MINING GEOLOGY, 29(5), 291•`306, 1979

Two-stage Mineralization and Formation Process of the Toyoha Deposits, Hokkaido, Japan

Junkichi YAJIMA*and Eijun OHTA*

Abstract: The formation process of the Toyoha deposits is discussed from considerations on geological environ ment, mineral association and on the result of fluid inclusion studies. It is well known that mineralization at the Toyoha deposits is divided into two stages, the earlier and the later ones. More than 15 veins are arranged in major three directions, E-W, NW-SE and N-S. Most of the veins of E-W system are earlier ones, while those of NW-SE and N-S systems were formed by the later mineralization. , and are common and abundant minerals throughout two stages. In the earlier stage, arsenopyrite is found at eastern lower part (Harima), whereas increases towards western upper side (Chikugo, Tajima). In the later one, pyrrhotite and graphite associated with and minerals occur at eastern and southeastern areas (Sorachi, Izumo), while carbonates and Sb-minerals are found at western and northwestern areas (Soya, Oshima). Temperature and salinity of fluid inclusions have a decreasing tendency from Izumo to Soya, and from Harima to Tajima. From these evidences together with the existence of heat source at southeastern deeper zone, the model for the formation of the Toyoha deposits is proposed. By this model, the consistent explanation for the occurrence of overall minerals in the mine has become possible. The model also to some important suggestions for exploration.

Introduction 1. Geological Setting In these ten years, investigation on West Hokkaido is tectonically a northern minerals from west Hokkaido has been carried continuation of "green tuff region" in North out in our laboratory, and on the way of east Japan, and is divided into three sub detailed studies, we are happened to be stimu - provinces which approximately coincide with lated by discovery of tin and tungsten minerals metallogenic sub-provinces (SAITOet al., 1967; from the Toyoha mine which is one of the most BAMBA,1977). The first sub-province is the area productive - deposits in Japan. The characterized by terrestrial sediments of early total production from 1937 to 1977 reached ten Miocene period and polymetallic vein-type and million tons of crude ore, containing 694,000 replacement-type ore deposits and it extends tons of Zn, 276,000 tons of Pb, 1,150 tons of Ag from Matsumae to Kudo area. The second one and 3,200 kilograms of Au. The current pro is an upheaved area where a large quantity of duction of crude ore is about 35,000 tons per pyroclastic rocks of Miocene age is found to- month. gether with polymetallic vein-type deposits, The purpose of this paper is to investigate which distribute in clusters at the Shakotan the environment and the process of ore deposi Peninsula area and the Otaru-Muroran area. tion at the mine through the consideration on In the third sub-province, subsiding troughs geological situation, mineral paragenesis and with a thick pile of normal marine sediments, on the result of fluid inclusion study. there occur stratabound type deposits of man ganese and also kuroko. This sub-province distributes widely around the second one. Received January 23, 1979, in revised form September The Toyoha mine is situated in the second 6,1979 * Geological Survey of Japan, Hokkaido Branch, sub-province, near the center of the Teine- Minami-1, Nishi-18, Sapporo 060 Chitose district at the northern half of the Keywords : Toyoha deposits, Zonal distribution, Fluid Otaru-Muroran area mentioned above. The inclusion, Heat source, Sequence of mineralization. district is characterized geologically by the

291 292 J. YAJIMA and E. OHTA MINING GEOLOGY: existence of so-called Jozankei quartz porphy arranged in major three directions, E-W, NW- ry. The rock intruded pyroclastic rocks of SE and N-S (Fig. 1). The dimension and ore Miocene age and it is considered to be one of grade of representative veins are listed in Tertiary granitoids composed from such rock Table 1. facies as granite porphyry, granodiorite por Tajima and Harima veins of E-W trend have phyry and dacite (NISHIKAWA,1977). The been the most important source of since close relationship between the quartz porphyry the beginning of exploitation. They have simi and mineralization is indicated in several lar features in ore texture and mineral com papers (AKIBA,1958; SUGIMOTO,1958; SAITO ponent. Quartz is the predominant vein mate et al., 1967). rial in which sphalerite of pale yellowish brown In this district, a number of polymetallic ore color, galena and pyrite are accompanied. It is deposits show a clear zonal distribution around observed that argentite-quartz vein cuts and the quartz porphyry such as the inner Pb-Zn replaces sphalerite, galena and pyrite. Hema zone and the outer Au-Ag zone (e.g. YAJIMA, tite and are scattered megascopically 1977). Furthermore, the close affiliation be in these veins, and they are rather rich in tween the quartz porphyry and ores of the Chikugo vein and are occasionally found to Toyoha deposits is presented from sulphur form fine banded or ring structure with pyrite isotopic study (ISHIHARAand SASAKI,1978). (YAJIMAand OKABE, 1971). Native is The quartz porphyry suffered itself the miner confirmed to be secondary in origin (SHIKA alization (SUGIMOTO,1958), and thus the age of ZONO, 1975), and hairy aggregate of the mineralization is younger but not far from the mineral is often found in vugs together with fission-track age determined for this rock as 8.5 argentite but at only restricted area adjacent to m.y. (SUMIet al., 1978). veins or faults of NW-SE and N-S systems The Toyoha mine is situated at the central (Fig. 2). part of abovementioned zonation, where older Soya vein, a representative one of NW-SE formations are well developed compared to the system, is also a superior one comparable to outer Au-Ag zone, in which Teine, Todoroki Tajima and Harima veins. It has another and Chitose mines are well known. At the feature characterized by alternating bands of Toyoha deposits, there develop earlier to mid sulphide and carbonate minerals dle Miocene sediments, pyroclastic rocks and and also by a paucity of quartz (SHIKAZONO, lavas. They are divided into three formations, 1975). Coarse-grained sphalerite of dark red Koyanagizawa, Motoyama and Nagato forma dish brown color is dominant. tions in ascending order. Koyanagizawa forma The recent exploration of Izumo vein tion is composed of altered basaltic andesite (HASHIMOTOet al, 1977) led an epoch-making and rhyolite lavas. Motoyama formation con result in prospecting works of the mine. The sists of tuffaceous silt stone and mudstone and it vein has been considered to belong to the overlies on Koyanagizawa formation with basal earlier-stage ones (YAJIMA,1977; HASHIMOTO conglomerate. Nagato formation is character et al., 1977). However, it is thought to be better ized by a large amount of pyroclastics and to classify it into the later-stage ones from sever lavas of altered andesite, dacite and rhyolite al reasons as follows. Izumo vein has a similar (AKOMEand HARAGUCHI,1963, 1967; MIYA mineral composition with Sorachi and Soya JIMAet al., 1971; HASHIMOTOet al., 1977). bunki and its western extent smoothly con tinuates to Soyashitaban vein of the later stage. 2. Ore Deposits of the Toyoha Mine The crossing relation between Soyabunki and 2.1 Vein system Izumo illustrated in the paper by HASHIMOTO Epithermal fissure-filling type ore deposits of et al. (1977, p. 93) should be a sort of branch- the mine mainly consist of pyrite, sphalerite and ing. galena with an important amount of silver, and Izumo vein is different from Tajima and also composed of more than 15 veins. They are from Soya in the ratio of constituent minerals 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 293

