Gold Deposits in Ancient Turbidites

The southeastern coast of the United States - NOAA

The largest deposits of gold on Earth are found in ancient turbidites that include the Witwatersrand of South Africa and the Abitibi found in Canada in both Québec and Ontario. By definition a Turbidite is a landslide that occurs on a continental slope of a continental landmass where the erosion products of the continent eventually work their way to the abyssal plain at the bottom of the ocean that can be several kilometers deep.

Turbidites start at the edge of the continental shelf and can be caused by several different geological phenomena. They are deposited by a special type of water flow called a Turbidity Current that can be started by an earthquake, a powerful oceanic storm or in some cases just by the amount of material that collapses in on itself. The turbidity current contains not only water but suspended debris that causes it to flow out of town little slow but a speed that exceeds that of water itself because the current is also carrying suspended particles. Depending on the height of the slope the speed that it flows can reach as much as 70 km/h.

Once this flow reaches the bottom of the slope it quickly loses its forward speed and as it does drops off particles according to their density. It has such a high specific gravity of more than 19 gold is one of the first things that is dropped from a turbidity current. The finer particles and spread out over the abyssal plain of the ocean extending across the abyssal plain for hundreds of kilometers. In many cases at the edge of the flow the finer particles may only be a few millimeters thick, but over time this can actually be built into deposits of great thickness.

Mostly gold of the Turbidite deposit will say relatively close to the edge of the continent. Examples of these Turbidite flows are found in several places in the world one of the classic examples is the MegumaTerrane of Nova Scotia.

Some of the metals found in this environment include gold, silver, tungsten and antimony.  In western Connecticut the Rowe schist is an example of a metamorphosed turbidite.  This formation extends from northern Massachusetts almost to Long Island Sound in Connecticut.  Over the years reports of gold have been rumored about with this formation, but what is lacking is substantial proof i.e. no core drilling.

A capsule description of a turbidite deposit is gold-quartz veins, segregations, lodes, sheeted zones that are hosted by fractures, faults, openings in anticlines, synclines, along bedding planes in the turbidites and those that are associated with poorly sorted clastic sedimentary rocks.

These rocks are typically found close to the margins of continents or in back arc basins.  These rocks were typically deposited in submarine troughs, periarc basins, foreland basins and other remnant oceanic basins.  One feature that is typical of these rocks is multiple deformation events accompanied by metamorphism.  They show evidence of being thick sediment sequences displaying both deformation and are metamorphosed.  Although there can be some igneous rocks found in one of these deposits they tend to be quite rare and are usually present as pegmatites.  In some cases there are younger granitic intrusions associated with these rocks raising them to be raised to greenschist, but in some cases can reach amphibolite grade metamorphism.

The ore mineralogy of one of these deposits whether it is principle or subordinate includes native gold, pyrite, arsenopyrite, phyrottite, chalcopyrite, sphalerite, galena, molybdenite, bismuth, stilbite, bornite and other sulfosalt minerals. These minerals generally have a low sulfide content of less than 2.5%. The usual minerals found in association with these ore minerals are Quartz, carbonates, feldspar and chlorite.

When rocks of this type are discovered in glaciated terrains they may undergo deep weathering where alluvial recycling may produce rich placer deposits, such as in the Bendigo region of Australia.

There is several factors that control the ore deposits that are often found in fold crests, discordant veins and tension gashes. In some cases such as the in the Bendigo there is a relationship between ore deposits in the amount of graphite that is found in the country rock. The structural control is often extended to a district causing a scale alignment of deposits. Sometimes the veins appear confined to a specific stratigraphic interval that is often near to a change in stone type. Sometimes a more subtle relationship occurs lower deposits more closely related to the upper portion of the turbidite as of the magma district of Nova Scotia.

Because the low sulfide content, the majority of Quartz veins houses most geophysical methods of geophysical exploration to become ineffective it is often necessary to resort to the more usual prospecting techniques that are often accomplished with a gold pan by tracing the deposit.

In the deposit of this kind is not unusual to find deposits that range from a 50 g per tonne. Many of these are attractive targets for exploration as many of them are world class deposits.

Deposits of this type become incorporated into continental land masses with the death of an ocean by two continental masses colliding and the turbidite deposits are squished between the continents causing the turbidite to become incorporated into the mountain range that is a result of the collision. 

Gold Occurrences in Burkina Faso

Map of Burkina Faso  -  CIA

In the past 40 years there has been a great deal of interest in gold mining in Burkina Faso. This is a country that is very geologically rich with the Poura region alone accounting for over 25 tonnes gold. As of 2003 the government brought about a new Mining act and immediately after there were more than 30 companies that have undertaken exploration projects in the country. They also created five new mines that are on the point of full production.

