The mineralization at Morro Agudo and the Ambrosia Trend are considered to be examples of Irish-type Zn-Pb deposits hosted by carbonate rocks which are a subclass of Mississippi Valley Type deposits.

MORRO AGUDO MINE
The Morro Agudo zinc and lead deposit comprises a number of concordant stratabound sulphide bodies, non-concordant remobilized sulphide bodies, and intraformational dolarenites and breccias of the Morro do Calcário Formation.

Mineralization
The combined length of the known mineralized bodies is approximately 1,700 m, the width is about 1,200 m, and the bodies have a variable thickness with a maximum of about 10 m. Mineralization is bounded to the northwest by the Main Fault. The western limit has not yet been defined, but drilling has shown continuity of mineralization at depth. Drilling deeper than 1,000 m has shown continuity of mineralization at depth, but with thinner intercepts (1 to 5 m) and lower grades.

The mineralized bodies are separated from each other by waste intervals of stratified dolarenites at upper and intermediate levels, and by waste breccias at lower levels. Sulphide levels occur as concordant stratabound lenses in dolarenite, and subordinately in dolomitic and dolarenitic breccias, in addition to occurring as late tectonic structure fill (faults/fractures). These sulphide lenses are typically less than 4 m thick, and separated by barren intervals that range from a few centimetres to several metres, depending on the lithology where they are deposited.

Mine geologists have identified eight mineralized strata, denominated from G to N, from the base to the top, respectively that are consistent across the deposit despite being divided by faults (Figure 7-8).

According to Rubo and Monteiro (2010), three episodes of mineralization are identified at the Morro Agudo deposit:
• Event 1: replacement of breccia matrix for chalcedony with very fine to finegrained sphalerite and other sulphides, mostly preserving the micritic carbonate particles and giving rise to mineralized dolarenites
• Event 2: filling of voids (veins and larger cavities) by fine to coarse-grained sphalerite, pyrite, and galena
• Event 3: Formation of late veins with sulphides cutting the veins that were deposited in the second event.

AMBROSIA TREND
The Ambrosia Trend deposits (Ambrosia Sul, Ambrosia Norte and Bonsucesso deposits) occur in the pelite–carbonate rocks of the Vazante Group in a similar stratigraphic position to Morro Agudo. Lower greenschist facies metamorphism has affected the Vazante Group in the area. When it is possible to recognize sedimentary textures, the prefix “meta” is not used for lithological descriptions.

Mineralization
Mineralization is hosted in dolomitic breccias along the Ambrosia Fault. At Ambrosia Norte and Bonsucesso, mineralization is predominantly vein-like, and is associated with brecciated dolomites of the Morro do Calcário Formation. In most cases, there is a single mineralized structure, but occasionally, two or more mineralized structures are present. At Ambrosia Sul, mineralization is controlled by hydrothermal breccias.

There are two types of mineralization associated with the Ambrosia Fault:
• Oxide mineralization: smithsonite and cerussite. The calamine mineralization (a mixture of the zinc secondary minerals hemimorphite (Zn4(Si2O7)(OH)2·H2O) and smithsonite (ZnCO3), was generated by weathering and supergene enrichment of the sulphide mineralization
• Sulphide mineralization: sphalerite and galena. Sulphide mineralization mainly fills microfractures in the rock and is associated with dissolution areas (cavity filling) together with sparry dolomite and pyrite.

The main mineralization stage is represented by the formation of sphalerite and significant concentrations of pyrite with subordinate galena concentrated in thin veinlets and vein networks that cut the recrystallized dolomite. Near the surface, there is a significant concentration of calamine (Monteiro, 2002).

The LOM is based on mill feed material including inferred mineral resources to be sourced from the operating underground Morro Agudo Mine, the open pit Ambrosia Sul Mine and Bonsucesso deposits that are assumed to be mined using underground mining methods.

The open pit Ambrosia Sul mine in Morro Agudo reached the end of its life of mine during the fourth quarter of 2020 and operations were suspended due to the uncertainties associated with the geological model of the area, safety considerations and a greater movement of ore compared to the original plan. Due to the mine closure, Nexa reviewed Ambrosia’s portfolio of assets and analyzed the possibility of using these assets in other operations, such as the Bonsucesso mine, which is currently in the feasibility study phase.

Morro Agudo Mine
The primary extraction method at the Morro Agudo Mine is inclined room-and-pillar. The mineralized zones are accessed via a ramp system. Backfill is in limited use at the Morro Agudo Mine and is specified on an operational basis depending upon the deposit geometry.

The Morro Agudo Mine is a mature operation, and staff has a well-developed understanding of the hydrogeology, geology, and mining methods required to safely extract the mineralization. Development and access profiles take advantage of the known hanging wall structural characteristics to minimize ground support requirements. Water inflows into the Morro Agudo Mine are not a major source of water volume to the mine.

