Rare 2.5-billion-year-old rocks reveal hot spot of sulfur-breathing bacteria

Gold miners prospecting in a mountainous region of Brazil drilled this 590-foot cylinder of bedrock from the Neoarchaean Eon, which provides rare evidence of conditions on Earth 2.5 billion years ago. -  Alan J. Kaufman
Gold miners prospecting in a mountainous region of Brazil drilled this 590-foot cylinder of bedrock from the Neoarchaean Eon, which provides rare evidence of conditions on Earth 2.5 billion years ago. – Alan J. Kaufman

Wriggle your toes in a marsh’s mucky bottom sediment and you’ll probably inhale a rotten egg smell, the distinctive odor of hydrogen sulfide gas. That’s the biochemical signature of sulfur-using bacteria, one of Earth’s most ancient and widespread life forms.

Among scientists who study the early history of our 4.5 billion-year-old planet, there is a vigorous debate about the evolution of sulfur-dependent bacteria. These simple organisms arose at a time when oxygen levels in the atmosphere were less than one-thousandth of what they are now. Living in ocean waters, they respired (or breathed in) sulfate, a form of sulfur, instead of oxygen. But how did that sulfate reach the ocean, and when did it become abundant enough for living things to use it?

New research by University of Maryland geology doctoral student Iadviga Zhelezinskaia offers a surprising answer. Zhelezinskaia is the first researcher to analyze the biochemical signals of sulfur compounds found in 2.5 billion-year-old carbonate rocks from Brazil. The rocks were formed on the ocean floor in a geologic time known as the Neoarchaean Eon. They surfaced when prospectors drilling for gold in Brazil punched a hole into bedrock and pulled out a 590-foot-long core of ancient rocks.

In research published Nov. 7, 2014 in the journal Science, Zhelezinskaia and three co-authors–physicist John Cliff of the University of Western Australia and geologists Alan Kaufman and James Farquhar of UMD–show that bacteria dependent on sulfate were plentiful in some parts of the Neoarchaean ocean, even though sea water typically contained about 1,000 times less sulfate than it does today.

“The samples Iadviga measured carry a very strong signal that sulfur compounds were consumed and altered by living organisms, which was surprising,” says Farquhar. “She also used basic geochemical models to give an idea of how much sulfate was in the oceans, and finds the sulfate concentrations are very low, much lower than previously thought.”

Geologists study sulfur because it is abundant and combines readily with other elements, forming compounds stable enough to be preserved in the geologic record. Sulfur has four naturally occurring stable isotopes–atomic signatures left in the rock record that scientists can use to identify the elements’ different forms. Researchers measuring sulfur isotope ratios in a rock sample can learn whether the sulfur came from the atmosphere, weathering rocks or biological processes. From that information about the sulfur sources, they can deduce important information about the state of the atmosphere, oceans, continents and biosphere when those rocks formed.

Farquhar and other researchers have used sulfur isotope ratios in Neoarchaean rocks to show that soon after this period, Earth’s atmosphere changed. Oxygen levels soared from just a few parts per million to almost their current level, which is around 21 percent of all the gases in the atmosphere. The Brazilian rocks Zhelezinskaia sampled show only trace amounts of oxygen, a sign they were formed before this atmospheric change.

With very little oxygen, the Neoarchaean Earth was a forbidding place for most modern life forms. The continents were probably much drier and dominated by volcanoes that released sulfur dioxide, carbon dioxide, methane and other greenhouse gases. Temperatures probably ranged between 0 and 100 degrees Celsius (32 to 212 degrees Fahrenheit), warm enough for liquid oceans to form and microbes to grow in them.

Rocks 2.5 billion years old or older are extremely rare, so geologists’ understanding of the Neoarchaean are based on a handful of samples from a few small areas, such as Western Australia, South Africa and Brazil. Geologists theorize that Western Australia and South Africa were once part of an ancient supercontinent called Vaalbara. The Brazilian rock samples are comparable in age, but they may not be from the same supercontinent, Zhelezinskaia says.

