NEOPROTEROZOIC AND PALEOZOIC STRATA IN THE STIBNITE-EDWARDSBURG AREA, CENTRAL IDAHO

Geologic mapping and LA-ICPMS and TIMS detrital zircon age results reported here build on earlier studies by Lund et al. (2003 GSAB) that identified Neoproterozoic Windermere-equivalent strata in central Idaho. Although complexly folded and faulted, and locally bearing garnet, sillimanite, and staurolite, we estimate that over 4000 m of Neoproterozoic and lower Paleozoic strata are preserved. SW-facing Neoproterozoic strata near Edwardsburg include two diamictite intervals separated by mafic volcanic rocks. The lower diamictite and quartzite immediately below it contain an abundance of 1000-1400 Ma detrital zircons. A thin interval of calc-silicate quartzite above the diamictite contains exclusively Archean zircons. The upper diamictite contains mostly 1700-1800 Ma grains. Overlying quartzite of the Moores Station contains rare 650 Ma grains, possibly sourced from a nearby diorite-syenite suite of this age. Twenty km to the south, quartzite within a calc-silicate and marble-rich interval along Missouri Ridge contains abundant ~500 Ma zircons and is thus similar to the Upper Cambrian Worm Creek Member of the St. Charles Formation in SE Idaho reported by Todt and Link (2013 NW Geology). These rocks had previously been thought to be Neoproterozoic in age. To the SE, E of Stibnite, stratigraphically lower quartzite contains numerous 1000-1200 Ma grains, another peak at about 1450 Ma, and a few ~685 Ma grains. This and a previously dated quartz pebble conglomerate share zircon provenance with the Neoproterozoic Caddy Canyon quartzite of the Brigham Group in SE Idaho. In contrast, the nearby overlying quartzite on Sugar Mountain lacks the 1000-1200 Ma grains, contains mostly1860-1760 Ma zircons, and resembles Cambrian Flathead Quartzite of Montana, the Cambrian portion of the Brigham Group, and the Addy Quartzite of NE Washington. Quartzite yet higher in the section at Stibnite (upper quartzite of Smitherman, 1985) contains mostly 1860-1800 Ma zircons, and may be Ordovician (Kinnikinic or Eureka quartzites). This section thus includes younger strata than recently speculated and confirms the Paleozoic assignment of Shenon and Ross (1936). The great variation in detrital zircon data highlights the need for a large number of samples, at least in this time period and place, in order to characterize a section.