17:15 〜 19:15
[HRE12-P09] 新潟県赤谷鉱床における赤鉄鉱鉱体の鉱物学的・地球化学的特徴および流体包有物
キーワード：スカルン Fe 鉱床、熱水性赤鉄鉱鉱床、微量元素、硫黄同位体、流体包有物、ドロマイト
The Akatani deposit, located in Shibata City, Niigata Prefecture, is one of the largest hematite deposits in NE Japan. Mining activities in the deposit primarily extracted hematite and chalcopyrite until 1977. The regional geology around the deposit consists of the Jurassic Ashio accretionary complex intruded by Late Cretaceous Ninoujidake granitoids (LCNG) and overlain or intruded by Middle Miocene Araya Formation rhyolitic rocks (MMAR). Limestone and hydrothermal dolomite are significantly distributed in the Akatani deposit. To the southwest of the deposit, the Mikawa deposit as a group of vein-type deposit form, and contains base-metal and Au-Ag ores. The deposit is associated with MMAR and the absence of carbonate rock. This study aims to characterize hematite mineralization in the Akatani deposit through the combination of mineralogical, trace elements, sulfur isotopes, and fluid inclusion (FI) studies and to compare it with the Mikawa deposit.
The hematite orebodies in the Akatani deposit are divided into two types: skarn-type orebodies (STO: Cu, Pb, Zn, and Fe) associated with LCNG and vein-type orebodies (VTO: Fe and Cu) associated with MMAR. STO occurs in distal skarns and has mostly undergone magnetitization. VTO forms at the contact between MMAR and dolomite and does not exhibit magnetitization. Based on their mineralogical characteristics, STO are classified as Hem 1–3, whereas VTO are classified as Hem 4–8. On the side of the Mikawa deposit closer to the Akatani deposit, there are many iron carbonate minerals such as siderite and ankerite, and some hematite. STO hematite was enriched in Sn and Mo, whereas VTO hematite exhibited a strong positive correlation between Mn+Zn and Mg. Primary FI in garnet with Hem 1 shows the pressure-corrected homogenization temperature (TH) of 382–445 ℃ (Avg. 412 ℃) and the salinity of 4.8–9.1 wt% (Avg. 6.5 wt%). MMAR quartz hosts multiphase FI containing halite and gas-liquid secondary FI, with the TH of 238–346 ℃ (Avg. 297 ℃) and 141–343 ℃ (Avg. 252 ℃), and the salinity of 34.3–45.4 wt% (Avg. 39.5 wt%) and 4.4–11.7 wt% (Avg. 7.9 wt%), respectively. Primary FI in quartz, barite, and siderite from VTO exhibit two-phase characteristics, and the TH of them are 238–347 ℃ (Avg. 294 ℃) and the salinity of them are 4.5–7.8 wt% (Avg. 5.5 wt%). Sphalerite in the quartz veins of the Mikawa deposit shows two-phase primary FI with the TH of 256–275 °C (Avg. 270 ℃) and a salinity of 5.9–7.7 wt% (Avg. 7.3 wt%). No pressure correction is applied to MMAR andVTO. δ34S values are +7.7 ‰ for sphalerite in the distal skarn (LCNG-related), +5.8 ‰ for pyrite in Hem 4, and +7.7 ‰ for chalcopyrite in Hem 7. In contrast, barite in Hem 6 shows a δ34S of +21.1 ‰. Sphalerite in the quartz veins of the Mikawa deposit has a δ34S of +6.5 ‰.
Trace elements in STO hematite suggest the influence of LCNG-derived magmatic fluids, whereas in VTO hematite is due to the significant contribution of dolomite to its formation. STO and VTO were formed by different fluids owing to differences in temperature and salinity. The FI of MMAR suggest that VTO formed at the same timing during MMAR intrusion. In contrast, the Mikawa deposit, which exhibits temperature and salinity similar to the secondary FI in MMAR, was likely formed by hydrothermal fluids after the MMAR intrusion, implying an older formation compared to the Akatani deposit. The sulfur isotope ratios suggest a mixture of meteoric water of seawater origin and fluids of magma origin.
Therefore, both VTO of the Akatani deposit and the Mikawa deposit were formed by MMAR magmatism in the shallower environment, however, difference in formation timing and the presence or absence of dolomite is an important key to the different types of ore deposits formation. The reason why VTO of the Akatani deposit formed predominantly hematite deposits compared to the Mikawa deposit is a result which the influence of dolomite is strongly reflected.
The hematite orebodies in the Akatani deposit are divided into two types: skarn-type orebodies (STO: Cu, Pb, Zn, and Fe) associated with LCNG and vein-type orebodies (VTO: Fe and Cu) associated with MMAR. STO occurs in distal skarns and has mostly undergone magnetitization. VTO forms at the contact between MMAR and dolomite and does not exhibit magnetitization. Based on their mineralogical characteristics, STO are classified as Hem 1–3, whereas VTO are classified as Hem 4–8. On the side of the Mikawa deposit closer to the Akatani deposit, there are many iron carbonate minerals such as siderite and ankerite, and some hematite. STO hematite was enriched in Sn and Mo, whereas VTO hematite exhibited a strong positive correlation between Mn+Zn and Mg. Primary FI in garnet with Hem 1 shows the pressure-corrected homogenization temperature (TH) of 382–445 ℃ (Avg. 412 ℃) and the salinity of 4.8–9.1 wt% (Avg. 6.5 wt%). MMAR quartz hosts multiphase FI containing halite and gas-liquid secondary FI, with the TH of 238–346 ℃ (Avg. 297 ℃) and 141–343 ℃ (Avg. 252 ℃), and the salinity of 34.3–45.4 wt% (Avg. 39.5 wt%) and 4.4–11.7 wt% (Avg. 7.9 wt%), respectively. Primary FI in quartz, barite, and siderite from VTO exhibit two-phase characteristics, and the TH of them are 238–347 ℃ (Avg. 294 ℃) and the salinity of them are 4.5–7.8 wt% (Avg. 5.5 wt%). Sphalerite in the quartz veins of the Mikawa deposit shows two-phase primary FI with the TH of 256–275 °C (Avg. 270 ℃) and a salinity of 5.9–7.7 wt% (Avg. 7.3 wt%). No pressure correction is applied to MMAR andVTO. δ34S values are +7.7 ‰ for sphalerite in the distal skarn (LCNG-related), +5.8 ‰ for pyrite in Hem 4, and +7.7 ‰ for chalcopyrite in Hem 7. In contrast, barite in Hem 6 shows a δ34S of +21.1 ‰. Sphalerite in the quartz veins of the Mikawa deposit has a δ34S of +6.5 ‰.
Trace elements in STO hematite suggest the influence of LCNG-derived magmatic fluids, whereas in VTO hematite is due to the significant contribution of dolomite to its formation. STO and VTO were formed by different fluids owing to differences in temperature and salinity. The FI of MMAR suggest that VTO formed at the same timing during MMAR intrusion. In contrast, the Mikawa deposit, which exhibits temperature and salinity similar to the secondary FI in MMAR, was likely formed by hydrothermal fluids after the MMAR intrusion, implying an older formation compared to the Akatani deposit. The sulfur isotope ratios suggest a mixture of meteoric water of seawater origin and fluids of magma origin.
Therefore, both VTO of the Akatani deposit and the Mikawa deposit were formed by MMAR magmatism in the shallower environment, however, difference in formation timing and the presence or absence of dolomite is an important key to the different types of ore deposits formation. The reason why VTO of the Akatani deposit formed predominantly hematite deposits compared to the Mikawa deposit is a result which the influence of dolomite is strongly reflected.