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
Abstract
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.