Lead Isotope Constraints on the Origin of Proterozoic Sediment-Hosted Co-Cu ores of Central Idaho

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Lead Isotope Constraints on the Origin of
Proterozoic Sediment-Hosted Co-Cu ores of Central
Idaho

• K. Panneerselvam
• Central Missouri State University
• Andrew Macfarlane
• Florida International University
• Vincent Salters
• NHMFL and Florida State University

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Acknowledgements

• Noranda Mining Company
• Ted Vanderschaaf, Blackbird Mine

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Geology of Idaho

• Belt Supergroup

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Idaho Cobalt Belt

• Iron Creek
• Blackpine
• Blackbird
• Salmon Canyon

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Yellowjacket Formation

• Metasedimentary rocks
• Middle Proterozoic
• Stratigraphic equivalent of Belt Supergroup

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Thermal Events

• Submarine geothermal system asso. With
  Yellowjacket volcanism
• Regional metamorphism (pre 1370 Ma) greenschist
  facies
• Regional metamorphism asso. with 1370 Ma granitic
  plutons
• Cretaceous-Tertiary metamorphism asso. with the
  Idaho Batholith

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Existing Hypotheses

• SEDEX - Proterozoic, Related to mafic volcanism
  (Nash Hahn, 1986)
• HYDROTHERMAL - Cretaceous-Tertiary, related to
  magmatism (Anderson, 1947)
• Metamorphosed exhalative (Modreski,1985 Nold,
  1990)

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Objective

• Trace the sources of metals using Pb isotopes
• Place constraints on the processes and timing of
  mineralization

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Methodology

• Determine the Pb isotope compositions of Co-Cu
  ores
• Compare the ore-Pb compositions with the Pb
  isotope compositions of potential source rocks

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Analytical Techniques

• Pb separation by ion exchange method - Clean lab
  at FIU
• Measurement of Pb isotope ratio by thermal
  ionization mass spectrometer (TIMS) - at NHMFL,
  Tallahassee, FL
• U and Pb concentrations by ICP-MS at FIU

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Ores and Rocks of central Idaho
208 Pb/ 204 Pb
207 Pb/ 204 Pb
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Blackbird Yellowjacket
207 Pb / 204 Pb
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Comparison with Sullivan

• Sullivan - Zn-Pb-Ag, Aldridge Fm, Canada - SEDEX
• Co-Cu ores of Idaho are extremely radiogenic
  compared to Sullivan
• Spatial and temporal association with mafic rocks
  (e.g., Moyie Sills)

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Comparison with Zambian Copper Deposits

• Co-Cu ores of Idaho are most similar to Zambian
• Both are Proterozoic sediment-hosted and contain
  Cobalt
• Syngenetic, diagenetic, and epigenetic fluids
  metal sources not constrained

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Comparison with Zambian Copper Deposits

• Zambian ore-Pb data plot closer to upper crustal
  growth curve
• Idaho Co-Cu ore-Pb data plot well beyond the
  model curves
• Despite the similarity in geologic setting and
  mineral assemblage Idaho ores are unique in their
  Pb isotope compositions

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Ore genetic models I

• Mixing of two fluids
• Co-Cu bearing from mafic igneous source
• Pb-bearing fluid from sedimentary basin
• No compositional range observed

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Ore genetic models II

• Hydrothermal leaching of sediment with mafic
  intercalations
• Pb from clastic sediments
• Co-Cu from mafic intercalations
• By diagenetic or epigenetic fluids
• No compositional range observed

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Ore genetic models III

• Hydrothermal leaching of sediments before the
  emplacement of igneous rocks
• Co, Cu, and Pb from clastic sediments
• No mafic igneous source of fluid or metals
• By evolved basinal brines
• Consistent with Pb-isotope data

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Ore genetic models III-B

• Stage I- formation of synsedimentary pyrite
  (fine-grained)
• Stage II- partial replacement of pyrite by Co-Cu
  bearing fluids
• Higher concentration of Co in epigenetic pyrite
  (coarse)
• Similar model proposed for Zambian Copper (Steven
  Armstrong, 2003)

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Conclusions

• Co-Cu ores are not similar to SEDEX
• Ore-Pb not derived from mafic sources
• Not related to Felsic igneous rocks
• Extremely radiogenic
• Very similar to host Yellowjacket Fm.

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Conclusions

• Leaching of metals from host sedimentary sequence
  by basinal brines or metamorphic hydrothermal
  solutions
• Middle Proterozoic