As demonstrated by the Abitibi region of Canada some of the richest gold mines in the world are located on greenstone belts.  In Burkina Faso greenstone belts cover about 3,000,000 km² of West Africa the Burkina Faso accounted for approximately 21% of these greenstone exposures. There are about 13 international mining companies that are active in this area now. .

As many as 13 international mining companies are working in the country mainly in the Greenstone belts. These belts account for over 21% of West Africa's Greenstone belt exposures that house on the world's most prolific gold mines. Some of these mines are capable of producing over 100 thousand ounces of gold per year. Much of this gold is produced by the heap leaching process and probably accounts to more than 1,000,000 ounces per year. Although some of these are producing mines, more of them are exploration projects that have been ongoing for some time.

All of the United Nations development program and Burkina Faso indicates that the early Proterozoic (Birrimian) Greenstone belts located at Boromo and Hound consists of steeply dipping volcanic and volcano-sedimentary units that have been intruded by the Eburnean granitoids there about 2 billion years old that have undergone incipient to low grade greenschist metamorphism. Many of the former gold prospects and mines are found in the Greenstone belts including the spell Poura vein gold deposit and the Dossi shear zone prospects. Much of gold that is discovered is associated with scheelite, calcium tungstate that is an ore for the metal tungsten.

Gold mining is one of the newest stories in Burkina Faso in recent years where the gold mining industry has been developing apparently inexhaustible resources.  There are many Australian companies that are already there searching for gold.  Many are finding it.

Most of the primary gold deposits are derived from metamorphic rocks that were developed from auriferous rocks that were eroded away long before the placer deposits bearing them were metamorphosed in the older volcanic and volcano=sedimentary rocks.  In this they are similar to the rocks found in the Witwatersrand district of South Africa and the Abitibi greenstone belts of Canada.

Gold Occurrences in South Africa

The headframe of a gold mine in Reef City, South Africa
Photo by Nolween

More then 40% of all the gold ever mined has come from the Witwatersrand commonly called “The Rand” since 1870.  The deepest mines in the world are also found in The Rand.  In 2005 it accounted for 12% of the total world’s gold production, but as late as 1993 it accounted to over 30% of the world’s gold.  Overall it has been estimated that greater then 50% of the world’s gold reserves are from South Africa with 40% from the Witwatersrand alone.

The word Witwatersrand in Afrikaans means the ridge of white waters a large range of sedimentary hills in the northeastern part of South Africa.  The range has a height ranging from 1,700 to 1,800 meters although it appears lower then that because its relief is low and the heights are because it is on top of a large mountain plateau.  The ranges run from east to west mainly in the Gauteng province of South Africa.

Geologically it is quite complex composed mainly of quartzites, quartz pebble conglomerate and shale.  These rocks are geologically turbidites that flowed down a Precambrian continental slope.  The Witwatersrand gold and uranium deposits for years have been the center of a controversy as to where they came from.  The controversy was apparently laid to rest with the discovery by analyzing some of the platinum group metals that determined they came from a long lost mountain range to the north.

The rocks of the “Rand” or reef are similar to those found at Elliot Lake and Timmins, Ontario and are very rich in gold and uranium.  So famous is the name Rand even South Africa’s currency is called a Rand.  The name Witwatersrand also attaches itself to the Greater Johannesburg Metropolitan Area.

Gold was discovered in the Rand as early as July 31, 1826 by Peter Jacob Marais on the banks of the Jukskei River; the find was kept secret at the time.  Gold mining really started on the Rand in 1886 although there were sporadic efforts before that.  The Rand sparked a gold rush in the Witwatersrand and later deposits of gold and uranium eventually created the largest urban area in sub-Saharan Africa.

Among the many gold mines in South Africa are two of the world’s deepest mines.  In the East Rand Mine at Boksburg reaches a depth 3,585 meters.  The TauTona mine in Carletonville is 13 meters less then the East Rand Mine.  Plans are afoot to work on an extension of the TauTona mine that will bring its total depth to more then 3,900 meters (12,800 feet). This will break the present by 39 meters (127 feet).  When you get to these depths the rocks can reach 60o C (140o F).

Gold Occurrences in Brazil

Coat of Arms of Brazil

Within its borders Brazil is the fifth largest country in the world; it touches upon all the countries in South America except Ecuador.  The country also has an astonishing variety of mineral deposits including iron, gold, gemstones, diamonds and oil.  There are literally hundreds of active mining companies found in Brazil.  Brazil also enjoys a rising economy and has a stable government with the future for mining being extremely bright.

The Amazonian shield covers much of the eastern portion of the South American continent and provides many of its gold mines.  Prospecting in the area is similar to prospecting in Canada’s Abitibi Gold Province in Ontario with a repeat of its greenstone belts with alternating gneiss’s.  There are also many deposits of turbidite hosted paleo-gold deposits similar to those found in the Witwatersrand in South Africa.  To the north of the Amazonian shield is the Guyana shield, to the south are the Rio Apa and Platian cratons and in the west is the Sao Francisco craton.  All of these areas are gold bearing.

