In 2008, the Alaska Division of Geological & Geophysical Surveys conducted new 1:50,000-scale geologic mapping in the eastern Bonnifield mining district across a key section of the northern foothills of the Alaska Range, 60 miles south of Fairbanks. The map incorporates field observations, and interpretations of airborne geophysical (Burns et al., 2007), lithogeochemical, ⁴⁰Ar/³⁹Ar age, petrographic, coal, and palynology data. The map and data will be published and available on DGGS’s website (http://www.dggs.dnr.state.ak.us/) in 2009.

Green, meta-siliciclastic rocks and phyllite of the Proterozoic to Devonian Healy Creek schist and gray meta-siliciclastic rocks and graphitic schist of the Devonian(?) Keevy Peak Formation are interfolded in alternating east-west-trending overturned folds in the south. Meta-sedimentary and meta-volcanic rocks of the Totatlanika Schist, outcrop in a broad asymmetric syncline in the north. 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 (Dusel-Bacon et al., 2004). We subdivide the Totatlanika Schist into nine intercalated lithologic sub-units based on observed relict textures, mineralogy, and lithogeochemistry. Hand-held XRF Zr and Nb values distinguish peralkaline meta-rhyolite from other felsic metaigneous rocks in the field.

Cretaceous porphyritic monzonite intrusions in the north are coincident with prominent aeromagnetic highs, but in contrast, Cretaceous equigranular granodiorite intrusions to the south are not magnetic. Tertiary mafic and felsic dikes occupy 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 smaller-scale faults, point to a transpressional regional stress regime during the Neogene that may persist into the Holocene.