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 percent in quartz diorite at Katsuraga-dani magnetite-series. and 5 percent in quartz gabbro at Zakka, Magnetite-free granitoids do occur in Shimane Prefecture, both of which are hosts magnetite-bearing belts. In typical magnetite- for the Akome-type residual iron sand deposits bearing belts, such as Kitakami and Sanin, (TSUSUE& ISHIHARA,1975). 8 and 10 percent, respectively, of examined The ilmenite-series granitoids, on the other samples contain no magnetite. In the Kitakami hand, contain less than 0.1 volume percent of belt; magnetite-free rocks tend to occur at stubby crystals of ilmenite. In another words, the margin of individual plutons, especially the ilmenite-series granitoids are practically along the eastern margin of the Goyosan free of opaque oxides. In some granitoids, pluton. In the Sanin belt, magnetite-free rocks those of the Hidaka belt and the southwestern occur in the Mochigase area in Tottori outer belt for example, pyrrhotite and/or Prefecture and in the Awaradani pluton of graphite may be more abundant than ilmenite. Shirakawa area, Gifu Prefecture. In the The areal extent of these rocks is very limited. Mochigase area, the magnetite-free rocks Magnetite contents of the magnetite-series have older K-Ar mineral ages (about 10 m.y.) rocks decrease with increase of potassium than the surrounding magnetite-bearing rocks feldspar plus quartz or similar parameters (ISHIHARA& SHIBATA,unpublished data). indicative of magmatic differentiation (Fig. 3). The contents also depend upon location. Mineralogical Characteristics Highest values were obtained in granitiods The most distinctive feature of the two from the central Sanin belt where many series of granitoids is the difference in volume residual iron sand deposits are distributed, percentage of the total opaque minerals and from the Tono-Kurihashi, Kesengawa, (Fig.2). The magnetite-series granitoids have a Miyako and Oura plutons in the Kitakami much higher content of opaques than do the belt. Lowest values were found in those from ilmenite-series granitoids. The magnetite- the northern part of the Sanyo belt and the series granitoids contain 0.1 to 2 volume Abukuma belt. percent of opaque minerals, as determined by Magnetite in the magnetite-series granitoids point counting thin sections.Most of this is generally euhedral and occurs within mafic silicates or closely associated with them, and in some instances with plagioclase. Blades of ilmenite and hematite may be seen in the magnetite. Ilmenite occurs as small euhedral to subhedral crystal in mafic silicates or sub- hedral to anhedral crystals coexisting with magnetite. Hematite-ilmenite intergrowths of various ratios occurs in small amounts in mafic rocks. Iron sulfide, if any, is generally pyrite. Chalcopyrite may be seen in small amount. Martitization, hematite replacing magnetite, is common in the magnetite-series rocks, particular in the salic phase. Hematite replaces magnetite along grain margin and (111) cleavage planes. This is prominant in the Fig. 2 Histograms of opaque mineral contents . The Sanin belt where complete martitization is also data source is the same as for Fig. 3. seen. Some of these rocks have depleted values 27(5), 1977 The Magnetite-series and Ilmenite-series Granitic Rocks 297

Fig. 3 Modal opaque minerals plotted against modal potassium feldspar plus quartz. All determined by point counter method described in the papers listed below. Ore-microscopic study on selected samples indicates that more than 90 percent of the opaque minerals of the magnetite-series granitoids consists of magnetite. Examined areas for the magnetite-series granitoids are the main plutons (except Hitokabe and Sanmaya plutons) of the Kitakami.belt (n=45, ISHIHARA& SUZUKI,1974), Shirakawa granitoids (n=30, ISHIHARA,1971a), western Tottori Pref. (n=5 HATTORI& SHIBATA,1974), eastern Shimane Pref. (n=38, ISHIHARA,1971a), and northern Hiroshima Pref. (n=11, ISHIHARAet al., 1969). Opaque minerals of the ilmenite-series granitoids were deter- mined by an integration apparatus on polished sections. Examined areas are the southern part of the Sanyo belt (n=34, ISHIHARA,1971b) and the Ryoke belt of Chubu district (n=26, ISHIHARA& TERASHIMA,1977a). Abbreviations for rock names (nomenclature recommended by IUGS subcommision): Qd, quartz diorite; Qmd, quartz monzodiorite; Tn, tonalite; Gd, granodiorite; MzG, monzogranite; SyG, syenogranite.

