A petrographic and geochemical characterization and the evaluation of the exploration potential for nickel sulfides in several mafic-ultramafic intrusive complexes in Newfoundland

Collins, Patrick G. (2007) A petrographic and geochemical characterization and the evaluation of the exploration potential for nickel sulfides in several mafic-ultramafic intrusive complexes in Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

On the island of Newfoundland a number of mafic- to ultramafic intrusions are exposed; most commonly in the Dunnage Zone. These intrusive suites occur in a variety of tectonic environments, and have variable relationships with their host rocks. Nickel is extracted from intrusive rocks of similar compositions throughout the world. This thesis evaluates the potential of three intrusive complexes in Newfoundland to host economic Ni-Cu sulphide and/or platinum group element (PGE) mineralization. -- The Red Cross Lake Intrusive Suite (RCLIS), located in central Newfoundland, is a ca. 431 Ma layered, mafic- to ultramafic intrusion. The Steel Mountain Anorthosite (SMA), located east of Stephenville, in the Southern Long Range Mountains, is a ca. 1275 Ma massive-type anorthosite that hosts abundant Fe-Ti oxides and variable mafic zones. The Taylor Brook Gabbro Suite (TBGS), located southeast of western White Bay, is a ca. 430 Ma gabbroic suite. -- The RCLIS is divided into an Upper and Lower Series (US and LS respectively). The LS is comprised of predominantly gabbro and troctolite layers with minor dunite, olivine gabbro and serpentinite. The US had a gabbroic- to leucogabbroic protolith that is variably altered to pyroxene amphibole gabbro. Geochemical data indicate the RCLIS exhibits an intermediate to alkaline and sub-alkaline composition and is tholeiitic. The RCLIS evolved through fractional crystallization; it exhibits a progressive decrease in MgO and an increase in FeO+Ti0₂ from the LS to the US. Sulfides comprise at most only a few weight percent; the maximum values are from country rock samples. Magmatic sulfides in the LS locally exhibit a high Ni tenor. Plagioclase composition averages An 69 in the LS and An 54 in the US, whereas paragneiss xenolith samples average An 47. Olivine composition averages Fo 83 in the LS and Fo 56 for the US; xenoliths average Fo 36. LS olivine is locally Ni-depleted with respect to its Fo content. δ³⁴S ratios exhibit a range of 5.64‰, from -1.79‰ to +3.85‰. The heaviest ratio is from a country rock sample, whereas igneous samples are closer to 0.00‰, suggesting little country rock contamination. -- The portion of the Steel Mountain Anorthosite (SMA) mapped is predominantly underlain by elongate, diffuse lenses of leuconorite, leucogabbro, norite and norite hosted by massive, recrystallized pink- to lilac coloured anorthosite. Massive Fe-Ti oxides occur locally in noritic units. Gabbroic units are relatively more altered, whereas noritic units more commonly preserve primary textures. Fe-Ti oxides exhibit complex exsolution textures. Lithogeochemistry was effective in separating the majority of samples along an evolutionary trend from Mg-rich melanorites to evolved anorthositic samples. Plagioclase becomes increasingly An-enriched as the rocks become more mafic. Clinopyroxene and orthopyroxene are consistently Mg-rich throughout the SMA. Locally, oxides display strong solid substitution and exsolution textures, which generally result from hematite exsolution in ilmenite or ulvöspinel exsolution in magnetite. In the Bishops quarries, hercynite is also common in magnetite. At Sheep Brook, pyrite contains up to 4.7 wt.% Ni in concentric bands. Sperrylite and several other unnamed Pt-Te and Pd-Sb phases were identified. δ³⁴S ratios range from +7.33%o to -2.41‰; the sulfides from Sheep Brook are near 1.00 ‰ whereas sulfides in the Bishop quarries massive oxides are negative at -2.41‰. -- The Taylor Brook Gabbro Suite (TBGS) is heterogeneous at all scales. It comprises a concentrically layered pyroxene amphibole gabbro intrusion that is cut by multiple sets of mafic and felsic dikes. In the northern region of the intrusion a separate, more mafic and coarse-grained zone is recognized. Much of the TBGS has been altered to greenschist assemblages with variable degrees of recrystallization. TBGS samples display well-developed geochemical evolution and are locally Fe-Ti enriched. The TBGS exhibits a sub-alkaline composition and lithogeochemistry suggests that it was emplaced in a within-plate tectono-magmatic environment. δ³⁴S ratios range from +2.22 ‰ to +4.81 ‰, suggesting a magmatic isotopic reservoir. -- The RCLIS has the greatest Ni-Cu-PGE exploration potential because of the presence of olivine-rich ultramafic rocks, evidence for some country rock contamination, locally Ni-depleted olivine and high-Ni tenor sulfides cropping out in mafic and ultramafic rocks. The SMA has the potential for future discoveries of PGE mineralization in the Sheep Brook area; grassroots exploration based on structural analyses and geological mapping would be most effective at finding similar mineralization in the area. The TBGS has marginal exploration future, primarily because there are few rock types with characteristics to host orthomagmatic sulfide bodies. It is possible that the TBGS is only weakly eroded and that favourable stratigraphic horizons lie at depth.

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