Summary:
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 the lateral continuity of the mineralisation (approximately 2 km along strike and 1.3 km down dip). Nickel and copper sulphides form stratiform bodies that are generally parallel to the lithostratigraphic contacts. The primary lithological host rocks are orthopyroxenite, olivine orthopyroxenite, harzburgite, and dunite.
The mineralisation that supports the Mineral Resource estimate is primarily hosted in disseminated sulphides, 2% to 5% sulphide by volume. Some evidence of vein-like semi-massive sulphides is also noted, but this mineralisation type is not economic. On average, sulphide mineralisation comprises 52% pentlandite, 7% violarite ((Ni,Fe)3S4), 18% chalcopyrite, 14% pyrite, and 9% pyrrhotite as granular intercumulus aggregates. Traces of PGMs also occur, however, these elements appear to be included within the structure of the principal sulphides.
The Fazenda Palestina mafic–ultramafic intrusion is located 25 km to the south-southwest of the Santa Rita Mine. The intrusion cluster measures approximately 5 km east-west by 3 km north-south and, similar to the Fazenda Mirabela intrusion, has intruded granulite facies country rocks. The two dominant lithologies within the intrusion are orthopyroxenites and, to a lesser extent, gabbronorites. The deposit is approximately 1,350 m long, 50 m wide, and extends to a depth of at least 350 m. Pentlandite and chalcopyrite are the dominant nickel and copper minerals, respectively.
Three deformation phases are currently recognised in the Santa Rita open pit area:
• The oldest phase forms thrust duplexes on the scale of tens to hundreds of metres in the open pit. The deformation strain is oriented northeast–southwest and forms west–northwest to east–southeast trending structures that converge to the southwest.
• The second deformation comprises a suite of quartz–feldspar pegmatite dikes developed in basement lithologies.
• Folds with a north–south axis form the third phase of deformation.
A dolerite (diabase) dike was observed in the Santa Rita open pit cross-cutting the Santa Rita intrusion at a 120° azimuth. The dike is approximately 20 m thick and follows previously existing fracture planes that were reactivated during dike intrusion.
There are two styles of sulphide mineralisation:
• Disseminated sulphides, 2% to 5% sulphide by volume, hosting the mineralisation that supports the Mineral Resource estimate;
• Vein-like semi-massive sulphides that have no economic interest.
On average, sulphide mineralisation comprises 52% pentlandite, 7% violarite ((Ni,Fe)3S4), 18% chalcopyrite, 14% pyrite, and 9% pyrrhotite as granular intercumulus aggregates. Minor sulphide phases identified in thin sections include mackinawite ((Fe,Ni)9S8), millerite (NiS), poorly defined lowtemperature iron sulphides, cubanite, bornite and chalcocite, along with traces of native copper. Individual sulphide aggregates are commonly not more than 0.5 mm to 1.0 mm in size; however, larger cumulates up to 30 mm are observed locally. Finer and more widely disseminated sulphide grains include abundant chalcopyrite.
Summary:
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.