Felsic granitic rocks

Unit symbol: TKg
Age range Tertiary, Paleocene, or Late Cretaceous, Maastrichtian (72.1 to 56 Ma)
Lithology: Igneous - Plutonic
Group name: Granitic rocks of southern and interior Alaska
Fine- to coarse-grained or porphyritic, light- to dark-gray, rarely pink, granitic rocks. Unit ranges in composition from granite to quartz diorite, and includes syenite, granodiorite, and quartz monzonite. Biotite and hornblende are locally common; muscovite is uncommon. K/Ar and 40Ar/39Ar ages range from about 76 Ma to about 57 Ma; the vast majority of samples yielded ages in the range 70 to 59 Ma. Available U/Pb zircon ages fall within the same ranges. Unit includes many of the plutons shown on various source maps for this map that are commonly labeled TKg. These plutons tend to be small and are exposed in a broad belt from southwest Alaska through interior Alaska and into the Yukon. They tend to be potassium-rich, even at lower SiO2 contents, having as much as 6 percent K2O at 60 percent SiO2 in the Dillingham quadrangle (F.H. Wilson, unpub.data). Plutons of this unit are common in the western Dillingham quadrangle (Wilson, 1977) and are unusual in that they tend to have biotite and pyroxene, often orthopyroxene, as their mafic minerals, regardless of the overall pluton composition. Unit consists of hundreds of individual plutons. Many of the hot springs of interior Alaska are spatially associated with these plutons (Motyka and others, 1983). Associated mineralization includes gold, tin, and mercury (see http://ardf.wr.usgs.gov for more information about mineral resources in Alaska)

Source map information

Source map Wiltse, M.A., Reger, R.D., Newberry, R.J., Pessel, G.H., Pinney, D.S., Robinson, M.S., and Solie, D.N., 1995, Geologic map of the Circle Mining District, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 95-2a, 1 sheet, scale 1:63,360.
Symbol Kg
Unit name Cretaceous Granite
Description Chloritized porphyritic (hornblende) biotite monzogranite; 5--10 mm quartz and 10--45 mm microcline phenocrysts in 2--5 mm groundmass; prim.1ry trace minerals include zircon, allanite, apatite, and ilmenite; plagioclase extensively zoned, and appreciably sericitized; trace hornblende slightly altered to biotite; biotite slightly to completely altered to chlorite, rutile, sphene, magnetite, and calcite; thin quartz-chlorite veins and veinlets are common; disseminated sericite-quartz alteration (pattern) and sericite-quartz veinlets occur sporadically in the granite; anomalous gold concentrations are spatially associated with the sericite-quartz occurrences; apparent K-Ar ages vary from 57 ± 2 to 71 ± 2 Ma.; Ar-Ar age dating studies indicate age resetting due to Tg intrusion, a likely age of 90 ± 2 Ma
Lithology Igneous

Correlated geologic units

Label TKgr
Description Granite and alaskite
Geologic age Maastrichtian to Paleocene
Geologic setting Intrusive
Lithology Form Importance
Granite < Granitic < Plutonic < Igneous Pluton Major
Alaskite < Leucocratic-granitic < Granitic < Plutonic < Igneous Pluton Indeterminate, major