Where to Find Gold

Mining a Primary Deposit

Old historic gold mine

Throughout the earth’s crust there is an estimated 52,000 tonnes (1.8 billion ounces) of gold still in the ground and more is likely to be discovered. So, the obvious first question of any gold miner is where to find that gold. Historically, the highest concentrations of gold have been found in primary and secondary deposits.

Primary deposits are known as lode ore where gold is originally deposited. In lode ore, gold is found in veins, nuggets and bits. This is embedded in lodes or veins in rock, often together with quartz, silver, copper, platinum and iron pyrite (fool’s gold).

Where to Find Placer Gold – Secondary Deposits

Gold can also be found in the form of free flakes, grains or larger nuggets that have been eroded from the primary deposits and end up in alluvial deposits called placer deposits. Over the years water erodes the exposed lode outcrops and that gold ends up in streams, rivers small creeks, and river beds where it collects. Gold is heavier than most minerals and so it flows with fast moving water and tends to sink and be found in areas where the current slows and is no longer powerful enough to carry the gold.

Gold mine shaft

Capped, culverted Mine Shaft

While gold can be found in placer deposits, and there are many articles on the subject we will focus our attention on extracting gold from the primary deposit. Lode mining is a better place to focus time and energy. That is because the amount of gold that can be extracted from the primary source compared to the secondary source is on average ten to one. Ten times the amount of gold can be extracted per day’s work at a mining claim than can be found in a day of placer mining. Which makes sense when you consider that placer mining is just finding the gold flecks that were soft enough to erode from the primary deposit and wash downstream. The largest mining companies in the world are not focused on chasing nuggets that settled in a riverbed. They are going after the vein, the source of the metal.

To further the point, in the old days miners would work their way up streams looking for gold specs in attempt to find the primary source of the gold, following the trail of evidence to the source. Placer claims do have their place and they do have certain advantages. Plenty of information can be found online about placer gold. This article however will focus on finding gold in the primary deposits.

Where to Find Gold in Lode Mines – Primary Deposits

So where are the primary deposits? The primary deposits are often found around faults in the Earth’s crust.  Two things must occur if order for a large ore deposit to form. Obviously the material needs to come from somewhere, so there must be a source, but then there must be a means of transportation and also there must be a trapping, or a location where the gold gets stuck, or deposited. That’s why faults or contact zone, where two different bedrocks meet, is often where gold is found.

Plate tectonics or the movement of the earth’s crust is  the reason gold deposits form. Almost all gold deposits are formed by intrusion deposits.

Intrusion Deposits

Intrusive gold deposits are generally found in granites and igneous rock such as quartz. Copper is usually present in the rock as well.  These minerals were carried in liquid form with the magma or were part of the magma itself and were forced into the rock walls.

Lode Ore Deposits

Lode ore deposits are generally high-grade, thin veins that are found near faults. They are primarily made up of quartz veins. Lode ore deposits are usually found in basalt or in sediments, although when found in the faults, they may be found in igneous rocks such as granite.

 

Bedded replacement ore deposits result when rising hydrothermal (hot) fluids from a cooling magma body come into contact with favorable carbonate rocks, such as limestone or dolomite. These fluids are generally rich in dissolved metals (gold, silver, lead, copper, or iron) and move upward along breaks or faults in the Earth’s crust. Chemical reactions between the fluids and the carbonate rock cause the metals to be brought out of solution, which replace the mineral composition of the original rock.


Vein or fissure ore deposits
have clearly defined, abrupt margins and are roughly tabular in shape. They are considered the  product of deposition from fluids in open fractures or fissures. These deposits also contain crystallized pockets of the vein minerals.


Skarn ore deposits
form when cooling magma deep under the surface contacts limestone. The heat of the magma and fluids from the magma cause recrystallization of the limestone and the formation of new minerals such as garnet, diopside, and wollastonite. Later, metal-bearing fluids from the intrusion deposit gold and other ore minerals, such as copper and iron.

Above Ore Deposit information relating to the Images and the images themselves are courtesy of the Utah Department of Natural Resources / Utah Geological Survey

Magmatic processes where magma seeps up through cracks in the Earth’s crust, separates and cools. The magma sulfides, which are heavier, don’t mix well with the silicate-rich magma and so they separate and sink to the bottom as they harden and cool. These heavy sulfides contain gold and other heavy metals.

Hydrothermal processes during this process water is heated deep in the Earth’s crust and moves upward through faults caring gold and other precious minerals with it and then depositing the gold and other precious metals in concentrations when the water cools too much to keep the gold in a solution. Sulfur plays a key role in transporting the gold to the deposit.

Metamorphic processes Ore deposits that form this way. Quartz, sulfides, gold and other metals that form ore deposits on fault lines under pressure are loosened up and pushed into areas where there is less pressure, such as fault lines. Metamorphic rock is a good indication of gold.

In all of the above examples gold is deposited into the ore veins as a result of the movement of the Earth’s tectonic plates.

Where to find Gold in the Western U.S.

Much of the gold found in the United States is found in the West.

It seems that each of the Western States have had several gold rushes. Start looking at the district history of each state and find out which mining districts were the biggest producers. Larger mining companies will often revisit old districts looking for gold that has been left. Timberline Resources claimed almost the entire Seven Troughs district in Nevada, and near 29 Palms, California the whole Dale mining district had been claimed and was to be sold for 6 million. Barrick Gold Mining Company makes billions of dollars each year mining gold. The gold is far from being gone, it does take some work to find it though.

Click here to see samples of what gold ore looks like.

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