GEOLOGIC MAP OF THE LIME HILLS C-1 QUADRANGLE WITH NEW GEOCHRONOLOGY, GEOCHEMISTRY, AND PETROLOGY

The Alaska Division of Geological & Geophysical Surveys Mineral Resources Section conducted geologic mapping in the Lime Hills C-1 Quadrangle through the state-funded Airborne Geophysical/Geological Mineral Inventory. The map area is 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. Lead-zinc skarns, molybdenum-bearing quartz veins, sediment-hosted base-metal, platinum-group-element, and rare-earth-element deposit types are also present within the Styx River, Farewell, and Dalzell Creek geophysical survey tracts. The majority of these mineral occurrences are related to numerous Cretaceous and Tertiary plutonic complexes, dike swarms, and volcanic fields.

Previous mapping distinguished five plutons: Tertiary Merrill Pass, Mount Estelle, and Crystal Creek; Cretaceous South Fork; and Tertiary McKinley sequence granites. These intrude the Late Jurassic to Early Cretaceous Kahiltna flysch assemblage. Based on airborne magnetic data in conjunction with new trace element data, the large, previously undifferentiated Merrill Pass pluton can be broken into two distinct plutons. New field observations indicate one of the Merrill Pass subdivisions predates the large Eocene to Oligocene volcanic field while the other postdates the volcanic field. The volcanics also separate into two groups with different paths of evolution. Plutons are oriented along elongate north-south trends, as are poorly exposed faults which parallel the large valleys throughout the region. Northwest-trending faults and mafic and felsic dikes crosscut the plutons.

New ⁴⁰Ar/³⁹Ar ages on biotite and hornblende of 60.7 ± 0.2 and 60.5 ± 0.5 Ma, respectively, from the southern Mount Estelle diorite differ from previous ~70.1 to ~66.7 Ma ages to the northeast, extending the span of plutonism in this large complex. 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. A previously undifferentiated diorite has an identical trace element composition to the South Fork pluton and was originally contiguous before being cut by an Eocene to Oligocene caldera complex.