La Encantada is a good example of high temperature carbonate-hosted Ag-Pb-Zn, intrusioncentered mineralized system. These deposits are referred to as CRD (Carbonate Replacement Deposits) and occur in thick carbonate-dominant Jurassic-Cretaceous basinal sedimentary sequences floored by Paleozoic or older crust (Megaw et al., 1988); e.g. the Chihuahua through and the Coahuila basin.

La Encantada property hosts silver mineralization in a variety of deposits, from underground epigenetic deposits (chimneys, breccia pipe, manto, vein and skarn) to artificial over-ground deposits (tailings). However, the following descriptions will be limited to the epigenetic deposits.

The epigenetic deposits at La Encantada are high temperature carbonate replacements in skarn and limestone. Replacement deposits are characterized by shallowly-dipping irregular shaped pods, lenses, and roughly tabular masses of oxides parallel or sub-parallel to the host stratigraphy. Irregular tabular bodies dipping at angles less than 45° are referred as mantos. Near vertical deposits with more or less circular, oval or irregular elongate shapes are referred to as chimneys and breccia pipes. Tabular sub-vertical deposits are referred to as veins, and sometimes they contain small chimneys at the intersection of NW trending faults and fractures; veins have a NE preferential orientation. Skarn, larger mantos, chimneys and breccia deposits are proximal to the main granodiorite apophysis, whereas veins, small chimneys, small breccias and small mantos occur distal with respect to the main intrusions; the stock below the skarn dome and the Milagros intrusion. Narrow andesite to basalt-andesite dikes are hosted along some of the NE trending faults, and some bear silver mineralization, e.g. the San Francisco dike and the great dike.

In general, the more proximal manto, chimney and skarn deposits of La Encantada can be described as CRD-type, whereas the more distal veins can be described as mesothermal open space-filling veins. Cumulatively, skarn, manto, chimney, breccia pipe and vein deposits represent a continuum of hydrothermal deposits centered on an intrusion (thermal source).

At La Encantada, mineralization is dominantly controlled by N60°E and N30°W stri ing faults and fractures. Mineralization in this type of deposits occurs immediately below permeability barriers that appear to confine mineralizing solutions to adjacent carbonate units without themselves becoming mineralized (Megaw et al., 1988). The upper argillaceous member of Cuesta del Cura Fm. and the alternating shales and limestones that rest on top of Cuesta del Cura may have acted as a permeable barrier.

Mineralization at La Encantada occurs in bodies with geometries that vary from tabular vein, manto, chimney, breccia pipe and irregular replacements. Structural controls on mineralization included intrusive contacts, faults, fold axes, fractures, fissures, and cavern zones. Intrusive contacts and intrusion related faults are the most important controls in the skarn, whereas regional faults, folds, and fracture systems are dominant controls on mantos, chimneys and veins. Detail field studies at Santa Eulalia and La Encantada showed that metal-depositing processes within carbonate packages are often influenced by thin shaly partings, stylolites, and carbonates of different grain size (Megaw et al., 1988). Stylolites are common in the Aurora Fm., which in combination with some of the structural controls mentioned above, may have favored the deposition of mineralized mantos and vein shoots.

Silver minerals such as native silver and acanthite occur associated with a complex mixture of Zn, Pb, Fe and Mn oxides in mantos, veins, breccias and chimneys; silver rich chimneys such as the La Prieta and Escondida were mined in the past by Peñoles. Native silver and acanthite also occur associated with sulphides in skarn below the La Prieta-660 Ojuelas oxide bodies. The most common oxides are hematite, goethite, jarosite, argento jarosite, cerussite, anglesite, zincite, pyrolusite, hemimorphite, smithsonite, willemite, malachite and brochanthite. Oxide minerals at La Encantada are secondary phase products of the strong supergene oxidation of primary sulphides; the deep oxidation level (>500 metres vertical extent of oxidation) of the La Encantada deposits suggests that the region has remained arid (deep water table) for thousands of years. A long period of oxidation favored total transformation of sulphides into secondary Fe, Zn, Pb Cu oxides and native silver; native silver is an oxidation product of acanthite. The sulphide minerals, pyrite, magnetite, marmatite (iron-rich sphalerite), galena, chalcopyrite, covellite and acanthite occur in the skarn dome area (below La Prieta 660-Ojuelas bodies), deep in the Milagros area and in pockets within the Ojuelas manto; sulphides are preserved in small impermeable pockets.

Mining the Veins System and other minor deposits at La Encantada is undertaken using primarily the conventional overhand cut-and fill mining method. Ramps are driven into the orebodies, and stopes are developed from sill drifts driven in the ore zones and slashed out the full width of the ore.

Mining operations at La Encantada mine are partially mechanized. Drilling of access drifts and ramps is carried out using hydraulic jumbos, and most of the headings and cut-and fill stoping is accomplished using pneumatic hand-held jackleg machines.

