The important Au-Ag occurrences in Santa Cruz Province are confined to the Deseado Massif. The potential for important gold-silver mineralization was recognized in the Deseado Massif some 25 years ago. The Deseado Massif is a plateau consisting of Middle- to Upper Jurassic acid volcanic and volcaniclastic rocks that host fracture-controlled epithermal Au-Ag mineralization. Precious metal mineralization is related to LateJurassic magmatism.

The gold-silver deposits are hosted in silicic volcanic and volcano-sedimentary Jurassic rocks related to arc or back-arc settings in Andean or extra-Andean settings. The ore geology, textures, mineralogy, restricted alteration and geochemistry of these mineralized occurrences indicate that they belong to the epithermal class of precious metal deposits. The deposits are mainly associated with quartz +/- calcite +/- adularia +/- illite alteration assemblages interpreted to represent low and intermediate sulphidation epithermal type deposits.

Precious metal deposits generally occur in tectonically controlled structures with quartz fill, frequently brecciated in nature. Mineralization is generally associated with discrete banded quartz vein structures ranging in width from 0.5 to over 5 m, although a disseminated form of weak stockwork quartz veining of less than 0.5 m width intermediate to the principal quartz structures does also occur in some instances. Strong lithological control may occur. Dating in a few instances suggests that hydrothermal activity is several million years younger than peak magmatism. The presence of surface features like silica sinters, veins interpreted to be feeder structures, carbonate-lacustrine deposits and silicic lithocaps related to steam-heated waters in the water-table, indicates that the tops of some systems have been preserved. Erosion of younger sedimentary and volcanic cover is now exposing these systems and is suggesting that large regions or tectonic blocks are highly prospective for epithermal mineralized systems at depth.

The La Paloma and Martinetas epithermal systems are classic “low sulphidation” type deposits consisting mainly of quartz with adularia and free Au, minor amounts of sulphides (except at Sulfuro where a relatively high sulphide content occurs), and weak alteration haloes. Classic low sulphidation textures like lattice textures, crustiform-colloform banding, comb quartz, etc., are common. They may also be associated with anomalous amounts of arsenic (As), mercury (Hg), or antimony (Sb).

The study is based on conventional open pit mining.

The Don Nicolás mine design specifies 5-m benches and has adequate phase geometry to achieve the required productions. The design calls for annual mill throughput of 350,000 t of ore. Mining will advance simultaneously in two different pits with two to three pit phases generally active at any time. The vertical advance rate of the pits will be relatively low, with a maximum of eleven 5-m benches per phase per year.

The process plant has been designed to treat gold-bearing material at the rate of 972 t/d, equivalent to 350,000 t/a.

The crushing circuit will reduce the mined material size from a nominal top size of 500 mm to a product size P80 of 6.5 mm in preparation for the grinding process.

The grinding circuit will reduce the size of the crushed material to a final product size with a product P80 of 75 µm, suitable for the subsequent gold recovery by gravity concentration and cyanide leaching processes. The grinding process will be a single-stage operation with the ball mill in closed circuit with classifying cyclones.

The gravity concentration circuit will produce a concentrate containing coarse gold from the grinding circuit, and will treat the gravity concentrate on a shaking table to upgrade the concentrate suitable for its smelting at the refinery.

The gravity concentrate will be stored until a sufficient amount has been collected for the tabling process. Tabling of the gravity concentrate will normally be conducted as a batch process on a daily basis. When tabling is complete, the table tailings will be returned to the ball mill circuit for further treatment. The table concentrate will be collected, dried and transferred to the furnace for smelting.

The gold and silver will be leached with cyanide and adsorbed onto activated carbon in this section of the processing circuit. Loaded carbon will be recovered periodically, nominally on a daily basis, for eluting/stripping to recover the gold and silver, and reactivated carbon will be added to replenish the eluted carbon in the circuit.

