Apr 20th, 1:00 PM - 4:00 PM


Investigation of Lead Isotope Ratio in Minerals form the Lemhi Pass District , Idaho

Faculty Mentor

Dr. Mark Schmitz


Lead (Pb) isotopic analysis was used to investigate the relationship between Early Paleozoic intrusive rocks and mineralized veins at the Lemhi Pass District, Idaho. The Lemhi Pass mineral district of the central Beaverhead Mountains (Idaho) comprises both early quartz-copper-gold vein mineralization and later thorium-rare earth element (REE) mineralization in the form of quartz-thorite-hematite veins, monazitethorite- apatite-bearing shears and replacements with specularite, biotite and alkali feldspar. Little is known about the timing or provenance of ore mineralization, and thus there remain many untested hypotheses regarding the relationships between mineralization and regional deformation, magmatism and metamorphism. My objective was to test whether Early Paleozoic magmatic intrusives provided the source of mineralizing fluids, using the Pb isotopic ratios of the magmatic intrusive, mineralized veins, and associated wall-rock alteration minerals. Genetic associations between the intrusive and mineralized samples within certain suites are supported by some of the overlapping Pb signatures. Early Paleozoic magmatic intrusives show a linear trend of in 206Pb/ 204Pb versus 207Pb/ 204Pb isotope space, with a primary magma range of 17.6 to 19.6 206Pb/ 204Pb and 15.6 to 15.9 207Pb/ 204Pb. Pristine magmatic compositions fall at the low end of this range, while syenites and granites with hematite vein and disseminated alteration fall at the high end. The sulfides follow a similar range of chemical compositions and tend to concentrate at the lower end near the primary magma range; however some sulfides extend to more radiogenic compositions. The lead isotope compositions of minerals associated with Th-REE-hematite veins extended to more radiogenic compositions ranging from 19.9 to 22.5 206Pb/204Pb and 15.6 to 15.8 207Pb/204Pb. These observations are consistent with a model whereby Cu-Au sulfide mineralization is genetically associated with Early Paleozoic magmatism. Subsequent Th- REE mineralizing fluids with a radiogenic crustal Pb isotopic composition were responsible for alteration of hosting granitoids and remobilization of earlier Cu-Au sulfide mineralization.