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[SCG47-P13] Skarn and iron mineralization in the Akatani deposit, Shibata city, Niigata Prefecture, Japan
Keywords:Fe skarn deposit, hypogene secondary magnetite, martitization, garnet
The Akatani skarn deposit is located on the southwest side of the Mt. Iide in Shibata city, Niigata Prefecture. This deposit yielded hematite ore, which is rare in iron skarn deposits in the world. However, the reason why hematite was dominantly formed in the deposit has remained unclear. The purpose of this study is to clarify the genesis of the skarn and iron mineralization in this deposit based on skarn and ore mineralogy.
This study includes mineralogical mapping proximal skarn near the intrusive rock and distal skarn formed far from it with descriptions of skarn occurrence. In addition, in order to characterize hydrothermal fluids associated with the iron mineralization, we described the skarn minerals such as garnet and hematite and magnetite by macro and microscopic observation, and identify minerals with powder XRD. The whole rock chemical composition of igneous rocks were analyzed by XRF, and the mapping and the quantitative analysis of garnet by EPMA, and the U–Pb dating of garnet by LA-ICP-MS.
The host rock consists of crystalline limestone (partially dolomite), chert, and pelitic metamorphic rocks of the early Jurassic system of the Ashio Belt. Late Cretaceous Ninoujidake granite and Early Miocene dolerite intrude into the host rock, and Early Miocene rhyolite covered or intruded these rocks. The main hematite orebodies are formed in the proximal and distal skarn associated with Ninoujidake granite, and some are also associated with Early Miocene rhyolite.
From the observed occurrence of hematite orebodies, the three formation stages were confirmed in this deposit: Ninoujidake granite intrusion stage, dolerite intrusion stage, and rhyolite stage. The orebodies of each stage consist of hematite after magnetite (martitization), magnetite after hematite (hypogene secondary magnetite), and magnetite-free hematite with pyrite, respectively.
Powder XRD analysis shows that distal skarn is rich in hedenbergite and poor in diopside, and proximal skarn is mostly composed of amphibole without clinopyroxene. Distal skarn and proximal skarn are found to be rich in both actinolite and tremolite. The whole rock chemical analysis shows that the granite is per-aluminous composition with alumina saturation index of 1.01 to 1.38. The magnetic susceptibility of the granite suggests ilmenite series. The results of mapping and quantitative analysis by EPMA show that the garnet minerals contains andradite (Ca2+3Fe3+2(SiO4)3) component (80.39-100 wt. %), and grossular (Ca2+3Al3+2(SiO4)3) component (19.61-0 wt. %), and spessartine (Mn2+3Al3+2(SiO4)3) component is (6.48-0 wt. %). As a whole, the core part is mostly composed of andradite, and the rim part contains a small amount of grossular component. The rim part rich in magnetite contains a very small amount of spessartine. These observations show that the early stage of garnet is formed from hydrothermal fluids rich in Fe3+, the final stage from hydrothermal fluids that contain Al3+. And the final stage in some skarns are rich in Mn2+. Thus, the ore-forming fluids are significantly variable, and the redox state changed over time.
The LA-ICP-MS analysis on garnet could not yield the significant U/Pb age due to their low U concentrations (< 1 ppm). This result suggests that U didn’t concentrate in garnet because of oxidized state of U (6+ valance) in the fluids. This is concordant with the abundant occurrence of hematite that indicate high oxidation state of the mineralizing fluids.
It is concluded that the Akatani deposit formed by multiple mineralization stages, rather than single event proposed in previous studies (hematite is formed from primary hydrothermal fluid, or hematite is formed with rhyolite intrusion, or hematite is secondarily formed after magnetite), it was also revealed that all these phenomena and magnetite is secondarily formed after hematite.
This study includes mineralogical mapping proximal skarn near the intrusive rock and distal skarn formed far from it with descriptions of skarn occurrence. In addition, in order to characterize hydrothermal fluids associated with the iron mineralization, we described the skarn minerals such as garnet and hematite and magnetite by macro and microscopic observation, and identify minerals with powder XRD. The whole rock chemical composition of igneous rocks were analyzed by XRF, and the mapping and the quantitative analysis of garnet by EPMA, and the U–Pb dating of garnet by LA-ICP-MS.
The host rock consists of crystalline limestone (partially dolomite), chert, and pelitic metamorphic rocks of the early Jurassic system of the Ashio Belt. Late Cretaceous Ninoujidake granite and Early Miocene dolerite intrude into the host rock, and Early Miocene rhyolite covered or intruded these rocks. The main hematite orebodies are formed in the proximal and distal skarn associated with Ninoujidake granite, and some are also associated with Early Miocene rhyolite.
From the observed occurrence of hematite orebodies, the three formation stages were confirmed in this deposit: Ninoujidake granite intrusion stage, dolerite intrusion stage, and rhyolite stage. The orebodies of each stage consist of hematite after magnetite (martitization), magnetite after hematite (hypogene secondary magnetite), and magnetite-free hematite with pyrite, respectively.
Powder XRD analysis shows that distal skarn is rich in hedenbergite and poor in diopside, and proximal skarn is mostly composed of amphibole without clinopyroxene. Distal skarn and proximal skarn are found to be rich in both actinolite and tremolite. The whole rock chemical analysis shows that the granite is per-aluminous composition with alumina saturation index of 1.01 to 1.38. The magnetic susceptibility of the granite suggests ilmenite series. The results of mapping and quantitative analysis by EPMA show that the garnet minerals contains andradite (Ca2+3Fe3+2(SiO4)3) component (80.39-100 wt. %), and grossular (Ca2+3Al3+2(SiO4)3) component (19.61-0 wt. %), and spessartine (Mn2+3Al3+2(SiO4)3) component is (6.48-0 wt. %). As a whole, the core part is mostly composed of andradite, and the rim part contains a small amount of grossular component. The rim part rich in magnetite contains a very small amount of spessartine. These observations show that the early stage of garnet is formed from hydrothermal fluids rich in Fe3+, the final stage from hydrothermal fluids that contain Al3+. And the final stage in some skarns are rich in Mn2+. Thus, the ore-forming fluids are significantly variable, and the redox state changed over time.
The LA-ICP-MS analysis on garnet could not yield the significant U/Pb age due to their low U concentrations (< 1 ppm). This result suggests that U didn’t concentrate in garnet because of oxidized state of U (6+ valance) in the fluids. This is concordant with the abundant occurrence of hematite that indicate high oxidation state of the mineralizing fluids.
It is concluded that the Akatani deposit formed by multiple mineralization stages, rather than single event proposed in previous studies (hematite is formed from primary hydrothermal fluid, or hematite is formed with rhyolite intrusion, or hematite is secondarily formed after magnetite), it was also revealed that all these phenomena and magnetite is secondarily formed after hematite.