Thunderbox Gold Project, Western Australia

The Thunderbox gold project includes the Thunderbox, Rainbow and Otto Bore gold deposits in Western Australia.

The project is developed as both an open-pit mine and an underground mine.

The Thunderbox deposit was discovered in 1999 and is one of the significant gold deposits discovered in Australia.

The project was acquired by Saracen Mineral from Norilsk Nickel in 2014. First gold was poured in early 2016 while commercial production was achieved in April 2016. Saracen Mineral was acquired by Northern Star under a merger-of-equals agreement signed in October 2020.

Thunderbox gold project location

The Thunderbox gold project is part of the Thunderbox operations (TBO), which also includes the Kailis, Bannockburn and Waterloo projects.

Thunderbox is situated at the southern end of the Yandal greenstone belt while TBO is located in the Yandal belt and the Agnew-Wiluna belt in the North Eastern Goldfields. The Thunderbox open-pit mine and carbon-in-leach (CIL) gold treatment plant are located 45km south of Leinster.

Thunderbox project geology and mineralisation

Thunderbox is a mesothermal lode gold deposit situated in an area where multiple major shear zones meet with the Perseverance Fault. The deposit contains three main mineralised zones, known as A, C, and D.

Mineralisation in the deposit is present in the form of heavily distorted, silicified and carbonate-altered albite-quartz porphyry in the hanging wall of the 200m-wide shear zone. The mineralisation is mainly restricted to the intrusive porphyry.

Reserves at the Thunderbox deposit

The Thunderbox deposit is estimated to contain proven and probable ore reserves of 40.94 million tonnes (mt), grading 1.6gpt of gold, as of March 2024. The total contained metal is estimated at 2.14 million ounces.

Mining at Thunderbox gold project

Mining operations at the Thunderbox gold project commenced in July 2015. The primary ore feeds come from the Thunderbox open-pit and underground operations. Supplementary feed is derived from the Otto Bore, Wonder and Orelia mining centres.

Thunderbox utilises cost-effective bulk methods through both open pit and underground operations. The open-pit mining fleet involves the use of a backhoe-configured excavator, supported by a fleet of rear dump trucks.

The underground operation is a modern, mechanised, trackless mine that employs long-hole open-stoping methods tailored to the width of the ore zones. Recently, paste fill has been introduced to enhance ore recovery and maintain stability in the mining areas.

Otto Bore is a small satellite open pit operation located approximately 10km north of TBO and employs conventional truck and shovel operations.

The Wonder underground mine is located approximately 25km south-east of the TBO processing centre. Mining commenced in November 2023 with operations conducted via conventional jumbo lateral development using long-hole open-stoping methods.

The Orelia (Bronzewing operation) is an open-pit mining operation undertaken using a conventional truck and excavator. All ore from Orelia is trucked to the Thunderbox processing centre.

Processing at Thunderbox project

The ore reserves from the mine undergo treatment at the Thunderbox CIL cyanide leach plant with a gravity circuit.

The processing plant was kept on care and maintenance since 2007 and has been operating at a nameplate capacity of 2.5 million tonnes per annum (mtpa) since May 2016. The processing facility was expanded from 2.9mtpa to 6mtpa in 2022.

The upgrade of the Thunderbox mill included enhancements to the conventional crushing circuit and reclaim area, as well as improvements to the grinding and milling circuit, pebble crushing, leaching, elution, tailings management, reagents, and associated services.

Site infrastructure at Thunderbox project

Saracen acquired the existing infrastructure, including a modern accommodation camp, a gravel airstrip, a Goldfields Gas pipeline spur and the CIL ore processing facility. Contract power supply, water supply from borefield and telecommunication services were also part of the acquisition.

The mine site is linked to Leinster via a 2km sealed highway.

Saracen developed a powerhouse with seven gas-fired generators and four dual-fuel units.

Contractors involved

SRG Global was awarded a $70m contract in March 2020 to provide drill and blast services at the mine for five years.

Kalgoorlie Power Systems (KPS) was contracted to rewire the powerhouse while BLA Burgess undertook the re-installation of the SAG and ball mills.

Maca Interquip was contracted for plant refurbishment while SMS Rental (WA) won a contract for the supply and maintenance of run-of-mine (ROM) loaders.

Northern Star Resources (NST) awarded GR Engineering the contract for the design, procurement, construction, and commissioning of the expansion of the processing facilities at its Thunderbox operation.