Cerro Moro is an epithermal low- sulphidation, gold-silver deposit comprising at least 9 NW-SE trending high-grade quartz-adularia veins 1−5 m in width and 0.5−2 km in length. Proven and probable reserves were estimated at 529,000 oz gold and 30.5 Moz silver in December 2019.
In 2016, SRK started a conceptual- level geochemical characterisation and design of waste rock dumps to prevent future acid rock drainage and metal release. Based on the 2016 mine plan, approximately 20 Mt of waste rock will be extracted from the open pits and 580,000 t re-mobilised in the underground mines. The 2016 mine plan specified that waste material would be deposited in 11 rock dumps ranging from around 300,000 t to 5.5 Mt.
Characterisation and design was carried out by an interdisciplinary team from the SRK’s Argentina and UK offices. The work program included:
- Representative sampling: (1) definition of geo-waste units based on alteration mineralogy; (2) spatial pre-sampling through 3D visualisation of the geo- waste units on LeapfrogTM; (3) on-site sampling from drill cores and waste rock.
- Static geochemistry and mineralogy: acid drainage potential, short-term leaching, sequential extraction, optical mineralogy.
- Kinetic geochemistry: up to 32-week humidity cell tests, scaled to site conditions.
- Based on results from geochemical characterisation: (1) estimation of a conservative on-site theoretical net potential ratio from Stotal, Ca and Mg analyses from the Cerro Moro’s database; (2) improved definition of waste material and incorporation into the mine plan via a mining engineering software.
- Conceptual rock dump design: (1) distribution of acid- and non-acid- forming material in the dumps; (2) water management of contact and non-contact water at the dumps.
Total sulphur contents are generally low (<2.5%) but highly variable between vein sectors. Sulphide is the dominant sulphur species. Acid drainage and metal leaching potential at Cerro Moro were considered to be low. However, high proportions of acid-forming material were found in two veins: 70% in Escondida Far West and 100% in Zoe.
The proportion in other veins range from 10% to 40%. Considering the semi-arid climate and the low acid-forming potential, onset for acid drainage production is considered to be delayed, but may occur several years after project closure. Further geochemical characterisation and modelling for leachate chemistry predictions were recommended to update rock dump designs to a pre-feasibility level.
