The Santo Tomás Project is an open pit mine hosting a porphyry copper deposit with fractured and disseminated sulphides of copper and molybdenum located in northern Sinaloa State, Mexico.
With significant gold and silver credits, the project is regarded as one of the largest copper deposits in the globally.
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The project is being developed by Oroco Resource.
A Preliminary Economic Assessment (PEA) of the copper project was filed by Oroco in October 2023. As per the PEA, the project has a total Life of Mine (LOM) of 23 years.
Santo Tomás Location and Site Access Details
The Santo Tomás Project is located in the Municipality of Choix situated at the northern border of the Sinaloa State and the southern border of the Chihuahua State.
The mineral resources are located in the Sinaloa State on the southern bank of the Rio Fuerte River in the Sierra Madre Occidental.
The project can be accessed by a 169km paved highway from the Topolobampo Port to the northern Choix Town via the Los Mochis City.
The project can also be accessed from the Chihuahua Pacific Highway via secondary unsurfaced roads, from El Ranchito by the current access road to a further 38km along unimproved roads via Cajón de Cancio, and via the El Sauzal Mine access road turning off at 38km.
Santo Tomás Ownership History
Since 1900s, informal miners have been working at the project site. Many small excavations and two small adits have been produced in the North and South Zones of the project.
ASARCO Mexicana S.A. conducted the first systematic exploration at the site in 1968 by constructing an access road from El Bienestar Ranch. The company drilled mostly in the North Zone.
The property was optioned by Tormex Mining Developers Ltd. (Contratista Tormex S.A.) and Industria Minera Peñoles in 1973. Both companies conducted exploration and re-sampling till 1977 on the North Zone.
The Mexican Government agencies included the project in a series of regional airborne surveys, LANDSAT imagery, helicopter surveys, and geological mapping in 1980s and 1990s.
By using the existing data, Esmeralda Group and Minera Real de Ángeles S.A. de C.V. produced mineral resource calculations, the results of which are not available.
The Esmeralda Group produced a new set of geological sections and plan maps after the resumption of exploration in 1990.
Minera Real de Ángeles and re-assayed two holes re-logged 12 ASARCO drill holes in 1991 resulting in a block model resource calculation. The results of this study are also not available.
The Esmeralda Group signed a purchase agreement of the property with Cerro de Cobre Inc., a Canadian company and optioned it to Exall Resources Ltd. in 1992.
Exall conducted a 4,000m drill program in 1993 which consisted of 33 reverse circulation drill holes and seven diamond drill holes. In 1997, Exall sold its option on the project.
Morgain Minerals Inc. and Minera MGM S.A. de C.V., a wholly owned subsidiary of Morgain, signed an agreement with Mr Rubén Rodríguez in 1997-98 to acquire 100% interest in the project.
The technical data regarding the bench-scale testing of copper concentrates was evaluated by Morgain with consultants resulting in a series of north-facing vertical sections at 1:1,000 scale but no further information regarding the agreement was found by the author.
Rubén Rodríguez transferred the 100% ownership of the project to Compañía Minera Ruero, S.A. de C.V., a Mexican private registered mining company, in 2002. Compañía Minera is owned by Ruero International Ltd. (99.998%) and Rodriguez (0.002%).
A USA company, Fierce investments Ltd. signed a share purchase agreement in 2002 with Rodriguez in to acquire the shares of Ruero International.
Several technical reviews of the project were successfully completed by Cambria Geological Ltd. (2005) and Cambria Geosciences Inc. (Cambria) (2006-2009).
A revised report of the mineral resource estimate was prepared by John Thornton in 2010.
The 100% ownership of Ruero International went back to Rodriguez through a decision of the Supreme Court of the Commonwealth of Bahamas in 2015.
Presently, Ruero International is owned by Altamura Copper (50%) and Rodriguez (50%).
Xochipala Gold, a subsidiary of Altamura, acquired 100% interest in the project from Compañía Minera Ruero in June 2016. The court judgement impeded the registration of the sale agreement and the transfer of title to Xochipala which was nullified in 2019.
Compañía Minera Ruero held seven core concessions until December 2019. In 2016, Compañía Minera Ruero and Xochipala signed a concession agreement by which Xochipala received 100% title of the seven concessions.
