Wrinkled clocks in the crust: dating deformation in Archean gold-bearing shear zones

Mid-crustal shear zones localize strain and control the migration of heat and fluids, making them central to understanding metallic ore-forming processes. Mica are widely used to date deformation in shear zones, however their radiometric ages can be difficult to interpret because hydrothermal alteration and strain-induced microstructural defects can promote recrystallization and disrupt isotopic retention. In Archean gold camps, mica ages commonly postdate mineralization events by >100 Myr, raising questions about whether these ages reflect primary mineralization, metamorphic or hydrothermal growth, or post-orogenic remobilization. Robust interpretation of these ages requires direct integration of geochemical and micro- to nanoscale structural analysis. We examined mica from the Sunday Lake Deformation Zone, a regional scale deformation zone controlling gold mineralization at Agnico Eagle Mines Ltd. giant Detour Lake Mine (DLM), in the northwestern Abitibi greenstone belt, Canada. The DLM orogenic gold deposit is characterized by c. 2734-2724 Ma volcanic rocks, comprising ultramafic-dominated lower units and mafic volcanic and volcaniclastic upper units, metamorphosed under greenschist to lower amphibolite facies conditions. Mafic host rocks are intruded by felsic to mafic sills and dikes. The main regional foliation is subvertical and axial-planar to west-trending, shallowly-plunging tight to isoclinal folds, which transposes the intrusive relationships. Gold mineralization occurred at c. 2670-2640 Ma in a syn-orogenic setting. Microstructural analyses were conducted on muscovite from felsic meta-intrusive rocks, collected from drill core, that are comprised of quartz, muscovite ± biotite, plagioclase, K-feldspar, chlorite, garnet, amphibole, carbonates and sulfides. Quartz microstructures record bulging recrystallization and nascent subgrain rotation, indicating deformation temperatures of ~300-400°C. Plagioclase display tapered deformation twins and brittle fracturing, consistent with low to moderate temperature deformation. Mica constitute between <5 and 30 vol% of the rock and occur as euhedral porphyroblasts/neoblasts to subhedral poikilitic, skeletal grains ranging in size from 15 x 50 μm to 250 x 650 μm. High-resolution electron channeling contrast imaging of the mica reveals weak undulatory orientation contrast patterns perpendicular to cleavage planes in ~20-50% of the grains. Such contrast patterns suggest deformation in the mica is accommodated primarily by dislocation glide. Backscatter electron imaging of the mica also revealed concentric chemical zoning, typically expressed as irregular and discontinuous rims along grain margins, which are weakly enriched in Fe and Al and depleted in Mg and Si relative to mica cores. Muscovite Ar-Ar analyses from unmineralized rocks at DLM yield single-crystal dates of 2600-2100 Ma, and complementary K-Ca and Rb-Sr dating will be conducted to assess the roles of Ar loss and element mobility in producing these younger and dispersed ages. The timing of metamorphism and deformation has important implications for understanding the nature and controls on mineralization at DLM, and whether the original geometry and mineralogy of the deposit has been modified through later stages of syn‐metamorphic deformation.