Komatiite-associated Cr and Ni-Cu-(PGE) mineralisation in the Black Thor-Black Label ultramafic intrusive complex, McFaulds Lake greenstone belt, Ontario

The Black Thor - Black Label deposit appears to be one of the largest and best-preserved chromite deposits in the world, and one of an increasing number of deposits recognized to have formed from high-Cr, low-Mg komatiitic magmas. The ores exhibit an exceptionally wide range of textures (disseminated, patchy disseminated, patchy net-textured, net-textured, mottled semi-massive, banded semi-massive, massive, and angular and ameboid magmatic breccia) and are thinly laminated to thickly bedded, with aggregate thicknesses up to 100nn. Regardless of texture, individual chromite grains are uniformly 0.1-0.2 mm and euhedral, many massive but some with inclusion-rich cores. The abundance of net-textured chromite, where aggregates of fine-grained chromite form networks to less dense olivine, the presence of extremely fine-scale layering, and the apparent absence of evidence for slumping favour in situ crystallisation, but the inclusion-rich cores appear to require initial mechanical transport and upgrading at high magma:chromite ratios (high R factor).