GEOLOGIC SETTING AND TECTONIC HISTORY OF THE SPRUCE PINE, NC MINERAL DISTRICT

The mineral deposits of the Spruce Pine district are part of the Ashe Metamorphic Suite (AMS), which contains rocks that were variably deformed, intruded, and metamorphosed during the three Paleozoic orogenies that affected the Appalachians. The protoliths of the AMS include continental margin and deep ocean sediments, MORB, and ultramafic rocks. All of these were amalgamated into a regional terrane, most likely as an accretionary wedge during the subduction that preceded the Taconian orogeny in the Ordovician. During the subduction and subsequent collision, the rocks experienced widespread amphibolite-facies metamorphism and at least local eclogite facies. The Day Book dunite deposit is a multiply metamorphosed fragment of oceanic mantle that was plucked off the downgoing slab of oceanic lithosphere and incorporated into the Taconian accretionary wedge. Although Taconian metamorphism is widespread, there are no documented Taconian thrust faults in the Spruce Pine area.

The most abundant mineral resources in the Spruce Pine district are found within a series of coarse-grained, mostly granodioritic intrusions. Ages for the intrusions range from 377 Ma to 404 Ma but the most reliable ages (U-Pb zircon ID-TIMS) are ~377 Ma. The Devonian ages match the timing of the Acadian orogeny, but this correlation is problematic in the southern Appalachians for a variety of reasons including the lack of a significant Devonian clastic wedge. The major Devonian structure in the Spruce Pine area is the Burnsville fault. The Burnsville fault trends NE-SW, bounds the AMS to the northwest of the Spruce Pine district, and is an amphibolite-grade dextral strike slip fault. The Burnsville fault may have moved the AMS from a more northerly location to its present location. A late greenschist overprint associated with the late Paleozoic Alleghanian orogeny is locally developed and the AMS in this area was translated tens of kilometers northwestward on major Alleghanian thrust faults.