Geology, geochemistry and metallogeny of the Archean Florence Lake Group and associated ultramafic and trondhjemitic rocks, Nain Province, Labrador

Brace, Terry D. (1990) Geology, geochemistry and metallogeny of the Archean Florence Lake Group and associated ultramafic and trondhjemitic rocks, Nain Province, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Florence Lake Group (FLG) forms a 65 km-long northeast-trending greenstone belt within the southernmost Archean Nain Province, Labrador. The FLG contains a bimodal volcanic sequence consisting dominantly of mafic metavolcanic schists, flows and pillow lavas, and minor synvolcanic sills. Felsic metavolcanic and siliceous metasedimentary rocks are less extensive. The FLG is intruded by elongate bodies of strongly serpentinized peridotite, which have undergone various degrees of carbonatization. Massive, gneissic and mylonitic trondhjemite and tonalite of the Kanairiktok Intrusive Suite envelop and intrude both the FLG and the metaperidotites. All units are folded and have undergone middle greenschist facies metamorphism. Post-tectonic Proterozoic diabase and gabbro dykes, informally referred to as the Florence Lake (FL) dykes, cut all other lithologies. -- Based on the relative depletion of LREE, FLG basalts can be divided into two groups: (1) Group 1 has flat chondrite-normalized REE patterns; (2) Group 2 is slightly LREE-depleted. The FLG basalts are similar to typical N-MORB, whereas the felsic supracrustal rocks and Kanairiktok trondhjemites display classic arc (or continental crust) signatures. Both lithological and geochemical characteristics are consistent with the FLG having been deposited subaqueously in a back-arc basin which lay proximal to a volcanic arc and/or continental margin. -- The Proterozoic FL diabase and gabbro dykes appear to be of two ages: (1) an older set of north- to northeasterly-trending gabbro dykes, viz., the FL gabbro dykes; and (2) an apparently younger set of east-west-trending smaller diabase dykes, viz., the FL diabase dykes. The FL gabbro dykes may be equivalent to the Harp dykes further to the north; however, the FL diabase dykes are chemically more evolved than other Proterozoic mafic suites in Labrador, with the exception of the Nain HP (high P₂O₅) dykes, with which they are very similar. -- The metaperidotites which intrude the FLG are enriched in Mg, Ni and Cr, and appear to consist dominantly of serpentinized dunites. Fe-Ni-Cu (+ minor PGE) sulphides hosted by meta-ultramafic rocks at the Baikie showings are interpreted to be magmatic in origin, and concentrations of PGE, Ni and Cu indicate that the metaperidotites south of Florence Lake and the meta-ultramafic xenoliths at the Baikie showing are komatiitic. -- Magnesite-quartz rocks southeast of Florence Lake are chemically similar to the metaperidotites. The alteration is somewhat similar to that associated with Archean lode gold deposits. However, abundant quartz veins which infill tension gashes in the rocks appear to have formed in situ due to the carbonatization process, and shear-related silicification and/or sulphldation is absent. -- Although metavolcanic and metasedimentary rocks of the FLG host many thin syngenetic pyrite horizons and several small base-metal sulphide occurrences, no significant mineralization has been discovered.

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