The gold deposits on the Castelo dos Sonhos plateau are classified as modified paleo-placers (Frimmel, 2005), mineral deposits in which free gold grains accumulated in ancient gravels that were subsequently slightly modified by low-grade metamorphism.

Most of the gold mineralization in the Castelo dos Sonhos Formation lies in a central band where the various conglomerate lithologies dominate. At the base and top of this band, the conglomerates are interlayered with arenites, which become more frequent as one moves away from the conglomeratic band, either downward into the older rocks (the lower arenite) or upward into the younger rocks (the upper arenite).

The mineralization was also modified by shearing and open folding and hydrothermal fluids associated with subjacent granitic intrusion which affect much of the mineralized conglomerate band. The most widespread of these are silicic and hematitic alteration, both of which can, in places, be intense. Other alteration minerals that are less widespread and generally less intense include sericite, muscovite, fuchsite and epidote.

Gold occurs as free grains and flakes of various sizes, from sub-visible (less than 100 microns) to highly visible, supergene enrichment creates nuggets that can reach a few centimetres in size.

The two predominant styles of mineralization are:

1. Paleo-placer: Free grains of gold that were likely deposited along with the sediments. In corethese can be seen in the matrix of the conglomerate, sometimes in heavy mineral bands;

2. Remobilized: Gold associated with alteration, usually hematitic alteration. Free grains of gold have been observed in hematite-filled fractures, and as thin films plated onto fracture surfaces.

The Castelo de Sonhos Project will be an open pit operation utilizing a contract mining fleet of hydraulic excavators, front-end loaders and 36 tonne haul trucks, associated with correspondent ancillary equipment. The mine planning model adopted is a “diluted” model, adding approximately 0 % dilution and 95% of recovery to the source model.

The disposal of waste rock will be executed on an area close to the pit. The site shall be adequately prepared to include drainage at its base and channels to direct the flow of water with the aim of aiding geotechnical stability and mitigating the erosion of the stockpiled material.

The operation of this phase, in accordance with the ascending method, shall begin during the construction of the heap at the base of this area. Waste rock will be disposed by truck, which will then be uniformly distributed and leveled by an operator using a tractor. The procedure is then repeated, stacking another bank above the original one, while maintaining a ramp for the trucks to be able to access the area.

The Castelo de Sonhos project beneficiation plant was developed considering a processing rate of 3 Mt/y, which, for a regime of 7,500 effective hours per year of operation, implies a nominal hourly rate of 400 t. The average ROM grade is 1.50 g/t, which involves an average gold production of 150,000 oz/year. The process tests, provide for a minimum recovery of 95% of the current gold.

The ROM is dumped in a hopper with capacity for 150 t, equipped with a stationary grizzly with a square opening of 400 mm. The coarse material, retained in this grizzly, is broken using a hydraulic hammer. The material passing through the grizzly is reclaimed by a 1.6m x 3.7 m vibrating feeder, equipped with bars spaced at 150 mm. The oversize feeds a primary jaw crusher,a 1005 t/h capacity C-130, with opening at closed size of 150 mm. The product of the primary crushing is joined to the material passing through the feeder and is conveyed to the coarse stockpile. The conveyor is equipped with auxiliary equipment, as belt scale, cross belt magnet and metal detector.

Two 1.6 x 3.7 m vibrating extractors installed under the 40,000 t capacity stockpile, reclaim the coarse ROM and feed a SAG mill through a 75 m long, 914 mm wide conveyor, at a production rate of 380 tph.

The material extracted from the stockpile feeds a 8.5m x 3.6 m (28 ft x 12 ft) EGL Semi Autogenous Mill (SAG) equipped with trommel, with opening of 1.5 inches. The oversize of the trommel feeds a cone crusher (pebbles) and the product is sent back to the SAG mill (circulating load).

The material passing on the trommel’s sieve is discharged to a box and pumped to a cluster of cyclones. The underflow of cycloning feeds a 5.5m x 7.5m (18’ x 25’) ball mill.

The overflow of cycloning, with P80 finer than <75µm, is piped to the pre-lime tank. The 50% solids pulp is mixed with lime for 6 hours to reach a pH between 10.5 and 11.

The slurry from pre-lime is conducted by gravity to 5 (five) agitated leach tanks in cascade, each one with capacity for 3,334 m3 of 50% in solids slurry. The leaching time in this phase is around 36 h. Cyanide is added in the first tank and where required to maintain the NaCN concentration. Slurry is transferred from one tank to the next by gravity. The mechanical arrangement allows bypassing among the tanks for maintenance or process requirements.

From the last 5th tank, the slurry is diluted for 45% solids and feeds 5 (five) CIP tanks, each one with capacity of 1,200 m3, in cascade. Each tank is equipped with a recessed impeller vertical pump and a hoister for screen handling for maintenance. The slurry residence time during adsorption is around 12 h, totaling 48 h of leaching and adsorption. Activated carbon, size ranging in between 8x16 mesh (2.36 - 1.18 mm), is added to the tanks at a rate of ranging from 18 to 20 gcarbon/lslurry. These tanks are equipped with 0.8 mm opening Kemix MPS1200 interstage screens. The slurry flows through the screen to the following tank and the carbon remains in the tank, retained by the screen.

The tailings from the 6th CIP tank are discharged in a safety high-frequency Derrick screen to prevent the loss of loaded carbon, and the passing feeds a 25 m thickener. The thickened solid is pumped to detoxification. The thickener overflow is pumped to a storage tank and distributed to the consumption points in leaching/CIP circuit. The underflow is pumped to two 785 m3 capacity cyanide destruction agitated tanks, assuring a residence time of around 3h. SMBS (sodium metabisulfide) and CuSO4 (copper sulfate) are added to these tanks to oxidize cyanide under intensive aeration, to assure complete oxidation of cyanide.

The loaded carbon stored in a bin is pumped to the acid washing facility.

The washed carbon is then transferred by recessed impeller pumps to the elution columns. For this study, the Zadra process is being considered because of its easier operation compared to the AARL method. A 1.5m diameter and 9m height strip column is filled with 4t of loaded carbon. The gold in solution is reduced electrochemically and plated in the cathodes.

After the elution cycle, or when required by process, the cathodes loaded in metallic gold are discharged twice a week or when required by the process. The cell is emptied and the cathode removed to a tank where it is washed under high-pressure water. The gold is displaced from the stainless steel wool and settles to the bottom. This sludge is pumped to a press filter, dried and smelted in a gas-fired furnace. The gold is stored in the vault and the slag stored in drums for further treatment.