MINERALOGICAL AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF CO-CU-AU ORES IN THE BLACKBIRD DISTRICT, IDAHO

Co-Cu-Au ores in the Blackbird district of Idaho consist of stratabound and locally stratiform sulfide-rich lenses in banded siltite of the Mesoproterozoic Apple Creek Formation. Metamorphic grade varies from middle greenschist to upper amphibolite. Most ores are intensely folded and sheared. Sulfide lodes occur at multiple stratigraphic levels and are commonly enclosed by biotite ± chlorite ± garnet ± tourmaline ± chloritoid rocks. Major ore minerals are cobaltite and chalcopyrite, with minor pyrite, arsenopyrite, safflorite, pyrrhotite, bismuth, bismuthinite, Bi-tellurides, and gold. Gangue minerals consist of quartz and lesser Cl-rich biotite, chlorite, muscovite, garnet, tourmaline, apatite, and siderite. Some high-grade Co ores have up to 3.7 wt % ΣREE+Y oxides, 9.2 wt % Bi, 1180 ppm Be, or 121 ppm U. SEM studies show locally abundant monazite, allanite, xenotime, and gadolinite (Y₂Fe²⁺Be₂Si₂O₁₀), which form the matrix to cobaltite grains; xenotime, gadolinite, and uraninite also occur as inclusions in cobaltite. Zircon forms subhedral to euhedral blocky prisms in xenotime and cobaltite, and irregular intergrowths with xenotime, cobaltite, Cl-rich biotite, and muscovite, and is probably hydrothermal in origin. These textural relationships suggest broadly coeval Co-Cu-Au and REE-Y-Be-U-Zr mineralization. SHRIMP U-Pb dating of monazite by J.N. Aleinikoff yields a ²⁰⁶Pb/²³⁸U age of 91±1 Ma (Slack et al., 2006), interpreted as postore metamorphic growth of monazite from an inferred allanite + apatite precursor.

Previous workers have proposed sedex, vms, stratiform Cu, and granite-related models for the Co-Cu-Au ores. However, the high concentrations of REE, Y, Bi, and Be locally in these ores are uncharacteristic of the first three deposit types, which uniformly lack this distinctive geochemical signature. The Blackbird district is more likely part of an epigenetic, iron oxide-copper-gold (IOCG) hydrothermal system. Small stratabound to discordant deposits of magnetite, hematite, thorite, and/or monazite (±Cu±Au) in the region may belong to this same system. Mineralization could have occurred during diagenesis ca. 1450-1430 Ma, or more likely was related to emplacement of a nearby granitoid pluton (now augen gneiss) at 1380 Ma.