When you find a rock that looks like it might contain gold there are several nondestructive tests you can run. These will give you more information about your potential gold and help you decide whether it is worth further investigation.
One of the first tests is to rub the rock against a piece of unglazed ceramic or glass for an inch or two. If the rock leaves a yellow-gold streak it is real gold; if it leaves a greenish-black streak then it is likely pyrite.
Over 75 per cent of the earth’s surface is made up of sedimentary rock. These rocks are made up of minerals that have been deposited on the ground for so long that they become cemented together to form hard beds or strata which can be seen on exposed cliffs. There are six main types of sedimentary rock; sandstone, shale, limestone, breccia, conglomerate and gneiss. The gold in these rocks usually ends up there because of hydrothermal activity. This means that gold was pushed into cracks and openings in the rocks by hot liquid.
Gold can also be found in organic sedimentary rock which is derived from the build up and decay of plant and animal material. These types of rocks can be identified by their green color. They are layered with a sand like substance or small pebbles that can be picked up and broken by hand. When examining these kinds of rocks it is important to note whether or not they contain a high concentration of iron. If a magnet is used and the rock does not attract it then this may indicate that it contains pyrite which is often called fool’s gold because of its similar appearance. On the other hand if the rock is attracted to the magnet it probably contains genuine gold.
Sedimentary rock can be subdivided according to the dominant grain size of the minerals in it. This is referred to as the Udden-Wentworth scale and it divides unconsolidated sediment into three categories; gravel (>2 mm diameter), sand (2 – 1/16 mm) and silt (1 – 1/256 mm). Geologists then further subdivide these categories into agglomerates, breccias, conglomerates, sandstones and mudrocks.
The gold in clastic sedimentary rocks can be found in the same ways that it is contained in organic sedimentary rock, but it is also found in igneous and metamorphic rocks as well. The gold in igneous rock is typically within magma which cools and crystallizes as it oozes out of the volcano. Metamorphic gold comes from igneous rock that has been subjected to extreme heat and pressure.
For centuries people have been chasing gold and in many cases it ends up in rock. Most of the time the gold is contained within sedimentary and metamorphic rocks but igneous rock can contain gold as well. Igneous rocks are formed from the cooling and solidification of molten earth material and can have different characteristics depending on where the magma came from and how quickly or slowly it cooled. Rocks that cool very quickly can become rhyolite or granite while rocks that cool very slowly can be gneiss or gabbro.
Gold can also be found in igneous rocks that have been auriferous. Auriferous rocks can have gold-bearing quartz veins or gold-rich granite. When looking for gold in igneous rock it is important to look for the right minerals. A rock can have an amazing array of minerals but a few common ones are potassic feldspars (microcline, orthoclase) which are pink to tan and show flat shiny faces; plagioclase feldspars (albite, labradorite) which are white to gray and often have flashes of blue or green; micas, which are flakes that glitter in the sun; metallic gray graphite; and black phlogopite.
Minerals can be arranged in different ways and this gives the rock its texture or “strength”. A rock with a lot of flakes that easily flake apart is described as schist. Gold is usually found with these rock types as it is attracted to their hard surfaces.
Some igneous rocks contain very little gold. When gold is present in igneous rock it is usually in veins, a type of igneous rock that contains small amounts of gold in its crystalline structure. Typically, these veins are created by the pressure of hot fluid being pushed into cracks in the rock over long periods of geological time.
Other igneous rocks that can contain gold include pegmatite and slate. Pegmatites are a type of igneous rock that is formed when magma from deep within the Earth rises and fills voids in the crust. As the magma cools it can grow very large crystals, which create a rock with a very heavy and dense texture. A schist can contain gold in addition to its mica minerals and when it is auriferous can have gold-bearing quartz reefs.
