Mineralization of the Arakawa No. 4 Vein of the Kushikino Mine, Kagoshima Prefecture, Japan

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1 MINING GEOLOGY, 34(3), 187 `195, 1984 Mineralization of the Arakawa No. 4 Vein of the Kushikino Mine, Kagoshima Prefecture, Japan Koichi TAKEUCHI and Naotatsu SHIKAZONO Abstract: Mineralization of the Arakawa No. 4 vein, one of the representative veins in the Kushikino mine, was investigated. Four periods of mineralization are recognized. They are, from earlier to later stages, period I (quartz-calcite zone), period II (high grade "Ginguro" ore zone containing polybasite, electrum, pyrite and tetrahedrite), period III (brecciated quartz-calcite-clay zone), period IV (banded "Ginguro" zone containing polybasite, electrum and pyrite). Period II can be divided into earlier one (period IIa containing polybasite and pyrite) and later one (period IIb containing electrum, polybasite, pyrite, sphalerite and chalcopyrite). No compositional variations were found in electrum and tetrahedrite. On the other hand, decrease of iron content of sphalerite, and increases of selenium contents of polybasite and naumannite-acanthite series minerals from earlier to later stage were observed. Filling temperature of fluid inclusions decreases slightly from earlier to later periods; Ž for period I, Ž for period II and Ž for period IV. Based on the variations in the iron content of sphalerite and filling temperatures of fluid inclusions, sulfur fugacity for period II is estimated to be atm. Decreasing of temperature under the relatively oxidizing environments can explain the variations of selenium contents of polybasite and naumannite-acanthite series minerals. 1. Introduction The Kushikino Au-Ag deposits are one of the great gold and silver producers in Japan. About eight million metric tons of ore (average 6.7 g/t Au and 61 g/t Ag) have been mined. Many papers have been published about this mine. Most of them are related to the exploration of ore deposit such as the analysis of fissure system (e.g., SUKESHITA and, UEMURA, 1976). Recently, MOTOMURA et al. (1981) described the occurrences of gold-silver ore of Arakawa No. 3 vein. IZAWA et al. (1981) reported the results of fluid inclusion studies on the Kushikino No. 1 vein. However, many fundamental data on the ore deposits are still very few. Therefore, it is intended to study this mine to reveal the nature of the gold-silver mineralization. Arakawa No. 4 vein, one of the most intensively exploited veins, was Received on March 15, 1984, accepted on May 8, * Institute for Ceramics of Nagasaki Prefecture, Hasami, Higashisonogi-gun, Nagasaki Prefecture ** Geological Institute, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113. Keywords: Gold-silver deposits, Electrum, Polybasite, Sphalerite, Fluid inclusions, Selenium. selected for the present study. The mineralization especially concerning the ore minerals from this vein will be described and then the physico-chemical environment of ore deposition will be estimated. Descriptions on the clay minerals in the host rocks and veins and the general features of hydrothermal alterations occurring in this mining district will be published elsewhere. 2. Outline of Ore Deposit The Kushikino mine consists of more than twenty gold and silver-bearing quartz-calcite veins (Fig.1). Many of them are mined along the strike for more than 1,000 meters. The veins consist of four fissure systems; N70 E, S; N40 W, W; N80 W, S; N20 E, E, and among them those with the first trend are well developed (Fig.1). The Arakawa No. 4 vein which was discovered in 1968 and has been developed since 1972 is located in the northwestern part of the mining district (Fig. 1). The vein has been developed along a strike for about' 1,000 meters and along a dip for 250 meters in a vertical distance. The average vein width is about meters and the average ore 187