Of •¬ 180 (ISHIHARA & MATSUHISA, 1977). In cleavages and interstices of mafic silicates. very small plutons in the Green Tuff belt Examples are the Hidaka and Ryoke belts,

(e.g., Oe mine stock), magnetite is completely southern part of the Sanyo belt, and the converted to hematite, and ilmenite is decom- southwestern outer belt.

posed to mats of hematite and TiO2 minerals. Sulfide minerals are generally pyrrhotite. Strong martitization and break-down of The mineral has two types of occurrence, one ilmenite are observed commonly in small in the main phase and the other in the most

granitoid plutons in the western Cascade differentiated phase. The first type is common Range and the San Juan Mountains of the in tonalite that occurs closely associated with United States. These also exhibit 18O deple- migmatite in the Hidaka belt and in small

tion (TAYLOR, 1971, 1974). Magnetites in plutons in the southwestern outer belt where small stocks in porphyry copper areas of the exnoliths of shale and sandstone from the southwestern United States and the Philippines intruded Shimanto Supergroup are dominant. are more or less martitized. The second type fills cavities of aplitic or

The ilmenite-series granitoids are com- pegmatitic clots and accompanies pyrite and

pletely free of magnetite under the ore- other sulfides in some instances. microscope with ordinary (100•~) magnifica- Graphite occurs in the ilmenite-series

tion. Small euhedral ilmenite occurs in granitoids but is visible under the ore- mafic silicates. Secondary ilmenite fills microscope(100•~) only in the migmatitic rocks 298 S. ISHIHARA MINING GEOLOGY:

series granitoids (see Fig. 5).

Genetic Considerations Among several factors controlling crystalliza- tion of ferromagnesian minerals of granitoids (oxygen fugacity, temperature, bulk com- position etc.), oxygen fugacity seems to be the Fig. 4 Contents of sphene and opaque oxide most important variable in forming the minerals of some sphene-rich magnetite-series magnetite-series and ilmenite-series granitoids. granitoids. Open circle, Kitakami belt including The bulk total iron contents of the two series plutons of Miyako (n=11), Yamada (n=6), Tanohata (n=4) and Oura (n=6); solid circle, are more or less similar at given silica contents Kawai-type granitoids (quartz gabbro to mon- but their ferric/ferrous ratios are distinctly zogranite) from eastern Shimane Pref. Batholithic different (ISHIHARA, 1971a; TSUSUE & granodiorite and monzogranite from the same ISHIHARA,1974; ISHIHARA & TERASHIMA, area is shaded. The data source is the'same as for 1977a). A permissible conclusion is that the Fig. 3. magnetite-series granitoids were formed under conditions of higher oxygen fugacity than the ilmenite-series granitoids (ISHIHARA,1971a), in the Hidaka belt and the xenoliths in the other magnetite-free belts. Graphite content of the migmatites is generally less than 0.2 wt. Table 1 Average modal compositions of selected granodiorite and monzogranite batholiths (weight percent. Mafic silicates have a certain correlation with these opaque minerals assemblages. Sphene is commonly present in the magnetite- series. rocks. It occurs either as euhedral crystals which can be seen with the naked eye or as secondary aggregated after decomposition of mafic silicates and ilmenite. The amount of of sphene is generally correlated with that of magnetite (Fig. 4), but its occurrence is somewhat erratic. In the Kitakami belt , sphene is dominant in the Miyako (-Yamada) , Tanohata and Oura (-Omoe) plutons but is rare in other major plutons. In the Sanin belt, sphene is most concentrated in the Kawai-type fine-grained rocks, at the margins of the coarse-grained batholithic plutons . Epidote may be seen in the magnetite- series granitoids. It occurs generally as a secondary mineral after mafic silicates as, for (1) Magnetite-series, Daito granodiorite (n=8, example, in small plutons in the Taro zone of after ISHIHARA,1971a). (2) Ilmenite-series, Sumi- the Kitakami belt. Primary epidote is visible kawa granodiorite (n=4, after ISHIHARA & with the naked-eye in the Ecstall pluton in the TERASHIMA,1977a). (3) Magnetite-series, Yokota Coast Range batholith, Canada. This pluton monzogranite (n=7, after ISHIHARA, 1971a). consists of slightly magnetite-bearing (4) Ilmenite-series, Toki monzogranite (n=5, after ISHIHARA& TERASHIMA,1977a). *1 Mainly granitoids. Since iron was consumed to form clino-pyroxene, *2 Mostly fluorite. Fe+3/Fe+3+ Fe-Ti oxides, biotite and hornblende are Fe+2 and Fe/Fe+Mg ratios indicate approximately generally rich in magnesium in the magnetite- those of hornblende + biotite or biotite 27(5), 1977 The Magnetite-series and Ilmenite-series Granitic Rocks 299

assuming that common granitoids were solid- ified in a closed environment (TSUSUE &

ISHIHARA, 1974). Table 1 gives modal composition of typical granodiorite and monzogranite batholiths. It is clear that the ratios of opaque oxides/

ferromagnesian silicates is distinctly different between the two series of granitoids. Assumed