Most of the Neoarchaean rocks studied are from Western Australia and South Africa and are black shale, which forms when fine dust settles on the sea floor. The Brazilian prospector’s core contains plenty of black shale and a band of carbonate rock, formed below the surface of shallow seas, in a setting that probably resembled today’s Bahama Islands. Black shale usually contains sulfur-bearing pyrite, but carbonate rock typically does not, so geologists have not focused on sulfur signals in Neoarchaean carbonate rocks until now.

Zhelezinskaia “chose to look at a type of rock that others generally avoided, and what she saw was spectacularly different,” said Kaufman. “It really opened our eyes to the implications of this study.”

The Brazilian carbonate rocks’ isotopic ratios showed they formed in ancient seabed containing sulfate from atmospheric sources, not continental rock. And the isotopic ratios also showed that Neoarchaean bacteria were plentiful in the sediment, respiring sulfate and emitted hydrogen sulfide–the same process that goes on today as bacteria recycle decaying organic matter into minerals and gases.

How could the sulfur-dependent bacteria have thrived during a geologic time when sulfur levels were so low? “It seems that they were in shallow water, where evaporation may have been high enough to concentrate the sulfate, and that would make it abundant enough to support the bacteria,” says Zhelezinskaia.

Zhelezinskaia is now analyzing carbonate rocks of the same age from Western Australia and South Africa, to see if the pattern holds true for rocks formed in other shallow water environments. If it does, the results may change scientists’ understanding of one of Earth’s earliest biological processes.

“There is an ongoing debate about when sulfate-reducing bacteria arose and how that fits into the evolution of life on our planet,” says Farquhar. “These rocks are telling us the bacteria were there 2.5 billion years ago, and they were doing something significant enough that we can see them today.”

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This research was supported by the Fulbright Program (Grantee ID 15110620), the NASA Astrobiology Institute (Grant No. NNA09DA81A) and the National Science Foundation Frontiers in Earth-System Dynamics program (Grant No. 432129). The content of this article does not necessarily reflect the views of these organizations.

“Large sulfur isotope fractionations associated with Neoarchaean microbial sulfate reductions,” Iadviga Zhelezinskaia, Alan J. Kaufman, James Farquhar and John Cliff, was published Nov. 7, 2014 in Science. Download the abstract after 2 p.m. U.S. Eastern time, Nov. 6, 2014: http://www.sciencemag.org/lookup/doi/10.1126/science.1256211

James Farquhar home page

http://www.geol.umd.edu/directory.php?id=13

Alan J. Kaufman home page

http://www.geol.umd.edu/directory.php?id=15

Iadviga Zhelezinskaia home page

http://www.geol.umd.edu/directory.php?id=66

Media Relations Contact: Abby Robinson, 301-405-5845, abbyr@umd.edu

Writer: Heather Dewar

From ‘Finding Nemo’ to minerals — what riches lie in the deep sea?

Left: The first species ever recovered from the deep sea. Center: Rockfish use deep-sea carbonate formations at Hydrate Ridge, US, as a refuge. Right: Deep-sea corals such as the one pictured are a source of jewelery and other riches. -  SERPENT Project/D.O.B. Jones, L. Levin, UK's BIS Department
Left: The first species ever recovered from the deep sea. Center: Rockfish use deep-sea carbonate formations at Hydrate Ridge, US, as a refuge. Right: Deep-sea corals such as the one pictured are a source of jewelery and other riches. – SERPENT Project/D.O.B. Jones, L. Levin, UK’s BIS Department

As fishing and the harvesting of metals, gas and oil have expanded deeper and deeper into the ocean, scientists are drawing attention to the services provided by the deep sea, the world’s largest environment. “This is the time to discuss deep-sea stewardship before exploitation is too much farther underway,” says lead-author Andrew Thurber. In a review published today in Biogeosciences, a journal of the European Geosciences Union (EGU), Thurber and colleagues summarise what this habitat provides to humans, and emphasise the need to protect it.

“The deep sea realm is so distant, but affects us in so many ways. That’s where the passion lies: to tell everyone what’s down there and that we still have a lot to explore,” says co-author Jeroen Ingels of Plymouth Marine Laboratory in the UK.

