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Age and origin of the Turnagain Alaskan-type intrusion and associated Ni-sulphide mineralization, north-central British Columbia, Canada Scheel, J. Erik


The Turnagain Alaskan-type intrusion in north-central British Columbia consists of ultramafic to dioritic rocks and contains significant magmatic sulphide mineralization. The age of the intrusion is constrained by U-Pb and Ar-Ar geochronology to be 190±1 Ma and the minimum depositional age of the youngest host rocks (volcanic wacke) is 301 Ma. Whole rock Nd isotopic compositions are characteristic of Paleozoic arc-derived mafic rocks in the northern Canadian Cordillera (εNd(190) =+4 to +6), but indicative of variable crustal contamination in some samples (εNd(190) =+2 down to -3.3). The age and tectonic characteristics of the Turnagain intrusion and its host rocks constrain the terrane it intrudes to be either Yukon-Tanana or Quesnellia, but not Ancestral North America. The Turnagain parent magmas were hydrous, arc-derived, in equilibrium with mantle peridotite, and ankaramitic. Cross-cutting and geochemical relationships define the crystallization and emplacement sequence of the Turnagain intrusion to be: dunite (∼Fo₉₁) → wehrlite (∼Fo₈₇)→ rolivine clinopyroxenite (∼Fo₈₅, Mg#cpx =0.92) → hornblende clinopyroxenite (Mg#cpx =0.81, Mg#hbl =0.65)→ hornblendite (Mg#hbl =0.60) → diorite. Mineral and whole-rock geochemistry indicate that all ultramafic lithologies are genetically related and relative depletion in the HFSE, specifically Nb and Ta, are consistent with an arc mantle source for the parent magmas. Variations in spinel chemistry in the Turnagain intrusion are mainly a function of post-crystallization re-equilibration and oxidation. Primary (unmodified) chromite compositions, observed in chromitite samples, are Cr-rich (Cr/(Cr+Al) =0.86-0.90) and Fe³⁺-poor (Fe³⁺/(Fe³⁺+Cr+Al)<0.1), indicating their crystallization from a magma with relatively low ƒO₂ (ΔFMQ < 0), which is substantially lower than for other Alaskan-type intrusions. At these relatively reduced conditions, S was dissolved as sulphide (S²⁻) rather than sulphate (SO₄²⁻). Sulphur (δ³⁴S =-9.7 to +1.4%o) and lead isotopic compositions of sulphide from the ultramafic rocks indicate that upper crustal sulphur and lead were added to the parent magmas by assimilation of graphitic, pyritic metasedimentary inclusions (δ³⁴S =-17.9%ο), which are found only in the sulphide-mineralized zones. Thus, addition of crustal carbon and sulphur reduced the Turnagain magmas and increased total S, which lead to early sulphide saturation.

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