Fe+3/Fe+3+Fe+2 ratios of the silicates, which are obtained from the bulk ratios after subtracting amounts of Fe2O3 and FeO

allocated to the modal oxides, are much higher in the magnetite-series granitoids than in the ilmenite-series ones, especially in those

granitoids containing no other ferromagnesian silicates than biotite. Biotite analyses from the magnetite-series

granitoids coexisting with potassium feldspar Fig. 5 Fe+3-Fe+2-Mg relation of Japanese biotites and magnetite plot above the Ni-NiO buffer of calc-alkaline suite. Biotites from small stocks

on WONES & EUGSTER'S (1965) Fe+3•\3 are excluded. Data source: KANISAWA (1972, Fe+2•\Mg diagram for biotite (DODGE et al., 1974) for the Kitakami belt including Tono, Hitokabe, Kesengawa, Oura, Omoe and Taro; 1969; KANISAWA, 1972, 1974). Although the KANISAWA (1976) for the Sanin belt; Tsusoi biotite-potassium feldspar-magnetite assem- et al. (1938) and HONMA (1974) for the Ryoke blage does not occur in the ilmenite-series and Sanyo belts. Only chemical analyses whose

granitoids, their biotites generally plot below magnetite bearing and free nature is confirmed

(Tsusoi et al., 1938) or around (HONMA, from the listed localities are plotted. Broken lines

1974) the Ni-NiO buffer. It appears that a represent compositions of buffered biotites

boundary between the two series of granitoids in the ternary system KFe3+3A•¬Si3O12H-1

is near the Ni-NiO buffer (Fig. 5), which KFe3+2A•¬Si3O10(OH)2- KMg3A•¬Si3O10(OH)2 de-

was also suggested by SHIMAZAKI (1976) picted by WONES & EUGSTER (1965).

and TSUSUE (1976). Water is important in regulating oxygen of which are related to cauldron subsidence, fugacity in igneous processes (OSBORN, 1959, yet they are free of magnetite. These 1962; WONES & EUGSTER, 1965). CZAMANSKE s gran-itoidscontainevidencethat their magmas and WONES (1973) pointed out that H2O can were contaminated or had interacted with act as an oxidizing medium through dissociation pelitic rocks which. contain graphite and/or and loss of H2 only after its separation from a related carbonaceous matter. Moreover, silicate melt. Such a model can explain the magnetite-free rocks occur widely in the high magnetite-series granitoids of near surface T/P type metamorphic terranes. Thus, buff- intrusion in the Sanin, Kitakami and Green ering of oxygen fugacity by graphite may be Tuff belts. Some external sources of oxidizing worth considering. medium, such as free oxygen from air The role of graphite as a reducing agent in

