Emplacement ages and exhumation rates for intrusion-hosted Cu–Mo–Sb–Au mineral systems at Freegold Mountain (Yukon, Canada): assessment from U–Pb, Ar–Ar, and (U–Th)/He geochronometers

Abstract

To decipher the thermal history of mineralized systems across the Freegold Mountain area (Yukon, Canada), a combined geochronology (zircon U-Pb and hornblende, biotite, and whole rock Ar-Ar) and thermochronology (apatite and zircon (U-Th)/He) study was carried out. Previous U-Pb data combined with new U-Pb and Ar-Ar data show that intrusive bodies across the Freegold Mountain were emplaced during two protracted episodes, the first spanning from 109.6 to 98 Ma and the second between 79 and 68 Ma. Overprinting of the first intrusive event by a second magmatic hydrothermal event is suggested by a zircon U-Pb age of 108.7 ± 0.4 Ma for a chlorite-altered dyke and a whole rock Ar-Ar plateau age of 76.25 ± 0.53 Ma. Zircon (U-Th)/He data are between 66 and 89 Ma, whereas apatite (U-Th)/He data are scattered (38.7-109.9 Ma) and bracket the two magmatic emplacement events. Our combined data reveal a complex history of reheating that led to resetting of numerous chronometers. In most of the investigated magmatic hydrothermal systems, early fast cooling from igneous emplacement through hydrothermal alteration (between 900 and 200°C) was followed by later and slower cooling accompanying post mineralization uplift and erosion (between 200 and 70°C). Preliminary models indicate intrusive bodies associated with the Stoddart Cu-Mo ± W prospect cooled slowly (23°C/Ma) compared with the ones spatially associated with the Revenue Au-Cu prospect (43°C/Ma), and the similarity of the zircon U-Pb and (U-Th)/He ages from Revenue dyke further supports a rapid cooling from 700 to 180°C. Erosion rates of 0.035-0.045 mm/year are consistent with tectonic quiescence during the Late Tertiary combined with the lack of Pleistocene glaciation in central Yukon. Such low rates of exhumation favour the formation and preservation of supergene mineralization, such as that found north of Freegold Mountain. Résumé : Afin de pouvoir décoder l'historique thermique des systèmes minéralisés à travers le secteur du mont Freegold (Yukon, Canada), une étude combinée de géochronologie (U-Pb sur zircon et Ar-Ar sur hornblende, biotite et la roche entière) et de thermochronologie (apatite et zircon (U-Th)/He) a été entreprise. Les données antérieures U-Pb, combinées aux nouvelles données U-Pb et Ar-Ar montrent que des amas intrusifs ont été mis en place au mont Freegold au cours de deux épisodes prolongés, le premier de 109,6 à 98 Ma et l'autre entre 79 et 68 Ma. Une surimpression du premier événement intrusif par un autre événement magmatique hydrothermal est suggérée par une datation U-Pb sur zircon, de 108,7 ± 0,4 Ma, pour un dyke altéré par de la chlorite; aussi, un âge plateau Ar-Ar, pour la roche entière, donne 76,25 ± 0,53 Ma. Des données (U-Th)/He pour le zircon varient entre 66 et 89 Ma alors que des données (U-Th)/He pour l'apatite sont plus dispersées (entre 38,7 et 109,9 Ma) et encadrent les deux événements magmatiques de mise en place. Nos données combinées révèlent un historique complexe de réchauffements qui ont conduit au rajustement de nombreux ch...

Landscape antiquity and Cenozoic drainage development of southern Yukon, through restoration modeling of the Tintina Fault

“…The results of recent thermochronology studies in central Yukon (Betsi et al 2012;Knight et al 2013;Joyce et al 2015;Moher et al 2016) also indicate minimal incision and erosion of the Yukon Plateau since the Late Cretaceous to Paleocene. All these factors combined support a scenario where gold present in the placer deposits was likely eroded from its bedrock source tens of millions of years ago, and may help explain the relative lack of success to date in finding the mother lode source(s) of the placers.…”

“…Published thermochronology studies of central Yukon (Betsi et al 2012;Knight et al 2013;Joyce et al 2015;Moher et al 2016) indicate that the Klondike Plateau cooled through medium to low temperature (400-300°C) for the 40 Ar/ 39 Ar system (Knight et al 2013;Joyce et al 2015) in the Early Jurassic to Early Cretaceous. The plateau cooled through the low-temperature (U-Th)/He geochronometers (Betsi et al 2012;Moher et al 2016) in apatite and zircon (<180°C) in the Middle Jurassic to Late Cretaceous (locally) and to the Paleocene in general. These relatively ancient thermochronology ages indicate that there has been little erosion of the Yukon plateaus since the Late Cretaceous to Paleocene, consistent with the preservation of mid-Cretaceous to Paleocene unconformities in the geological record across much of the Klondike Plateau.…”

The exhumation history of collision-related mineralizing systems in Tibet: Insights from thermal studies of the Sharang and Yaguila deposits, central Lhasa

“…40 Ar/ 39 Ar step-heating analysis was performed at the Laboratory of Paleomagnetism and Geochronology at the Chinese Academy of Sciences, Beijing, on a MM5400 mass spectrometer operating in static mode (Wang et al, 2006(Wang et al, , 2009. The sericite separates were irradiated, along with Bern-4M standards, in vacuo within a cadmium-coated quartz vial for 47.5 h in position H8 of the reactor at the Beijing Atomic Energy Research Institute (Bineli Betsi et al, 2012). The Bern-4M biotite standard age was taken as 18.7 ± 0.1 Ma, recalculated using the Ga1550 biotite standard age of 98.8 Ma (Renne et al, 1998).…”

“…For arcrelated porphyry Mo deposits, most Mo mineralization takes place at temperatures ranging from 450 to 200°C, and subsequent overprinting of low-temperature alteration (phyllic, argillic and late carbonate alteration, 350-200°C) could influence the hydrothermal system through magmatic-meteoric fluid circulation during the latest stage of mineralization (Taylor et al, 2012). Given the zircon and apatite helium closure temperatures (closure temperature, Tc~180-200°C for zircon and 70-100°C for apatite; Reiners, 2005;Farley, 2000), ZHe ages probably represent the timing of the lowest temperature hydrothermal event, and AHe ages can constrain the time of exhumation of the hydrothermal system (Bineli Betsi et al, 2012;McInnes et al, 2005a).…”

Quantifying Exhumation at the Giant Pulang Porphyry Cu-Au Deposit Using U-Pb-He Dating

“…The apatite (U-Th)/He system has the lowest closure temperature (45°-70°C; Farley et al, 1998;Wolf et al, 1998) and has the potential to place temporal constraints on exhumation processes from 1-to 3-km depth to the surface under a geothermal gradient of 20° to 40°C/km, commensurate with the average depth of most porphyry systems. The apatite (U-Th)/He thermochronometer has been used widely in combination with other thermochronometers to quantify the cooling and exhumation processes at porphyry and/or skarn deposits (e.g., McInnes et al, 1999McInnes et al, , 2005aMasterman et al, 2005;Xu et al, 2006;Harris et al, 2008;Bineli Betsi et al, 2012;Braxton et al, 2012;Zhao et al, 2015Zhao et al, , 2016, orogenic-type Au deposits (Zeng et al, 2013;Liu et al, 2017), and diamondiferous kimberlite occurrences (e.g., McInnes et al, 2009;Evans et al, 2013). McInnes et al (2005a, b) performed apatite (U-Th)/He, zircon (U-Th)/He, and zircon U-Pb dating on samples from porphyry deposits in Iran, Chile, and Indonesia.…”