Monday, November 30, 2015

Definitions and Characteristics Quartz Vein Deposits

Hydrothermal deposits also form on land when metal-rich fluids are expelled from magma chambers. These fluids form veins and may contain concentrations of economic minerals. One of the last minerals to form during the cooling of a magma chamber is quartz. Quartz is precipitated in veins from quartz-rich fluids expelled from magma chambers or from fluids formed during metamorphism and often form associations with gold deposits. A vein type deposit is a fairly well defined zone of mineralization, usually inclined and discordant, which is typically narrow compared to its length and depth. Most vein deposits occur in fault or fissure openings or in shear zones within country rock. A vein deposit is sometimes referred to as a (metalliferous) lode deposit. A great many valuable ore minerals, such as native gold or silver or metal sulphides, are deposited along with gangue minerals, mainly quartz and/or calcite, in a vein structure. A vein system is a group of discrete veins with similar characteristics and usually related to the same structure.

Example Quartz Vein Deposits
Quartz vein deposits (img source : au-prospecting.blogspot dotcom).

As hot (hydrothermal) fluids rise towards the surface from cooling intrusive rocks (magma charged with water, various acids, and metals in small amounts) through fractures, faults, brecciated rocks, porous layers and other channels (i.e. like a plumbing system), they cool or react chemically with the country rock. Some form ore deposits if the fluids are directed through a structure where the temperature, pressure and other chemical conditions are favourable for the precipitation and deposition of ore minerals. The fluids also react with the rocks they are passing through to produce an alteration zone with distinctive, new minerals.

The presence of intrusive rocks and alteration associated with them provide important guides to prospecting ground for seasoned prospectors. Deposits are often controlled by the physical characteristics of the country rocks. For example, good fissure veins may occur in igneous rocks whereas they are poorly developed in sedimentary rocks and serpentine. Large quartz veins exist in quartzite, whereas in mudstones the veins are very narrow. The igneous rocks and quartzites fracture readily while the "softer" rocks do not tend to hold open spaces.

Vein deposits include most gold mines, many large silver mines and a few copper and lead-zinc mines. Veins commonly consist of quartz (sometimes of several varieties such as amethyst or chalcedony) usually occurring as interlocking crystals in a variety of sizes or as finely laminated bands parallel to the walls of the vein. Minor amounts of sulphide minerals and other gangue minerals such as calcite and various clay minerals often occur; gold is rarely visible.

Veins range in thickness from a few centimetres to 4 metres, the average mining width being around 1 m. They can be several hundreds of metres long and extend to depths in excess of 1,500 metres. Mineralization commonly occurs in shoots within the vein structures. These may be up to 150 metres in strike length, 30 metres in width and greater than 250 metres vertical.

Many outcrops of good looking veins are barren of gold or other ore minerals, but rich ore shoots may occur unexposed on surface, either down dip or along strike. Therefore, geochemical pathfinders are required. These include arsenic, antimony, or mercury which may be enriched in the rocks adjacent to the gold ore, either within the vein structure or in adjacent country rocks, producing a "halo".

Grades of gold historically have been in the 13.7 to 17.1 g/tonne range with cut-off around 8.6 g/tonne. Many more recently developed deposits have larger tonnages and lower grades and can be mined economically thanks to more efficient mining and milling methods. Mining requires adits, drifts, shafts and narrow slopes. If a vein system occurs near the surface it may be possible to mine by open pit methods which would greatly reduce mining costs.

Gold may be associated with pyrrhotite, arsenopyrite, pyrite, chalcopyrite and with minor sulphides - the classic 'free gold'. Silver is commonly associated with galena and galena-sphalerite, tetrahedrite or other copper minerals, antimony or copper-arsenic sulphides and chalcopyrite.

Written by : Flyshgeost

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