Geochemistry and petrogenesis of the Black Thor intrusive complex and associated chromite mineralization, McFaulds Lake greenstone belt, Ontario

Abstract

The Black Thor intrusive complex (BTIC) contains a conduit-hosted, stratiform Cr-Ni-Cu-(PGE) deposit with a very large amount of chromite for an intrusion of its size. Most conduit-hosted stratiform deposits are Archean, formed from komatiitic magmas containing approximately 3000 ppm Cr2O3, and are typically saturated in chromite. The fundamental problem in understanding the genesis of the BTIC deposit and other deposits of this type is explaining how such large quantities of chromite crystalized from a magma that normally crystallizes only small amounts chromite and normally have a chromite:olivine abundance ratio of ~1:60. Current genetic models, such as in situ crystallization (by oxidation, pressure increase, magma mixing, and/or wholesale assimilation of felsic rocks or iron formation) or physical transportation of chromite slurries do not provide a wholly satisfactory explanation for the high abundance of chromite in this type of deposit. We are testing an alternative model: partial assimilation (as opposed to wholesale assimilation) of local oxide-silicate-facies iron formation by a Cr-rich magma. As low-Mg komatiite is saturated in chromite, the magma may dissolve the silicate component (quartz/chert and Fe-silicate minerals) of the iron formation, but would be unable to dissolve the oxide component (magnetite). Through interaction with the hightemperature (1400°C) Cr-rich magma, the fine-grained magnetite could be upgraded via diffusion to chromite during transportation within the conduit. This upgrading is similar to the upgrading of barren sulphide xenomelts that has been proposed for Ni-Cu-(PGE) deposits.

Diversity of Net-Textured Sulfides in Magmatic Sulfide Deposits: Insights from the Eagle’s Nest Ni-Cu-(Platinum Group Element) Deposit, McFaulds Lake Greenstone Belt, Superior Province, Canada

“…Nonmineralized peridotites and pyroxenites in the Eagle's Nest dike vary between ~22 and 38% and ~21 and 34% MgO (Fig. 10), respectively, and plot along the same trends as barren host rocks at Black Thor (Carson et al, 2015). For other elements, Si-Ti-Ca increases with decreasing Mg except for samples containing significant orthopyroxene (which plot at lower Si-Al-Ca; Fig.…”

“…1; Houlé et al, 2020;Metsaranta and Houlé, 2020). The iron formation is composed of interbedded layers dominated by silicates (fine-grained quartz ± stilpnomelane ± grunerite ± allanite), oxide (finegrained magnetite), and lesser chlorite (probable mafic volcaniclastic horizons; Carson et al, 2015) with fine-grained acicular ferristilpnomelane along the contacts between the silicate and oxide layers. Rafts of 25-to 60-m-thick leuco-mesomelagabbroic rocks also occur in the tonalite.…”

“…Rafts of 25-to 60-m-thick leuco-mesomelagabbroic rocks also occur in the tonalite. The plagioclase is predominantly fine to medium grained but locally porphyritic and is normally altered to chlorite and amphibole (Carson et al, 2015).…”

“…Tuchscherer, unpub. report, 2010;Carson et al, 2015;Houlé et al, 2020), so the Esker intrusive complex is interpreted to young to the south-southeast and to have been structurally rotated toward that direction from an originally subhorizontal orientation (e.g., Houlé et al, 2020).…”

Comparisons Between Tethyan Anorthosite‐Bearing Ophiolites and Archean Anorthosite‐Bearing Layered Intrusions: Implications for Archean Geodynamic Processes

“…Just as with Tethyan anorthosite‐bearing ophiolites, most Archean anorthosite‐bearing layered intrusions crystallized mainly from tholeiitic (hydrous Al‐rich tholeiitic, hydrous tholeiitic, high‐Al tholeiitic, or island arc tholeiitic) magmas (Figure 12 and Table S2). The Neoarchean Black Thor Intrusive Complex in Canada was interpreted to have crystallized from a komatiitic basaltic magma (Figure 12 and Table S2; Carson et al, 2015). The low Zr/Y, La/Yb, and Th/Yb ratios exhibited by Archean anorthosite‐bearing layered intrusions corroborate their interpreted tholeiitic affinity (Table S2; Ross & Bédard, 2009).…”