The Granny Smith microgrid is powered by a 7.3MW solar plant supported by a 2MW/1MWh battery energy storage system, and a gas power plant. Image courtesy of Aggreko.
The deposit was mined as an open-pit from October 2001 until December 2006. Image courtesy of Geomartin.
The Granny Smith gold mine is an underground gold mining operation located in Yilgarn Craton, Western Australia. Image courtesy of Kecko.

The Granny Smith gold mine is an underground gold mining operation located at Yilgarn Craton in Western Australia. The mine is owned and operated by GSM Mining Company, a wholly-owned subsidiary of Gold Fields.

The Wallaby deposit which is currently the primary focus of mining operations at Granny Smith was discovered in 1998. The deposit was mined as an open-pit from October 2001 until December 2006 while underground mining has been continuing since December 2005.

Gold Fields acquired the Granny Smith gold mine from Barrick Corporation as part of the purchase of the latter’s Yilgarn South operations in October 2013.

In October 2020, Gold Fields completed the installation of one of the world’s largest renewable energy microgrids for the Granny Smith mine as part of its commitment to reduce carbon footprint.

Location and geology

The Granny Smith gold mine is situated within the Yilgarn Craton, approximately 400km north-east of Kalgoorlie in the Laverton district of Western Australia.

The Granny Smith ore body is hosted by the Mt Margaret Dome to the northwest and the Kirgella Dome to the southeast. The north-northwest-trending shear zones flank the domes to the east and west, while the central zone is dominated by north-to-north-northeast-trending sigmoidal shear zones.

Mineralisation and reserves

Granny Smith is an orogenic greenstone gold deposit with mineralisation hosted in lodes comprised of vein stock works localised by a northerly trending shear at the margin of a granodiorite, as well as flat-lying alteration zones present in magnetite amphibole altered conglomerate.

The Granny Smith underground mine was estimated to hold approximately 12.5 million tonnes (Mt) of proven and probable ore reserves grading 5.18g/t Au and containing 2.07 million ounces (Moz) of gold as of December 2019.

Mining operations

The current mining operation at Granny Smith is being carried out at the Wallaby underground mine, which is accessible via a surface portal from the completed Wallaby open-pit. The mining operation is trackless and ore mined at the underground zones are hauled by trucks via the pit ramp to the surface.

The underground operations utilise two primary underground mining methods with minor adjustments to suit localised geometry. The inclined room and pillar (IRP) method is utilised for the mineralised areas with a moderate dip of 10° to 35° and width up to six meters. The thicker zones with variable dips are mined using the transverse long-hole stoping (TLHS) method.

The mining operations also involve lesser-used narrow vein long-hole stoping in some areas with the benefit of reduced planned footwall dilution while thicker zones with 15m or more use bulk long-hole stoping.

Mineral processing

The Granny Smith processing plant has undergone periodic upgrades since its commissioning in 1990. A gravity circuit was introduced in the latest expansion completed in 2015. The process plant currently operates at a capacity of 3.5 million tonnes per annum (Mtpa).

The processing plant consists of a two-stage crushing circuit and a grinding circuit comprising a standard semi-autogenous grinding (SAG) mill and semi-autogenous ball mill crusher (SABC).

The crusher connects to a gravity circuit that feeds a leach/carbon in pulp (CIP) train. The gold absorbed on to the activated carbon via the CIP train is then treated in a pressure Zadra elution circuit to desorb gold from the carbon. The processing plant has an overall metallurgical recovery rate of approximately 92.6%.

Granny Smith microgrid

The Granny Smith microgrid, hybrid power system comprises a 7.3MW solar power plant integrated with an existing 22MW gas-fired power station. The solar facility comprises 20,000 solar panels and is supported by a 2MW/1MWh battery energy storage system.

The microgrid is expected to reduce the mine’s fuel consumption by up to 13% with approximately 18Gwh of energy produced from the solar, thermal, and battery storage assets, while also reducing annual carbon emissions by approximately 9,500 tonnes (t).

The solar-gas hybrid power station will meet the 24.2MW daily power requirement of the mine, with 8MW allocated to the Wallaby underground mine and the remaining 16.2 MW allocated for the processing plant, associated facilities, and the mine accommodation camp.

Contractors involved

The contract for the installation of the Granny Smith microgrid was awarded to Aggreko, an energy services company based in the UK.