11:15 AM - 11:30 AM
[SCG57-03] U-Pb dating of magnetite and garnet: application to the Kamaishi skarn deposit, Iwate Prefecture, Northeast Japan
Keywords:U-Pb dating, LA-ICP-MS, magnetite, garnet
In this study, we determined the U-Pb age of magnetite and garnet using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Recently, U-Pb datings of minerals other than zircon, such as calcite, garnet and others, have been performed. It is a powerful to understand the timing of thermal events, fault movements, and other geological events, which are difficult to be dated by zircon U-Pb dating and other methods. In this study, we examine magnetites, as magnetites are common iron ore minerals that occur in various types of ore deposit. However it is usually difficult to obtain reliable age of magnetite because it contain high common Pb and very low U concentratioins. We have examined the U-Pb dating of magnetite. This technique has been applied to the magnetite from the skarn deposit.
We present U-Pb dating of magnetite and garnet in the ore and the skarn from the Kamaishi mine, in the Kitakami Mountains, Iwate Prefecture, Japan to dicuss the timing of ore and skarn formation. The Kamaishi deposit is a largest Fe-Cu deposit in Japan. Early Cretaceous igneous complexes, the Ganidake granodiorite and diorite, and the Kurihashi granodiorite are occurred around the Kamaishi deposit. The Ganidake granodiorite is considered to have formed Fe-Cu mineralization and skarn of the Kamaishi deposit from the field evidences, although there is no significant age difference between the Ganidake and Kurihashi granodiorites. The zonal skarn was formed at the boundaries between limestones and sedimentary rocks or the granodiorites. The zircon SHRIMP U-Pb datings of the Ganidake granodiorite and the Kurihashi granodiorite yield 123.43 ± 0.70 Ma and 119.65 ± 0.71 Ma respectively. Theresult of the magnetite U-Pb dating shows 116.0 ± 3.8|5.0 Ma, which indicates thermal overprint by the Kurihashi granodiorite. In contrast, the garnet U-Pb dating shows 120.7 ± 8.7 Ma, which is consistent with the U-Pb zircon age of the Ganidake granodiorite, which led the skarn formation by the contact metamorphism.
We present U-Pb dating of magnetite and garnet in the ore and the skarn from the Kamaishi mine, in the Kitakami Mountains, Iwate Prefecture, Japan to dicuss the timing of ore and skarn formation. The Kamaishi deposit is a largest Fe-Cu deposit in Japan. Early Cretaceous igneous complexes, the Ganidake granodiorite and diorite, and the Kurihashi granodiorite are occurred around the Kamaishi deposit. The Ganidake granodiorite is considered to have formed Fe-Cu mineralization and skarn of the Kamaishi deposit from the field evidences, although there is no significant age difference between the Ganidake and Kurihashi granodiorites. The zonal skarn was formed at the boundaries between limestones and sedimentary rocks or the granodiorites. The zircon SHRIMP U-Pb datings of the Ganidake granodiorite and the Kurihashi granodiorite yield 123.43 ± 0.70 Ma and 119.65 ± 0.71 Ma respectively. Theresult of the magnetite U-Pb dating shows 116.0 ± 3.8|5.0 Ma, which indicates thermal overprint by the Kurihashi granodiorite. In contrast, the garnet U-Pb dating shows 120.7 ± 8.7 Ma, which is consistent with the U-Pb zircon age of the Ganidake granodiorite, which led the skarn formation by the contact metamorphism.