Updated PFS Boosts Economics at Vista Gold’s Mount Todd Project

Vista Gold has reported positive results from an updated preliminary feasibility study (PFS) of its 50,000-metric-ton-perday (mt/d) Mount Todd gold project in Northern Territory, Australia. The PFS is based on the results of a comprehensive review of all aspects of the project and re-design of elements of the process flowsheet that incorporate automated sorting and grinding circuit design changes.

The process improvements have reduced operating costs and increased gold recovery and gold production for Mount Todd as compared to a May 2013 PFS. Vista management believes that the design changes will allow Vista to achieve a significant improvement in the project’s economics at the current gold price.

Mount Todd has estimated proven and probable reserves of 5.85 million ounces (oz) of gold in 221 million mt grading 0.82 grams per mt at a cut-off grade of 0.40 g/mt. The project is designed to be a conventional, owner-operated, openpit mining operation utilizing large-scale blast-load-haul equipment. Average annual production is estimated at 382,211 oz/y of gold over a 13-year mine life, including an average of 479,450 oz/y during the first five years of operation. Life-of-mine average cash costs are estimated at $645/oz, including average cash costs of $571/oz during the first five years of operation.

Initial capital requirements are estimated at $839 million, down from $1.04 billion in the 2013 PFS. The updated Mount Todd PFS includes significant design changes in the comminution circuit. Vista now plans to screen the high-pressure grinding roll (HPGR) product at 16 mm, followed by automated sorting technology (both X-ray transmission and laser) to remove material that is sub-economic. Testing indicates that approximately 10% of the run-of-mine plant feed can be eliminated using automated sorting with only a 1.3% gold loss.

Excess capacity in the HPGR circuit will be employed to create a final product that is nominally 3.5 mm, or approximately 50% smaller than previously planned. This material will be wet screened and then classified to provide feed to the redesigned primary and secondary grinding circuits. Vista has selected smaller, more energy-efficient ball mills for the primary grinding circuit and horizontal IsaMills for the secondary grinding circuit. Together, these mills are expected to efficiently produce a final product that is nominally 60 microns in size, a 33% reduction from previous studies.

Leaching tests conducted at the finer grind size indicate that an overall gold recovery of 86.4% net of solution losses can be expected, a significant improvement from the 81.7% recovery in the previous PFS. This results in a 4.1% increase in total gold production net of sorting losses. The robust comminution circuit is designed to process material with an average Bond work index 5% greater than actual rock hardness. Gold will be recovered through a traditional carbon-in-pulp circuit.

The Mount Todd project is located at the site of a previously operated mine, so in-place infrastructure reduces initial capital requirements and significantly reduces capital risk related to infrastructure construction. This infrastructure includes an existing tailings storage facility that will receive two raises and is expected to contain 62 million mt of processed material; an existing fresh water storage reservoir that will receive a 2-m dam raise and will harvest storm water sufficient to provide process water for year-round operations; a natural gas pipeline that can supply sufficient natural gas to meet the project’s energy requirements; and a paved road to site.

The Stuart highway is less than 15 km from the project site, and the regional center of Katherine, population approximately 12,000, is less than 60 km from the site. The city and port of Darwin are about 300 km from site. The updated Mount Todd PFS was authored by Tetra Tech Inc., with input from Mine Development Associates, Resource Development Inc., Proteus EPCM Engineers (a Tetra Tech company), and POWER Engineers Inc.

“The results of this updated PFS demonstrate that Mount Todd is a robust project at today’s gold price and foreign exchange rate,” Vista President and CEO Frederick H. Earnest said. “We believe it is a world-class project that enjoys long-standing government and community support.”

Earnest added that the project has received the last major environmental permit required. He believes this coupled with the updated PFS results provides a solid foundation for continued advancement of Mount Todd.

As featured in Womp 2018 Vol 03 - www.womp-int.com