MINING GEOLOGY, 27, 293•`305, 1977 The Magnetite-series and Ilmenite-series Granitic Rocks* Shunso ISHIHARA** Abstract: Opaque minerals of common granitic rocks were studied microscopically. The granitoids were divided into (i) a magnetite-bearing magnetite-series and (ii) a magnetite-free ilmenite-series. Each series has the following characteristic assemblages of accessary minerals: Magnetite-series: Magnetite (0.1-2 vol.%), ilmenite, hematite, pyrite, sphene, epidote, high ferric/ferrous (and high Mg/Fe) biotite; Ilmenite-series: Ilmenite (less than 0.1 vol.%), pyrrhotite, graphite, muscovite, low ferric/ferrous (and low Mg/Fe) biotite. The mineral assemblages imply a higher oxygen fugacity in the magnetite-series granitoids than in the ilmenite- series granitoids during solidification of the granitic magmas. The boundary separating the two series is probably near the Ni-NiO buffer. The magnetite-series granitoids are considered to have been generated in a deep level (upper mantle and lowest crust) and not to have interacted with C-bearing materials; whereas the ilmenite-series granitoids were generated in the middle to lower continental crust and mixed with C-bearing metamorphic and sedimentary rocks at various stages in their igneous history. The former carries porphyry copper-molybdenum deposits and the latter accompanies greisen-type tin-wolframite deposits. Lack of porphyry copper deposits in the Mesozoic orogeny belts in East Asia is related to a general paucity of the magnetite-series granitoids in this terrane. understood that both magnetite and ilmenite Introduction are present as accessary constituents in com- In studies of granitic rocks, opaque oxide mon granitic rocks. minerals are often ignored, although they may Studies ofJapanese granitic rocks during the have an important bearing on both petrog- past years have revealed that species and enesis and ore genesis. In recent years there amounts of the opaque minerals change from have been excellent studies on small pluton one granitic belt to the other, and that the where oxidation and reduction ofgranitic rocks may be divided into two series: one magma can be analyzed in detail (e.g., containing magnetite and ilmenite, and the CZAMANSKE& MIHALIK, 1972). However, other in which no magnetite but very small the distribution and occurrence of the opaque amounts of ilmenite are observed. They may minerals in common granitic rocks seems not be called "magnetite-series" and "ilmenite- yet well established, although it is generally series (ISHIHARA,1975), referring, respec- tively; to magnetite-bearing series and *Received June 3,1977; in revised form July 7, 1977. magnetite-free series granitic rocks. Paper presented at the 13th Pacific Science Congress, Previous papers (ISHIHARA,1971a; TsusuE Vancouver, B.C., Canada, August 1975, in a & ISHIHARA, 1974) have reported bulk symposium organized by the Circum Pacific Pluto- chemistry of granitic rocks and. mode of nism Project of the International Geological Correla- occurrence and some chemistry of Fe-Ti tion Program and chaired by P. C. BATEMAN. * * Geological Survey of Japan, Hisamoto 135, Takatsu, oxide., minerals, mostly-from. southwestern Kawasaki, Japan. Japan. Here, granitoids from the remaining Key words: Magnetite-series granitoid, Ilmenite- part of the Japanese Islands and some foreign series granitoid, Oxygen fugacity, Sphene, Graphite, countries are considered. This paper briefly Porphyry Cu-Mo deposits, Tin-wolframite deposits. summarizes classification, distribution and 293 294 S.ISHIHARA MINING GEOLOGY: Fig. 1 Distribution of the magnetite-series and ilmenite-series granitoids in Japan . Ratios of the two series in one tectonic unit or one area are shown in circles. The Inner Zone of Southwest Japan is subdivided into northern Kyushu, Chugoku-Kinki, Chubu and Niigata-Kanto districts . Abbreviations: HK, Hidaka belt (Tertiary); KT, Kitakami belt (early Cretaceous); AB, Abukuma belt (Cretaceous and minor older rocks); RY , Ryoke belt (Cretaceous and minor older rocks); SY, Sanyo belt (Cretaceous-Paleogene); SL, Sanin belt (Cretace- ous-Paleogene); SWO, Southwestern outer belt (Miocene); TTL , Tanakura tectonic line; MTL, Median tectonic line; SM, Sanbagawa metamorphic belt; KM , Kamuikotan metamorphic belt. Jurassic Funatsu granitoids and Miocene granitoids of the Green Tuff belt are not shown. Both consist of magnetite-series rocks. Tsushima is Miocene and probably an independent belt. mineralogical characteristics of the two series in Fig. 1 of granitoids, and discusses some genetic implications for the formation of granitoids Definition, Rock Types and Distribution and proximal types of ore deposits . Detailed magnetic and mineralogical studies will be The magnetite-series and ilmenite-series