Geoff Heggie MSc thesis abstract

The Seagull Intrusion emplaced during formation of the 1.1 Ga Mid-continental Rift centred around Lake Superior was studied in several drill core and surface samples and consists of a lower ultramafic section (~650 m) and an upper mafic section of unknown thickness (<100m). The lower ultramafic section is texturally dominated by cumulate olivine and poikilitic clinopyroxene with lithologies consisting of dunites, peridotites and pyroxenites. The upper mafic section is variable in lithology, with increasing upwards in abundance of cumulate pyroxene and poikilitic feldspar, dominated by olivine gabbro and gabbros.  

Olivine, pyroxene and oxide mineral chemistry from the lower ultramafic section, indicates limited fractionation throughout the section. Forsterite compositions range from Fo75.8 to Fo86.3. Pyroxene also exhibits a limited range of variability with clinopyroxene restricted to the compositions of En42Wo48Fs10 to En54Wo39Fs7. Oxides occur as inclusions in olivine and in pyroxene with compositions reflecting subsolidus re-equilibrium with the host mineral. Olivine compositions from the mafic section exhibit a more differentiated nature and range in composition from Fo57.4 to Fo75.8.

Whole rock geochemistry is consistent with the homogeneous nature of the ultramafic section and more differentiated character of the mafic section. Major elements only exhibit minor variations in abundance through the ultramafics, and exhibit enrichment in Fe and Si only in the mafic section. Trace elements and primitive mantle normalised patterns identified five geochemical units, characterised by total trace element abundances and Eu, Nb and Th anomalies. Sm-Nd and Rb-Sr isotopes indicate the intrusion was variably contaminated by older continental crust. The most extensive contamination is found in the basal section of the intrusion and decreases away from the contact.

Platinum, palladium, nickel and copper mineralization is found at the base of the ultramafic section of the intrusion, formed during initial emplacement and sulphur saturation at that point in time. A second mineralized horizon (RGB Horizon) occurs ~100 to 150m above the basal contact. The RGB Horizon is continuous over 700 m and contains three distinct horizons separated by at least 2 m of unmineralized ultramafic rock. These layers are interpreted to have formed as individual events related to the injection of primitive magma refreshing the chamber.

Geoff is currently working on his PhD at the University of Western Australia in Perth

A copy of this thesis can be downloaded here