Master of Science
Lightfoot, Peter C.
The Proterozoic Thompson Nickel Belt (TNB) contains rift-related mafic-ultramafic igneous rocks, in a collisional belt at the NW margin of the Archean Superior province in Northern Manitoba, Canada. The TNB hosts world class Ni-Cu-(PGE) mineralization, that has undergone polyphase deformation (D1 – D4) and upper amphibolite to lower granulite facies metamorphism, up to ~750ºC and 7.5 kbars (Lightfoot et al., 2017). Komatiitic magmas (1880 ± 5 Ma; Hulbert et al., 2005) fertile in Ni & PGE elements intruded Ospwagan Group metasedimentary sequences and through sulfur assimilation and segregation produced primary magmatic sulfides (Lesher and Burnham, 2001).
The TNB metasedimentary-hosted ores of the Thompson Deposit (T1 and T3) are interpreted to have formed from the complete remobilization and relocation of sulfides from primary ultramafic boudins (Bleeker, 1990). The aim of this study was to assess geological, mineralogical and geochemical controls on the Ni tenor of sulfide (% Ni normalized to 100% sulfide), and to understand why Ni tenor in the metasedimentary-hosted Thompson deposits is higher than those of primary ultramafic associated mineralization, like the Pipe and Birchtree Deposits. The research focused on the Thompson 1D orebody at T3 mine because of the large variability of nickel tenor, ranging from(8-16 wt% Ni).
The two most significant findings of this study are that; Ni-Co-Pd tenors are highest in shear-hosted massive sulfides and that there is a positive correlation between pyrite modal abundance and Ni-Co tenor. Pyrite is interpreted to be paragenetically-late, which strongly indicates a metamorphic modification of the ores. Understanding the processes responsible for pyrite generation is crucial to deconvoluting the cause of the Ni tenor variability across the Thompson 1D deposit.
This study is the first to document the occurrence of polymetallic sulfide melt inclusions (SMI) at Thompson, which indicates that partial melting of sulfide ores occurred during peak metamorphic conditions and modified the ores at Thompson.
A staged mechanism for the formation of Thompson sulfide deposits is proposed, in which three principle stages exist: komatiitic emplacement and a primary magmatic origin for the sulfides; high P-T MSS reformation and repositioning of primary ultramafic sulfide, and partial melts, into sedimentary country rocks; and modification by hydrothermal fluids, contemporaneous localized partial melting, and shear-melting, which further modified the remobilized ores.
Research in this study has helped move the ore genesis models for Thompson forward and although nickel tenor upgrading of ores is not entirely explained, it seems to be related to pyrite and the late tectonic-metamorphic history of the deposit.
Summary for Lay Audience
The Thompson Nickel Belt is a collisional craton margin located along the northwestern edge of the Superior Igneous Province (Canadian Shield). This region underwent a classic rift-drift-collision sequence (Wilson Cycle) providing an optimal location for ultramafic komatiitic magmas to be emplaced into sulfide-bearing, sedimentary rocks (Ospwagan Group). This resulted in the production of primary magmatic sulfides which are host to world-class nickel mineralization. The rocks underwent high pressure-temperature (7.5 kbars and ~750ºC) metamorphism and deformation. The magmatic sulfides were remobilized from primary ultramafic intrusions into the neighbouring meta-sedimentary rocks, forming the Thompson deposits. The purpose of this study was to determine geological, mineralogical and geochemical factors that might be responsible for the higher nickel grades (tenor) in the remobilized, metasedimentary-hosted Thompson deposit compared to those of the primary ultramafic-associated deposits. Research focused on the 1D sulfide deposit at Thompson mine (T3), where nickel tenors are highly variable, ranging from(8-16 wt% Ni). The two most significant findings of this study are that nickel grades seem to be highest in shear-hosted massive sulfides and a positive correlation between pyrite modal abundance and nickel grade exists. The mineral pyrite is interpreted to be paragenetically-late, which strongly indicates a metamorphic modification of the ores, and possible link to explaining the highly variable sulfide grades. Additionally, this study is the first to document the occurrence of sulfide melt inclusions at Thompson, which indicate that partial melting of sulfide ores occurred during peak metamorphic conditions. A three-part, model is proposed to explain the formation of the remobilized Thompson metasedimentary-hosted sulfide deposits: a primary magmatic origin for magmatic sulfides; high pressure-temperature metamorphism, transport and partial melting of these primary sulfides; and, ensuing post-ore modifications of sulfides by contemporaneous hydrothermal fluids and localized shear-melting.
Grigutis, Povilas G., "Geological, mineralogical and geochemical process controls on Ni tenor variations in the metasedimentary-hosted Thompson 1D orebody at Vale’s Thompson mine (T3), Northern Manitoba, Canada" (2021). Electronic Thesis and Dissertation Repository. 7727.
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