Geology and Genesis of Beryllium Ore Deposit, Mihara Mine, Hiroshima Prefecture, Japan
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概要
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Mihara mine, Hiroshima Prefecture, had been exploited for dissemination fluorite deposits in skarn and monzonite. Rocks of this area consist of Paleozoic limestone, chert and diabasic rocks intruded by Cretaceous biotite granite. The Paleozoic formation is thermally metamorphosed and metasomatised by the granite which is partly monzonitised. Fluorite ores occur in the monzonite and adjacent skarns.<BR>Recently presence of beryllium mineralization in this ore deposit was discovered with a beryllium detector. The mineralization occurs mostly in the monzonite and partly in the scheelite-feldspars-fluorite-iron-rich biotite skarn. The beryllium mineral contained in the skarn is exclusively phenacite which coexists with iron-rich biotite, plagioclase and potassium feldspar, filling the grain boundaries of fluorite crystals. A small amount of scheelite is also found in association with the minerals. On the other hand, danalite is the main beryllium mineral in the monzonite. It occurs as disseminated grains of euhedral or subhedral form, and is associated with violet fluorite and feldspars. Phenacite is also present in a trace amount. It is conspicuous that these phenacites are always corroded along cleavages and cracks, and also surrounded by danalite grains.<BR>Volumetric analyses of constituent minerals in thin sections by a point counter reveal that (1) danalite and phenacite are found in specific monzonite which contains fluorite and quartz more than 5.6% and less than 2.3%, respectively; (2) the quantity of modal danalite in such rocks ranges from 2.5% to 3.6%, and that of phenacite is less than 0.4%; and (3) the amount of quartz gradually decreases towards the adjacent skarn body, while total feldspars and fluorite increase in contrast. The degree of Al/Si order-disorder and the Or contents in potassium feldspars in biotite granite and monzonite were determined with powder X-ray diffractometry. Potassium feldspars in biotite granite have a tendency to become ordered and rich in Or component with coming close to monzonite, and the maximum ordering and Or contents are encountered where they coexist with danalite in monzonite. These results are correlative with the modal analysis data.<BR>From the results obtained, the genesis of this beryllium ore deposit may be summarized as follows : (1) Paleozoic rocks were partly metasomatised by the intrusion of Cretaceous biotite granite. (2) Migration of silica from the granite to the adjacent skarn body caused relative increases of alkali and alumina contents in peripheries of the granite, converting them to monzonite. (3) Only a small portion of beryllium contained in the granite could have moved to the skarn body to form phenacite at a limited part of the skarn. (4) The main portion of beryllium was precipitated as danalite crystals within the monzonite.
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