The Ni–Cr–Cu content of biotite as pathfinder elements for magmatic sulfide exploration associated with mafic units of the Sudbury Igneous Complex, Ontario, Canada

Abstract

Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) was used to evaluate the dissolved Ni–Cr–Cu content of biotite as an exploration pathfinder for magmatic Ni–Cu–platinum-group element (PGE) sulfide deposits associated with mafic igneous units of the Worthington quartz diorite offset dyke and their host rocks at the Totten Mine (Vale Canada Ltd), Sudbury Igneous Complex (SIC), Ontario, Canada. Enrichment in Cu in biotite (up to two orders of magnitude higher than background) occurs within barren to weakly mineralized, sublayer quartz diorite and adjacent Huronian metasediments within ~200m of massive sulfide. With respect to Ni and Cr, three distinct populations of biotite were distinguished based on textural and chemical criteria: (i) type I — isolated, euhedral laths within only inclusion-rich sublayer quartz diorite (IQD; the main mineralized host lithology), and high Ni (~1400ppm<[Ni]Bt<~2700ppm), low to moderate Cr (~50ppm<[Cr]Bt<~2450ppm) and variable Ni/Cr ratios (always >2). (ii) Type II — coarse-grained poikiloblasts (enclosing amphibole, chlorite, and type III biotite) within country rocks, and having moderate to high Ni (~500ppm<[Ni]Bt<~1400ppm), moderate to very high Cr (~1750ppm<[Cr]Bt<~6000ppm) but consistently low Ni/Cr ratios (always <0.5); and (iii) type III — subhedral to euhedral laths, intergrown with amphibole in dense, foliated aggregates and having low Ni ([Ni]Bt<~300ppm), very low to moderate Cr (~4ppm<[Cr]Bt<~1200ppm), and variable Ni/Cr ratios (<20), found within Huronian country rocks, sublayer quartz diorite (QD), and cross-cutting diabase dykes (Sudbury dyke swarm). Type I biotite within IQD is compositionally distinct from those observed in all other lithologies associated with the Sudbury Igneous Complex and its footwall rocks, and can be most readily discriminated in a Ni/Cr vs. Ni binary diagram or in a Ni–Cr–Cu ternary diagram by anomalously high Ni content and Ni/Cr ratio >2.Application of biotite chemistry to routine exploration requires establishing local “background” metal-in-biotite concentrations for each potential host lithology, and scrutiny of anomalous or heterogeneous metal contents in biotite resulting from discrete sulfide microinclusions that contaminate the analytical volume, and chloritization or coeval sulfide minerals in direct contact with biotite causing localized modifications to primary biotite metal abundance.In the Sudbury environment, the Ni–Cr–Cu chemistry of biotite can be used (i) in drill core to identify proximity to mineralization, and to differentiate QD (hosting minimal sulfides) from IQD (primary host to sulfide ore bodies), rock types for which bulk textural and compositional discrimination is problematic; and (ii) in soils and tills through the analysis of biotite and its weathering products to locate buried or surface-exposed IQD. The results may be extended to other mafic–ultramafic systems where sulfide-saturated or metal-enriched intrusive phases grew metal-enriched biotite during primary crystallization, or through secondary processes of metasomatic enrichment involving remobilization of base metals by magmatic-hydrothermal fluids.