Rocks that contain gold are a rarity and finding them takes some luck. Knowing a few geological formations can help you to identify potential sites.
One of the first tests that you can use to determine whether a rock is golden is its ductility. A piece of gold will easily bend and deform in your hand, whereas pyrite or other possible minerals won’t.
Quartz is one of the most common minerals in Earth’s continental crust. It is a crystal that is formed by oxygen and silicon atoms joined together to form tetrahedra. These tetrahedra share oxygen atoms with each other, forming the mineral silica (SiO2). It has the simplest chemical formula of any rock-forming mineral. Quartz forms in igneous and metamorphic rocks as well as in hydrothermal veins.
Quartz occurs in many different colors and shapes. It is often clear and transparent or translucent. It may contain microscopic inclusions of other minerals that give it color. The most common colored varieties of quartz include rose quartz, amethyst, citrine, smoky quartz and milky quartz. Other common varieties of quartz are aventurine, agate and jasper. Quartz is a very hard and durable mineral that usually takes a good polish. It is also commonly used for gemstone carving.
Gold is found in quartz veins that are formed by hydrothermal fluids. These fluids often contain sulfide compounds and other metals. Gold is transported by these fluids and deposited in the quartz veins and in the wall rocks. The sulfide-quartz-gold deposits of the Comstock Lode at Virginia City, Tonopah and Bodie in Nevada are examples of this type of mesothermal gold deposit.
Quartz-gold deposits typically occur in areas of tectonic and volcanic activity that caused fractures and cracks in the bedrock. These fractures and cracks provide ideal pathways for superheated water and steam under pressure to flow through the rock. The fluids dissolve and precipitate their dissolved mineral and heavy metal loads along these pathways. In these types of deposits, the gold occurs within quartz-bearing hydrothermal veins and in the wall rock of the vein.
Granite is a common rock that is found throughout the Earth. This type of rock is made up primarily of quartz and feldspar. It forms when molten magma cools and crystallizes underground. This process is slow, and that’s what gives granite its coarse texture. Granites can range in color from pink to gray, and they typically form tors and other rounded features.
While gold is a heavy mineral, it does not get embedded into granite rocks. Instead, gold is more likely to be deposited in the surrounding soil or water. The only time you might see gold in a granite rock is if it was eroded from another area and then deposited within the granite.
When you see gold in granite, the best place to look is in cracks and crevices. You might also notice gold flakes in rivers and streams flowing through granite bedrock. Granite is one of the hardest types of rock, so it’s unlikely to be eroded easily, making it a good candidate for holding secondary gold deposits.
If you’re looking for more information about finding gold in granite, check out our blog post on the topic! We’ll give you some tips and tricks on how to find gold in granite.
Greenschist is a metamorphic rock that forms from ancient igneous stones like basalt and gabbro. It is a common host rock for gold mineralization, but not all of it contains it. Those that do are usually associated with quartz veins. Gold-bearing schist is found all over the world, but it’s rare to find a mine that produces a lot of it.
It can be a tricky type of rock to recognize. The best way to tell is by its foliated partings, which are caused by alternating layers of mica and albite feldspar. The Pelona schist, which is shown above, is an example of this. It has a pronounced color, and the mica can be split along layers to reveal gold nuggets in between them.
This type of rock usually forms under the lowest temperatures and pressures produced by regional metamorphism. It has a lepidoblastic or nematoblastic texture, with chlorite and actinolite as the most common minerals. It has a schistose, foliated, or tephritid cleavage, and it can contain quartz veining.
In schists, gold precipitates in fluids that react with sulfide-rich rocks or carbonaceous siliclastic metasedimentary hosts. Gold is deposited in mesozonal deposits and hypozonal deposits that form across the greenschist-amphibolite facies boundary . The fluid source for these deposits is thought to be metamorphic devolatilization of hydrated or carbonated metabasic rocks.
Gold occurs in both pelitic and mica schists, but it is more commonly found in the former. In pelitic schists, it typically occurs in quartz veins or as a scattered grain throughout the rock. In the latter, it forms in shear zones that are long zones of rock that have been twisted and deformed by movement. These zones often contain veins that have a high density of gold, and they tend to occur near the shear zone.
Slate is a type of metamorphic rock that contains gold in very small amounts. It is a very hard and brittle rock that can be split into two pieces with ease. Slate is a form of shale that has been transformed by intense heat and pressure for geologically long periods of time. It is believed that the shale that has become slate was once clay beds of mud that were slowly compressed over geological time into a metamorphic rock known as shale.
Slate contains a number of minerals, but is made mostly of silicates, which are compounds that contain silicon and oxygen. The silicates found in slate include quartz, muscovite (mica) and illite (clay, an aluminosilicate). Other minerals that can be found in slate include biotite, chlorite, hematite, apatite, graphite, kaolinite, magnetite, tourmaline and feldspar. Slate also has a very fine-grained structure, and is often foliated, meaning that it has a pattern of flat planes that give the rock a striped appearance.
Slate can be found in a variety of colors and shades, but is most commonly gray in color. It can also be black, green, red, purple or brown in color. It can also transition into other forms of metamorphic rock, such as phyllite or schist over time. Gold is found in the muscovite and illite components of slate, but is not very common in other types of mineral within the rock. The gold that is present in slate usually ends up deposited in thin faults and cracks, as opposed to being embedded in the rock itself. This is because the mineral solutions that are produced by water that seeps through the rock can erode the rocks and wash the gold into riverbanks or streams.
If you see rocks that are red or purple it indicates that the rock has a high iron content which can be a good indicator of gold. I’ll show you more about that later in the video. Another thing that you can look for is the ductility of the mineral. Real gold is ductile and will bend and deform rather than break like quartz and pyrite.
A good way to test if you have found gold is to do the streak test. To do this take the rock you think may contain gold and rub it on a streak plate which is an unglazed ceramic tile. You want to rub it for an inch or two and look at the color of the streak that is left behind. Gold will leave a nice yellow streak while pyrite will leave a greenish black streak.
The other way to test your rock for gold is to try and find a decent sized piece of it and see if you can dent or bend it with your fingernail. If it does then you have found gold because gold is much softer and more malleable than other minerals such as pyrite and chalcopyrite.
Finally you can also use the magnet test to test your rocks for gold. Pyrite, also known as fool’s gold, will stick to a magnet while real gold will not. You’ll want to make sure you are using a strong magnet because a regular magnet from your kitchen refrigerator will not be strong enough to detect pyrite. You’ll need a rare earth magnet. Those are typically available from hardware stores. They are much more expensive than regular magnets but they will be stronger and more accurate.