1988 Volume 38 Issue 210 Pages 335-346
Hosokura Pb-Zn ore deposits consisting of fissure-filling veins of epithermal to mesothermal type, occur in regionally altered andesitic lava and pyroclastics of the Hosokura Formation of Miocene age in northeastern Japan. Superimposed on the reginal alteration, the ore-related alteration is seen along veins, and this alteration was studied in the Fuji-Honpi vein area.
Based on the mineral assemblages, the wall rock alteration around this vein is divided into three zones, A, B and C, from the vein side outwards. Characteristic mineals are K-feldspar in the A zone, K-feldspar and albitized plagioclase (An 10-20%; whereas, An 40% in the regionally altered rock) in the B zone and albitized plagioclase (An 30-40%) in the C zone. Sericite and chlorite are seen in all these zones, but 001 reflection (2θ) of the sericite shift to the higher angle side and the chlorite becomes Fe2+ rich towards the vein. Both A and B zones are characterized by addition of K2O, and leaching of Na2O and CaO; thus an alteration index, K2O/(K2O+Na2O+CaO), is proposed. The index is inversely correlated with magnetic susceptibility (r=-0.98) and positively (r=+0.88) with "migration degree" of the chemicals which was obtained from loss and gain calculation.
The alteration zones are extensively developed in brecciated rocks of two generations: one along the ore vein which was originally fault breccia, while the other along the top of andesitic lava which was flow-top breccia at the paleo-surface. Width of the A and B zones is generally larger in the upper horizons wherever the permeable rocks exist.
It is considered that upward migrated heated meteoric and magmatic fluids along fault zone in andestic lava met a large amount of cool meteoric water contained in porous pyroclastic rocks (Fig. 11). Temperature of the fluid was dropped off, then high-grade ores were formed in the andesitic lava. The bonanza is also shown by the amounts of normative quartz in the altered rocks (Fig. 12). Application of this study for mineral exploration is given in the flow chert of Fig. 13.
