Kiera Broda, PhD
K.R. Broda1, A.E. Williams-Jones1, O.V. Vasyukova1
1Earth and Planetary Sciences, McGill University, Montreal
The Alaskan-type Turnagain mafic-ultramafic complex in British Columbia, Canada, hosts a large tonnage, low grade magmatic nickel-sulphide resource (measured resource of 423.4 million tonnes at 0.214 % Ni) that provides an excellent opportunity to study the effects of hydrothermal alteration on nickel-sulphide deposits in such complexes. Pentlandite, the primary ore mineral, and pyrrhotite, the dominant sulphide mineral, occur interstitial to magmatic olivine and diopside within dunite and wehrlite, which is consistent with their magmatic origin. However, local serpentinization of magmatic olivine and diopside coupled with the replacement of magmatic sulphides by magnetite, and the subsequent remobilization of pyrrhotite and pentlandite has significantly affected the distribution of nickel and sulphur in the deposit.
A preliminary petrographic and electron microprobe study of olivine in representative samples from the Horsetrail-Northwest Zone was performed to characterize this alteration. Magmatic olivine forms subhedral to euhedral cumulate crystals with Mg numbers (Mg/Mg+Fe*100) ranging from 85 to 97 and concentrations of MnO and Ni ranging from 0.05 to 0.3 wt. % and 0 to 0.7 wt. % Ni respectively. Secondary olivine occurring as rims on relict olivine grains and filling fractures in serpentine, has a higher Mg number (94 to 98), an elevated MnO content (0.15 to 1.3 wt. %), and a lower Ni content (0 to 0.05 wt. %). The negative correlation of Mg number with MnO and the positive correlation with Ni in the magmatic olivine are consistent with convergent margin magmatic processes. In contrast, the elevated Mg number and MnO content of the secondary olivine, and a lower Ni content are indicative of the dehydration of serpentine.
The higher Mg number in secondary olivine is attributed to the sequestration of iron in magnetite during serpentinization, whereas its lower nickel content indicates that nickel was mobilized during alteration. The elevated MnO concentration in secondary olivine reflects the uptake of this component from fluids during serpentine dehydration.
This study identifies two distinct alteration stages in the Turnagain complex: 1) serpentinization of olivine driven by fluids along faults; and 2) the partial dehydration of serpentine due to thrust-induced decompression or heating of the complex by nearby Cretaceous (Cassiar) intrusions. These findings demonstrate that textural relationships, Mg number, MnO and Ni concentrations can effectively distinguish magmatic from secondary olivine in Alaskan-type complexes and reveal a mechanism for nickel remobilization from silicate to sulphide minerals.