Chemical Compositions of Electrum Grains in Ore and Placer Deposits in the Japanese Islands 1
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Chemical Compositions of Electrum Grains in Ore and Placer Deposits in the Japanese Islands 1 Chemical Compositions of Electrum Grains in Ore and Placer Deposits in the Japanese Islands Kazumi Yokoyama1, Shogo Takeuchi2, Izumi Nakai2, Yukiyasu Tsutsumi1, Takashi Sano1, Masako Shigeoka1, Ritsuro Miyawaki1 and Satoshi Matsubara1 1 Department of Geology and Paleontology, the National Museum of Nature and Science, 3–23–1, Hyakunin-cho, Shinjuku, Tokyo 169–0073, Japan 2 Department of Applied Chemistry, Tokyo University of Science, 1–3 Kagurazaka, Shinjuku-ku, Tokyo 162–8601, Japan Abstract Electrum was analyzed from 33 ore deposits and 51 placer deposits in the Japanese Is- lands. Chemical composition of electrum from ore deposit is generally consistent with the classifica- tion by Shikazono and Shimizu (1988): Ag-rich in epithermal type and Ag-poor in hypo/mesothermal type. Although a Hg-bearing electrum was reported from one ore deposit by them, it is confirmed that such electrum is present from seven more ore deposits among the thirty three deposits studied. Core composition of electrum from the placer deposit is roughly consistent with that from near- by ore deposit: Ag-rich in the Green Tuff region and Ag-poor in the Early Cretaceous granitoid re- gion. One specific characteristic of the placer gold is the presence of a pure gold rim. It is probable that this was formed by dissolution and precipitation rather than by simple removal of Ag or bacteria- related biomineralization. Hg was detected in electrum grains from many placer deposits. The distri- bution of Hg in the placer gold sometimes shows an unusual texture such as vermiculate, network and island-like. There is no available information about the origin of such textures. As an accretion of placer electrum is scarcely observed in the placer deposits, it is hard to explain the coarse-grained placer gold or nugget by accretion of fine-grained placer gold grains. As far as core compositions are concerned, it is most likely that the placer gold grains were derived directly from ore deposits. Introduction In AD 749, gold was first reported in the South Kitakami region, Tohoku Province, of the Japanese Islands. It was collected from a placer deposit. Until the 12th century, gold was recov- ered from placer deposits and was used mainly for decorating temple, shrine and buddhism scrip- tures. Mining of ore deposits for gold was started fundamentally from the Warring State period, i.e. 16th century, and gold was mainly used as currency. During the 20th century, more than 250 gold mines were operated in the Japanese Islands (Plate-1). Now, the Hishikari mine is the only working gold mine in the islands. As whole generation have forgotten that placer gold can be col- lected from riverbeds in almost every prefecture (even in Tokyo), a special exhibition was held in 2008 in the National Museum of Nature and Science (cf. Matsubara and Yokoyama, 2008). At the time we collected placer gold in the islands as a part of the project “Study of rare metal in the Japanese Islands” and analyzed placer gold grains in addition to those from ore deposits stored in our museum. The name “gold” used above is strictly gold-silver alloy. Native gold-silver mineral is divided generally into three species: native gold, electrum and native silver on the basis of the chemical composition. The native gold-silver mineral has almost continuous composition from pure gold 2K. Yokoyama et al. to pure silver and does not show any compositional gap. Hence, definitions of three species are usually ambiguous and are different from researcher to researcher (Ramdohr, 1969: Barton and Toulmin, 1964: Boyle, 1970). In this paper, following Barton and Toulmin (1964) and Shikazono and Shimizu (1988), the term “electrum” is used for natural Au-Ag alloy from pure gold to pure silver. As discussed in many papers, including this one, Au-Ag alloy from a placer deposit has often a clear rim containing more than 96 wt% Au (e.g. Groen et al., 1990: Knight et al., 1999a: Chapman et al., 1999). As it is clearly different in origin from core composition of the alloy, it is tentatively called “pure gold”. In addition, the name “gold” is used as “gold mine” and “placer gold” because of their popular usage in the world. There are two types of electrum deposits: ore and placer deposits. Large electrum masses were rarely reported from ore deposit. Although the biggest electrum from the ore deposit in the Japanese Islands exceeds more than 1.5 kg (Tokunaga, 1991), most of electrum grains in ore de- posits are usually very fine-grained, less than 100 µm. Placer electrum has been found in many rivers in the Japanese Islands as discussed later. The size is mostly visible ones by the