Noble metals in Thetford Mines ophiolites, Quebec, Canada; Part II, Distribution of gold, silver, iridium, platinum, and palladium in the Lac de l'Est volcano-sedimentary section

Abstract

The gold, silver, iridium, palladium, and platinum contents of basic volcanics and argillaceous sedimentary rocks from the Lac de l9Est, Thetford Mines area, Quebec, were determined by radiochemical neutron activation analysis. Copper and sulfur were also determined in this suite by atomic absorption spectroscopy and LECO gas analysis, respectively. The basic volcanics are part of the Thetford Mines ophiolite complex of the southern Quebec Appalachians. They rest in fault contact on a complex consisting of tectonite peridotite, ultramafic and mafic cumulates, and hypabyssal metagabbro. The basalts are divisible into a lower and an upper unit separated by argillaceous sediments. Three different petrographic-chemical lithologies were recognized in the Lac de l9Est volcanic section including high TiO 2 and low TiO 2 basalts in the lower unit and low TiO 2 basalts in the upper unit. The main purpose of the study was to compare the relative mobility of platinum-group metals (Pt, Pd, and Ir) with Au, Ag, and Cu in a basic volcanic ophiolite setting which was subject to significant thermal metamorphism and possibly some hydrothermal activity.Based on the distribution of metals in a stratigraphic profile across approximately 425 m of ophiolitic basalt, it is concluded that Pt, Pd, and Ir show no trends related to stratigraphic setting and that these metals were not redistributed by thermal or hydrothermal activity. Their abundances, which in some cases differ greatly from one basalt type to another, are probably characteristic of the lavas at the time of eruption. Au, Ag, and Cu show some tendency to concentrate in sulfur-rich rocks or near the stratigraphic tops of flow units. Some redistribution within flow units may have occurred, but metal profiles indicate no significant displacement of these metals toward the top of the pile. The lack of a well-developed sheeted dike complex at Lac de l9Est suggests a fracture system conducive to large scale deep circulation of seawater in the pile was lacking.