Eritrea – Nevsun Resources, the dual-listed base metals miner, has conducted studies evaluating the potential for underground mining at its Bisha project and Harena satellite deposit and both have proven highly feasible.
In Q2, 2015, Nevsun commissioned SRK Consulting (Canada) to prepare an internal scoping study of the potential for underground mining at Bisha and Harena as well as to conduct a life-of-mine optimisation study considering all Bisha regional resources.
This optimisation study used inferred mineral resources, no mineral reserves have been estimated.
The scoping study indicated that underground mining at Bisha is highly plausible and that some of the material currently in the open pit mine plan may be more economic if extracted by underground bulk mining methods as compared with high strip ratio open pit cutbacks.
An exploration drive would be required to further assess this opportunity with increased drilling to upgrade mineral resource classifications and further investigate geotechnical, hydrology and metallurgical rock characteristics.
A decision to drive an exploration ramp up, most likely in H2, 2017, will be made following further exploration drilling and metallurgical and geotechnical test work which will be conducted from within the Bisha pit during 2016.
The scoping study also confirms the correct decision last year to halt the cutback (Phase 9) of waste stripping for the Bisha open pit.
If further work at Bisha main pit demonstrates that the upside potential is limited under the current economic and technical assumptions, waste stripping could be restarted with minimal disruption to the existing mine plan provided this happens over the next one to two years.
For Harena, the underground mining scoping study used an interim mineral resource calculated mid-2015 which has since been superseded by the current larger resource.
The study showed a potentially mineable deposit with marginal economics that was sensitive to metal prices and metallurgical recoveries. Therefore, additional metallurgical drilling at Harena is planned during 2016 before making a decision to invest in an exploration decline which would be required to take the deposit through feasibility.
An updated scoping study including this work is expected in early 2017.
Resource growth through drilling in 2015
Nevsun Resources has also announced increased minerals volumes in it updated mineral resource estimate effective 31 December 2015, for both the Bisha and nearby Harena satellite deposits.
- Increased Bisha district inferred resources which now contain 426 Mlb copper, 1.16 billion pounds zinc, 531 000 oz gold, and 18 Moz silver.
- Increased Bisha district measured and indicated resources which now contain 959 Mlb copper, 3.56 billion pounds zinc, 840 000 oz gold, and 46 Moz silver.
- 15% increase to Harena primary indicated resources, now 3.7 Mt at 0.86% copper, 3.1% zinc, 0.6 g/t gold and 27 g/t silver.
- 69% increase to Harena primary inferred resources, now 10.9 Mt at 1.45% copper, 4.0% zinc, 1.0g/t gold and 41 g/t silver.
- Bisha and Harena remain open at depth with further drilling on-going in 2016.
- Positive Bisha and Harena underground scoping study, more investment warranted.
“Our drilling at Harena continues to produce significant increases in the total resource available for the Bisha operation. We anticipate that further drilling will continue to grow the deposit at depth. Our exploration drilling was also successful in discovering new massive sulphide mineralization at Asheli, highlighting the prospectivity of the Bisha District to host additional resources,” says Cliff Davis, Nevsun Resources CEO.
He continues: “The underground scoping study demonstrated the potential to extend Bisha Main life by transitioning to underground mining in the future and the potential to provide additional feed to the Bisha mill from a Harena underground mine. In 2016, we will further assess these opportunities through additional investment.”
Resource growth due to 20 308 m of new drilling at Harena was the most pronounced development. The new drilling also led to an improved understanding of the geology and mineral zonation used in the resource estimation process.
Indicated resources at Harena grew by 480 000 t or 15% from the previous year. Of most significance, the inferred resources at Harena increased by 4.5 Mt or 69% from the year earlier.
At Bisha, a relatively modest 2 819 m of drilling was completed in 2015. Despite the lower commodity prices used in 2015 as compared with 2014 for mineral resource calculations, the measured and indicated resource at Bisha remained essentially unchanged from the year previous while the inferred resource decreased slightly by 0.3 Mt.
Year on year, the Bisha measured and indicated resources of contained zinc, gold and silver increased by 5%, 11% and 11%, respectively, helping to offset the 25% reduction in copper driven mainly by mine depletion of the supergene zone.