A Pb and Nd Isotope Study of the Footwall Rocks of the Noranda and Matagami Cu-Zn Sulfide, Deposits, Quebec

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We propose a Pb and Nd isotope study of the footwall rocks of the Noranda and Matagami volcanogenic massive sulfide deposits, which are located in the southern and northern parts, respectively of the Abitibi greenstone belt of the Archean Superior Province. These two areas, formed at 2705 Ga and 2725 Ga respectively, represent the extremes in both lithology and initial Pb isotope rations of the Abitibi belt. The Matagami deposits are underlain by a cogenetic tholeiitic suite of basalts, subordinate rhyolites, and a large mafic intrusion. The Noranda deposits are underlain by basalts-andesites and rhyolites of calc-alkaline and (subordinate) tholeiitic affinities and a large calc-alkaline subvolcanic intrusion. The Noranda ore has the most, and Matagami ores the least radiogenic Pb-isotopic ratios in the Abitibi belt. Approximately 5-8 samples from each of the key lithologies in each area (at total of 100 analyses) will be analyzed for Nd and Pb isotope ratios and parent-daughter ratios, enabling us to calculate initial ratios. About 20 samples will be analyzed for major elements, rare earths, and Hf, Zr, Nb, Ta, and Y. Through assessment and comparison of initial isotope ratios and trace element geochemistry, the study will address the following direct problems: the relationship of the footwall rocks to the subvolcanic intrusive, the relationship between the felsic and mafic volcanic rocks in the footwall sequence, the relationship of the footwall lithologies to the ores including the source of metals in these ores, and the origin of the magmas forming the footwall lithologies. The actual scientific objectives of the study are of broader scope and include seeking answers to the following questions. Why are Superior Province volcanogenic massive sulfides so consistently associated felsic footwall volcanics? What is the role of the subvolcanic intrusive in the genesis of these ores? What is the significance of Pb isotope ratios in Archean VMS ores generally? Is the Pb in these ores representative of the mantle, of the crust, or of a mixture of the two? What was the tectonic environment in which these rocks formed? Were the magmas forming the footwall volcanics purely mantle-derived or do they contain a component of older crust? How are the isotopic evolution of mantle Nd and Pb related?