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The Rusty Mountain area is underlain by sedimentary strata of the Paleoproterozoic Wernecke Supergroup, Mesoproterozoic Pinguicula Group, Neoproterozoic Hematite Creek Group and Windermere Supergroup, and Paleozoic Bouvette Formation. Three suites of intrusions are documented: (1) 10–200 m thick, subalkaline, mafic sills and dikes of the ca. 1380 Ma Hart River suite intrude the Wernecke Supergroup; (2) 2–3 m wide, vertical, east-west striking, alkaline, mafic dikes that are geochemically distinct from the Hart River suite intrude the Wernecke Supergroup; and (3) a 30 cm thick, mafic, porphyritic dike intrudes the Wernecke Supergroup at one locality. The main structures in the Wernecke Supergroup are northwest-verging folding and thrusting and a steeply dipping axial-planar cleavage. This deformation affected the Hart River sills, but not the east-west striking dikes. The main structures in the Pinguicula Group and younger strata are northwest-southeast trending gentle folds and a steeply dipping axial-planar cleavage.

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The NTS 106B/04 map area straddles the upper reaches of the Stewart River in east-central Yukon. The area north of the Stewart River is underlain by Ediacaran clastic and carbonate continental slope deposits of the uppermost Windermere Supergroup, and by Ediacaran-Cambrian rocks of the Hyland Group (Selwyn basin). The area south of the Stewart River is dominated by the Cambrian Gull Lake Formation and Cambrian (-Silurian?) volcanic rocks of the Old Cabin Formation. The main structures in 106B/04 define an arcuate pattern; they are oriented NW-SE in most of the area, but are approximately E-W in the westernmost part of the map area. These structures include upright, gently-plunging folds and steeply-dipping, axial-planar cleavage. Folding was locally accompanied by thrusting. Late structures include a steeply-dipping sinistral fault that transects the central part of the map area and a number of NW-WNW-striking normal (± dextral) faults. Stratigraphic relationships suggest correlation of the upper Yusezyu, Algae, and Narchilla formations of the Hyland Group (Selwyn basin) with the upper Blueflower, Risky, and Ingta formations of the Windermere Supergroup (Ogilvie and Mackenzie platforms). Gold mineralization has recently been discovered in the Algae Formation, which has also been explored for Mississippi Valley-type lead-zinc-silver mineralization elsewhere in the area.

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