Brazil covers 8.5 million square kilometers with 7,500 kilometers of coastline making it the largest country in South America, a size that dwarfs all its neighbors.  Its size is on continental size contains a vast resource base making it a prime target for attracting foreign investors and miners as well as Brazilian mining companies.  The country also has a long history of gold mining dating back into the 16^th century.

It is estimated that less then 30% of the country has been adequately mapped so there is plenty of room for prospecting in both placer and lode gold deposits. Placer deposits can be found throughout Brazil except in the depths of the Amazon Basin, and there are plenty of gold deposits found in the Precambrian Shield areas that dot the country.

One of the biggest problems facing the Brazilian mining industry are the number of artisanal mining operations that keep popping up; there was a gold rush in the 1990s that centered on the Guyana shield that reportedly attracted over 90,000 miners to the area.  These miners for the most part disregarded good environmental practices resulting in large areas that were contaminated with mercury and other contaminants.

It has been found that artisanal mining is today the largest source of mercury release into the environment not only in Brazil but other countries too.  One of the sites mentioned that is the largest is in the Tapajos River Basin in the Amazon region of Brazil.  The Global Mercury Project (GMP) under the auspices of the United Nations is aiming to reduce this mercury contamination.

Brazil as a whole is mining friendly and enjoys a now stable government and a rapidly expanding economy that welcomes miners from around the world.  There are still vast amounts of the country that have not been explored, so one never knows what might be found just around the next bend of the river.

The World’s Deepest Gold Mine is cooled with Slush

A South African goldmine similar to the one in this article.
Photo by Babakathy

There is a certain amount of argument as to what is the world's deepest gold mine, but there is no doubt it is found in the Witwatersrand of South Africa. The latest contender for the title of world's deepest mine is the Mponeng mine of the West Wits district of the Witwatersrand.  The mine's name is derived from the Sotho word for “Look at me.” At 13,000 feet this presently holds the record as the worlds deepest mine. This record does not only apply to gold but any other product such as uranium that is mined from the deepest recesses of the earth.

This mine is so deep the only way it can be cooled to the highest temperature allowed by South African law of 83°F at the mine face is by refrigeration that makes a mixture of ice and water that we call slush.  There is several refrigeration units deep in the bowels of the mine made by IDE Industries headquartered in Israel that make slush as part of a scheme to make fresh water from sea water . The slush is held in readiness in underground reservoirs until it is piped to the working face of the mine.  After it has done its job of cooling the mine the warm water is pumped back to the surface where it originally came from by large electric pumps using tremendous amounts of electricity where it is allowed to cool off.

The mine contains 230 miles of shafts and tunnels that take the workers as much as 90 minutes to reach the working face of the mine allowing only five hours per shift at the working face.  The shaft is split into two sections causing the workers to have to walk to the second shaft before they can continue their journey to the bottom of the mine.  Most of the trip continues on rails that carry men, equipment and ore.  The final part of the journey is made in low tunnels that cause the men to walk hunched over with their miners lamps to brush the ceiling of the tunnels.  Some of the time they have to go down narrow stairways leading into the depths of the mine.

At present this mine is the deepest on earth, but it is possible to extend the depth of a mine to 33,000 feet with most of the work being done by robots.  This mine makes it too all apparent that no real changes have taken place in deep mining for more then 100 years, a situation that must change if we are going to continue extracting minerals from the depths of the earth.

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Gold, Uranium and Carbon in Detrital Deposits

One of the common associations with gold in detrital deposits is the association of gold with uranium and carbon. This holds true in the Witwatersrand and all the other gold deposits of a detrital nature.  For many years the origin of the carbon was hotly debated with the most recent evidence holding that it is from primitive life forms that lived in the distant past in the Archean. 

Algae exposed at the intertidal zone similar to the ancient algae that trapped gold.  Even today this kind of mat could be a good place to search for gold.
Photo by Leonardo de Jorge

It has been posited that these algae formed great mats similar to stromatolites that acted as a trap for the gold particles and uranium minerals in the same manner that modern mats of algae work to snare gold particles in a modern environment.  It is common practice to use an artificial mat of the same nature in a sluice box to entrap gold.

Although the deposits can be several thousand feet deep now, but at the time of depositation they were relatively shallow at the bottom of running water that was only a few tens of meters deep subsequent Earth processes have buried them to their present depth. 