Fig. 1 Disposition of veins in the Toyoha deposits (prepared by Mr. MIYAISHI).

Table 1 Dimension and ore grade of the representative vein. veins of the Toyoha mine Two stages of mineralization are discrimi nated at the Toyoha deposits from the crossing relation among veins, that is, the earlier and the later ones (AKOMEand HARAGUCHI,1963, 1967; MIYAJIMAet al., 1971; HASHIMOTOet al., 1977). In this paper, they are called Stage I and Stage II respectively. Most of E-W fissures were mineralized at Stage I and those of NW- SE and N-S systems largely at Stage II. It is and ore textures. It has a higher content of zinc worthy to mention that there exist two different and lesser amounts of quartz and galena than features between the veins of Stage I and II, those of Tajima. It consists of both broader in addition to differences in mineral constituent band of pyrite-marcasite-arsenopyrite-sphaler and ore texture. The first is that the veins of ite and of fine rhythmic bands of pyrite- Stage II situate at the lower horizon than sphalerite-quartz in contrast to symmetrical Tajima and Harima. Uppermost levels of bandings in Soya. Sphalerite is dark in color deposition at Stage II are equivalent to -150 and often shows a compact aggregation of very to -200 mL of Tajima. There exists also some fine crystals. Another distinctive feature of the difference in the levels among the later veins; vein is recently found tin and tungsten minerals those of western side, Soya and Oshima, occur such as , stannite and . a little shallower than those of eastern side, Although smaller in dimension, Sorachi vein Izumo and Sorachi. Another characteristic dif of N-S trend occupies an important position in ference is that the veins of Stage II have, in the mine because of its dense ore (Table 1). A general, slight indications of alteration halo high content of silver is due to silver sulphosalt even on the present ground surface (OICABE minerals included microscopically in galena and BAMBA,1976). and here argentite is rarely observed. Sphalerite As for the depth of formation of the Toyoha presents a similar appearance as that of Izumo deposits, it is estimated from the basal plane 294 J. YAJIMA and E. OHTA MINING GEOLOGY:

about 600 and at least 900 meters at the time of mineralization. The lowest limit of these veins may be assured according to the future develop ment of prospecting works at the mine. 2.2 Mineral paragenesis The veins of Toyoha deposits represent signi ficant difference with one another in micro scopic mineral association and texture as well as macroscopic characters. Mineral association of important veins are summarized in Table 2. Regarding to the other veins, following analo gies in constituent minerals are recognized after observations of more than 500 polished sections : Chikugo and Bizen to Tajima; Oshima, Oshi mashitaban and Hiyama to Soya; Satsuma, Daini-Satsuma and Soyabunki to Izumo: Ishi kari and Nemuro to Sorachi. Satsuma and Daini-Satsuma veins have a peculiar trend in direction, but they can be considered to belong to Stage II from their mineral assemblage including tin and tungsten minerals. Pyrite, sphalerite and galena are most com mon and abundant minerals throughout Stage I and II. Minor amount of and tetrahedrite is almost always associated with them. The veins of Stage I have rather simple mineral association as shown in Table 2. Hematite and magnetite are often found mac roscopically. Under the microscope, magnetite takes in most cases a form of pseudomorph after hematite and its genetic consideration has been presented by many authors (FUJIWARAand SAITO,1958; ISHIBASHIand SASAKI,1961; OTSU and HARADA,1963; YUI, 1966; YAJIMAand OKABE, 1971). Anyway a concentric aggrega tion of hematite and magnetite cuts the texture made by sulphide minerals (photo 6 in Fig. 3), so hematite must have occurred at the late period in Stage I. Several examples in which argentite occurs with magnetite are known. From microscopic observations of such speci contour map of pyroclastic rocks of Pliocene mens, it is confirmed that argentite was formed age, that the top of Tajima vein has deposited at about the same time or a little later than at the depth of 450 meters from the ground magnetite (photo 7 and 8 in Fig. 3). Argentite surface at that time (OKABE, 1968). Conse has never been found in direct contact with quently the formation of Tajima vein was be hematite. A tiny prismatic crystal of pyrrhotite tween the depth of 450 and 850 meters, while is occasionally found in sphalerite and pyrite of Izumo vein deposited between the depth of Stage I. The notable difference between Tajima 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 295

Table 2 Mineral association in the representative veins of the Toyoha mine.

* in a veinlet of NW-SEtrend . ** in Oshimashitaban vein. ( ): minor amount, (( )): rare

