Chemical composition of rock-forming minerals in granitoids associated with Au-Bi-Cu, Cu-Mo, and Au-Ag mineralization at the Freegold Mountain, Yukon, Canada: Magmatic and hydrothermal fluid chemistry and petrogenetic implications

The chemical compositions of rock-forming minerals have been determined for both altered and least-altered igneous rocks spatially associated with numerous mineralized zones (Nucleus Au-Bi-Cu-As deposit, Revenue Au ± Cu and Stoddart Cu-Mo ± W mineral occurrences, and Laforma Au-Ag deposit) across the Freegold Mountain area, Yukon, Canada. Within the study area, K-feldspar has a narrow compositional range (89.4-91% Or), whereas plagioclase spans a wide range (4.4-70.07% An). In all of the investigated samples, T _Ab = T _An = T _Or, suggesting that magmatic equilibrium between the coexisting plagioclase and K-feldspar was maintained. Igneous amphibole phenocrysts from hypabyssal dikes are typically calcic, whereas the Stoddart Cu-Mo ± W, Laforma Au-Ag, and Goldy Au mineralization are associated with Mg-enriched primary amphibole of edenite composition, and Au-Bi-Cu-As mineralization from Nucleus is related to Al-enriched primary amphibole of ferropargasite composition. Primary biotite phenocrysts across the Freegold Mountain area re-equilibrated with oxidized magma (f(O₂) values between 10-13 and 10-11.5 bars, lying between the Ni/NiO and the magnetite/haematite buffers). However, biotite and amphibole phenocrysts from Stoddart, Goldy, Laforma, and the Highway zones crystallized from a more oxidized magma, as indicated by their elevated X _Mg up to 0.65, relative to biotite and hornblende from Nucleus and Revenue characterized by a lower X _Mg (typically < 0.50). This suggests that various sources and (or) rapid emplacement were involved in magma genesis, as further supported by the considerable variation of pressure (1.8-7.3 kb) of amphibole crystallization and of the total Al content in least-altered biotite (2.6-2.9 afu) within the Freegold Mountain area. Biotite and apatite equilibrated within the T range of 520-780°C, consistent with temperatures of equilibration between ilmenite and magnetite, and their compositions indicate that they formed from an oxidized I-type magma. Magma differentiated by fractional crystallization (indicated by the presence of normally zoned plagioclase with Ca-rich cores and Na-enriched outer rims) and multiple magma mixing (supported by the presence of reversed zoned plagioclase and coexistence of normally and reversely zoned plagioclase). Lower X _Mg biotite associated with the mineralized (Cu-Mo ± W) potassic alteration incorporated more F and Cl relative to least-altered biotite with higher X _Mg. In both Nucleus and Revenue Au-Cu mineralizations, secondary biotite composition varies with respect to the associated alteration mineral assemblages. Although secondary biotite in the skarn re-equilibrated with F-poor fluids, secondary biotite from the pervasive biotitization is related to F- and Cl-enriched fluids, and secondary biotite from the phyllitic zone is related to F-, Cl-, and Mg-depleted fluids, thus consistent with a change in mineralizing fluid composition during mineralization.