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 pyrite-chalcopyrite 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 basin's 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 of the 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.

Atalaya Mining is presently using mining contractors for all excavation work, including drilling and blasting, through the joint venture UTE Riotinto. This joint venture includes the companies S&L, which handles earth moving, and Insersa, which is responsible for drilling and blasting. Both companies are significant and well-financed contractors in Spain with extensive metal mining experience. Atalaya Mining is responsible for all grade control and mine planning.

Basic Pit Design Parameters:
- Bench height - 10 m;
- Haul road width (including ditch & safety berm) - 26 m;
- Internal ramp gradient - 10%;
- Minimum pushback width - 45 m.

Planned ultimate pit is approximately 2400 m long E-W, 1300 m wide N-S, and is about 330 m deep along the SE wall. The highest pit elevation is about 475 m in the SE wall and the pit bottom in the NW quadrant reaches an elevation of 124 m. Grid lines are shown on 500-m intervals. Total material – ore and waste – contained within the ultimate pit limits is estimated at just over 478 Mt (174 Mm3 ).

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The Riotinto concentrator processes copper sulfide ore using conventional froth flotation to produce copper concentrates. The plant employs a combination of existing equipment associated with the historical operations as well as expanded and upgraded facilities.

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. Commissioning and ramp-up of the Phase 2 plant commenced in March 2016.

The 15 Mtpa expansion of Proyecto Riotinto, which was first announced in December 2017, reached full mechanical completion on schedule and on budget in August 2019. Following completion, the processing plant was fully commissioned and began successfully operating at an increased annualised rate of 15Mtpa in January 2020. The project was designed to upgrade processing facilities with the ultimate goal of increasing copper production to approximately 50,000 – 55,000 tonnes per year.

The 15 Mtpa ........