(MURAKAMI, 1969) and meteric ground water, the earth's crust has been emphasized by may have accelarated the oxidation in the metamorphic petrologists (e.g., MIYASHIRO, later stage of crystallization. 1964). In common pelitic rocks, magnetite The above explanation meets difficulty and graphite do not occur together and the with the small discordant plutons in the assemblage graphite-pyrrhotite-ilmenite is southwestern outer belt and the Hidaka common (KANEHIRAet al., 1964; MARIKO belt. These are shallow level intrusions, some et al., 1975). FRENCHand EUGSTER(1965) 300 S. ISHIHARA MINING GEOLOGY: stressed the importance of graphite-gas xenoliths and a graphite-bearing xenolithic equilibrium on oxygen fugacity in both block at Manguro in the Minami-osumi igneous and metamorphic processes, and pluton was once mined for both graphite and further mentioned (p. 1537) that "Because of associated sulfide minerals. Seven analyses the low gram-molecular weight of graphite, of shales from the intruded Shimanto Super- even trace amounts of graphite will exert group contain an average of 0.85%C. Thus it is a very large buffering effect with respect to obvious that the granitic magma interacted changes in the composition of the gas phase." with graphite of the intruded sedimentary Foliated granitoids of the metamorphic rocks. terranes are generally free of magnetite and Interaction during magma emplacement is contain ilmenite only in very small amount . considered to have happened at several The assemblage graphite-pyrrhotite-ilmenite stages for the different types of contained is seen in the migmatitic rocks. These are xenoliths and even at the beginning of the observed in the Ryoke and Hidaka belts. magmatic history for the following reasons. In However, foliated granitoids of the Abukuma the southwestern outer belt, some granitoids belt contain magnetite in some portions of the intrude Sanbagawa metamorphic rocks which highest grade zone. Chemical analyses indicate are predominantly mafic meta-igneous rocks, that the metamorphic rocks of the Abukuma yet the granitoids are free of magnetite. In belt have much lower carbon contents than the Hidaka belt, foliated tonalites in the those of the Ryoke belt (Table 2). It is con- axial zone are associated with migmatite and sidered, therefore, that the magnetite-bearing pelitic metamorphic rocks. Other granitoids and magnetite-free granitoids are a con- occur as discordant stocks and pelitic xenoliths sequence of different graphite contents in the are not common, yet these plutons are com- intruded metamorphic rocks. posed of magnetite-free granitoids. Many pelitic xenoliths are found in massive, Distribution of the magnetite-bearing and discordant-type granitoids in the southwestern magnetite-free suites is observed on a regional outer belt. Graphite is commonly seen in the scale, and magnetite-free rocks are generally salic in composition and are rich in F, Rb, Li, Table 2 Average carbon contents of pelitic and Sn and Be (ISHIHARA& TERASHIMA,1977a,b). semi-pelitic metamorphic rocks from the Ryoke Thus it is speculated that most of the ilmenite- and Abukuma belts series granitoids were generated in salic con- tinental materials and interacted with near- surface rocks before their solidification. This speculation is supported by oxygen isotope study (ISHIHARA& MATSUHISA,1977). Of the Japanese granitoids, the Miocene granitoids of the Green Tuff belt in north- eastern Japan and the Cretaceous granitoids of the Kitakami belt were derived from a deep source region, probably from the upper mantle, as evidenced by their tectonic setting and initial strontium ratios (SHIBATA & Note: Averages for mafic metamorphic rocks of ISHIHARA,1976). These granitoids contain the Abukuma belt, which are the majority in the belt, are 0.06%C (n=3) for the Takanuki magnetite. It is concluded that such granitic series and 0.08%C (n=14) for the Gozaisho magmas originated in deep regions where no series. Samples analyzed in this study are those carbonaceous matter is available form the listed in ISHIHARAet al. (1973). Analyst for this magnetite-series granitoids. Sporadic distri- study was T. NISHIMURA,Geological Survey of bution of magnetite free-rocks especially at the Japan margins of magnetite-series plutons may be a 27(5), 1977 The Magnetite-series and Ilmenite-series Granitic Rocks 301 result of minor interaction of the original the southern part of the Sanyo belt where magma with surrounding pelitic rocks. related granitoids are of the ilmenite-series. Similar results from other regions around Relation to Metallogenic the world are shown in Fig. 6. In the regions of Provinces tin and greisen-type wolframite deposits, such Tin, tungsten, molybdenum and porphyry as Erzgebirge, northern Portugal, Tasmania, copper deposits are known to occur spatially Malay Peninsula, Seward Peninsula and Mt. close to granitic rocks. These deposits can be McKinley area in Alaska, and Round Moun- correlated with the magnetite-series and tain area in Nevada, the granitoids consist of ilmenite-series. In the Japanese metallogenic biotite granite or biotite-muscovite granite and provinces (ISHIHARA& SASAKI,1973), mo- contain very small amount of ilmenite; hence lybdenite and scheelite-gold deposits are they belong to the ilmenite-series granitoids. distributed in the Sanin and Kitakami belts Some contain magnetite but the content is where related granitoids are of the magnetite- as low as a few grains in one hand specimen. series. Tin and greisen-type wolframite deposits "Normal" magnetite -series granitoid occurs in are located in the southwestern outer belt and a small tin granite stock of the Serpentine Hot

Fig. 6 Distribution of the magnetite-series and.ilmenite-series granitoids in the major porphyry copper-mo- lybdenum provinces and greisen-type tin-wolframite provinces. Examined samples include both small stocks related to the mineralization (designated as stock) and general rocks in the proper mining areas (designated as regional). Cu-Mo areas: Arizona (n=21): Cornelia pluton (n=8, stock), Patagonia and Santa Rita Mts. (n=8, stock), Esperanza-Sierrita and Silver Bell (n=5, stock); Front Range (n=26): Climax (n=3, stock), Questa (n=23, stock); Nevada and Utah (n=10): Bingham (n=5, stock), Robinson (n=2, stock), Yerington (n=3, stock); Butte (n=11, regional), Highland Valley (n=6, regional), Endako (n=6, regional), Alice Arm (n=3, stock); Philippine (n=8): Atlas (n=2, stock), Sto. Thomas and Sto. Nino (n=4, stock), Sipalay (n=2, stock); Mamut (n=5, regional); Panguna (n=10, stock); Medet, Bulgaria (n=4, stock). Sn-Wareas: Mt. McKinley (n=8, stock), Seward Peninsula (n=16, stock), Southwestern outer belt (n=277;regional and stock), Northern Thailand (n=18, regional), Malaysia (n=7, regional), Tasmania (n=9,. regional), Erzgebirge (n=12, regional), Northern Portugal (n=6, regional). 302 S. ISHIHARA MINING GEOLOGY:

