Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Geology

Supervisor

Dr. Norman Duke

Abstract

Supracrustals of the 2.76-2.71 Ga Mary River Group form the northeastern segment of the Committee Bay Belt. Field mapping has determined that this succession is preserved in structural keels marginal to high-grade gneiss domes and is characterized by lower metasedimentary rocks and an upper BIF-komatiite-quartzite succession. Algoma-type BIF sited at the dome/keel boundary is host to bodies of high-grade iron ore, the largest being Deposit 1 which has an estimated reserve of 500 mt averaging 64 wt % Fe.

Field mapping, petrography, and EMPA indicates 3 main stages of ore formation. Stage 1 was characterized by the formation of the dome-and-keel tectonic style during the regional Paleoproterozoic Transhudson overprint. Juxtaposition of cold greenschist facies supracrustals against hot amphibolite facies gneiss domes resulted in dehydration of supracrustals. Dehydration of komatiites and amphibolitic wacke provided a fluid reservoir for an alkaline Mg-Fe-Ca-rich brine, and accounts for massive desilication and massive chlorite retrogression at the boundary. Stage 2 involved ascent of this hot (400-500°C) fluid leaching silica, stabilizing Mg-Fe-Ca carbonate within enriched BIF. This was followed by prograde thermal metamorphism resulting in porphyroblastic growth of numerous Fe-Mg(-Ca) species. Stage 3 postdates the thermal peak and involved a shift to lower pH and cooling to below 300°C, causing the oxidation of magnetite to martite. This fluid leached remnant carbonate, and release of CO2 formed retrograde replacement of relict carbonate by talc, serpentine, sericite, and minor chlorite within the massive magnetite bodies

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