given in separate papers. Studied granitic granitoids were distinguished by the presence rocks of both concordant and discordant or absence, respectively, of magnetite in types are divided into several belts as illustrated polished sections. As little as one grain of 27(5), 1977 The Magnetite-series and Ilmenite-series Granitic Rocks 295 magnetite was sufficient, by definition, to ores. The Ashizuri-misaki pluton is an excep- classify a granitoid as belonging to the tion to others of the southwesternn outer belt magnetite-series. Amounts of magnetite in the in as much as it is magnetite-bearing. This magnetite-series granitoids are not constant pluton consists of monzogranite, syenogranite, at a given silica content. If the magnetite quartz syenite and syenite. Residual iron sand content of a rock is low, one polished section deposits (HAYASHIet al., .1969) were formed by may not be sufficient to determine whether weathering of the pluton. Small lens-shaped or not the hand specimen belongs to the magnetite deposits are known to occur in magnetite-series. However, most of magnetite- .magnetite-free areas of the Sanyo belt, as series granitoids studied had several grains of exemplified at Yashiro, Okayama Prefecture magnetite in one polished section. If one pluton (HENMI & NUMANO,1966) and at Daniwa, or one geotectonic belt contains more Hiroshima Prefecture (SOEDA, 1964). Wher- magnetite-series rocks than ilmenite-series ever they occur, - syenitic rocks are present rocks, it is called magnetite-series pluton or within calc-alkaline biotite monzogranite. magnetite-series belt. Syenite is the host for the magnetite deposits. Most of the Japanese granitoids are of the Distribution of the magnetite-series and calc-alkaline suite. of Cretaceouss to Miocene ilmenite-series granitoids on a regional scale age, and consist of hornblende-biotite gran- in the Japanese Islands is illustrated in Fig. 1. odiorite, biotite monzogranite and subordinate Of the largest batholithic exposure of the Inner hornblende-biotite tonalite. Pyroxene may Zone of Southwest Japan. (Cretaceous to be present in mafic phases. The granitoids Paleogene), granitoids of the Ryoke meta- belong to the magnetite-series or ilmenite- morphic belt and the southern part of the series, as illustrated in Fig. 1. Monzogranite Sanyo belt are generally magnetite-free. having roughly equal amounts of biotite. and Magnetite may or may-not be present in those muscovite is most abundant in the Ryoke of the northern part of the Sanyo belt. The belt but is rare in the other belts. The biotite- Sanin belt consists of the magnetite-series muscovite and muscovite granites are free of granitoids. The magnetite contents of the magnetite as is also true of two-mica grano- magnetite-series granitoids in these two belts diorite in the Sierra Nevada of the United gradually increase to north. The famed States (FD-20 of DODGE et al, 1969). But residual iron sand deposits in the Sanin magnetite was reported in some two-mica belt, which had supplied raw materials for granites of the Yenshanian cycle in southern iron industry for most periods of Japan's China (WANGet al., 1975). history, were formed by weathering of these Alkalic and subalkalic plutonic rocks are magnetite-rich granitoids. very rare in the Japanese Islands. However, In region to the east of the Tanakura tectonic they do occur in limited extent in both the line of Cretaceous granitoids terraries, the magnetite-series and ilmenite-series belts and ilmenite-series granitoids predominate in the are very magnetite-rich. The former examples western part but the magnetite-series is are Zone IV plutons of KATADA(1974) of the dominant in the eastern part. Content of Kitakami belt and monzonitic rocks at magnetite increases generally to the. east. Hamanaka in easternmost Hokkaido Granitoids of the Kitakami belt are mostly (FUJIWARA, 1959). In the Kitakami belt magnetite-bearing. The content is lowest the alkaline rocks have higher magnetite in the Senmaya and Hitokabe plutons which contents than the calc-alkaline rocks in the are located in the westernmost part. A few same belt. residual iron sand deposits were mined in Syenitic rocks of possible metasomatic magnetite-rich magnetite-series plutons in origin are present sporadically in the ilmenite- the northern part of the Kitakami belt. series belt of southwestern Japan. Magnetite Magnetite is generally, absent in Tertiary occurs as accessary constituents, or as massive granitoids of the Hidaka belt. Among Neogene 296 S. ISHIHARA MINING GEOLOGY: granitoids, almost all of those in the south- content, more than 90 percent in general, is western outer belt are of the ilmenite-series, magnetite. Magnetite modes are as high as 3 but all of the Green Tuff belt belong to the