2 188 K. TAKEUCHI and N. SHIKAZONO MINING GEOLOGY: Fig. 1 Vein system of the Kushikino mine on the L6 level. grade is 10 g/metric ton for gold and 70 g/ metric ton for silver. Ore reserves are estimated to be about 100,000 metric tons. The vein is developed along two fissure systems; NS- N20 E and N40-50 E with dip of S. This vein is characterized by the existence of several small bonanza placed with regular intervals (Fig. 2). Seven ore shoots are discovered already. Except for these ore shoots, the vein is thin in width and barren. The W ore shoot on each level (L6, L8 and L9) was studied in detail in the present investigation. 3. Mineralization 3.1 Inner structure of the vein The Arakawa No. 4 vein has been suffered mineralization several times. The multiple mineralizations are recognized by the difference of ore type, ratio of each gangue mineral and existence of"gingro"band and/or by faults. The sequence of ore vein formation was determined from the mutual geometric rela- Fig. 2 Longitudinal section of the Arakawa No. 4 vein showing the distribution of ore shoots. Shaded ore shoot is named the W13-14 ore shoot. Wide solid bars represent the sketched section. tions and by the nature of breccias in the vein. For example, on L8 level, the vein can be divided into five zones from foot wall side to hanging wall side, as schematically shown in Figure 3. Here, the terminology of "zone" is used to represent actual units which was recognized on the field. Zone I: Barren zone including sometimes wall-rock fragments, being rich in quartz on the foot wall side and in calcite wall side. on the hanging

3 34 (3), 1984 Mineralization of the Arakawa No. 4 Vein of the Kushikino Mine 189 Zone II: High grade zone including banded and brecciated "Gingro" ore. This zone is divided into two parts. The part of foot wall side is rich in calcite and bordered by banded "Ginguro" ore only on the foot wall side. The banded "Ginguro" ore of hanging wall side was probably scraped by the formation of the next zone. The part of hanging wall side is rich in quartz and bordered by banded "Ginguro" ore on the both wall sides. Relatively large brecciated "Ginguro" ore with tabular form is found in the central portion of this zone. They are probably brought from a place not so far from the original position. Argillaceous brecciated zone containing brecciated wall rock fragments on the most hanging wall side is thought to be the former fault breccia. Zone III: This zone contains various brecciated fragments such as barren quartz, calcite, wall rock, and "Ginguro" ore. Interstitial part is filled with quartz and a lot of clay minerals such as montmorillonite. Zone IV: Quartz zone bordered by banded "Ginguro" ore on the both wall sides. As the banded "Ginguro" ore of foot wall side is not scraped, it is considered that this zone was formed after the formation of zone III. The crushed banded "Ginguro" ore of hanging wall side implies that the hanging wall was moved after the formation of this zone. Zone V : Barren zone mainly consisting of calcite and being remarkably crushed. It is Schematic section showing inner structure of vein Fig. 3 Schematic section showing the inner structure of the Arakawa No.4 vein. This figure is constructed from the data of the L8 level, especially from the central part of sketched section. Fig. 4 Distribution of each period in the vein on the L8 level.

4 190 K. TAKEUCHI and N. SHIKAZONO MINING GEOLOGY: Table 1 Mineral sequence of the Arakawa No. 4 vein. Fig. 5 Schematic profile of the W13-14 ore shoot showing the mineralization periods. Ginguro band is not necessarily continuous in fact. thought from the texture that this zone has been brought from the other place. The zone does not continue for a long distance. As shown in Figure 3, the Arakawa No. 4 vein is characterized by the asymmetrical structure. Namely, a late stage zone is formed successively on the hanging wall side of the former zones. 3.2 Period of mineralization and mineral assemblage Four periods of mineralization are recognized judging from the inner structure of vein on each level, and is summarized in Figure 4 and 5. The sequence of mineralization and common paragenetic relation for the Au-Ag minerals are summarized in Table 1. Width of each bar in Table 1 represents relative amount of mineral precipitated in each period. A remarkable difference is not found in mineral assemblage among four periods, but there is a difference somewhat in paragenetic and quantitative relations. Period I: The vein formed in this period is developed widely on the foot wall side all over * including aguilarite and acanthite ** identified by powder X-ray diffraction "paragenetic relation" means the mode of occurrence showing direct contact in each other. Abbreviations used are; El: electrum, Pol: polybasite, Tet: tetrahedrite, Pyr: pyrargyrite, Nm: naumannite, Py: pyrite, Mc: marcasite, Sph: sphalerite, Gn: galena, Cp: chalcopyrite, Qz: quartz, Cal: calcite, Adu: adularia, Chl/ Sap: interstratified chlorite-saponite, Ser/Mont: interstratified sericite-montmorillonite, Mont: montmorillonite, Chl: chlorite the Arakawa No. 4 vein. It is rich in quartz at early time and later is abundant in calcite. Network veinlets or brecciated wall rocks are accompanied sometimes. Period I corresponds to the Zone I in Fig. 1. Period II : Banded and brecciated "Ginguro" ore formed in this period contains the most part of ores in the Arakawa No. 4 vein. Repeated mineralization, faulting and brecciation have complicated the inner structure. Period II corresponds to the Zone II in Fig. 3. As mentioned above it is observed that the early formed "Ginguro" band is scraped by the later one. Therefore, the period can be divided into early one (period IIa) and later

