The Kitto intrusion is a Mesoproterozoic 1117.7 ±1.8 Ma peridotitic to gabbroic intrusion, that formed as a result of plume magmatism and was emplaced along Mesoproterozoic and Archean faults in the East Nipigon Embayment of northern Ontario. The plume signature for the Kitto intrusion is suggested by five samples with OIB (ocean island basalt)-like primitive mantle normalized multielement patterns in the lherzolite-olivine websterites.
In the southern part of the intrusion, the sequence of lithologies from top to bottom is lherzolite, olivine websterite, vari-textured pyroxenite, pyroxenite and melanogabbro. The basement is banded iron formation and mafic metavolcanic rocks. Lithologies on surface are dominantly lherzolite, with olivine websterite present in the area of the Phoenix mineralized occurrence. The Kitto intrusion is dominated by lherzolite that exhibits cumulate to pyroxene-poikilitic textures dominated by olivine with lesser clinopyroxene and poikilitic orthopyroxene. The lithologies crop out as a 15 to 30 cm thick layer cake bed dipping from 0 to 30° to the SE and SW. The olivine websterite displays similar cumulate to pyroxene-poikilitic textures, but contains more abundant clinopyroxene. The pyroxenite exhibits orthocumulate textures consisting of cumulate clinopyroxene and intergranular plagioclase. The melanogabbro comprises poikilitic plagioclase and minor cumulate clinopyroxene. In the central part of the intrusion, lithologies from east to west are lherzolite, olivine websterite, pyroxene-porphyritic melanogabbro and granophyric gabbro. Lherzolite and olivine websterites in the central part of the intrusion are more fractionated than at the southern part of the intrusion, and are characterized by the presence of secondary pyroxene and minor plagioclase. The pyroxene-porphyritic melanogabbro also contains secondary pyroxene and, along with cumulate clinopyroxene, there is enrichment in intercumulus plagioclase component.
The lithologies of the Kitto intrusion have geochemical signatures consistent with crustal contamination of a primitive magma. The lherzolite-olivine websterites are characterized by primitive mantle normalized multielement patterns with negative Nb and Ti anomalies, consistent with assimilation of Archean continental crust. All the lithologies have εNd contents of -5.54 to -6.53 consistent with contamination by an older crustal source. Trace element modeling of assimilation-fractional crystallization suggests the lithologies probably assimilated metasedimentary rock of the QST (Quetico Sedimentary Terrane) and mafic metavolcanic rock of the BGB (Beardmore-Geraldton Greenstone Belt) at depth. Emplacement at shallower depths was accompanied by assimilation of mafic metavolcanic rock and banded iron formation. In the central part of the intrusion, pyroxene-porphyritic melanogabbros have more pronounced negative Nb anomalies indicative of greater assimilation of continental crust.
Emplacement of the Kitto intrusion occurred with an initial pulse of pyroxenite-melanogabbro followed by a pulse of lherzolite-olivine websterite magma in the southern part of the intrusion. The initial pyroxenite-melanogabbro pulse is suggested by a trend of increasing forsterite and enstatite contents, and decreasing ferrosilite in olivine and pyroxene minerals from the melanogabbro to the higher pyroxenite. A second pulse is suggested by a trend of replenishment to higher forsterite, enstatite, and ferrosilite contents, but lower wollastonite content in olivine and pyroxene minerals in the lherzolite-olivine websterite. Towards the top of the intrusion trends of decreasing forsterite, enstatite and ferrosilite contents, but increasing wollastonite content, in olivine and pyroxene are consistent with fractionation of the magma.
The Kitto intrusion was probably emplaced along north-south Proterozoic and east-west Archean faults in the East Nipigon Embayment. Possible north-south orientated faults include the Nipigon River and Pijitawabik Faults. As the magmas came in contact with the east-west orientated Blackwater, Empire, Princess Lake, Standingstone and Sandy Creek Faults of the BGB, they were displaced east or west along the faults, creating the ring-shaped form of the intrusion.
Mineralization of the Kitto intrusion is represented by interstitial Ni-Cu-PGE sulphides in one zone at the bottom of the lherzolite-olivine websterite, two zones in the underlying pyroxenite, and one zone at the contact of the melanogabbro and the basement iron formation. On the surface outcrops, high Ni contents in the lherzolites and trends of increasing Ni with MgO in all the lithologies, show that Ni partitioned into olivine as the magma evolved. On the surface outcrops in the central part of the intrusion, lower Ni contents are associated with higher Cu contents in the pyroxene-porphyritic melanogabbros, suggesting the lithologies did not contain sufficient olivine that would partition the Ni. Generally, anomalous PGE contents occur with anomalous Ni and Cu contents in the lithologies. Se and S data suggest that externally derived sulphur resulted in sulphur saturation and the precipitation of Ni-Cu-PGE sulphides. High Se contents are detected in the main mineralized zone at the bottom of the lherzolite-olivine websterite, and in the zones of the underlying pyroxenite. High Se contents have also been detected in basement iron formation and SVU (Southern Volcanic Unit) mafic metavolcanic rocks consistent with these lithologies being sulphur sources for contamination and mineralization.
A copy of the thesis can be downloaded here
Jordan is currently working on his PhD at the University of Western Ontario