A major Aphebian–Helikian unconformity within the Central Mineral Belt of Labrador: definition of new groups and metallogenic implications

Abstract

The Central Mineral Belt of Labrador consists of a belt of supracrustal rocks that occupies the northern foreland region of the Grenville Province of the Canadian Shield. Recent mapping in this belt has shown that the Proterozoic Croteau Group consists of two distinct sequences separated by an observed angular unconformity. It is therefore proposed that the name Croteau Group be abandoned and that the lower, Aphebian, marine sequence of sandstone, dolostone, slate, argillite, and mafic volcanic rocks be named the Moran Lake Group and that the upper, Helikian, continental sequence of conglomerate, tuffaceous sandstone, and a calc-alkalic volcanic assemblage be named the Bruce River Group.The Moran Lake Group underwent polyphase deformation, which has been assigned to the Hudsonian Orogeny, prior to deposition of the Bruce River Group around 1474 Ma. The Bruce River Group was intruded by a large granitic batholith, the Otter Lake Granite, for which a preliminary Rb–Sr isochron age of 1445 Ma has been obtained; this age correlates with the Elsonian magmatic event, an event well documented in northern Labrador. The Seal Lake Group, a Neohelikian (1278 Ma) sequence of quartzites, conglomerates, and intercalated mafic lava flows, was unconformably deposited upon the Bruce River Group and the Otter Lake Granite. During the Grenvillian Orogeny, the Bruce River and Seal Lake Groups were deformed together into a major easterly trending syncline. Deformation and metamorphism decrease across these groups to the north.The Bruce River Group forms part of the Labrador uranium area and hosts 14 known uranium occurrences. Occurrences are concentrated in the basal sandstones and conglomerates of the group, above the Aphebian–Helikian unconformity, and in ignimbrites and acid tuffs near the top of the group. No uranium occurrences are known from the Moran Lake Group except in fault-related fractures below the unconformity.

The position of the Grenville Front in eastern and central Labrador

“…changes in geological style. The only significant Our proposal is not entirely new (Wynnemodification is south of Makkovik where the boun-Edwards 1972; Smyth et al 1978), but we feel that dary has been positioned a few kilometres farther the new field data contribute to a more precise north to embrace a series of east-west shear zones understanding of the nature and limits of the Grenin massive granitoid rocks.…”

Anatomy and orogenic history of a Paleoproterozoic accretionary belt: the Makkovik Province, Labrador, Canada

“…Paleoproterozoic supracrustal rocks of the Makkovik Province have been assigned to the Moran Lake Group (Smyth et al 1978) and the Aillik Group (Stevenson 1970) (Fig. 3).…”

“…The Moran Lake Group consists of a lower unit of quartzite and iron formation overlain by siltstone, shale, dolostone, and greywacke (Warren Creek Formation) and an upper unit dominated by pillowed basalt (Joe Pond Formation; Smyth et al 1978;Ryan 1984). Metamorphic grade is uniformly lower greenschist facies.…”

Geochemistry, Tectonic Setting, and Mineralization of High-Potassium Middle Proterozoic Rocks in Central Labrador, Canada

“…the Sylvia Lake Formation of the middle Proterozoic Bruce River Group (Fig. 1;Smyth et al 1978). This contribution to the geochemistry and metallogeny of Proterozoic volcanic rocks is essentially a summation of data from several of our recent studies of the volcanic rocks (Baragar 1981;Kontak 1980;Ryan 1984).…”

“…The supracrustal rocks east of the Smallwood Reservoir ( Fig. 1) have been known for over thirty years to be hosts to numerous showings of Cu, Pb, Zn, U, Mo and Be mineralization, and constitute a metallogenic province termed the Central Mineral Belt (Smyth et aL 1978;Fig. 1, inset).…”