The slurry from the grinding circuit will be fed into the first of eight CIL tanks. The pH will be controlled by the addition of lime which will be added as required. Cyanide solution will be added to maintain the necessary concentration required for leaching. These additions will be made to the CIL feed distributor, and the first CIL tank, with provision made for the addition of lime and/or cyanide further down the train of CIL tanks. The cyanide concentration required in the CIL circuit will be dictated by operational conditions, but will be between 300 and 500 mg/L as sodium cyanide, and will depend on the type of material being treated in the plant.

Loaded carbon will be transferred from the first CIL tank to the loaded carbon screen by means of an air-lift. The loaded carbon screen will be a vibrating screen equipped with spray water nozzles to wash the ore particles off the carbon. The loaded carbon will be transferred to the carbon circuit for acid treatment, elution and regeneration. The screen underflow will contain the slurry and wash water and will be returned to the first CIL tank.

The leached slurry will exit the CIL circuit via the interstage screen on the last CIL tank and will be discharged over the carbon safety screen. This vibrating screen will also be equipped with spray water nozzles and will prevent any carbon particles, which may have by-passed the last interstage screen or been lost through a holed interstage screen, from reporting to the residue. Any carbon which reports to the carbon safety screen will be collected in a bin and will be returned to the CIL circuit. The safety screen undersize slurry will be transferred to the tailings thickener.

Loaded carbon will enter the acid wash tank from the loaded carbon screen. Under normal operating conditions, the loaded carbon will first be acid treated, then neutralized with caustic solution, or thoroughly rinsed, followed by the subsequent elution stage.

The gold and silver adsorbed onto the carbon will be desorbed/eluted in the elution column.

After acid washing, the loaded carbon will be pumped to the elution column. The elution solution will be heated using a combination of plate and frame heat exchangers and a diesel fired hot water heater.

After reaching the elution/stripping temperature, the solution will be pumped upward through the elution column. The elution column/strip vessel will be designed to treat a 2-t batch of loaded carbon although planned the production rate will be about 1.4 t depending on operational conditions. The elution/stripping of gold and silver from the loaded carbon will be accomplished using a modified Zadra based elution process. The gold will be eluted from the carbon under temperature and pressure conditions to form a pregnant solution. The gold-bearing solution will exit the elution vessel, and will then flow through the two cool-down heat exchangers to the electrowinning circuit. The two cool down heat exchangers will cool the pregnant solution before this catholite solution is transferred to the electrowinning circuit. The barren catholite solution leaving the electrowinning cells will be re-circulated via the barren solution tank.

After completion of the elution cycle, the carbon will be transferred to the regeneration kiln storage bin via a dewatering screen.

The gold and silver will be recovered by electrowinning from the catholite solution.

The catholite solution will enter the electrowinning cell from the elution column. The electrowinning will be undertaken in one 4.0 m3 electrolytic cell. The gold and silver will be electro-plated onto stainless steel wool cathodes. The total daily metal recovery will average 14 kg gold and silver.

The flowrate through the electrowinning cells will be about 7 m3 /h. The solution leaving the electrowinning cells will flow by gravity to the barren solution tank for making up to strength with caustic and cyanide for the following elution/stripping cycle. On completion of the electrowinning process, the electrowinning cell will be drained and the deposited metal will be washed off the cathodes into the cathode wash tank. The cathodes can also be lifted out of the cells and placed inside the cathode wash tank for the thorough washing and removal of deposited metal, if necessary. The cell sludge and the metal washed off the cathodes will be washed into the cathode wash tank as required. The sludge and metal will then be discharged to the electrowinning sludge filter press to remove the bulk of the solution which will be re-used in the electrowinning circuit, or recycled to the leach circuit. The filter press will be cleaned out at the end of the filtration cycle. The metal and sludge will be placed into trays for drying in the drying oven followed by smelting.

A typical elution cycle time will be about 18 hours in total. The elution and electrowinning circuit design of 2 t per elution will allow for the treatment of HG material, or maintenance demands.

The gold and silver will be smelted into doré bars.