When a sedimentary or igneous rock undergoes extreme heat and pressure while buried, it becomes metamorphosed. This process is known as metamorphism, and gold often concentrates in these transformed rocks. Metamorphic rocks are categorized as either foliated or non-foliated. Examples of foliated metamorphic rock include marble, novaculite and phyllite, while quartzite and serpentine are examples of non-foliated metamorphic rocks.
Minerals in foliated metamorphic rock provide clues to the metamorphic conditions under which the rock was changed. For example, green minerals like chlorite and serpentine indicate a low grade of metamorphism, while micas such as andalusite, kyanite and sillimanite indicate higher degrees of metamorphism. Geologists use these index minerals to map the different zones of an area that has undergone metamorphism. These metamorphic zones are called the Barrovian sequence and they represent a progression of index minerals with respect to temperature and pressure levels.
Gneiss is an example of a high-grade metamorphic rock, and it is usually made from both igneous and sedimentary rocks. It has a banded appearance of dark (mica, amphibole and iron-magnesium minerals) and light (quartz and feldspar) minerals. It is a very durable rock and has been used for paving, building and sculpture since ancient times.
Amphibolite is another metamorphic rock that can contain gold. It forms through recrystallization under high viscosity and directed pressure conditions. Amphibolite is made up primarily of hornblende and plagioclase, with very little quartz. It has a distinctive appearance and can be found in gneiss, granite and pegmatites.
Serpentinite, also referred to as ophiolite, is a magnesium-rich rock that forms through hydrothermal alternation between basalt and other silicate rocks. It forms at mid-ocean spreading centers and is a form of contact metamorphism. Serpentinite is an amorphous metamorphic rock that contains a mixture of lizardite, antigorite and chrysotile, all of which are magnesium silicate minerals. Its most common uses are for ornamental and decorative purposes, but it is sometimes used in construction projects. All of these types of metamorphic rock can contain gold, and the concentrations of gold will decrease with increasing grade of metamorphism.
Skarn deposits are a metamorphic zone that develops in the contact area around igneous rock intrusions. They are formed when carbonate sedimentary rocks are invaded by hot fluids of incompatible chemistry. The resulting mineral assemblage can vary depending on the rock types that are invading, the original chemistry of the magma, and the temperature gradients within the system over time and geography. In some cases, gold can be found in skarns. The most common skarn minerals are iron oxides (such as magnetite), calc-silicates (wollastonite, diopside and forsterite), and pyroxenes. They may also contain ore minerals such as pyrite, chalcopyrite, molybdenite and scheelite (tungsten ore) or tin-copper sulphides (cassiterite).
Skarns are usually described by their iron content in terms of its % of the end member, i.e. Hd 70Di 28Jo2. Compositional variations in the skarn minerals are expressed graphically by triangular plots. When fluid inclusions are available, they can be used to determine the temperature of metasomatism in the skarn minerals. The mineral assemblages at Junction Reefs exhibit a high degree of zoning with prograde skarn dominated by hedenbergitic pyroxene, andradite and grossularite garnet. The skarn zones vary in thickness from a few meters to several kilometers, and are often surrounded by a halo of granitic intrusions.
In general, the skarn mineral assemblage increases in iron content with distance from the granite. The mineral assemblages also tend to be enriched in nickel, cobalt and zinc with increased depth. This reflects the increased concentration of metals in the magmatic melt that is associated with deeper skarns.
The skarn mineral assemblage at Junction Reefs is highly anomalous in Au, As and Bi. This is probably a reflection of the relatively low temperatures involved in the formation of the skarn. In addition, there is a strong correlation between Au and the sulfides of other metals, e.g. sphalerite, galena and pyrite.
The skarn deposit at Junction Reefs contains a significant amount of tin as well as copper, gold and other base metals. The tin sulfide, cassiterite, is disseminated throughout the skarn gangue. The tin-copper mineral system is zonated spatially from a proximal sulfide-poor calcic tin skarn with minor cassiterite disseminated in sulfide-poor garnet-pyroxene gangue to a magnesian massive sulfide proximal to the skarn front.