Rowan Perrott, BSc

R. Perrott¹, A. McDonald1, L. Foucault², J. Olivera^3 
1Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada
 ²Sudbury Integrated Nickel Operations, Falconbridge, Ontario, Canada
³Glencore-XPS, Falconbridge, Ontario, Canada 

The Norman West Deposit is an advanced stage Ni-Cu-PGE exploration project on strike with the Whistle offset dyke (which hosted the past-producing Whistle and Podolsky mines). Located at depth (1650-2850 m) with no surface expression, it is divided into contact-style and footwall-style zones based on major sulfide mineralogy and Ni/Cu tenors. The footwall-style zone, which is the focus of this study, shows an atypical texture of alternating bands of chalcopyrite and pyrrhotite not previously reported in other footwall deposits. It occurs in a breccia package, as opposed to typical sharp-walled veins, so does not represent a typical footwall deposit. This study aims to characterise the types, associations and grain sizes of precious-metal mineral assemblages (PMMs, including PGMs and Au+Ag-bearing minerals) to understand the genesis of the deposit and to constrain the conditions related to its formation. Samples from drill core (n=17) were selected for optical and SEM-EDS analyses. Data from SEM-EDS analyses indicates that the dominant Pd-bearing mineral species is michenerite [PdBiTe], up to 150μm in size, which occurs in two populations; population 1 contains up to 0.4 apfu Pt, while population 2 is Pt-free. Michenerite, as well as other Pd-dominant PGM, often occurs in anhedral multi-mineral aggregates along with hessite [Ag₂Te], clausthalite [PbSe], altaite [PbTe] and tsumoite [BiTe], which may include Cl-bearing minerals, including bismoclite [BiOCl] and annite. These aggregates take two forms: those with sharp grain contacts, and those which are droplet-like, found only in chalcopyrite, and show irregular grain contacts with abundant mutual inclusions, suggesting different parageneses. The dominant Pt-bearing species is sperrylite [PtAs₂], which is found as euhedral to subhedral single crystals up to 220 μm, enclosed within sulfides and Cl-bearing annite. Argentopentlandite [Ag(Fe,Ni)₈S₉] is the main Ag-bearing mineral.  It develops in oriented, cross-hatched intergrowths with pentlandite (exsolution?), a texture not previously reported, and is invariably associated with chalcopyrite. Norman West crystallised from an Fe-Cu-Ni-PGE melt, and quenching above 254°C may explain the dominance of hexagonal pyrrhotite and lack of exsolved phases like cubanite, leading to the pyrrhotite-chalcopyrite banded texture. Segregation of Ni from this melt gave rise to the Ag-bearing pentlandite which subsequently underwent exsolution during cooling. The droplet-like aggregates of PMMs developed as segregations in chalcopyrite and underwent separation during cooling to form the complex aggregates observed, with irregular intra-droplet grain contacts also suggesting quenching. The association of Cl-bearing hydrous silicates and bismoclite with PMM assemblages may also suggest strong oxidising conditions.