Mineralization is hosted by a pyroclastic unit composed of ash and lapilli-type polymictic tuffs with subrounded, well classified fragments. Some lapilli have centimetre-scale, pencil-like shapes, due to development of an incipient schistosity.

Eight styles of mineralization were identified at the Cerro Lindo deposit:

1. Pyritic, homogeneous, primary massive sulphide (SPP): This unit includes almost exclusively pyrite, less than 10% barite, and minor interstitial chalcopyrite. Its structure is equigranular, generally coarse grained (3 mm to 6 mm), but with finegrained areas (0.4 mm to 2 mm).

2. Copper-rich, baritic homogeneous primary sulphides (Cu-SPB): This unit contains more than 50% total sulphides (including barite), and more than 10% barite. Barite is associated with sulphides because it was deposited from the same solution at the same time as the sulphides. Its structure is homogeneous, and it is composed of barite, pyrite, pyrrhotite, chalcopyrite, and brown sphalerite. Sulphides typically occur as intergrowths and patches, and brown sphalerite is included in chalcopyrite grains. There is less pyrite than in the Zn-SPB unit. The CuSPB is generally found within or near the contact with Zn-SPB and SPP.

3. Zn-rich, banded, baritic primary sulphides (Zn-SPB): This unit comprises more than 50% of total sulphides (including barite), and more than 10% barite. The Zn-SPB unit contains variable proportions of pyrite, barite, yellow sphalerite, and galena. It is typically banded and has a coarse grain size (3 mm to 6 mm).

4. Semi-massive sulphides (SSM): This unit contains between 20% and 50% sulphides, which are mostly represented by barren pyrite as disseminations, patches, stringers, and stockworks. This mineralization is generally fine grained as compared to massive sulphides. SSM forms a variable envelope, 20 m to 80 m thick, around the massive sulphide bodies. The sulphide proportion decreases outward. It is better developed in the footwall.

5. Pyritic oxidized sulphides (SOP): This unit comprises bornite and covellite, and it is mostly located in the OB-2 mining production area.

6. Baritic oxidized sulphides (SOB): This unit comprises bornite, covellite, and oxidized zinc, and it is also located in the OB-2 mining production area.

7. Leached massive sulphides (SLB) and leached semi-massive sulphides (SSL): These units are located near surface in the OB-2 mining production area.

8. Mineralized volcanic rocks (VM): This unit contains rhyolite and dacite rocks with some chalcopyrite and sphalerite disseminated in veinlets or patches, located on the edge of the mineralized zones.

Cerro Lindo contains 18 mining production areas within the mineralization domains. The mineralized lenses exhibit an irregular elongated geometry, and their longest axis (nearly 500 m) has a northwest-southeast horizontal trend (azimuth 135°). The mineralized bodies are approximately 200 m thick (occurring between 1,600 MASL and 1,980 MASL) and 100 m wide. They are the largest near the edge of the Topará Ravine, after which they diminish in size toward the southeast. The mineralized bodies generally dip to the southwest at 65° on average.

The majority of these bodies show two types of mineralization. The upper part features the massive mineralisation of barite, sphalerite, galena, and pyrite (SPB). The lower part includes massive pyrite (SPP), with two different general grain sizes, one fine grained with a higher chalcopyrite content and the other coarse grained and largely barren. Finally, the base of the system exhibits a cluster of small veins of pyrite, pyrrhotite, and to a lesser extent, chalcopyrite. The mineralization at Cerro Lindo is generally coarse grained, which may be related to recrystallization due to the contact metamorphism and this improves metallurgical recovery.

The massive sulphide frequently presents a marked banding, which may be related to tectonic deformation. In the contact with the adjacent batholiths, there is a noticeable predominance of mobilized sulphides elongated in banding that runs parallel to the volcanic contact with the intrusives.

Significant barite is present mainly in the upper portions of the deposit. A secondary enrichment zone, composed of chalcocite and covellite, formed near surface. Silver-rich powdery barite remains at surface as a relic from sulphide oxidation and leaching.

The lead content is usually low and is mainly associated with high grade zinc zones, and locally with late quartz veins or small volcanic enclaves. These enclaves represent approximately 2% to 3% of the deposit volume, and commonly measure 0.5 m to 10 m in diameter. Silver grades correlate well with copper and lead.

Cerro Lindo, as is typical of Kuroko-style VMS deposits, is characterized by a distinct mineralization zonation.

The primary mining method used at Cerro Lindo is SLS with paste backfill.

The stopes are accessed on levels of 30 m vertical interval by production crosscuts that extend from a footwall lateral access through to the hanging wall. Stopes are 30 m wide, and usually 25 m long (across strike); stope dimensions may vary because of orebody geometry or local geotechnical conditions. After mining, the stopes are backfilled with cemented paste fill made from the tailings stream. The paste fill is allowed to cure before an adjacent stope is mined.

