Abstract

Instead of two metallogenic epochs, a late Mesozoic and a mid-Tertiary, Idaho appears to have no less than five; namely, (1) late Precambrian, (2) early Tertiary, (3) mid-Tertiary, (4) late Tertiary, and (5) Quaternary. The role of the Idaho batholith (late Mesozoic) as the most important "mineralizer" in the State is discredited. Ore deposits supposed to be related to it genetically appear to have had their source in the more basic crust beneath the batholith and to have been associated with igneous activity that came near the end of the Laramide orogeny in early Tertiary time.All the metallogenic epochs, except perhaps the last, are closely associated with igneous activity; the late Precambrian with basic sills and flows; the early Tertiary with hypabyssal porphyritic and non-porphyritic intrusives ranging from diabase and diorite to rhyolite porphyry and lamprophyre; the mid-Tertiary with granite and related porphyries from dacite to rhyolite, and lamprophyre; the late Tertiary with basaltic and rhyolitic flows; and the Quaternary with possible igneous activity manifested in hot spring action. The close association of the ores with lamprophyre dikes during the early and mid-Tertiary epochs invites speculation as to the origin of lamprophyres and their role in metallization.

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