The Kilbella Bay Limestone occurrence is a band of very pure limestone or marble, located west of the entrance of Kilbella Bay, 8 kilometres west of the head of Rivers Inlet on its north shore. The limestone also contains a seam of magnetite in which a 30-metre deep shaft was dug before it was abandoned in the early years of the twentieth century (Geological Survey of Canada Summary Report 1908).

The area is underlain predominantly by granitic to dioritic gneisses and intrusive rocks of the Jurassic to Tertiary Coast Plutonic Complex (Geological Survey of Canada Summary Report 1908, Map 1386A). Within these rocks is a band of recrystallized limestone, more appropriately called marble (Geological Survey of Canada Summary Report 1908). This marble is white, very coarse grained, and is 30 to 45 metres thick (Geological Survey of Canada Map 92A; CANMET Report 811). The band trends north, and has inclusions of granite and greenstone, which also occur outside the marble.

A sample taken across the marble band was analyzed at 97.09 per cent calcium carbonate (calcite), indicating a high purity; in addition, there was 1 per cent silica and 1.19 per cent magnesium carbonate (CANMET Report 811).

Within the marble is a seam of massive magnetite, up to 0.3 metre thick, which is apparently traceable for 550 metres (Geological Survey of Canada Summary Report 1908). Smaller veins of magnetite may also be present. Minor pyrite and pyrrhotite are associated with the magnetite.