Geologic Environment of Hirase Molybdenum Deposits, Gifu Prefecture, Central Japan

Study on Molybdenum Deposits in Iapan (5)

Abstract

The Hirase molybdenum mine, consisting of many molybdenite quartz veins, some of which are pegmatitic and contain beautiful euhedral crystals of molybdenite, is located at the second largest productive area of molybdenum in the Inner Zone of the Southwestern Japan. The area is composed of late, Cretaceous pyroclastic rocks of largely rhyodacitic composition and small granitic intrusives within the pyroclastics. These granitic bodies, of which the largest dimensions are 3 by 15 km, consist of aplite (Ga, 19%), related quartz diorite (Dq, 10%), hornblende-biotite granodiorite (Gd, 30%), and biotite granite (Gb, 41%). Relatively dominant fine-grained aplitic textures, as well as modal and chemical compositions of the granitic rocks indicate that they were derived from slightly mafic-poor series of granitic magma in shallower horizons than those of granitic rocks of similar ages in the Inner Zone of the Southwestern Japan.
Small aplitic masses of the biotite granite of Hirase and the southern half of Hatogaya bodies seem to be the source of the molybdenite bearing ore-fluid. Modally and chemically, the aplitic granite belongs to one of the most fractionated granitic rocks and occurs at the margin of the biotite granite. The aplitic granite, however, is the most suitable host rock to ore veins, petering out downward at the Hirase mine. According to concept presented by the author (1967) that molybdenum-bearing ore-fluid would have been separated from silicate melts in a place where the aplitic granite intruded, kind of the host rocks could indicate limit of molybdenum deposits. This is shown in lower parts of the Hirase underground.
Pyroclastic rocks adjacent to the aplitic granite are hydrothermally altered. Potash feldspar and quartz are most abundant in the alteration zone. The innermost zone can be called (pyrrhotite) -andalusite-pale brown biotite-muscovite subzone, while most parts of the alteration zone consist of (pyrite)-carbonates (siderite)-sericite. Loss and gain during the alteration are as follows: Drastic increases in S and CO₂, slight increases in K₂O, Fe₂O₃, and Na₂O; decreases in MgO, CaO, TiO₂, FeO, and P₂O₅, and slight decreases in MnO and AL₂O₃. The inceased elements are roughly coincidental in their behavior with vein-forming elements except Si⁴⁺. Distributions of 15 trace elements indicate that only Mo and U which occur also in ore veins increase in the alteration zone. Most of other elements, i. e., Ba, Sr, B, V, Ni, Co, As, Zn, and Pb decrease in the alteration zone because of re crystallization of potash feldspar, together with decomposition of plagioclase and mafic minerals. Detection of andalusite clots and geochemical prospecting using Mo and U should be applicable in molybdenum exploration in this destrict.