In October 2018, Oroco signed a definitive option agreement with Altamura. Oroco held an approximate 13% interest in Altamura at that time.
In March 2020, Oroco acquired Altamura receiving an outright control of the project.
Santo Tomás Geology and Mineralisation
The mineralisation of the project, porphyry copper-molybdenum-gold-silver, is closely linked to the Late Cretaceous to Palaeocene (90-40Ma) Laramide Orogeny.
The project and other porphyry copper deposits in Mexico are situated along a 150,000m (1,500km) long NNW trending belt extending from the southwestern USA to the Guerrero State of Mexico.
The evolution of this belt is related to the accretion of allochthonous terranes comprising a basement of Palaeozoic accreted sedimentary rocks and Triassic rift-related meta-igneous rocks.
The basement is overlain by Middle Jurassic and Early Cretaceous arc-related rocks of the Guerrero Arc.
The project area hosts Mesozoic country rocks consisting of marble, limestone, sandstone, andesitic volcanic rocks.
The dominant intrusive lithology is related to mineralisation and Late Cretaceous (~75Ma) quartz monzonite.
The mineralisation is dominantly controlled by the Laramide-age deformation, sulphide mineralisation, and hydrothermal breccias.
The sulphide mineralisation is dominated by pyrite, chalcopyrite, molybdenite, and minor bornite, chalcocite, and covellite. These are distributed in altered andesite country rock and quartz-monzonite.
The minor mineralisation is linked with replacement-style and skarn mineralisation in the hanging wall limestone with copper oxides occurring near the surface.
The alteration consists of wide zones of potassic, silica-albite, phyllic, propylic, and argillic hydrothermal alteration.
Mineral Resource Estimate
The total indicated mineral resource estimate of the Santo Tomás Project is 561Mt containing 4,579Mlb of copper equivalent (CuEq) at a grading of 0.37%, 4,077Mlb of copper at a grading of 0.330%, 98.4Mlb of molybdenum at a grading of 0.008%, 487.4Koz of gold at a grading of 0.027g/t, and 37,762Koz of silver at a grading of 2.1g/t.
The total inferred mineral resource estimate of the project is 549.1Mt containing 4,166Mlb of CuEq at a grading of 0.34%, 3,729Mlb of copper at a grading of 0.308%, 88.8Mlb of molybdenum at a grading of 0.007%, 375.8Koz of gold at a grading of 0.021g/t, and 34,458koz of silver at a grading of 2g/t.
Mining Methods and Processing of Ore
The proposed method to mine ore from the mine is open-pit truck and shovel method with 10m bench intervals.
A total of 847.7Mt of the mineralised material will be processed with an average grading of 0.36% of CuEq. The processing plant will have a throughput of 60 kilo tons per day (kt/d) in the first year of production followed by 120kt/d from second year.
A maximum material movement capacity of 107Mt per year will be required to support the processing plant.
A total of 983.6Mt of waste will be generated which needs to be removed resulting in a strip ratio of 1.16 over a 23-year life of mine including two years of pre-stripping.
Using hydraulic mining shovels (5-29m3) and front-end loader (1-22m3), the blasted material will be mined to mine waste and stockpile claim.
The material will be trucked using 194 tonne mining haul trucks to the processing plant for processing.
The processing plant is anticipated to produce 646t/d of copper concentrate at an average grading of 26.9% and 7.1t/d of molybdenum concentrate at an average grading of 45%.
The run-of-mine (ROM) will enter a three-stage primary crushing circuit reducing the size of ROM to a P80 of 143mm from an F100 of 1,200mm.
The size of ROM will be reduced further by the secondary crushing circuit from an F80 of 143mm to a P80 of 42mm. The tertiary HPGR crushing circuit will reduce the size further to a P80 of 5.6mm.
The grinding circuit will reduce the particle size of the material to a P80 of 150µm from an F80 of 5,600µm.
The reduced material will enter a bulk rougher flotation to recover a mixture of copper and molybdenum concentrate.
The regrinding circuit will reduce the rougher concentrate particle size to a P80 of 23µm from an F80 of 125µm.
Copper and molybdenum will be floated in a bulk cleaner flotation circuit thereby increase in grades. The molybdenum rougher flotation tails will be copper concentrate.
