Named for exposures in the north-central and northwestern Sierra Madre, Carbon Co, WY in the Northern Rocky Mountain region. Type area designated as the area between South Spring Creek Lake and Jack Creek mine (abandoned) site, T14N, Rs86 and 87W. Crops out over an area of about 80 sq km. Occurs only in the Sierra Madre. Consists of gray, strongly foliated granodiorite. Geologic map; chemical analyses. Is conformable in structure to rocks it intrudes. It is difficult to distinguish Spring Lake from the quartzofeldspathic gneiss of the basement that it intrudes. A granodiorite southeast of Carrico Ranch thought to be part of Spring Lake cuts Phantom Lake Metamorphic Suite (revised) and is intruded by mafic igneous rocks. Intrudes Vulcan Mountain Volcanics (new). Intrudes Jack Creek Quartzite (new) of Phantom Lake Suite. Intruded by mafic igneous rocks (not mapped). Is probably older than Baggot Rocks Granite. Best-fit whole-rock Rb/Sr isochron for samples from near the Jack Creek mine site suggests age of about 2,700 Ma. Assigned to the Late Archean.

Source: GNU records (USGS DDS-6; Denver GNULEX).