Springs area at Seward Peninsula in Alaska appears to be a prominent feature in the but its distribution is limited to the Zone 2 Circum-Pacific region and implies that a phase of porphyritic biotite granite. different tectonic history developed on either Scheelite-gold deposits occurring in non- side of the Pacific Ocean, although both calcareous host rocks which are not accom- sides are located above the consuming plate panied by greisen-type alteration are rather margins. rare, but are known in the southern Kitakami The ratio of the magnetite-series/ilmenite- belt and Sierra Nevada (e.g., Atolia district, series granitoids may also be related to evolu- LEMMON& TWETO,1962). Since the major part tion of lithosphere and hydrosphere in the of the Sierra Nevada batholith seems to con- earth's history. Low Fe2Os/FeO ratios of tain magnetite (DODGE et al., 1969), this Precambrian granitoids in the Canadian mineralization is characteristic of the shield (FAHRIG & EADE, 1968) suggest that magnetite-series granitoids. most of the Archean rocks are composed In porphyry copper deposits, magnetite, generally of the ilmenite-series. If this assump- hematite, pyrite (instead of pyrrhotite) and tion is valid, a general scacity of porphyry anhydrite occur within the orebodies. These copper deposits in Precambrian terranes may minerals suggest that the related intrusive be explained by the general paucity of the rocks should be magnetite-series granitoids magnetite-series granitoids in the older and, indeed, all examined specimens from continental crust. various localities listed in Fig. 6 fall within Concluding Remarks the category of the magnetite-series rocks. A possible exception may be the Copper Canyon Granitoid batholiths and stocks can be deposit (THEODORE& BLAKE, 1975) where divided into (i) magnetite-bearing magnetite- pyrrhotite occurs in an altered stock. Porphyry- series and (ii) magnetite-free ilmenite-series, type molybdenite deposits are also related to and are considered to form as a result of the magnetite-series granitoids. exposure to different oxygen fugacity during In the Mesozoic and Cenozoic orogenic the life of the granitic magmas. The magnetite- belts in the Circum-Pacific region, porphyry- series granitic magma may have been type copper and molybdenum deposits are generated at great depths where no carbona- abundant in North and South America, ceous material exists, whereas the ilmenite- whereas tin-tungsten deposits of the greisen series magma may have originated at a shallow type are dominant in East Asia and Alaska. level where small amounts of crustal carbon are This regional pattern may imply that present in the host country rocks. Both magmas magnetite-series granitoids are more abundant were modified slightly in terms of oxygen on the American side than on the Asian side. fugacity at the shallowest level at which they Samples were examined from various wide- solidified, yet the original characteristics are spread units ranging in age from Triassic to retained even through post-magmatic proc- late Cretaceous from the Malaysia-Thailand esses. Thus the two series of granitoids have an region. Their ratios of magnetie-series/ilmenite- important bearing on both petrogenesis and series rocks is very similar to that of the metallogenesis. Recognition of these two Japanese tin-wolframite region. Mesozoic fundamental types of granitoids is very useful granitoids of southeastern China (Nanking in exploration for mineral deposits of granitic Univ., 1974) have similarity to those of the affinity. Malay Peninsula and the southern part of the Identification of the two series is easy in the Sanyo belt. Thus it seems likely that the field with the aid of a hand magnet and by magnetite-series/ilmenite-series ratio is below observing heavy minerals concentrated on 1 in the Asian side and is above 1 in the weathered surfaces. Typical rocks can be American side of the Mesozoic to early recognized by the mineralogical characteristics Cenozoic orogeny belts. This difference aforementioned. The most difficult cases of 27(5), 1977 The Magnetite-se ies and Ilmenite-series Granitic Rocks 303 distinguishing the two types may be encoun- DODGE, F.C.W., SMITH, V. C. and MAYS, R. E. (1969): tered in granitic massifs where biotite granites Biotites from granitic rocks of the central Sierra of low magnetite content are monotonously Nevada batholith, California. Jour. Petrol., 10, exposed (e.g., Hiroshima granite). In this case, 250•`271. FAHRIG, W. F. and EADE, K. E.(1968): The chemical quantitative analysis is necessary in the evolution of the Canadian Shield. Canadian Jour. laboratory and one polished section per out- Earth Sci., 5, 1247•`1252. crop may not be enough. Magnetic suscepti- FRENCH, B. M. and EUGSTER, H. P.(1965): Experi- bility measurement (KANAYA & ISHIHARA, mental control of oxygen fugacities by graphite-

1973; ISHIHARA& KANAYA,in preparation) gas equilibriums. Jour. Geophy. Res., 70, 1529•` and ferric ferrous ratios of bulk analysis of 1539. granitoids are useful adjuncts to proper identi- FUJIWARA, T.(1959): On the igneous activities and the fication. ore deposit at Hamanaka area, Hokkaido. Jour. Acknowledgement: The writer is indebted Japan. Assoc. Min. Petr. Econ. Geol., 43, 208•` 214. greatly to the following people who provided HATTORI, H. and SHIBATA, K.(1974): Concordant some samples from foreign countries: Dr. K-Ar and Rb-Sr ages of the Tottori granite, western

G. R. BALCE, Bureau of Mines, Manila; Japan. Bull. Geol. Surv. Japan, 25, 157•`173.

Prof. J. M. GUILBERT,University of Arizona; HAYASHI, S., ISHIHARA, S. and SAKAMAKI, Y.(1969):

