page_title: Vein (geology)
section_title: Gold-bearing veins
hierarchical_section_title: Vein (geology) / Mineralization and veining / Gold-bearing veins
caption_attribution_description: English: Gold bearing vein of Toi gold mine in Izu city, Shizuoka prefecture, Japan. 日本語: 静岡県伊豆市にある土肥金山
context_page_description: In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation.
  Veins are classically thought of as being the result of growth of crystals on the walls of planar fractures in rocks, with the crystal growth occurring normal to the walls of the cavity, and the crystal protruding into open space. This certainly is the method for the formation of some veins. However, it is rare in geology for significant open space to remain open in large volumes of rock, especially several kilometers below the surface. Thus, there are two main mechanisms considered likely for the formation of veins: open-space filling and crack-seal growth.
context_section_description: In many gold mines exploited during the gold rushes of the 19th century, vein material alone was typically sought as ore material.  In most of today's mines, ore material is primarily composed of the veins and some component of the wall rocks which surrounds the veins.
  The difference between 19th-century and 21st-century mining techniques and the type of ore sought is based on the grade of material being mined and the methods of mining which are used. Historically, hand-mining of gold ores permitted the miners to pick out the lode quartz or reef quartz, allowing the highest-grade portions of the lodes to be worked, without dilution from the unmineralised wall rocks.
  Today's mining, which uses larger machinery and equipment, forces the miners to take low-grade waste rock in with the ore material, resulting in dilution of the grade.
  However, today's mining and assaying allows the delineation of lower-grade bulk tonnage mineralisation, within which the gold is invisible to the naked eye. In these cases, veining is the subordinate host to mineralisation and may only be an indicator of the presence of metasomatism of the wall-rocks which contains the low-grade mineralisation. 
  For this reason, veins within hydrothermal gold deposits are no longer the exclusive target of mining, and in some cases gold mineralisation is restricted entirely to the altered wall rocks within which entirely barren quartz veins are hosted.
caption: In situ gold-bearing vein (in brown) at the Toi gold mine, Japan.
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