The Cretaceous Horsethief Batholith intrudes sediments of the Middle Proterozoic Mount Nelson Formation, Purcell Supergroup. The intrusion is zoned from fine-grained to medium-grained granodiorite to coarse-grained quartz monzonite. In the area of the Alder 16 showing, the Horsethief batholith is predominantly quartz monzonite.

Canadian Johns-Manville Co. Limited initiated a biogeochemistry survey along Horsethief Creek in the winter of 1970-71. They followed up with a major biogeochemical survey in late 1971 after the staking of the Alder and Talus claims. An induced polarization survey was carried out and 4 diamond drill holes were drilled. Drilling showed the sedimentary rocks to consist of banded quartzite, dolomitic limestone and hornfels. Disseminated sulphides, such as pyrite, pyrrhotite with trace chalcopyrite and galena are reported to be common, averaging 1 per cent but up to 10 per cent locally. Chalcopyrite was found as finely disseminated blebs in quartzite boulders and in quartz veins in diabase boulders. Traces of disseminated chalcopyrite in banded quartzite and dolomitic limestone were observed in drill hole HTC-71-1 which was drilled in order to test an IP anomaly.