Chemical Composition of Ores from the Takara Volcanogenic Massive Sulfide Deposit, Central Japan

Abstract. The Takara volcanogenic massive sulfide (VMS) deposit occurs in Miocene formation of the Misaka Mountain, the South Fossa Magna region, central Japan. The tectonic setting of the Misaka Mountain is reconstructed to be a part of the paleo Izu-Ogasawara arc which collided with the Honshu arc and to form accreted body in the present position. The Takara deposit, therefore, is considered to have formed in the paleo Izu-Ogasawara arc.
The ores from the Takara deposit are classified into pyrite-type ore, chalcopyrite-type ore, and sphalerite-type ore on the basis of chemical composition and their mineral assemblages. Some pyrite-type ores are characterized by their high Au content. The Au content is hardly recognized in the chalcopyrite-type and sphalerite-type ores.
The ores from the Takara deposit have intermediate bulk chemical composition between those from the Besshi-type deposits and the Kuroko-type deposits that are two representative VMS deposits. However, the bulk chemical composition is closer to that from the Kuroko-type deposits. And moreover, chemical composition of tetrahedrite-tennantite series minerals (tetrahedrite) is similar to that from the Kuroko-type deposits. The bulk chemical composition (Cu, Zn, Co, Pb, and As contents) of ores is affected by the chemical composition of volcanic rocks associated with VMS deposits.     

Whole-rock Geochemistry of Basic Schists from the Besshi Area, Central Shikoku: Implications for the Tectonic Setting of the Besshi Sulfide Deposit

Abstract. Whole-rock chemical compositions of the Besshi basic schist closely associated with the Besshi massive sulfide deposit from the Sanbagawa Belt are reported. Studied samples were collected from four outcrops around the Dozan-goe, central Shikoku. Common metamorphic mineral assemblage of the basic schist is albite + epidote + actinolite + chlorite ± muscovite ± quartz. Major element contents are similar to those of typical tholeiitic basalts. Trace element patterns of the basic schist normalized to normal mid-ocean ridge basalt (N-MORB) are generally flat, although concentrations of highly mobile large-ion lithophile elements are quite variable. Chondrite-normalized rare earth element (REE) patterns are flat to slightly light REE-depleted patterns. In the Hf-Th-Ta and Nb-Zr-Y discrimination diagrams, basic schist samples closely associated with the sulfide deposit are plotted within the N-MORB field. The Th/Nb ratios of the basic schist are also comparable to those of N-MORB. These geo-chemical lines of evidence indicate that the protolith of the Besshi basic schist is N-MORB and the Besshi sulfide deposit was formed by hydrothermal activity in conjunction with MOR volcanism.     

别子型火山成因块状硫化物矿床的地质和地球化学特征

别子型矿床最早形成于古元古代,并在显生宙海沟环境或弧前盆地广泛发育,其成矿区域发育厚层沉积岩地层,覆盖在火山岩之上,对成矿流体物质交换和金属元素富集有重要意义。别子型块状硫化物矿床下盘发育强烈的黄铁绢英岩化,而上盘仅发育微弱的绿泥石化、碳酸盐化。矿床常呈单个矿体产出或2~3个矿体连生,剖面上,由下至上表现为枕状玄武岩→块状含铜黄铁矿矿石→块状、条带状燧石黄铁矿矿石→块状碧玉岩。别子型矿床的火山岩围岩多为钙碱性系列,少量拉斑系列,与活动大陆边缘的岛弧火山岩具有相似的微量和稀土元素地球化学特征。成矿流体中的硫为幔源硫和海水硫的混合来源,成矿元素来源为幔-壳混合源。