Constraints on the Timing of Co-Cu ± Au Mineralization in the Blackbird District, Idaho, Using SHRIMP U-Pb Ages of Monazite and Xenotime Plus Zircon Ages of Related Mesoproterozoic Orthogneisses and Metasedimentary Rocks

The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu +/- Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary countiy rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 +/- 10 Ma, regarded as the maximum age of deposition of metasedimentaiy country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 +/- 4 and 1359 +/- 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 +/- 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (BEE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xenotime and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 +/- 4 Ma), determined on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1.315 to 1.270 Ma and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on Pb-206/U-238 data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 +/- 3 and 92 +/- 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.