“What we know highlights that it provides much directly to society,” says Thurber, a researcher at the College of Earth, Ocean and Atmospheric Sciences at Oregon State University in the US. Yet, the deep sea is facing impacts from climate change and, as resources are depleted elsewhere, is being increasingly exploited by humans for food, energy and metals like gold and silver.

“We felt we had to do something,” says Ingels. “We all felt passionate about placing the deep sea in a relevant context and found that there was little out there aimed at explaining what the deep sea does for us to a broad audience that includes scientists, the non-specialists and ultimately the policy makers. There was a gap to be filled. So we said: ‘Let’s just make this happen’.”

In the review of over 200 scientific papers, the international team of researchers points out how vital the deep sea is to support our current way of life. It nurtures fish stocks, serves as a dumping ground for our waste, and is a massive reserve of oil, gas, precious metals and the rare minerals we use in modern electronics, such as cell phones and hybrid-car batteries. Further, hydrothermal vents and other deep-sea environments host life forms, from bacteria to sponges, that are a source of new antibiotics and anti-cancer chemicals. It also has a cultural value, with its strange species and untouched habitats inspiring books and films from 20,000 Leagues Under the Sea to Finding Nemo.

“From jewellery to oil and gas and future potential energy reserves as well as novel pharmaceuticals, deep-sea’s worth should be recognised so that, as we decide how to use it more in the future, we do not inhibit or lose the services that it already provides,” says Thurber.

The deep sea (ocean areas deeper than 200m) represents 98.5% of the volume of our planet that is hospitable to animals. It has received less attention than other environments because it is vast, dark and remote, and much of it is inaccessible to humans. But it has important global functions. In the Biogeosciences review the team shows that deep-sea marine life plays a crucial role in absorbing carbon dioxide from the atmosphere, as well as methane that occasionally leaks from under the seafloor. In doing so, the deep ocean has limited much of the effects of climate change.

This type of process occurs over a vast area and at a slow rate. Thurber gives other examples: manganese nodules, deep-sea sources of nickel, copper, cobalt and rare earth minerals, take centuries or longer to form and are not renewable. Likewise, slow-growing and long-lived species of fish and coral in the deep sea are more susceptible to overfishing. “This means that a different approach needs to be taken as we start harvesting the resources within it.”

By highlighting the importance of the deep sea and identifying the traits that differentiate this environment from others, the researchers hope to provide the tools for effective and sustainable management of this habitat.

“This study is one of the steps in making sure that the benefits of the deep sea are understood by those who are trying to, or beginning to, regulate its resources,” concludes Thurber. “We ultimately hope that it will be a useful tool for policy makers.”

Magnetic anomaly deep within Earth’s crust reveals Africa in North America

Boulder, Colo., USA – The repeated cycles of plate tectonics that have led to collision and assembly of large supercontinents and their breakup and formation of new ocean basins have produced continents that are collages of bits and pieces of other continents. Figuring out the origin and make-up of continental crust formed and modified by these tectonic events is a vital to understanding Earth’s geology and is important for many applied fields, such as oil, gas, and gold exploration.

In many cases, the rocks involved in these collision and pull-apart episodes are still buried deep beneath the Earth’s surface, so geologists must use geophysical measurements to study these features.

This new study by Elias Parker Jr. of the University of Georgia examines a prominent swath of lower-than-normal magnetism — known as the Brunswick Magnetic Anomaly — that stretches from Alabama through Georgia and off shore to the North Carolina coast.

The cause of this magnetic anomaly has been under some debate. Many geologists attribute the Brunswick Magnetic Anomaly to a belt of 200 million year old volcanic rocks that intruded around the time the Atlantic Ocean. In this case, the location of this magnetic anomaly would then mark the initial location where North America split from the rest of Pangea as that ancient supercontinent broke apart. Parker proposes a different source for this anomalous magnetic zone.

Drawing upon other studies that have demonstrated deeply buried metamorphic rocks can also have a coherent magnetic signal, Parker has analyzed the detailed characteristics of the magnetic anomalies from data collected across zones in Georgia and concludes that the Brunswick Magnetic Anomaly has a similar, deeply buried source. The anomalous magnetic signal is consistent with an older tectonic event — the Alleghanian orogeny that formed the Alleghany-Appalachian Mountains when the supercontinent of Pangea was assembled.

