Dolores is a low sulphidation epithermal deposit with strong structural control (Overbay et. al., 2001). Northeast-striking faults, perpendicular to the structural grain, locate many mineral deposits in the Sierra Madre Occidental. However, regional controls at Dolores are unclear where mineralized faults are north-northwest striking. There is no apparent transverse structure apparent on geological maps or satellite imagery. The mineralized structures have a typical Sierra Madre, or Laramide, trend, a direction probably inherited from the basement of elongate terranes.

Silver and gold mineralization at Dolores is hosted in north-northwest trending hydrothermal breccias and sheeted vein zones on the order of 5 m to 10 m wide. Most high grade mineralization occurs along two or three major structures which provided the conduit for metal-bearing hydrothermal fluids. Silver and gold mineralization identified on the surface at Dolores lies over an area 4,000 metres long and up to 1,000 metres wide, at elevations ranging between 1,100 metres to 1,700 metres above sea level. The extent of mineralization at depth and along strike has not yet been fully defined.

Most high 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 post-mineral San Francisco Fault. The breccia trends farther away from the fault towards the north until it joins a second major breccia zone known as the Alma Maria Breccia. Other high grade zones split and re-join, forming diamond-shaped structures and steeply pitching mineralization shoots. These types of splits and branches, and steeply pitching intersections, are typical of fault systems with some strike-slip movement, in this case dextral. The San Francisco Fault marks a profound drop in grade and the hanging wall is mostly devoid of silver and gold mineralization. This barren hanging wall, mostly latite lava, forms the high west walls of the open pits.

Hydrothermal breccias carry the highest silver and gold grades and vary from crackle to very milled types. Crackle types were more permeable and they commonly show coarse quartz+sulphide (pyrite, sphalerite, and galena) + fluorite gangue. They pass outward into vein stock works. Milled varieties of breccia contain fewer sulphides. The finer varieties resemble the fluidised mud/sand dikes typical of active geothermal fields. Hydrothermal breccias commonly include quartz vein or quartz-veined fragments. For example, exposed
breccias at the north end of the San Francisco Breccia, where it more or less coincides with the San Francisco Fault, include amethyst vein fragments. Overall, the hydrothermal breccias mostly developed after the sheeted vein swarms, presumably by evolution from early fracturing to total failure and transportation of clasts. However, there are rare examples of breccias cut by veins.

Most veins at Dolores are relatively simple, with only weak crustiform texture. They comprise euhedral comb quartz, commonly zoned from milky to transparent, depending on fluid inclusion content. Veins commonly show aggregates of very fine grained drusy quartz. They are thin, rarely over 30 mm, and tend to occur as sheeted swarms. Economically mineable grades occur where the veins are sufficiently closely spaced.

Local acanthite and visible gold occur in a variety of settings, including hydrothermal crackle breccias and in thin quartz-dominated veins. Base metal sulphides appear to increase at depth, with widespread coarsely crystalline sphalerite, galena and minor chalcopyrite. The sphalerite is always zoned, from yellow in the core, to black (marmatite) rims. Base metal sulphides are particularly abundant in the deeper parts of the stock-diatreme complex in the pit.

A sub-vertically pitching mineralization shoot occurs where the San Francisco and Alma Maria Breccias meet, parallel to the intersection. Clearly this was a site of increased permeability for hydrothermal fluids. High grades also occur in the diorite stock-diatreme complex in the pit. The diorite formed an ideal brittle host for fracture, meaning more extensive brecciation and stock working. It may also have been the main upflow centre for hydrothermal fluids.

Not all silver and gold mineralization occurs in vein swarms and hydrothermal breccias. Some diffuse areas of sugary, silicified rock with coarse adularia and high silver and gold grades also occur.

Mining at Dolores has been ongoing since 2008 using conventional open pit methods with excavators, shovels, loaders, and haul trucks. 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 underground mine will operate concurrently with open pit mining and will be completed at approximately the same time. 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.

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 ........