and Harima is in the presence of arsenopyrite in so high, although taking into account the the latter. Thus, a simple mineral zoning exists existence of ultra-microscopic particles of silver in the veins of Stage I that arsenopyrite occurs (USHIZAWA,1964). About twenty grains of at eastern lower side and hematite increases native gold, in total, were observed in 150 towards western upper side. Occurrence of polished sections from Tajima and Harima coaly substance and also graphite in sphalerite which associated with argentite. and/or pyrite from Harima vein is known Rare occurrencesof cassiterite, a canfieldite- (YAJIMA,1978). Description on graphite will like mineral, berthierite, stibnite and jam be given in a later paragraph. esonite in Tajima and wolframite in Harima Distribution and occurrence of silver minerals are known (OHTA,1979; NARITAet al., 1977). in Tajima and Harima are illustrated in Fig. 2. Their points of occurrence are also indicated in Argentite is a product of later mineralization Fig. 2. judging from its occurrence as aforementioned. The veins of Stage II exhibit a contrasted It is also indicated that argentite associated mineral assemblage between those of western with manganese minerals was formed at Stage and eastern sides. Oshima, Oshimashitaban II (HARAGUCHIand TAJIMA, 1969). In the and Soya veins are characterized by abundant same figure, two areas where silver occurs with carbonate minerals. It is also demonstrated in chalcopyrite are shown, but it is unknown Fig. 2 that innumerable veinlets of manganese whether the mineral is argentite or silver carbonate minerals occur at western side of sulphosalt because these areas were mind a Tajima vein. Furthermore, occurrences of long time ago. Silver sulphosalt minerals are berthierite-stibnite in Oshimashitaban vein and rarely observed in the veins of Stage I. Pyrargy- berthierite-native arsenic in a quartz-calcite rite, polybasite, miargyrite and freibergite in veinlet cutting across Tajima vein (OHTA, galena, sphalerite and pyrite are recognized in 1979) are noteworthy. Rare occurrence of two out of 64 polished sections from Tajima pisolitic cassiterite is known in Soya at -300 and in three out of 87 from Harima. They are mL. identified by optical properties, hardness, Tin and tungsten minerals give a charac reflectivity and qualitative analysis by EPMA. teristic feature to the veins of eastern side. Consequently silver content in Stage I was not Cassiterite and stannite are commonly found 296 J. YAJIMA and E. OHTA MINING GEOLOGY: 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 297 in Sorachi and Izumo, and decrease their ores can be referred to it without doubt. amount towards Soyabunki, Satsuma, Daini- Coaly substance in ores are often brecciated Satsuma, Nemuro and Ishikari veins. Cas and replaced by sulphide minerals and show siterite has a grain size of a few to 200 microns more or less distinct pleochroism which may be and that of 10 to 20 microns is abundant. In corresponding to the pre-graphitization men stannite group minerals, four species are dis thioned by RAMDOHR(1969). Graphitization criminated optically but their identification is proceeds from periphery or along cracks of not sufficient yet. They display various textures vitrinite showing a very strong and character with sphalerite such as colloform banding, istic pleochroism and anisotropism (photo 5 alternative zonal growth or atoll-like ones. in Fig. 3). Chalcopyrite is nearly always associated with Ag sulphosalt minerals are usually found in them in a variable amount. Wolframite is the veins of Stage II. They are in most cases occasionally found in Izumo, Soyabunki, Satsu included in galena and occasionally in sphal ma and Daini-Satsuma. erite, pyrite and chalcopyrite. The following Another prominent feature of the eastern side minerals were recognized optically, miargyrite, veins is occurrence of pyrrhotite, wurtzite polybasite-pearceite, pyrargyrite-proustite, and graphite. Much amount of pyrrhotite, freibergite and in decreasing order associated with pyrite-marcasite-arsenopyrite- in relative frequency. Miargyrite often ex sphalerite-galena-Pb sulphosalt is observed in hibits orientated intergrowth with galena drill core samples from the lower part of Izumo (photo 4 in Fig. 3). vein (photo 1 and 2 in Fig. 3). Common It should be concluded from these des occurrence of pseudomorph after pyrrhotite criptions that there exists a distinct zonal dis in the eastern side veins (photo 3 in Fig. 3) tribution of elements in Stage II mineraliza suggests that a considerable amount of pyr tion, from southeastern veins (Sn, W) to rhotite had once deposited in Stage II. Oc northwestern ones (Sb, As, carbonate). currence of wurztite has been reported already 3. Fluid Inclusion Study (HARAGUCHIand TAJIMA, 1969; MIYAJIMAet al., 1971). The mineral is occasionally found in 3.1 Description of fluid inclusions Harima and abundantly in the eastern side One hundred samples of quartz and veins of Stage II. X-ray diffractometry of two sphalerite obtained from Tajima, Harima and specimens in which anisotropic acicular crystal Izumo were examined for fluid inclusions. of zinc sulphide is rich in thin sections, revealed About 1300 inclusions studied are almost two the possible existence of, three polytypes, 3C, phase ones of moderate filling degree. They 6H and 15R. are 10 to 50 microns in diameter. A deep red Coaly substance and graphite commonly hexagonal crystal and a birefringent flaky accompanied by rutile are found not only crystal are occasionally found as daughter in Harima as mentioned above, but also in minerals. Necking down of inclusions is often Ishikari, Sorachi, Izumo and Soyabunki observed and they are kept out of experiment. veins. Around these veins, distributes a thick As the first trial in this area, EL SHATOURY formation of black mudstone as a host rock et al. (1975) studied fluid inclusions in quartz in which coaly substance and rutile are widely phenocryst of rhyolite from the mine which found. Thus the source of those materials in are thought to be suggestive to know the

Fig. 3 Photomicrographs showing the occurrence of some characteristic minerals. 1: pyrite(py)-marcasite(mc)-Pb sulphosalt(ps)-pyrrhotite(po), (Izumo, -580 mL). 2: distorted crystals of pyrrhotite replaced from periphery by marcasite (Izumo, -580 mL). 3: pyrrhotite replaced entirely by very fine aggregate of pyrite and marcasite(py-mc) with overgrowth of arsenopyrite(asp), (Izumo, -300 mL). 4: orientated intergrowth of miargyrite in galena(oil), (Sorachi, -250 mL). 5: graphite in pyrite(left//nicols, right, crossed nicols), (Sorachi, -250 mL). 6: hematite(hm) and magnetite(mt) replacing pyrite(py), (Chikugo, -150 mL). 7 and 8: relation between magnetite(mt), argentite(arg) and native silver(Ag), (Chikugo, -150 mL). 298 J. YAJIMA and E. OHTA MINING GEOLOGY: primordial state of mineralizing solutions TAKENOUCHI, 1976). The result for Tajima and (DEICHA,1955; TAKENOUCHIand IMAI, 1975). Harima is in good agreement with them. They reported occurrences of halite-bearing Histograms in Fig. 4 show a similar trend of polyphase inclusions and an abundant gaseous variation in both temperature and salinity for ones as well as a small number of CO2-bearing the three veins. In Harima, a little higher inclusions. Crushing experiment (DEICHA, temperature and salinity than those of Tajima 1950; ROEDDER, 1970) on quartz and are observed. The difference of mean tem sphalerite showed a minor to moderate evolu perature between two veins is about 20•‹C. tion of gases from inclusions which is referred Temperature and salinity of inclusions in to a small quantity of CO2. None of them, sphalerite occupy upper ranges of those meas however, had shown a liquid phase of CO2 ured in quartz. It is notable that temperature upon cooling. Gaseous inclusions are occasion and salinity show a decreasing trend towards ally observed in Tajima and Harima, sug the lowest level in Tajima vein. This curiosity gesting that the boiling of ore-foming solutions would request a further investigation. had not arisen so violently as in the case of Higher temperature and salinity appeared rhyolite. 3.2 Temperature and salinity measure ment Filling temperature was measured using a heating stage Type 1350, Leitz. Temperatures were calibrated against the melting points of organic materials (YAJIMA, 1969) and were determined with an accuracy of •}1.5•‹C. Salinity measurement was carried out by the freezing microscope stage Type NE, Nikon.