5 34 (3), 1984 Mineralization of the Arakawa No. 4 Vein of the Kushikino Mine 191 one (period IIb). Period IIa is well developed in eastern part of L8 level. Polybasite is abundant and electrum-pyrite assemblage is common. Period IIb ore is well developed in L9 level and central part of L8 level. Tetrahedrite is more abundant in this subperiod, compared with IIa subperiod. Tetrahedrite-polybasite and electrum-tetrahedrite-polybasite assemblages are frequently observed. Especially in L9 level, the amount of tetrahedrite exceeds that of polybasite. Period III: Appreciable mineralization has not occurred in this period. Quartz, calcite, and clay minerals such as montmorillonite and interstratified chlorite-saponite fill the interstice among various kinds of brecciated fragments; wall rock, calcite, barren quartz, and brecciated "Ginguro" ore. This period corresponds to the Zone III in Fig. 3. Period IV: The vein formed in this period is well developed at the upper level with continuous banded "Ginguro" ore. But, it is absent at L9 level. Polybasite is the predominant mineral and electrum coexists with pyrite. Period IV correponds to the Zone IV in Fig Selected Ore Mineralogy Variations of chemical composition of ore Table 2 Representative analytical data on ore minerals FeS*: Corrected value excluding the effect of X-ray scattering caused by chalcopyrite.

6 192 K. TAKEUCHI and N. SHIKAZONO MINING GEOLOGY: minerals obtained by using electron microprobe analyzer of J.E.O.L., JXA-5, and paragenetic features will be described below. Analytical conditions of E.P.M.A. are as follows : voltage, 25 kv, current, 0.05 Đa on pure ZnS, counting time, 8 sec ~4, correction method, SWEATMAN and LONG (1969), standard, Au-Ag alloy for Au and Ag of electrum, CuFeS2 for Cu, Fe and S, ZnS for Zn, MnS for Mn, CdS for Cd, Sb for Sb, Cu3AsS4 for As, and Ag2Se for Ag and Se. Representative analytical data on ore minerals are given in Table 2. Electrum occurs mainly as isolated grains in the ore. The mineral sometimes coexists with pyrite or silver-bearing minerals except pyrargyrite. Au content in electrum ranges from 55 to 65 weight percent and is uniform all over the ore shoot. No compositional zoning was observed inside of the grains. Polybasite is the most abundant silverbearing mineral of this vein. The mineral coexists intimately with tetrahedrite. It often shows myrmekitic texture with pyrargyrite, when the mineral paragenetically coexists with the latter. According to HALL'S classification (HALL, 1967), all of the mineral analyzed belong to "polybasite" in a restricted sense. The polybasite from the Arakawa No. 4 vein contains appreciable amounts of selenium. Se/S weight percent ratio of this mineral is represented in Figure 6. It shows that the selenium content increases with elevation of level on a certain period and this mineral in later period has higher Se/S ratios compared with that in earlier period. Tetrahedrite from the Arakawa No. 4 vein contains considerable amounts of silver. The amount of this mineral at L9 level is larger than that at other levels. No systematic variation of chemical composition is observed from earlier to later period. Arsenic content is below 2 weight percent. Silver content ranges from 8 to 22 weight percent and mostly is about 20 weight percent. Silver content is nearly constant at the L6 level, being about 20 weight %. Zinc content is larger than iron content all over the ore shoot and its variation range is smaller at the upper levels. No compositional Fig. 6 Variation of Se/S weight percent ratio of polybasite. At the L9 level, IIb consists of 1-2 ginguro bands and its breccia. Thus, data from the L9 level are plotted correspondingly to its position of occurrence. At the L8 level, three mineralized parts are recognized and investigated. zoning is also observed in the grains. Naumannite-acanthite series mineral occurs generally as small grain and is small in amount on every level. The mineral coexists with most of ore minerals. Selenium content of the mineral is higher in the upper level than in the lower level as same as in the case of polybasite. Pyrargyrite paragenetically coexists rarely with other ore minerals. Even in a given period, pyrargyrite has precipitated in earlier or later stage than the other ore minerals. Its chemical composition is nearly constant and is close to pure pyrargyrite end member, but small amounts of Se is detected. Galena coexists paragenetically with polybasite and tetrahedrite intimately, but its occurrence is rare. A small amount of selenium (1-3 weight %) is detected. Small amounts of sphalerite coexist paragenetically with tetrahedrite at the L9 level. In most cases, however, the mineral coexists

