Gold-rich polymetallic vein deposits, and gold-copper porphyry deposits, occur along a northwesterly trend across the southern Dawson Range. Vein mineralization is hosted by lithologic units ranging from the Proterozoic-Paleozoic. Basement Metamorphic Complex, through the mid-Cretaceous Mt. Nansen volcanics, to the Late Cretaceous Carmacks volcanics. The mineralized areas also contain numerous porphyry dykes that are spatially associated with gold veins, and historically have been thought to be genetically linked to them. Dykes belonging to both the Mt. Nansen and Carmacks Groups are present, although Mt. Nansen dykes are the more common. Dykes proximal to mineralized veins are strongly altered to sericite and clay.
Volcanic and subvolcanic rocks of the Mt. Nansen and Carmacks Groups can be distinguished chemically on the basis of their K content; the Mt. Nansen Group is high-K calc-alkaline suite while the Carmacks Group is a shoshonitic suite. Radiometric age determinations constrain the age of the Carmacks Group to approximately 70 Ma, while the age of the Mt. Nansen Group is approximately 105 Ma. K/Ar dates for altered Mt. Nansen dykes, however, range from 94 to 61 Ma, reflecting resetting of Mt. Nansen ages by a Carmacks-age hydrothermal event. This hydrothermal event appears to have been responsible for much of the mineralization in the southern Dawson Range.
Alteration in porphyritic dykes proximal to mineralization is characterized by a strong depletion of Na, reflecting the replacement of feldspar by sericite and clay minerals. Altered dykes also display a general depletion in the ore metals Pb, Zn, and Cu, suggesting that these elements were mobilized from the host rocks during alteration and precipitated in nearby gold-rich base metal veins. As, Sb, and Au, however, appear to have been introduced directly form the hydrothermal fluid.
