The Riacho dos Machados gold project is approximately 540 km north of Belo Horizonte in the northern part of the Minas Gerais State of southeastern Brazil.
The mineral deposit was discovered in the 1970s by the Brazilian government in partnership with Vale (CVRD) in a region with no prior history of mining production. Vale open pit mined the oxidized part of the deposit in the period 1989-1997 and produced 155,000 oz gold from low grade (1.89 g/t) oxide ore.
Carpathian Gold released a positive feasibility study in 2011. Total mineable mineral reserve amounts to 20.9 Mt at a grade of 1.24 g/t gold.
The company intends to mine the gold deposit by conventional open pit mining methods. The mine would produce an average of 2.555 Mt per year ore over a period of 8 years at a strip ratio of 7.4 to 1.
The process plant is conventional with crushers, grinders, CIL and electrowinning circuits. Total gold production is estimated at 747 Koz.
Commercial mining is expected to start in 2012.
The Riacho dos Machados gold project is located 145 km by road northeast of the city of Montes Claros in Minas Gerais, Brazil. The 11,000 people town of Riacho dos Machados is about 25 km from the project.
Brazil is the worldâ€™s largest Portuguese speaking nation and the largest country in South America. It is at the same time one of the ten largest economies in the world. It boasts diverse mineral resources and it is open to mining â€“ the Mining Code defines the mining rights which are guaranteed by the Federal Constitution of Brazil.
The state of Minas Gerais was initially inhabited by the Anais and Tapuias Amerindians. It was settled by Europeans as a result of several gold rushes that took place in the 17th and 18th century. In 1725, alluvial gold miners found diamonds; it was a time when diamonds from India were already scarce as a result of depleting of their resources; and because of that from 1730 to 1870 Brazil was the worldâ€™s top diamond producer.
The mining project lies within the Espinhaco Mountains a range that runs north south through the states of Minas Gerais and Bahia. The project areaâ€™s terrain consists of low rolling hills (770 to 900 masl) dissected by sparse intermittent streams.
The climate is tropical with two seasons - the rainy season runs from November to March. Annual rainfall average over 1,000 mm and temperatures in the 17 deg C to 30 deg C.
Vegetation is overgrown tropical savannah consisting of brush forests interspersed with grasses. Subsistence agriculture and cattle ranching are mainstays of the local economy.
The Riacho dos Machados gold project is located in a sheared and deformed Precambrian greenstone belt along the eastern margin of the San Francisco Craton.
Mineralization is confined to the Riacho dos Machados shear zone, an important structure which extends for 30 km in a parallel to bedrock style manner. Host rocks are the Ouro Fino Sequence, which is comprised of pelitic or arenaceous sediments metamorphosed to staurolite- or garnet bearing biotite Â± quartz â€“ oligoclase - sericite schists and quartz-feldspar schists. The schists had been hydrothermally altered to quartz-muscovite schists and quartz-sericite/muscovite-biotite/chlorite schists.
The mineralized zone extends for 1,700 m and is up to 40 m thick. Gold occurs in two different styles of mineralization: disseminated associated with the hydrothermal alteration zone; and, in stockworks-veinlets adjacent to igneous bodies that intruded the greenstone rocks. Most of the gold is finely disseminated and associated with arsenopyrite, pyrrhotite and little pyrite.
In 1993 and 1994 Vale conducted an underground exploration program by means of a shaft and exploration galleries. They delineated a historical sulphide gold mineralization resource below the pit.
A 2011 Feasibility Study calculated proven and probable open-pit reserves of 20.9 Mt at 1.24 g/t gold for 830,200 ounces of gold.
Mining & Operation
Mining will utilize owner operated conventional open-pit method with drill and blast backhoe excavators, haul trucks, and auxiliary mobile equipment to support a mining operation of a nominal 2.555 Mt per year of ore and a maximum of 27.7 Mt per year of waste. Total mineable mineral reserve amounts to 20.9 Mt of ore at a grade of 1.24 g/t gold with an operational strip ratio of 7.4 to 1. A combination of owner operated and contractor equipment will be used during the pre-production period.
The run-of mine (ROM) ore will be hauled from the open-pit with 40 t haul trucks and dumped directly into the hopper that feeds the primary crusher at a rate of 7,000 t per calendar day or 2.555 Mt per year. A small ore stockpile of 5,000 t will be established at the primary crusher area in the event that the pit operation is stopped so that the crushing plant can continue to operate.
The final pit is 1.8 km long in the north-south direction and 0.6 km wide in the east-west direction. The highest wall will be about 270 m on the southeast side. The total area impacted by the pit is about 92 hectares.
The process plant is conventional with a primary jaw crusher, secondary and tertiary cone crushers in closed circuits with vibrating screens, a single stage ball mill in a closed circuit with cyclones, a CIL circuit, a cyanide destruction system, and an adsorption, desorption and recovery (ADR) plant to produce gold bars with a gold content of about 95%. The final crushed product will be 100% passing 19mm and final product of the grinding circuit or the CIL feed will be 80% passing 53 microns. The processing rate of the plant will be 7,000 t per calendar day at a head grade of 1.24 g/t gold and will average about 90% gold recovery. Approximately 7,780 ounces of gold will be produced on a monthly basis.
The leach circuit will consist of two agitated holding tanks and six agitated CIL tanks with air injection to provide about 24 hours of leaching time. Based on the test work, the oxygen content of the slurry will be about 8 ppm which is sufficient for efficient gold leaching. The content of activated carbon in the CIL circuit is 25kg per cubic meter of pulp. The carbon will be advanced three times per day for 45 minutes each time. Fresh activated carbon will be added in the last or sixth CIL tank.
The ADR plant will comprise an acid wash and neutralizing circuit, a pressure strip vessel, a carbon reactivation kiln, and the associated carbon holding tanks and carbon fines recovery system. The pregnant solution will flow from the strip vessel to electrowinning cells via a holding tank where the gold will plated onto stainless steel cathodes. The gold will be removed from the cathodes with high pressure washers and recovered with filters. The filter cake containing the gold will be dried and smelted to produce bars of gold bullion. The ADR plant is sized to process three tonnes of carbon per day at a grade of about 3,500 ppm.
The cyanide destruction circuit will consist of two agitated reactor tanks in series where the CIL tailing will be treated with air and metabisulphite to oxidize the total and WAD cyanides.
The electrical power for the project operations will be provided by upgrading the existing 33 km long 18.8 kV transmission power line to a 34.5 kV transmission line.
The existing access to the mine-site is by a 14 km all weather compacted gravel road from the interstate paved highway.