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”; overs ........