Journal of Mineralogical and Petrological Sciences, Volume 113, page 293–309, 2018 Ore–microscopy and Geochemistry of Gold–Silver Telluride Mineralization in Southwestern Hokkaido, Japan Euis T. YUNINGSIH*, Hiroharu MATSUEDA** and Ildrem SYAFRIE* *Faculty of Geology, Raya Bandung–Sumedang Km. 21, Bandung 45363, West Java, Indonesia **The Hokkaido University Museum, Hokkaido University, Sapporo 060–0810, Japan Tellurium–bearing gold, silver, and base metal assemblages characterize the epithermal mineralization in south- western Hokkaido, Japan. The telluride deposits in this district consist of quartz–filled fracture systems of Late Miocene to Pliocene age. Most of the telluride mineralization in southwestern Hokkaido consists of vein sys- tems or are massive in form. The dominant host rocks for this deposit type are intermediate to acid volcanic rocks. The gangue and alteration minerals are very fine to fine–grained quartz, chalcedony, anatase, zeolite, sericite, barite, adularia, chlorite, smectite or other clay mineral, while carbonate (calcite and rhodocrosite) and Mn minerals are poorly represented and rarely coexist with tellurides. The country rocks suffered argillic and propylitic alterations, silicification, and kaolinitization. Tellurium–bearing mineralization is related to or co– exists with sulfide and sulfosalt mineralization. The tellurides identified in the studied area consist of native tellurium, hessite, stutzite, petzite, sylvanite, calaverite, tellurobismuthite, tetradymite, Se–bearing tetradymite, tellurantimony, goldfieldite, altaite, coloradoite, tellurite, teineite, and poughite. The first crystallized telluride minerals such as native tellurium, stutzite, petzite, sylvanite and hessite are followed by calaverite, coloradoite, tellurobismuthite, tellurantimony, and altaite, indicating a decline in tellurium activity during telluride deposi- tion with time. Microthermometry of fluid inclusions in quartz and barite indicate that the mineralization formed at moderate temperatures, ranging from 250–200 °C with low average salinity (1.9–5.7 wt% NaClequiv.). The shift in mineralogy reflects decreases in temperature and fugacity of sulfur, with a concomitant increase in fugacity of tellurium. Further decrease in fugacity of tellurium (coupled with decreasing fugacity of sulfur) resulted in deposition of Au–Ag–Te and other tellurides. Keywords: Fluid inclusion, Native tellurium, Tellurium fugacity, Telluride minerals, Sulfur fugacity INTRODUCTION mal ore deposits of the Neogene period in this area, such as Teine mine, Sapporo; Toyoha mine, Sapporo; Hokkaido is the northernmost of Japan’s four main islands Todoroki mine, Yoichi; Chitose mine, Chitose; and and bounded by the Pacific Ocean to the south and east, Koryu mine, Ishikari. Previous studies reported more the Sea of Japan to the west, and the Sea of Okhotsk to the than twenty mines in the southwestern Hokkaido district, north. The Neogene metalliferous resources of Hokkaido some of which had records of production (Yajima, 1979). are gold and silver vein–type ore, copper, lead and zinc ore Those mines have been closed during the past forty years in the form of vein–type or Kuroko–ore (Yajima, 1979). due to the exhaustion of ores. However, nowadays there are no working metal mines in In many epithermal and massive sulfide deposits, Hokkaido. gold and silver are contained in the form of tellurides The southwestern district of Hokkaido has produced and/or selenides (Lindgren, 1933; White and Hedenquist, a valuable amount of gold, silver, lead, and zinc metals 1995). According to Lindgren (1933) and Heald et al. (Shikazono, 1986). This production has occupied an im- (1987), gold correlates strongly with Te but these relation- portant portion of the mineral resources of Japan. These ships are not clear since there is insufficient thermody- resources are related to the occurrence of many epither- namic information on Te aqueous species and their com- plexes under hydrothermal conditions. While sulfide doi:10.2465/jmps.171017 sulfur, a close chemical analogue of Te and Se, known E.T. Yuningsih, [email protected] Corresponding author to a greater extent, controls on Au and Ag solubility and 294 E.T. Yuningsih, H. Matsueda and I. Syafrie Figure 1. Southwestern Hokkaido, situated at the junction between the N–S trending northeast Honshu Arc and the ENE–WSW Kuril Arc (modified after Watanabe, 1990; left). Simplified geological map of the southwestern Hokkaido (modified after Wauschkuhn et al., 1977; right). The locations of deposits in the current study are noted as follows: 1, Suttsu; 2, Kobetsuzawa; 3, Teine; 4, Chitose; 5, Date. transport in hydrothermal fluids and fairly stable hy- 1989, 1990), Yahata (2002) etc. The gold–silver deposits drosulfide complexes of