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The Itaga Wolframite Deposits, Known for Their Unusual Gangue

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The Itaga Wolframite Deposits, Known for Their Unusual Gangue RESOURCE GEOLOGY, 45(3), 169•`172, 1995 K-Ar Age and Sulfur Isotopic Ratio of Ores from the Itaga Tungsten Deposit, Tochigi, Japan. Shunso ISHIHARA and Akira SASAKI Abstract: Muscovite and biotite from the greisenized granite of the Itaga wolframite deposits were dated at 63.4•}1.4 Ma and 63.0•}1.4 Ma, respectively. The age belongs to the younger group of granite activities of the Ashio Mountains. Lollingite disseminated in the altered granite shows ƒÂ34SCDT of ca. -5 permil, which is very similar to the average value of the ilmenite- series ore deposits of the Chubu-Kinki district. (SAKURAI, 1943), which is called the endo-gra- Introduction nitic ore deposit in this paper. The official statis- The Itaga wolframite deposits, known for their tics of production during this period are not avail- unusual gangue minerals such as andalusite, co- able. rundum and topaz (SAKURAI,1943; SHIBATA, Later in 1952, wolframite quartz veins hosted in 1952), had been the second largest tungusten pro- sedimentary wall rocks, which will be called the ducer in the Kanto district after the Takatori de- exo-granitic ore deposit hereafter, were discov- posits up to the early 1960's. The area had been ered at some 200 m south of the endo-granitic de- explored for gold-silver and copper since the posit, and wolframite concentrates of 23.5 tons Meiji Era (NAKAZAWAand MONONOBE,1957), had been produced by 1961. The Itaga mine was but wolframite was discovered from a steep sole example in Japan where profitable amounts mountain slope of the Ashio Mountains in 1939 of tungsten were mined from both endo-granitic (Fig. 1) and was open-pit mined from the orebod- disseminated-type and exo-granitic vein-type ore- ies hosted in granites during World War II bodies. The mine area is located in the late Paleozoic- Mesozoic sedimentary terrane of so-called Ashio Mountains in the nothern part of the Kanto Dis- trict. The sedimentary rocks are extruded and in- truded by felsic magmas during the late Creta- ceous time, which are considered an eastern ex- tension of the late Cretaceous-Paleogene granitoids of the Sanyo belt (ISHIHARA, 1977). Yet its age determination has been scarsely made. Sulfur isotope study of granitoid-related ore de- posits has revealed consistent negative values in wolframite deposits with greisen alteration in the Fig. 1 Itaga mine of the Ashio Mountains in 1958, looking Inner Zone of Southwest Japan (SASAKI and toward the north. IsHIHARA, 1980). It is interesting to know of the Received on June 3, accepted on October, 26, 1994 ratio on sulfides of the Itaga deposits. * Department of Earth and Planetary Sciences , Hokkaido Uni- We have obtained recently the ore samples from versity, Sapporo, 060 Japan the Itaga deposits to study their age and sulfur iso- ** Mineral Resources Department , Geological Survey of Ja- topic ratio. The results, which may be a supple- pan, Tsukuba, 305 Japan ment to our previous works (SASAKI and Keywords: K-Ar dating, Greisen, Muscovite, Hydrothermal ISHIHARA, 1980, ISHIHARA et al., 1988, 1992), biotite, Andalusite, Sapphire, Wolframite, Lollingite, ƒÂ34SCDT . will be reported in this short communication. 169 170 S. ISHII-IARA and A. SASAKI RESOURCE GEOLOGY: Characteristic of this deposit is common occur- rence of high alumina minerals like corundum and andalusite. SAKURAI (1943) described purple- blue sapphire crystals coexisted with feldspars in the pegmatite. The hexagonal platy one is 1-2 mm in diameter, while the lamellar crystal occurring in feldspars is 1 cm in length. Andalusite occurs also in pegmatite together with sapphire, as pale pink to reddish crystals growing up to 1 cm wide and 3 cm long. Other gangue minerals are topaz, up to 2-3 mm in diameter, tourmaline, up to 1 cm in di- ameter and 2 cm long, muscovite and quartz. Exo-Granitic Deposit The exogranitic deposit is developed at some Fig.2 Simplified geological map of the Ashio Moun- 200 m south of the open pit and is vein type occur- tains. Compiled from 1:200,000 geological map of ring in phyllitic quartzites, within 15-20 meters Utsunomiya (Geol. Surv. Japan) and Geological map from the granite contact (SASAKI, 1959). The ore of Tochigi Prefecture. 