Abstract

Alteration veins containing Cl-poor actinolite and actinolitic hornblende are a common feature throughout the Archean-aged gneisses and granitoids along the northern margin of the Sudbury Igneous Complex, Ontario, Canada. Crosscutting relations and structural measurements show that the veins postdate the Sudbury impact event but predate the emplacement of footwall-style Cu-Ni-PGE (platinum-group element) sulfide ores. The veins are four to five times more abundant in brecciated, mineralized outcrop than in unbrecciated, barren outcrop. Analyses of vein-hosted amphibole by electron microprobe (EMP) and laser ablation ICP-MS (LA-ICP-MS) show elevated concentrations of Ni (up to ~100 times background; increasing from ~100 ppm to ~1.0 wt %), Cu (up to ~4 times background) and Sn (up to ~2.5 times background) in proximity to sulfide veins. Copper and Sn contents in amphibole are only anomalous within 10 to 20 m of mineralization and require quantification by LA-ICP-MS. However, anomalous Ni concentrations in vein-hosted amphibole may be detected with a high level of confidence solely by EMP up to ~700 m from mineralization in rocks that contain no visible sulfides in hand sample or thin section and show no bulk rock Ni or Cu anomalies. The relationship between the Ni content in amphibole (in ppm) and mineralization proximity (distance “D” in meters) is given by the following equation:

 
\[[Ni]_{amp}\ =\ 5100\ (D)^{{-}0.45}.\]

The data indicate wide-scale Ni mobility in hydrothermal solutions in the Sudbury footwall. The formation of Ni-rich amphibole in the fracture networks, by either the growth of primary Ni-rich amphibole or metasomatic Ni enrichment of preexisting (Ni-poor) amphibole, was associated with circulation of a sodium- and halogen-rich fluid between ~390o and 550oC. Nickel may have been released into the footwall environment by extensive hydrothermal alteration of Ni-rich contact-style sulfide ores that lie in the hanging wall of the Cu-rich footwall-style ores. In the footwall rocks, pre-existing fracture networks (now amphibole-bearing veins) acted as common structural pathways, first for the circulation of early Ni-rich fluids associated with leaching in the contact region and the onset of footwall mineralization, and second for the emplacement of Cu-rich footwall-style sulfide veins. Other metal concentrations in amphibole (Pb, Zn, Co) show no relationship to mineralization proximity. These metals may have been derived locally from country rocks during alteration reactions that promoted amphibole growth in the fractures. Identification of fracture-hosted amphibole with anomalous Ni contents in thin section, or Ni-Cu-Sn anomalous amphibole grains liberated from surface-weathered rocks hosting sulfide veins may serve as useful proximity indicators for hidden footwall-style sulfide deposits at Sudbury.

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