Date of Award

2016

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

First Advisor

Samson, Iain

Second Advisor

Gagnon, Joel

Rights

CC BY-NC-ND 4.0

Abstract

The Marathon and Geordie Lake Cu-PGE deposits occur within the Proterozoic Coldwell alkaline complex, located on the northeastern shore of Lake Superior, Canada. The Marathon deposit is hosted by the Two Duck Lake Gabbro (TDLG), and the Geordie Lake deposit is hosted by the Geordie Lake Gabbro (GLG). Three zones of mineralization have been identified at Marathon, which, in order of stratigraphic height from the basal contact, are the Footwall Zone, the Main Zone, and the W Horizon. Silicate mineral textures and chemistry reveal that the Marathon deposit formed in a dynamic conduit system with a complex magma evolution history. For example, the anorthite content and concentrations of trace elements such as Fe, Sr, Ba, and rare earth elements in plagioclase, change significantly across resorption surfaces. Several lines of evidence, such as the different pyroxene chemistry of the Footwall Zone compared to the other two zones (e.g., higher Fe, lower Mg, V, and Sc) and the absence of inverted pigeonite from the W Horizon, suggest that compositionally different magmas infiltrated the three zones at different times. Some features, such as the replacement of pyrrhotite by chalcopyrite, and sulfide intergrown with hydrothermal alteration minerals, which are more common in the Footwall and Main zones than the W Horizon, suggest that the metals were remobilized by volatiles, especially in the Footwall and Main zones. Given the ubiquity of hydrothermal alteration, principally in the lower part of the TDLG, the volatiles were possibly derived from devolatilization of the country rocks below the TDLG. Late apatite crystals are associated with residual hydrous melt aggregates, and are replaced along the rims. In the Footwall Zone, these replacement rims resulted from the interaction of late apatite with Cl-rich volatiles exsolved from the gabbroic melts. These rims have high metal contents and are associated with chalcopyrite indicating that the metals and S were also remobilized by these volatiles in the Footwall Zone. All δ34S values from the Marathon deposit are within the range of typical magmatic values (0 ± 2‰). Sulfides from the Marathon deposit, however, show a Δ33S range from - 0.91 to 0.00, and these values exhibit a negative correlation with Δ36S, indicating that sulfur was derived from both the mantle and Archean sedimentary rocks. Crustal sulfur contamination is probably not reflected in the δ34S values because of the similarities between mantle and Archean sedimentary-derived sulfur. Given that the country rocks in the vicinity of the Marathon deposit are felsic to intermediate volcanics and volcaniclastics, and contain insignificant amounts of sulfide, crustal sulfur assimilation must have occurred at depth, prior to magma emplacement. The magnitude of the Δ33S anomaly decreases with increasing distance from the basal contact suggesting that sulfur contamination from Archean country rocks was greatest in the Footwall Zone. The overall higher Cl/F ratios of apatite in the Footwall and Main zones compared to the W Horizon can be explained if chlorine was added to the Footwall Zone magma at the time that sulfur contamination occurred at depth. The Main Zone magma, however, is less contaminated and the W Horizon was not contaminated. Sulfides from the Geordie Lake deposit exhibit a wider range of δ34S values than in the Marathon deposit, but are within the range of magmatic values (0 ± 2‰). Nevertheless, the values exhibited by hydrothermal chalcopyrite compared to those of magmatic pyrrhotite, and to values of chalcopyrite from country rock syenite, indicate derivation of sulfur from an external source and from the gabbroic magmas.

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