Gold is a rare element. The average concentration of gold in the Earth’s crust is 0.004 g/tonne (i.e. 0.004 grams of gold in one tonne of rock). A gold deposit becomes interesting for economic exploitation with grades usually above 0.2 g/tonne. A rich deposit has gold grades above 10 g/tonne. The extraction of gold does not depends only on the gold grade but also on the access and infrastructure of the site as well as the mineralogy of the gold, i.e. how gold is occurring in the ore.
It is very common to say that “a miner found a gold vein”. In fact gold mineralization occurring in hard rocks is usually associated with quartz veins that can be as thin as a few centimeters and as wide as several meters. Inside the Earth’s crust, gold is dissolved in hot silica-rich fluids. When the host or wall-rock suffers a geological fracture or accommodation, room is opened up for the gold containing hot liquid or vapor to come up and penetrate into the cracks and fractures. When the fluids cool down, gold-mineralized quartz veins are formed. This has happened millions to billions years ago. In some cases the hot fluids reacts with the wall rocks. In these cases, gold mineralization is not restricted to the quartz veins but can be found in the host rocks, especially when these contain dark minerals, which are more reactive. Quartz veins can be very deep and can extend for kilometers. It is not uncommon to find quartz veins without gold because the fluids that form the quartz veins often do not contain any gold. When gold occurs disseminated throughout the host rocks, it is usually finely distributed in the minerals that form the rock. Gold can occur as very fine particles, as fine as 0.001 mm, which are very difficult to concentrate. When the gold-bearing rocks are eroded or weathered, unconsolidated sand or gravel placer deposits rich in gold can be formed. Sometimes this unconsolidated material reaches the water streams, but sometimes it stays near the uplands in terraces or beaches formed by ancient streams that no longer exist. The unconsolidated sand and gravel eventually turns into hard rock, hardened over millions of years by geological processes—it is often possible to see the round pieces of quartz and other hard minerals embedded in a matrix of fine minerals in these “conglomerates”.
Gold can occur as a pure mineral or be combined with other metals such as silver, copper, mercury, palladium, tellurium, etc. The term “carat” is used to describe the purity of gold and is based on a total of 24 parts. Pure gold is known as 24 carat. In 18 carat gold, for example, 18 of the 24 parts are gold and the remaining 6 parts are another metal, such as silver or copper. The properties of pure gold are altered when it has other metals. For example 24 carat gold is too soft to make long lasting jewelry. The international price of gold is usually related to pure gold. The purity or fineness of gold is expressed in parts per 1000 (which is 100%). The most common unit for measuring the weight of gold is the “troy ounce”. One troy ounce of gold weighs approximately 31.1 grams. The price of gold is established daily by the London Exchange Market and the gold buyers around the world apply this price when they buy pure gold from miners. However, gold dealers usually do not pay the miners the same gold value as a bank because gold produced by artisanal miners is still impure (usually with fineness of about 900, or 90% pure). The value of a gold nugget is usually higher than the amount of gold contained because nuggets are rare and collectors around the world pay more for them. When a gold nugget is found, this can sometimes be an indication that the primary gold deposit is nearby (gold nuggets do not travel too far because gold is a heavy mineral). Sometimes gold minerals are formed as a secondary mineral. For example, in a deposit rich in iron oxide-hydroxides (yellow-red-brownish-earthy type of material), gold can “grow” as it precipitates from the fine iron hydroxides, forming large nuggets (Fig 2.1) such as the Canaa nugget weighing 60.8 kg found by the artisanal gold miners in Serra Pelada, Brazil in the 1980s. Actually the Canaa nugget was part of a larger gold nugget weighing 150 kg that broke in pieces when it was removed by the miners. Secondary gold nuggets are usually very pure (>99%) because they have been “refined by nature” (i.e. gold was naturally dissolved, adsorbed and precipitated). When gold is produced by artisanal miners, usually the fineness (purity) of the gold is known by the local gold dealers. It is possible, however, to find gold grains containing less than 45% gold. In general, silver is the main metal found in native gold (commonly gold panicles contain around 10% silver). Gold has its characteristic strong yellow color when it contains less than 10% silver or other metals.
It is very common to find gold associated with sulfides, those shiny metallic minerals that sometimes resemble gold. When gold occurs associated to sulfides, it has a preference for pyrite (FeS2), followed by arsenopyrite (FeAsS), chalcopyrite (CuFeS2), pyrrhotite (FeS) then other sulfides such as galena (PbS), sphalerite (ZnS), etc. Pyrite chalcopyrite and pyrrhotite are golden colored minerals and sometimes they are known as “fools’ gold”. Pyrrhotite is magnetic so with a hand magnet it is possible to differentiate it from gold which is not magnetic. Pyrite and chalcopyrite are yellow but harder than gold. A simple way to differentiate these sulfides from gold is to separate some grains, and leave them in vinegar-water solution over night (this works better with oxygenated water—shake the vinegar and water in a jar or bottle for a minute to start the process). In the morning, a dull oxidized layer will appear on surface of the sulfides, but gold will not be oxidized and keeps its brightness.
When gold occurs within sulfides, it is not usually visible. Such tiny “occluded” gold is not easy to be extracted by gravity concentration, amalgamation, or even by cyanidation. The sulfide must be oxidized (transformed to reddish iron oxide-hydroxide) in order to allow gold to be exposed or liberated. This oxidation can occur by many methods, such as roasting (burning in a bonfire), or mixing with oxygenated water, bleach, etc.
