PROTEROZOIC PB ISOTOPE EVOLUTION IN THE BELT-PURCELL BASIN - CONSTRAINTS FROM SYNGENETIC AND EPIGENETIC SULFIDE DEPOSITS

Sediment-hosted massive sulfide and Ag-Pb-Zn veins are two major depos it types which contain large concentrations of lead in the Middle Prot erozoic Belt-Purcell basin. New Pb isotope data on the Sullivan sedime nt-hosted massive sulfide deposit and the St. Eugene Ag Pb-Zn vein dep osit are combined with existing data to provide a comprehensive overvi ew of Proterozoic Pb isotope evolution in the Belt-Purcell basin. The Pb isotope ratios for sulfides in tile Sullivan deposit form a linear array, the Sullivan array. Bedded sulfides plot within this array and no systematic damage in Pb isotope ratios with stratigraphic position (time) is observed. The Sullivan array results from the mixing of lead s from the upper crustal and/or orogen Pb reservoirs, represented by m etasedimentary rocks of the lower Aldridge Formation in the footwall o f the Sullivan deposit, with, perhaps, leads derived from the Precambr ian basement of the Belt-Purcell basin. A zone within the massive sulf ides of the Sullivan deposit has been altered to chlorite-pyrite-calci te. The center of the alteration zone is comprised of massive pyrite-c alcite which has more radiogenic Pb isotope compositions, the Sullivan alteration. The Sullivan alteration apparently formed from the same P b reservoirs as the Sullivan array, perhaps as a product of the East K ootenay orogeny (<1370 Ma), after a period of up to 108 m.y. which all owed radiogenic Pb growth. Ag-Pb-Zn occurrences in the Belt-Purcell ba sin 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 is otope compositions less radiogenic than those of the Sullivan array. T he Coeur d'Alene array is a mixture of leads derived from the upper cr ustal and mantle Pb reservoirs. It is proposed that mantle lead was in troduced into the Belt-Purcell basin at 1468 Ma by the mafic Moyie sil ls. Leads in sediment-hosted massive sulfide deposits in middle Aldrid ge rocks and leads of the Coeur d'Alene array in epigenetic veins form ed after intrusion of the Moyie sills but before East Kootenay orogeny metamorphism.