SHRIMP U-PB GEOCHRONOLOGY OF HYDROTHERMAL MONAZITE IN CO-CU-AU ORES OF THE BLACKBIRD DISTRICT, IDAHO

Recent work has identified previously unknown occurrences of abundant hydrothermal monazite and xenotime in Co-Cu-Au deposits of the Blackbird district, Idaho. The deposits form stratabound and locally stratiform lenses within banded siltite of the Apple Creek Formation, a dominantly siliciclastic metasedimentary sequence of Mesoproterozoic age (ca. 1.45-1.38 Ga). Distinctive biotite±chlorite±garnet rocks commonly enclose the deposits. Cobaltite and chalcopyrite are the major sulfide minerals, accompanied by minor pyrite, arsenopyrite, safflorite, pyrrhotite, bismuthinite, Bi-tellurides, and gold. Gangue minerals consist mainly of quartz with minor biotite, chlorite, muscovite, garnet, tourmaline, apatite, and/or siderite. SEM work shows monazite, xenotime, and allanite in complex intergrowths with cobaltite; monazite commonly surrounds cobaltite and is coeval or younger. Folds, faults, and shear zones deform the deposits; metamorphic grade is middle to upper greenschist facies.

SHRIMP U-Pb dating of 12 monazite grains from the Sunshine deposit yields a ²⁰⁶Pb/²³⁸U age of 91±1 Ma. This Late Cretaceous age is similar to those for the oldest phases of the Idaho batholith (ca. 94 Ma), and for metamorphic garnet in northern Idaho (89.6±2.6 Ma; Vervoort et al., 2005). The age for the Blackbird monazite may reflect synsedimentary Mesoproterozoic Co-Cu-Au-REE-Y mineralization followed by pervasive recrystallization of monazite (or formation from precursor allanite+apatite) during Late Cretaceous regional metamorphism. This model requires monazite growth under fluid-rich, upper greenschist-facies conditions, which has been documented in other areas. Alternatively, epigenetic Co-Cu-Au-REE-Y mineralization might have occurred entirely during a Late Cretaceous metamorphic event. This model is problematic, however, because of the generally low LREE/HREE ratios of the ore samples that contrast with the characteristic LREE-rich nature of metamorphogenic REE mineralization elsewhere in the world.