Some amelioration of apparatus was performed to minimize the temperature gradient within the sample chamber (OHTA and YAJIMA, 1977b), and the uncertainty in the freezing temperature determination became to •}0.1•‹C for the inclusions in which ice is well visible. Calibration was performed by a standard NaCl solution confined in a thin glass tube. As mentioned in the preceding chapter, geologic reconstruction suggests that a thickness of rock cover on the veins was less than 1,000 meters. Maximum hydrostatic pressure at this depth was 100 bars. Assuming a lithostatic condi tion, maximum pressure was 250 bars. The maximum pressure correction for filling tem perature at a maximum pressure of 250 bars is about 20•‹C (LEMMLEIN and KLEVTSOV, 1961). So the pressure correction is not applied in this paper. The results of temperature and salinity measurements are illustrated in Fig. 4. The Fig. 4 Temperature and salinity of fluid inclusions formation temperatures of the Toyoha deposits from major three veins of the Toyoha deposits. reported in previous works are in the range of Broken lines represent temperature ranges reported 150•‹to 250•‹C (TOKUNAGA, 1979; ENJOJI and by TOKUNAGA(1970). 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 299

present those for main sulphide deposition in each veins. As to the formation temperature of hematite, a range of 150•‹ to 200•‹C was re

ported (YAJIMA and OKABE, 1971). In this study, temperature measurement for quartz from quartz-argentite veins in Tajima showed the range of 220•‹ to 230•‹C with a mean temperature of 226•‹C (14 inclusions). mag

netite occurs in close relation with argentite, so it can be said that magnetitization was real ized at a little higher temperature than that for hematite. The formation temperature of

quartz-calcite veinlet including berthierite is reported as 175•‹ to 210•‹C (OHTA, 1979) and it is thought to represent the final condition of Stage II. Argentite was formed at temperature a little higher than this. 3.3 Relationship between temperature and salinity The results in which filling temperature and salinity measurements were realized on the same inclusion are plotted in Fig. 6. Distribu tions for each veins constitute their own domains characteristically arranged. The domain for Izumo is seen from upper right to a central Fig. 5 Filling temperature of fluid inclusions in dense zone. The domain for Harima is situated sphalerite and quartz associated with graphite. at around central part, whereas, that for in Izumo vein. Data of lower levels under Tajima occupies an area from central to lower -300 mL are all from drill core samples left side. These relation in distribution is il related to the vein. An average temperature is lustrated simply in Fig. 7. about 30•‹C higher than that of Harima. It is From the consideration on the same relation clear that temperature and salinity have an ship in inclusions from rhyolite, EL SHATOURY increasing tendency from shallower to deeper et al. (1975) showed the possibilities of mixing levels. Fluids in sphalerite shows higher con saline fluids with pre-heated meteoric water centration than that of quartz. and of active boiling at -300 mL. As their The temperature and salinity of ore-forming samples were collected along Tajima vein, solutions at the time of deposition of major three mixing with meteoric water is considered to veins are summarized as follows. have occurred at least in fractures of E-W Izumo vein 300•‹-200•‹C, 4.2-1.0 percent trend. Variation tendency in temperature and Harima vein 250•‹-200•‹C, 2.2-1.0 percent salinity between Tajima and Harima suggests

Tajima vein 250•‹-150•‹C, 1.5-0.2 percent that the location of mixing was in Tajima or lower side.

Soya vein is considered to have about the same Temperature and salinity ranges of in condition in temperature and salinity with clusions in rhyolite are shown in Fig. 7 together those of Tajima (SHIKAZONO, 1974, 1975). with examples of hypothermal and mesothermal Filling temperature of fluid inclusions in deposits. Fluids in rhyolite represent a good sphalerite and quartz associated with graphite similarity with those related to hypothermal is in a narrow range of 220•‹ to 230•‹C (Fig. 5). and mesothermal ore deposition. However, the Above-mentioned temperature ranges re condition at which deposition actually occurred 300 J. YAJIMA and E. OHTA MINING GEOLOGY:

Fig. 7 Distribution of temperature and salinity of fluid inclusions from epithermal (Toyoha, this study), mesothermal (Taishu, IMAI, 1978) and hypothermal (Takatori etc., ENJGJI, 1972; IMAI, 1978) deposits. Crossed lines represent the ranges measured in in clusions of quartz phenocrysts of rhyolite from the Fig. 6 Temperature-salinity relation of fluid inclu Toyoha mine (EL SHATOURYet al., 1975). sions from major three veins of the Toyoha deposits. solid marks=sphalerite, open marks=quartz. The fissure pattern in the Toyoha deposits suggests that stress of NW-SE direction had an at the Toyoha mine is still in the lower ranges. important role to the formation of the deposits Gradual variation of the condition in solution and the direction is famous as "Shakotan from initial state to depositional stage is also trend", but it is not well clarified yet what presented by EL SHATOURYet al. (1975). motive had operated to give rise to this stress. As will be mentioned later, the heat source is 4. Genetic Consideration existing at the southeastern deeper zone from 4.1 Geological environment the mine. Fractures of NW-SE trend might be The composite fold structure of N-S trend formed most probably related to the upheaval which is a characteristic in the third sub process of rock bodies with high temperature province of sedimentary basins, develops also which could be the parent of mineralizing in volcano-sedimentary formations in the solutions in this area. Teine-Chitose district. Such folding move The mixing with meteoric water (EL ments give rise to fractures perpendicular to SHATOURYet al., 1975) suggests that surface the axis together with those of N-S directions. water could freely circulate through the frac Jozankei quartz porphyry had intruded along tures of E-W trend. On the other hand, those tectonic zone made by fractures of N-S trend. of NW-SE and N-S trend might have not Fractures of E-W trend which might have permitted free circulation of surface water widened by an upheaval movement in the from the reasons as follows. (1) The veins of district must have provided the space of ore Stage II have slight indications of alteration deposition. halo and also traces of cracks or veinlets of the 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 301 same direction even on the present surface The difference in the nature of solution and ground. (2) They occur 150 to 200 meters also in the formation condition between two lower than Tajima and Harima. (3) Frac stages of deposition are described in second tures of E-W trend had already been cemented and third chapters. by vein materials at the time of deposition in Stage I is characterized by rather simple Stage II. paragenesis and an abundance of quartz. At 4.2 Heat source the late period of this stage, occurred hematite EHARAand YOKOYAMA(1971) studied the probably with relation to the mixing of meteoric terrestrial heat flow in Hokkaido and found an water. Silver was not contained so much in extremely high heat flow region situated at solution of Stage I. The difference of filling Shikotsu, Toya and Niseko in a distinct contrast temperature and salinity between Tajima and with an extremely low heat flow region which Harima suggests that the deposition had com covers almost entirely the Sapporo-Tomako menced initially at the latter and then con mai lowland belt. They considered the heat tinued to the former. Meteoric water might anomaly in west Hokkaido to be generated in have come down through fractures for Chikugo "green tuff" activity of Miocene period and and Tajima veins. estimated that the heat source with temperature Mineralization at Stage II had brought of higher than melting points of rocks is under numerous veins of NW-SE and N-S systems. lying even now at the depth of 15 kilometers Izumo and Sorachi are characterized by between Niseko and Shikotsu area. dense compact ores of zinc and lead accom The Toyoha mine is located in the Jozankei- panied by tin and tungsten minerals. Sorachi Toyoha geothermal where active investi is especially concentrated in silver content. On gations based on the Sunshine Project have the other hand, Soya, Oshima and Hiyama commenced recently (SAKO et al., 1977; SUMI veins have a larger amount of manganese et al., 1978). Around the Toyoha mine, TAKE carbonate minerals and several examples of UCHI et al. (1975) analysed the subterranean Sb sulphosalt minerals. This zonal distribution thermal structure based on the temperature of minerals in Stage II together with the differ distribution at the depth of 30 meters and ad ence in temperature and salinity of inclusions ditional drill hole data at Kurumizawa. They between Izumo and Soya, suggests that ore have estimated the temperature of 300•‹C at the deposition at Stage II had initiated at south depth of 1,000 meters just under Yunosawa eastern side and proceeded towards north and River located at about 3 kilometers southeast northwest directions. from the Toyoha mine. The thermal structure It is apparent that the veins of Stage I were between the Toyoha mine and Jozankei Hot more or less infiltrated by the solutions of Stage Spa proposed by them is shown in Fig. 8. They II at numerous sites as demonstrated in Fig. 2. have also analysed the heat flow pattern in Occurrence of various minerals in veins of west Hokkaido presented by EHARA and YOKP Stage I such as argentite, a canfieldite-like YAMA (1971) and deduced four different ages mineral, cassiterite, wolframite, berthierite- of heat source as follows: 5 to 6 •~ 107, 1.2 to stibnite and jamesonite, are all explained by 1.5 •~ 107, 2 •~ 106 and 105 years. The second the effect of solution of Stage II. Silver, having

one well represents the age of igneous activity been rich in Stage II solution, had precipitated in Tertiary period. The third and fourth ones initially as sulphosalt minerals in galena, and correspond to those in Quaternary period and then transported and deposited as sulphide at especially the third one is in good coincidence Tajima and Harima. Antimony and arsenic with that which formed flat lava plateau (1.9 also show the same behaviour with silver. m.y., SUMI et al., 1978) in this area. The relation between magnetite and argentite 4.3 Sequence of mineralization suggests that the reduction of hematite to It is evident that mineralization at the magnetite have occurred from just after de Toyoha deposits is divided into two stages. position at Stage I to the late period of Stage II. 302 J. YAJIMA and E. OHTA MINING GEOLOGY:

4.4 Physicochemical condition of ore deposition Mineralization in the Toyoha deposits is now considered from the view point of physicoche mical condition available from the mineral stability relations. SHIKAZONO(1975) proposed the progressive variation in chemical environment for Soya vein and mentioned that reducing processes operated throughout deposition of most of the sulphide minerals in the Toyoha deposits. We have few occurrence of indicative miner als in Stage I which is useful to estimate chemi cal condition of deposition. minerals in Fig. 9 Temperature-fS2 diagram showing stability fields the veins of Stage I are pyrite and hematite, of mineral phases in Stage I and II. (see text) while a coexisting mineral is chal Line(1)=pyrargyrite-argentite boundary under the condition of fO2 for hematite-magnetite copyrite. Existence of arsenopyrite at the lower boundary. levels of Harima is also taken into account. Line(2)=pyrargyrite-argentite boundary under the The formation temperature of Harima, Tajima condition of fO3 for upper limit of pyr and Chikugo was mentioned in the preceding rhotite. chapter. Line(1) and (2) are based on data in HOLLAND(1959) The stability field of ores of these three veins and CRAIGand BARTON(1973); two lines related to in fS2-temperature diagram (Fig. 9) may be arsenopyrite are from BARTON(1969) ; other lines are situated between Cu5FeS4 + FeS2-CuFeS2 from RIPLEY and OHMOTO(1977). and Fe2O3 + FeS2-Fe3O4 boundaries with a slight deviation to pyrite-arsenopyrite bound ary, while that in fO2-temperature diagram (Fig. 10) is lying between PbSO4-PbS and Fe2O3- Fe3O4 boundaries. Mineralization at Stage II brought several indicative mineral occurrences as follows. (1) Pyrrhotite decreases its amount from Izumo and Sorachi to Soya, and magnetite is scarcely observed. (2) Arsenopyrite is a common mineral in Izumo and Sorachi. (3) Silver minerals appear as sulphide in the veines of Stage I and as sulphosalt in those of Stage II with few

