Petrogenesis of Au-BI-As-Cu, Cu-Mo± W, and base-metal-AU-AG mineral occurrences, in the Mountain Freegold Region (Dawson Range), Yukon, Canada

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University of New Brunswick


In order to gaining a better understanding of the evolution of the magmatic systems, as well as processes that combined to form ore minerals across the NFR Freegold Mountain gold-copper project, field observations were combined with reflected and transmitted light microscopy, ChromaSEM-CL and EPMA, whole rock geochemistry, isotope (0, H, C, S, Sr, Nd, and Pb) analyses, and geochronology (U-Pb, Re-Os, and Ar/Ar) and thermochronology (U-Th-He) investigations. Magmatic bodies were emplaced during two protracted episodes, the first from 109.6 to ⁓ 98 Ma and the second between 75.2 and 68.4 Ma. Magma was derived from different sources, as testified by: (i) variations in XMg values of biotite and hornblende (< 0.50-0.65); (ii) the T Al content in least-altered biotite (2.6-2.9); (iii) K values that range from <1 X 10-3 to 33.27 10-3 SI); (iv) least-altered igneous rock O isotope compositions (6.2-10.1 ‰), and; (v) variation in the depth of magma emplacement ( ⁓ 2-7 .1 kb). Magmas differentiated by magma mixing (as indicated by the presence of reversed zoned plagioclase and increase of temperature of crystallization of quartz from the core to the rims) and fractional crystallization (as supported by the presence of normally- zoned plagioclase with Ca-rich core and Na-rich outer rims), thus leading to igneous suites of mostly high-K calcalkaline, spanning from dioritic to granitic compositions, and exhibiting adakitic affinities (depletion in HREE + Y relative to LREE, and high Sr/Y and La/Yb ratios). These magmatic-hydrothermal systems cooled fast, as indicated by the similarity (within analytical error) between zircon U-Pb and zircon (U-Th/He) data, and were slowly exhumed (0.035-0.045 mm/year), favouring the formation and preservation of supergene mineralization, such as that found north of the Mountain Freegold area. Mineral occurrences are of different metal suite of interest, and corresponding style of mineralization (e.g., Ttinta Au-Ag-base-metal-bearing quartz veins, Stoddart Porphyry Cu-Mo± W prospect, and Nucleus Au-Bi-Cu-As deposit). Mineralization within the NFR Freegold Mountain gold-copper project took place over a period of at least 115 Ma and Mineralized systems resulted from the overlapping of multiple volcanoplutonic processes that influenced the Mountain Freegold area, as indicated by 76-78 Ma molybdenite-bearing veins from Nucleus and Revenue, the182 Ma skarn at Nucleus, as well as the 76 Ma Tinta deposit. The 2 Ma difference between the juxtaposed Nucleus and the Revenue mineral occurrences favours not only a long-lived hydrothermal system, but also an evolution from the porphyry to the epithermal environments. Lead and by association all metals, derived from associated magmatic bodies. Sulfur is also of magmatic origin and δ34 S values range from 0.2 to 2.5‰ at Nucleus-Revenue, from -1.4 to 3 .6‰ at Tinta, and from 1.8 to 2.6‰ at Stoddart. The input of meteoric water in the mineralizing process(es) was also notable, as suggested from altered igneous rocks 0 isotope compositions (5.6-5.2‰) and magnetite and quartz O isotope compositions (-6.3‰ and 1.5‰, respectively), as well as sulfate minerals of supergene origin (3.7- 4.3‰ δ34S).