100 Ma timing paradox of Archean gold, Abitibi greenstone belt (Canada): New evidence from U-Pb and Pb-Pb evaporation ages of hydrothermal zircons

Abundant hydrothermal zircon occurs within quartz-tourma-line veins at four gold deposits in the Val d"Or district of the Archean Abitibi greenstone belt, Canada. Host rocks to the gold-bearing veins are characterized by pronounced metasomatic enrichment of Zr, Hf, Y, and rare earth elements. Hydrothermal zircons contain inclusions of quartz, tourmaline, scheelite, pyrite, and gold. In hydrothermal zircons and quartz, primary low-salinity H₂-CO₂, fluid inclusions coexist with CO₂-rich inclusions, indicating CO₂, unmixing and gold precipitation at 260 to 380 °C and ∼2 kbar. The zircons have ²⁰⁷Pb/²⁰⁶Pb evaporation ages of 2683 ±20 to 2703 ±4 Ma, close to a concordant U-Pb age of 2693 ±2 Ma, both commensurate with previous SHRIMP ages for the zircons. The same vein systems have U-Pb ages on rutile and titanite, Sm-Nd dates on scheelite, and ⁴⁰Ar/³⁹Ar ages on muscovite that scatter between 2630 to 2579 Ma; these ages gave rise to a "late gold" model.

In the Archean Superior province, vein gold deposits are related in space and time to terrane boundary structures which mark the diachronous accretion of allochthonous tectonostratigraphic terranes from south to north from 2720 to 2680 Ma. Consequently, the vein zircon ages of ∼2690 to 2680 Ma at Val d"Or represent the primary mineralizing event, consistent with its late kinematic timing throughout the Superior Province and with Late Archean supercontinent assembly globally, whereas the young scattered ages record the time of secondary fluid events associated with reactivation of the structures, resetting some isotopic systems below their conventional blocking temperature.