The Pirquitas deposit is an example of the Ag Sn sub-group of the epithermal class of mineral deposits (Panteleyev, 1996). Also known as Bolivian-type polymetallic deposits, examples of this deposit type are numerous in the Bolivian Silver -Tin Belt that extends between the San Rafael Sn(-Cu) deposit in southern Peru and the Pirquitas deposit in northwestern Argentina.

Bolivian-type Ag-Sn deposits generally consist of sulphide and quartz-sulphide vein systems typically containing cassiterite and a diverse suite of base and trace metals, including Ag in a complex assemblage of sulphide and sulfosalt minerals. The vein systems are generally spatially and likely genetically associated with epizonal (subvolcanic) quartz-bearing peraluminous intrusions one to two kilometres in diameter, although the mineralization may be entirely hosted by the country rocks into which the intrusive stocks were emplaced.

The suite of principal and subordinate ore minerals that characterize this deposit type includes, but is not limited to, pyrite, cassiterite, pyrrhotite, marcasite;,sphalerite, galena, chalcopyrite, stannite, arsenopyrite, tetrahedrite, scheelite, wolframite, andorite, jamesonite, boulangerite, ruby silver (pyrargyrite), stibnite, bismuthinite, native bismuth, molybdenite, argentite, gold and a variety of complex sulfosalt minerals. Metal zoning from depth to surface and from centers outward shows: Sn + W, Cu + Zn, Pb + Zn, Pb + Ag, and Ag ± Au; commonly there is considerable ‘telescoping’ of zones. Oxidized zones may have secondary silver minerals such as Ag halides.

Quartz and sericite are the main gangue minerals, with tourmaline appearing at deeper levels and kaolinite + chalcedony commonly present close to surface. Quartz-sericite pyrite alteration is characteristic of these deposits; it is pervasively developed in certain lithologies and restricted to narrow vein halos in rocks that are less susceptible to reactions with the hydrothermal fluids.

Sn-Ag vein deposits are believed to be the source of cassiterite that has been mined from placer deposits around the world, and the lodes themselves are extensively mined in South America, particularly Bolivia. At present, the San Rafael deposit in southern Peru is the world’s largest and richest underground tin mine, while the Pirquitas deposit is Argentina’s largest silver mine.

The fracture and breccia-hosted mineralization at the Pirquitas Mine consists of Fe and Zn sulphides with accessory cassiterite (Sn oxide) and a large variety of Ag-Sn-Zn (-Pb-Sb-As-Cu Bi) sulphides and sulfosalts. Crystalline quartz, along with chalcedony in the upper levels of the system, and kaolinite are the main gangue minerals in the veins and mineralized breccias. The main sulphides, specifically pyrite, pyrrhotite, sphalerite and wurtzite, form colloform bands parallel to vein margins, which together with crustiform and drusy vein textures suggest that the mineralization is epithermal in origin.

Mining of the San Miguel open pit ceased in January 2017. Previously, the Pirquitas mine used a standard open pit mining method and conventional drilling and blasting activities with a pre-split to ensure stable wall rock conditions. RC grade control drilling was used in the pit to define the structurally controlled vein and breccia hosted ore zones. The depth and relatively lower grade of the ore body made the Mineral Reserves and Mineral Resources at the Pirquitas mine amenable for open pit mining.

The Pirquitas mine processing plant consists of primary, secondary and tertiary crushing operations which deliver ore to a stockpile. The maximum crushing circuit throughput is currently around 6,850 tonnes per day, although long-term plans allowing for good maintenance protocols call for rates around 6,000 tonnes per day. Crushed ore is reclaimed into a conventional closed circuit ball milling circuit.

The maximum milling throughput is currently 5,000 tonnes per day. Mill discharge is pumped through a cyclone system and oversize is fed back into the mill for additional grinding. The ground fines are fed into a conditioning and reagent addition tank and then flow into the silver flotation circuit. The tailings from the silver flotation process are routed to a separate conditioning tank and are then directed to the tailings thickener. Tailings are thickened and stored at a permitted facility on-site.

Milling is performed on the feed from the jig plant and can be augmented by a by-pass feed system in the event of jig downtime or milling capacity in excess of jig capacity. Mill discharge is pumped through a cyclone system and oversize is fed back into the mill for additional grinding. Fines are then fed into a conditioning and reagent addition tank and then flow into the silver flotation circuit.

The tailings from the silver flotation process are routed to a separate conditioning tank and from there flows to the zinc flotation circuit. Tails from the zinc flotation circuit can be directed to the tin circuit or to the tailings thickener, as appropriate.

The mine delivers suitable ore materials to stockpiles, including stockpiles at the primary crusher. The crusher is a jaw crusher that can be fed directly via discharge of 25-30 tonne trucks or with a front-end loader. The crusher produces a 15 cm product.

Secondary and tertiary crushing and screening operations reduce this material to a size of which 80% is less than 9 mm. This material is discharged onto a crushed ore stockpile that has four feeders located below the stockpile.

Ore is fed from the crushed ore stockpile into a washing screen that separates material by size and sends material to a series of jigs for pre-concentration. These jigs are capable of making a separation based on material density and can upgrade the mine ore feed by rejecting rock that has a lower density than the higher grade Ag, Zn and Sn containing material.

The ore is then sent to the ball mill while the rejects (jig plant tails) are placed in a stockpile and analyzed for metal content. The jig tails are variously distributed based on grade; they are primarily hauled to long term waste storage but can be stockpiled with other medium or low grade stockpiles if grades are justified. They can also be used for road maintenance, pit floor smoothing and blast hole stemming.

The milling circuit starts with wet ball milling to break all crushed and jigged ore to the optimum size for mineral separation. Wet milling is performed at a rate of approximately 4,000 tpd. Mill feed is primarily from the jig plant but the plant is equipped with a bypass system that allows for passage of crushed ore to be fed in addition to the jig plant feed.

Mill discharge is pumped into a cyclone nest where the oversize is returned to milling operations and the fines report to flotation.

The conditioning of the ball mill discharge is done in a conditioning tank where appropriate pH and reagent controls are achieved to optimize recovery. Silver is floated first and the tailings from that silver flotation are pumped to the zinc conditioning tank where the pH is increased and the reagents are modified to achieve optimal zinc recovery.

The higher pH required for zinc recovery helps to ensure that the tailings facility does not become acidic.

The correctly-sized particles flow from the ball mill circuit to the silver flotation conditioning tank. Here flotation reagents suitable for silver minerals are added in tightly-controlled quantities.

The silver flotation circuit consists of a series of froth flotation stages, where silver minerals are recovered as froth, and progressively upgraded to a marketable silver containing concentrate.

The silver-poor silver flotation circuit tailings are pumped to the zinc circuit conditioning tank where flotation reagents suitable for zinc minerals are added.

The zinc flotation circuit consists of a series of froth flotation stages, where zinc minerals are recovered as froth, and progressively upgraded to a marketable zinc containing concentrate. Some additional silver is recovered in the zinc concentrate.

The zinc circuit feed grades (silver flotation tailings) are used as the basis for the zinc flotation circuit modeling.

The silver and zinc froth flotation concentrates are thickened in dedicated thickeners, ahead of automated batch pressure filters.

The two filtering units discharge concentrate ‘cake’ onto conveyors. These discharge into separate storage areas. From the concentrate stockpile areas, the products are bagged for shipment off-site to Customers.