Controls on the regional distribution of iron – nickel – copper – platinum-group element sulfide mineralization in the eastern Cape Smith Belt, Quebec

Abstract

Fe – Ni – Cu – platinum-group element (PGE) mineralization in the eastern Cape Smith Belt (Quebec) is hosted by ultra-mafic and differentiated mafic–ultramafic bodies (high-level sills and dykes, lava lakes, or channelized lava flows). These units of Early Proterozoic age are associated with both volcanic and sedimentary rocks (Povungnituk Group) that accumulated during continental rifting and a transitional continental to oceanic basalt sequence (Chukotat Group). Mineralization is found in two principal horizons. Important deposits occur in the Raglan horizon located near the tectonic boundary between the Povungnituk and Chukotat groups. A second mineralized zone, termed the Delta horizon, occurs within the Povungnituk Group and contains smaller sulfide showings. The distribution of proximal–distal sedimentary facies in the Povungnituk Group represents an important control on the localization, grade, and size of the sulfide deposits and showings. Deposits in the Raglan horizon are associated with relatively distal sedimentary units of the Povungnituk Group. In contrast, the Delta horizon occurs in a more proximal (quartz-rich) facies of the Povungnituk Group which is interlayered with volcanic rocks and marked by a profound decrease in the proportion of fine-grained, sulfuric beds relative to the Raglan horizon. Regional mapping has shown that the Cape Smith Belt is a thin-skinned thrust belt containing imbricates of Povungnituk Group and Chukotat Group rocks and thus controlling the regional extent of the mineralized horizons. Critically, the along-strike continuity of the Raglan horizon (85 km) is determined by the three-dimensional structural geometry of a late thrust fault.