Ore and skarn mineralogy of the Eboshi deposit of the Naganobori copper mine, Yamaguchi, Japan

摘要

Mineral assemblages and chemical compositions of ore minerals from the Eboshi deposit, the historical Naganobori copper mine, Yamaguchi Prefecture, Japan were investigated in order to clarify its characteristics as a skarn deposit. Some Bi–, Ag–, and Te–bearing minerals are newly identified, which contribute updating the mineralization sequence of this deposit. Samples collected from the mine dump are one massive magnetite ore, and copper ores associated with skarn gangue minerals. Skarns are categorized as clinopyroxene skarn, garnet skarn, and wollastonite skarn, and the clinopyroxene skarn is the most dominant. The major ore minerals are chalcopyrite, cobaltite, and early–stage pyrite (Py–I) and later stage pyrite (Py–II). Py–II is enriched in arsenic (~ 5.19 As wt%). The Bi–, Ag–, and Te–bearing minerals, such as native bismuth, bismuthinite, wittichenite, emplectite, tsumoite, kawazulite, hessite, and matildite are minor ore minerals. Based on the mineral assemblages and textures of the specimens examined, four ore mineralization stages were recognized;the ore mineralization stage I is characterized by the major ore minerals such as chalcopyrite, bornite, pyrrhotite, sphalerite, and Py–I. The stage II is defined by the mineralization of cobaltite, Py–II, and Bi(–Cu)–bearing sulfides such as native bismuth, bismuthinite, and wittichenite. The mineralization stage III is characterized by the Ag– and/or Te–bearing ore minerals such as matildite, kawazulite, tsumoite, and hessite. The stage IV is characterized by chalcopyrite veins cutting the main skarn masses and the host limestone. The mineralogical properties and mineralization process of the Eboshi deposit is similar to those of the skarn deposits in the Yamato mine and the Tsumo mine, and consistent with common skarn–type deposits associated with ilmenite–series granitoids in the San–yo Belt, which are characterized by the occurrence of minor Ag– and/or Te–bearing ore minerals.