Going with the flow — Changes of vorticity control gold enrichment in Archean shear zones (Shebandowan Greenstone Belt, Superior Province, Canada)

Abstract

Archean orogenic gold deposits are commonly hosted in brittle-ductile shear zones, where gold is locally remobilized and reprecipitated due to changes in physio-chemical conditions. However, in Archean rocks of the Superior Craton, these shear zones are often sparsely exposed and inferred primarily through spatial correlation, hence these shear zone architectures remain poorly understood. Here, we apply geochronology, kinematic analysis, and vorticity analysis techniques to study the Shebandowan Greenstone Belt in Northern Ontario, Canada. Geochronological results from zircon U-Pb CA-ID-TIMS and molybdenite Re-Os NTIMS dating yielded an age of 2717.35 ± 0.48 Ma for a proximal felsic intrusion and 2708 ± 12 Ma for auriferous and molybdenite-bearing vein mineralization, respectively. Field observations and regional age data indicate that both mineralization and intrusive activity predate deformation. Spatial interpolation of ductile foliation orientations reveals that gold mineralization is spatially associated with abrupt changes in shear zone orientation. Vorticity analysis — including rigid porphyroclast and crystallographic vorticity analysis — shows that these changes induce strain partitioning in releasing and restraining bends. These observed localized deflections in shear zone strike are attributed to the competency contrast between the old, feldspar-rich, isotropic, and syenitic plutonic bodies (rheologically stronger) and gold-bearing, intermediate metavolcanic and diorite units (rheologically weaker). Transitions from non-coaxial to coaxial flow dominated areas appear to have localized fluid pathways, allowing for fluid migration and gold precipitation during deformation. Quantifying deflections of shear zones adjacent to rigid geological bodies, combined with analyzing the deformation flow types, represents a powerful and cost-effective tool for gold exploration in granite-greenstone belts.