CHROMIAN SPINEL IN THE TURNAGAIN ALASKAN-TYPE ULTRAMAFIC INTRUSION, NORTHERN BRITISH COLUMBIA, CANADA

Abstract

The Early Jurassic Turnagain intrusion (24 km2) in northern British Columbia, Canada, is one of a number of Alaskan-type intrusions that occur within the Mesozoic Quesnel accreted arc terrane. It is distinguished by the presence of magmatic Ni sulfide mineralization hosted by ultramafic cumulate rocks. Chromian spinel is a common accessory mineral ( 0.85, Fe2+/(Fe2+ + Mg) < 0.4] are preserved in magnesiochromite grains from the chromitites, which were isolated from nearby olivine and interstitial melt and thus did not significantly re-equilibrate during cooling. Extensive re-equilibration with evolved interstitial melt, defined by broadly linear trends of increasing Al, Fe3+, Ti, V and Fe2+/(Fe2+ + Mg) with decreasing Cr content, is recorded in chromian spinel grains from dunite, wehrlite, and olivine clinopyroxenite. These trends converge toward the compositional field of grains from the chromitites. Rims of Fe3+-rich chromite and Cr-bearing magnetite, up to 30 μm thick, formed around chromian spinel grains during subsolidus infiltration of oxygenated fluids and serpentinization. Relative to the global database of spinel compositions from Alaskan-type intrusions, the majority of the chromian spinel compositions from the Turnagain intrusion are characterized by lower Fe3+/(Fe3+ + Cr + Al) and higher Cr/ (Cr + Al), which is consistent with a relatively low oxygen fugacity for the parental magmas to the ultramafic cumulates. The Turnagain parental magmas were likely reduced by assimilation of graphite- and pyrite-bearing phyllite host-rocks, which led to local saturation in a sulfide liquid.