1:45 PM - 3:15 PM
[MIS13-P09] The changes of magmatic-hydrothermal systems from the Cretaceous to the Neogene: the case of iron mineralization in the Akatani deposit, Niigata Prefecture, Japan.
Keywords:hematite deposit, garnet, magmatic-hydrothermal systems
The Akatani deposit in Niigata Prefecture is the iron ore deposit located on the southwest side of the Mt. Iide south of the Tanakura Tectonic Line in the Ashio Belt and was mined for hematite and chalcopyrite ore. The distribution of rhyolite and hematite ore bodies in this deposit is closely related and two deposits are recognized: (1) the contact metasomatic deposits of the lead-zinc skarn-type were formed by igneous activity of the Late Cretaceous granite; (2) the hematite deposits of the hydrothermal ore veins-type and contact metamorphosed-type in the Early to Middle Miocene rhyolite. The previous studies about the Akatani deposit have focused on lithofacies between the hematite ores and rhyolitic rocks, whereas they have not discussed how the Late Cretaceous granite is related to the genesis of the hematite ores. Moreover, they have not focused on the hydrothermal fluid chemistry forming the ore bodies.
In this study, we reexamined the relationship between the igneous rocks, skarn, and iron-sulfide mineralization in the Akatani deposit, based on detailed field survey, rock descriptions, and mineralogical and geochemical descriptions and analyses of hematite and garnet.
The host rock consists of crystalline limestone, chert, and hornfels of the early Jurassic system of the Ashio Belt, and is intruded by the Late Cretaceous Ninoujidake granite and Early to Middle Miocene dolerite. In addition, Early to Middle Miocene rhyolite covers or intrudes the host rocks. In this study, proximal skarn formed between granite and crystalline limestone, and distal skarn formed in crystalline limestone and hornfels. Hematite ores were found in rhyolite, dolostone, chlorite, kaolinite, and hornfels, and a new hematite ore was discovered in a Late Cretaceous distal skarn which had not been mentioned in previous studies.
The magnetic susceptibility of the Late Cretaceous Ninoujidake granite is 1.26 × 10-3 SI and the results of the whole-rock chemical composition analysis by XRF indicate alumina saturations of 1.0-1.4. Quantitative analysis of garnet by EPMA in the whole skarn zone shows that the andradite (Ca2+3Fe3+2(SiO4)3) component is 80.39-100.00 wt.%, the grossular (Ca2+3Al3+2(SiO4)3) component is 0.00- 19.61 wt.%, and the spessartine (Mn2+3Al3+2(SiO4)3) component ranged from 0.00-6.48 wt.%. Based on these results, optical characteristics, and occurrence, andradite was classified into four types from Adr1 to Adr4. The hematite ores were classified into eight types: Hem1 to Hem8, according to the occurrence type. Trace element analyses by EPMA of hematite and magnetite of each type showed differences in Si, Mg, Mn, Ti, Al, and other elements.
The Ninoujidake granite of the ilmenite series shows peraluminous which is characteristic of related igneous rocks forming the Pb-Zn skarn suggests that it was formed by a reductive skarn. The garnet chemical compositions shows that the redox state of the hydrothermal fluids forming the skarn deposit was repeatedly changed. Hem1-3 in the Late Cretaceous skarn zone formed by alternation of skarn, and Hem4 in the chlorite zones associated with Early to Middle Miocene dolerite replaced an existing skarn of the Late Cretaceous. Hem5-8 are associated with Early to Middle Miocene rhyolitic intrusive rocks. This indicates that the differences in trace element concentrations of hematite and magnetite in Hem1 to Hem8 are caused by differences in their chemical composition (the ore-forming fluids have a wide range of fluid-rock ratios and crystallized in silicate melt at high temperatures of magmatic origin, or crystallized by hydrothermal alternation under low temperatures of magmatic-hydrothermal origin, etc.) and it is suggested that this is consistent with the chemical composition of garnet and coexistence with other minerals.
Therefore, the detailed field investigations, rock descriptions, whole-rock chemical compositions of related igneous rocks, and trace element compositions of garnet, and hematite/magnetite in this study reflect the different environments or conditions under which the hematite ores were formed and reveal new insights into the changes of magma-hydrothermal systems from the Cretaceous to the Neogene in the same location.
In this study, we reexamined the relationship between the igneous rocks, skarn, and iron-sulfide mineralization in the Akatani deposit, based on detailed field survey, rock descriptions, and mineralogical and geochemical descriptions and analyses of hematite and garnet.
The host rock consists of crystalline limestone, chert, and hornfels of the early Jurassic system of the Ashio Belt, and is intruded by the Late Cretaceous Ninoujidake granite and Early to Middle Miocene dolerite. In addition, Early to Middle Miocene rhyolite covers or intrudes the host rocks. In this study, proximal skarn formed between granite and crystalline limestone, and distal skarn formed in crystalline limestone and hornfels. Hematite ores were found in rhyolite, dolostone, chlorite, kaolinite, and hornfels, and a new hematite ore was discovered in a Late Cretaceous distal skarn which had not been mentioned in previous studies.
The magnetic susceptibility of the Late Cretaceous Ninoujidake granite is 1.26 × 10-3 SI and the results of the whole-rock chemical composition analysis by XRF indicate alumina saturations of 1.0-1.4. Quantitative analysis of garnet by EPMA in the whole skarn zone shows that the andradite (Ca2+3Fe3+2(SiO4)3) component is 80.39-100.00 wt.%, the grossular (Ca2+3Al3+2(SiO4)3) component is 0.00- 19.61 wt.%, and the spessartine (Mn2+3Al3+2(SiO4)3) component ranged from 0.00-6.48 wt.%. Based on these results, optical characteristics, and occurrence, andradite was classified into four types from Adr1 to Adr4. The hematite ores were classified into eight types: Hem1 to Hem8, according to the occurrence type. Trace element analyses by EPMA of hematite and magnetite of each type showed differences in Si, Mg, Mn, Ti, Al, and other elements.
The Ninoujidake granite of the ilmenite series shows peraluminous which is characteristic of related igneous rocks forming the Pb-Zn skarn suggests that it was formed by a reductive skarn. The garnet chemical compositions shows that the redox state of the hydrothermal fluids forming the skarn deposit was repeatedly changed. Hem1-3 in the Late Cretaceous skarn zone formed by alternation of skarn, and Hem4 in the chlorite zones associated with Early to Middle Miocene dolerite replaced an existing skarn of the Late Cretaceous. Hem5-8 are associated with Early to Middle Miocene rhyolitic intrusive rocks. This indicates that the differences in trace element concentrations of hematite and magnetite in Hem1 to Hem8 are caused by differences in their chemical composition (the ore-forming fluids have a wide range of fluid-rock ratios and crystallized in silicate melt at high temperatures of magmatic origin, or crystallized by hydrothermal alternation under low temperatures of magmatic-hydrothermal origin, etc.) and it is suggested that this is consistent with the chemical composition of garnet and coexistence with other minerals.
Therefore, the detailed field investigations, rock descriptions, whole-rock chemical compositions of related igneous rocks, and trace element compositions of garnet, and hematite/magnetite in this study reflect the different environments or conditions under which the hematite ores were formed and reveal new insights into the changes of magma-hydrothermal systems from the Cretaceous to the Neogene in the same location.