Marc Rinne's Honours thesis abstract

The Big Lake Ultramafic Complex (BLUC) is a 30km by 500m east-west striking intrusion located 30km southeast of Marathon, Ontario, on property owned by MetalCORP Limited.  It is a layered intrusion formed primarily by cumulate differentiation of two or more ultramafic magmas, comprising an amphibolite to melagabbro overlain by two sequences of relict cumulate peridotite and cumulate websterite.  Platinum group element mineralization occurs within the complex as reefs within the two peridotite units, near their upper contacts with pyroxenites.  PGMs are euhedral sulphides and arsenides within intercumulus magnetite, and are also observed in hand sample to occur in weak association with chalcopyrite.  The magmatic process responsible for the precipitation of PGMs, and the change from pyroxene to olivine cumulate phases in both reefs, is interpreted to be the introduction of a hotter, more primitive magma into the chamber.  The role of hydrothermal processes in the formation of the reef deposits is negligible.   

If two mixing events are responsible for the two PGE reefs observed at Big Lake, the position of the reefs with respect to igneous layering suggests the BLUC sequence is overturned to match magma mixing models such as that applied to the J-M and Merensky reefs.  Other evidence suggesting the BLUC is overturned includes pyroxenite xenoliths near upper peridotite contacts into pyroxenites.
 
A steep northward dip, regional metamorphism of upper greenschist to lower amphibolite facies matching that of host metasedimentary and metavolcanic rocks of Archean age, and a trace element signature unlike that of Midcontinent Rift intrusions suggest that the BLUC is one of several syn-Kenoran mafic-ultramafic intrusions located throughout the Heron Bay area of the Schreiber-Hemlo greenstone belt, of an age older than 2.68Ga. 

 

Marc is currently a Ph.D. student in the CODES, University of Tasmania