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

Thierry Bineli Betsi, David R. Lentz

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Abstract

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.

Original languageEnglish
Pages (from-to)657-691
Number of pages35
JournalInternational Geology Review
Volume55
Issue number6
DOIs
Publication statusPublished - Apr 20 2013

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hydrothermal fluid
biotite
chemical composition
mineralization
mountain
mineral
rock
amphibole
magma
plagioclase
fluid
feldspar
magnetite
edenite
mineral alteration
fluid composition
skarn
ilmenite
fractional crystallization
hornblende

All Science Journal Classification (ASJC) codes

  • Geology

Cite this

@article{5253c27d589a4d998427aaf248bea033,
title = "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",
abstract = "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.",
author = "Betsi, {Thierry Bineli} and Lentz, {David R.}",
year = "2013",
month = "4",
day = "20",
doi = "10.1080/00206814.2012.731767",
language = "English",
volume = "55",
pages = "657--691",
journal = "International Geology Review",
issn = "0020-6814",
publisher = "Bellwether Publishing, Ltd.",
number = "6",

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TY - JOUR

T1 - 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

T2 - Magmatic and hydrothermal fluid chemistry and petrogenetic implications

AU - Betsi, Thierry Bineli

AU - Lentz, David R.

PY - 2013/4/20

Y1 - 2013/4/20

N2 - 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.

AB - 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.

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