Parker’s main conclusion is that the rocks responsible for the Brunswick Magnetic Anomaly mark a major fault-zone that formed as portions of Africa and North America were sheared together roughly 300 million years ago — and that more extensive evidence for this collision are preserved along this zone. One interesting implication is that perhaps a larger portion of what is now Africa was left behind in the American southeast when Pangea later broke up.</P

Gold mining ravages Peru

The Carnegie Airborne Observatory flies over the Madre De Dios region of Peru, where vast deforested and polluted areas result from gold mining. -  Image courtesy Carnegie Airborne Observatory
The Carnegie Airborne Observatory flies over the Madre De Dios region of Peru, where vast deforested and polluted areas result from gold mining. – Image courtesy Carnegie Airborne Observatory

For the first time, researchers have been able to map the true extent of gold mining in the biologically diverse region of Madre De Dios in the Peruvian Amazon. The team combined field surveys with airborne mapping and high-resolution satellite monitoring to show that the geographic extent of mining has increased 400% from 1999 to 2012 and that the average annual rate of forest loss has tripled since the Great Recession of 2008. Until this study, thousands of small, clandestine mines that have boomed since the economic crisis have gone unmonitored. The research is published in the online early edition of the Proceedings of the National Academy of Sciences the week of October 28, 2013.

The team, led by Carnegie’s Greg Asner in close collaboration with officials from the Peruvian Ministry of Environment, used the Carnegie Landsat Analysis System-lite (CLASlite) to detect and map both large and small mining operations. CLASlite differs from other satellite mapping methods. It uses algorithms to detect changes to the forest in areas as small as 10 square meters, about 100 square feet, allowing scientists to find small-scale disturbances that cannot be detected by traditional satellite methods.

The team corroborated the satellite results with on-ground field surveys and Carnegie Airborne Observatory (CAO) data. The CAO uses Light Detection and Ranging (LiDAR), a technology that sweeps laser light across the vegetation canopy to image it in 3-D. It can determine the location of single standing trees at 3.5 feet (1.1 meter) resolution. This level of detail was used to assess how well CLASlite determined forest conditions in the mining areas. The CAO data were also used to evaluate the accuracy of the CLASlite maps along the edges of large mines, as well as the inaccessible small mines that are set back from roads and rivers to avoid detection. The field and CAO data confirmed up to 94% of the CLASlite mine detections.

Lead author Asner commented: “Our results reveal far more rainforest damage than previously reported by the government, NGOs, or other researchers. In all, we found that the rate of forest loss from gold mining accelerated from 5,350 acres (2,166 hectares) per year before 2008 to15,180 acres (6,145 hectares) each year after the 2008 global financial crisis that rocketed gold prices.”

In addition to wreaking direct havoc on tropical forests, gold mining releases sediment into rivers, with severe effects on aquatic life. Other recent work has shown that Perú’s gold mining has contributed to widespread mercury pollution affecting the entire food chain, including the food ingested by people throughout the region. Miners also hunt wild game, depleting the rainforest fauna around mining areas, and disrupting the ecological balance for centuries to come.

Co-author Ernesto Raez Luna, Senior Advisor to the Minister, Peruvian Ministry of the Environment, remarked: “Obtaining good information on illegal gold mining, to guide sound policy and enforcement decisions, has been particularly difficult so far. Finally, we have very detailed and accurate data that we can turn into government action. We are using this study to warn Peruvians on the terrible impact of illegal mining in one of the most important enclaves of biodiversity in the world, a place that we have vowed, as a nation, to protect for all humanity. Nobody should buy one gram of this jungle gold. The mining must be stopped.”

As of 2012, small illicit mines accounted for more than half of all mining operations in the region. Large mines of previous focus are heavy polluters but are taking on a subordinate role to thousands of small mines in degrading the tropical forest throughout the region. This trend highlights the importance of using this newer, high-resolution monitoring system for keeping tabs on this growing cause of forest loss.

