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A study of the lead isotopic composition of galena at Manitouwadge, Ontario. Hayles, John Gordon


Lead isotope compositions of galena from the Fox Creek Fault at Manitouwadge, Ontario form an anomalous linear array of values on a standard ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb plot and a ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb plot. The last geologically reasonable time for galena mineralization along the Fox Creek Fault is during the Keweenawan age, 1100 million years ago. Using the slope of the anomalous lead line and this time of 1100 million years as the time of final galena mineralization along the fault gives an age of 2000 million years as the maximum time of emplacement of uranium parent of the anomalous leads into the crust. The corresponding Th/U ratio for the source of the anomalous leads is 5.3. Lead isotope compositions of galena from the main ore zone at Manitouwadge are extremely homogeneous and do not show a lineation, other than along a ²⁰⁴Pb error line, which would indicate them as anomalous leads. It is improbable for the ore zone deposits to be as young as 2700 million years as required by Tilton and Steiger (1969). In order to relate ore zone leads to Fox Creek Fault leads at least a three stage lead growth model is required. Possibly no genetic relation exists between ore zone and Fox Creek Fault galenas analysed to date. Single galena crystal lead isotope variations should be studied in detail within the Fox Creek Fault itself. Sulphur isotope studies may help to determine the relation between ore zone and fault zone galena at Manitouwadge.

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