RECENT GEOLOGIC MAPPING IN THE EASTERN BONNIFIELD MINING DISTRICT, ALASKA

Geologic mapping in 2008 by the Alaska Division of Geological & Geophysical Surveys (DGGS) will result in a new 1:50,000 scale geologic map that will cover a key section of the northern foothills of the Alaska Range.

Green meta-siliciclastic rocks and phyllites of the Healy Creek Schist and gray meta-siliciclastic rocks and graphitic schist of the Keevy Peak Formation are interfolded in alternating east–west-trending overturned synclines and anticlines in the southern half of the map area. Meta-sedimentary and meta-volcanic rocks of the Totatlanika Schist, cover most the northern part of the map area. The metavolcanic rocks comprise a bimodal, within-plate suite of alkali basalt, rhyodacite, and Zr- Nb-, and Y-enriched peralkaline rhyolite ranging from 373 to 357 Ma. We subdivide the Totatlanika Schist into nine intercalated lithologic sub-units based on observed relict textures and mineralogy supported by lithogeochemistry. Hand-held XRF determinations of Zr and Nb provide the only reliable method to distinguish peralkaline meta-rhyolite from other felsic meta-igneous and silicic meta-sedimentary rocks in the field.

Cretaceous porphyritic monzonite intrusions in the north are associated with prominent aeromagnetic anomalies. In contrast, Cretaceous equigranular granodiorite intrusions to the south are not associated with aeromagnetic anomalies. Tertiary mafic and felsic dikes intrude east-west and northeast-trending structures throughout the field area.

Tertiary sediments occupy fault-bounded asymmetric synclines in the northern map area, and include coal-bearing, poorly consolidated Healy Creek Formation of the Usibelli Group and the Nenana Gravel. Slivers of coal-bearing Healy Creek Formation are also present in the Kansas Creek fault along the southern edge of the map area. Four prominent northwest-trending faults cross the map area; observed oblique right-lateral, south-side-up kinematic indicators, and the overall pattern of small-scale faults, are interpreted as strain associated with a transpressional regional stress regime during the Neogene that apparently persisted into the Holocene.