The cut-and-fill stoping cycle is started with blast holes drilled using hand-held jackleg drills, followed by blasting using conventional mining explosives. After blasting, LHD’s are used to muck the blasted ore. The cut and fill stopes range between 50 to 150 m in length along strike, and extend between levels which are typically spaced 15 to 30 m apart vertically. Each cut is 2.5 to 3.0 m in height. Depending on ground conditions, the blast holes are drilled either upward or horizontally. Waste and mineralized material below cut-off grade is blasted down and used as backfill as needed.

The minimum mining width is 2.0 m, and planned dilution is included in the mine design, which varies according to the ground conditions, vein width, and the dip of the vein. The dilution factors range from 5% to 20%, with an average of approximately 10%. Mined areas are measured to compare the width of the vein and the width of the cut on a regular basis; as mining advances, this comparison is used as means of reconciliation and to build the historical database of the dilution and mining recovery factors. Sills and access drifts are excavated at 2.5 m wide by 3.0 m high, cross-cuts and access ramps to the stopes are excavated 3.0 m wide by 3.0 m high, and main access ramps are excavated 4.0 m wide by 4.5 m high.

Based on the geotechnical characteristics and the geometry of the breccias, First Majestic has started the implementation of a variant of Incined Caving for the San Javier and Milagros breccias. This configuration allows the extraction of ore by building draw-points at different elevations, starting from the outside of the deposits and working inwards as the lower levels are developed.

Based on the constraining shapes generated for resource constraining, two main blocks have been identified, the first to be extracted is a block for the San Javier breccia with bottom elevation 1,740 masl. A second block has been delimited in the Milagros breccia with bottom elevation 1,550 masl.

A review was carried out to assess the applicability of the possible mining methods, including caving and non-caving methods, to mine the Ojuelas deposit. Their applicability was scored using the University of British Columbia mining method selection matrix, or UBC method, to narrow down the potential methods that are appropriate given the characteristics of the orebody and country rocks.

Based on a selection matrix, Block Caving, Sublevel Caving and Cut and Fill mining methods were the top three methods identified, in that order. Due to the geometry of the Ojuelas orebody, applying a standalone block caving or sublevel caving method could negatively impact the economics of the project, therefore, a combination of inclined caving and block caving methods is proposed as one of the methods to be further evaluated in mining the Ojuelas deposit. The upper zone, which has a moderate dipping and thin mineralization zone, will be mined with the incline caving method, whereas the lower portion that has a flatter dipping and thicker mineralization will be mined with a block caving method. The mining will be done in a top-down fashion.

Inclined caving uses block caving principles with a truncated sub-level caving mining layout. The cave is initiated by undercutting the orebody at the inclined footwall area. The caving is advanced downward. The principles of gravity flows and draw interactions applied in block caving and sublevel caving are also applied in this mining method. Once all of the ore in the sublevels has been extracted, the block cave will be initiated at the bottom to remove the remainder of the reserve.

The mill at the La Encantada cyanidation plant processes silver minerals through a leaching method producing doré bars. The installed plant capacity of the processing plant is 3,000 tpd for the crushing/grinding area, and 4,500 tpd for the cyanidation circuit.

The processing plant flowsheet consists of three-stage crushing, ball mill grinding, a leaching circuit, and a Merrill-Crowe system, followed by smelting of the precipitate and final tailings filtration.

The processing plant is divided into two parts: Plant No. 1 and Plant No. 2. Plant No. 1 is comprised of crushing and grinding circuits, while Plant No. 2 is comprised of leach tanks, countercurrent tanks (CCD), Merrill-Crowe section, smelting furnace, and tailings filtration.

ROM material delivered from the mine is dumped into a steel-made coarse ore bin of 300 tonne capacity. The coarse ore bin is equipped with a steel rail grizzly in its upper part. The grizzly has openings of 12” x 12”; oversize material is reduced in size using a hydraulic hammer.

The coarse ore bin has a lower discharge chute that discharges into a vibrating feeder grizzly of 4” opening. The -12” + 4” material is fed into a 24” x 36” primary jaw crusher, and reduced to a minus 3” to 3-½”. This product is transported by a 30” wide belt conveyor to the two primary vibrating screens. These screens have only one sieve with an aperture of 3/8” x 3/8”. The lower discharge of the screens contains material from 80 to 90% minus 1/4” (6,350 µm).

Crushing plant capacity is 3,000 tpd in 18 operating hours.

The grinding section is comprised of two ball mill circuits: a recently installed Metso ball mill and the two ball mills previously used, which are currently maintained as back-up. Current grinding capacity is of 125 tpd. The dimensions and sizes of the equipment are:
- 12’ x 24’ Metso Mill (1800 HP)
- D-26” Krebs cyclones, and
- 10”x8” 250 HP Envirotech pumps
- two back-up ball mills 9-1/2’ x 11’ and 9’ x 12

The following reagent dosages are added to the process:
- Cyanide is prepared at 1,400 ppm and is fed in at three points of addition: into the mill, and into the 6 th and 13th tanks. Cyanide consumption is 1.0 kg/tonne.
- There are two preparation tanks for lime, one in Plant No. 1 and another in Plant No. 2. A total of 3,000 kg/day of lime is added into the mill (Plant No. 1). The same amount is added in the primary and intermediate thickeners in Plant No. 2. Lime is prepared as a suspension (whitewash).