Asner emphasized: “The gold rush in Madre de Dios, Perú, exceeds the combined effects of all other causes of forest loss in the region, including from logging, ranching and agriculture. This is really important because we’re talking about a global biodiversity hotspot. The region’s incredible flora and fauna is being lost to gold fever. “

There’s gold in them thar trees

This is a eucalyptus leaf showing traces of gold. -  CSIRO
This is a eucalyptus leaf showing traces of gold. – CSIRO

Eucalyptus trees – or gum trees as they are know – are drawing up gold particles from the earth via their root system and depositing it their leaves and branches.

Scientists from CSIRO made the discovery and have published their findings in the journal Nature Communications.

“The eucalypt acts as a hydraulic pump – its roots extend tens of metres into the ground and draw up water containing the gold. As the gold is likely to be toxic to the plant, it’s moved to the leaves and branches where it can be released or shed to the ground,” CSIRO geochemist Dr Mel Lintern said.

The discovery is unlikely to start an old-time gold rush – the “nuggets” are about one-fifth the diameter of a human hair. However, it could provide a golden opportunity for mineral exploration, as the leaves or soil underneath the trees could indicate gold ore deposits buried up to tens of metres underground and under sediments that are up to 60 million years old.

“The leaves could be used in combination with other tools as a more cost effective and environmentally friendly exploration technique,” Dr Lintern said.

“By sampling and analysing vegetation for traces of minerals, we may get an idea of what’s happening below the surface without the need to drill. It’s a more targeted way of searching for minerals that reduces costs and impact on the environment.

“Eucalyptus trees are so common that this technique could be widely applied across Australia. It could also be used to find other metals such as zinc and copper.”

Using CSIRO’s Maia detector for x-ray elemental imaging at the Australian Synchrotron, the research team was able to locate and see the gold in the leaves. The Synchrotron produced images depicting the gold, which would otherwise have been untraceable.

“Our advanced x-ray imaging enabled the researchers to examine the leaves and produce clear images of the traces of gold and other metals, nestled within their structure,” principal scientist at the Australian Synchrotron Dr David Paterson said.

“Before enthusiasts rush to prospect this gold from the trees or even the leaf litter, you need to know that these are tiny nuggets, which are about one-fifth the diameter of a human hair and generally invisible by other techniques and equipment.”

CSIRO research using natural materials, such as calcrete and laterite in soils, for mineral exploration has led to many successful ore deposit discoveries in regional Australia. The outcomes of the research provide a direct boost to the national economy.

X-ray vision to detect unseen gold

Powerful x-rays can now be used to rapidly and accurately detect gold in ore samples, thanks to a new technique developed by CSIRO – a move that could save Australia’s minerals industry hundreds of millions of dollars each year.

CSIRO has conducted a pilot study that shows that gamma-activation analysis (GAA) offers a much faster, more accurate way to detect gold than traditional chemical analysis methods.

This will mean mining companies can measure what’s coming in and out of their processing plants with greater accuracy, allowing them to monitor process performance and recover small traces of gold – worth millions of dollars – that would otherwise be discarded.

GAA works by scanning mineral samples – typically weighing around half a kilogram – using high-energy x-rays similar to those used to treat patients in hospitals. The x-rays activate any gold in the sample, and the activation is then picked up using a sensitive detector.

According to project leader Dr James Tickner, CSIRO’s study showed that this method is two-to-three times more accurate than the standard industry technique ‘fire assay’, which requires samples to be heated up to 1200°C.

“The big challenge for this project was to push the sensitivity of GAA to detect gold at much lower levels – well below a threshold of one gram per tonne,” he says.

Dr Tickner explains that a gold processing plant may only recover between 65 and 85 per cent of gold present in mined rock. Given a typical plant produces around A$1 billion of gold each year, this means hundreds of millions of dollars worth of gold is going to waste.

“Our experience suggests that better process monitoring can help reduce this loss by about a third,” he says.

Last year, Australia produced over A$10 billion worth of gold. Even if GAA only led to a modest 5 per cent improvement in recovery, that would be worth half a billion dollars annually to the industry.

Dr Tickner says that the other major benefit of GAA is that it is easily automated, allowing for much quicker analysis of ore samples.

