Stratiform platinum-group element mineralization in the layered Northern Ultramafic Center of the Lac des Iles Intrusive Complex, Ontario, Canada

摘要

Graphical abstractDetailed stratigraphic variation of modal rock type, modal minerals, selected PGE, base metals and trace elements throughout the CUA-6 which hosts the Lower Middle Interval from selected drill holes NL12-100. The rapid increase of Pd, Pt and Au abundance halfway through the cycle, accompanied with the increase of S and Cu abundances (which reflect sulfide mineralization) and the decrease of enstatite content clinopyroxene suggests that the mechanism of initial sulfur saturation and, by inference, the collection of precious and base metals is related with fractional crystallization. The abrupt increase of sulfide mineralization in the hybrid unit, just below the level at which CUB magma was emplaced, coinciding with the reversal (decrease) in enstatite content of clinopyroxene implies that the introduction of CUB magma also prompted the formation of an immiscible sulfide liquid and the collection of precious and base metals.Display OmittedHighlights?The Northern Ultramafic Center of the Lac des Iles Complex comprises two types of cyclic of unit that are distinguished by their cumulate sequences and interpreted to have formed from the periodic recharge of two geochemically distinctive ultramafic parent magmas that differed in their silica activity.?The stratiform PGE-enriched intervals occur exclusively in the type of cyclic units which generated from a Si enriched parental magmas.?Orthomagmatic processes of fractional crystallization and dynamic magma recharge were the dominant mineralization processes triggering sulfide-saturation and PGE concentration.AbstractThe Northern Ultramafic Centre (NUC) of the Lac des Iles Complex, Northwest Ontario hosts several platinum group element (PGE) occurrences, including the Sutcliffe Zone, which consists of four subparallel, stratiform PGE-enriched intervals exposed within the cyclically layered eastern flank of the NUC. Field relationships, mineral paragenesis and lithogeochemistry allowed for the identification of 14 cyclic cumulate sequences of two distinct types – Cyclic unit type A (CUA) and Cyclic unit type B (CUB). CUA-type and CUB-type units are interpreted to have formed from a Si-enriched and Si-poor parent magmas, respectively. PGE-enriched intervals occur in four of the CUA-type cyclic units (CUA-5, -6, -8 and -11). PGE enriched intervals are commonly associated with websterite, olivine websterite and gabbronorite containing primary disseminated sulfide (0.2–2vol%) which are dominated by pyrrhotite, chalcopyrite, and pentlandite with minor cubanite, and troilite. In hydrothermally altered rocks enriched in PGE, primary sulfides are locally partial