Riotinto is a typical Volcanogenic Massive Sulfide (VMS) deposit, which are formed in extensional tectonic settings of oceanic seafloor, including spreading ridges subductions zones and arc environments.

According to genetic, rock association and geodynamic setting, the Riotinto volcanic-hosted pyritechalcopyrite mineralization is classified as felsic siliciclastic of Kuroko type. It occurred as lenses of polymetallic massive sulfide that took place at the sea floor in a submarine volcanic environment during the earlier Carboniferous, some 350 million years.

As with most significant VMS mining districts, the IPB is defined by deposit clusters formed within ocean rifts with volcanic centers. The clustering is attributed to a common heat source that caused large-scale sub-seafloor fluid convection systems.

As with most VMS deposits, Riotinto has two morphological and genetic components:

• A mound-shaped to tabular strata bound body composed mainly of massive sulfides.
• An underlying zone with development of a stockwork system of irregular veins filled by quartz and disseminated sulfides that represent the pipes of the volcanic feeders.

Furthermore, Riotinto as well as other VMS deposits, is characterized by extensive zones of hydrothermal alteration as a result of subvolcanic intrusions and fluid convection systems, which define zones of discordant alteration in the immediate footwall and hanging wall of the deposit.

Mineralization is typical of the VMS deposits and occurs as stockwork and massive sulfides. Beside these, oxidation weathering of the primary mineralization has developed gossans and enrichment zones in places.

The stockworks occur as irregular veins, fractures and fissures filled with quartz and sulfides. Two main types of stockworks are identified: 1) pyritic (20% S content) and 2) cupriferous (2% Cu content). The veins become thicker towards the surface. Close to the massive sulfides, most stockworks are made up of veins with lesser amounts of strongly replaced volcanic rocks.

In the Riotinto district mineralization, hydrothermal alteration and secondary replacement (i.e., silicification, chloritization, sericitization and sulfidization) often occur associated to some stockwork zones, and along thin fractures and veins. The alteration zone is characterized by an inner core of chlorite alteration surrounded by an envelope of sericitic alteration, silicification, and pyrite and carbonate alteration. The copper-rich mineralization is most closely associated with the chlorite altered zone.

The massive sulfide consists of lenses of pyrite plus chalcopyrite emplaced on the marine basins surface as a result of black smoker flows. They occur overlaying the felsic volcanic and the stockworks. The primary sulfide mineralization consists mostly of pyrite, with minor chalcopyrite, sphalerite, tetrahedrite and sulfosalts of Sb and As. Chalcopyrite is the predominant copper mineral and mostly occurs within small fractures in the pyrite, on a lesser extent it occurs in isolation.

The massive sulfide deposits are located as dismembered lenses along the axis and the flanks of the anticline close to the Northern and Southern Faults. The principal deposits are San Dionisio-Atalaya, Filón Sur, Filón Norte (Cerro Colorado, Dehesa, Lago and Salomon).

Nowadays, massive sulfides remain only at San Dionisio-Atalaya. At Filon Sur and Filon Norte, all the massive sulfides were mined. In the core ofthe anticline at Cerro Colorado-Salomon open pit, it is believed that the deposits originally formed an almost continuous lens of massive sulfides of about 5 km long, 750m wide and 40m thick, containing more than 500 million tonnes of sulfide mineralization.

Well-developed stockwork mineralization occurs in the volcanic rocks that underlay parts of the massive sulfide ore at all the zones from Filon Norte. Beside the stockwork and the sulfides, secondary mineralization occurs in places. An important volume of sulfides in Cerro Colorado and minor amounts in Corta Atalaya have developed extensive gossans, which were exploited for gold and silver. Gossan and the zone of supergene enrichment are characterized by the occurrence of secondary minerals goethite-limonite and chalcocite-covellite respectively.

Mining has been conducted in the Riotinto area episodically since Roman times, and possibly earlier. The Riotinto Company (a British firm) gained mineral rights and surface ownership in 1874 and operated Riotinto Minera until it began divesting its interests in 1954. Operations in the Cerro Colorado open pit, which is the foundation of the Riotinto Project, began in 1968. The Riotinto mine properties were under the ownership of a local cooperative by the end of the 1990s. Mining activities ended at Cerro Colorado and other smaller operations in 2001 due to deteriorating economic conditions. Mining operations at the Riotinto Project site restarted in June 2015. The Cerro Colorado pit is completely open and in good condition.

Continued exploitation of the Cerro Colorado deposit uses conventional, open pit mining methods. Mining benches are on 10-m vertical intervals. Contractors’ small- to medium-scale mining equipment is used to execute the development plan, including: rock drills capable of drilling 102- to 127-mm-diameter blastholes, hydraulic excavators with bucket capacities of 6 to 14 m3 , off-highway trucks with 55- to 91-t payload capacities, and suitably sized support equipment.

