Geochemical Studies of Archean Iron Formations and Association Volcanic Rocks (Kirkland Lake and Temagami/Ont. areas)

Abstract

<p>The Boston Iron Range forms part of a steeply folded Archean supracrustal volcano - sedimentary assemblage in the Abitibi Greenstone Belt. Horizons of Algoman type oxide facies iron formation and enclosing volcanics in the Adams Mine area are situated in the thermal aureole of the post - Kenoran Lebel Syenite Stock; recrystallized mineral assemblages of Upper Greenschist/Lower Amphibolite facies are superimposed upon prehnite - pumpellyite facies terrain. Stratigraphically, the composite sequence represents the transition between two volcanic Supergroups in the Kirkland Lake area, each comprised of a threefold magmatic succession of komatiitic - tholeiitic - calc - alkalic affinity. Lenses of distal, calc - alkalic Skead Group pyroclastics, terminating the Lower Supergroup, and volcaniclastic sediments are intercalated in a series of Larder Lake Group submarine volcanics of magnesian - tholeiitic magma affiliation, extruded from fissure systems in an extensional tectonic regime, and comprising the basal stratigraphic unit of the Upper Supergroup.</p> <p>Magmatic cyclicity present among Supergroups is observed also on a smaller scale within members of the Larder Lake Group. Four major, repetitive volcanic cycles are recognized in the Adams Mine area, three of which are terminated by laterally persistent horizons of BlF. Each cycle is comprised of a characteristic sequence of lithologies of either a high-magnesian (komatiitic)-, low-magnesian- or tholeiitic series.</p> <p>Petrogenetically, the Larder Lake Group magma series evolved from a peridotitic mantle source through various degrees of partial melting and fractional crystallization. REE signatures and concentrations of incompatible elements indicate that the differentiation of the diverse magma suites took place at different crystal levels.</p> <p>Chemical sediments associated with the volcanic sequence are found either as thin, discontinuous interflow units, marking a brief cessation of magmatic activity, or as thick, laterally persistent horizons of banded iron formation, each deposited during a major hiacus of extrusive volcanism. Both types of chemical precipitates show a close spacial association to volcaniclastic material of calc - alkalic magma affinity, and a lithologic control for the deposition of chemical sediments in a semi-restricted basin is implied. Local variation of BIF components (oxide facies BIF or minor sulfide iron formation) is attributed to the presence or absence of organic productivity in a deep water environment.</p> <p>Oxygen isotope studies of BIF components, and comparison of the Adams Mine medium grade metamorphic terrain to the lower grade environment of the Sherman Nine near Temagami reveals, that iron formation is very susceptible to large scale communication of fluids, and that homogenization is complete at temperatures of 350-400 °C. However, higher temperatures (T>450 °C) are required for a complete isotopic re-equilibration between phases in adjoining BIF mesobands. Here, temperatures derived from quartz-magnetite fractionations coincide with thermal stabilities of metamorphic mineral assemblages.</p> <p>Although the spatial distribution of Algoman type BIF is indicative of a volcanogenic hydrothermal source, oxide facies iron formations formed distal to centers of exhalative activity. Volcanic emanations mixed with seawater prior to the precipitation of ferro-silicic components in local, semi-restricted basins. While a structural control assumes major importance in the concentration of base - and precious metals in BIF horizons, the potential for a syngenetic accumulation of such metals during the degassing of the volcanic pile in interfIow chemical sediments is considerable.</p>