An angular uncomformity with a basel layer of conglomerate that would be a good place to look for gold.   The best place is right at the uncomfority.
Photo by Lamprus

At the time many of these gold bearing deposits were at the bottom of a braided stream channel that by being deposited in this manner explains the stringers of gold found in such a deposit.  These stringers of gold are common in detrital gold deposits.  Sometimes there is a layer of carbon that is as thin as a pencil line that is so rich in gold and other minerals that they are mineable. In many detrital deposits the slim lines occur at a regular frequency to the extent that the entire deposit is mined so that I can undergo further ore dressing to free the gold so it can undergo even further treatment usually by being leached with a solution of cyanide.

Even today if you encounter a mat of algae in the bottom of the stream or river is a good place to search for gold that has been caught in the mat by the action of running water.

Detrital Gold Deposits

Maybe you have heard of the Witwatersrand in South Africa if you haven't yet is the world's largest deposits of gold that since its discovery in 1886 has produced over 40% of the total gold supply ever mined in the world. This makes it the most glaring example of a detrital gold deposit in the world. The type of rock hosting this vast deposit is conglomerate that until about a year ago the source of its gold was unknown. By working with an isotope of iridium a team of geologists was able to pinpoint the source of this gold In eroded highlands delayed to the north.

Conglomerate the gold in this rock is usually found in the cement holding the pebbles together.
Photo by Hannes Grobe

The Witwatersrand is not the only deposit of conglomerate in the world that contains gold. In the area around Timmins Ontario there are also mines that produce gold from ancient conglomerates of Archean age between 2.8 M2 .6 billion years old. It is not only gold that these ancient conglomerates are apt to contain a common Association in rocks of this type are uranium mineralization. This is been taken advantage of at Elliot Lake, Ontario where some of the largest uranium mines in the world are found. The same deposits also contain significant amounts of rare earth elements to the point that the mining company Pele Mountain Resources (TSX-V ~ GEM) headquartered in Toronto, Ontario. At their Eco Ridge Mine in Elliot Lake, Ontario they are actively searching for both uranium minerals and rare earth minerals. The conglomerate at the Eco Ridge Mine also contains some gold.

Sandstone in a red bed that has been cemented and stained by iron oxide.
Photo by Anne Burgess

Ancient conglomerates are not the only place where gold can be found in sedimentary rocks as detrital gold is also found in many types of sandstone as small grains, and can even be found in fine-grained shale as microscopic particles that are too small to be seen with the naked eye. This kind of gold was never recovered until recently when Newmont Mining developed the gold deposits of the Carlin Trend in northern Nevada.

A closeup of the Marcellus Shale found in the eastern United States.  Aside from holding a vast amount of natural gas the Marcellus Shale sometimes contains gold in very fine specs that are invisible to the naked eye.
Photo by Lvklock

Most of those miners that are working with placer gold deposits in the rivers and streams of the world or along the beaches are also dealing with a type of detrital gold deposit in the form of specs of gold to have been eroded from hard rock and have been swept down river by the action of running water to where we can find them in the gravel deposits of the stream bed.

Conglomerate is nothing other than stream gravel that has become fossilized over time; just about any time you find conglomerate when you are prospecting it is a good idea to have a very careful look at the deposit. This also applies to sandstone and shale since they are also sedimentary rocks that have been produced by the action of running water.

Gold in Quartz Pebble Conglomerates

Over 40% of the gold in the world is produced in the Witwatersrand of South Africa that is an ancient bed of conglomerate. It is only been in the past year that the source of the school's been discovered proving it to be a fossilized placer deposit formed when the gold a eroded from even older mountains where it was deposited in an ancient sedimentary basin. Today is the Witwatersrand is the home to some of the deepest mines in the world that either approach or exceed 13,000 feet in depth. Ancient conglomerate deposits are not limited to South Africa as some of the deposits in the area around Timmins, Ontario are hosted in conglomerate too.

Conglomerate rock in as natural setting.
Photo by Daniel Case

Although gold was discovered in just about any type of rock that is found on Earth prospecting in quartz conglomerates is a pretty good bet. In the eastern United States quartz pebble conglomerates are commonly found in the Queenstown Delta as exemplified by the Schwangunk Ridge of New York and New Jersey. There have been reports of sparse gold deposits associated with this ridge in both states.

There are two different ways that gold can be deposited in conglomerate. One of the ways is by direct precipitation from hydrothermal waters where it forms a primary deposit. The second way gold can be deposited in a conglomerate is as a fossilized placer deposit where the gold has eroded from ancient mountains.

Closeup of conglomerate that sometimes is called "Puddingstone."
Photo ny Roy Parkhouse

Although gold is not found in every conglomerate it is a good idea to check them all for the presence of gold especially if the quartz pebbles show any sign of staining either yellow or green. This is a sure sign that metal bearing hydrothermal waters have penetrated the conglomerate. There are several other interesting minerals found in conglomerate like uranium, thorium and rare earth metals. Conglomerates can also be the host of other metals such as iron, copper, lead and zinc.