Phase 1
The original concentrator, consisting of crushing, grinding, and flotation that was designed and built in the 1960’s and 1970’s, was refurbished to process ores from the mine operations.

In June 2014 Atalaya Mining started working with engineering firms that specialized in detailed engineering, construction, and commissioning, to develop the Phase 1 project of the Riotinto plant. Completion of Phase 1 required the reconditioning of the existing equipment and infrastructure, as well as the installation of new equipment and systems. This phase was successfully completed on schedule in June 2015. The commissioning and ramp- up of Phase 1 was accomplished in seven months, reaching a nominal milling rate of 5 Mtpa in February 2016.

Phase 2 – Current Configuration
The basic engineering design to increase the plant throughput rate from 5 Mtpa (Phase 1) to 9.5 Mtpa (Phase 2) began in March 2015. Similar to Phase 1, engineering firms that specialize in the design of processing plants were hired for detailed engineering and construction. Commissioning and ramp-up of the Phase 2 plant commenced in March 2016.

In order to achieve the expansion production rate of 9.5 Mtpa, the existing crushing and screening plant equipment was refurbished and new equipment was added where required. New grinding, flotation and concentrate handling equipment was also added to the existing plant.

The process flowsheet features three crushing stages, two ball mill grinding stages, a rougher flotation circuit, one rougher concentrate re- grinding stage, and a three stage cleaner circuit. The process ends with a thickening and filtering stage to obtain a final copper concentrate product.

After commissioning and operating Phase 2, an additional 300m3 cell wasinstalled in a “Pre- Cleaner” duty, and the following flotation banks are only intermittently used as required: Cleaner Scavenger; Second Cleaner; and Third Cleaner.

Future Process (Phase 3 – 15 Mtpa Upgrade Project)
As part of the Riotinto 15M Upgrade Project, the basic engineering design for the upgrade of the Phase 2 plant in order to further increase the plant throughput rate from 9.5 Mtpa to 15 Mtpa commenced in July 2017. Detailed engineering design and installation activities are currently in progress with the commissioning and ramp-up of the upgraded plant scheduled for the fourth quarter of 2019.

After the installation of modern mechanically aspirated flotation cells during Phase 2, it was observed that acceptable upgrading and recoveries were achievable when only utilizing the new flotation cells on cleaning duty. As a result, the original naturally aspirated cells on cleaning and cleaner-scavenger duties were regularly bypassed during operations. The Phase 3 flotation design leverages off this observation and only has two stages of cleaning utilizing modern flotation cells.

New equipment has been added to the existing Phase 2 plant design and the resulting plant design has been modified as well as reconfigured to achieve the increased plant throughput rate of 15 Mtpa. A new primary crushing circuit, a coarse ore stockpile, a new primary milling circuit, a new rougher flotation circuit, a new thickener and two new filters has been incorporated in the Phase 3 plant design.

In summary, the flowsheet modifications allow the expansion to 15 Mtpa with optimal use of existing/available equipment by:

1. Installing a SAG mill with a pebble crushing circuit, that is capable of grinding 15 Mtpa of coarsely crushed product to the extent where a proportion of the ground product is sufficiently fine to report directly to flotation (cyclone overflow), and the coarser component (cyclone underflow) is suitable to feed the existing milling circuit.

2. The installation of the SAG mill effectively re-distributes the power balance away from high cost secondary and tertiary crushing, yet retains the ability to achieve comparable grind product specification with sufficient ball milling capacity.

3. Fully re-utilizing the existing crushing circuit on ROM ore, so that a parallel 5 Mtpa crushing line with a single 250 kW jaw crusher is sufficient to increase overall crushing capacity from 9.5 Mtpa to 15 Mtpa. Compared to the 9.5 Mtpa scenario, the crushed product is coarser, but suitable for SAG milling.

4. The portion of SAG mill discharge reporting to the primary cyclone underflow is of a similar particle size distribution and tonnage as the existing ball milling circuit feed. As such, the existing circuit will achieve the required grind product P80 of 183 µm with 15 Mtpa of plant feed;

5. Installing additional rougher flotation capacity, re-tasking recently installed modern tank cells (Phase 2) and de-commissioning older naturally aspirated (Wemco) cell technology to finish with a simpler flotation circuit with sufficient capacity to achieve comparable metallurgical performance.

6. Incremental expansion of concentrate and tailing handling facilities by the installation of a 14 m diameter high rate thickener and two 62 m2 vertical filter presses to cope with additional product as well as one tailings pump and an extra tailings line for the tailings generated from the expanded operation.