7 34(3), 1984 Mineralization of the Arakawa No. 4 Vein of the Kushikino Mine 193 Fig. 7 Histogram of the FeS content of sphalerite. Shaded area for period IIa, the open area for period IIb. The solid line represents the FeS content of sphalerite coexisting with pyrite and the dashed line represents other paragenesis. Fig. 8 Filling temperature of fluid inclusions in quartz coexisting with ore minerals from the Arakawa No. 4 vein. with pyrite and chalcopyrite. At the L6 level, sphalerite is not recognized. Figure 7 represents the FeS contents of sphalerite. In the IIa period, FeS content of sphalerite from the L8 level is smaller than that from the L9 level. The sphalerite from period IIa contains higher amounts of FeS than that from period IIb at Level 8. Manganese content is below the detection limit of E.P.M.A. (less than 0.1 weight %). Its cadmium content is usually below I wt. %. 5. Physico-Chemical Environment of Ore Deposition The size of the fluid inclusions is mostly below 10 microns and thus the fluid inclusions suitable for the measurements were not many. Filling temperatures of about 100 fluid inclusions in 7 samples from periods I, II, and IV were measured. Many of the fluid inclusions are in quartz coexisting with ore minerals. Figure 8 suggests that no appreciable variations of temperature exist from the lower to the upper level in the period II. The filling temperature decreases slightly, however, from the earlier to the later periods. Sulfur fugacity (fs2) is estimated from the relation between the temperature determined Fig. 9 Schematic variation trends of sulfur fugacity of the Arakawa No. 4 vein. The solid arrow represents the variation from lower to upper levels, and dashed arrow from earlier to later periods. NFes: mole fraction of FeS in sphalerite, Py: pyrite, Po: pyrrhotite. from the fluid inclusion study and the FeS content of sphalerite coexisting with pyrite based on the experimental study by BARTON and TOULMIN (1966). The fs2 for period II is estimated to be atm. based on the filling temperature data (200 Ž-220 Ž) and the mole fraction of FeS in sphalerite as in Figure 9. As represented in Figure 6, the selenium content of polybasite becomes higher with the

