If you want to find gold, knowing which rocks might contain it is important. There are three main types of rock that gold can be found in; Sedimentary, Metamorphic, and Igneous.
You can test a rock for gold by scratching it against another rock or piece of paper. Genuine gold leaves a golden streak while fool’s gold leaves a greenish-black one.
Sedimentary Rock
Gold is found in three main types of rock: sedimentary, metamorphic, and igneous. Among these rocks, sedimentary rock is most likely to contain gold, as it usually forms from material that has been eroded or weathered from other rocks and then deposited in rivers or lakes. This is why a person who is prospecting for gold might find the most promising deposits in sedimentary rock.
The most common kind of sedimentary rock is sandstone, which can be made up of sand, silt, or clay. The size of the particles in this type of rock makes it possible to distinguish one sandstone from another by its color, texture, and other properties. For example, quartz sandstone is bright white and has a finer grain than feldspar sandstone. Quartz sandstone is also more durable than feldspar sandstone and is often used in construction.
Other sandstone types include quartzite, adobe, and wacke. The composition and texture of these kinds of sandstone reveal important information about the history of the rocks that formed them. In particular, a quartzite is very hard and has a smooth surface; an adobe is soft and crumbly, with little granularity; and a wacke has a mix of sand, silt, and clay.
These kinds of sandstone can be very useful in finding gold as well, especially if they are rich in pyrite, which is a mineral that contains iron. People can use a chemical process to determine the amount of pyrite in a sample of sedimentary rock, and this information helps them estimate the quantity of gold within it.
Metamorphic rock is a rock that has been changed in some way by the action of heat and pressure on existing rock. This can happen as magma cools and crystallizes deep beneath the earth’s surface, or as igneous rock undergoes metamorphosis when it is pushed up into contact with molten rock. During this process, metamorphic rock can become a host for sulfide minerals that may contain gold.
Greenschist, which is a kind of metamorphic rock, can sometimes contain gold. This is a result of the fact that it forms from ancient igneous stones like basalt and gabbro, which can both contain gold. The amount of gold that is contained in this type of stone can vary widely, however, and this is largely dependent on how the greenschist was originally formed.
Igneous Rock
Gold is often found in igneous rock. It may be present in rocks that have been altered by metamorphic processes, but a rock can also contain gold because of the geological history of the area. Geologists classify rocks into three groups: igneous, sedimentary and metamorphic. Igneous rock forms either when magma cools underground or erupts from the ground. Magma that erupts from the surface is called lava. Igneous rock that cools slowly can form large crystals, but if it cools quickly the crystals will be small and you would need a magnifying glass to see them.
Some igneous rocks form in layers, a process known as porphyritic. This type of igneous rock contains coarse-grained minerals surrounded by fine-grained material. This mix of minerals indicates that the magma cooled in two stages, first in a deep place where it could form large crystals and later on the surface where it cooled more quickly.
When a rock contains gold, it is said to be auriferous (aw-uhr-fee-us). Some types of igneous rock that may contain gold include basalt, gabbro and syenites. Gold can be found in these rocks because they contain feldspar, mica and quartz. In addition, they have a high silica content.
Other igneous rocks can also contain gold, including granite and greenschist. Greenschist is a metamorphic rock formed from ancient igneous stones like basalt and gabbro. It consists of flaky sheetlike minerals like dark green chlorite, silvery muscovite and metallic gray graphite. It can also have garnet, pyrite and staurolite. Some schists contain quartz, which increases their strength. It is common for gold to occur in quartz reefs within a layer of schist.
Generally, you can tell if a rock contains gold by looking at its color and structure. Gold is often lighter than the surrounding rock and it will look rounded, as in a grain of sand. It also has a lustre and a magnetic attraction. Gold occurs in alluvial deposits, so you should check for gold in areas where water flows. This might be streams, rivers or rocky creek beds. It helps to know the regions in which gold has already been found to narrow your search for areas with potential.
Metamorphic Rock
Metamorphic rock is the product of geologic changes that occur when a rock is heated, subjected to high pressure and changed in composition. Metamorphism is usually caused by tectonic movement of plates that cause rocks to rub together and put them under intense pressure. Sometimes metamorphic rock is created by the action of magma that flows through existing rock, changing its chemical equilibrium and mineral composition. In some cases, gold is carried into and mixed with the metamorphic fluids that form in the rock as a result of hydrothermal metamorphism.
In these circumstances, the gold may be released from the sulfide minerals and become concentrated in the resulting metamorphic rock. Gold concentration in the rock is a function of the amount of time the rock was heated and the temperature at which it was heated. Higher temperatures and pressures generally lead to lower gold concentrations.
Metamorphism can also be caused by a phenomenon called directed stress, in which the unequal balance of forces acting on a rock causes physical changes in its mineral crystals. This is different from confining pressure because it does not change the atomic structure of the minerals but rather modifies them at a mechanical level, changing the size and arrangement of the mineral grains.
Some types of metamorphic rock have a special texture called foliation, in which the mineral grains are arranged in parallel lines. These types of rocks are named based on their type of foliation, and can include schist, gneiss and amphibolite. The gold in these types of rocks is found in the minerals sylvanite and calaverite.
At the highest levels of metamorphism, a rock is characterized by a lack of foliation and a generalized granulitic texture. This type of metamorphism is often associated with syenitic to amphibolitic rock and is called contact metamorphism.
Metamorphic rocks can contain gold from igneous, sedimentary and metamorphic sources. The most common way gold is contained in these rocks is from the erosion and weathering of igneous and metamorphic rocks, depositing their contents in rivers or lakes. The most concentrated amounts of gold in these rocks are usually found in sedimentary and metamorphic rock that contains the greatest amount of igneous and metamorphic material.
Schist
A rock that has undergone some sort of metamorphism is known as schist. As with the other types of rocks, there are many different kinds of schist but all share certain characteristics. Schist is a tough rock with a distinct texture and usually contains platy minerals like micas, chlorite, graphite and talc. Often there are also large inclusions of gemstone-like minerals including garnet, staurolite and kyanite. These are called porphyroblasts and they add an eye-catching element to the schist.
The most common type of schist is mica schist. It can be shiny or dull depending on the type of mica present – biotite is grey and acts as a mirror, while muscovite is more yellow. Other varieties of schist include prasinite (metamorphosed mafic rock that contains epidote, chlorite and hornblende in equal parts), glaucophane schist (which is blue or green) and scapolite schist (made from metamorphosed limestone). Other minerals can also be found in schist such as quartz, feldspar, quartzone and zoisite.
When a rock is a schist it means that the original sedimentary or igneous rock was subjected to high levels of heat and pressure. This causes the original minerals to transform into more stable ones such as the platy micas and chlorite. The resulting rock is then layered and arranged in thin sheets which give it the distinctive texture we recognize as a schist. It usually does not have the distinct alternating bands that are characteristic of gneiss, the next phase of metamorphism above schist.
If you’re looking for gold in a schist formation, the best place to start is near areas where there is a lot of shear zone. These are long zones where the change in elevation has caused rocks to deform and break apart. Gold is commonly found along these shear zones.
Schist with gold is not as common as gneiss or granite, so it might take a bit more digging to find it. However, it does exist in places like New Zealand, Finland and Virginia. When you do find a schist with gold, you’ll need to follow the same steps as finding gold in other rock formations. Look for quartz veins in the schist to locate the precious metal.