Fig. 10 Temperature-fO3 diagram showing stability Fig. 8 Sketch of the presumed profile of subterranean fields of mineral phases in Stage I and II . (see text) thermal structure at Toyoha area (TAKEUCHIet al ., Fe2O3-Fe3O4 boundary and upper limit of FeS (pyr 1975). rhotite) are from RIPLEYand OHMOTO(1977) ; other A: reservoir, B: heat source lines are from HOLLAND(1959, 1965). 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 303 exception. (4) Among veins of Stage II, car bonate minerals are rich in western side veins and graphite is often found in eastern ones. (5) Reduction of hematite to magnetite is prevailing throughout the veins of Stage I. From fluid inclusion studies, formation tem perature of main ore minerals at various levels in Stage II is estimated approximately as follows. Fig. 11 fCO2-fO2 diagram showing the variation of

A: Izumo vein, -580 mL...... 300•‹C stability fields of mineral phases in Stage II with tem

B: Izumo vein, -430 to 400 mL perature. Solid circle represents Fe3O4-Fe2O3-FeCO3 triple point at 220•‹C. (1) C+O2-CO2 boundary. (2) ...... 270•‹C CaCO3 + SiO2-CaSiO3 boundary. (3) FeS-FeCO3 C: Sorachi vein, -300 mL...... 250•‹C*1 boundary based on data in HOLLAND (1965) and D: Argentite-quartz in Tajima vein, RIPLEY and OHMOTO (1977). (4) FeS-Fe3O4 bound -200 mL ...... 220•‹C ary. Lines (3) and (4) are boundaries where pyrite E : Calcite-quartz-berthierite veinlet and pyrrhotite are coexisting. (upper limit of pyr crossing Tajima vein at -450 mL rhotite). (OHTA, 1979)...... 220•‹C The fS2and fO2conditions of each levels may be lower concentration of antimony in ore solu in such ranges as indicated in Fig. 9 and 10 by tion. Occurrence of minor amounts of be the same notation. Pyrrhotite-arsenopyrite as rthierite and stibnite at the latest period of semblage is taken into consideration for A, B, C Stage II will also imply the lower concentra and E. tion of antimony.

As mentioned earlier, ore solution of Stage II Partial pressure of carbon dioxide is possible infiltrated the veins of Stage I through numerous to be estimated from CO2-C phase boundaries fractures crossing them (Fig. 2). The reaction at different fCO2 in Fig. 10. Occurrence of

between ore solution of Stage II and ore graphite, pyrrhotite and carbonate minerals is materials of Stage I might have caused the also useful to estimate the fCO2-fO2 condition. shift from C to D and the reduction of hematite Fig. 11 shows the variation of stability field of to magnetite (line R in Fig. 9, 10). This process these minerals at different temperatures and is well explained by the occurrences of argentite also the variation in condition at each levels. with magnetite at D and of Ag sulphosalts at From these considerations, it may be reason A, B and C. Dashed line (2) in Fig. 9 represents able to estimate the succession of mineraliza a decomposition curve for Ag3SbS3 under the tion as A-•¨B•¨C•¨D and A•¨E in Stage II. condition of fO2for upper limit of pyrrhotite. If The occurrence of each mineral mentioned in fO2 increases up to the hematite-magnetite the preceding section is explained consistently boundary, fS2for the decomposition of Ag3SbS3 by the mineral stability relations. also increases up to dashed line (1) in the same 4.5 Model for the formation figure. Therefore pyrargyrite can not exist at D It is concluded from above-mentioned con where fO2is near to hematite-magnetite boun sideration that the hydrothermal solution ary and fS2is under the line (1). One the other related to the Toyoha deposits had originated hand, the same mineral is stable at A, B, and at the southeastern deep and traveled towards C where fO2 is near to the upper limit of northwestern side. At first, deposition occurred pyrrhotite and fS2 is higher than the line (2). in fractures of E-W system. Then, resurgent Kermesite (Sb2S2O) and other Sb minerals are solution had arisen and separated into two not found yet at D and this fact may be at directions, NW-SE and N-S, at the locus a tributed to undersaturation of Sb2S2O, that is, little afar from Izumo vein to southeast. The solution which advanced northward had *1 Teken as intermediate temperature between B and D brought Sorachi, Ishikari and Nemuro veins, 304 J. YAJIMA and E. OHTA MINING GEOLOGY:

Fig: 12 Schematic diagram for the formation of the Toyoha deposits in two stages. Elements and minerals brought in each stage are noted on and under the vein names respectively . py: pyrite, asp: arsenopyrite, sp: sphalerite, gn: galena, mia: Ag sulphosalts, po: pyrrhotite, stn: stannite, cs: cassiterite, wo: wolframite, hm: hematite, mt: magnetite, qz: quartz, carb: carbonates. while that which took a way to northwest had extent of Izumo and Sorachi is considered to deposited Izumo, Soya and so on. These de be the most important task for the present. But positional processes are summarized in the a struggle against higher geothermal gradient model for the formation of the Toyoha de will be inevitable. (2) As the later veins were posits (Fig. 12). Consistent explanation for the formed at the horizon about 200 meters deeper occurrence of overall minerals and also for the than Tajima, prospection for lower levels of all relationship among veins of major three direc veins of Stage II is indispensable, especially for tions had become to be possible by this model. the eastern side ones. These veins have little Harima and Tajima can be regarded as the indications on the surface ground, so analysis of forerunner, though they are bigger in dimen fissure pattern and underground research may sion, to the later mineralization. The ore be more effective than prospection on the formed at Stage II may be superior to that surface. (3) All veins and veinlets formed at deposited at Stage I both in amount and in Stage II ought to be examined carefully for quality. silver content because the later mineralization Further investigations will be necessary to is the major source for silver. Furthermore, at justify the proposed model for the Toyoha tention must be paid to the relationship be deposits with much solid evidence, for example, tween the occurrence of argentite in the veins by the isotopic study. of E-W system and fissures or faults of NW-

