Hematite Formation During Skarn Mineralization in the Akatani Iron Deposit, Niigata Prefecture, Japan

ABSTRACT The Akatani Fe deposit in the Ashio Belt, northern Japan, has been regarded as an iron skarn deposit which consists of iron‐oxides in the skarn bodies in marble lenses near the Cretaceous Ninoujidake granite. The major ore mineral in this deposit is hematite, which is uncommon in typical Fe skarn deposits. Some of the orebodies are hosted by Miocene rhyolitic lavas and intrusions. The rhyolite intrusion complicates the understanding of the overall skarn deposit formation. We examined the formation model of the Akatani iron deposit through fieldwork, petrographic observations, apatite U–Pb geochronology, and mineralogical studies of the igneous rocks, skarn minerals, and iron oxides present in the deposit. The skarn mineralization at Akatani is classified into (1) early prograde stage represented by wollastonite, (2) early to late prograde stage represented by garnet and pyroxene, (3) early retrograde stage represented by hydrous skarn minerals represented by tremolite and/or actinolite, and (4) late retrograde stage represented by chlorite. Garnet shows the andradite‐rich chemical composition and the zoning by a slight enrichment of grossular component, sometimes including a spessartine component. The hematite orebodies are classified into two main categories based on the petrographic occurrence and mineral assemblage: magnetized hematite (mushketovite)‐bearing ores in the skarn zone and magnetite‐free ores. Fluorapatites occurring in the magnetized hematite‐bearing ores were used for U–Pb dating. These fluorapatites show euhedral to subhedral forms and occur within andradite coexisting with mushketovite, within tremolite formed by the alteration of clinopyroxene that coexists with andradite, or within mushketovite. The fluorapatites yielded U–Pb ages of 100 ± 23 Ma ( n  = 3, MSWD = 1.2). This age is consistent with that of the Ninoujidake Granite. This geochronological evidence indicates that at least part of the hematite orebodies in the Akatani deposit formed during the Late Cretaceous. The petrological and geochronological studies suggests that hematite mineralization of the deposit resulted from multiple hydrothermal activities with early hematite formation during the prograde‐retrograde skarn stage in the Late Cretaceous. Crystalline temperatures of graphite are calculated from carbonaceous material based on the geothermometer. The scarcity of reactive carbon in the host marble and hornfels may have prohibited the reduction of the hydrothermal fluids, resulting in hematite mineralization in the deposit. This study shows that the hematite orebodies at Akatani is composed by multiple mineralization stages at skarn mineralization, all of which are related to Fe skarn minerals formed by the oxidized hydrothermal fluids from the Cretaceous Ninoujidake granite.