Cyclone overflow is pumped to the Plant 2 and fed into the 125’ Primary Thickener. Primary thickener underflow is pumped to a series of twelve 30’ X 43’ agitated leach tanks, to complete 50 hours leaching time, which is considered the first leaching stage.

Overflow from the 12th leach tank goes to the intermediate thickener, which recovers the pregnant liquor in the overflow while the underflow is pumped to the second leaching stage. This involves five 30’ X 43’ agitated leach tanks, which complete 22 more hours of residence time. Most of the volume of the overflow solution from the intermediate thickener goes to the primary thickener, producing the pregnant solution which is fed to the Merrill Crowe system.

Slurry from the last agitated tank feeds the CCD thickeners. There are a series of four 125’ thickener tanks. Underflow from tank #4 feeds a storage tank which doses the slurry to the three final tailings filters. Tailings filters have a 2000 mm square section and 139 plates each. Final tailings are discharged at 16% moisture, and deposited on a tailings deposit.

CCD final overflow solution is pumped to the grinding circuit located in Plant 1. Slurry density of thickener underflow ranges from 48% to 55%. Leaching circuit capacity is 125 tph (3,000 tpd) for 72 hours of residence time. Thickening capacity is 177 tpd.

Samples from plant ore feed are taken with a branded automatic sampler, while the final tailings slurry sample is taken with a home made automatic sampler. Pregnant and barren solutions are taken using automatic home-made samplers. Samples are taken by dripping, and all the samples are sent to the La Encantada laboratory to be analyzed. Solids are analysed for: Ag, Pb, Zn, Fe, Cu and Mn, while solutions are analysed for Ag, Pb and Cu. The production balance is calculated solely with the silver tests.

Pregnant Solution is sent to a 1,200 m3 storage tank called “Dirty Pregnant Solution”, which is filtered and clarified through three autojet pressure clarifiers. Product from the autojet filters is then stored into a 1,200 m3 tank called “Clean Pregnant Solution”. This solution is then pumped through a tower with two deaerator cylinders in order to remove dissolved oxygen, which goes from 7 ppm to less than 1 ppm O2.

After deaeration, pregnant solution is pumped to three 1,500 mm press filters. Before being pumped, zinc dust is added to the solution in order to carry out a precipitation reaction. Zinc consumption is 1.5 kg per kg of doré.

Daily production of pregnant solution is approximately 18,000 m3 with a grade of 12 ppm Ag. Merrill Crowe capacity is approximately 550 kg doré per day.

Precipitates are dried and then smelted in two induction furnaces, producing 23 kg doré bars with a purity of 80 to 90%. Doré bars are weighted and packaged before sending to the client.

Ojuelas ore is planned to be processed by flotation. The ore has been tested at the First Majestic Central Lab, and all the samples received have been tested by cyanide leaching and flotation, in order to determine the most suitable process. The majority of the test results show that flotation is more suitable for the Ojuelas ore.

The flotation processing approach has been considered as follows:
- Crushing plant: the existing plant has enough capacity to treat a planned 2,000 tpd feed from Ojuelas.
- The processing plant flowsheet consists of three-stage crushing, ball mill grinding, sulphides and oxides flotation, and final tailings filtration.

The flotation plant is considered to be comprised of crushing, grinding and flotation circuits.

The flotation section will have 2 circuits: sulphides and oxides. Sulphides conditioning: a collector (potassium amylic xanthate) will be added to the grinding product in the conditioning tank. A frothing reagent will also be added to the conditioning discharge.

The discharge from the conditioning tank will flow by gravity toward to three rougher flotation cells; each cell will have a capacity of 1,000 ft3. Concentrate from these three cells will flow to a cleaner flotation stage. Overflow will be considered the sulphides final concentrate and will be sent directly to the concentrate thickener.

Tailings from the three rougher cells will be pumped into the next flotation step: scavenger. The scavenger stage will be comprised of one 500 ft3 cell. Concentrate from this cell will be pumped to the sulphides conditioning circuit as circulating load.

Tailings from the scavenger stage will be considered head of the oxides flotation circuit. The oxides flotation circuit will be comprised of five 1,000 ft3 cells as rougher (2) and scavenger (3) stages. Reagents in the oxides circuit include: sodium sulphide (Na2S), potassium amylic xanthate and sodium bicarbonate.

The concentrate of the two rougher cells will flow to a cleaner flotation stage. Overflow will be considered the oxides final concentrate and will be sent directly to the concentrate thickener to be mixed into the tank with the sulphides concentrate. Samples will be taken in the flotation circuits using automatic samplers: plant feed, final concentrates and tailings will be sampled with cuts taken every 15 minutes. Samples will be sent to the La Encantada mine laboratory to be analyzed for: Ag, Au, Mn, Pb, Zn, Fe, Cu and As. Production balance will be calculated with the silver assays.