ISOTOPIC EVIDENCE FOR CRETACEOUS-PALEOCENE UPWELLING FROM ARCHAIC CRUST IN SNOWBIRD-TYPE DEPOSITS

Snowbird-type deposits are a series of small hydrothermal quartz-carbonate veins that intrude Belt metasediments on a strike line extending NW-SE from the Coeur d’Alene Mining District along the Idaho-Montana border toward the Idaho Batholith. Early carbonates and quartz are Mesoproterozoic (Ramos and Rosenberg, 2012). Purple and green fluorite occur in Snowbird-type deposits (Rosenberg and Wilkie, 2015). Masses of similar fluorite have also been reported in the Idaho Batholith (Foord et. al., 1993).

U-bearing xenotimes in calcite, closely associated with fluorite, yield concordia ages of late Cretaceous to Paleocene. Pb 207/204 and 206/204 ratios in these xenotime-free fluorites lie along a straight line which corresponds to an age of ~1.34 Ga. 207/204, and 206/204 ratios for fluorite from the Idaho Batholith also lie on this line. Upwelling of fluids from underlying Archiozoic to Paleoproterozoic crust during the Mesoproterozoic appears to have been repeated in late Cretaceous to Paleocene times carrying Proterozoic Pb and U to the surface. U was scavenged by xenotime during crystallization from these fluids, thus setting the xenotime clock while leaving associated fluorites essentially U-free.

For example, a purple fluorite from Wilson Gulch north of the Snowbird deposit with a xenotime concordia age of 72 M.Y. has 206/204-207/204 ratios of 34.280 and 16.969 respectively while a green fluorite from the Swallow deposit, south of the Snowbird deposit, with a xenotime concordia age of 61.5 M.Y. has 206/204, 207/204 ratios of 24.117 and 16.047 respectively.

Initial ⁸⁷Sr/⁸⁶Sr ratios of fluorites are >0.80 suggesting relatively recent emplacement from an archaic source. Cretaceous jasperoid veins have also been reported at the Sunshine mine in the Coeur d’Alene District which may represent an earlier period of Cretaceous remobilization (Zartman and Smith, 2009).

Mesoproterozoic upwelling of fluids from underlying Archean to Paleoproterozoic crust in the Pacific NW has been discussed by Ramos and Rosenberg (2012). The present study documents renewed upwelling in late Cretaceous to Paleocene times contemporaneous with late faulting on the Lewis-Clark line and the emplacement of the Bitterroot lobe of the Idaho Batholith.