Abstract

Strata-bound cobalt-copper deposits occur in the metasedimentary rocks of the Apple Creek Formation (Mesoproterozoic) in a linear belt called the Idaho cobalt belt, near Salmon, Idaho. We report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores. The Pb isotope compositions of cobalt-copper (Co-Cu) ores (206Pb/204Pb = 30.8–40.4; 207Pb/204Pb = 16.8–17.6; 208Pb/204Pb = 49.7–63.9) are very radiogenic, much more radiogenic than any known sedimentary exhalative (sedex) deposits. The data plot well beyond the crustal growth model curves, a feature shared by the Mississippi Valley-type (MVT) deposits which is characteristic of an upper crustal source. It is unlikely that the ore lead could have come from any mafic igneous source of mantle origin (Proterozoic Moyie sill and its equivalent, or the Tertiary Challis Volcanic Group). The Cretaceous felsic igneous rocks of the region (the Idaho batholith and related rocks) also have much lower Pb isotope ratios than the Co-Cu ores. The Pb isotope ratios of Proterozoic crystalline rocks partially overlap Pb isotope ratios of the Apple Creek Formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 Ma. Only the host Apple Creek Formation is known to have appropriate Pb isotope compositions (206Pb/204Pb = 26.8–86.7; 207Pb/204Pb = 16.3–21.1; 208Pb/204Pb = 47.9–64.8) to be the source of Co-Cu ores. Leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of Co-Cu deposits in the Idaho cobalt belt.

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