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[MIS13-P08] Igneous activity during mineralization of the Sado deposit: constraint by zircon U-Pb age and trace-element chemistry
Keywords:Zircon, Deposit, U–Pb age, Trace element
The Sado deposit is located on Sado Island of the Niigata Prefecture, where it hosts the second largest epithermal gold deposit in Japan. Previous studies suggest that the Sado deposit was formed by hydrothermal activity associated with igneous bodies, which coincides with the Yamato Basin development during the Sea of Japan spread (ca. 23 Ma). However, the age, tectonics, and magma chemistry of the igneous activity responsible for mineralization in this area remains unknown because the whole-rock and mineralogical chemical compositions of the igneous rocks were secondarily altered by strong hydrothermal alterations. In this study, we investigated zircon U–Pb ages and geochemistry of the igneous rocks associated with the mineralization of the Sado deposit.
The volcanic and volcanoclastic rock samples were collected from various lithologies around the Sado deposit, which includes the Aikawa Fm. (21102904, 22051007), the Masaragawa Fm. (22051006, 22051002), the Kinpokusan Fm. (22051005, 22051009, 22051004, 22051001), and intrusive rocks (21102901, 22051008). The following ages were obtained for each sample: [Aikawa Fm.] 21102904: 27.64±0.90 Ma, 22051007: 25.46±0.77 Ma. [Masaragawa Fm.] 22051006: 23.59±0.44 Ma, 22051002: 22.87±0.35 Ma. [Kinpokusan Fm.] 22051005: 22.72±0.52 Ma, 22051009: 22.24±0.36 Ma, 22051004: 22.87±0.32 Ma, 22051001: 20.99±0.45 Ma. [Intrusive rock] 21102901: 25.03±0.56 Ma, 22051008: 21.87±0.33 Ma. Those ages are almost consistent with the mineralization age of the older Sado deposit (ca. 25-21 Ma). The veins of the Sado deposit clearly crosscut the Aikawa Formation according to the previous geological studies. Therefore, the igneous activity responsible for mineralization of the Sado deposit (ca. 25-21 Ma) occurred during the development of Masaragawa Fm, Kinpokusan Fm., and the intrusive rock (22051008) in the Kinpokusan Fm.
The zircon trace element compositions are classified into three groups: (1) high U/Yb ratio (0.2-1.5), low Sc/Yb ratio (0.05-0.20), low Nd/Yb ratio (0.001-0.003); (2) high U/Yb ratio (0.5-3.0), high Sc/Yb ratio (0.10-0.30), low Nd/Yb ratio (0.001-0.005); and (3) low U/Yb ratio (0.2-0.8), high Sc/Yb ratio (0.15-0.30), and high Nd/Yb ratio (0.025-0.015). Group (1) and (2) zircons are from the volcanic and volcanoclastic rocks during the syn-mineralization period, whereas the group (3) ones are from the rocks before the mineralization. Higher U/Yb ratio couple with low Sc/Yb and Nd/Yb imply that the mineralization of Sado deposit derived from primary magma that was highly influence by crustal contamination with an indication of fractional crystallization affecting the crystallization of zircon.
The volcanic and volcanoclastic rock samples were collected from various lithologies around the Sado deposit, which includes the Aikawa Fm. (21102904, 22051007), the Masaragawa Fm. (22051006, 22051002), the Kinpokusan Fm. (22051005, 22051009, 22051004, 22051001), and intrusive rocks (21102901, 22051008). The following ages were obtained for each sample: [Aikawa Fm.] 21102904: 27.64±0.90 Ma, 22051007: 25.46±0.77 Ma. [Masaragawa Fm.] 22051006: 23.59±0.44 Ma, 22051002: 22.87±0.35 Ma. [Kinpokusan Fm.] 22051005: 22.72±0.52 Ma, 22051009: 22.24±0.36 Ma, 22051004: 22.87±0.32 Ma, 22051001: 20.99±0.45 Ma. [Intrusive rock] 21102901: 25.03±0.56 Ma, 22051008: 21.87±0.33 Ma. Those ages are almost consistent with the mineralization age of the older Sado deposit (ca. 25-21 Ma). The veins of the Sado deposit clearly crosscut the Aikawa Formation according to the previous geological studies. Therefore, the igneous activity responsible for mineralization of the Sado deposit (ca. 25-21 Ma) occurred during the development of Masaragawa Fm, Kinpokusan Fm., and the intrusive rock (22051008) in the Kinpokusan Fm.
The zircon trace element compositions are classified into three groups: (1) high U/Yb ratio (0.2-1.5), low Sc/Yb ratio (0.05-0.20), low Nd/Yb ratio (0.001-0.003); (2) high U/Yb ratio (0.5-3.0), high Sc/Yb ratio (0.10-0.30), low Nd/Yb ratio (0.001-0.005); and (3) low U/Yb ratio (0.2-0.8), high Sc/Yb ratio (0.15-0.30), and high Nd/Yb ratio (0.025-0.015). Group (1) and (2) zircons are from the volcanic and volcanoclastic rocks during the syn-mineralization period, whereas the group (3) ones are from the rocks before the mineralization. Higher U/Yb ratio couple with low Sc/Yb and Nd/Yb imply that the mineralization of Sado deposit derived from primary magma that was highly influence by crustal contamination with an indication of fractional crystallization affecting the crystallization of zircon.