Abstract
Most magmatic chromite deposits, particularly those in stratiform layered ultramafic-mafic intrusions, are interpreted to have formed by processes that decrease the solubility of chromite. However, such models have difficulty in explaining the great thicknesses (up to 10 m) of chromite mineralization at Inyala and Railway Block (Zimbabwe), Ipueira-Medrado (Brazil), Uitkomst (South Africa), and Sukinda (India), and the very great (up to 100 m) thicknesses of chromitite at Kemi (Finland) and in the Black Thor Intrusive Complex (Canada). We propose a new process that involves partial melting of Fe ± Ti oxide–rich rocks (oxide-facies iron formation or ferrogabbro) and conversion of fine-grained oxide to chromite by reaction with Cr-rich komatiitic magma in a dynamic magma conduit. This process is analogous to the widely accepted model for the upgrading of barren iron-sulfide xenomelts to form magmatic nickel–copper–platinum group element deposits. We demonstrate that this process is geologically, physically, and chemically feasible, that it probably occurred in the Black Thor Intrusive Complex, and that it may have applications elsewhere.
Highlights
Chromite deposits have traditionally been subdivided into stratiform and podiform types (e.g., Stowe, 1994)
There are oxide-silicate–facies iron formations and Fe-Ti oxide-bearing gabbros in underlying rocks, the intrusions contain iron-formation and gabbro xenoliths, the parental magma was a chromite-saturated low-Mg komatiite (Mungall et al, 2010), and the chromite-bearing intrusions are dominated by cumulates and interpreted to represent flow-through magma conduits (Carson et al, 2013)
The chromites in the Black Thor Intrusive Complex exhibit a wide range of textures including (1) massive, (2) inclusion-bearing, and (3) pitted with fine Fe ± Cu sulfides (Fig. 3A)
Summary
Chromite deposits have traditionally been subdivided into stratiform (type I) and podiform (type II) types (e.g., Stowe, 1994). A fundamental problem in the genesis of all stratiform chromite deposits is how layers of massive to semi-massive chromite that are commonly up to 1 m thick (e.g., Bushveld [South Africa], Stillwater [Montana, USA]), less commonly up to 10 m thick (e.g., Inyala and Railway Block [Zimbabwe], Ipueira-Medrado [Brazil], Uitkomst [South Africa], Sukinda [India]), but in some cases up to 100 m thick (e.g., Kemi [Finland], Black Thor Intrusive Complex [Canada]) (Table DR1) are generated from magmas that typically contain
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