Deposition of metals in the polymetallic veins at Akenobe mine:products of a fossil geothermal system

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The copper, zinc and tin veins of the Akenobe mine are of polymetallic xenothermal type deposits. The following mechanism of metal precipitation is deduced based on the assay contour maps of the veins. (i) Ore shoots in a vein coincide with wider parts of the vein fracture and plunge northwest. (ii) These ore shoots in a vein are aligned along a zone which plunges southeast. (iii) A southwestward dipping plane is obtained by connecting these zones of parallel running veins in the mining area. (iv) A Zn-rich zone is situated at levels upper than that of Cu-rich zone throughout the ore field. A Sn-rich zone is situated at the lowest level in the northeast area. However, in the south area, it is situated at the same level as the Cu and/or the Zn-rich zones. These modes of metal zoning can be explained by reasonable assumption that the hydrothermal solution rose perpendicularly to the flat-lying isothermal planes passing mainly through the wider part of vein fractures. The isothermal plane is considered to be nearly horizontal at a discharge area as is the case of recent geothermal systems. Therefore, it is estimated that the Akenobe ore deposits are wholly tilted by post ore tectonic phenomena toward south. The volume of the ore-forming solution, the total thermal energy, and metal concentration in the solution are estimated to be 4.8×1010m3, 4.7×1016kJ and 20mg/kg, respectively.
資源地質学会の論文