91B and 65B are K-Ar ages on biotite listed in SATO et al. (1992). Blank part is veins are composed of many small ones whose mostly Paleozoic-Mesozoic sedimentary rocks. width ranges from 3 to 20 cm with an average of Geology of the Itaga Deposits ca.7 cm. They are grouped into two; the gentle- dip one with N-S strike and 30•‹W dip, and the Kamitsuga County of Tochigi Prefecture is un- steep-dip one with N15-20•‹E strike and 70•‹E dip. derlain by late Paleozoic to Mesozoic sedimentary The vein-forming minerals are mostly quartz rocks, which were extruded and intruded by the and wolframite. The wolframite has high Fe/Mn late Cretaceous to Paleogene rhyo-dacitic volca- ratios between Fe0.87Mn0.13WO4and Fe0.83Mn0.17 nic rocks and granite-granodiorite of stock size WO4, thus properly called as ferberite (SASAKI, (<100 km2, Fig. 2). The granitoids are magnetite- 1959). The other minerals are trace amounts of free, ilmenite-series, according to the measure- scheelite, feldspar, topaz, muscovite, biotite. Sul- ment of magnetic susceptibility. fides are very rare, but lollingite, chalcopyrite and The sedimentary rocks exposed in the Itaga pyrite could be visible. mine area are mostly phyllitic quartzites, with some amounts of siliceous slate, slate and sand- Analyzed Samples and Analytical Results stone, which strike N40-60•‹ and dip 40-60•‹ SE Analyzed samples were collected from the and NW (SASAKI, 1959). These rocks are intruded endo-granitic orebody in 1950s. The sample for by biotite granite, whose marginal facies is por- the age determination is coarse-grained "gre- phyritic. The granite crops out at few places as "Fenster" with dimensions less than 200 b isenized" granite with fair amounts of muscovite, y 750 m biotite and andalusite (Fig. 3). Under the micro- (SASAKI, 1959). The tungsten mineralization is scope, biotite has Z=Y color of pale brown and seen both in the granite body and in its roof pen- andalusite shows a weak pleochroism. Both mus- dant sedimenatary rocks. covite and biotite were separated by an Isody- Endo-Granitic Deposit namic Separator. The deposit occurs in the apical part of the por- K-Ar dating of muscovite yielded an age of 63.4 phyritic granite that has been greisenized and •}1.4 Ma, while biotite was 63.0•}1.4 Ma; the two veined with feldspars and quartz. Tungsten miner- values agree within the analytical error (Table 1). als are disseminated in the altered granite and may It is inferred that the Itaga mineralization has a also occur in the pegmatitic quartz veins Paleogene age, and the granites occurring in the (SAKURAI, 1943). Ore minerals are wolframite, Itaga-Karasawa mine area (Fig. 2) must have a and small amounts of scheelite, lollingite, molyb- similar age, because granites and related Mo-W- denite, chalcopyrite and pyrite. Sn deposits usually give us identical ages. Thus 45(3), 1995 K-Ar age and sulfur isotopic ratio of ores from the Itaga tungsten deposit 171 Magnetite-series Provnce knerite-series Prov. Fig.3 Photomicrograph of the studied greisen for the age dating. Euhedral andalusite (high relief) filled with muscovite (white with cleavage) and biotite (dark with cleavage). Table I K-Ar age data of the Itaga greisen from the north open-pit, Tochigi Prefecture, analyzed by the Central Lab., Mitsubishi Material Co., Ltd. Fig. 4 Sulfur isotopic ratio of the studied lollingite from the Itaga mine, plotted in the histograms of ore sulfur isotopic data from the Mesozoic-Paleogene granitic provinces (ISHIHARAand SASAKI, 1991). One box is average of one ore deposit. Solid box is Itaga deposit. The box with cross in the Chugoku subprovince is the *1 Decay constants used are Ae=0 .581•~10-10/Y and ƒÉƒÀ=4.962 average of subvolcanic Akenobe and Ikuno deposits. •~10-10/Y (STEIGER and JAEGER, 1977). *2 40K/K=0 .01167 atom% 1988). Greisenized chert discovered in an explo- *3 Error estimation based on NAGAO et al . (1984). ration drill hole to the west of the main No. 7 vein of the Takatori mine is found to have similar age the granites in the Itaga-Karasawa mine area are of 71.8 Ma (OGASAWARAet al., 1993). It is sug- definitely younger than the Sori granodiorite (91 gested that the tungsten mineralization of the Itaga Ma) and is similar to that of the Yokoneyama gra- deposits is slightly younger than that of the nitic stock (65 Ma, Fig. 2). Takatori deposits. The granitic activities of the Ashio Mountains Sulfur isotope ratio of ƒÂ34SCDT (permil) was are clearly divided into two: the older one (ca. 90 analyzed by A. SASAKI on two specimens of Ma) represented by Sori granodiorite and the lollingite disseminated in the greisenized granite. younger one (ca. 63 Ma) which includes most of The result are -4.5 and -5.1 permil (average -4.8 the small bodies associated with coeval volcanic permil). This value is lighter than ƒÂ34SCDT of -3.6 rocks occurring in the northern part (Fig. 2). Thus permil of ore sulfurs from the Takatori mine area the granite related to the Itaga tungsten mineral- (SASAKI and ISHIHARA, 1980), which contains ar- ization belongs to the younger group of granitoid senopyrite and some pyrite, and is similar to an in the Ashio Mountains. average value of ilmenite-series ore deposits in Vein muscovite of the Takatori tungsten depos- the Chubu-Kinki District (Fig.
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