Kinross acquired its 50% ownership interest in the Crixás underground mine in Brazil upon completion of the merger with TVX on January 31, 2003. AngloGold Ashanti is the other 50% owner and operator. The mine itself is located in the State of Goias, approximately 260 kilometres northwest of the capital city of Brasilia.
The Crixás mine consists of two operating underground gold mines accessed by decline, Mina III and Mina Nova; three orebodies that have been accessed by underground development, Corpo SUL, Corpo IV, and Corpo V; and two orebodies under evaluation, Forquilha and Palmeiras.
The mining methods used are primarily mechanized cut-and-fill and room-and-pillar and ore is transported to surface by 25 tonne trucks. The operation runs 24/7.
The first geological, geochemical and geophysical reconnaissance work began in the Crixás region in 1973. Detailed geological mapping and ground magnetic surveys were completed and diamond drilling was conducted from 1973 to 1976. In 1976, gold mineralization was discovered below a group of excavations known as the Mina III Old Workings and work concentrated in that area
Mining started in 1987 with ore being stockpiled on the surface. Development was largely completed by the end of 1989, enabling successful testing of the metallurgical circuit to take place through the fourth quarter of 1989. Initial doré bullion associated with this testing was poured on November 14, 1989. Initial gold sales from the project occurred in January 1990.
The Crixas mines are situated 3 km south of the city of Crixás in the Goias State of Central Brazil and it is 260 km northwest of Brasilia, the country's capital.
Access to the mine area is by paved road, which links the city of Crixas and the Belem-Brasilia highway 120 km to the southeast. There is an airstrip suitable for small aircraft outside the city of Crixas.
The main structures observed in the area are a sequence of sinistral shear faults in the central west portion of the belt, striking to the northwest. A dextral fault system, striking to the north, occurs in the region of Crixas city and intersects the sinistral belt in the area of the MSG workings. A series of shallow thrust faults occur in the eastern central part of the belt with thrusting initiated from the west towards the southeast. The intersection of the two fault systems would have set up an extensional stress regime in a northwesterly direction, which is coincident with the plunge direction of the main orebodies at MSG. It is thought likely that these structures are having a controlling influence on the mineralization seen at MSG. The tectonic processes created a series of tight folds with axes striking and plunging to the northwest. Mineralization appears to be concentrated in the limbs of these folds and the fold axes are strongly sheared. The mineralization is, essentially, stratabound within the chemical and argillaceous sedimentary units with only minor mineralization in the volcanics and dolomite units with the latter commonly forming the hangingwall to the orebodies.
There are currently seven individual orebodies recognized at Crixas with the
largest, Mina III containing some 70% of the LOM Gold resources, Corpo IV and V
combined account for 12% of the LOM resources and Mina Nova contains the remaining
18% of the LOM resources. The Corpo Sul orebody is included with Mina III. The
Forquilha and Palmeiras orebodies are classified as inferred and are not included in the
Gold at Mina III is associated with massive sulphides and vein quartz oreshoots. These are linear bodies with long axis lengths in excess of 200m in places and plunging to the
northwest at between 6-35°. The Upper and Intermediate Zone are dominated by massive
sulphide bodies whilst the Lower Zone is subdivided into a disseminated sulphide in the
graphitic schist and a high grade quartz orebody. At Mina Nova, the gold is associated with shallow dipping (6-10°) tabular bodies of disseminated sulphides.
In Mina Nova mine and in the upper zones at Mina III the predominant sulphide minerals are arsenopyrite and pyrrhotite. In the Lower Zone at Mina III the pyrrhotite content increases and minor chalcopyrite can be observed.
The Corpo IV, Corpo V and Corpo Sul orebodies all occur in close proximity to the Mina III orebody and exhibit similar mineralogical properties to the Lower Zone at this mine. These orebodies are currently accessed from Mina III although mining has not commenced at Corpo V.
The Forquilha and Palmeiras orebodies have been intersected by surface diamond drilling. The latter is high grade but a large proportion of the orebody occurs within the Metago license boundary. Forquilha is a relatively low-grade deposit. The current Life of Mine (LoM) Plan does not include either of these orebodies.
Mining & Operations
Production is currently sourced from two underground mines, Mina III, and from Mina Nova which is located 1400m to the north of Mina III. Both mines are accessed by decline ramps, sized to accommodate 25 tonne haulage trucks. Ore and waste are hauled to surface by truck from both mines to a common stockpile area, to the north of the processing plant.
