Dolores is located in the Sierra Madre Occidental volcanic belt, which comprises calc-alkaline batholiths and volcano sedimentary rocks of the Lower Volcanic Series and ignimbrites of the Upper Volcanic Series.

The San Francisco fault and its footwall host most of the mineralization at Dolores. The immediate footwall and hanging wall of the fault forms a 500 metre wide northwest-striking corridor of igneous intrusions.

Low sulphidation epithermal silver and gold mineralization is hosted in north-northwest trending hydrothermal breccias and sheeted vein zones in the order of 5 metres to 10 metres wide. Most high grade mineralization occurs along three major structures. Silver and gold mineralization identified on the surface lies over an area 4,000 metres long and up to 1,000 metres wide.

The highest grade mineralization occurs within the San Francisco Breccia, a well-defined and continuous hydrothermal breccia and stockwork zone that occurs in the immediate footwall of the San Francisco fault. The breccia trends further away from the fault towards the north until it joins a second major breccia zone known as the Alma Maria Breccia.

Hydrothermal breccias carry the highest silver and gold grades and pass outward into vein stock works. The veins are thin, rarely over 30 mm, and tend to occur as sheeted swarms. Economically mineable grades occur where the veins are sufficiently closely spaced.

Mining at Dolores is by standard open pit methods using shovels, loaders, and haul trucks and by underground methods using sub level open stoping and ramp haulage to the surface.

Open pit
The minimum mining unit is 7 m x 14 m x 7.5 m. Ore grade control drilling is carried out using angled reverse circulation drilling to provide closer spaced sample data for a grade control estimate, which is used to mark out the ore and waste mining boundaries. The grade control holes are oriented perpendicular to the strike of the deposit on sections spaced every 15 m along strike and every 10 m to 15 m across strike. Drillholes are approximately 43 m long, which results in a vertical span of 30.2 m, equal to four bench heights. The holes pattern is offset with 25% of the holes drilled from each bench to provide full coverage. The reverse circulation drillholes are logged for lithology and oxidation and sampled every 2 m. Ore and waste material is drilled and blasted using 135 mm diameter, 8.5 m deep blast holes spaced 4.5 m along strike and 4.5 m across strike, charged with ammonium nitrate fuel oil (“ANFO”). Ore and waste are usually blasted separately and a blast movement monitoring system is in place to minimize ore loss, dilution, and material misclassification.

The majority of the waste will be stored to the south of the pit. The mining sequence is planned to focus on the higher grade material in the south of the pit with waste material stored in the east waste rock facility. When the last phase of mining in the south is completed, mining will switch to the north part of the pit with some of the waste stored inside the pit and the remainder at the north waste rock facility.

Underground
The mining method is open stoping, mostly oriented longitudinally, which takes into account the competent ground conditions and the requirement to tight fill stopes beneath the open pit. Golder (2014) advised that stope heights of 25 m, widths of 15 m, and lengths of up to 100 m will be stable. Avoca long hole mining will be used for stopes with widths from 3.5 m to 7 m, longitudinal long hole stopes will be used for stopes from 7 m to 15 m wide, and transverse long hole stopes will be used for stopes greater than 15 m wide. The average stope width is 5.0 m. There are localized areas with stope widths between 10 m and 20 m, and these are mined using a primary and secondary sequence with cemented fill. The average stope length used in the design is 25 m.

The stopes are planned to be mined up into the completed open pit, or will be mined and tight filled prior to mining by open pit methods. The sections of the pit that are used for in-pit waste rock dumping will not be mined from beneath and will be left as crown pillars. The necessary backfill material will comprise both cemented and uncemented rock fill. Cemented fill will be used in the thicker parts of the deposit where the mining method uses primary and secondary stoping sequences. The cemented rock fill will be prepared from a cement slurry prepared in a mixer, discharged onto mine waste in a mixing pit, and mixed with a front end loader bucket.

Sill drifts will be developed along strike and spaced 25 m apart vertically. The sill drifts are designed at 4.5 m high to accommodate long hole drills and remote control scoop trams. Footwall drives will be developed parallel to the ore drives to allow bypass access for stope backfill and to provide egress points along the extents of each level. The footwall and ore drives will be linked by nominal 20 m long crosscuts that act as draw points during stope development and as backfill tip heads during filling.

Access to the underground workings from the surface is via a ramp that approaches from the east of the pit. The ramp acts as a fresh air intake and is independent of the open pit. Alternate accesses will be developed over the life of mine from the final stages of the open pit, and these interactions will be scheduled to allow for in pit dumping of waste from the final mining stages of the northern phases of the open pit. The underground workings immediately below the planned open pit will be accessed by ramps extending from the southern underground workings and by short adits from the completed sections of the open pit. The total length of all lateral development in the plan is estimated at 51 km. Decline development is nominally 5.5 m wide by 5.5 m high.

High grade ore will be stockpiled on the run of mine pad, separated from the medium grade ore. The high grade feed will be tipped into a bin using front end loaders at a rate of 5,600 tpd. The feed will be recovered from the bin by a vibrating grizzly feeder and delivered to a primary jaw crusher (Metso C-160), and the discharge conveyed to a vibrating screen. Screen oversize will pass through a cone crusher (Metso HP-800) in a closed loop to produce a rod mill feed with a P80 of 12.8 mm. The screen undersize will report to a conveyor and radial stacker for depositing onto a fine feed stockpile. The material will be recovered from this stockpile via multiple reclaim feeders and a reclaim tunnel conveyor system to feed the rod mill (Nordberg 13’ by 18’8”).

The rod mill discharge will be pumped in closed circuit to a Vertimill/vibrating wet screen circuit. The screen oversize will report back to the Vertimill and the screen undersize will be pumped to a surge tank for feed to the pressure filters. This closed loop circuit is estimated to produce a P80 of 0.425 mm (35 mesh) product.

Heap leach flow sheet
Broken ore is trucked to the crushing plant and crushed to a particle size of P80 6.3 mm in a three stage crushing circuit with a recirculating load at a nominal rate of 16,200 tpd. The crushed ore is sampled at regular intervals by a sample tower, conveyed to the leach pads via an overland conveyor system, and placed on the pads using portable grasshopper conveyors and a radial stacking system. Sodium cyanide solution is prepared in the Merrill-Crowe plant, pumped to the leach pads, and applied using drip and sprayer systems.

Metal recoveries are a function of solution flow rates, cyanide concentration, and time, and the metal leaching period can cover years, continuing as subsequent lifts are placed on the pads. Gravity collectors and/or vertical perforated steel pipes (well risers) fitted with internal pumps transfer the pregnant leach solution containing the dissolved silver and gold from the bottom of the leach piles to the pregnant solution ........