Drs. W. E. HALL, T. L. HUDSON,B. L. REED, Uranium in the decomposed granitic rocks at the D. R. SHAWE and T. G. THEODORE,U. S. cape Ashizuri, Kochi Prefecture, with special Geological Survey; Prof. W. C. KELLY, reference to the green uranothorite. Geol. Surv. University of Michigan;Dr. C. NISHIWAKI,In- Japan, Pept. 232, 93•`103. ternational Mineral Resources Development; HENMI, K. and NUMANO, T.(1966): Feldspar dikes in Dr. J. R. RICHARDS,Australian National Uni- the Yamate-Kasaoka area, Okayama Prefecture. versity; Dr. S. SUENSILPONG,Department of Rept. Earth Sci. Res. Okayama Univ., 1, 111•` 119. Mineral Resources, Bangkok; Prof. H. P. TAYLOR,Jr., California Institute of Tech- HONMA, H. (1974): Chemical features of biotites from metamorphic and granitic rocks of the Yanai nology; among my colleagues, T. NOZAWAand district in the Ryoke belt, Japan. Jour. Japan.

K. SHIBATA.Thanks are also due to Dr. G. Assoc. Min. Pet. Econ. Geol., 69, 390•`402.

W. WALKER, U. S. Geological Survey at ISHIHARA, S. (1971a): Major molybdenum deposits

Menlo Park and Prof. C. MEYER, University and related granitic rocks in Japan. Geol. Surv. of California at Berkeley for generous per- Japan, Rept. 239, 1•`178. mission to use their facilities to study some of ISHIHARA, S. (1971b): Modal and chemical composition the North American samples during a short of the granitic rocks related to the major molyb- visit in summer 1975. The visit was supported denum and tungsten deposits in the Inner Zone of by a research grant from the Science and Southwest Japan. Jour. Geol. Soc. Japan, 77, Technology Agency of Japan. Critical review 441•`452. by Prof. S. D. SCOTT,University of Toronto, ISHIHARA, S. (1975): Acid magmatism and miner- has improved English of the original manu- alization-Oxidation status of.granitic magma and its relation to mineralization•\Marine Sci. script. Monthly, 7, 756•`759.

ISHIHARA, S. and SASAKI, A.(1973): Metallogenic map References of Japan: Plutonism and mineralization (1),

molybdenum, tungsten and tin. 1/2,000,000 Map CZAMANSKE, G. K. and MIHALIK, P.(1972): Oxidation during magmatic differentiation, Finnmarka com- Series, 15-1, Geol. Surv. Japan.

ISHIHARA, S. and SUZUKI, Y.(1974): Modal compositions plex, Oslo area, Norway: Part 1, The opaque oxides. Jour. Petrol., 13, 493•`509. of Cretaceous granitic rocks in the Kitakami

CZAMANSKE, G. K. and WONES, D. R.(1973): Oxidation Mountains. Geol. Surv. Japan, Rept. 251, 23•`42.

during magmatic.differentiation. Finnmarka com- ISHIHARA, S. and TERASHIMA, S.(1977a): Chemical variation of the Cretaceous granitoids across plex, Oslo area, Norway: Part 2, The mafic sili- cates. Jour. Petrol., 14, 349•`380. southwestern Japan. •\Shirakawa-Toki-Okazaki 304 S. ISHIHARA MINING GEOLOGY:

transection•\.Jour. Geol. Soc. Japan, 83,1•`18. in the United States. U.S. Geol. Surv. Miner.

Invest. Res. Map MR 25, 1•`25. ISHIHARA, S. and TERASHIMA, S.(1977b): Chlorine and MARIKO, T., TANAKA, K. ana ITAYA, T.(1975): Oxide fluorine contents of granitoids as indicators for base and sulphide minerals in pelitic and psammltic metal and tin mineralizations. Mining Geol., 27, schists from the Nagatoro district, Saitama pre- 191•`199. fecture, Japan. Jour. Japan. Assoc. Min. Petr. ISHIHARA, S. and MATSUHISA, Y.(1977): The magnetite- Econ. Geol., 70, 413•`423. series/ilmenite-series granitoids and their 18•›/16•› MIYASHIRO, A.(1964): Oxidation and reduction in the ratios (abs). Abstract Issue, 84th Annual Mtg, Earth's crust with special reference to the role Geol. Soc. Japan, 82. of graphite. Geochim. Cosmochim. Acta, 28, ISHIHARA, S., KOMURA, K. and MURAKAMI, T.(1969): 717•`729. Source rocks of Miocene bedded-type uraniferous MURAKAMI, N.(1969): Two contrastive trends of deposits in northern Miyoshi district and genesis evolution of biotite in granitic rocks. Jour. Japan. of uranium anomalies at Myoga, Shobara city, Assoc. Min. Pet. Econ. Geol., 62, 223•`248. Hiroshima Pref., Japan. Bull. Geol. Surv. Japan, Nanking Univ.(1974): Granitic rocks of different geolo- 20, 161•`172.