Au and Ag metals formation in southwestern Hokkaido, including major vein–type (Barnes, 1979; Gammons and Barnes, 1989; Benning deposits such as Chitose, are located at the intersection and Seward, 1996; Yuningsih and Matsueda, 2014). Com- between the volcanic chain and the strike–slip shear zone. prehensive experimental studies for telluride systems The Pliocene or younger ages vein in the district mostly were performed by Cabri (1965), Afifi et al. (1988) etc. strike E–W and are oblique to the NW–SE volcanic for the purpose of understanding the formation conditions chains (Watanabe, 1986). Thus, the arc–arc junction is of some natural telluride ores. one of the most suitable sites for vein type mineralization The present study characterizes the mineralogy, in this area (Watanabe, 1990; Fig. 1). compositional variations and clarify the formation condi- Bamba (1977) defined southwestern Hokkaido not tion of the telluride mineralization occurring in Au–Ag only as one geologic unit but also as one metallogenic deposits of southwestern Hokkaido. Geological features, province. The deposits investigated here are a part of the ore mineral characteristics and fluid inclusion microther- second sub–province and are mostly polymetallic ore of mometry are synthesized in the present study to under- fissure type which is characterized by violent volcanism stand their mutual relationships and to constrain the ther- and upheaval movement. The Neogene volcanic and vol- mochemical conditions of ore formation. caniclastic rocks predominate in the district, while gran- ites, rhyolites, and sedimentary rocks from the Creta- REGIONAL GEOLOGY ceous and Paleogene ages are scattered in limited areas. Most deposits are intimately related to extrusive rocks Southwestern Hokkaido geologically is the northern ex- ranging from andesite to rhyolite and some other deposits tension of North Honshu Arc, characterized by the exten- are related to acid intrusives. Mineable vein–type deposits sive distribution of Neogene and Quaternary volcanic in this area are generally hosted by hydrothermally al- rocks. Oblique subduction of the Pacific Plate caused tered Miocene strata (e.g., MITI 1973, 1985) overlain dextral strike–slip movement at the southwest end of by non–altered Pliocene to Pleistocene andesite lavas. the Kuril Arc and westward dislocation of the frontal The K–Ar dating of gangue and alteration minerals Kuril arc (Kimura et al., 1983; Fig. 1). The geology of of the epithermal veins type deposits in southwestern the southwestern Hokkaido region in relation to mineral- Hokkaido has revealed that they were placed in the Mio- ization, has been described in detail by Ishihara (1974), cene – Pliocene or younger age (around 0.8–8.1 Ma, Table Bamba (1977), Yajima (1979), Watanabe (1986, 1987, 1). Watanabe (1989) proposed three genetic stages of Telluride Mineralization in Southwestern Hokkaido, Japan 295 Table 1. K–Ar dating of selected gold–silver deposits counting times. Standards used were natural chalcopyrite, MnS, CdS, FeAsS, Sb2S3, PbS, SnS, HgS, ZnS, and el- emental Se, Au, Ag, Te, and Bi. The characteristic X–ray lines measured are As, Se, Te, Cd, Ag, Bi, and Sb(Lα), S, Cu, Zn, Fe, and Mn(Kα) and Pb, Au, and Hg(Mα). The raw data was corrected using the ZAF method. The X–ray energy–dispersive and backscattered electron images from the JEOL JSM–5310 scanning electron microscope are also used to map the co–existing ore minerals. Microthermometric analysis of fluid inclusions was performed using a Linkam THMSG 600 system attached to a Nikon transmitted–light microscope. The heating rate was maintained near 2 °C min−1 for measurement of ho- −1 vein–type deposits in the district based on geologic struc- mogenization temperature (Thtotal) and 0.5 °C min for tures such as latest Middle Miocene, Late Miocene, and measurement of ice melting temperature (Tm). Precision Pliocene to Pleistocene. Among these, the last is the most is calculated as ±0.1 °C in the temperature range of the productive one. observed phase changes. Accuracy between −60 and −10 An arrangement of Pb–Zn zone (Toyoha), Cu–Sb °C is estimated in the order of ±0.2 °C, whereas between sulphosalt zone (Shin– Oyoto), Au–Ag–Cu–Sb–Bi zone −10 and +30 °C and above +200 °C it is placed at ±0.5 (Teine), and gold–quartz zone (Chitose) is observed from and ±2 °C, respectively. Salinity is determined from the the inner to the outer part in the area (Yajima, 1979). The last melting temperatures of ice, utilizing the equation by ores of Teine mine are different to typical gold–quartz Bodnar (1993). veins of the outer zone. This characteristic was interpret- ed as an intermediate character between Au–Ag and Pb– RESULTS Zn deposits. Koryu and Chitose represent vein type Au– Ag deposits with abundant quartz as