As a general rule for concentrating minerals we can say that: “to concentrate the mineral of interest we must eliminate mass”. In other words, the mass of the undesirable material, also called “gangue minerals”, must be discarded. Many miners believe that they lose gold when they discard fine fractions of their material. This is true, but the main loss is usually in the coarse fraction (i.e. fine gold that is not liberated from gangue cannot be concentrated). The only way to liberate gold from the gangue is by grinding. This concept of “mineral liberation” is very important to communicate to artisanal miners. In order to eliminate the undesirable minerals, miners need to know: how (and in which grain size) the gold is “freed” (not attached) from the gangues minerals (e.g. quartz). When the miners grind the ore , they intuitively try to reach a grain size in which gold is liberated from the gangue, but they do not know at which size this happens most effectively. If the gold is not liberated, some bad things may happen:
- The concentrate is not enriched with gold (mass of gangue is not sufficiently eliminated)
- Gold is lost during the gravity concentration (e.g. sluicing, or centrifuging)
- Mercury cannot trap the gold in the concentrate
- Gold is not exposed to be leached by cyanide
When gold occurs in water streams or terraces, forming “placer” deposits, it is usually liberated because the transport of the gold particles grinds the minerals and frees gold particles. However, it is also possible to find non-liberated gold particles in placer deposits.
So, it is clear that the first step before grinding and concentrating gold is to determine the best grain size in which most of the gold will be liberated from the gangue. But how do miners do this when they work with varied types of ores? It is important to make a sample of ore that is representative of the ore in the deposit (i.e. how to collect a sample that has the same characteristics as most of the ore in the deposit). We can start the pile collecting pieces of ore, crushing them manually in a mortar to pieces finer than 1″ (2.5 cm). Then this material is spread in a pile and homogenized (mixed vigorously) manually. A pile made of 20 buckets of ore (around 500 kg) is sufficient. Keep this pile of ore for all tests to better understand the ore. This material is representative of the ore and we call it “head sample”. Unless the ore changes its characteristics a lot (e.g. in some places is weathered, is from the wall rock, or is taken from a different location, etc.), we do not need to do the tests very often. Next, take sub-samples from this pile in a very uniform way. Collect 20-Liter bucket sub-samples from different parts of the pile. Keep these sub-samples in a safe place with a label “head sample #___” and the name of the mine site.
When we desire to establish the degree of liberation of gold in sulfides, for example, we can do this by sieving a sub-sample (head sample) in different screen sizes and observing the screened fractions with a magnifying lens or under a microscope (Fig. 2.3 and 2.4). Unfortunately, it is often very difficult to find visible gold panicles in a ground and screened fractions because gold usually occurs in low concentration in most ores—indirect methods must be applied to determine the ideal grain size the ore should be ground to in order to most effectively liberate the gold from the gangue. This can be time consuming, but yields valuable information about the ore we are working. We do not have to do this frequently, only when we change the type of ore being processed. We will need:
- a small ball mill
- a clock
- a water box, 1 or 2 plastic tubs and 1 or 2 buckets
- some sieves (5mm, 2mm, 1mm, 0.5mm, 0.1m; mosquito nets and woman stockings can be used)
Using a small ball mill to investigate gold liberation
A small ball mill can be build to test the liberation. Artisanal miners in Mozambique (Fig. 2.5) make small manual ball mills using a gas cylinder. This can grind 10 to 20 kg of material in one or two hours, depending on the size of the tank. Small ball mills (usually unlined) can also be operated using a motor and can be connected in-line using a pulley belt (Fig. 2.6). This system is being used for gold production in Indonesia.
Manual or mechanical small ball mills should operate 40% full to have good performance. Ball mills are loaded with 1 part of ore, 1 part of water and 2 parts of steel balls (Fig. 2.7).
Small balls are best for fine grinding, but big balls help break the larger chunks of ore. Ideally a mixture of small and big ball mill should be used. Always use the same mixture of ball sizes for any tests.
To start the liberation test, the following sequence of tasks is recommended:
- Load the mill with 20 or 40 kg (depending on the size of the mill) of steel balls
- In a bucket, mix 10 or 20 kg of head sample with 10 or 20 kg of water
- Add the mixture into the mill
- Use a little water to wash completely the bucket (gold can stay at the bottom of the bucket)
- Grind (manually or mechanically) for 10 minutes
- Discharge the material into buckets or plastic tubs
- In a water box, concentrate the material by panning or sluicing (Fig. 2.8)
- Always collect the tailings (waste) in the water box
- Separate the concentrate (with visible gold) and weigh the gold
- Get rid of as much water as possible (try to obtain the same consistency of the mixture head sample + water you had before (see the second bullet, above)
- Load the mill with same amount of balls as before
- Grind for additional 10 minutes
- Repeat the concentration process and re-grind again for additional 10 minutes
- Redo the process until you do not see any more gold in the concentrate. For a given ore, best length of time for grinding would be the total of all 10 minutes grinding steps that you have completed.
If you want to know what the grain size is of the ideal ground product, take a fresh head sample and grind it for the same time you have established (above) and pass the ground product through a series of screens:
- Use as a first screen a 5mm mosquito net, then use a 2mm, 1mm, 0.5mm, 0.1mm (you can use woman stockings for the finer mesh)
- Dry both the material retained in each screen, as well as the material passing through the last screen
- Weigh all screened factions; divide the weight of each fraction by the total weight of the head sample you used; then multiply by 100 to obtain the % of weight retained in each screen
- Note which screen passed 80% of your material—this is the best size to grind your sample. Whenever you grind in a different mill, try to reach the same size; if the ore is the same, this size will give the best liberation in any mill.