Keevy Peak Formation and similar rocks

Unit symbol: Pzkp
Age range early Paleozoic (541 to 358.9 Ma)
Lithology: Metamorphic
Group name: Klondike Schist, Keevy Peak Formation, and similar rocks (Yukon-Tanana crystalline complex)
Multiply deformed siliceous and carbonaceous assemblage of phyllite, meta-argillite, quartzite, metachert(?), and lesser amounts of interlayered calcareous phyllite, marble, and mafic and felsic metavolcanic rocks at lower to upper greenschist facies (Foster, 1992; Dusel-Bacon and others, 1993). Includes rocks in the northern Big Delta and southern Circle quadrangles that are referred to as the Blackshell quartzite and phyllite and the Blackshell calcschist and marble (Smith and others, 1994). Contains minor stretched pebble conglomerate in the Healy and Mount McKinley quadrangles. In the Big Delta quadrangle, U/Pb zircon ages from felsic metatuff interlayers yielded dates of 372±5 and 353±7 Ma, indicating Early Mississippian and Late Devonian extrusion ages and, therefore, the depositional age for at least part of the sequence (Dusel-Bacon and others 2004); Pb-isotope data from syngenetic galena in a carbonaceous, phyllite-hosted, stratiform zinc-lead deposit also indicate a Mississippian or Devonian age for mineralization (Dusel-Bacon and others, 1998, 2004; Mortensen and others, 2006). Late and Middle Devonian fossils have been reported in rocks of this unit in the Mount McKinley quadrangle (Gilbert and Redman, 1977). Regionally, unit appears to occupy a stratigraphic position between the Totatlanika Schist and unit |<yqs of this map. As mapped, it includes the Keevy Peak Formation as originally defined by Wahrhaftig (1968) in the Healy and Mount McKinley quadrangles; new age data support extension of the unit to the Big Delta, Circle, and Fairbanks quadrangles (Dusel-Bacon and others, 2004) and correlation with the Nasina assemblage in Yukon, as proposed by Weber and others (1978). Dusel-Bacon and others (2006) wrote that the Blackshell unit of Smith and others (1994) was previously correlated with the Nasina assemblage in Canada (Foster and others, 1994; Smith and others, 1994), but Dusel-Bacon and others (1996) proposed that the use of the Nasina assemblage name in Alaska be restricted “to the rocks in the eastern Yukon-Tanana Upland that contain Permian and Mississippian felsic rocks and appear to grade into the Fortymile River assemblage.” The Permian U/Pb zircon dates reported by Dusel-Bacon and others (2006) are unusual for this map unit and are coeval with ages determined for the Klondike Schist (unit Pks here) and, as mentioned in the Klondike Schist map unit description, Dusel-Bacon and others (2006) also reported Mississippian and Devonian zircon ages. Other rocks included in this map unit are units Pzq and Pzm of Weber and others (1978) in the Big Delta quadrangle, units Pzq and Pzm of Foster and others (1983) in the Circle quadrangle, units Pzgs Pzcp, and Pzq of Foster (1992) in the Eagle and Tanacross quadrangles, units MDq, MDkq, and MDda of Szumigala and others (2002) in the Eagle quadrangle, units uPzst, and uPzv of Werdon and others (2001) also in the Eagle quadrangle, unit Pzk of Csejtey and others (1992) in the Healy quadrangle (which Bela Csejtey, Jr. [written commun., 1993] has indicated also occurs in the Mount McKinley quadrangle and we include here), and the Birch Hill sequence (units Dbs and Dbcs of Newberry and others, 1996) in the Fairbanks mining district of the southeastern Livengood and northeastern Fairbanks quadrangles, as well as units Pzbc, Pzbp, and Pzbr of Smith and others (1994). Unit is typically exposed in close proximity to the Totatlanika Schist (MDts)

Source map information

Source map Szumigala, D.J., Newberry, R.J., Werdon, M.B., Athey, J.E., Stevens, D.S.P., Flynn, R.L., Clautice, K.H., and Craw, P.A., 2002, Geologic map of the Eagle A-1 quadrangle, Fortymile Mining District, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2002-1a, 1 sheet, scale 1:63,360.
Symbol MDq
Description Fine-grained, predominantly dark gray, commonly pyritic, carbonaceous quartzite and phengite–quartz schist, with lesser graphite–quartz schist and light gray quartzite. Locally grades into thin layers of biotite–albite-bearing quartz schist and quartzite, and are locally calcite-rich and include some marble. Apparent thickness is greater than 1,000 m, as this unit underlies a large adjacent area in Yukon Territory (Mortensen, 1988a). Foliation is locally crenulated; quartz lenses are common and commonly isoclinally folded. Magnetic susceptibility unit is very low, 0.01–0.15 x 10-3 SI, with sporadic higher values. The carbonaceous rocks are strongly conductive, although low conductivity quartzite with little to no carbon is interlayered throughout the unit. Consequently, high conductivity, covered zones near outcrops of Nasina quartzite were also mapped as Nasina. Phengite from a phengite–quartz schist layer yielded 40Ar/39Ar plateau ages of 106.3 ± 0.6 Ma and 116 ± 0.9 Ma (sample A11; table 1), presumably reflecting age of cooling from peak metamorphism
Lithology Metamorphic

Correlated geologic units

Label Pzkp?
Description Nasina assemblage, quartz-graphite schist, calcareous and carbonaceous schist
Geologic age Paleozoic to Carboniferous
Geologic setting Metaigneous and metaseds, low-grade
Lithology Form Importance
Schist < Metamorphic Major