naked eye, more than 100 µm, larger than those from ore deposit. A large electrum mass is called a nugget. The biggest nugget found in Japan was 768 g from the Hokkaido Province (cf. Yanaga, 2008), un- fortunately far smaller than the biggest nugget, about 76 kg, in the world. As placer gold is gener- ally considered to be a product transported by a river after weathering and eroding of an ore de- posit, the difference in the sizes of electrum grains between ore and placer deposits has been at- tributed to accretion of the placer gold grains during transportation. Chemical compositions of electrum grains from ore deposits in the Japanese Islands have been obtained with an electron microprobe analyzer (EPMA) by many workers (Yamaoka, 1981: Urashima et al., 1981: Sugaki et al., 1981: Soeda and Watanabe, 1981). Shikazono and Shimizu (1988) analyzed electrum from 41 ore deposits from the Japanese Islands and summarized chem- ical compositions of electrum grains from more than 100 mines. Electrum is composed essential- ly of Au and Ag with minor amounts of other elements. Cu, Hg, Sb, Te and Ni were reported as rare elements in electrum (Yamaoka, 1981: Urashima et al., 1981: Sugaki et al., 1981: Soeda and Watanabe, 1981). On the other hand, placer gold has scarcely been analyzed in Japan (Yanaga, 2008). In addition to EPMA analyses, electrum grains in both ore and placer deposits of the world were analyzed by LA-ICP-MS, PIXE and SR-XRF (Dussubieux and Zelst, 2004: Guerra et al., 2008: Constantinescu et al., 2008). The most striking feature reported is presence of an almost pure gold rim on electrum grains from placer deposits. There are many debates about the origin of the gold rim. One group insisted that the rim was due to involvement of budding bacteria. Oth- ers considered that such a rim was formed chemically after deposition. In this study, we analyzed electrum newly from 33 ore deposits and 51 placer deposits in the Japanese Islands and reported chemical variations of electrum grains in both the ore and placer deposits, discussing relationships of electrum grains between ore and placer deposits and the ori- gin of gold rim on placer gold. Provenance studies of ancient gold have been done in Europe (Guerra et al., 2005: Chapman et al., 2006: Constantinescu et al., 2009). In Japan, gold was his- torically used for religious purposes (Buddhism) and currency. Some natural electrum grains were buried at the time of construction of Buddhist temples. These are now national cultural her- itages. The compositions of various electrum grains in the Japanese Islands may contribute where electrum grains used for the cultural heritage were collected. Chemical Compositions of Electrum Grains in Ore and Placer Deposits in the Japanese Islands 3 Electrum in Ore Deposit Ore deposits in the Japanese Islands have been studied by many workers (e.g. Shikazono, 1974: Hattori, 1975: Takeuchi and Shikazono, 1984: Shikazono and Shimizu, 1987). They esti- mated physicochemical environments of ore formation. Shikazono and Shimizu (1988) summa- rized and classified ore deposits in the Japanese Islands roughly into five types: (1) epithermal type, (2) hypo/mesothermal vein-type, (3) Kuroko type, (4) cupriferous bedded iron sulfide-type and (5) skarn type. The first two types are the most common and are subdivided into Au vein- type, base metal vein-type, Au disseminated-type and polymetallic vein-type. The epithermal type deposits occur in Tertiary-Quaternary volcanic regions, mostly in submarine-altered vol- canic regions (so-called the Green Tuff region). The major occurrence of the hypo/mesothermal type is the Kitakami region where Early Cretaceous granitoids intruded into sedimentary and metamorphic rocks. The authors discussed chemical compositions of electrum grains in each type and concluded that the Ag content of electrum is different in different types of deposits. Electrum from the epithermal type is mostly Ag-rich. Only Au disseminated-subtype of the ep- ithermal type contains extremely Ag-poor electrum. On the other hand, most of the electrum grains from the hypo/mesothermal vein-type are Ag-poor. They analyzed electrum grains from 38 ore deposits in the Japanese Islands. In this paper, we analyzed electrum grains from 33 ore deposits. They are registered samples stored in the National Museum of Nature and Science. Many of the localities are the same as those studied by Shikazono and Shimizu (1988). They are listed in Table 1 and their localities are shown in Figs. 2 to 9. Some of electrum grains studied are visible by the naked eye, more than a few mm in length. Relatively large crystals are shown in Plate 2, but the others are very fine-grained and confirmed only under the microscope. Among them, electrum from the Hishikari mine, now the only active mine, is shown in Plate 3. The grain size of electrum is mostly less than 10 µm.