Stopes are mined in a primary–secondary sequence, progressing from hanging wall to footwall, and from bottom to top.

CAF mining methods will be used to extract sill pillars, remnants, and irregular shaped portions of the deposit. Although these methods have not previously been used at the Cerro Lindo Mine, they are commonly used in Peru, including at Nexa’s Cerro de Pasco operations (Atacocha and El Porvenir). Paste fill will be the primary backfill method used, with waste fill used when appropriate.

Because of the geometry of the orebodies, sill pillars at Cerro Lindo typically have a large horizontal extent, with the vertical thickness usually limited to approximately 20 m. The sill pillars will be mined from bottom up, in horizontal slices 4 m thick. For geotechnical and safety reasons, the top slice that intersects the stopes above is not included in the mine plan or in Mineral Reserves.

CAF mining will be less productive and higher cost. RPA recommends a trial of this method as soon as a sill pillar area is ready (i.e., stope mining in that portion of the orebody is complete), in order to confirm assumptions around design, production, and execution.

An alternate recovery method for sill pillars is a longhole retreat – develop undercut drifts at the bottom, drill upholes, blast, muck, and repeat, moving towards solid ground. This method is likely to be lower extraction (inducing instability and requiring portions of the sill pillar to be left in place), however may be more productive and lower cost. It may be a better fit for Cerro Lindo’s crews and equipment, as it more closely resembles the current SLS mining.

Stopes are backfilled with cemented paste. The paste plant is located on the surface, near the exhaust portals. It is supplied with whole mill tailings by pipeline from the process plant.

The paste plant operates two identical vacuum filter trains to supply 300 tonnes per hour (tph) of filter cake to the paste mixers. The nominal binder percentage is 3% cement, although this can be varied as and if required. Paste is pumped underground to its point of use. Paste distribution pipelines enter the mine through the 1970 m level exhaust portals and are laid along the floor of the drifts through much of their route.

Total required paste delivery is 5,000 m3/day. The existing plant operates at 95% availability and meets these requirements; however, the plant has very little redundancy and make-up capacity. The plant is equipped with stand-by pumps for delivering paste to the stopes.

CRUSHING
Crushing is carried out in three stages with the first stage being underground, where the primary jaw crusher, fed by a vibrating grizzly feeder, crushes ore to less than 100 mm. Crushed ore is conveyed to the coarse ore stockpiles located at the processing plant. When the primary crusher requires maintenance, ore is trucked directly to surface where it is stockpiled prior to being crushed in a mobile primary crusher, which discharges onto the coarse ore stockpile feed conveyor.

Coarse ore is reclaimed from the stockpile and fed to two parallel crushing circuits, each consisting of secondary and tertiary crushing. Reclaimed ore is screened and the oversize reports to the secondary crushers, while the undersize is directed to the fine ore bins. Secondary crusher product is screened, with the screen oversize reporting to the tertiary crushers and the undersize being directed to the fine ore bins. The tertiary crushers are in closed circuit with the tertiary screens and the screen undersize is directed to the fine ore bins. The crushed ore is 80% passing (P80) 4 mm, and the two fine ore bins provide approximately 16 hours of storage capacity.

GRINDING
Fine ore is fed from the fine ore bins at a total of approximately 900 tph to two parallel ball mill circuits, each in closed circuit with high frequency classifying screens. Each grinding circuit also includes flash flotation, producing bulk (copper and lead) concentrate. The first grinding circuit is the originally-installed single-stage ball mill circuit capable of processing 5,000 tpd, while the second circuit consisting of two identically sized ball mills in series processing approximately 16,000 tpd. The P80 of the grinding circuits is 150 µm to 170 µm.

The Cerro Lindo processing plant is located on a ridge adjacent to the mine and is at an altitude of 2,100 MASL to 2,200 MASL. The plant commenced operations in 2007 with a processing capacity of 5,000 tpd, however, has since been expanded to a name-plate capacity of 20,800 tpd. Processing consists of conventional crushing, grinding, and flotation to produce separate copper, lead, and zinc concentrates. The tailings are thickened and filtered for use as backfill or trucked to the dry-stack tailings storage facility.

FLOTATION
Flotation consists of bulk rougher and scavenger flotation to produce a copper-lead concentrate, which is then cleaned combined with the flash flotation bulk concentrate prior to being separated into copper and lead concentrates. Bulk flotation tails forms the feed to zinc rougher and scavenger flotation on the bulk flotation tails to produce a zinc concentrate, which is then cleaned. The three concentrates are thickened and filtered, and then depo ........