ABSTRACT This field trip examines the results of integrated geologic studies of the 9 August 2020, M w 5.1 earthquake near Sparta, North Carolina, USA. The earthquake generated ~4 km of coseismic surface rupture of the Little River fault and uplifted a surface area of ~11 km 2 . The Little River fault is a thrust fault oriented 110–130°/45–70°SW, and mapped fault segments are en echelon with scarp heights from <5–30 cm. The epicenter is in polydeformed rocks of the Ashe and Alligator Back Metamorphic Suites in the eastern Blue Ridge. Bedrock structure formed during multiple Paleozoic orogenies; the regional foliation strikes NE-SW and dips SE (mean orientation 063°/52°SE). Mapping identified late Paleozoic veins and shear zones, a regional joint set striking 330–340° and 250–240°, and brittle faults that cut the Paleozoic foliation. Brittle faults oriented similar to the Little River fault are mapped up to 4 km along strike from the coseismic rupture along Bledsoe Creek valley, and the combined length of the Little River fault system is ~8 km. Paleoseismic trenches across the Little River fault corroborate the reactivation of an older fault by the 2020 earthquake and reveal two events during late Pleistocene (<50 ka). Surficial mapping identified several terrace deposits, including a deposit along Bledsoe Creek that yielded a 26 Al/ 10 Be isochron burial age of 0.46 ± 0.13 Ma and overlies a brittle fault, thus constraining the timing of movement of the fault at that location. Paleoliquefaction studies document soft-sediment deformation features in alluvium that may represent paleoseismic events. Collectively, these results highlight long-lived paleoseismicity of the Blue Ridge and that the 9 August 2020 earthquake reactivated an older, suitably oriented brittle fault in the bedrock. The Little River fault is an example of a previously unknown but active fault lying outside of known seismic zones with demonstrated recurrence of paleo-ruptures, raising questions about the assumption that damaging earthquakes are limited to areas of ongoing background seismicity, which is counter to seismic hazard assessments in the eastern United States. Bedrock mapping separates eastern Blue Ridge lithostratigraphy of the Lynchburg Group and Ashe and Alligator Back Metamorphic Suites into separate fault-bound packages juxtaposed over various 1.3–1.0 Ga basement rocks of the northern French Broad massif by the Gossan Lead fault.

Abstract Gold is ubiquitous in the unconformity-related uranimum deposits of Australia’s Alligator Rivers uranium field and in some cases constitutes an economic resource, Palladium and platinum accompany gold in significant amounts at the Coronation Hill and Jabiluka deposits. Spatial association of uranium, gold, and the platinum-group elements and textural relationships between gold and uraninite suggest that all these metals together with magnesian chlorite (amesite) were deposited during the same mineralizing event. The most likely ore-transporting solution would have been an oxidized, slightly acidic, chloride-rich brine (or brines), derived from within the mid-Proterozoic hematitic terrestrial clastic cover sequence to the deposits. In addition to carrying ore-forming amounts of U, Au, Pd, and Pt as chloride complexes, such brines are predicted to have been capable of transporting Se and Te as acid oxyanions. The reaction path history of these brines probably involved initial equilibration with atmospheric oxygen and subsequent equilibration with rocks with a high intrinsic oxidation state, namely a hematitic clastic sedimentary sequence (Kombolgie Formation), poor in organic detritus and ferrous iron. It is proposed that this reaction path history is more important in ore formation than interaction with specific, metal-enriched source rocks. Preliminary calculations of irreversible mass transfer are presented which support the interpretations of previous workers who have suggested that reduction is the principal mechanism of ore deposition. These calculations show that either mixing of an oxidized metal-bearing solution with a reduced CH 4 -bearing fluid, derived from lower Proterozoic graphitic schist host rocks, or direct interaction of the oxidized metal-bearing solution with lower Proterozoic graphitic and ferrous iron-bearing schistose host rocks could have produced the association of uranium, gold, and platinum-group elements. Some involvement of ferrous iron-bearing phases in the host rocks is suggested by hematitic alteration around the deposits. The predicted sequence of ore and associated gangue mineral deposition is magnesian chlorite and uraninite, followed by native gold (as is actually observed), followed by palladium telluride (kotulskite), palladium sulfide (vysotskite), and finally platinum sulfide (cooperite). Thus, spatial association of uranium, gold, and platinum-group elements deposition with the unconformity reflects a major difference in ambient oxidation state between mid-Proterozoic cover rocks and underlying lower Proterozoic metasediments, coupled with fault permeability. Similar depositional mechanisms are believed to occur in deposits hosted by younger rocks, the Kupferschiefer of Poland being one possible example.