Mineralisation within the Fazenda Mirabela intrusion is considered to be an example of a magmatic nickel–copper sulphide deposit.

The Fazenda Mirabela intrusion, which hosts mineralisation at Santa Rita, is located within the Archean–Paleoproterozoic Itabuna–Salvador–Curaça orogenic (ISC) belt. It consists of a lowpotassium calc-alkaline plutonic suite of rocks that includes intercalated metasedimentary rocks, gabbro, and basalt. The Fazenda Mirabela mafic–ultramafic body intruded granulite of the ISC. The lower zone of the intrusion consists of olivine-rich cumulates, primarily dunite to harzburgite, and is capped by pyroxenite; the upper zone consists primarily of gabbroic cumulates, consisting of gabbronorites to norites.

The intrusion is oval-shaped in plan view, with outcrop dimensions of approximately 4.0 km by 2.5 km and original stratigraphic thickness of at least three kilometres. In cross-section, the intrusion extends to a vertical depth of approximately 1,400 m.

Three generations of deformation phases are recognised, including thrust duplexes, quartz–feldspar pegmatite dikes intruded into basement lithologies, and folding. The major alteration type is fracture or structurally controlled serpentinisation.

A significant laterite profile, typically 25 m thick, developed over the dunite–harzburgite lithologies, but is absent or poorly developed over other lithologies.

The Santa Rita deposit is characterised by th ........

The Santa Rita deposit is currently being mined by conventional open pit methods, and production is scheduled from 2023 to 2028. The open pit will be mined to a maximum depth of 320 m (164 m RL). Underground development would commence before the open pit is depleted so that there would not be a significant gap in feed to the process plant.

The Santa Rita mineralisation extending below the open pit comprises a large tabular-to-massive deposit, striking north–south for over 800 m, dipping 50° to 55° to the east, extending to a known depth of approximately 1,100 m, and varying in thickness from 50 m to 150 m. The mineralisation shows a trend for increasing grade and thickness with depth.

An SLC mining method was selected for the underground portion of the Santa Rita deposit based on the amenable geometry of the deposit and the productivity and cost advantages of SLC enabling greater exploitation of the Mineral Resource at a greater margin than more selective methods. The relatively wide deposit determined that a transverse drill drive orientation should be used.

The SLC mining method employs longhole drilling and blasting techniques to extract mineralisation sequentially from surface to the bottom of the deposit. The method does not require backfill and therefore relies on the overlying waste rock to cave and fill the mined void. Caving of the overlying waste rock results in surface subsidence directly above and in the immediate vicinity of the u ........

Crushing and Grinding
Blended open pit mill feed from the stockpile area is fed to primary crushing. The primary gyratory crusher (a Metso 50/65 MK II gyratory crusher) crushes from a nominal F80 size of 800 mm to a P80 size of 140 mm to 150 mm at a rate of approximately 1,500 t/h. The second primary crusher (a Metso C160 jaw crusher) is located close to the primary gyratory crusher and has a nominal throughput rate of 750 t/h and produces a crushed product with a nominal P80 size of 152 mm.

The primary crushed material will be conveyed to an open stockpile with a total capacity of 67,000 t (live capacity 15,000 t, 19 hours; 84 hours total capacity). Three feeders will extract the mill feed from beneath the stockpile at a controlled rate of around 900 t/h for feed to the SAG mill.

The SAG mill is a 30 ft diameter by 16.4 ft long Outotec mill with an 8 MW motor. The design target transfer size (T80) is 4 mm at a nominal throughput of 832 t/h.

Two Metso HP400 pebble crushers operate in closed circuit with the SAG mill to crush oversize from the SAG mill (material in the size range -70 mm to +12 mm).

Material <12 mm flows to a common pump box shared by the SAG mill and ball mill circuits. Material from this pump box is pumped to a single cluster of ten 26 inch diameter cyclones, allowing the two 20 ft diameter x 28.5 ft long 5.8 MW Outotec ball mills to operate in closed circuit to produce a nominal flotation feed product with a P80 of approximately 125 µm.

For underground mill feed, the flowsheet for this area will be the same as the current flowsheet. The process plant operates at around 90% availability; hence, the required hourly throughput will be approximately 797 t/h compared to 842 t/h for open pit mill feed. Laboratory testing carried out by SGS Geosol has shown that the underground material has slightly lower crushing and SAG milling indices than the open pit ore and the Bond ball mill work index is a little higher than the hardest open pit mill feed. JKTech used its JKSimMet software to model the circuit with underground feed and concluded that a treatment rate of 955 t/h could be achieved at maximum SAG mill motor power. The required rate of 797 t/h will be achievable at a normal SAG power draw of 90%. The SAG mill will not be a bottleneck because it currently operates at up to 900 t/h with the harder open pit ore. The ball mills currently operate at about 72% of full motor power; hence, there is additional capacity available to produce the required flotation feed F80 size of 125 µm from the underground material.

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