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Petrology of the Mechanic Settlement Pluton and related platinum-group element mineralization

A. Dogan Paktunc
Petrology of the Mechanic Settlement Pluton and related platinum-group element mineralization
Exploration and Mining Geology (October 2000) 9 (3-4): 157-170


The Mechanic Settlement pluton is a small body of mafic and ultramafic rocks of tholeiitic affinity, occurring in the Late Neoproterozoic volcanosedimentary rocks of the Avalon terrane of the Appalachian Orogen. The lenticular-shaped intrusion is layered and consists of ultramafic and mafic lithologies in at least six cyclic units that are relatively thin or short-lived. The lowermost exposed portion of the intrusion is characterized by the cumulate sequences of olivine and olivine-clinopyroxene and display relatively flat rare earth element profiles. In the overlying lithologies, plagioclase is the dominant cumulus phase following olivine in the crystallization sequence. Olivine ranges in composition from Fo (sub 73) to Fo (sub 85) and plagioclase from An (sub 62) to An (sub 89) . Variations in the compositions of orthopyroxene and clinopyroxene are within Wo (sub 0-4) En (sub 52-87) Fs (sub 11-45) and Wo (sub 39-50) En (sub 23-50) Fs (sub 3-19) . These mineral compositional variations in part reflect cryptic patterns and correspond to normal fractionation trends in cyclic units. Elevated Pt and Pd concentrations of up to 2.6 ppm occur in ten zones within the central cyclic series in association with both the peridotitic and gabbronoritic rock types. The mineralized zones are about several meters thick and appear to have lateral continuities over several hundred meters, suggesting that they are stratiform. Platinum-group elements (PGE) occur as discrete platinum-group minerals (PGM) including vysotskite, merenskyite, michenerite, sperrylite, stillwaterite, and hollingworthite that are associated with magmatic sulfides composed of pentlandite, pyrrhotite, and chalcopyrite. PGM grains range in size from a few micrometers to well above 20 mu m. Sulfur isotope ratios and the Se/S ratios of the mineralized rocks are typically higher than the mantle values. Sulfide saturation probably occurred as a result of assimilation of crustal rocks and magma mixing during emplacement of subsequent pulses of magma. Repetition of the PGE mineralization in rather thin cyclic units suggests that the replenishing magma probably had high PGE levels and was saturated with respect to sulfide during emplacement. Ingression of the replenishing magma into the chamber in a turbulent plume and its mixing with the resident magma was probably responsible for the formation of the PGE mineralizations in the Mechanic Settlement pluton.

ISSN: 0964-1823
Serial Title: Exploration and Mining Geology
Serial Volume: 9
Serial Issue: 3-4
Title: Petrology of the Mechanic Settlement Pluton and related platinum-group element mineralization
Author(s): Paktunc, A. Dogan
Affiliation: CANMET, Mining and Mineral Sciences Laboratories, Ottawa, ON, Canada
Pages: 157-170
Published: 200010
Text Language: English
Publisher: Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, QC, Canada
References: 34
Accession Number: 2002-050640
Categories: Economic geology, geology of ore depositsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch maps
N45°42'00" - N45°44'30", W65°16'00" - W65°12'00"
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200217
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