Klondike Schist

Unit symbol: Pks
Age range Permian (298.9 to 252.2 Ma)
Lithology: Metamorphic
Group name: Klondike Schist, Keevy Peak Formation, and similar rocks (Yukon-Tanana crystalline complex)
Poorly understood assemblage of generally low-grade metamorphosed pelitic and volcanic rocks and minor marble. Primarily exposed in the Eagle quadrangle in Alaska (Szumigala and others, 2002; Dusel-Bacon and others, 2006) and more broadly exposed in the Yukon (Gordey and Ryan, 2005; Beranek and Mortensen, 2011), it consists of gray-green to dark-greenish-black massive greenstone, amygdaloidal pillow basalt and metatuff and associated metachert, quartzite, phyllite, silica-carbonate lenses, and other low-grade metasedimentary rocks. Mortensen (1999) reported that the Klondike Schist yielded mid-Permian protolith ages as determined by U/Pb zircon analysis, and more recent analyses by Beranek and Mortensen (2011) bracket intrusion and metamorphism to between 260 and 252.5 Ma. In the Tanacross quadrangle, quartz-muscovite, quartzite, quartz-muscovite schist, actinolite schist, chert and other fine-grained silicic rocks, phyllite, metagraywacke, and schistose greenstone are also included here (Foster, 1970). On this map, assignment of rocks in the eastern Eagle quadrangle to the Klondike Schist or the Keevy Peak Formation (unit Pzkp) must be considered tentative; Permian (~255 Ma) U/Pb zircon ages, as well as Mississippian and Devonian zircon ages are reported by Dusel-Bacon and others (2006) for the Nasina assemblage, here assigned to the Keevy Peak map unit (unit Pzkp)

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 Pks
Description Predominantly rusty-weathering, fine- to medium-grained, albite–chlorite–phengite–quartz schist and lesser albite–phengite–quartz schist, with an apparent thickness of ~500 m. Pks is distinguished from Pkf based on abundance of metafelsites—rare in Pks and abundant in Pkf. Magnetic susceptibility is low, 0.03–0.25 x 10-3 SI, with sporadic higher values. In the southeast Eagle A-1 Quadrangle this unit was explored in the 1970s for volcanogenic massive sulfide deposits, and a polymetallic (Pb–Zn–Cu–Ag) soil anomaly was defined (Resource Associates of Alaska, 1977; Smit, 2000). Several short core holes drilled in upper Brophy Creek drainage did not encounter massive sulfide, although highly pyritic zones were intersected, with rare galena and sphalerite (Resource Associates of Alaska, 1977). Galena from this core displayed lead isotope ratios indistinguishable from those of syngenetic Klondike schist-hosted prospects in Yukon Territory (J.K. Mortensen, written commun., 2002). Rocks in upper Arkansas Creek and, to a lesser extent, Brophy Creek, are heavily iron-stained. Phengite yielded a 40Ar/39Ar plateau age of 157.5 ± 1.2 Ma (sample A10; table 1), which represents cooling from peak metamorphism
Lithology Metamorphic

Correlated geologic units

Label Pks
Description Klondike Schist, assemblage of metamorphosed pelitic/volcanic rocks and minor marble.
Geologic age Permian
Geologic setting Metaigneous and metaseds, low-grade
Lithology Form Importance
Calc-silicate-schist < Schist < Metamorphic Greenschist Major
Mica-schist < Schist < Metamorphic Major
Greenstone < Metaigneous < Metamorphic Greenschist Minor
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Minor
Quartzite < Metaclastic < Metasedimentary < Metamorphic Greenschist Minor