5. Suggestion for Exploration SE and N-S trends crossing them. Acknowledgments: The present authors wish Consideration as mentioned above leads to to express their gratitude to Professor T. BAMBA some suggestions for exploration. (1) From the of Hokkaido University for his continued proposed model, recognition of southeastern encouragement and guidance since he had been 29(5), 1979 Two-stage Mineralization of the Toyoha Deposits 305

the chief geologist in our laboratory. The ENJOJI, M. (1972): Studies on fluid inclusions as the authors are also grateful to Dr. K. SAWAMURA media of the ore formation. Sci. Rept. Tokyo Univ. Educ., Ser. C, 11, No. 106, 79-126. and Dr. E. NARITA of Geological Survey of ENJOJI, M. and TAKENOUCHI, S. (1976): Present and Japan, Hokkaido Branch, for reviewing manu future researches of fluid inclusions from vein-type script with valuable discussions and sugges deposits. Mining Geol., Special Issue. 7, 85•`100. (text in Japanese) tions. They are also indebted to their col FUJIWARA, T. and SAITO, M. (1958): On the occurrence leagues, Dr. T. IGARASHIand Dr. K. OKABE of hematite and magnetite from the Toyoha mine, for discussions and support in experimental Hokkaido. Jour. Jap. Assoc. Miner. Petro. Econ. Geol., 42, 15•`22. (text in Japanese) work. HARAGUCHI, M. and TAJIMA, Y. (1969): Lead-zinc The cooperation of many geologists includ mineralization of the Toyoha mine with special ing Messrs. T. ONO, T. TERAE, Y. OGUMA,H. reference to the nature and occurrence of sphalerite and the behaviour of silver. Mining Geol., 19, 9•`18.

HASHIMOTO,T. ICHINOSE,T. ISHIZAKA,T. (text in Japanese) TAKEYAMAand O. MIYAISHIof Toyoha Min HASHIMOTO, H., ISHIZAKA, T. and ICHINOSE, T. (1977): ing Company during field work is gratefully Recent exploration for the Izumo vein of the Toyoha mine. Mining Geol., 27, 87•`97. (text in acknowledged. Mr. O. MIYAISHIhave cordially Japanese) prepared two figures anew in this paper. HOLLAND, H. D. (1959): Some applications of thermo Sincere thanks are due to Messrs. R. TANITSU, - chemical data to problems of ore deposits I. Stability relations among the , sulphides, sulphates and S. WATANABEand T. KIMURA who have carbonates of ore and gangue minerals. Econ. Geol., provided innumerable numbers of polished and 54, 184•`233. thin sections as well as doubly polished ones, HOLLAND, H. D. (1965): Some applications of thermo- chemical data to problems of ore deposits II. Mineral and to Miss. N. KUMAGAIfor her patience in assemblages and the composition of ore-forming drafting all figures in this paper. fluids. Econ. Geol., 60, 1101•`1166. IMAI, H. (1978): Geological studies of the mineral depo References sits in Japan and east Asia. Univ. Tokyo Press, Tokyo, 392p. AKIBA, C. (1958): Some problems on the ore deposits ISHIBASHI, M. and SASAKI, S. (1961): On some silver- in the southwestern part of Hokkaido. Cenozoic lead-zinc ores from the Toyoha mine, Hokkaido, Res., 27, 22•`31. (in Japanese) Japan. Mining Geol., 11, 337•`343. (text in Japa AKOME, K. and HARAGUCHI, M. (1963): Geology and ore nese) deposits of Toyoha mine. Mining Geol., 13, 93•`99, ISHIHARA, S. and SASAKI, A. (1978): Sulphur of kuroko (text in Japanese) deposits-A deep seated origin? Ibid, 28, 361•`367. AKOME, K. and HARAGUCHI, M. (1967): The charac LEMMLEIN, G. G. and KLEVTSOV, P. V. (1961): Relations teristics of fracture and mineralization of the Toyoha among the principal thermodynamic parameters in mine. Ibid., 17, 93•`100. (text in Japanese) a part of the system H2O-NaCl. Geochemistry, No. BAMBA, T. (1977): Metallogenic province of Hokkaido. 2, 148•`158. Jour. Fac. Sci. Hokkaido Univ., Ser. IV, 17, No. 4, MIYAJIMA, T., HAKARI, N. and KITA, M. (1971): Some 695•`708. considerations on geologic structure and mechanisms BARTON, P. B., Jr. (1969): Thermochemical study of the of fracturing at the Toyoha mine. Mining Geol., 21, system Fe-As-S. Geoch. Cosmoch. Act., 33, 841•` 22•`35. (text in Japanese) 857. NARITA, E., YAJIMA, J. and OHTA, E. (1977): On tha CRAIG, J. R. and BARTON, P. B., Jr. (1973): Thermo occurrence of tin and tungsten minerals from the chemical approximations for sulphosalts. Econ. Toyoha mine. Sanko Gakkai, Collected abstract, Geol., 68, 493•`506. 95 (in Japanese) DEICHA, G. (1950): Essais par ecrasement de fragments NISHIKAWA, J. (1977): Jozankei quartz porphyry and its mineraux pour la mise en evidence d'inclusions de strontium isotopic ratio. Ibid., 50. (in Japanese)

gaz soul pression. Bull. Soc. fr. Miner. Crist., OHTA, E. and YAJIMA, J. (1977a): New occurrence of LXXIII, 439•`445. canfieldite and berthierite from Toyoha mine. Ibid., DEICHA, G. (1955): Les lacunes des cristaux et leurs 96. (in Japanese) inclusions fluides, signification dans la genese des OHTA, E. and YAJIMA, J. (1977b): Improvement of

gites mineraux et des roches. Masson et Cie., Paris, freezing stage and its result. Bull. Geol. Surv. Japan, 126p. 28, 199. (in Japanese) EHARA, S. and YOKOYAMA, I. (1971): Measurements of OHTA, E. (1979): Occurrence and geochemical environ terrestrial heat flow in Hokkaido (Part 2). Geophy. ment of berthierite from Toyoha mine. Mining Bull. Hokkaido Univ., 26, 67•`84, (text in Japanese) Geol., 29, 97•`102. (text in Japanese) EL SHATOURY, H. M., TAKENOUCHI, S. and IMAI, H. OKABE, K. (1968): Depth of formation of the Toyoha

(1975): Nature and temperature of ore-forming deposits estimated by volcanic activity in west fluids at Toyoha mine in the light of fluid inclusions Hokkaido. Geol. Surv. Japan, Hokkaido Branch, in quartz porphyry. Mining Geol., 25, 11-25. Koenyoshiroku, No. 19, 32•`35. (in Japanese) 306 J. YAJIMA and E. OHTA MINING GEOLOGY:

OKAEE, K. and BAMBA, T. (1976): Propylite and altera Geol., 42, 238•`246. (text in Japanese) tion halo around the Pb-Zn vein of Toyoha mine, SUMI, K. et al. (1978): Geological investigation of hydro west Hokkaido, Japan. Mining Geol., 26, 239•`251. thermal alteration haloes in Japanese geothermal fields. Part I. Rept. Geol. Surv. Japan, No. 259, (text in Japanese) OTSU, H. and HARADA, H. (1963): On the modes of (text in Japanese) occurrence of hydrothermal iron minerals in TAKENOUCHI, S. and IMAI, H. (1975): Glass and fluid inclusions in acidic igneous rocks from some mining green tuff region. Bull. Geol. Surv. Japan, 14, 579•` 590. (text in Japanese) areas in Japan. Econ. Geol., 70, 750•`769. RAMDOHR, P. (1969): The ore minerals and their inter TAKEUCHI, M., HIROSAWA, T. and HAYAKAWA, M.

growths. Pergamon Press, Oxford, 1174p. (1975): Subterranean thermal structure at Toyoha RIPLEY, E. M. and OHMOTO, H. (1977): Mineralogic, geothermal field in Hokkaido. Bull. Volc. Soc. sulphur isotopic and fluid inclusion studies of the Japan, 20, 13•`29. (text in Japanese) stratabound copper deposits at the Raul mine, Peru. TOKUNAGA, M. (1970): Lead-zinc veins of Toyoha mine. Econ. Geol., 72, 1017•`1041. In TATSUMI, T. ed. Volcanism and , ROEDDER, E. (1970): Application of an improved crush 247•`257. ing microscope stage to studies of the gases in fluid USHIZAWA, N. (1964): On the mode of occurrence of inclusions. Schweiz. Miner. Petro. Mitt., 50, 42•`58. silver and silver minerals of the Toyoha mine, SAITO, M. et al. (1967): Metallic and non-metallic Hokkaido, Japan. Mining Geol., 14, 183•`190. mineral deposits of Hokkaido. Geol. Surv. Japan, (text in Japanese) 575p. (text in Japanese) YAJIMA, J. (1969): Fundamental problems in the research SAKO, S. et al. (1977): Geothermal fields and hot springs of fluid inclusions in minerals and rocks. Ibid., 19, in Hokkaido. (B) Northern part of west Hokkaido. 376•`388. (in Japanese) Rept. Geol. Surv. Hokkaido, No. 4, (in Japanese) YAJIMA, J. (1977): New occurrence of tin-minerals from SHIIAZONO, N. (1974): Physicochemical properties of the Toyoha mine, Hokkaido, Japan. Ibid., 27, ore-forming solution responsible for the formation of 23•`30. (text in Japanese) Toyoha lead-zinc deposits, Hokkaido, Japan. Geoch. YAJIMA, J. (1978): Coaly substance in the ore from the Jour., 8, 37•`46. Toyoha mine. Ibid., 28, 45. (in Japanese) SHIKAZONO, N. (1975): Mineralization and chemical YAJIMA, J. and OKABE, K. (1971): On the iron-rich environment of the Toyoha lead-zinc vein-type banded ore from the Toyoha mine, Hokkaido. deposits, Hokkaido, Japan. Econ. Geol., 70, 694•` Ibid., 21, 221•`228. (text in Japanese) 705. YUI, S. (1966): Stability relations among iron oxide, SUGIMOTO, R. (1958): Geology and ore deposits in the sulphide and carbonate minerals during magmatic area drained by the Otarunai River, Ishikari Prov., ore deposition with special reference to the role of

Hokkaido. Jour. Jap. Assoc. Miner. Petro. Econ. graphite. Ibid., 16, 16•`27. (text in Japanese)

豊 羽鉱床 にお ける前 ・後 期鉱化作用 とその形成過程

矢島 淳吉 ・太 田 英順

要 旨:豊 羽 鉱 山 の含 銀 鉛 亜 鉛 鉱 脈 鉱 床 は,手 稲 ・千 才 液 体 包 有 物 の 充填 温 度,塩 濃 度 は,前 期 鉱 脈 で は 播磨 地 域 に お い て 定 山渓 石 英 斑 岩 を囲 み 同 心 円状 に累 帯 配 列 〓か ら但 馬〓 に 向 って,後 期 鉱脈 で は出 雲〓 か ら宗 谷〓 に す る鉱 床 群 の 中心 的 位 置 に あ る.周 知 の よ う に同 鉱 床 の 向 っ て,そ れ ぞ れ減 少 す る.以 上 の事 実 と鉱 山東 南方 の 鉱 脈 群 は前 期 鉱 脈 と後 期 鉱 脈 に区 別 され る.前 期 鉱 化 作 深 部 に知 られ る熱 源 の存 在 とか ら,本 鉱 床 の鉱 脈 群 は, 用 にお い て は,播 磨〓 下 部 で硫 砒 鉄 鉱,石 墨 が 認 め ら 東 南 方 深 部 か ら上 昇 して き た鉱 液 に よ って,前 期 に お い れ,但 馬〓,筑 後〓 の上 部 で赤 鉄 鉱 が 増 す とい う簡 単 な て は播 磨 → 但 馬 へ,ま た後 期 にお いて は空 知 →根 室 ・石 鉱 物 累 帯 が観 察 され る.後 期 鉱 化 作 用 にお い て は,東 側 狩 へ,及 び 出雲 → 宗 谷 → 渡 島へ,と い う経 路 で順 次形 成 の空 知〓,出 雲〓 が,錫 ・タ ン グス テ ン鉱 物 お よび石 墨 され た とい うモ デル を提 唱 す る.こ の モ デ ル に よ り,当 を伴 う多 量 の磁 硫 鉄 鉱,硫 砒 鉄 鉱 の 産 出 で特 徴 づ け られ 鉱 床 中 に産 す る ほ とん ど全 ての 鉱 物 の産 状,特 に前 期 鉱 るの に対 し,西 側 の 宗 谷〓,渡 島〓 な どは,マ ンガ ンを 脈 中 に お け る輝 銀 鉱,含 ア ンチ モ ン鉱物 の産 状 や 赤 鉄 鉱 含 む 炭 酸 塩 鉱 物 と共 に,最 末 期 にベ ル チ エ鉱,輝 安 鉱 な の磁 鉄 鉱 化 な どを統 一 的 に説 明 す る こ とが で き る.〓ま ど を産 す る とい う顕 著 な 鉱物 の 累帯 分 布 が認 め られ る. た,こ の モ デル か ら幾 つ か の 探鉱 指 針 が導 かれ る.