U–Pb vein xenotime geochronology constraints on timing and longevity of orogenic gold mineralization in the Malartic-Val-d’Or Camp, Abitibi Subprovince, Canada

Despite a well-developed structural framework, orogenic gold deposits from the Malartic-Val-d’Or Camp (MVC) report ages that span nearly 345 million years (~ 2705–2360 Ma), significantly post-dating the tectono-metamorphic history of the Abitibi-Wawa Orogeny, and are in conflict with the structural setting along the retrograde path of the orogenic cycle. Gold in the MVC is commonly associated with three events that form pre- and syn-/late-regional penetrative fabric (S2), developed during major N-S shortening (D2, ~ 2669–2643 Ma): (1) pre-D2 quartz-carbonate veins; (2) deformed quartz-carbonate stockworks, entrained in S2; (3) laminated brittle-ductile quartz-tourmaline-carbonate reverse-shear and tension veins, sub-parallel to S2, which contain the bulk of the gold. Here, we present in situ U–Pb geochronological and rare earth element (REE) data from 85 xenotime grains, hosted by quartz-carbonate veins (Kiena S50) and quartz-tourmaline-carbonate veins (Goldex, Triangle, Plug #4, Beaufor, Pascalis Gold Trend, and Akasaba West). Xenotime in textural equilibrium with Au-hosting pyrite of pre-D2 veins yields a subtle negative Eu anomaly and elevated medium-REE contents, and records an auriferous hydrothermal event at 2686 ± 15 Ma (n = 19; 11 grains; MSWD = 2.8), consistent with cross-cutting relationships with dated intrusions. Late-D2 veins record two subsequent hydrothermal events, based on texture and chemistry of xenotime. Xenotime in textural equilibrium with gold-bearing pyrite contains elevated medium-REE values (Nd>1000 ppm; Sm>2490 ppm) and constrains the timing of vein emplacement to 2643 ± 3 Ma (n = 92; 44 grains; MSWD = 1.2), dating the main phase of quartz-tourmaline-carbonate vein formation and major gold precipitation. Xenotime in textural disequilibrium with auriferous sulfides, located at dissolved sulfide grain boundaries and in sulfide fractures, contains lower medium-REE values (Nd<1000 ppm; Sm<2490 ppm), and yields an age of 2607 ± 5 Ma (n = 63; 30 grains; MSWD = 1.9), indicating a second period of gold mineralization. Our data imply that district-scale gold mineralization in the MVC formed during at least two short-lived hydrothermal periods with the bulk of orogenic gold introduced along the retrograde path of the orogenic cycle, ~25–30 million years post-peak metamorphism.