Rocks that contain gold are a special type of mineral and can be found in sedimentary, metamorphic, and igneous rocks. They can contain significant amounts of gold because they have been exposed to geothermal fluids or melted during geologic processes.
These fluids pick up gold and concentrate it in cracks or veins in the rock. This concentration can be very high.
Alluvium is the sand, soil and other particles that are left behind by rivers when they flow over land that is not permanently submerged. Alluvium is found in many places worldwide, and is a good source of fertile agricultural land.
Alluvial deposits are made of a variety of materials, including sand, earth and gravel. Gold is usually found in alluvium, and the process that makes it possible to extract it from alluvium is called panning.
The gold that is deposited in alluvium comes from the water that flows over the ground. When a river flows, it continually picks up and drops solid particles of rock and soil from the bottom of the stream, depending on the speed of the water. When the river is flowing fast, more particles are picked up than dropped.
However, the more slowly a stream flows, the more particles are dropped. This is why the alluvium found in flood plains and deltas is usually much larger than the alluvium found in gulches and creeks.
In addition to alluvium, a stream can also deposit debris cones and bajadas, which are areas of land that have been formed by landslides or other forms of mass wasting. Debris cones are more like half-cones than flat fans, and they are often created by the slow accumulation of alluvium over centuries.
Another type of alluvium is a gravel plain, which can be found in regions where ore-bearing rocks are in the catchment of a river. This type of alluvium is characterized by unconfined alluvium and huge volumes of sand and gravel. It is a poor environment to mine for gold, as the gold is typically dispersed and the grade is diluted.
Gulches are the headwater drainages of alluvial gold provinces, where most of the auriferous alluvium is deposited. They tend to have steep gradients, ranging from over 200m/km down to about 25m/km.
Alluvium in gulches is more concentrated than in gravel plains, and it can be mined for gold if the auriferous rock is rich enough to warrant extraction. For example, the Folsom field in California was 11km wide with a width of 1.6 to 3.2 km and contained at least 430M m3 of profitable ground.
Gold and other metals can be found in intrusive rock, which is a type of igneous rock formed when magma hardens below the surface of the Earth. These rocks are exposed later on through processes of erosion. These include uplift and mountain building, as well as erosion of surrounding softer rock that was laid down in the area.
Intrusive igneous rock can be classified into plutonic and hypabyssal types. These are based on the way they are formed, which involves cooling and crystallizing deep in the Earth’s crust. They also have different mineral compositions, which can be important in determining the kinds of minerals that are found within them.
The most common form of intrusive igneous rock is granite, which is often found near volcanoes and in the middle layer of the Earth. It is a very durable stone that is used in countertops, statues, and tombstones.
Another kind of intrusive rock is diorite, which is a dark, medium-grained igneous rock composed of pyroxene and plagioclase feldspar. It is sometimes accompanied by small amounts of hornblende and biotite.
Other forms of intrusive igneous rock are gabbro, which is a light, soft-grained igneous rock with olivine. It is the most common intrusive rock in areas with volcanic activity and is a major source of copper and gold.
Finally, gneiss is another intrusive rock that contains gold and other minerals. This is the most commonly found type of igneous rock and is found throughout the world.
In general, gneiss can be very expensive to mine because it is rare and requires large volumes of rock to produce large amounts of gold. However, if you can find an appropriate place to mine it, you can be assured of making a profit.
Gold is also found in other forms of intrusive igneous rock, such as schist and quartz. These are more common in the United States and are typically associated with lava flows from volcanoes and other sources of heat. These are usually more profitable to mine because they are easier to extract than gold-rich porphyry copper deposits.
Schist is a metamorphic rock that can be found all over the world. It is made of platy minerals, such as biotite, muscovite, and chlorite, as well as larger gemstone minerals like garnet or staurolite.
The mineralogy of schist can vary greatly, depending on which minerals are dominant. For example, a rock that has more biotite will be green in color. Similarly, a rock with more chlorite will be blue in color.
Another variation is a rock that has alternating bands of minerals, such as quartz and feldspar. This is called gneiss and it is the next stage of metamorphism that a schist has gone through.
Originally, schist was a different type of rock. It started out as clay, which is a fine-grained rock that is usually found in sedimentary deposits. Eventually, this clay was pushed under pressure and heated, turning it into shale (which is the same thing as a slate) or phyllite.
These clay-rich rocks are buried underneath more and more layers of rock over millions of years. As the shale is pushed deeper and deeper, it becomes harder. The harder the shale becomes, the more pressure and heat it must endure. As a result, the original clay minerals transform into smaller mica minerals.
After the metamorphism is complete, a schist will become igneous rock. It can be formed by metamorphism of very common rock types, but it is also commonly created in mountainous regions where metamorphic rocks have been exposed on the surface.
A schist is also commonly associated with veins of gold. These veins are long zones that have changed height over time and that have caused the rock to crack. This is where the gold gets deposited in the schist.
Many places around the world have schist that is associated with gold. In New Zealand, for example, the Otago schist belt has produced more than 12 million ounces of gold.
It is important to note that most of the gold that is present in schist comes from the first pulse of orogenic gold mineralisation during the Cretaceous. The gold that was deposited during this first pulse was concentrated in diagenetic pyrite. This pyrite was initially in the form of small particles that had a very low concentration of gold. Once the pyrite became metamorphic and was able to recrystallize, it contained a high concentration of gold.
Quartz is a mineral that occurs naturally all over the world. Its crystalline appearance and versatility make it a popular choice for jewelry. This stone can be cut, shaped, or tumbled to create bracelets, necklaces, pendants, and earrings. It also comes in a variety of colors, making it easy to find one that matches your style.
Gold and quartz are often found together because both minerals have the same chemical composition. They are both “mineral fluids” that solidify into a rock after being exposed to the elements and weathering over time. The chemistry of the two minerals complements each other well and can be found in large quantities in the same area.
Unlike other minerals, quartz can contain gold inside the crystals themselves. This happens when a fluid begins to run through the rocks, and it contains sulfide minerals such as pyrite or arsenopyrite, which then leave spaces in the crystals for gold to fill in.
This process can happen relatively close to the surface or deep in the earth’s crust. It usually happens when weather or a natural event causes sulfide minerals to rush out of the rock and leave cracks, veins, or spaces in the quartz.
Once the fluid begins to solidify, small particles of gold begin to form inside these spaces in the quartz. This is called dissolved gold and is very common in quartz.
In addition to containing gold, quartz can also contain other sulfides such as pyrite and arsenopyrite. The sulfides are usually present in the same veins that the gold is in, so this can be a very common way to find both minerals together.
Another way to find gold and quartz is through the process of hydraulic fracturing. When oil and gas are extracted from reservoir rocks, a slurry of sand is injected into the fractures to help them open up. This slurry has a high resistance to being crushed, and the durable sand grains help to hold the fractures open after pressure is released.
Gold can also be found in greenschist, which is a metamorphic rock that formed from ancient igneous rocks like basalt and gabbro. Some types of greenschist have auriferous gold in them, but this is rare and only happens in a few locations.