Petrology and Geochemistry of the Volcanic Host Rocks to the West and North Pits of the Sherman Mine Iron Formation, Temagami, Ontario

Abstract

<p>A detailed study was undertaken of the volcanic host rocks underlying the northern band of iron formation at the Sheraan Mine, Temagami, Ontario. The main objective of the study was to characterize the volcanic activity which immediately preceeded the iron formation deposition. A 1.5 km volcanic section was characterized through detailed geological mapping and extensive lithogeochemical sampling .</p> <p>Felsic pyroclastics are the immediate volcanics underlying the iron formation in the West Pit. The pyroclastics commonly host disseminated to massive, stratiform sulphide zones which carry anomalous Au, As and Sb contents. The section extending 1 to 1.5 km stratigraphically below the iron formation is dominated by calc-alkaline andesites with subordinate intercalated dacites, rhyolites, and calc-alkaline and tholeiitic basalts. A small eruptive center or focus of volcanism has been identified in the northeast corner of the section .</p> <p>The volcanics are notably enriched in Cr and Zn and depleted in Cu and Ni. The dacites and rhyolites contain slightly elevated contents of Au. A pervasive depletion of Na is evident in all rock types, and Ca is generally depleted in the intermediate to mafic volcanics. It is suggested that these elements were leached from the rocks during hydrothermal seawater circulation. Various degrees of chloritization, carbonatization and silicification reflect hydrothermal alteration of the volcanic pile.</p> <p>A similarity in the major element chemistry and rare earth element patterns exists between the volcanics underlying the iron formation and the volcanics of the Marda Complex, western Australia . A similar tectonic setting between the Marda Complex and the Temagami greenstone belt is inferred from this similarity. As such, the Temagami volcanics are considered analogous to continental margin calc-alkaline suites. Evidence of hydrothermal seawater circulation in the Temagami volcanics suggests some separation of volcanism from the continental margin , perhaps by a small ocean basin.</p>