黒鉱鉱床における脈状鉱の成因的意義

Abstract

Veins, veinlets, and networks occur in and around stratified Kuroko deposits for which the vein-type mineralization is genetically important. It is necessary for the exhalative sedimentary hypothesis of Kuroko deposits to be accompanied by the solution-feeding pass under beneath the stratified massive ore. On the contrary, it was also necessary for the epigenetic replacement hypothesis of them to be associated the veinlets in the hanging wall. This key evidence shows that the mineralization occurs in the hanging wall of stratified massive ore, so that the massive ore itself was considered to be the result of replacement in the underground. According of the sedimentary hypothesis, it is, however, explained that the post-Kuroko mineralization took place after the time of deposition of stratified massive ore which is sedimentary.A lot of observations in the Fukazawa Kuroko deposits, Northern Akita Prefecture, depict the various mode of occurrence of the vein-type mineralization in and around the stratified Kuroko deposits. The distributions of massive and vein-type ores are subdivided as follows:1) Stratified massive gypsum-anhydrite-(pyrite) ore in the footwall.2) Pyrite-quartz veins, copper-rich and zinc-poor veins, and networks of veinlets in the footwall with a strong silicification.3) Zinc-rich veins and networks of veinlets in the footwall. They are silicified weaker with clay alteration.4) Stratified massive pyrite ore in the bottom of the stratified sulfide bed.5) Copper-rich and zinc-poor massive ore in the lower and upper part of the stratified sulfide beds.6) Zinc-rich massive ore in the most part of the stratified sulfide beds. Sedimentary textures are most excessive in this mode.7) Massive barite lens in the upper part of the stratified sulfide beds.8) Hematite-bearing chert lens or veinlets in the hanging wall.9) Veinlets in the hanging wall. The boundaries between the different modes of massive ores are always clear and sharp.The vein-type mineralizations in the footwall continue about 100 meters to 200 meters long or more to the lower part. They are divided into two main modes; copper rich and zinc-rich. Copper-rich one belongs to normal-type veins or networks of veinlets which configuration shows zonal structures and drusy pipes. Zinc-rich one is more “massive” and compact which has no zonal structure in the internal. The distributions of the two are separated into the central copper-rich and the marginal zinc-rich parts of the stratified extent. The stratified sulfide beds include some powdery are or lenticular copper-rich veins. The powdery “intrusion” of copper-rich are to the zinc-rich massive bed connects with lower and upper part of the massive copper-rich are in part.Zinc-rich fragments of massive are and zinc-rich and copper-rich veinlets are frequently observed in the hanging wall, especially in the basaltic lava. Copper-rich fragments also occur in the hanging wall a little, but are rare in the basaltic lava and post-basaltic lava sediments.