren of gold. If this can be verified, it could become a valuable exploration tool for the region. Horner (in Prenn, 1995) also noted the possibility of two or more types of tourmaline in the cobbles sampled in the stream beds. Horner believes that only one set of the tourmaline veins is associated with the gold and suggests that bismuth is also associated with one tourmaline quartz vein event.

Other metallic minerals associated with the deposit include trace to small amounts of chalcopyrite, galena, sphalerite, covelite, bornite, argentite-acanthite and pyrrhotite. Trace amounts of molybdenite and wulfenite have also been reported. Metal mineralization is low, with copper reaching into the hundreds of ppm, arsenic reaching about 100 ppm, and antimony rarely over 10 ppm. Petzite was recognized but tellurium values rarely reached 10 ppm. The mineral relationships, the possibility of associated tourmaline, and the style of mineralization suggest that the San Francisco deposit might be of mesothermal origin (see Prenn, 1995 for discussion). Others have suggested the same genesis based on these and other factors, including fluid inclusion studies (in Prenn, 1995).

The San Francisco deposits are roughly tabular with multiple phases of gold mineralization. The deposits strike 60° west to 65° west, dip to the northeast, range in thickness from 4 to 50 m, extend over 1,500 m along strike and are open ended. The San Francisco deposits consisted of the El Manto, the San Francisco, the En Medio and the El Polvorin deposits. All of these deposits were later incorporated into the main San Francisco pit. The El Manto deposit (north pit), to the north of the San Francisco (main pit), is tabular, strikes 65° west, dips relatively shallowly to the northeast, and ranges in thickness from 5 to 35 m. The En Medio (in the main pit north of San Francisco) strikes 60° west, dips to the northeast and varies in thickness from 4 to 20 m. The El Polvorin (west pit) is a northwest extension of the San Francisco mineralization which strikes 65° west, dips moderately to the northeast and ranges in thickness from 4 to 20 m.

Alteration related to the mineralization consists of negligible to locally intense sericitization, course-grained pyritization and rare local silicification. This alteration forms a halo extending a few metres from the mineral deposits, but may also be absent. Supergene alteration consisting of oxidation of pyrite to goethite is common. Additionally, there is supergene alteration of feldspar to kaolin and sericite.

Analysis by Geomaque of 110 samples in seven mineralized zones showed a silver/gold ratio of less than 1 to 10, with very low values of zinc, copper, molybdenum, bismuth, antimony and mercury. Lead is occasionally high, but not above 1% while gold shows a good correlation locally with arsenic and lead. However, none of the other elements is a good indicator for gold.

The San Francisco and La Chicharra pits will be mined at the same time. The La Chicharra pit, previously mined by Geomaque, is located 1,000 m west of the San Francisco pit.

All mining activities are being carried out by the contractor, Peal Mexico, S.A. de C.V., of Navojoa, Mexico. The contractor is obliged to supply and maintain the appropriate principal and auxiliary mining equipment and personnel required to produce the tonnage mandated by Alio, in accordance with the mining plan.

In addition to the open pit, Alio previously conducted an investigation into whether or not it is economical to conduct limited underground mining beneath the southern pit wall of the San Francisco pit. In 2015, Alio conducted limited underground drifting to expose the mineralized lenses outlined in preliminary drilling. In September, 2015, Alio ceased the underground drifting after exposing the mineralization along two lenses. Alio was contemplating doing further drilling to define the extent of the mineralization exposed in the workings and also to assist it in deciding the best underground mining method, should it proceed with mining these lenses.

In the latest open pit design, the mineral resources previously identified to be mineable via underground mining methods will be recovered by conducting a pushback of the pit all in the southern direction towards the waste dumps. This pushback will constitute a fifth phase of mining at the San Francisco pit.

Ore extracted from the pit is transported in 100 t capacity haulage trucks and fed directly into the gyratory primary crusher with dimensions of 42 inch x 65 inch. The primary crusher has a nominal capacity of 900 t/h. The crushed product is then transported on conveyor belts to a coarse ore stockpile with a capacity of 6,000 t.

Two feeders beneath the coarse ore stockpile deliver the material to a conveyor belt for transport to the secondary crushing circuit. The ore is screened at 12.5 mm (½ inch). Screen undersize reports to the final product, while screen oversize is fed to two secondary crushers.

Product from the secondary crushers is transported on conveyor belts to the tertiary crushing circuit, which comprises three parallel tertiary crushers operating in closed circuit with screens. The minus 12.5 mm undersize from the screens is delivered to the leach pad. This crushing circuit had a nominal capacity to deliver 16,000 t/d of crushed material to the leach pads, but typically operates at 15,000 t/d.

By August, 2013, Alio had installed a new crushing circuit with a capacity for processing an additional 7,000 t/d. This circuit comprises one jaw primary crusher, two secondary crushers, three tertiary crushers, screens and conveyors.

The current crushing operating rate is 22,000 t/d.

Alio is working on improving the recovery within the leach pads by finer crushing in both crushing circuits from P100 = 12.50 mm to P100 = 9.53 mm (P80 = 8.50 mm). Both crushing circuits must be upgraded to maintain a throughput of 22,000 t/d at the finer crush sizes.

The 7,000 t/d crushing circuit, will also be upgraded. New equipment has been procured, is already on site and will be installed in the near future.

Product from the crushing plant is transported to the leach pad on overland conveyors and deposited on the pad with a stacker, forming 8 m to 12 m high lifts. A bulldozer is used to level the surface of each lift. The irrigation pipelines are then installed to distribute the leach solution over the entire surface of the lift. The design primary leach time is reported to be 180 days although in practice this can be extended when leaching a lift placed above the original placed material.

The leach solution consists of 0.05% sodium cyanide with lime addition to obtain a pH of 10.5 to 11. The solution percolates to the bottom of the lift and flows to the channel that carries the solution to the pregnant solution storage pond, from which it is pumped to the adsorption, desorption and recovery (ADR) plants.

Barren solution exiting the ADR plants flows to the barren solution storage pond where fresh water and sodium cyanide are added, before being pumped back to the leach pad.

Pregnant leach solution is fed to the first adsorption plant which consists of 2 parallel lines of carbon columns, each with 5 tanks in series, through which the carbon is advanced countercurrently to the solution flow. One line of columns contains approximately 2.0 t of carbon and the other 2.5 t. Gold is adsorbed on the carbon to a concentration of approximately 5,000 g/t. Desorption of the carbon is achieved in a Zadra type elution circuit. Gold is recovered by an electrowinning circuit comprising stainless steel electrodes in a stainless steel electrolytic cell. The stainless steel cell and cathodes are relatively new and replace the original polypropylene cell with steel wool cathodes. The use of stainless cathodes is more efficient, as it eliminates the smelting of substantial quantities of steel wool, which requires substantially more flux and can lead to inferior grade doré.

Installation of a new line of carbon columns (second ADR plant) with 5 tanks containing approximately 6 t of carbon, and a design flow of 3,500 USGM (805 m3 /h), was completed in August, 2011, to increase the production capacity.

A new stripping circuit with a capacity of 5.5 t of carbon has been added to the process.

An additional carbon tank with a capacity of 6 t of activated carbon (similar at the existing ones in ADR Plant #2) for capturing the gold from the old phases has been added to the circuit.