Mineral deposits at Del Toro occur in veins, chimneys, breccias and mantos. The deposits are associated with a quartz monzonite–granodiorite intrusion and are hosted by Cretaceous limestone and shale that has been altered to marble, hornfels, skarnoid and skarn.

Because of their spatial relation with intrusions and metamorphic/motasomatic rocks, the deposits are proposed to be of the intrusion-related hydrothermal type. Potassic alteration observed at depth in the San Juan mine suggests high-temperature alteration at depth. Although fluid inclusion- microthermometry studies have not been carried out for the Del Toro deposits, the association intrusions and skarn suggests they could be of the mesothermal to epithermal type. The occurrence of distal quartz– calcite veins containing fluorite in the Dolores mine is suggestive of an epithermal environment.

Mineralization to the south at San Nicolas and San Juan seems to be more mesothermal whereas the mineralization to the north in Dolores seems to be epithermal. No attempt was made to fit Del Toro deposits to the settings of the well-studied porphyries or epithermal deposits described by Sillitoe or Hedenquist elsewhere, since the geologic features observed suggest that Del Toro sits in between these end-member environments.

Mineral deposits at Del Toro occur in veins, chimneys, breccias and mantos. It is interpreted that some of the mineralization is skarn-related mesothermal in style, similar to that of San Martin Sombrerete. Some epithermal features, such as quartz–calcite veins containing fluorite, occur at Dolores and the Navidad claims north of Dolores. Veins at Del Toro can be of two types, open space filling or fault vein.

The open space filling type can be massive sulphide veins containing galena, sphalerite and pyrite, quartz– carbonate veins containing pyrite, sphalerite and galena, and massive carbonate veins consisting of calcite, siderite and manganiferous calcite.

The second type of vein, fault-vein, consists of breccia or gouge with disseminated sulphides and oxides. Open space filling veins can transition into fault veins, and vice versa, along a structure. Most veins were likely open or partially open faults that were flooded with hydrothermal fluids carrying metals, and some of these were reactivated by later faulting events. Good examples of massive open space filling veins that transition into fault veins are Lupita, Santa Teresa and San Nicolas. Cuerpo 1 and Cuerpo 2 in the San Juan mine are good examples of vein-faults. Veins typically range in width from a few centimetres to up to five metres.

Breccia pipes and chimneys also occur in the district. Sulphide-rich (galena and sphalerite) chimneys were mined in the Perseverancia mine (Perseverancia and San Nicolas chimneys) where they occur at the intersection of northwest trending structures and north-trending structures. Cuerpo 3 is a sub-vertical pipe like structure located on the eastern footwall of the Cuerpo 1 vein-fault system. It extends from 2,200 masl to at least 1,955 masl, varies in width from 40 to 80 m and forms where the Cuerpo 1 and Cuerpo 2 bodies intersect with one another. It contains sulphide and oxide mineralization in cross-cutting structures and disseminations.

Manto-type structures are more common in the Cotorras and Magistral areas, although some manto development has also been observed in the San Juan mine associated with the Lupita vein. The mantos may vary in thickness from 20 cm at Las Cotorras to 1.5 m at Magistral.

Current production is sourced from three different underground mining areas (San Juan, Perseverancia, and Dolores).

In mineralisation that exhibits fair to good geotechnical conditions, Del Toro uses cut-and fill (resue) and shrinkage stoping. Both methods have been successfully employed and recover the mineralisation with limited ore loss and dilution, albeit at a low productivity. A recent trade-off study indicated that where mineralisation was greater than 1.0 m in width, longhole stoping with fill could be more productive and cost effective than the current methods. Overhand drift-and fill is planned for the Cuerpo 3 deposit.

Stope designs assumed:

• Dolores: a minimum mining width of 1.5 m was designed for longhole stopes and 1.0 m for cutand-fill zones; additional waste was included to meet the minimum mining width for access, therefore waste was designed and scheduled on either side of the mineralisation. Sills mined for longhole stoping areas were proposed to be 3.0 m wide, 4.0 m high, and suitable for modern drilling equipment. Production stopes were designed with a minimum width of 1.5 m and a vertical distance of 9.5 m. Stope panels were designed at 20 m long (9.5 m high). A ramp mined with an arched profile will be excavated to a width of 3.5 m and a height of 3.5 m, and will incorporate a minimum stand-off distance of 20 m to locate the ramp away from mineralisation. Planned development includes: access drifts, sills (development on mineralisation), operating waste development (sills mining material below cut-off), sumps, escapeways and accesses to the escapeways, return airways and accesses to the return airways, stockpiles, and ore passes and access to the ore passes, where required.

• San Juan: There is limited opportunity to optimise the mining methods that can be applied to Cuerpo 1, Cuerpo 2 and the minor veins, as they are well-established and nearing completion. Shrinkage stoping, using a minimum mining width of 1.0 m, was designed for additional stoping areas. For cut-and-fill stoping, a minimum mining width of 1.0 m was used for design (resue portion), with a final width of 2.5 m for excavated width (1.5 m of waste blasting). Additional waste was included to meet the minimum mining width for access, therefore waste was designed and scheduled on either side of the mineralisation. Generally, the ramps have been moved from ore into waste to reduce the potential for sterilisation and to increase productivity. The drift-andfill method proposed for Cuerpo 3 used 3.0 m square profiles for development. The whole drift, once inside the mineralisation, was interrogated to meet the cut-off analysis. The access ramp designs for Cuerpo 3 are 60 m from the orebody. This stand-off distance will allow sufficient space between the ramp and the orebody for the excavation of the level accesses, stockpiles and sumps. The main ramp has a profile of 3.5 m by 3.5 m. The production levels will be spaced vertically every 18 m. Planned development includes: ore drifts, sumps, escapeways and accesses to the escapeways, return airways and accesses to the return airways, stockpiles, and ore passes and the access to the ore passes, where required.

