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

本文对位于太古界优势省阿比提比绿岩带南部和北部的Noranda和Matagami火山成因块状硫化物矿床的下盘围岩进行了铅和钕同位素研究。这两个区域分别形成于2705Ga和2725Ga，代表了阿比提比带岩性和初始铅同位素比值的极端。Matagami矿床被一套同成因拉斑玄武岩、次级流纹岩和大型镁铁质侵入体覆盖。诺兰达矿床由钙碱性和(从属)拉斑玄武岩亲缘的玄武岩-安山岩和流纹岩以及大型钙碱性次火山侵入岩构成。在Abitibi带中，Noranda矿最多，Matagami矿最少。每个地区每个关键岩性的大约5-8个样本(总共100次分析)将被分析，以确定Nd和Pb同位素比率和亲子比率，使我们能够计算初始比率。将对大约20个样品进行常量元素、稀土和Hf、Zr、Nb、Ta和Y的分析。通过对初始同位素比值和微量元素地球化学的评估和比较，这项研究将解决以下直接问题：下盘岩石与次火山侵入岩的关系，下盘层序中长英质火山岩和镁铁质火山岩之间的关系，下盘岩性与矿石的关系，包括这些矿石中金属的来源，以及形成下盘岩性的岩浆的来源。这项研究的实际科学目标范围更广，包括寻求以下问题的答案。为什么苏必利省火山成因的块状硫化物如此一致地与长英质下盘火山岩联系在一起？次火山侵入岩在这些矿石的成因中起什么作用？太古界VMS矿石中铅同位素比值的总体意义是什么？这些矿石中的铅代表地幔、地壳还是两者的混合体？这些岩石形成的构造环境是什么？形成下盘火山岩的岩浆是纯粹来自地幔，还是含有较老地壳的成分？地幔中Nd和Pb的同位素演化是如何相关的？

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?