Limestone is quarried by the International Marble and Stone Company (Imasco) 3 kilometres southwest of Benson Lake, 26 kilometres south-southwest of Port McNeill.

The quarry is situated within a discontinuous belt of limestone of the Upper Triassic Quatsino Formation (Vancouver Group) that extends from Quatsino Sound southeastward for 120 kilometres to Tlupana Inlet. The belt is segmented by a number of northeast trending faults. Bedding at the quarry strikes 120 degrees and dips 30 degrees northeast.

The quarried limestone is massive, fine grained, and white in colour with a brightness of 95.5 per cent (Fieldwork 1985, page 239). One third of the exposed limestone is extensively fractured. A series of randomly collected samples from the quarry averaged 55.1 per cent CaO, 2.49 per cent MgO, 0.27 per cent SiO2, 0.05 per cent Al2O3, less than 0.09 per cent Fe2O3, less than 0.003 percent MnO, less than 0.01 per cent TiO2 and 43.97 per cent ignition loss (Fieldwork 1985, page 239).

Several kilometres to the southeast in the vicinity of Iron and Truite lakes, the limestone is generally light grey and fine grained, although an outcrop on the northern tip of Truite Lake displays black limestone that is highly fractured and veined with white calcite. A sample of chips collected at 6.1 metre intervals along 152 metres of roadcut on the south shore of Iron Lake analyzed 54.60 per cent CaO, 0.58 per cent MgO, 1.10 per cent insolubles, 0.31 per cent R2O3, 0.12 per cent Fe2O3, 0.013 per cent MnO, 0.01 per cent P2O5, 0.11 per cent sulphur, 43.62 per cent ignition loss and 0.25 per cent water (Minister of Mines Annual Report 1968, page 318, Sample 16).

Imasco has been quarrying limestone at Benson Lake since late 1985. The limestone is shipped to Surrey, British Columbia, where it is crushed and ground at the company's plant for a variety of products including white extenders and fillers. Between 1985 and 1987, 26,807 tonnes of limestone were quarried. In 1990, the company mined 40,000 tonnes (Z.D. Hora, personal communication, 1991).