The Morro Agudo Mine ventilation infrastructure is essentially at the full extent of development and is not currently planned to have significant expansion going forward.

The primary mine access for material and personnel is a ramp via the portal. A shaft is used primarily for hoisting mill feed and waste material and can be used as an emergency egress if necessary. Electrical power supply is in place underground. The forecast production rate is 1,100 t/d on average.

Ambrosia Sul open pit
The Ambrosia Sul open pit is approximately 35 km north of the Morro Agudo plant site. Production and waste rock from the Ambrosia Sul open pit was mined using contract mining equipment operated by Nexa employees.

Since the suspension of its operations in the fourth quarter of 2020 onwards, activities at the mine have been limited to ensuring appropriate maintenance and security.

Bonsucesso Mine
The proposed mine plan for Bonsucesso considering a bulk mining operation using AVOCA as mining method. Uncemented waste rock backfill will be employed to fill the stopes to provide hanging wall support to reduce mining dilution. The proposed Bonsucesso underground mine will be accessed through a portal and ramp developed from surface.

The current 13.8 kV power supply to the Ambrosia Sul operations will be improved to support development at Bonsucesso that will require an extension of the distribution network and an electrical substation. The forecast production rate from Bonsucesso varies from 2,000 to 3,300 t/d during LOM.

The Morro Agudo mill uses a conventional crushing, grinding and flotation circuit to produce separate lead and zinc sulphide concentrates.

Mineralization is delivered by skip from the mine haulage shaft and stored in a silo ahead of the primary crusher. Mineralized material is withdrawn using a vibratory feeder to the single-axis Nordberg jaw crusher that operates in open circuit. The primary crusher capacity is 1.5 Mt/a but this is not fully utilized due to upstream and downstream constraints. The primary crusher discharge is conveyed to an intermediate stockpile and weighed using a continuous weighing scale. A fixed electromagnet removes metallic material to avoid equipment damage.

Primary crushed material is then processed in open circuit through the Nordberg HP 1352 hydro-cone secondary crusher. The secondary crusher discharge is classified on a double-deck vibrating screen (8 mm and 20 mm). The screen undersize is sent to a homogenizing stockpile, whereas the oversize is directed to the closed crushing circuit in the Metso/Nordberg HP400 hydro-cone tertiary crusher.

The undersize feeds a conveyor belt, which has a continuous weighing scale and automatic sampler. Crusher plant product is analyzed for particle size and zinc, lead and iron assays. The product size is 80% passing 7.2 mm and 100% passing 9.6 mm. Material is discharged to the tripper, which directs flow to the appropriate storage location according to sample content, for blending in the homogenization stack. This helps minimize feed variation to the process plant. The homogenization stockpile capacity is 14,000 t. Five belt feeders under the stockpile reclaim material into two silos ahead of the grinding circuit.

Crushed and blended mineralized material is fed to two parallel grinding circuits, each consisting of one overflow ball mill in closed circuit with hydrocyclones for classification:

- Ball mill 1 is 13.6 ft diameter x 20 ft effective grinding length, has a 1,500 kW motor, is steel lined, and operates at 73% critical speed

- Ball mill 2 is 12 ft diameter x 18.3 ft effective grinding length, has a 900 kW motor, is steel and rubber lined, and operates at 72% critical speed.

Each ball mill is filled to approximately 38% charge with 63.5 mm (2½ ") diameter steel balls.

The Mill 1 circuit includes a battery of 12 Krebs model D10LB 10” cyclones, of which normally nine operate. The circulation rate is approximately 700% of the fresh feed of the mill. The Mill 2 circuit includes a battery of eight Krebs model 8xD10Gmax3061 10” cyclones, of which normally four operate. The circulation rate is approximately 400% of the fresh feed of the mill. Water is added to dilute each cyclone feed, and the resulting cyclone overflow is 40–42% solids. The resulting combined grinding circuit product produces a flotation feed with 65% passing 44 µm (325 mesh) or 80% passing 74 µm. Mill 1 has a capacity of approximately 750,000 t/a and Mill 2 approximately 400,000 t/a of mineralized material.

The Morro Agudo mill uses a conventional crushing, grinding and flotation circuit to produce separate lead and zinc sulphide concentrates.

The lead flotation circuit follows grinding. The pulp is conditioned with sodium amyl xanthate collector to render galena hydrophobic. Most of the galena in feed is recovered in a rougher flotation column cell. The rougher column tails are sent to the scavenger stage, which uses six conventional mechanical flotation cells. The scavenger concentrate is returned to the rougher column feed. Rougher concentrate is treated in a two-stage cleaner circuit. The first cleaner stage is a single flotation column, with first cleaner concentrate reporting to a second cleaner (recleaner) column, that was installed in 2017. Both the first and second cleaner tailings report back to rougher concentrate feed. Previously in 2016, an Eriez stack cell had been installed into the second cleaner duty. The stack cell is currently being tested for alternative du ........