• Perseverancia: The production areas in the Perseverancia mine were designed to a minimum mining width of 1.0 m, after ore dilution and loss were considered. Additional waste was included to meet the minimum mining width for access, therefore waste was designed and scheduled on either side of the mineralisation. A ramp mined with an arched profile will be excavated to a width of 3.5 m and a height of 3.5 m; ramps will be located away from mineralization. Planned development includes: access drifts, sills (development on mineralisation), operating waste development (sills mining material below cut-off), sumps, escapeways and accesses to the escapeways, return airways and accesses to the return airways, stockpiles, and ore passes and the access to the ore passes, where required.

The metallurgical process flowsheet at Del Toro has experienced some modifications from its original design. Initially, the flowsheet was designed to process two ore types: oxides in a cyanide leaching circuit designed to produce doré, and sulphides via flotation to produce silver-rich lead and zinc concentrates.

During commissioning of the leaching circuit in 2014, however, a series of tests showed that processing lead-oxide ore in the flotation circuit produced higher revenue from silver and lead contents when compared to processing the oxide ore in the cyanidation circuit (which mainly produced silver). As a result, the decision was made to put the cyanidation circuit into care-and-maintenance.

The sulphide flotation circuit has also undergone considerable changes. Originally, the sulphide circuit was designed to produce two concentrates: lead and zinc, with the former containing most of the floatable silver values. The circuit was operated in this configuration for approximately 16 months – from March 2013 to June 2014. However, as the circuit yielded rather low zinc recoveries (< 20%), a decision was made to suspend the production of zinc, and use the installed capacity to process lead oxide ore instead, thereby generating higher revenue from the sale of silver and lead.

The plant feed at Del Toro consists of a mixture of transitional and sulphide ore types processed via flotation. The silver-bearing particles are associated with lead minerals, which occur both as lead oxide (PbO) and lead sulphide (PbS). To maximize metal recovery, the circuit targets first the PbS followed by sulphidisation conditioning that promotes PbO flotation. Plant metallurgical performance is mainly controlled by the transitional/sulphide composition of the ore and by the ratio PbO/Pb, i.e., an index that gauges “degree of oxidation”. In general, the higher the sulphide content and the lower the PbO/Pb ratio, the higher the recovery.

Blended crushed ore is stored in a bin before being fed to the grinding circuit. Grinding takes place in a 10.5’ x 14’ ball mill operating in closed circuit with two cyclones(one of which is normally in standby). The grinding circuit has ample capacity (900 hp mill motor) and flexibility (two cyclone feed pumps, one in standby) to regularly achieve its grinding production targets: 82-84 µm (P80) and 920 t/d throughput. The mill grinding media consists of 3-inch steel balls.

Sulphide Flotation
To promote sulphide flotation, sodium dicresyl dithiophosphate is added in the grinding circuit followed by a modified dithiophosphate reagent added in the rougher cells (three 1,000-ft3 self-aspirated machines). To control concentrate grade, a selective frother formulation (a blend of alcohol and glycol molecules) is added in the rougher cells.

The concentrate from the roughers is upgraded in two 1,000-ft3 cleaner cells that produce the final sulphide concentrate. This concentrate is processed in a dewatering circuit that comprises two 60’ diameter thickeners and two 1.5 x 1.5 x 30 m pressure filters.

Metal values in the rougher cells are recovered in two 1,000-ft3 scavenger cells in which concentrate is recycled—along with the tailings from the last cleaner cell—to the head of the circuit. To reduce costs and optimize the circuit operability, this chemical scheme is implemented so that sulphide flotation can take place at natural pH, thus eliminating the need for lime addition.

Oxide Flotation
The final tailings stream from the sulphide scavenger cells is sent to a 785-ft3 agitated tank in the oxide flotation circuit. Sodium sulphide (Na2S) is added in this tank to change the electrochemical properties of the PbO particles into those resembling sulphide minerals. This transformation—often referred to as “sulphidisation”—is required so that reagents normally used in sulphide flotation can be used to recover PbO. The collector and frother used in the oxide circuit are Potassium Amyl Xanthate (PAX) and a mixture of glycol and glycol ethers.

Conditioned pulp from the tank is sent to a bank of two rougher cells (1,000-ft3 each) which produce the final oxide concentrate. The rougher tailings stream is sent to a second agitated tank where further Na2S and PAX are added. Pulp from this tank is sent to a bank of two scavenger cells (500-ft3 each), to which concentrate is upgraded in two stages of cleaning arranged in counter-current configuration.

The concentrate from the second cleaning stage is combined with the concentrate from the rougher cells and the concentrate from the cleaner cells in the sulphide circuit to produce the final concentrate.

The final plant tails (from the last scavenger cell in the oxide circuit) are processed in a dewatering circuit that consists of a 125’ diameter thickener and two 2.5 x 2.5 x 121 m pressure filters.