Mantos Blancos is a copper stratabound deposit with subordinate content of silver mineralization hosted by Jurassic volcanic sequences. Mantos Blancos has the typical rock, mineralization, alteration and structural setting of the Jurassic deposits, in spite of its bigger size in comparison with other Chilean stratabound copper deposits. The area has been the subject of geological studies and interpretation since 1917 and the deposit geology has been extensively studied by Ramirez (2006) and Maksaev (2002).

Mantos Blancos is associated with the La Negra Formation. A volcanic arc is recognized in the La Negra Formation in the Coastal Range of northern Chile. This formation comprises mainly thin flows of calc-alkaline basalts, basaltic andesites and high-K basaltic andesites, as well as some volcaniclastic intercalations with a total thickness of 7 km to 10 km. The beginning of the volcanism of the La Negra Formation has been determined to be of Lower Jurassic age. A comparable volcanic belt is also recognized in the Lower Cretaceous in several areas of northern Chile.

The deposit is mainly characterized by pyroclastic and intrusive host rock units. The pyroclastic and intrusive units are from the Permo-Triassic and Early Jurassic age. Andesite belonging to the La Negra Formation (Middle-Late Jurassic), tonalite, granodiorite, aplite and dacite porphyry belonging to the El Ancla and Alibaud plutons of Middle to Late Jurassic age also outcrop along the deposit. Rock types in Mantos Blancos consist of a rhyolitic dome and magmatic–hydrothermal breccias which are intruded by dioritic and granodioritic stocks and sills. The dioritic and granodioritic stockwork locally grade upwards into magmatic–hydrothermal breccias that are mineralized to different degrees. Late mafic dikes cross all the previously mentioned rock units and are essentially of no economic value.

The sulphide mineralization consists of chalcocite (and/or digenite), covellite, bornite, chalcopyrite, pyrite, specularite, magnetite, galena and low sphalerite, occurring in disseminated form, with varying thicknesses. The oxidized copper minerals are atacamite, chrysocolla and minor malachite, antlerite, tenorite, cuprite and almagres occurring as dissemination and fracture filling. Silver occurs in the crystal structure of the copper sulphides and occasionally as native silver. The geometries of the mineralized bodies are irregular lenses and oxidized copper sulphides are arranged in tabular form with a 100 m to 200 m thick interval (Boric et al., 1990) that is strongly controlled by structures.

The mineralization has a distinct vertical zonation (Infanta, 2002), with specularite at the top (porphyritic andesite and upper andesite), followed by an oxidized copper (atacamite with minor chrysocolla) that migrates in depth to a high-grade sulphide (chalcocite-bornite). The latter corresponds to irregular chalcocite rich centre lenses which decrease towards the edge to predominant bornite. Surrounding these lenses is a zone of lower grade with chalcopyrite and bornite, ending at depth with a pyritic zone, occasionally in some sectors associated with chalcopyrite. The areas with secondary enrichment are of small extent, predominantly with covellite over chalcocite and located near major faults. The oxidized copper would have developed by the in situ oxidation of primary sulphides.

The open pit operation includes one large open pit (Santa Barbara) which provides most of the sulphides for the concentrator and oxides for dump leaching.

Other sources of material are:
• Flotation: Sulphide stockpile (Cancha 90);
• Dump leach process: Oxide stockpile (Mercedes Stockpile).

Variable slope angles were used for the detail mine design, with inter-ramp slope values varying between 50° and 59°. A slope angle of 36° (natural slope angle for broken materials) is used on areas with existing underground stopes and for waste dumps.

The pit design was developed considering the following:
• Minimum expansions width: 30 m for front end loaders;
• Minimum road width: 30 m;
• Maximum gradient on ramps: 10%;
• Operating bench height: 12 m (24 m in final pit walls with pre-split blasting).

The selected final pit was divided into operational phases, resulting in a total of eight phases to define the LOM 2020.

Mine movements considered historical KPIs for loaders, operational ramps and maximum quantity of equipment for each phase. The production plan was developed considering a maximum mining rate of 66 Mt per year and the operation of 4 to 5 phases simultaneously.

The resulting plan considers a total mining rate (mineral and waste) which decreases from approximately 66 Mt in 2022 to 60.0 Mt in 2025, staying at this level up to 2030 when production starts to decrease through to 2037.

