A Study of the Composition, Distribution, and Genesis of Pyrrhotite In the Copper Cliff Offset, Sudbury, Ontario, Canada

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Pyrrhotite (Po) from five orebodies (830, 865, 880, 890, and 100 OB) in the Copper Cliff Offset Dike (CCOD) at Sudbury, Ontario was studied using powder X-ray diffraction data and Rietveld analysis, SEM-EDS, magnetic colloid, optical petrographic techniques, and Laser Ablation-ICP-MS in order to: (1) quantify the distribution of NC (non-magnetic, symmetry higher than monoclinic) and 4C (magnetic, monoclinic in symmetry) Po; (2) characterize the major, minor, and trace-element chemistries of both polymorphs; and (3) determine the factors enhancing the stability of NC Po. Results confirm that both NC and 4C Po are present, with ratios varying between 0 to 0.97. In general, 4C Po is the dominant polymorph within the sampled OB, with the exception of the 100 OB, which is dominated by NC Po. Mineral chemistry indicates that there is an almost complete compositional overlap between the major, minor, and trace-element compositions of NC and 4C Po from the CCOD. The data also show that both polymorphs are slightly metal-deficient relative to the ideal chemistry of 4C Po, Fe7S8. There were no significant differences found in the Ni contents of either polymorph. Data revealed that the main mineralogical difference between the five OB was the presence of ilmenite in the 830, 865, 880, and 890 OB, whereas magnetite dominates in the 100 OB. Mineralization occurs along the eastern margin of the 830–890 OB, in contrast to the 100 OB, where mineralization occurs in the center of the dike. The increased abundance of NC Po in the 100 OB may be explained by this difference in the style of mineralization, specifically, the proximity of ore to the surrounding country rock. Therefore, f O2 may have directly influenced the distribution and stabilization of NC Po, which would then impact the observed NC:4C Po ratios in the selected OB.