ISHIHARA, S., HATTORI, H., SAKAMAKI, Y., KANAYA, gical periods of southeastern China and their

H., SATO, T., MOCHIZUKI, T. and TERASHIMA, S. genetic relations to certain metallic mineral deposits. Sci. Sinica, 17, 55•`72. (1973): Lateral chemical variation of the granitic and metamorphic rocks across the central Abukuma OSBORN, E. F.(1959): Role of oxygen pressure in the

highland-With emphasis on the contents of ura crystallization and differentiation of basaltic magma. Amer. Jour. Sci., 257, 609•`647. nium, thorium and potassium. Bull. Geol. Surv.

Japan, 24, 269•`284. OSBORN, E. F.(1962): Reaction series for subalkaline

KANAYA, H. and ISHIHARA, S.(1973): Regional varia- igneous rocks based on different oxygen pressure

tion of magnetic susceptibility of the granitic rocks conditions. Amer. Miner., 47, 211•`226.

in Japan. Jour. Japan. Assoc. Min. Pter. Econ. SHIBATA,K. and ISHIHARA,S.(1976): Regional varia-

Geol., 68, 211•`224. tion of the initial 87Sr/86Sr ratio of the Japanese

KANEHIRA, K., BANNO, S. and NISHIDA, K.(1964): plutonic rocks (abs). Abstract Issue, 83rd Annual

Sulfide and oxide minerals in some metamorphic Mtg., Geol. Soc. Japan, 307.

terranes in Japan. Jour. Geol. Geogr., 35, 175•` SHIMAZAKI, H.(1976): Granitic magmas and ore deposits

191. (2) Oxidation state of magmas and ore deposits. KANISAWA, S.(1972): Coexisting biotites and horn- Mining Geol. Spec. Issue, 7, 25•`35.

blendes from some granitic rocks in southern SOEDA, A.(1964): Ore deposits of Hiroshima Prefecture.

Kitakami Mountains, Japan. Jour. Japan. Assoc. Explanetary text for 1/200,000 scale map, Hiro-

Min. Pet. Econ. Geol., 67, 332•`344. shima Pref., 125•`154.

KANISAWA S.(1974): Granitic rocks closely associated TAYLOR, H. P. Jr.(1971): Oxygen isotope evidence for

with the lower Cretaceous volcanic rocks in the large-scale interaction between meteoric ground

Kitakami Mountains, Northeast Japan. Jour. waters and Tertiary granodiorite intrusions, western

Geol. Soc. Japan, 80, 355•`367. Cascade Range, Oregon. Jour. Geophy. Res.,

KANISAWA, S.(1976): Chemistry of biotites and horn- 76,7855•`7874.

blendes of some granitic rocks in the San'in Zone, TAYLOR, H. P. Jr.(1974): Oxygen and hydrogen isotope

Southwest Japan. Jour. Geol. Soc. Japan, 82, evidence for large-scale circulation and interaction

543•`548. between ground waters and igneous intrusions,

KATADA, M.(1974): Granitic rocks in the southern with particular reference to the San Juan volcanic

Kitakami Mountains and zonal arrangement of field, Colorado. Carnegie Inst. Washington Pub. granitic rocks in the entire Kitakami Mountains. 634, 299•`342.

Geol. Surv. Japan, Rept. 251, 121 - 133. THEODORE, T. G. and BLAKE, D. W.(1975): Geology

KATADA, M., ISOMI, H., OMORI, E. and YAMADA, T. and geochemistry of the Copper Canyon porphyry

(1964): Chemical composition of Paleozoic rocks copper deposits and surrounding area, Lander from northern Kiso district and of Toyoma clay- county, Nevada. U.S. Geol. Surv. Prof. Paper

slates in Kitakami mountainland: Supplement. 798-B, 1•`86. Carbon and carbon dioxide. Jour. Japan. Assoc. Tsuaol, S., SUGI, K. and IWAO, S.(1938): On rock-

Min. Pet. Econ. Geol., 52, 217•`221. forming biotites from Japan. Jour. Geol. Soc.

LEMMON,D. M. and TWETO, O. L.(1962): Tungsten Japan, 45, 453•`455. 27(5), 1977 The Magnetite-series and Ilmenite-series Granitic Rocks 305

TsusuE, A. (1976): Granitic magmas and ore deposits district, Ibaraki Prefecture. Jour. Geol. Soc. Japan, 67,228•`236. (1) Especially those of Southwest Japan. Mining Geol. Spec. Issue, 7, 15•`24. WANG, L., ZHANG, Y. and Lru, S. (1975): Multiple

TSUSUE, A. and IsHIHARA, S.(1974): The iron-titanium emplacements and some geochemical characteristics

oxides in the granitic rocks of. Southwest Japan. of the Zhuguangshan granitic batholith, southern

Mining Geol., 24, 13•`30. China. Geochimica, 3, 189•`201.

