The paleoproterozoic metaplutonic suite of Voisey's Bay, Labrador: a geochemical, tectonic and metallogenic investigation

Rawlings, Alana Maxine (2001) The paleoproterozoic metaplutonic suite of Voisey's Bay, Labrador: a geochemical, tectonic and metallogenic investigation. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Paleoproterozoic Torngat Orogen of northern Labrador welds the Nain and Rae Archean cratons. A suite of calc-alkaline metaplutonic rocks has been identified in the Voisey’s Bay area and these rocks are believed to represent a southern extension of similar metaplutonic rocks identified along the some 500 km length of the orogeny. In addition the ore-bearing troctolites of the recently discovered Voisey’s Bay Ni-Co-Cu deposit are located within the metaplutonic suite. -- This study involves detailed geology, geochemistry, isotopic composition and geochronology of the Paleoproterozoic metaplutonic suite in the Voisey’s Bay area. To determine: (1) the petrogenesis of the metaplutonic rocks and constrain the Paleoproterozoic tectonic setting of the region; and (2) the role of the metaplutonic rocks in crustal contamination of the Voisey’s Bay troctolitic intrusion and the genesis of the Ni-Cu-Co deposit. -- The Paleoproterozoic metaplutonic rocks are a relatively undifferentiated, mafic to intermediate calc-alkaline batholith. The rocks have been metamorphosed to upper amphibolite of granulite facies and interleaved with newly discovered sulfide-rich paragneisses, amphibolite and meta-anorthosite. Recent mapping of this thesis shows that the metaplutonic suite covers a sizeable geographic area encompassing most parts of the Voisey’s Bay Ni-Cu-Co ore system. Two samples give ages of 1893 +/-1 Ma and 1890 +/-2 Ma by U-Pb zircon TIMS geochronology. Based on LAM-ICP-MS, one of these samples contained inherited zircon cores, some 50-100 Ma older than the igneous population, but no Archean cores were found in this sample of the two others. Metatonalite rocks of this suite probably form during a slightly younger magmatic event dated 18 1883 +/-5 Ma. All the compositions are enriched in relative amounts of Ba, Sr, Rb, Ar and Hf; and have negative Nb-Ta anomalies. This trace element chemistry of the rocks is distinctive and markedly similar to the Voisey’s Bay deposit, specifically in possessing Th-U and Nb-Ta depletions and exhibits elevated La/Sm ratios (2.92 to 11.01) with a range of Th/Nb ratios (0.004 to 1.065). The sulfur content of the metaplutonic suite reaches levels of 0.1 weight percent. Their REE patterns exhibit fractionation of LREE relative to HREE, with the extent of fractionation increasing with silica content. The εNd(₁₈₉₀ Ma) values for the samples range from +0.13 to -5.3. The Pb isotopic ratios were 15.160-15.834 for ²⁰⁶Pb/²⁰⁴Pb and 15.000 to 15.245 for ²⁰⁷Pb/²⁰⁴Pb and suggest that the contaminant formed in the lower crust. When calculated to the age of the Voisey’s Bay troctolite intrusion, the Paleoproterozoic metaplutonic rocks define two distinct isotopic groups: an enriched group with εNd(₁₃₂₀ Ma) of -11.1 and -11.2, ⁸⁷Sr/⁸⁶Sr(₁₃₂₀ Ma) ratios between 0.7032 and 9.7050 and μ* values of 7.66 to 7.77; and a more depleted group which exhibits εNd(₁₃₂₀ Ma) values between -2.8 to -6.8, ⁸⁷Sr/⁸⁶Sr(₁₃₂₀ Ma) ratios between 0.7038 and 0.7041 and μ* values 7.75 to 7.98. -- Meta-gabbro to meta-quartz diorite compositions of this suite defines a magmatic series that probably evolved through fractional crystallization. The meta-tonalites formed during a slightly younger magmatic event and the increasing fractionation of LREE to HREE suggest the meta-tonalities had more residual garnet in their source regions than did other members of the suite. The range in εNd(₁₈₉₀ Ma) suggests interaction between the primary magmas and continental crust. The Pb isotopic values suggest that the contaminant formed in the lower crust. A model for the metaplutonic suite is presented in which the meta-gabbro to meta-quartz diorite series formed by partial melting of the mantle wedge and interacted with crustal contaminant either in the source of lower crust. The meta-tonalites were derived by partial melting of subducting oceanic crust. Crustal genesis by both slab melting and wedge melting in the Paleoproterozoic may mark the transition from Archean to post-Archean processes. -- In terms of the role of the metaplutonic suite in the crustal contamination and genesis of the Voisey’s Bay Ni-Cu-Co deposit. The trace element and isotopic data of the Voisey’s Bay troctolitic intrusion and mineralized conduit rocks can be most readily explained by 5-10 percent contamination of a picritic basalt magma with the enriched group of the metaplutonic rocks. Contamination by the Tasiuyak paragneiss produced more subtle effects on the compositions of the intrusion. This new evidence indicates that the Paleoproterozoic metaplutonic rocks played a key role in the crustal contamination of the Voisey’s Bay intrusion, which may have led to sulfur saturation and precipitation of sulfides. Metaplutonic contamination may be an essential element in the formation of similar deposits in the region and elsewhere.

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