The Geology of Hydrothermal Quartz Veins that are Gold Bearing

When you first look at a quartz vein in a rock it looks like a very narrow vein but in reality a quartz vein develops into a sheet like body of quartz or other mineral that is carried in the cracks and fissures in the rock by a hot aqueous solution that is deposited through precipitation. The flow of water involved in this mechanism is usually hot water that is due to hydrothermal circulation making a great deal of the water being defined as supercritical.

A gold bearing quartz vein that has been eroded from rock.  Note the staining of the quartz from mineral bearing water as an indicator for gold.  On the right hand side there is a small section of a vein showing its sheetlike character.
Photo by Peripitus

According to geologists veins are usually thought of as being from the result of hot water precipitating the growth of crystals on the walls of cracks and fissures that are found in the rock. The crystal growth itself is usually oriented at 90° from the surface of the walls of rock in a cavity that leaves the crystals sticking out into the open space. Quartz is only one of these minerals there are many others among them gold that precipitates with the other minerals according to its solubility.

Quartz veins in a rock, notice that in this case the rock shows staining.

The reason why in these crystals grow in the 1st Pl. is because nature abhors a vacuum, and an effect of open-space in a body of rock is a vacuum. Veins usually grow on rocks at a considerable depth below the surface of the earth whose distance is measured in kilometers below the surface. There are actually two mechanisms that can form the veins of crystalline minerals, they are: open-space filling and crack-seal growth.

One type of open space filling is exemplified by epithermal vein systems mainly in stock works, in griesens and in some skarns. For this type of deposit to occur some type of confining pressure is required such as being married to a depth of 3 to 5 km. Many of these veins are capable of exhibiting a colloform, agate like appearance having sequential selvedges of various minerals that radiate out from nucleation points on the vein walls that appear to fill the available open space it rocks. You can often find evidence of boiling water in the form of vugs, cavities or geodes.  In some cases you could even find crystal lined Alpine Fissures.

There is an alternate way that the formation of these veins can be formed by hydraulic fracturing that may create a breccia filled with various types of vein material. Breccia deposits of this nature can be very expensive farming the shape of tabular dipping sheets, diatremes or in some cases along controled boundaries exhibiting latterally extending mantos that are often found in areas that have been thrust faulted, more competent sedimentary layers, or even in cap rocks.

The mineral rhodochrosite on quartz another indicator mineral for gold.
Photo by Rob Lavinsky

A second cause of veining is the so-called crack-seal method where the confining pressure is too great or when brittle, ductile, rheological conditions are met were vein formation occurs using crack seal mechanisms.

One thing about crack seal things they tend to form quite rapidly during the deformation of the rocks by precipitation of minerals within the fractures. Geologists feel that this happens quite swiftly when measured in geological time, because pressures and deformation mean there are large open spaces that cannot be maintained. The space of one of these systems is usually measured in millimeters or even micrometers were the veins can grow once that despite the reopening of the growth surfaces of the vein.
As a whole veins require hydraulic pressure that is in excess of hydrostatic pressure to cause hydraulic fractures or hydro-fracture breccias that need open spaces or fractures or they require some open spaces or fractures within the rock mass. In all cases were expelled were breccias are formed you can measure the plane of extension within the rock mass providing you give or take a sizable bit of error.