The Sproat Lake limestone occurrence extends southeast from the south shore of Two Rivers Arm of Sproat Lake for 6.5 kilometres, 12 kilometres west of Port Alberni.

A 60 metre thick bed of limestone of the Upper Triassic Vancouver Group, Quatsino Formation is irregularly exposed over 6.5 kilometres. The bed outcrops as two irregular northwest trending bands up to 3 kilometres apart; a result of being folded into a northwest trending syncline. The southwest limb is segmented by several parallel northwest trending faults. The limestone is overlain by black argillite of the Parsons Bay Formation and underlain by andesites and basalts of the Karmutsen Formation, both units of the Upper Triassic Vancouver Group.

The bed is composed of fine-grained, dark grey limestone that weathers light grey. The limestone contains thin calcite veinlets and scattered small siliceous patches in addition to scattered lenses and nodules of black chert. A few narrow mafic dykes intrude the limestone.

In 1996 through 2006, SYMC resources prospected the area as part of the of the Macktush Property along the west side of the Alberni Inlet, now called the Dauntless property. In 2009 through 2011, G4G Resources completed programs of geological mapping and rock and soil sampling on the area as the Macktush Copper property. In 2012, Nahminto Resources Ltd. completed a satellite remote sensing survey and analysis on the Macktush group and continued prospecting, geological mapping and rock sampling programs from 2012 through 2014. A similar prospecting, geological mapping and rock sampling program was conducted on areas of the Macktush property for World Organics Inc. in 2015.