Vein - komyaku (English spelling) vein

A vein is a fissure in a rock that is filled with minerals, and when the minerals can be economically extracted, it is called a vein. There are two types of fissure that can become a vein: shear fractures (faults) and tension fractures. Veins formed in shear fractures are narrow, while veins formed in tension fractures are wide, sometimes reaching more than 10 meters in depth. The minerals in the vein are precipitates from hydrothermal solutions at temperatures above 100°C, called ore fluids. The minerals that precipitate from the ore fluids vary depending on the temperature due to their solubility. High-temperature veins (over 300°C) are characterized by the production of tin and tungsten, intermediate-temperature veins (350-200°C) are characterized by the production of copper, zinc, and lead, and low-temperature veins (below 250°C) are characterized by the production of lead, gold, and silver minerals (temperatures are estimated under many assumptions, so the numbers in parentheses are approximate). The ore fluids cause hydrothermal alteration of the surrounding rocks according to their temperature. A vein that is composed only of minerals that precipitated from a series of ore fluids that are continuous in time is called a single-upward vein, while a vein that was formed from ore fluids from multiple periods is called a multiple-upward vein.

[Tetsuya Masamichi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

岩石中の割れ目を鉱物が充填(じゅうてん)したものを脈といい、充填鉱物が経済的に採掘可能な場合、鉱脈という。鉱脈となる割れ目には剪断(せんだん)割れ目（断層）と張力割れ目がある。剪断割れ目に形成された鉱脈は幅が狭いのに対し、張力割れ目に形成された鉱脈は広く、10メートル以上に達することもある。鉱脈中の鉱物は鉱液とよばれる100℃以上の熱水溶液からの沈殿物である。鉱液から沈殿する鉱物は、溶解度の関係で温度により異なる。高温脈（300℃以上）は錫(すず)、タングステン、中温脈（350～200℃）は銅、亜鉛、鉛、低温脈（250℃以下）は鉛、金、銀の鉱物の産出で特徴づけられる（温度は多くの仮定の下に推定されるので、括弧(かっこ)内の数値は目安）。鉱液はその温度に対応する熱水変質作用を周囲の岩石に及ぼす。時間的に連続する一連の鉱液から沈殿した鉱物のみからなる鉱脈を一回上昇脈、複数の時期の鉱液から生成した鉱脈を多回上昇脈という。

［正路徹也］

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