Characteristics of Vein Deposits

Vein deposits take in most gold mines, a lot of big silver mines as well as some copper and lead-zinc mines. Veins frequently are made up of quartz, in some occasions of a number of varieties such as amethyst or chalcedony, and in most cases occur as interlocking crystals in a number of sizes or as lightly coated bands similar to the walls of the vein. Very small quantities of sulphide minerals and other gangue minerals such as calcite and a variety of clay minerals frequently take place; however gold is hardly ever visible.

Veins vary in thickness and can be anywhere from a few centimeters up to around four meters, the average mining width is usually around one meter though. Veins can be quite a few hundreds of meters long though and expand to depths in excess of 1,500 meters. Mineralization frequently takes place in shoots inside the vein structures. These might be up to one hundred and fifty meters in strike length, twenty five meters in width and over than two hundred meters vertical.

Grades of gold in the past have been in the 13.7 to 17.1-g/tonne range with cut-off around 8.6 g/tonne. Several more newly developed deposits have superior tonnages and lower grades and can be mined economically due to more proficient mining and milling techniques. Mining requires adits, drifts, shafts and narrow slopes. If a vein system crops up close to the surface it might be feasible to mine by open pit techniques and doing this would significantly decrease the cost of mining. 

Gold might be connected with pyrrhotite, arsenopyrite, pyrite, and chalcopyrite and with minor sulphides. Silver is frequently connected with galena and galena-sphalerite, tetrahedrite or other copper minerals, antimony or copper-arsenic sulphides and chalcopyrite.

Epithermal Deposits
Epithermal gold deposits occur in hydrothermal systems associated to volcanic activity. These systems, when they are active, go up to the surface as hot springs or fumaroles. As a result, the study of active hydrothermal systems makes available information on hydrothermal processes that are associated to metal transport and deposition. Sequentially, this information can be used to foresee how gold deposits form, and where it is they can be located.

Epithermal gold deposits occur for the most part in volcano plutonic arcs, both in island arcs as well as continental arcs, and are related with subduction regions, with ages comparable to those of volcanism. The deposits structure at superficial depth, of 1 kilometer to 2 kilometers, and are hosted mostly by volcanic rocks. Epithermal deposits take place as small vein systems under than a million tonnes in size, however they do have good grades.