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[HRE12-05] Characterization of lithium-bearing pegmatite in Sakihama, Iwate, Japan
Keywords:Sakihama, lithium-bearing, tourmaline, monazite U-Pb dating
Pegmatite at Sakihama in Sanriku coast, Iwate, Northeast Japan, is one of the representative lithium-bearing pegmatites in Japan. A main outcrop of the pegmatite occurs as horizontal sheet-like dike exposed along the coastline of Okirai Bay. Several thinner pegmatites, aplite dikes, and pegmatite floats occur 3 km northeast of Sakihama, along the coastline of Yoshihama Bay. The pegmatite at the main outcrop comprises five asymmetrical growth zones with different mineralogical and textural features and shows two directions of crystallization, downward and upward, from granitic wall zone 1 (top contact) to core zone and from granitic wall zone 2 (bottom) to core zone, respectively. Objectives of this study are to elucidate mineralogical characteristics and conditions and age of formation of the pegmatite in Sakihama based on microscopy, bulk and mineral chemistry, fluid inclusion analysis, and U-Pb monazite geochronology.
The pegmatite in Sakihama is hosted in Early Cretaceous granodiorites (the Goyosan Pluton) that intruded into the Kamaishi Formation and the Ofunato Group, in the eastern and southwestern parts, respectively. Thinner pegmatite dikes intruded into the metasedimentary rocks of the Kamaishi Formation in the northeastern part of Okirai Bay. The pegmatite in Sakihama consists mainly of K-feldspar, quartz, and plagioclase as well as minor tourmaline, garnet, biotite, and muscovite, with accessory minerals of monazite, columbite, magnetite, ilmenite, apatite, uraninite, and zircon. Tourmaline occurs as black prismatic crystals with size ranging from a few millimetres to 15 cm in length. Under the microscope, the tourmaline occurs as subhedral-anhedral crystals with small cracks that host inclusions of accessory minerals, mainly zircon and exhibits strong pleochroism from brown to bluish brown. In some cases, the tourmaline crystals show zoning with a light blue core and dark blue rim. K-feldspar is the most abundant mineral and occurs as large pink block mass and exhibits graphic texture. The accessory minerals commonly occur as minor inclusions in the major and minor minerals. Cathodoluminescence imaging of the pegmatite samples revealed the presence of crack-like texture in quartz, polysynthetic twining and zonation in plagioclase, and graphic texture and tartan twinning in K-feldspar.
Bulk chemical analysis of single pegmatite minerals using ICP-MS revealed elevated contents of some selected elements compared to the primitive mantle, i.e., tourmaline (averages: 146 ppm Li, 177 ppm Nb, 26 ppm Sn, 18 ppm Ce, 51 ppm Ta, and 32 ppm U), mica (averages: 680 ppm Li, 490 ppm Nb, 113 ppm Sn, 70 ppm Cs, 96 ppm Ta, 78 ppm Ce, 126 ppm W, 38 ppm Th, and 40 ppm U), K-feldspar (averages: 18 ppm Li, 891 ppm Rb, and 20 ppm Cs), and quartz (average: 29 ppm Li).
The chemical compositions of tourmaline, mica, garnet, and K-feldspar were plotted on classification diagrams. Tourmaline belongs to alkali group and schorl, and garnet is Mn-rich almandine-spessartine solid solution. Mica is trioctahedral micas and biotite in composition, and K-feldspar contains 34.1 to 98.1 % orthoclase component. On the basis of the trace element geochemistry, the pegmatite and quartz diorite in Sakihama are I-type affinities and represent the same magmatic event, where the pegmatite was formed by crystal fractionation. Ti-in quartz geothermometer indicates a temperature range of 619-560 °C for the emplacement of the pegmatite. U-Pb dating yielded 121 Ma for monazite in the pegmatite.
The pegmatite in Sakihama is hosted in Early Cretaceous granodiorites (the Goyosan Pluton) that intruded into the Kamaishi Formation and the Ofunato Group, in the eastern and southwestern parts, respectively. Thinner pegmatite dikes intruded into the metasedimentary rocks of the Kamaishi Formation in the northeastern part of Okirai Bay. The pegmatite in Sakihama consists mainly of K-feldspar, quartz, and plagioclase as well as minor tourmaline, garnet, biotite, and muscovite, with accessory minerals of monazite, columbite, magnetite, ilmenite, apatite, uraninite, and zircon. Tourmaline occurs as black prismatic crystals with size ranging from a few millimetres to 15 cm in length. Under the microscope, the tourmaline occurs as subhedral-anhedral crystals with small cracks that host inclusions of accessory minerals, mainly zircon and exhibits strong pleochroism from brown to bluish brown. In some cases, the tourmaline crystals show zoning with a light blue core and dark blue rim. K-feldspar is the most abundant mineral and occurs as large pink block mass and exhibits graphic texture. The accessory minerals commonly occur as minor inclusions in the major and minor minerals. Cathodoluminescence imaging of the pegmatite samples revealed the presence of crack-like texture in quartz, polysynthetic twining and zonation in plagioclase, and graphic texture and tartan twinning in K-feldspar.
Bulk chemical analysis of single pegmatite minerals using ICP-MS revealed elevated contents of some selected elements compared to the primitive mantle, i.e., tourmaline (averages: 146 ppm Li, 177 ppm Nb, 26 ppm Sn, 18 ppm Ce, 51 ppm Ta, and 32 ppm U), mica (averages: 680 ppm Li, 490 ppm Nb, 113 ppm Sn, 70 ppm Cs, 96 ppm Ta, 78 ppm Ce, 126 ppm W, 38 ppm Th, and 40 ppm U), K-feldspar (averages: 18 ppm Li, 891 ppm Rb, and 20 ppm Cs), and quartz (average: 29 ppm Li).
The chemical compositions of tourmaline, mica, garnet, and K-feldspar were plotted on classification diagrams. Tourmaline belongs to alkali group and schorl, and garnet is Mn-rich almandine-spessartine solid solution. Mica is trioctahedral micas and biotite in composition, and K-feldspar contains 34.1 to 98.1 % orthoclase component. On the basis of the trace element geochemistry, the pegmatite and quartz diorite in Sakihama are I-type affinities and represent the same magmatic event, where the pegmatite was formed by crystal fractionation. Ti-in quartz geothermometer indicates a temperature range of 619-560 °C for the emplacement of the pegmatite. U-Pb dating yielded 121 Ma for monazite in the pegmatite.