The Moriarty Lake occurrence area is underlain by biotite- hornblende granodiorite of the Early to Middle Jurassic Island Plutonic Suite unconformably overlain by the Upper Cretaceous Nanaimo Group consisting of pebbly sandstone, siltstone and mudstone. The Nanaimo Group is intruded by thick dacite sills of the Late Eocene to Early Oligocene Mount Washington Intrusive Suite. The sedimentary sequence dips gently to the northeast and has been transected by an east trending fault, the Moriarty fault. The Moriarty fault has acted as a feeder zone for the intrusion of the dacite sills and is the locus of several stages of dyke emplacement and brecciation associated with locally intense hydrothermal alteration and sporadic sulphide miner- alization. The alteration appears to be vertically zoned, with prop- ylitization and silicification dominating where the fault cuts base- ment granodiorite and clay-carbonate alteration in the overlying Nanaimo Group sediments and Tertiary sills.

The main showing is located in the fault zone in intensely carbonated and clay altered dacite at the base of a major sill, about 40 metres vertically above the Cretaceous unconformity. It is exposed in a creek gully where intensely altered carbonated rock carrying trace pyrite is intermittently exposed for 78 metres. The lower 30 metres of the section appears to consist of intensely altered sandstone with an outcrop of intensely altered dacite near the base. The upper part of the section is altered dacite within which the main showing is located just above the contact with the sandstone. The mineralized zone consists of a 12 metre section of intensely altered dacite with ankeritic carbonate veins 1 centimetre wide. The veins carry pyrite, sphalerite, galena, chalcopyrite and tetrahedrite. These minerals also occur in disseminated zones up to 18 centimetres wide in altered dacite with irregular ankeritic gashes. The veins generally strike 025 degrees and dip 75 degrees southeast and appear to be tension gashes en echelon to the Moriarty fault which is inferred to lie on the south side of the showing. This suggests a component of left-lateral movement on the fault. A grab sample assayed 1405.4 grams per tonne silver, 1.7 per cent zinc, 0.2 per cent copper and 0.04 per cent lead (Assessment Report 10025).

Minor disseminated pyrite and chalcopyrite occur in both silty mudstone and dacite where the Moriarty fault juxtaposes the two units about 600 metres east of the main showing. A rock sample assayed 0.14 per cent copper (Assessment Report 10025).

Further east, pebbly sandstone contains minor disseminated pyrite and traces of chalcopyrite associated with ankerite veins about 200 metres south of the Moriarty fault.