Indicator mineral and till geochemical signatures of the Broken Hammer Cu–Ni–PGE–Au deposit, North Range, Sudbury Structure, Ontario, Canada

Abstract

The Broken Hammer Cu–Ni–PGE–Au footwall deposit in the North Range of the Sudbury Structure in Canada consists of a shallow surface zone of vein-hosted and vein stockwork-hosted mineralization within Sudbury breccia developed in the quartz monzonite Levack Gneiss Complex. The surface of the deposit consists of a 2–120 cm wide chalcopyrite vein and numerous smaller veins dominated by chalcopyrite–magnetite–millerite with trace gold, platinum group minerals, tellurides, bismuthides and selenides. The Laurentide Ice Sheet flowed southward across the region depositing a sandy till that contains abundant sperrylite (hundreds of grains), chalcopyrite, pyrite and gold in the heavy mineral fraction down-ice of mineralization. Mineral liberation analysis of the <0.25 mm heavy mineral fraction of metal-rich till identified a broader suite of PGM and sulfides than visual identification methods. The <0.063 mm fraction of till displays a strong geochemical signature of the mineralization for Pd, Pt, Au, Cu and Ag and, to a lesser extent, Bi, Te and Sn; however, geochemical signatures are not detectable as far down-ice as indicator minerals. Till sampling has not been used for exploration in the Sudbury region because of the abundant outcrop and the use of geophysical and prospecting techniques. This study demonstrates that indicator mineral and till geochemical methods are useful exploration tools for the region. The presence of sperrylite and chalcopyrite in oxidized till indicates that even thin (<1 m) highly weathered till is an effective sample medium here.