Proterozoic Pb isotope evolution in the Belt-Purcell Basin; constraints from syngenetic and epigenetic sulfide deposits

Abstract

Sediment-hosted massive sulfide and Ag-Pb-Zn veins are two major deposit types which contain large concentrations of lead in the Middle Proterozoic Belt-Purcell basin. New Pb isotope data on the Sullivan sediment-hosted massive sulfide deposit and the St. Eugene Ag-Pb-Zn vein deposit are combined with existing data to provide a comprehensive overview of Proterozoic Pb isotope evolution in the Belt-Purcell basin. The Pb isotope ratios for sulfides in the Sullivan deposit form a linear array, the Sullivan array. Bedded sulfides plot within this array and no systematic change in Pb isotope ratios with stratigraphic position (time) is observed. The Sullivan array results from the mixing of leads from the upper crustal and/or orogen Pb reservoirs, represented by metasedimentary rocks of the lower Aldridge Formation in the footwall of the Sullivan deposit, with, perhaps, leads derived from the Precambrian basement of the Belt-Purcell basin. A zone within the massive sulfides of the Sullivan deposit has been altered to chlorite-pyrite-calcite. The center of the alteration zone is comprised of massive pyrite-calcite which has more radiogenic Pb isotope compositions, the Sullivan alteration. The Sullivan alteration apparently formed from the same Pb reservoirs as the Sullivan array, perhaps as a product of the East Kootenay orogeny (<1370 Ma), after a period of up to 108 m.y. which allowed radiogenic Pb growth. Ag-Pb-Zn occurrences in the Belt-Purcell basin form an array in Pb isotope space, the Coeur d'Alene array, which is not collinear with the Sullivan array; it is characterized by Pb isotope compositions less radiogenic than those of the Sullivan array. The Coeur d'Alene array is a mixture of leads derived from the upper crustal and mantle Pb reservoirs. It is proposed that mantle lead was introduced into the Belt-Purcell basin at 1468 Ma by the mafic Moyie sills. Leads in sediment-hosted massive sulfide deposits in middle Aldridge rocks and leads of the Coeur d'Alene array in epigenetic veins formed after intrusion of the Moyie sills but before East Kootenay orogeny metamorphism.