Sb-enriched ultramafic lamprophyre in the Hemlo Au-Mo deposit of the Superior Province, Canada: Evidence for Post-Archean Sb mobility

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 (9 00 ppm), and Ni (720 ppm) contents, as well as the unusual mineralogy (oliv ine, perovskite, Ti-rich phlogopite, calcite) of the sill are consistent wi th the geochemistry and mineralogy of other ultramafic lamprophyres on the global scare. The fine-grained, dark greenish-black sill has unchilled marg ins and intrudes basaltic and felsic fragmentals. The lamprophyre contains 794 ppm Sb, which is concentrated in fine-grained ullmannite, a Ni(Co)-Sb s ulfide, within pervasively altered olivine. The presence of ullmannite in a ltered olivine within the UML implies that Sb was mobilized after the empla cement of the sill. Because Sb is considered to be one of the pathfinder el ements for Au at Hemlo; and because antimony sulfides (including ullmannite ) and arsenides are commonly associated with Au at the Hemlo camp, the addi tion of Sb to the Proterozoic lamprophyre suggests that Sb mobilization (an d by inference, Au) possibly occurred throughout the tectonic evolution of the deposit. This has implications for genetic models of Hemlo, as cross-cu tting relationships of Archean plutons and multi-phase deformation of the d eposit make it difficult to distinguish between the actual timing of Au, Mo and Sb introduction and their subsequent re-mobilization and re-concentrat ion into structural traps during various stages of deformation. It also re- opens questions concerning the genetic link between gold metallogeny and te ctonic evolution of the region.