Stabilization of garnet in metamorphosed altered turbidites near the St. Eugene lead–zinc deposit, southeastern British Columbia: Equilibrium and kinetic controls

The St. Eugene lead-zinc deposit, near Moyie, southeastern British Columbia, is a Mesoproterozoic vein deposit hosted by metaturbidites of the 1.5–1.4 Ga Belt–Purcell Supergroup. The regional metamorphic grade of the rocks is biotite zone, with the age of metamorphism being Mesoproterozoic (ca. 1.35 Ga). The vein system is enveloped by a metamorphosed alteration zone of increasing intensity as the vein is approached. Thin argillaceous tops of turbidite beds away from the vein are garnet-free, whereas those in the inner alteration zone are garnet-bearing. Compared to rocks away from the vein, those near the vein are enriched in Fe, Mn, Pb and Zn, with proportional reduction in other compositional parameters. Thermodynamic modeling of three rocks across the alteration gradient predicts increasing stabilization of garnet with increasing degree of alteration. Predicted and observed modes of garnet in the samples are in close agreement. In the most altered rock, the garnet-in line is displaced down-temperature by ~ 100 °C relative to the least altered rock. Approximately 2/3 of the garnet stabilization is accounted for by increase in Mn content and the rest by increase in Fe/(Fe + Mg). Kinetic factors played a role in the development of the mineral assemblages, including metastable persistence of zoisite, and disequilibrium (overstepped) initial growth of garnet. Estimates of peak pressure–temperature conditions from mineral assemblage constraints and from compositional isopleths are complicated by the kinetic effects but yield similar results: 490–510 °C and 3.6–4.0 kbar. The pressure–temperature estimates imply an average linear geothermal gradient of ~ 35 °C/km, broadly consistent with burial metamorphism in the Belt–Purcell extensional basin. However, the estimated pressure, equivalent to a depth of 13–15 km, is greater than the estimated ~ 8 km (~ 2.2 kbar) of stratigraphic overburden at the time of metamorphism. The results of this study support the idea that garnet is a useful indicator mineral (‘vector’) for hydrothermal alteration associated with base and precious metal mineralization in this region, and potentially in other regions of comparable metamorphic grade. The ideal metamorphic conditions under which garnet is useful in this role are the metapelitic chlorite and biotite zones (greenschist and lower amphibolite facies).