On the Lord Roberts prospect a number of shallow open cuts have exposed a body of magnetite which varies in grade and apparently follows the somewhat irregular contact between rocks of the Middle to Late Jurassic Nelson Intrusions and sediments of the Pennsylvanian to Permian Mount Roberts Formation. The Mount Roberts Formation rocks consist of interbedded siltstone, quartzite, greywacke, slate, pebble conglomerate and limestone. They are intruded by a stock comprised of granodiorite to quartz diorite.
The massive blue-grey magnetite (occurring on the Lord Roberts claim) ranges up to about 10 metres in width and hosts disseminated pyrite, pyrrhotite and chalcopyrite. In 1920, a sample of the best grade of magnetite assayed: iron, 54 per cent; phosphorus, nil; and sulphur, 2.69 per cent (Minister of Mines Annual Report 1920, page 137). A sample of the magnetite with disseminated chalcopyrite and pyrrhotite was assayed for copper, and gave negative results. Hornblende, epidote, garnet, feldspar and quartz were detected in the bluish magnetite. Magnetite and pyrite are abundant near the footwall and pyrrhotite and chalcopyrite near the hanging wall. In the lowest shaft, exposed magnetite hosts pyrrhotite, pyrite, bismuthinite(?) and chalcopyrite in a quartz-hornblende-feldspar gangue.
In 1924, 7 tonnes of ore were shipped which produced 187 grams of silver and 22 kilograms of copper.