Gold in Black Shale, Metal Extraction using Specially Cultivated Bacteria

A settling pond in front of a heap leaching pile.   USGS

Gold and other metals are found in black shale usually in quantities too small to mine economically using the old methods, but that is all changed with the introduction of a process called  bioextraction that is closely related to bioremediation. The author has actually use this technique in his environmental consulting business to remediate industrial sites that were contaminated with heavy metals. This particular method of metal extraction makes use of local bacteria that are already present in the soils around the area of black shale.

A cultivated bacteria used to heap leach black shale.   NOAA

This method of using the natural bacteria is completelygreen allowing the extraction of metals without soil contamination. Of using a solution of bacteria for extracting metals from black shale was developed in the mid-2,000s in Finland on another poly metallic black shale. The process is extremely inexpensive to operate and can be fine-tuned to extract different metals from the shale. The process itself is based upon heap leaching the shale after it has been ground up and piled in its better then sprinkled with the bacteria bearing solution. The pregnant bacteria solution is then pumped into a central point where the metal is extracted then the solution is recycled back into the heap leaching process.

Black shales are noted for their poly metallic mineraldeposits all over the world. One such black shale is the Marcellus shale found in the eastern United States that is now being exploited for its natural gas content. In the process of extracting the gas in the shale is Hydro Fracked so that after the gas is recovered a solution of the metal extraction bacteria could be pumped down the hole and allowed to extract any metals that are left in the shale.

Besides allowing the extraction of gold this process allows the recovery of virtually any other metal found associated with Black shales including rare earth elements. For just gold mining this process would obviate the use of cyanide or mercury for the extraction of gold. By fine tuning the process any number of other metals can be extracted.  Since the source of the bacteria is from the same area as the mine is causing this to be a completely green process.

This is only of a number of other processes that can be adapted from existing metal remediation used in the environmental remediation practice.  Once the proper bacteria are extracted from the environment and cultured there is no limit to the different types of metals that can be extracted using this process. 

The Chemistry of Gold Extraction: Part 1. Simple Methods

A flotation cell similar to that used in the agglomeration process for gold recovery
by Dhatfield

Due to the complexity of this subject we are going to have to present it in several parts.

There are as many ways of extracting gold from its ores as there are ores it all depends upon which method is the most commercially viable and efficient. If the ore is amenable all that may be necessary is to reduce it to fine particles and extract the gold by gravity separation in water like any other method like panning or using a sluice box. A more advanced system is by using a “shaker table” a device that is slightly tilted in two directions.  The gold bearing fines are added at the top corner of the table with the lighter materials being washed by water off the lower edge of the table leaving behind the gold to be recovered from the surface.  There are several other schemes that have been developed over the years for affecting this process including the widely used “spiral classifier.”

A gold panner using a gold pan the simplest form of gravity separation
Photo by Alan Souter 

Gold agglomeration is a technique that is used for the recovery of gold when it is impossible to use cyanide or mercury because of environmental concerns.  The use of a process called coal-gold agglomeration makes use of a slurry made from finely divided coal and oil that works on the principle that gold is a hydrophobic material that won’t be wetted by water but will be attracted to the slurry.  After the gold has combined with the slurry, the slurry is burned causing the gold to form larger particles that can be extracted using gravity methods using water.

In an actual test of this process gold ore was finely divided and mixed with powdered charcoal, oil and water then stirred briskly for an hour that formed agglomerates containing gold.  Once the gold had formed a suitable agglomerate a collector such as potassium amyl xanthate (PAX) was stirred with the agglomerate for an additional fifty minutes.  The gold bearing agglomerate and the residue gangue was then ashed in a muffle furnace and the gold recovered from the ash using aqua regia. 

Greater gold concentration was obtained by increasing the viscosity of the oil or the increased size of the coal particles.  The rate of gold recovery was also influenced by the rate of stirring.  Stopping before after the optimum amount of stirring decreased the amount of gold recovery. By recycling the agglomerate showed that the amount of gold recovered could be further increased resulting in lowering the costs of the process.

Gold Occurrences in Texas

A worker in a gold mine

Gold was first discovered by the Spaniards in Texas in 1756 where they discovered gold and silver on Riley Mountain. They made their discovery of gold in the central mineral district around the Llano Uplift. It was in the same area that the Spaniards discovered the lost San Saba Gold Mine that according to tradition was worked by Mexican miners after they gained their independence from Spain sometime in the early 1800s. The Spaniards lacked any technology to keep mines from flooding caused by the inrush of groundwater. This was the reason why the Spaniards limited their mining to those areas that were above the water table causing these mines to exist as long trenches that have long since been eroded away. The consequences of this weathering are that today these old gold mines are hard to find, and for the most part were operated as placer deposits.

The peculiar thing about the Llano Uplift is because it is composed of Grenville aged rocks that are around 1.2 billion years old that are actually an extension of rocks that are often found in the Appalachian Mountains instead of the Rockies. It is in these ancient rocks that gold is found usually in small deposits that are quite numerous; these deposits are similar to those that were found during the Klondike Gold Rush where the gold had eroded from the rocks and formed large placer deposits.

In many of the rivers and streams that drain the Llano Uplift placer gold can still be found. There have been tales of gold occurring in this area for the past 250 years many of these tales are more credible than others. It is tales of this sort to drive the prospecting business even though gold is found in small quantities throughout the area. It is the hope of virtually all prospectors that they will eventually strike a Bonanza or even find El Dorado itself.

