A Detailed Metallogenic Study of the McFaulds Lake Chromite Deposits, Northern Ontario

The Black Label, Black Thor and Big Daddy chromite deposits are a series of chromitite layers that are hosted by the 2734.5 +/-1.0 Ma ‘Ring of Fire’ Intrusion in the McFaulds Lake greenstone belt of Northern Ontario. Over 4200 electron microprobe analyses and 142 laser ablation ICP-MS analyses were performed on chromite and record individual fractionation sequences of chromitite below the metre scale. In comparing results, the dunite-hosted Black Thor chromites are higher grade with 53 to 49 wt. % Cr2O3, more primitive than Black Label chromites at 50 to 46 wt. % Cr2O3. Heterogeneous growth of orthopyroxene oikocrysts in Black Label chromitite accounts for increased silicate content. Massive chromitite at Big Daddy shows remarkable homogeneity at 50 to 51 wt. % Cr2O3. The homogeneity of the massive ore is attributed to development of chromitite layers by continuous magma pooling. A strong linear regression of increasing Cr2O3 with MgO from disseminated to massive chromite at all deposits indicates the primary chromite compositions are magmatic. Elemental variation in the chromites include decreasing Cr, increasing Fe, slightly decreasing Al and decreasing Mg due to differentiation. Near constant low-Al compositions are characteristic of komatiitic chromite. Laser ablation results show trends of depletion in Ti, V, Zn, Mn and Co, but enrichment in Ni and Sc, consistent with fractionation of chromitite. The chromitites contain mineral inclusions of pyroxene, igneous amphibole, and phlogopite that have very magnesian contents suggesting a source in the ultramafic magma. The host dunite and pyroxenite have negative Nb-Ta and Zr-Hf anomalies that signify an inherent contamination of ultramafic magma by TTG crust. The Ring of Fire Intrusion has been autohydrated, causing pervasive serpentinization and tremolitization of host dunite and pyroxenite. Secondary ferrichromite with higher wt. % FeOT and Cr2O3 forms as rims on primary chromites, thereby enriching the Cr content of the ore. The rims of primary chromites are leached of Mg and Al leaving high Cr and Fe. The released Mg and Al forms magnesian clinochlore that contains up to 9 wt. % Cr2O3