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

本文对位于太古宙苏必略省阿比提比绿岩带南段的Noranda和北部的Matagami火山块状硫化物矿床下盘岩石进行了Pb和Nd同位素研究。这两个地区分别形成于2705 Ga和2725 Ga，代表了阿比提比带的岩性和初始Pb同位素比值的极端。Matagami矿床由玄武岩、流纹岩和大型基性侵入岩组成的同生拉斑岩套组成。诺兰达矿床下部为玄武岩-安山岩和流纹岩，具有钙碱性和（次级）拉斑岩的亲缘关系，以及大型钙碱性次火山侵入体。在阿比提比带中，Noranda矿石的放射性成因铅同位素比值最高，Matagami矿石的放射性成因铅同位素比值最低。每个区域的每个关键岩性大约有5-8个样品（总共100个分析）将进行Nd和Pb同位素比率和亲子比分析，使我们能够计算初始比率。本研究将对约20个样品进行主元素、稀土、Hf、Zr、Nb、Ta、y等元素的分析，通过对初始同位素比例和微量元素地球化学的评估和比较，解决以下直接问题：研究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与基性火山岩的关系，下盘岩性与矿石的关系，包括矿石中金属的来源，以及形成下盘岩性的岩浆的来源。该研究的实际科学目标范围更广，包括寻求以下问题的答案。为什么苏必利尔省火山块状硫化物总是与长英质下盘火山相联系？次火山侵入在这些矿石的成因中起什么作用？太古代VMS矿石中Pb同位素比值的一般意义是什么？这些矿石中的铅代表的是地幔、地壳，还是两者的混合？这些岩石形成的构造环境是怎样的？形成下盘火山的岩浆是纯地幔衍生的，还是含有更古老地壳的成分？地幔Nd和Pb的同位素演化关系如何？

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?