The Llano uplift itself is pre-Cambrian in age, but is surrounded by rocks of Paleozoic age making it  very similar to the Gold Belt of the Southeastern United States. Most of The state is an ocean of sedimentary rocks having an occasional outcrop of older crystalline rocks. It is in these rocks that you are apt to find gold. Gold can be found in rocks of both the Proterozoic and Phanerozoic ages.

The rocks found in the hills overlooking Presidio in the Rio Grande Valley are probably closely related in age to the Central Texas Mineral Region. With sedimentary rocks you often find a situation where occasionally older rocks do get exposed at the surface. This was what is happening here as well in the southern gold belt (Piedmont) in the Appalachian Mountains.

On the southern flank of the Chinati Mountains overlooking Presidio is the Prasidio Mine that was active from 1880 until 1942 when the War Production Board closed all the gold mines in the United States to free the gold miners for other wartime production. This one mine produced 92% of the silver and 73% of the gold ever produced in Texas.

There is also gold associated with a sandstone of Eocene age located in the Gulf Coastal Plain. This is gold that probably eroded out of the Llano Uplift rocks.

Nowhere in the state however is gold mining ever been extensive because the amount of metal available in the state appears to be limited. The Presidio and Hazel mines in West Texas and the Heath mine Llano have produced a total of 8,277 ounces of gold that end in 1942 when the government closed on all gold mines in the United States as part of the war effort during World War II. The last minute was in production in Texas was the Presidio mine where production and that in 1942.

Gold Mining Practices under the Roman Empire

Roman gold was of this sort
Photo by Alaska Mining

Minerals brought great wealth to the Roman Empire as they were used most often as payments for imports. The Romans were using a sluicing whereby broken rock was washed through channels containing prickly shrubs, which caught the gold. This extraction technology was being taught all over Europe in mining schools, yet the majority of the individuals working the mines were slaves who were subjected to atrocious working conditions and incurred a high mortality rate.

In 22BC, Publius Carisius, in charge of the Ulterior Army, succeeded in securing several gold mining areas, while he advanced northwards through the Pajares and Manzanal passes. By the middle of the first century AD, Rome’s gold supplies were being maintained in Spain and Dalmatia.

At the end of the First Dacian War in Autumn 106, Dacia was incorporated as a province with a garrison of two or three legions, and stationed at Apulum. Dacian gold mining was exploited at Sarmizegethusa, a colony founded there, and workmen were brought in from Dalmatia. The process of Roman gold mining allowed for Romanization to occur.

Roman Gold Mining in Britain

The only evidence of Roman gold mining in Britain is at the Dolaucothi Gold Mines situated near Pumsaint, Carmarthenshire, Wales. Washing the gravels from the river Cothi was the most elementary method of gold extraction by the Romans. These unique gold mines are set amid wooded hillsides overlooking the beautiful Cothi Valley. 2,000 years ago, the powerful Romans left behind a glimpse of gold-mining methods. The harsh mining environment continued in the 19th and 20th centuries, ending in 1938.

Dolaucothi Gold Mine in Wales that has been mined since the days of the Romans
Photo by David Smith

Types of Romans Gold Mining

The Romans used several different methods for mining gold and extraction, the first being hydraulic mining. This process was also used by the Spaniards. Gold would be stored in large tanks or vessels and when the water was released into a water source or river, the sediments would wash away, leaving veins in the earth and particles of gold. Pliny the Elder gives an account of hydraulic Roman gold mining, made possible by his observations in Spain.

The Romans also utilized deep mining techniques which allowed surveyors to track the veins with shafts and tunnels underground. The remains of Roman dewatering machines were found during the 1880s and the 1920s when the Rio Tinto mines in Spain were being mined by opencast methods (a large tank contains smaller reservoirs below it. It is likely that this complex was used for washing powdered ore to collect the gold dust). Rio Tinto was one of the principal Roman gold mining settlements, having produced more than 2,000,000 tons of silver ore in antiquity.

The Romans also used trip hammers to crush the ore. However, after the Roman occupation, the Carreg Pumsaint was discovered; a block of stone erected many years ago after the Romans had left the site. These types of crushing stones have been found at other ancient Roman mines in Europe, and the hollows in the block were formed by a trip hammer probably worked by a water wheel or a "water lever"

With the decline of the Roman Empire, mining activity was halted until its revival once again in the 11th century. Mining made its emergence in Harz, in what is now eastern Germany, and by the 15th century, amalgamation and retorting processes were widely used in gold extraction.

Author Bio

Lauren Axelrod is a fulltime archaeology student and owner of the website Ancient Digger at: http://www.ancientdigger.com

Archaeology. Her articles have been featured on Treehugger, India Times, Independent.uk, and USAToday. Ancient Digger is also featured on the Discovery News Blogroll under Archaeology.

1. The chemistry of gold extraction. John Marsden, Iain House

2. Roman Spain. By S. J. Keay

3. The Roman Empire. By Colin Michael Wells

4. Money and Government in the Roman Empire. By Richard Duncan-Jones

5. National Trust: Dolaucothi Gold Mines. Quote

6. Gold Nugget Photo

7. Dolaucothi Gold Mines Photo