Postmagmatic thermal activity in the Abitibi greenstone belt, Noranda and Matagami districts; evidence from whole-rock Pb isotope data

Abstract

The Pb isotope compositions of the footwall sequences of the Noranda and Matagami volcanogenic massive sulfide deposits in the Abitibi greenstone belt have been examined. At Noranda, whole-rock Pb isotope data for the Flavrian, Northwest, Rusty Ridge, and Amulet Formations and the subvolcanic Flavrian intrusion scatter about a line that corresponds to a Pb-Pb age of 2638 + or - 14 Ma. Pb-Pb ages calculated for the individual units are all within error of this age but are significantly younger than the 2700 + or - 2 Ma U-Pb zircon age for the Flavrian intrusion. The Pb-Pb array passes above a tightly constrained field for the galena Pb from ore deposits within the Mine sequence. The younger Pb-Pb ages and the lack of collinearity with the sulfide ores reflect rehomogenization of the Pb isotope compositions in these rocks approximately 60 m.y. after their formation.At Matagami, whole-rock Pb isotope data fall along two arrays. The first, which includes the porphyritic metarhyolite of the Watson Lake Group from the footwall and the key tuffite, corresponds to a Pb-Pb age of 2584 (super +19) (sub -20) Ma. The second array includes the Bell River igneous complex and the overlying Wabassee Group basalts and corresponds to an age of 2647 + or - 14 Ma. These Pb-Pb ages are approximately 75 and 140 m.y. younger than corresponding zircon U-Pb ages of 2725 + or - 2 and 2725 (super +3) (sub -2) Ma. Both arrays pass above a tightly constrained field for galena Pb from several Matagami ore deposits and are probably the result of differences in the initial Pb isotope compositions of the respective rocks. The apparent initial Pb values most likely represent heterogeneities in the mantle source regions for these rocks. The younger Pb-Pb ages for the two arrays indicate Pb isotope homogenization in these rocks approximately 100 m.y. after their formation. The cause of the different ages for the two arrays remains controversial but may be the result of either incomplete isotopic homogenization of Pb or an earlier closure of the U-Pb systems in the rocks constituting the older array.The Pb homogenization implied by the young Pb-Pb ages may be the result of the pervasive circulation of hydrothermal solutions that may have been derived from the dewatering of the lower parts of a newly formed, thickened Archean crust. Such large-scale processes apparently occurred 50 to 150 m.y. after this crust was formed and assembled.