PRECAMBRIAN (?) PALEOZOIC (?) QUARTZITE WITH UNKNOWN AFFINITIES MAPPED IN SOUTHWEST MONTANA

New mapping of the 1:24,000-scale Coyote Creek Quadrangle in southwestern Montana provides interesting new information regarding Mesoproterozoic stratigraphy and correlation between Belt strata as observed in Montana and Lemhi Group strata as observed in Idaho. Strata in this area of Horse Prairie and Bloody Dick Creek have been previously mapped as Wallace Fm., part of the Montana Belt sequence Missoula Group (Coppinger, 1972) and, by inference from Hansen (1983), as possibly Big Creek Fm. part of the Idaho Lemhi Group of the same age. Recent mapping by Lewis, et al. (2009) of the Kitty Creek Quadrangle just to the west of the present area do not assign formation names to their stratigraphic sequences.

In the northeast part of the Coyote Creek Quadrangle we observe a medium- to coarse-grained white quartzite lying directly above and also seemingly interbedded with a finer-grained red to purplish-red quartzite. This sequence of rock types has not been previously described in this area.

Samples of red and white quartzite across a mappable contact were analyzed for U-Pb ages of zircons at the Arizona LaserChron Lab. Results show that the underlying red unit has typical Missoula and Lemhi Group detrital zircon populations with a small 1450 Ma population, a large (60%) 1730 to 1780 Ma population, a small 1840 to 1880 Ma population and scattered Neoarchean grains.

The white quartzite on the other hand has abundant (50 out of 80 grains) Grenvillean aged grains (age peaks at 1032, 1141 and 1242 Ma), a small (7 grain) 1400 +/- 7 Ma population, no grains between 1650 and 1800 Ma and sparse Paleoproterozoic and Neoarchean (1800 to 2600 Ma) grains. It cannot be Belt Supergroup, which always has abundant 1650 to 1800 Ma grains. The distribution also is very different from Cambrian Flathead Quartzite (1790 Ma peak), Ordovician Summerhouse Formation (1740 Ma peak), and Ordovician Kinnickinnic Quartzite (1840 Ma peak).

This is the first observation of rocks with this distribution of zircon ages in Montana. The nearest location with rocks having a similar zircon distribution is the Neoproterozoic-early Cambrian Brigham Gp. of southeastern Idaho.