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 pond and the Merrill-Crowe plant. The solution is clarified, de-oxygenated, and processed through a Merrill-Crowe circuit at a rate of approximately 1,300 m3 per hour. The clarified and de-oxygenated silver and gold cyanide solution is added to zinc dust, which triggers the precipitation of the silver and gold. The remaining barren cyanide solution is recycled back to the heap leach pad for continued processing, while the zinc, gold, and silver precipitate in suspension is pumped to filter presses and dried. Retort ovens remove trace amounts of mercury from the precipitate, then it is mixed with fluxes and melted in a furnace to form doré bars. The bars are cleaned, assayed, weighed, and transported to a third party refinery in North America for final recovery. The doré bars typically contain between 1% and 5% gold and 95% to 99% silver, with generally less than 1% impurities. The metals recovery circuit has the capacity to produce approximately 200,000 ounces of gold and 7.5 million ounces of silver, for 8 million ounces of doré annually.

The heap leach process is a closed circuit without tailings facilities and there is no release to the environment.

Pulp agglomeration flow sheet
The pulp agglomeration plant will operate in an integrated fashion with the heap leach processing facilities. 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. Cyanide and lime will be fed into the rod mill to start the metals recovery process. Process water for the rod mill will be pregnant solution from the heap leach. The entire area from the rod mill forward will be on concrete pads for chemical and spill containment.

The milled slurry will be pumped to a filter feed tank and then pumped to one of three horizontal pressure filters equipped with vertical plates, air blowers, and a membrane squeeze and dewatered to produce a filter cake with approximately 15% moisture. Three filters will allow for continuous operations with one filter being filled while the other two are being emptied or are in a wash cycle. The recovered filtrate solution will be pumped to a pregnant solution holding tank and a pinned bed clarifier for clarification and for eventual metal recovery in the Merrill-Crowe plant.

The dewatered filter cake will be discharged from the filter presses onto a belt conveyor and into a de-lumper that will break the filter cake. The broken filter cake will be discharged onto another conveyor belt where cement will be added at 25 kg of cement per tonne of filter cake. The filter cake with the cement will be blended with coarse medium grade ore in a mixer prior to being fed into the rotating agglomerator drum. Additional solution will be sprayed onto the mixture within the drum as it tumbles in order to control the moisture content and formation of the agglomerate balls. The agglomerate balls will be discharged from the drum and conveyed to the heap leach pad feed conveyor where they will be combined with crushed medium grade mineralized material to be conveyed to the heap leach pads.

Most of the pulp agglomeration process make-up water will be sourced by diverting a portion of the pregnant solution flow from the heap leach pads on its way to the Merrill-Crowe plant. Additional cyanide solution will be sourced from the sodium cyanide solution make-up plant in the Merrill Crowe plant. The heap leach pads, ponds, and cyanide circuit will remain contained within a closed circuit and there is no tailings stream or release to the environment.

The pulp agglomeration plant comprises a two stage crushing plant with a capacity of 5,600 tpd, a rod mill targeting a particle size of P80 passing 1,700 µm, a Vertimill targeting a particle size of P80 passing 425 µm, a filter feed tank, pressure filters for dewatering of the ground rod mill product, a pinned bed clarifier, a de-lumper, a mixer for the addition of cement, and an agglomeration drum.