While maintaining its primary focus on gold development and production, Horizon Minerals Ltd (ASX:HRZ) is assessing longer-term opportunities presented by the Nimbus Silver-Zinc Project adjacent to its cornerstone Boorara Gold Project in WA’s Goldfields.
After a concept study confirmed the optimal economic development pathway for Nimbus and with silver trading at A$42 per ounce and zinc at A$4,000 per tonne, Horizon is examining the possibility of enhancing its gold strategy.
Nimbus resource
The current mineral resource estimate for Nimbus stands at 12.1 million tonnes grading 52 g/t silver for 20.24 million ounces and 0.9% zinc for 106,000 tonnes. Within this is a high-grade subset immediately below the historical pits of 260,000 tonnes grading 774 g/t silver for 6.4 million ounces and 12.8% zinc for 33,000 tonnes.
There is also a potential VMS exploration target of between 550,000 tonnes and 700,000 tonnes with a grade range of 3.4-3.6% zinc and 140-210 g/t silver with minor lead (0.45-0.65%), copper and gold.
A Program of Work (PoW) has been approved to drill the exploration target and once cashflows from gold production at Boorara and Phillips Find are well advanced, in addition to drilling for gold extensional and new discovery drilling, funding will be directed to test the exploration target below the current high-grade Nimbus ore zone.
Potential to grow core
Horizon managing director and CEO Grant Haywood said: “We see the potential to grow the higher-grade core within the Nimbus resource at depth down plunge and along strike.
"Whilst we are firmly focused on delivering on gold production at Boorara and Phillips Find, the company will continue working to develop a longer-term production profile at Nimbus.
"We look forward to undertaking drilling in 2025 with the aim of increasing the resource prior to re-instigating a feasibility study for the project.”
Kalgoorlie project area locations and surrounding infrastructure.
Nimbus history
Nimbus is 17 kilometres east-southeast of Kalgoorlie-Boulder, just 2 kilometres east of Horizon’s cornerstone Boorara Project and 6.5 kilometres north-northwest of Golden Ridge. Both are historic gold mining centres and the company is recommencing production activities at Boorara.
The project was mined by Polymetals in two stages. Phase 1 from January 2004 to April 2005 concentrated on mining extremely weathered oxide material in the Discovery and East Pits and phase 2 from November 2005 to May 2006 concentrated on mining remnant oxide and supergene material from the Discovery Pit.
Ore treatment was undertaken at an onsite mill utilising a Merrill-Crowe circuit and the project was placed on care and maintenance in 2007 after producing 3.6 million ounces of silver from 318,000 tonnes of ore processed at a grade of 353 g/t silver.
MacPhersons Resource Ltd (MRP) completed due diligence in August 2011 and purchased the property in September 2011 from Kalgoorlie Ore Treatment Company Pty Ltd in order to treat ore hauled from their proposed Coolgardie gold mining operations, 55 kilometres to the west.
MRP fast-tracked the development of the Nimbus project, securing financing and purchasing necessary plant infrastructure.
In June 2019 the former Intermin Resources Ltd finalised its merger with MRP, becoming Horizon Minerals with the Nimbus project put on hold pending review.
Clean-up completed
Following the merger, Horizon undertook a Mandatory Review Audit (MAR) process under the Contaminated Sites Act to carry out cleanup works to reclassify the site.
This clean-up was completed in 2022 and the MAR report is nearing completion to submit to the Department of Water and Environmental Regulation (DWER) to reclassify the site.
The old plant was removed and the area rehabilitated. The mine office and other infrastructure is still in place and operational and on grid power.
An independent review by Australian Mining Consultants (AMC) and Sedgmen was commissioned in 2022 to assess the historic data, mine optimisations and metallurgical work to evaluate the optimal pathway forward for the project.
This highlighted the potential of the project through underground mining the fresh higher-grade ore below the historic Discovery pit and generating a silver and a zinc concentrate for sale to potential offtake partners.
A gold concentrate was also assessed to improve overall life of mine given the need for additional tonnage to underpin a concentrator at site.
Drilling planned
Given drilling successes to date along the Gretel-Nimbus-Brindabella trend and the potential for repeat high grade lenses at Nimbus deeps and along strike, a drilling campaign has been designed to begin in the first half of 2025 pending cashflows from gold mining operations.
The aim of this is to firm up the conceptual exploration target below the current high-grade Nimbus ore zone.