CRETACEOUS TO TERTIARY MAGMATISM AND ASSOCIATED MINERALIZATION IN THE LIME HILLS C-1 QUADRANGLE, WESTERN ALASKA RANGE

The Alaska Division of Geological & Geophysical Surveys Mineral Resources Section conducted geologic mapping through the state-funded Airborne Geophysical/Geological Mineral Inventory. The study area lies in the Styx River and Farewell geophysical survey tracts, located 100 miles northwest of Anchorage in the western Alaska Range. Ongoing mineral exploration is focused on porphyry copper ± molybdenum ± gold, reduced intrusion-related gold, and polymetallic vein deposit types. Porphyry systems such as Whistler and Copper Joe have distinctive positive magnetic signatures and electrical conductivity highs in geophysical surveys and geophysical models. The majority of these mineral occurrences are related to several overlapping Cretaceous and Tertiary plutonic complexes, dike swarms, and volcanic fields.

The Tertiary Merrill Pass, Mount Estelle, and Crystal Creek plutons, Tertiary McKinley sequence granites, and Cretaceous South Fork gabbro intrude the Late Jurassic to Early Cretaceous Kahiltna flysch assemblage. New ⁴⁰Ar/³⁹Ar biotite and hornblende ages of 60.7 ± 0.2 and 60.5 ± 0.5 Ma, respectively, from the copper-gold-bearing Tertiary Mount Estelle diorite differ from previous ~70.1 to ~66.7 Ma ages to the northeast, extending the time span of plutonism in this large complex. An age of 62.6 ± 0.4 on sericite from a Mount Estelle quartz gabbro with anomalous silver and copper that lies near a northwest-trending dike swarm suggests that post ~70 Ma plutons may have mineral potential. Contemporaneous intrusions include the chemically distinctive Cretaceous South Fork pluton with new biotite and hornblende ages of 62.2 ± 0.3 and 62.5 ± 0.4 Ma, respectively. Sericite alteration ages of 11.3 ± 0.1 and 10.5 ± 0.1 Ma from the Copper Joe prospect copper-molybdenum porphyry greatly extend known Western Alaska Range Cu-Au-Mo mineralization into the Neogene. The altered felsite wall rock is cut by small quartz veins and has a pervasive, texturally-destructive quartz-sericite-pyrite overprint suggestive of sustained hydrothermal alteration. Further work is ongoing to determine if this mineralization event is related to continued magmatism and/or fault activation, but the newly dated sericite highlights that the Western Alaska Range mineral district has been affected by tectonic processes during the Miocene.