An Sb-enriched ultramafic lamprophyre (UML) sill was identified within the Hemlo Au-Mo deposit (David Bell mine) of the Superior Province, Canada. The low SiO2 (31 wt.%), high TiO2 (4.2 wt.%), Nb (146 ppm), Ta (10 ppm), Cr (900 ppm), and Ni (720 ppm) contents, as well as the unusual mineralogy (olivine, perovskite, Ti-rich phlogopite, calcite) of the sill are consistent with the geochemistry and mineralogy of other ultramafic lamprophyres on the global scale. The fine-grained, dark greenish-black sill has unchilled margins and intrudes basaltic and felsic fragmentals. The lamprophyre contains 794 ppm Sb, which is concentrated in fine-grained ullmannite, a Ni(Co)-Sb sulfide, within pervasively altered olivine. The presence of ullmannite in altered olivine within the UML implies that Sb was mobilized after the emplacement of the sill. Because Sb is considered to be one of the pathfinder elements for Au at Hemlo, and because antimony sulfides (including ullmannite) and arsenides are commonly associated with Au at the Hemlo camp, the addition of Sb to the Proterozoic lamprophyre suggests that Sb mobilization (and by inference, Au) possibly occurred throughout the tectonic evolution of the deposit. This has implications for genetic models of Hemlo, as cross-cutting relationships of Archean plutons and multi-phase deformation of the deposit make it difficult to distinguish between the actual timing of Au, Mo and Sb introduction and their subsequent re-mobilization and re-concentration into structural traps during various stages of deformation. It also re-opens questions concerning the genetic link between gold metallogeny and tectonic evolution of the region.

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