Crushing
The primary crushing area comprises two 42” x 65” gyratory crushers. Each crusher has a maximum capacity of 5.5 Mt per year, equivalent to a nominal throughput of 897 tph, operating with an open side setting of 125 mm.

Line 1 operates with the existing fine crushing plant.

The Line 1 fine crushing plant will comprise one triple deck 8 ft x 20 ft primary screen fed from the 8,000 t coarse ore stockpile. The screen undersize will pass to final product, the screen oversize will pass to the existing 7 ft secondary cone crusher; the product will be conveyed to the tertiary screening plant. The tertiary screens, four 8 ft x 20 ft single deck screens, will be in closed circuit with two H7800 tertiary crushers. The product of Line 1, undersize from the tertiary screens, will be 80% passing 4 mm and will pass to the existing 15,000 t (live) fine ore stockpile. This will be the feed to the existing N°3 Ball Mill.

Line 2 was the oxide ore crushing plant until January 2021. The modified plant will consist of two 10 ft x 12 ft secondary screens that are fed from the 12,000 t coarse ore stockpile. The screen undersize will pass to final product. The screen oversize will feed two 5½ ft secondary standard cone crushers, the product of which will feed the tertiary screen feed bins. These will feed four tertiary crushing lines, each consisting of two 6 ft x 12 ft tertiary screens and one 5½ ft short-head crusher. The screen undersize will be final product, the oversize will be recirculated. The final product of Line 2 will be -11 mm and will pass to the existing but modified fine ore stockpile, capacity 11,600 t (live).

Grinding
The product of crushing Line 1 will be milled in the existing Ball Mill N°3 at a rate of 1.8 Mt per year, and the product of Line 2 will be milled in a new ball mill, Ball Mill N°8, at a rate of 5.5 Mt per year.

The 16.5 ft x 25 ft Ball Mill N°3 with a 3,200 kW motor will mill 4,932 tpd of ore and is in closed circuit with the existing cyclone cluster. The cyclone overflow, with a P80 of 250 µm, will be pumped to the new rougher flotation cells. The cyclone underflow will be returned to the mill feed. Lime, primary flotation collector and dilution water will be added to the mill circuit. Line 2, with the new 23 ft x 40.5 ft Ball Mill N° 8 with a 13,000 kW motor, will be installed and will operate in closed circuit with a new cyclone battery. This mill will receive 15,068 tpd of -11 mm ore and, as for Ball Mill N° 3, will produce a product P80 of 250 µm. The cyclone overflow will also be pumped to the new rougher flotation bank, joining the product of Ball Mill N°3. The Hatch mill sizing and power (12,000 kW) was reviewed by Fluor. Fluor concluded that the design was conservative and fit for purpose. A second independent review of the mill sizing was carried out in 2019 (by RPA) and as the result the motor size was increased from 12,000 kW to 13,000 kW.

In the new milling circuit existing Ball Mills 1 and 2 will be redundant. However, they will remain available to cover maintenance periods on Mills 3 and 8 or to provide additional capacity should it be required.

The regrind mills will be the existing Ball Mills N° 6 and 7 (currently used for primary grinding) (8.5 ft x 12.0 ft with a 500 hp motor and 9.5 ft x12.0 ft with a 600 hp motor).

Ore processing at Mantos Blancos currently comprises two plants: the oxide plant where the oxide ore is leached using sulphuric acid to produce copper cathodes, and the copper concentrator where the sulphide ore is treated to produce a copper concentrate.

The current sulphide concentrator has a capacity of 4.2 million tonnes of sulphide ore per year, with three stage crushing followed by ball milling and flotation plant. The oxide run-of-mine ore is leached on dumps, and the solution sent to an SX-EW plant with a production capacity of 60 thousand tonnes per year of high purity copper cathodes.

Current mining plans indicate that oxide reserves will be completely depleted by 2023 and only sulphides will be processed after that time.

Flotation and Concentrate Regrinding
The current flotation plant consists of thirteen 42.5 m3 rougher cells, one regrind ball mill, one 3 m x 12 m cleaner column cell and three 28 m3 plus twenty 14 m3 cells operating as ........