Mode of Occurrences of Vugs or Microvugs and Their Significance to the Epithermal (or Xenothermal) Mineralization at Ashio Mine, Tochigi Prefecture.

Abstract

The vugs or microvugs in Ashio Mine are found in three types or modes of occurence, i.e., those in chert, in rhyolite-porphyry (or quartz-porpyry) and in vein. In chert, the writer observed eight examples of vugs, from which the following conclusions are made. Vugs in chert are arranged near the veins or pebble-dykes, which gave passage for the upward movement of the mineralizing solutions. It is demonstrated that the solutions come from these veins or pebble-dykes to vugs according to the writer's investigations of the crystal growths or the manner of growths of crystals of one kind of mineral on the crystals of another kind of mineral, after the method already suggested by Newhouse. The formation of vugs was brought about at first by the dissolving of the cryptocrystalline quartz of chert by the hydrothermal solution of an earlier stage, depositing in the next stage crystals of several kinds of minerals, sulphides and silicates. The general successions of vug minerals of Renkeiji-bonanza are as follows in descending stages ; quartz → Fe-Cu-Zn-sulphides (chalcopyrite, sphalerite, galena) → Fe-sulpharsenide (arsenopyrite) → apatite → pyrrhotíte of hexagonal plates, cubic crystals with curved plane of pyrite-marcasite → calcite. The arrangement of a series of vugs referring to the source of mineralizing solution goes sometimes upwards, in another cases, however, downwards. Sometimes, the mineral succession in a series of vugs varies rapidly from one vug to the next, from earlier minerals to the later. These facts may suggest the abrupt addition of cold ground water from above to ascending hydrothermal solutions at the point forming vugs and ore bodies. Each vug in chert is found to have been formed elongated to the direction of the flow of the hydrothermal solutions and to the bedding plane of chert, the passage of the solutions having been mainly via cracks and joints in the thiner chert beds intercalated by thin slate lamina.
The vugs in rhyolite-porphyry or quartz-porphyry at.Honzan district occur mainly in the pseudoconglomerate, named by the writer for the present, which resembles a boulder deposit and consists mainly of rounded or subangular pebbles or boulders of rhyolite-porphyry with a sericite-clay matrix. The origin of this sort of conglomerate remains yet in question, though the writer supposes that it may have been formed in the local kettle-depression in the very center of the rhyolite-porphyry mass, accompanying the fault movement. The faults became the passages of mineralizing solutions, causing natural stoping and the depression. The mineral assemblage in vugs in rhyolite-porphyry resembles those in chert, differing only in the richness of sericite and chlorite. This was caused probably by the chemical or mineralogical differences of the country rocks, for the material of the gangue minerals seems to have been derived in most cases from the leaching of the rock-forming minerals themselves.
From the study of vugs in Ashio Mine, the writer was strongly interested in the effect of groundwater on the hydrothermal solutions, the directions of the movement of solution, some upwards and others downwards from the source, suggesting circulation or convection of hydrothermal solutions. Further investigations in the future will clarify this problem.