Gold has captivated the imaginations of people around the world for centuries, and it continues to be one of the most valuable and sought-after precious metals in the world.
Knowing which rocks that contain gold can be crucial to your success as a prospector. By understanding which types of rock are prone to containing gold, you can make better sense of the landscape and increase your chances of finding it.
Quartz Veins
Quartz veins are rocks that contain gold, and they can be found in a number of different places. Most of the time you can find them in igneous rocks, but sometimes they appear in other types of rocks as well.
The most common way to identify a quartz vein is by its appearance. Typically they are very thin and are filled with small crystals, but they can also be very large. This is usually because of the fact that the vein was formed under pressure and had to go through a lot of rock.
You can also tell a quartz vein by its texture. A vein that is thick and covered with quartz will have a smooth, polished texture. On the other hand, a vein that is very thin will have a more bumpy or rough texture.
Another way to tell if you are looking at a quartz vein is by its color. Most quartz veins are yellow to brown in color. You can also spot them by the color of their edges. A vein that is colored green or blue will be more difficult to find, as these colors are usually associated with minerals that are not quartz.
Occasionally, quartz veins are found that are not filled with crystals but instead are filled with other minerals. This can happen if the minerals in the vein have been replaced by other materials, such as limestone.
If you are looking for a quartz vein, it is important to try to look at it from all sides. This will allow you to see the vein better and will help you figure out what it is.
The best way to do this is by going rockhounding in the area where you want to find a vein. This can be very rewarding, as it can lead to the discovery of new areas and a whole bunch of other things.
One of the best things about going rockhounding is that it gives you a chance to get up close and personal with the rocks in your area. But you need to be careful. The crystals can be very fragile, so if you are not careful, you could break a crystal or damage the rock underneath it.
Sedimentary Rocks
Gold occurs in many different types of rocks, but sedimentary rocks are one of the most common places to find it. These rocks are typically layered and cemented together, and they contain important information about the geological time and place in which they were formed.
Sedimentary rocks can be divided into two main categories: clastic and chemical. Clastic sedimentary rocks are usually composed of the broken pieces (called clasts) of other rocks that have been mechanically weathered. The minerals and other characteristics of the clasts can help you identify these rocks.
There are also a number of other ways to tell if a rock is a sedimentary rock or not. Some rocks have distinctive layering or bedding, and some have fossils in them. Other sedimentary rocks may have unique features such as mud fissures or ripple lines.
Another way to distinguish a sedimentary rock from a different type of rock is the amount of gold it contains. Some sedimentary rocks only contain gold in a small amount, while other rocks are very rich in the element.
The amount of gold in a rock depends on the way it is deposited. Most gold deposits occur in veins. These veins contain large amounts of gold within sulfide minerals such as pyrite, which are dispersed throughout the rock.
In other cases, gold may be found in tiny flakes within the sulfide mineral itself. This is known as native gold and it is commonly found in a variety of gold ores.
These flakes are generally found in sandstone and quartz veins, but they can be found in other sedimentary rocks as well. For example, sandstones that contain rounded gray quartz pebbles in a matrix of sand and mica are often gold-bearing.
Other sedimentary rocks that contain gold include diatomite, which is a type of organic sedimentary rock formed from the skeletal remains of microscopic organisms such as diatoms. The skeletons of these organisms are often very soft and porous, making them easy to break apart and remove from the rock.
The process that makes these sedimentary rocks possible is called erosion, and it is what causes the material to be deposited on the earth’s surface. The most common forms of erosion are wind and water, but there are also other processes that can cause sedimentary rocks to form, such as volcanic eruptions.
Skarn Deposits
Skarn deposits are igneous rock formations that contain gold (Goldschmidt, 1911; Lindgren & Barrell, 1902; Umpleby, 1913; Knopf, 1918) in a number of mineral phases. These minerals can include garnet, pyroxene, quartz, and amphibole. The composition of these minerals can be very different depending on the host rock and structural setting. For example, pyrope-rich garnets would be more atypical in an olivine-rich protolith than a calcite-rich pluton.
Garnet is the most common mineral phase in skarn. It can be found in a variety of different compositions, ranging from adolescent to granitic in structure. It also contains pyrope and spessartine, two metal elements that occur in many skarn systems.
Most skarn deposits contain significant amounts of sulfide minerals as well as some form of gold. These sulfide minerals can be traced back to a magmatic event and are the primary sources of gold in a skarn deposit (Ettlinger & Meinert 1992).
A skarn deposit is formed when magmatic fluids react with isotopically heavy carbonate wallrocks to produce skarn-forming mineral phases that trap these fluids. These mineral phases are usually enriched in d18O and d13C values indicating progressive reaction with the isotopically heavy carbonate wallrocks (Zimmerman et al. 1992; Myers 1994).
In addition to d18O and d13C variations, skarn minerals can also be characterized by their salinities in stoichiometric ratios, which can reflect the mixing of magmatic, connate, and meteoric fluids. This can be especially useful in skarns that have been produced by a single source, such as a skarn associated with the mafic Boulder Batholith in Montana (Ettlinger & Read 1992).
Other skarn minerals that can be used to identify a skarn deposit are calcite, quartz, and pyroxene. These minerals are typically intergrown with skarn minerals and have a wide T-P-X range, which allows them to trap fluids that are above their stability limits in the mineral.
The d18O and d13C value of calcite is lower than that of unaltered limestone, and this is consistent with the progressive reaction of magmatic fluids with isotopically heavy carbonate wallrocks that form skarn (Zimmerman et al. 1993).
Most skarn deposits are related to mafic diorite stocks marginal to the Boulder Batholith in the Elkhorn district of Montana. These skarn deposits are relatively young, but still contain a substantial amount of gold.
Alluvial Deposits
Alluvial deposits, also known as placer or stream-bed deposits, are formed when gravels or sands contain gold. This is a common type of mineral deposit. Sands and gravels are often found in streams and rivers, which is where they can be prospected for gold.
These deposits may be large or small, but they are all deposited by water. These deposits are usually a mix of sand and gravel, but can also include other types of material.
Gold is the most common metal found in alluvial deposits, although tin and other minerals may also be present. These deposits can be located in areas where the Earth has undergone many changes, such as elevations, tilting, faults and volcanic action.
They can also be found in ancient rocks that have been eroded by the elements. Normally, these rock forms will be in an oxidation zone. This means that they have a high level of oxidation (adsorption of gold) on the surface and will also have gold mineralization.
If the gold is a fine particle, it will be relatively easy to separate out from the sand. This is why the earliest alluvial mines were so rich, with 10’s of grams of gold per tonne of gravel.
Some of these deposits have been worked in the past, and a few still are being worked. Some are extremely rich, with hundreds of grams of gold per tonne of stone. Others are much less so.
The most productive of these deposits are those that are near the banks of rivers or creeks. These are generally easier to prospect than gulches, which tend to be steeper and higher in gradient.
Another important point is to look for dry places. These are typically places where the ground has not rained in a while, but can be a good location for alluvial gold when fluvial rains do happen.
In New Zealand, for example, there are some beaches where gold and black sand are mixed together near the low water mark. These have been worked by portable sluicing tables.
The only problem with alluvial deposits is that it can be difficult to get a sample size large enough to test for them. Moreover, when testing for a mineral of high unit value, such as gold, any errors in the value of a sample can be magnified. This can lead to overvaluation of the results.