LEAD ISOTOPE CONSTRAINTS ON THE ORIGIN OF PROTEROZOIC SEDIMENT-HOSTED COBALT-COPPER ORES OF CENTRAL IDAHO

The cobalt-copper (Co-Cu) deposits of east-central Idaho form a linear trend that extends for about 60 km in a NW-SE direction. Ore minerals vary from an iron-rich association at the southern end through a cobalt-rich one in the middle (Blackbird mine) to a copper-rich association at the northern end. A Proterozoic sedimentary exhalative (SEDEX) origin related to mafic volcanism and an epigenetic hydrothermal origin related to Cretaceous-Tertiary felsic magmatism have been proposed. Metamorphism and tectonic deformation have modified the original configurations of the ore bodies and possibly the mineral assemblages. The objective of this study is to trace the sources of lead (Pb) in the cobalt-copper ores and thereby place constraints on the sources, processes, and timing of mineralization.

Lead isotope ratios of the sulfide minerals, host rocks (Middle Proterozoic Yellowjacket Formation), and crystalline rocks (Early Proterozoic?) that lie beneath the Yellowjacket Formation were measured. The Pb isotope compositions of Co-Cu ores (206Pb/ 204Pb=30.8-40.4; 207Pb/ 204Pb=16.8-17.6; 208Pb/ 204Pb=49.7-63.9) are among most radiogenic ores on Earth, and more radiogenic than any known SEDEX deposits. The data plot well beyond the crustal growth model curves, a characteristic shared only by the Mississippi Valley-Type (MVT) deposits, and are 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 cobalt-copper ores. The Pb isotope ratios of the crystalline basement rocks partially overlap the Pb isotope ratios of the Yellowjacket Formation at the present day, but would have been much less radiogenic than the ores at an assumed mineralization age of 1700 Ma (Middle Proterozoic). Only the host Yellowjacket 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 cobalt-copper ores. Leaching of metals from host sedimentary sequence by basinal brines or metamorphic hydrothermal solutions is envisaged.