Ore Formation Processes of The Mozumi Skarn-type Pb-Zn-Ag Deposit in the Kamioka Mine, Gifu Prefecture, Central Japan

The Mozumi skarn-type Zn-Pb-Ag deposit of the Kamioka mine occurs in crystalline limestone of the Hida gneisses. The ore formation processes are divided into five periods: clinopyroxene-Zn-Pb, calcite-quartz-Pb-Zn, actinolite-Cu, quartzecalcite-Cu and quartz-Ag periods, in order of formation. The clinopyroxene-Zn-Pb period began with the formation of huge amounts of clinopyroxene (Di10-35Hd43-75Jo11-35) at 400-330•Ž, associated with minor amounts of grandite garnet (Ad16-78), calcite, quartz, Ag-and Bi-bearing galena, native bismuth, bismuthinite, Ag-Pb-Bi-S minerals and Fe-poor sphalerite(3-8 FeS mole%). The second stage (240-12•Ž) of the first period was the main sulfidation stage. The sulfide minerals, which had already begun to crystallize during the first stage, vigorously formed in this second stage and replaced earlier-formed pyroxene and garnet. During the late second stage, the ore fluid increased in Fe content and produced hedenbergetic pyroxene (Hd53-88) and Fe-rich sphalerite (9-14 FeS mole%), which replaced portions of earlier-formed pyroxene and sphalerite, respectively. During the first stage of the calcite-quartz-Pb-Zn period, early calcite(400-270•Ž), quartz (360-310•Ž) and Fe-poor sphalerite (400-270•Ž; 3-7 FeS mole%) associated with Ag-and Bi-bearing galena, native bismuth, bismuthinite and Ag-Pb-Bi-S minerals were formed by replacing limestone and a part of the clinopyroxene-Zn-Pb ore. In the second stage (300-230•Ž), infiltrated Fe-rich ore fluid produced Fe-rich sphalerite (7-21 FeS mole%) which replaced early sphalerite and minor amounts of chalcopyrite, pyrite, pyrrhotite, arsenopyrite and magnetite. Ag-and Bi-bearing galena, native bismuth, bismuthinite and Ag-Pb-Bi-S minerals continued to precipitate at this stage. During the succeeding third stage (230-150•Ž), late galena (Ag-poor), freibergite and Fe-rich sphalerite (10-14 FeS mole%) were formed. At the end of this stage, a minor amount of hematite crystallized in association with late calcite, quartz and epidote. The actinolite-Cu period was characterized by hydration and Fe and Cu metasomatism of the early-formed barren clinopyroxene skarn and clinopyroxene-Zn-Pb ore. During this period, clinopyroxene was extensively replaced by actinolite in the lower levels of the deposit, associated with quartz, arsenopyrite, magnetite, pyrite,pyrrhotite, chalcopyrite, Fe-rich sphalerite and other sulfide minerals. Ore formation during the quartz-calcite-Cu and quartz-Ag periods was local. Ores produced during these five periods are thought to have been formed by different hydrothermal circulation systems that were controlled by a common igneous heat source.

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