The Chinchillas and Pirquitas deposits are within the Bolivian tin-silver-zinc belt which occupies the back-arc portion of the central Andes and extends from the San Rafael tin-copper deposit in southern Peru to northern Argentina. The Bolivian tin-silver deposits are typically associated with felsic volcanic domes of broadly rhyodacitic composition (Cunningham et al., 1991). 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 mineralisation may be entirely hosted by the country rocks into which the intrusive stocks were emplaced. The Chinchillas deposit is modelled as a Tertiary-aged diatreme volcanic centre that has intruded Paleozoic sedimentary basement rocks. The mineralisation occurs mostly as disseminations, veinlets, and matrix fill.

Most of these deposits are characterised by the intrusion of dacite dome complexes with mineralisation hosted in shear zones and breccia within the dacite domes and/or within shear zones and breccia within the host rocks. At Pulacayo, Potosí and San Cristóbal, where associated domes are present, there is significant mineralisation within the domes. More rarely, as in the case of Chinchillas and San Cristóbal, the deposits include disseminated mineralisation in flat lying manto bodies within sedimentary and pyroclastic rocks. Chinchillas demonstrates phreatomagmatic diatreme morphology associated with a dome structure.

Mineralisation at Chinchillas is dominated by silver with lesser amounts of lead and zinc. Mineralisation occurs as disseminated sulfides, matrix infilling within the volcanic tuffs, and as matrix and fracture filling in breccias within the basement metasediments. Dacite volcanic rocks are rarely mineralised in shear zones, veinlets or vein-like structures. Within the basement lithologies shear zones and faults are more commonly mineralised. Depth of oxidation is a few metres within the volcanic rocks and is insignificant within the basement rocks. Silver, lead, and zinc-bearing minerals include silver sulfosalts, freibergite, boulangerite, tetrahedrite, schalenblende, sphalerite, and galena. Associated mineral associations include chalcopyrite, quartz, pyrite, siderite, limonites, manganese oxides, cerusite, smithsonite, anglesite and malachite (Marshall and Mustard, 2012 and Coira et al., 1993).

The geological model for the Chinchillas deposit includes significant silver-lead-zinc mineralisation in the Silver Mantos and Mantos Basement zones in the western part of the Project. Similar mineralisation is present at the adjacent Socavon deposit. A recent review of the Socavon deposit has resulted in its removal from Mineral Resource inventory in 2022.

The main structural elements controlling the location of mineralisation are the contact between basement sediments and overlying volcanic rocks and the dominant east–west and subordinate north-west, north, and north–north-east trending structures that control the Chinchillas volcanic centre. The phreatomagmatic explosion that produced the diatreme generated a symmetrical cylindrical shaped caldera, with mineralised brecciated basement rocks along the contacts and disseminated mineralisation in sub-horizontal tuff layers.

Open pit mining is carried out by MPSA as an owner mining operation with ore hauled from Chinchillas pit to the Pirquitas plant.

The Chinchillas deposit is located in the high lands of the Andes. The topography of the property consists of several mountains and hills on the sides of property with a small valley in the middle. The orebody is located mainly in the bottom of the valley with extensions stretching to the west on the hillside. The elevation varies from about 4,090 masl in the east side of the valley to 4,300 masl on the peaks in the west. There is a small creek in the middle of valley running from west to the east.

The Chinchillas deposit is mined as a conventional open pit operation. Most of the in-pit haulage for both ore and waste is carried out using 100 t haulage trucks. Ore is mined in five metre benches and stockpiled in a staging area close to the pit. From the staging area, ore is transported to the crusher at the Pirquitas Operation which is 42 km away from Chinchillas. Throughout the mining operation, low grade ore is stockpiled near the pit rim to be processed at the end of mine life. The mining operation is conducted by the owner. Ore haulage was changed to owner operated in 2021.

Waste rock is mined and hauled to two major on-site rock storage facilities based on geochemical characteristics. For the mine planning work the NSR is calculated for each block. No dilution is included in the block model. Ore is placed in the ore staging area as it is mined from the pit. The ore is then loaded onto haul trucks and transferred to Pirquitas on a daily basis. Material that falls between the Resource cut-off and the Reserve cut-off is stockpiled separately as mineralised waste.

SSR prepared new pit designs in 2021, the interpretation of the slope design recommendations used by SSR in the 2021 designs. Ore and waste are mined in five metre benches. Final wall 20 m benches are formed by joining four working benches together. Haulage roads are 30 m wide, which is sufficient for 2- way traffic of 100 t trucks, plus enough space to build a ditch and a safety berm. Inter-ramp angles for the west and east walls are 49 and 43 degrees, respectively. For every 150 m of slope height, either a 20 m geotechnical berm or a haulage road was added to the slope.

Some of the waste types have the potential to leach metals and are separated from the neutral waste material. Based on geochemical characteristics, waste is classified into three groups designated A, B and C. Type ‘A’ waste is stored close to the pit as it has the potential to leach metals. This is so that the drainage can be collected in the pit and if necessary be treated. Type B and C are stored together in the same location.

According to this classification, two waste rock storage facilities have been designed for Chinchillas to accommodate different rock types. These can be seen in the general site layout.

Rock storage A is close to the pit, on a hill side to the north-east of the Chinchillas pit. The toe of this dump is 100 m offset from the pit rim. Rock storage B and C are located to the southeast of the active mining area on somewhat flatter terrain. Waste Rock Facilities are built with 25 m lifts and 15 m berms. The angle of repose for each lift is 35 degrees and the overall slope angle of dumps is 26°. Access to the dump is by 30 m wide haulage roads. The total height of the dumps are approximately 100 m.

The trucked material is delivered to suitable stockpiles at the primary jaw crusher. The jaw crusher is fed directly via 35-42 t truck dumping or with a front-end loader. Mill operations decides daily feed blending from the ore stocked looking for steady head grade according to the production plan.

Secondary/tertiary crushing and screening operations will reduce this material to an 80% passing size of 9 mm. This material is discharged onto a crushed feed stockpile with four feeders located beneath the stockpile. The crushing circuit was designed to process up to 6,000 tpd.

The ball mill circuit grinds crushed ore to the optimum size at a rate of 4,500 tpd. The ball mill is 4.8 m in diameter by 6.25 m long with 2,400 kW of installed power. Mill discharge is pumped to a cyclone nest where the underflow is returned to milling operations and the overflow reports to flotation. The addition of granular lime to the ball mill feed belt is done for flotation pH control. The pyrite/sphalerite depressant and frother are added into the mill. The lead/silver flotation collector and a reinforcement of frother are added to the cyclone overflow.

Chinchillas material is processed at a rate of up to 1.7 Mtpa through the existing Pirquitas Operation process plant. The Pirquitas plant was commissioned in 2009 and has since been in continuous operation. The plant has not been expanded since start-up; however, minor changes in the flotation flow sheets have occurred to optimise performance.

The Pirquitas plant was upgraded in 2017 to process the Chinchillas ore types, producing a silver / lead concentrate and a zinc concentrate.

The lead/silver flotation section consists of rougher, and in the concentrate cleaning stage with a scavenger stage.

The zinc flotation circuit consists of rougher, and one stage of conventional cell concentrate cleaning followed by one stage of column cell cleaning.

The Pirquitas silver / lead concentrate dewatering circuit consists of a thickener, holding tank and pressure filter. However, as higher lead feed grades are mined after the first few years of oper ........