HR: 0830h
AN: V51I-0399    [PDF]
TI: Au-Skarn Mineralization: Constraints from LA-ICP-MS U-Pb Zircon Dating
AU: * Gaspar, M
EM: mgaspar@mail.wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AU: Meinert, L D
EM: lmeinert@smith.edu
AF: Department of Geology, Washington State University, Pullman, WA 99164 United States
AB: In situ U-Pb zircon geochronology by laser ablation ICP-MS was performed on samples from the intrusive rocks of the Buckhorn Mountain, Washington in order to constrain the age of the Crown Jewel Au-skarn deposit. The analyses were conducted at Washington State University using a ThermoFinnigan Element2 single collector, high resolution magnetic sector ICP-MS, and a New Wave UP 213 Nd-YAG (213 nm) laser ablation system. The analytical parameters included a repetition rate of 10 Hz, a 40 microns spot size, and a total analysis time of 30 seconds per spot. A 94-35 cascade standard was used to bracket the samples. Two distinct $^{206}$Pb/$^{238}$ U ages were obtained, 52.3 Ma $\pm$ 1.6, 165.0 Ma $\pm$ 5.9, 163.4 $\pm$ 5.7, and 169.8 $\pm$ 12.1 (all errors are 2 sigma). These ages represent distinct magmatic events occuring during the two main tectonic periods in the region. The oldest age ($\sim$165 Ma) is associated with the accretion of the Quesnel terrain during the Middle Jurassic while the younger age ($\sim$52 Ma) represents the magmatism during the Eocene extension that is well represented by the Challis Volcanics in the vicinity. The Jurassic ages were obtained in one sample from a deformed granodiorite dike and two samples from the main granodiorite intrusive facies that, based on the skarn mineralogy zonation, is spatially associated with the skarn. The Eocene intrusive unit was intercepted in one drill core but is clearly distinct from the granodiorites both geochemically and mineralogically. This facies is more an adamellite, with a distinctive pinkish colour due to a K-spar alteration. It is higher in silica, and depleted in compatible elements (Ca, Fe, Ti, Mg, P, Y, and V) relative to the granodiorite. Geochemically the granodiorite has a composition typical of plutons associated with Au-skarns worldwide. Because Au mineralization is erratically distributed and does not correlate with any of the skarn mineralogy, a question can be raised. Does the Au mineralization have any connection with the more recent intrusive event? Future work will be conducted in order to clarify this question.
DE: 1020 Composition of the crust
DE: 3640 Igneous petrology
DE: 3665 Mineral occurrences and deposits
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting