Lead-isotopic data from sulfide minerals from the Cascade Range, Oregon and Washington

Lead-isotopic studies of mineral deposits associated with Tertiary plutons found in the Cascade Range of Oregon and Washington demonstrate a rather uniform isotopic composition in various sulfide minerals (206Pb204Pb= 18.84 to 19.05">²⁰⁶Pb²⁰⁴Pb= 18.84 to 19.05; 207Pb204Pb= 15.57 to 15.62">²⁰⁷Pb²⁰⁴Pb= 15.57 to 15.62; 208Pb204Pb= 38.49 to 38.74">²⁰⁸Pb²⁰⁴Pb= 38.49 to 38.74), show less variation than data from the volcanic rocks of the Cascade Range and fall within the mixing array defined by the MORB regression line and continental sediments. An evaluation of the role of crustal assimilation by hydrothermal convection during emplacement was made on five sulfide deposits associated with a single composite batholith, the Cloudy Pass pluton. The Pb-isotopic data and mass balance calculations suggest that only minor amounts of the lead were derived from the overlying Precambrian (?) Swakane Biotite Gneiss during emplacement. The bulk of the metal that occurs in sulfide deposits in the Cascade mineral belt appears to have been derived from subducted continental detritus.

The variation of the Pb-isotopic signature of Sulfides from specific districts or deposits suggests that there is a correlation with age and structure of the crust. 206Pb204Pb">²⁰⁶Pb²⁰⁴Pb is greater than 18.92 in northern Washington and southern Oregon where deposits have intruded Mesozoic or older crust. However, the ore deposits between the northern Oregon border and central Oregon, south of Eugene, have intruded younger crust composed largely of mafic and andesitic volcanic rocks and 206Pb204Pb">²⁰⁶Pb²⁰⁴Pb lies between 18.84 and 18.92. This region, previously called the Columbia embayment, appears to be underlain by Tertiary volcanic rocks. Lead-isotopic data may be used to define the boundaries between discontinuous blocks of Mesozoic crust and Tertiary volcanic cover.