Metamorphic remobilization of Archean Au: Insights from the world-class Hemlo deposit, Canada

Abstract

Although most Archean orogenic Au deposits formed after the related metamorphic events, numerous deposits have formed prior to peak metamorphic conditions, and abundant research has focused on the remobilization of Au by metamorphic-hydrothermal fluids. At the Archean Hemlo Au deposit, Canada, most recent research agrees that Au mineralization first occurred prior to metamorphism and that Au was subsequently remobilized. Given that Li is mobile, relatively compatible in micas and clay minerals, and experiences isotopic fractionation primarily as a result of fluid-rock reactions, here, Li isotopic composition is analyzed using multiple collector ICP-MS and used to investigate Au remobilization during metamorphic-hydrothermal alteration. It is shown that bulk rock Li concentrations and isotopic compositions changed simultaneously with Au remobilization, as a result of the metamorphic recrystallization of biotite, chlorite, muscovite, and white mica. The concentration of Li increased from 2.6-58 ppm in hydrothermally-altered rocks, and δ⁷Li values decreased simultaneously, from + 10.5 to + 1.4 ‰. Interestingly, the δ⁷Li values correlate positively with Au and Li concentrations, indicating that the metamorphic-hydrothermal fluid remobilized Au as the rocks were fluxed with Li, and the biotite-chlorite-muscovite-white mica assemblages were recrystallized. In addition to bulk rock analyses, the concentration of Li-Au was measured in minerals using laser ablation ICP-MS, in order to investigate why bulk rock δ⁷Li values and Li concentrations are systematically correlated with Au concentrations. The presence of Au in K-feldspar is ascribed to primary mineralization and related K-feldspar alteration, whereas that in prograde and retrograde biotite, as well as in retrograde chlorite, muscovite, and white mica, must have resulted from the metamorphic-hydrothermal alteration and related Au remobilization. The decrease in Au counts and concentrations in minerals representing the sequence of recrystallization support bulk rock results suggesting that some Au was remobilized during metamorphism. Overall, these results highlight the strong potential of Li isotopes for Au exploration and metallogenic studies.