TSUSUE, A. and ISHNHARA, S.(1975): "Residual" iron- WONES, D. R. and EUGSTER, H. P. (1965): Stability of

sand deposits. of Southwest Japan. Econ. Geol., biotite: Experiment, theory, and application. Amer.

70, 706•`716. Jour. Sci., 50, 1228•`1272.

UNO, T.(1961): Metamorphic rocks of the Tukuba

磁鉄鉱 系花 崗岩類 とチ タン鉄鉱 系花 崗岩類

石 原 舜 三

要 旨

表 題 の2組 の 花 崗岩 類 に つ い て,主 と して 鏡 下 観 察 結 花 崗岩 類 の生 成 時 の酸 素 フ ュガ シ テ ィ を規 制 す る要 因 と 果 か ら構 成 鉱 物 の 特 徴 が 記 載 され,分 類 の 基 準 ・両 者 の し て は花 崗岩 質 マ グ マ の 発 生 か ら固 結 に至 る過 程 に お け 分 布 ・成 因 ・鉱 化 作 用 との 関 連 性 な どが の べ られ た.2組 る炭 質物 に よるバ ッ フ ァー が 重 視 され,H2Oの 解 離 と の 花 崗岩 類 は 一 般 の鏡 下観 察(100×)で 磁 鉄 鉱 が 認 め ら H2の 逸 散 は磁 鉄 鉱 系花 崗 岩 類 の 一 部 につ い て 考 慮 され

れ るか 否 か の点 で分 類 され,磁 鉄 鉱 系 花 崗岩 類 は0.1―2 た.磁 鉄 鉱 系花 崗 岩 類 は 炭 質 物 が 存 在 しな い深 所 起 源 で あ り,チ タ ン鉄 鉱 系 花 崗 岩 類 は 炭 質 物 を伴 う大 陸 地 殻 起 容 量%の 磁 鉄 鉱 とご く少 量 の チ タ ン鉄 鉱 を有 し,チ タ ン 鉄 鉱 系花 崗岩 類 は0.1容 量%以 下 の チ タ ン鉄 鉱 を伴 うに 源 で あ ろ う と考 え られ た. す ぎ な い.す な わ ち,両 者 は 苦 鉄 質 珪 酸 塩 鉱 物 とFe―Ti 花 崗 岩 類 中か そ の 近 傍 に産 出す る 鉱 床 に お い て は 花 崗 酸 化 鉱物 の 量 比 に お いて 著 し く異 な り,チ タ ン鉄 鉱 系 花 岩 類 に み られ る性 質 が 継 続 し て認 め られ,た とえ ば ポ ー 崗岩 類 はFe―Ti酸 化 鉱 物 に欠 け る 系列 とみ な して よい. フ ィ リー カ ッパ ー 鉱 床 で は磁 鉄 鉱 系花 崗 岩類 と共 通 の 鉱 このFe-Ti酸 化 鉱 物 に欠 け る事 実 か ら,チ タ ン鉄 鉱 系 物 組 合 せ が 産 出 す る.2組 の花 崗岩 類 の 性 質 は マ グ マ期

花 崗 岩 類 が 磁 鉄 鉱 系花 崗岩 類 よ り低 い酸 素 フ ュガ シ テ イ 末 期 か ら後 マ グ マ期 の 一 部 に及 ん で お り,花 崗 岩 類 に密 の 条 件 下 で 生 成 され た もの と推 論 され た.こ の よ うに 考 接 な鉱 床 探 査 で は両 者 を識 別 す る こ とが 重 要 で あ る.ス え る と,2組 の花 崗岩 類 に そ れ ぞ れ 特 徴 的 に 認 め られ る ズ-鉄 マ ン ガ ン重 石 鉱 床 が チ タ ン 鉄 鉱 系 花 崗岩 類 と密 接 他 の苦 鉄 鉱 物 や 硫 化 鉱物 の組 合 せ が 説 明 し易 い.両 者 の な経 験 則 か ら,環 太 平 洋 地域 の 西 側 で は 磁 鉄 鉱 系 花 崗岩 酸 素 フ ュガ シ テ ィ を定量 的 に 推 定 す る共 通 の 鉱 物 組 合 せ 類 に 乏 しい こ とが 予想 され,こ の こ とが 沿 海 州― 中国 大 は得 られ て い な い が,黒 雲 母 のFe+3/Fe+3+Fe+2比 か ら両 陸 南 東 部― マ レ― 半 島 に 至 る中 生 代 花 崗岩 類 に ポ ー フ ィ 者 の 境 界 は ほ ぼNi-NiOバ ッフ ァー 付 近 と考 え られ た. リー カ ッパ ー 鉱床 が発 見 され な い 一 因 と考 え られ た.