Mineralized veins at Bolañitos consist of the classic banded and brecciated epithermal variety. Silver occurs primarily in dark sulfide-rich bands within the veins, with little mineralization within the wall rocks. The major metallic minerals reported include pyrite, argentite, electrum and ruby silver, as well as some galena and sphalerite, generally deeper in the veins. Mineralization is generally associated with phyllic (sericite) and silicification alteration which forms haloes around the mineralizing structures. The vein textures are attributed to the brittle fracturing-healing cycle of the fault-hosted veins during and/or after faulting.

Economic concentrations of precious metals are present in “shoots” distributed vertically and laterally between non-mineralized segments of the veins. Overall, the style of mineralization is pinch-and-swell with some flexures resulting in closures and others generating wide sigmoidal breccia zones.

Primary economic mineralization at Bolañitos is gold and silver. Bolañitos is postulated to be a low sulphidation system with pyrite but no arsenopyrite.

The silver-rich veins of Bolañitos contain quartz, adularia, pyrite, acanthite, naumannite and native gold. Native silver is widespread in small amounts. Much of the native silver is assumed to be supergene. Silver sulfosalts (pyrargyrite and polybasite) are commonly found at depth.

Conventional drill and blast methods are used to extract the ore at Bolañitos, and access to the mining areas is provided by ramps and audits. Mine development headings are drilled by jumbo and by jackleg. Traditionally a conventional bottom-up cut and fill mining method was employed with waste rock brought in using diesel or electric loaders. The rock used to backfill the stopes is either dropped down a bore hole from surface or is generated from the waste development underground. Over the past several years a transition to a modified long hole method has taken place wherever the width and dip of the vein is applicable to this method.

Once sill development is completed and the limits of the ore have been defined, stope production can begin. For conventional cut and fill stoping, ore is mined upward in horizontal slices using jackleg drills. Cut and fill mining is a method of short hole mining with hole lengths usually less than 2m. For long hole mining the holes are typically 10-12m in length but vary from 6 to 16m depending on the stope. Under certain circumstances concrete is used as fill to create a solid floor. This enables mining from the stope below up to the concrete pillar and recovering most, if not all of the ore pillar that would otherwise be left behind. This process is usually reserved for high- grade floor pillars.

For cut and fill the production cycle starts by drilling upper holes using a jackleg. Geologists mark up the vein, and the stope is drilled and blasted accordingly. Drillholes on the vein are blasted first. After the ore has been mucked, the holes drilled in waste are then blasted to achieve the dimensions required for the scoop to work in the next production lift.

By comparison, longhole open-stoping, holes are drilled upwards and/or downwards from the sill level. Longhole methods are typically 6 to 16m in length and are more productive than cut and fill methods. Longhole stoping is also cheaper than conventional cut and fill stoping. As with cut and fill methods, longhole stopes are filled with waste rock from development headings or from surface waste.

Some of the ore produced with the longhole drill machines is generated by drilling old pillars. Other stopes are blind by drilling uppers and blasting a slot at the far end of the stope to enable the ore to break in the subsequent larger stope blasts. Uppers are drilled to a 10-15m height on vein projections in rows across the width of the vein. The rows closest to the slot are blasted first. The stope is mucked clean, or at least sufficiently to allow the next blast. The ore is extracted using remote-controlled scoops.

The plant processing rate is 1,600 t/d after it was expanded from 1,200 t/d in 2012 adding a 6’x16’ vibration screen, four additional flotation cells 500 ft3 each, six 1st cleaner cells 100 ft3 each, six 2nd cleaner cells 50 ft3 each, conveyor belts and a flocculent mixing system.

Run-of-mine ore is hauled by 20 tonne dumper trucks and discharge on a grizzly with opening 11”. Oversize rock (>11’) is broken by a backhoe hydraulic hammer. The undersize material falls in a feed bin and further crushed in a primary jaw crusher of size 24”x36”. After the primary crusher the ore is held in two coarse ore bins each with a 450 t capacity.

From the coarse ore bins the ore is conveyed to a 6’x16’ vibratory screen with openings 3/8”, the undersize product is conveyed to the fine ore bins. The oversize material is fed to a 4.25’ standard head Symons secondary cone crusher where the ore size is crushed down to 2”. The secondary crusher product is screened by a 5’x10’ vibratory screen with openings 3/8”. The screen undersize product is conveyed to fine ore bins and the oversize material is crushed by a tertiary cone crusher (Metso, HP200).

The fine crushed ore (approx. 80-85% of -3/8”) is stored in two ore bins. The storage capacity of the first fine ore bin is 250 tonnes and of the second ore bin is 500 tonnes of ore.

The grinding circuit consists of two ball mills: No. 1 is of size 9’6” x 14’ with a 600 HP motor, the No. 2 mill is of size 11’x18’7” with a 1000 HP motor. The mills are fed independently from respective ore bins.

The grinding product is the cyclone overflow with 70-75% passing 74 microns and flows further to the flotation circuit. Each ball mill has a separate rougher and scavenger cell lines. The ball mill #1 line consists of four (4) flotation cells with capacity 500 ft3 each. The ball mill #2 line consists of nine (9) flotation cells with capacity 300 ft3 each. The rougher and scavenger concentrates from both lines are combined and fed to the column flotation cell.

The flotation layout considers two cleaning stages though the 2nd cleaning stage was shut down in December 2013 since the concentrate grade obtained in the column cell was meeting the target silver grade between 7 and 9 kg/t.

The final concentrate flows by gravity to a thickener, where it is thickened to 60% of solids, then it is pumped to a filter press where concentrate is dewatered down to 13-17% of moisture. The filtered concentrate is stored, then loaded on 35 t trucks and shipped to concentrate traders.