At Mina III, the orebodies occur in two main ore zones: the Upper/Intermediate Zone and the Lower Zone. The Upper/Intermediate ore is contained in layers of massive sulfide or is
disseminated in schist rocks of variable composition, with host rocks generally being dolomite. The Lower Zone ore is contained in a graphitic schist rock, and the gold is present in quartz veins and in disseminated sulfides in graphitic schist. Reserves in the Upper Zone and Intermediate Zone have now been largely depleted, and represent only a small portion of the production in the LoM Plan.
The overall mining sequence has been from the top, downward. The main ramp has been advanced down to 550m level, while levels 350 to 450 represent the current mining horizon.
At Mina III the main mining method is mechanized cut and fill using uncemented hydraulic backfill to recover the moderately dipping Lower Zone (150 to 200) quartz veins ranging in thickness from 1 meter or less, to several meters. Where the ore is thicker, pillars are left for support. 7m spans and 4m pillars are standard practice 4m cuts are taken successively in an overhand sequence. The mining cycle includes drilling, blasting, ventilation, dust suppression, scaling, mucking, rock bolting, and filling. Drilling is done using an electric/hydraulic two-boom jumbo and blasting utilizes 80% anfo. The blasted ore is loaded into 25 tonne trucks by LHD or front end loader. Ground support consists of grouted rebar bolts and the ground conditions were observed to be very good.
The hydraulic backfill consists of classified tailings from the processing plant. The overall ventilation at Mina III totals 350 cubic meters per second (cms). Main fans are located on surface and underground. Mine dewatering requirements average 80 m3 per hour, increasing to 170 m3 per hour during backfilling. The main underground sump delivers dirty water to one of the thickeners in the mill, where it is clarified and recycled to the mine. A small maintenance shop is located underground on 200m level for minor repair work. Major equipment repairs are completed in the surface shop that serves both mines. Productivity in the mechanized cut and fill stopes averages 30 to 40 tonnes per man shift.
Currently, production is sourced from deposit 1 by mechanized room and pillar (no backfill).
Other deposits being developed are 3, 5, and 6. The vertical thickness of deposit 1 ranges from 5 to 20m. Deposit 1 hosts approximately 60% of the Mina Nova reserves. The dimensioning of the room spans and pillar widths varies depending on the thickness of the deposit. Thicker areas are taken out by an initial cut followed by two benches to achieve the full height. Ground support installed in the back consists of 2.4m grouted rebar. In cases where the back will be opened up to 13m wide, 5.4m grouted cable bolts are installed prior to slashing out to the full width. Where back heights are expected to exceed the reach of the equipment, steel mesh is installed on the back.
Total mine ventilation flow is 120 cms, with fresh air entering the mine through the main ramp. Productivity in the mechanized room and pillar stopes averages 30 tonnes per man shift.
The mineral processing plant at Crixas has historically achieved a gold recovery very close to 95%. The process is described in point form as follows:
The ore is hauled from the mines in Volvo A-25 trucks and is stockpiled close to the feed hopper of the plant according to grade.The primary crusher is a jaw crusher operated in open circuit. Ore is discharged onto a primary vibrating screen for a first classification. Secondary crushing is done in a hydrocone unit, operating in open circuit.
Grinding and Gravity Concentration:
The grinding is single staged, wet, in closed circuit with hydrocyclones. The final product of the grinding is approximately 65% passing 200 mesh (74?m). The underflow from the thickener at 60% solids is pumped to the leaching stage
The leaching area consists of 16 tanks with 202 m3 capacity each. The leaching is done in two stages: pre-lime and cyanidation. The cyanidation contact time is approximately 26 hours.
The filtration circuit consists of two stages: primary and secondary filtration. After primary filtration the cake, containing approximately 16% moisture, is fed to a primary repulping vessel, while the filtrate (rich solution) is pumped to a receptor tank. The pulp is pumped from the repulping vessel to the secondary filters. The process is repeated with two products being obtained: rich solution (filtrate), that is pumped to a second receptor tank, and the final solid tailings, repulped, and pumped to the tailings pond.
Clarification and Precipitation:
The Merril Crowe process is used consisting of clarification, air depletion from solution, gold precipitation with zinc powder, and subsequent precipitate separation using press filters. The precipitation area has 4 press filters, one operating at a time. The precipitate has an average gold grade of approximately 20%, and it is sent to the smelting building.
The fusion process consists of receiving and fusing the precipitate and gold concentrate originating from the press filters and shaking tables respectively. Every fusion produces a bullion weighing approximately 20 kg. The slag generated in the fusion is fused again to recover gold and then it returns to the crushing plant with a gold grade about 200ppm. The fusion for bar production is done in an electrical induction furnace, where the bullions are fused and transformed into bars weighing approximately 10kg each. Gold bars are sent to Mineração Morro Velho in Nova Lima-Minas, Gerais State, for refining.