Geochemical and isotopic investigation of magmatism in the Fox River belt: tectonic and economic implications
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Date
2012-09-05
Authors
Desharnais, Guy
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Abstract
The Fox River Belt forms a segment of the Circum-Superior Belt that comprises a
series of ca. 1900 Ma volcanosedimentary belts located around the margins of the
Superior Province. The Fox River Belt is a north-facing homoclinal sequence, composed
of a rift-related sedimentary sequence, intercalated with thick sequences of komatiites
and tholeiites, and intruded by mafic-ultramafic intrusions. Geological mapping, whole
rock geochemistry and S, Nd- and Hf-isotope geochemistry of the magmatic and
sedimentary units within the belt were used to obtain a better understanding of the
magmatic and tectonic history of the belt and its relation with other segments of the
Circum-Superior Belt.
The compositional variation observed in the volcanic rocks (19% to 5% MgO)
can be accounted for as the product of fractional crystallization. Trace element patterns as
well as εNd and εHf values (-4 to +8 and +4 to +18 respectively) indicate that the
magmatism within the belt was the product of partial melting of depleted mantle that was
locally contaminated with sediments or gneiss. A plume origin for this melting event is
proposed based on the high degree of partial melting, the large volume of magma, and the
presence of a radiating dyke swarm. The 250 km long Fox River Sill appears to have
behaved as a magma conduit during part of its history. Several intervals within the sill
show enrichment and depletion of PGE. The KO Zone PGE-Cu-Ni mineralization (up to
5ppm PGE) formed from magmatic sulphides whose tenor was upgraded by the upward
percolation of evolved intercumulus magma. Contaminated Lower Intrusions and the
lower margin of the Fox River Sill are considered particularly prospective for hosting Ni-
Cu-PGE mineralization.
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Keywords
Magmatism, Geochemistry, Circum-Superior, Belt, Isotopic