Volcanic-Sedimentary Features in the Matsumine "Kuroko" Deposits, Hanaoka Mine

Abstract

Recent geological studies on the representative Kuroko deposits at Matsumine, Hanaoka Mine, have revealed many interesting features, which uphold the submarine exhalative sedimentary theory for their origin. The process of the ore deposition inferred from those features can roughly be summarized as follows :
Taking general view of the mode of submarine acid (dacitic in chemical composition) volcanism prior to the mineralization, there seems to be two eruptive cycles in the Hanaoka District. The mineralization followed to the dacite lava doming which demonstrates the final stage of the second cycle. This lava dome is thought to have never come out on the sea bottom. The pressure that raised lava dome up would have been released at the level of tuffs and tuff breccias which were still highly saturated with sea water and unconsolidated.
The mineralization started with the exhalation of silica-rich hydrothermal solution and chemical properties of the solution was gradually changed from Ca-rich to metallicion-rich one. The exhalation was led through the marginal contact zones of lava domes. or the cracks through them.
Judging from the existene of relict breccias in zones of silicified rocks and gypsum beds, silica and gypsum are thought to have been precipitated in a closed system in the tuffs and tuff breccias, while most of the sulfide minerals deposited in open space at sea bottom or at shallow part of the sediments.
The mineralization was multi-centered as shown by the distribution of lava domes, strongly silicified parts of tuff brecciss or lava domes, anhydrite-prominent zones in gypsum beds, and veins of sulfide minerals. Productivities and durabilities of the mineralization centers were slightly different each other. The more productive and active one went over two or three repetition with tendency of decreasing activity and branching of the center to the later stage. On the other hand, physicochemical properties of the ore-forming hydrothermal solution may have gradually changed through those repetition mainly by cummulative mixing of sea water into the solution not only at the deeper area but even at the sea bottom itself. The commonly observed stratified zoning of the ores, such as barite ore, barite-Pb-Zn-Cu-Ag-bearing polymetallic sulfide ore, cupriferous pyrite ore, pyrite ore and gypsum ore, in descending order, may depend on those changes.
In proportion as the minerals were precipitated and accumulated onto the unconsolidated and mobile (or semi-liquid) beds (argillized tuffs or gypsum), some surface parts of the ores were deformed and caused the slumping or turbulent flow on account of heterogeneity and gravitational unbalance between the ores and surrounding materials, forming characteristic sedimentary features such as transposition-structure, banded structure, graded bedding, rhythmic alternation of ore beds and tuffs, groove structure, imbricate structure, slump folding and nodular ores with concentric mineral zoning. Rather round shaped-small scale ore bodies may have been formed by auto-gravitational grooving into argillized tuffs. Many kinds of these . structures and some microscopical textures of the ore indicate also that the deformation took place while the ores were in plastic condition.
As the subsidiary cause of the above mentioned "penecontemporaneous plastic (or rheomorphic) deformation", movement of the lava domes adjacent to the ore field must be taken into consideration.