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Polymetallic Mineralization at the Toyoha Mine, Hokkaido, Japan
| Title: | Polymetallic Mineralization at the Toyoha Mine, Hokkaido, Japan |
| Other Titles: | 豊羽鉱山の多金属鉱化作用 |
| Authors: | Ohta, Eijun1 Browse this author |
| Authors(alt): | 太田, 英順1 |
| Keywords: | Toyoha | | Polymetallic vein | | Silver | | Lead | | Zinc | | Tin | | Indium | | Bismuth | | Cobalt | | Formation condition |
| Issue Date: | 25-Mar-1991 |
| Publisher: | Hokkaido University |
| Abstract: | Toyoha is the largest lead－zinc－silver vein－type deposit in Japan．A remarkable feature of this deposit is occurrence of rare metals； tin， indium， wolfram， cobalt， nickel， bismuth， gallium， antimony and arsenic．Among them， indium concentration is extraordinary high enough to make the mine a leading indium producer in the world． Trace amounts of molybdenum, vanadium, selenium and tellurium are also detected in the,veins and drill cores from the mining area. Two stages of the mineralization at Toyoha， the earlier and the later, are recognized as cutting relations of the veins， and the later is classified into successive five substages A， B， C， D and E． Detailed underground and microscopic observations， and EPMA analyses haverevealed that the rare metals derived from the ore solution during the substage B． Tin and indium are detected in cassiterite， stannite，kesterite， rhodostannite， roquesite， an unnamed Zn－In mineral， an unnamed Ag－In mineral， hocartite， pirquitasite， toyohaite （Ag analogueof rhodostannite）， berndtite， herzenbergite， teallite， sphalerite， chalcopyrite and solid solutions among these phases． Bismuth is in matildite， bismuthinite， gustavite， native bismuth， galena and solid solutions among these minerals， and in tetrahedrite． Cobalt and nickelexist in the cobaltite－arsenopyrite solid solution， and are detected also in pyrite as minor component． Arsenic does not make sulfosalt with silver as main component in the Toyoha deposit， though silver - antimony and silver－lead－antimony sulfosalts are common ore minerals． Gallium is detected as a minor component in sphalerite， though no germanium is found to date． Wolframite and two types of unknown Mo－Pb－Sb sulfides also occur． Silver occurs as native silver， argentite and electrum in the earlier veins which have been significantly modified by the later－stage mineralization, while it exists in the silver－antimony and silver－lead－antimony sulfosalts， the tetrahedrite－tennantite solid solution， Ag-Sn minerals， the Ag－In mineral， Bi-Pb-Ag-S minerals， and electrum in the later veins． Formation of argentite in the earlier veins is attributed to the later－stage mineralization． That is， deposition of argentite in the earlier veins and the silver sulfosalts in the later veins proceeded simultaneously； as oxidation of silver－sulfide complexes by reaction with hematite in the earlier veins， and as dissociation of silver－chloride complexes by dilution of the ore solution in the later veins； during the substage C． Deposition of the metals occurred upon mixing of the hydrothermal fluid from a granitic source with ambient geothermal water of meteoric origin． The temperature and NaCl equivalent concentration of the initial ore solution before the mixing were higher than 250℃ and 2.3 weight percent for the Harima-Tajima trend （earlier－stage mineralization）， and higher than 300℃ and 4.2 weight percent for the lzumo trend （later－stage mineralization）． Especially for the rare－metal mineralization， initial hydrothermal fluid at 350 to 400℃， salinity of 5 to 7 equivalent weight percent NaCl， fo2 below the upper limit of pyrrhotite， fS2 at around the pyrrhotite－pyrite boundary， is consistent with data from fluid inclusions and mineral－assemblages at Toyoha． The temperature and salinity of the ambient water are estimated to be 160 to 210℃ and 0.35 weight percent for the earlier stage， and 210 to 250℃ and 0.5 weight percent for the later． The earlier - and the later－stage evolution trends of the ore solutions on the temperature versus fs2 diagram correspond to the trends caused by magnetite- and ilmenite－series granitoids respectively． These trends and the occurrence of tin and indium in the later veins suggest that the later-stage mineralization was initiated by an ore solution emanated from an intrusion of ilmenite-series granitoid， and that the earlier was derived from a magnetite－series granitoid． The ilmenite-series granitoid may have been evolved through local reduction of a magnetite－series magma by contamination of organic materials in the Usubetsu Formation． The chronological data， the zonal distribution of minerals， grades of metals and fluid inclusions， and the flow vector ofthe currently active hydrothermal solution found in the Shinano vein imply that the source of the ore solution is a latent granitoid intrusion below the Muineyama Andesite. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 乙第3863号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/49656 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （理学）
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Submitter: 太田 英順
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