The Huaron mine is located within the Western Cordillera of the Andes Mountains and the regional geology is dominated by Machay Group limestones and Pocobamba continental sedimentary rocks. These groups have been deformed by the Huaron anticline, the dominant structural feature of the area. The limestones and sedimentary rocks are strongly folded and intruded by quartz monzonites and quartz monzonite dikes with associated fracturing. Following the intrusion of the dikes, the sedimentary rocks were further compressed and fractured, and the fractures were subsequently mineralized by hydrothermal fluids.

The main lithology in the area of Huaron is a sequence of continental redbeds which unconformably overlie massive marine limestones. North-south trending sub-vertical porphyritic quartz monzonite dykes crosscut the mine stratigraphy. The Huaron mine is located within an anticline with an axis striking approximately north-south and plunging gently to the north. There are two main fault systems. One system comprises north-south striking thrust faults, parallel to the axis of the anticline, and the other comprises east-west striking tensional faults.

The Huaron mine is a hydrothermal polymetallic deposit of silver, lead, zinc, and copper mineralization hosted within structures likely related to the intrusion of monzonite dikes, principally located within the Huaron anticline. Mineralization is encountered in veins parallel to the main fault systems, in replacement bodies known as “mantos” associated with the calcareous sections of the conglomerates and other favourable stratigraphic horizons, and as dissemination in the monzonitic intrusions at vein intersections.

The mineralized veins vary from a few centimetres to up to 10 metres wide, and may extend along strike for up to 1,800 metres. Vein orientations vary but generally trend east-west or north-south.

Mining is undertaken using primarily mechanized sub-level long hole stoping methods. Ore is brought to the surface using haul trucks, electric locomotives, or hoisted through a shaft.

Crushing
Ore is delivered from the mine to a 15,000 tonne capacity stockpile where the material is separated by metallurgical characteristic to obtain an optimal ore blend for processing through the plant. The blended material is fed into a 100 tonne capacity coarse ore bin and then via an apron feeder to a vibrating grizzly. The oversize from the grizzly is reduced in size by a jaw crusher to 3.5 inches and joins the undersize via conveyor to a vibrating screen. The oversize material reports to a secondary crusher where it is reduced to a 2.5 inch product size by a 4.25 foot Symons standard cone crusher and then joins the undersize via conveyor to another vibrating screen. The undersize material reports to a tertiary 4.25 foot Symons short head cone crusher where it is reduced in size to 100% passing one quarter inch. The final crushed product travels by conveyor belt equipped with an electromagnetic separator and metal detector for storage in three 300 tonne capacity fine ore bins prior to entering the grinding circuit.

Grinding and classification
The grinding circuit consists of a 12 foot diameter by 16 foot long primary ball mill operating in an open circuit with two parallel secondary ball mills, one 8 foot diameter by 8 foot long and the other 6.5 foot diameter by 14 foot long. The 6.5 foot diameter by 14 foot long mill was added to the process flowsheet at the end of 2013 and allows for increased processing throughput to 870,000 tonnes per annum at the target final product particle size. The milled product from the primary and secondary ball mills reports to a classification hydrocyclone nest of two units of 20 inches diameter each. The hydrocyclone is in close circuit with the secondary ball mills and the overflow of the hydrocyclone classification is treated in a third stage 8 foot diameter by 3 foot long mill. The third stage grinding is in close circuit with a hydrocyclone nest of four units of 12 inches diameter each. The final milled product is approximately 60% at minus 200 mesh, approximately 30% at between 65 and 200 mesh, and approximately 10% at plus 65 mesh.

The Huaron mine operates a 2,900 tpd nominal capacity mill using froth induced flotation technology to produce silver in copper, lead, and zinc concentrates. Metallurgical recoveries average approximately 84% for silver, 77% for zinc, 76% for lead, and 76% for copper.

The mill flowsheet consists of three-stage crushing, ball mill grinding, and selective flotation of the ore to concentrates, followed by thickening and filtering of the concentrates.

Flotation
The pulp from the grinding circuit is fed to the flotation circuit. The flotation circuit includes an initial stage of depression of zinc and flotation of a bulk concentrate. The bulk concentrate consists of lead and copper and is treated with sodium dichromate to separate and produce a silver rich lead and copper concentrate. The tailings from the bulk flotation are activated and conditioned with copper sulphate and lime to modify the pH and to produce a zinc concentrate. The bulk flotation occurs in two ........