The Thunderbolt showing is underlain by chlorite schist, mica schist and quartzite of the Hadrynian? to Paleozoic Eagle Bay assemblage. Two adits have been driven and several pits (shafts) have been sunk on narrow, pyritic quartz veins in the chlorite and mica schists. The veins assayed trace gold.

One adit is about 91 metres above the lake, the other on the lakeshore. The upper adit is 7.6 metres long, the lower one 18.2 metres long. Near the lower adit, on the north side, is an outcrop of impure iron ore in mica schist which, several years ago, a pit was sunk 3.6 metres wide by about 3.6 metres long at its mouth, tapering to 1.2 by 1.5 metres at a depth of 2.4 metres, below which sinking had been continued, but to an unknown depth due to water (Minister of Mines Annual Report 1913, page K205).