8 194 K. TAKEUCHI and N. SHIKAZONO MINING GEOLOGY: elevation of the level and from earlier to later periods of mineralization. The same tendency is also observed in selenium content of naumannite-acanthite series minerals (Table 2), that is, selenium content of this mineral increases from the lower to upper levels. In order to consider the causes for the variation in selenium content of these minerals the following exchange reaction has to be taken into account, (MS)+Se2-=(MSe)+S2-(1) where (MS) and (MSe) represent MS and MSe components in the solid solution M(S,Se) and S2- and Se2- are the simple ionic species of sulfur and selenium in the aqueous solution. The equilibrium contant, K, for the reaction (1)is defined by, K=aMSeaS2-/aMsaSe2- where a denotes activity. From this equation, we obtain, mmse/mms=k(ase2-/as2-)(ƒáms/ƒámse) where m represents mole fraction, and ƒá represents activity coefficient. Therefore, it is reasonable to consider that the ratio of mmse/mms in sulfide positively correlates to ase2-/as2- in aqueous solution at constant temperature and pressure. It was shown by SHIKAZONO (1978) that the ase2-/as2- is represented as functions of several parameters such as ƒ Se, ƒ S, ph, fo2 and temperature. Therefore, it is obvious that the selenium content of sulfide mineral depends on the factors mentioned above. Considering the effects of the factors mentioned above on the selenium content of sulfide, the increase of selenium contents from earlier to later periods could be caused by the following four processes; (i) increase of oxygen fugacity under the oxidized sulfur species predominant region, (ii) variation of ph from neutral to acid condition under the reduced sulfur species predominant region, (iii) variation of ph from neutral to strongly alkaline condition under the reduced sulfur species predominant region and (iv) decrease of temperature under the oxidized sulfur species predominant region. Increase of oxygen fugacity under the oxidized sulfur species predominant region at constant temperature is accompanied by decreasing of fs2. Therefore, the process (i) alone cannot explain the decreasing of FeS in sphalerite coexisting with pyrite from period Ila to period IIb. Although the detailed discussion cannot be carried out here due to the lack of information about the precise values of oxygen fugacity, it is thought that the gold-silver minerals from the epithermal vein-type deposits in Japan have been formed under the oxidized sulfur species predominant region (e.g., SHIKA- ZONO, 1974). Therefore, the process (ii) seems also unlikely. In the Arakawa No. 4 vein, sericite coexists with polybasite indicating that the ph region in which polybasite was formed is considered to be from acid to neutral. It is also observed that calcite is poor at the upper levels but abundant at the lower levels. In general, calcite is stable under the neutral condition. Considering the changes of abundances of sericite and calcite with elevation of level in a same period of mineralization, it is inferred that ore solution becomes acidic during the mineralization. Therefore, the process (ii) could be impossible to explain the variation of selenium content of polybasite. The activity ratio, ase2-/as2-, decreases with decreasing of temperature under the reduced sulfur species predominant region. Therefore, this process alone cannot explain the variation in Se contents of polybasite and naumanniteacanthite series minerals. In contrast, ase2-/ as2- increases with decreasing of temperature under the oxidized sulfur species predominant region. Considering the variation of filling temperature during the period of mineralization and decreasing of FeS in sphalerite from earlier to later stages, this process seems highly likely, although it is probable that this process accompanies the other processes such as increasing of oxygen fugacity and decreasing of ph, considering the changes of abundances of sericite and calcite and selenium content of polybasite from lower to upper levels and from earlier to later periods. Acknowledgments: It is pleasure to a- knowledge the assistance provided by the stuff

9 34 (3), 1984 Mineralization of the Arakawa No. 4 Vein of the Kushikino Mine 195 of exploration division of the Mitsui Kushikino Mining. Co., Ltd. including Messers. K. UEMURA, M. SAITO, K. KOMUTA, K. YAMAZAKI, T. KAMIKIYA, and especially M. SAITO, the chief geologist, for the suggestive advices and discussions. The authors take supreme pleasure in expressing whole heartly thanks to Prof. J. T. IIYAMA, Prof. H. SHIMAZAKI, and Dr. T. URABE of the University of Tokyo for their kind instruction and helpful criticisms throughout this work. Prof. S. TAKENOUCHI of the University of Tokyo provided laboratory facilities for fluid inclusion studies. Mr. M. KAWASAKI of the Mitsui Mining Smelting Co., Ltd. kindly provided the samples of the Kushikino mine. References BARTON, P. B. Jr., and TOULMIN, P. III. (1966): Phase relations involving sphalerite in the Fe- Zn-S system. Econ. Geol., 61, 815 `849. HALL, H. T. (1967): The pearceite and polybasite IZAWA, E., YOSHIDA, T., and SAKAI, T. (1981): Fluid inclusion studies on the gold-silver quartz veins at Kushikino, Kagoshima, Japan. Mining Geol. Spec. Issue, 10, 25 `34. MOTOMURA, Y., YAMAMOTO, S., WAKABAYASHI, K., and HIROWATARI, F. (1981): Gold-silver ores from the Arakawa No. 3 vein of Kushikino mine, Kagoshima Prefecture. Mining Geol., Spec., Issue, 10, 15 `23 (in Japanese with English abstract.). SHIKAZONO, N. (1974): Physicochemical environment and mechanism of volcanic hydrothermal ore deposition in Japan, with special reference to oxygen fugacity. Univ. Tokyo Fac. Sci. Jour., 70, 694 `705. SHIKAZONO, N. (1978): Selenium content of acanthite and the chemical environments of Japanese vein type deposits. Econ. Geol., 73, 524 `533. SUKESHITA, M. and UEMURA, K. (1976): Recent exploration of the Arakawa veins, Kushikino mine, Kagoshima Prefecture. Mining Geol., 26, 165 `177 (in Japanese with English abstract). SWEATMAN, T. P. and LONG, J. V. P. (1969): Quantitative electron-probe microanalysis of rock-forming minerals. Jour. Petrol., 10, 332 `379. series. Amer. Mineral., 52, 1311 `1321.