The molybdenum grade will be increased to 45% in the molybdenum cleaner flotation circuit.
Both concentrates will be thickened to a solid density of 60% weight by weight by a hi-rate thickener.
The thickened concentrate will be filtered by a vertical pressure filter resulting in a final copper concentrate with 9% moisture.
The molybdenum concentrate will also be filtered resulting in a filtered molybdenum concentrate with 15% moisture which will be dried to 5% moisture and bagged.
To recover quality sand for dam construction, the flotation tailings will enter a thickener circuit which will advance to a sand cyclone system.
The cyclone O/F fines will be deposited in the tailing storage facility after thickening in a slime thickener.
The water recovered from the thickener will be sent to the sand plant process water tank.
As a pH modifier, quick lime will be added to the grinding circuit as a flotation promoter.
Aerophine 3418A will be used as a collector and methyl isobutyl carbinol as a frother. These will be added to the cleaner and bulk rougher flotation circuits.
In the molybdenum circuit, sodium hydrosulphide will be used as a depressant to depress copper.
To promote sedimentation of solids and dewatering, flocculant will be added to tailings thickener, concentrate thickeners, and slime thickener.
Project Infrastructure
The infrastructure required for phase 1 (60kt/d throughput) includes onsite and offsite facilities.
The onsite infrastructure consists of earthworks development, onsite roads, water management systems, site facilities, buildings, and electrical facilities.
The offsite infrastructure includes a site access road, a switch electrical substation, a power line, fresh water supply, pipes, a waste rock storage facility, and a tailing storage facility.
The infrastructure for phase 2 (120kt/d throughput) consists of onsite and offsite infrastructure.
The phase 2 onsite infrastructure includes a processing plant, additional facilities and buildings, earthworks development, water management systems, and site electrical power facilities.
The phase 2 offsite facilities include upgrades in switch electrical substation.
The proposed raw water option for the processing plant is groundwater at a maximum consumption rate of 2,244m3/h for both phases of the project (1,122m3/h for each phase).
The pipes used for the project will be over 1,500m (1.5km) of carbon steel pipe of 610mm diameter and a 2,300m (23km) of SDR 11 HDPE pipe with 762mm diameter.
Power Transmission
The power supply will be provided by an existing 230kVA high voltage power transmission line connecting the Huites Hydroelectric Plant.
The transmission line will be connected to the switch electrical substation and a new transmission line will also be connected to the switch electrical substation which will be routed to a new electrical main substation located at the site of the project.
The electrical substation and new power supply line will supply power for both phases of the project.
From the grid, the high voltage transmission line will connect a new switch electrical substation and a new power distribution line to a new electrical main substation of 230kV/34.5kV onsite.
For phase 2, the new electrical main substation will be expanded depending on the energy demand.
Contractors Involved
Ausenco Engineering USA South Inc. prepared the October 2023 PEA of the Santo Tomás Project
SRK Consulting US Inc. and SRK Consulting Canada (collectively SRK) completed the mineral resource estimate and data verification work of the project.
SRK (Vancouver) conducted work related to deposit type, mapping, exploration, and geological setting of the PEA.
Mining Plus Consulting Ltd. was selected to develop a mine plan based on the mineral resource estimate developed by SRK of the PEA.
A helicopter magnetics survey of the project site was completed by Terraquest Ltd. in March 2021. The independent quality monitoring of the survey was provided by Condor Consulting, Inc. of Lakewood, CO.
In June 2021, the three-dimensional induced polarisation (DCIP) geophysical survey was conducted by Dias Geophysical of Saskatoon, Saskatchewan. Dias conducted a similar survey in November 2020.
A technical report of the project was prepared by Dane A. Bridge Consulting Inc. of Calgary, Alberta in June 2019.
Bateman prepared an update to the 1994 PFS of the project in 2003.
In 2002, IGNA Engineering and Consulting Ltd. was selected to conduct the geological study and evaluation of the project.
The technical data of the project was consulted by Morgain with Cominco Engineering Services Ltd. in 1997-1998.
The Pre-feasibility Study of the project was prepared by Bateman Engineering Inc. of Phoenix, Arizona in 1993.
In 1992, the available exploration data of the project was reviewed by Watts, Griffis a d mcQuat.