“Fire assay usually involves sending samples off to a central lab and waiting several days for the results. Using GAA we can do the analysis in a matter of minutes, allowing companies to respond much more quickly to the data they’re collecting.”

“A compact GAA facility could even be trucked out to remote sites for rapid, on-the-spot analysis.”

Another great advantage of GAA is that it is more sustainable – unlike fire assay it doesn’t require the use of heavy metals such as lead.

It is also very adaptable. “While most of the work we’ve done has been based on the gold industry, the technique can be modified for other valuable commodities such as silver, lead, zinc, tin, copper and the platinum group metals.”

Now that the research team has proved the effectiveness of the technique, their next goal is to partner with local and international companies in order to get a full-scale analysis facility up and running in Australia. They hope to achieve this within the next two years.

Where does all the gold come from?

Ultra high precision analyses of some of the oldest rock samples on Earth by researchers at the University of Bristol provides clear evidence that the planet’s accessible reserves of precious metals are the result of a bombardment of meteorites more than 200 million years after the Earth was formed. The research is published today in Nature.

During the formation of the Earth, molten iron sank to its center to make the core. This took with it the vast majority of the planet’s precious metals – such as gold and platinum. In fact, there are enough precious metals in the core to cover the entire surface of the Earth with a four meter thick layer.

The removal of gold to the core should leave the outer portion of the Earth bereft of bling. However, precious metals are tens to thousands of times more abundant in the Earth’s silicate mantle than anticipated. It has previously been argued that this serendipitous over-abundance results from a cataclysmic meteorite shower that hit the Earth after the core formed. The full load of meteorite gold was thus added to the mantle alone and not lost to the deep interior.

To test this theory, Dr Matthias Willbold and Professor Tim Elliott of the Bristol Isotope Group in the School of Earth Sciences analysed rocks from Greenland that are nearly four billion years old, collected by Professor Stephen Moorbath of the University of Oxford. These ancient rocks provide a unique window into the composition of our planet shortly after the formation of the core but before the proposed meteorite bombardment.

The researchers determined the tungsten isotopic composition of these rocks. Tungsten (W) is a very rare element (one gram of rock contains only about one ten-millionth of a gram of tungsten) and, like gold and other precious elements, it should have entered the core when it formed. Like most elements, tungsten is comprised of several isotopes, atoms with the same chemical characteristics but slightly different masses. Isotopes provide robust fingerprints of the origin of material and the addition of meteorites to the Earth would leave a diagnostic mark on its W isotope composition.

Dr Willbold observed a 15 parts per million decrease in the relative abundance of the isotope 182W between the Greenland and modern day rocks. This small but significant change is in excellent agreement with that required to explain the excess of accessible gold on Earth as the fortunate by-product of meteorite bombardment.

Dr Willbold said: “Extracting tungsten from the rock samples and analysing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks. In fact, we are the first laboratory world-wide that has successfully made such high-quality measurements.”

The impacting meteorites were stirred into the Earth’s mantle by gigantic convection processes. A tantalising target for future work is to study how long this process took. Subsequently, geological processes formed the continents and concentrated the precious metals (and tungsten) in ore deposits which are mined today.

Dr Willbold continued: “Our work shows that most of the precious metals on which our economies and many key industrial processes are based have been added to our planet by lucky coincidence when the Earth was hit by about 20 billion billion tonnes of asteroidal material.”

New model for how Nevada gold deposits formed may help in gold exploration

Barrick Gold Corporation's large open pit at its Goldstrike Mine on the Carlin Trend. The mine has Carlin-type gold deposits, the formation of which has been newly modeled by University of Nevada researchers. -  Photo by John Mundean, University of Nevada, Reno and it's public service department, the Nevada Bureau of Mines and Geology.
Barrick Gold Corporation’s large open pit at its Goldstrike Mine on the Carlin Trend. The mine has Carlin-type gold deposits, the formation of which has been newly modeled by University of Nevada researchers. – Photo by John Mundean, University of Nevada, Reno and it’s public service department, the Nevada Bureau of Mines and Geology.

A team of University of Nevada, Reno and University of Nevada, Las Vegas researchers have devised a new model for how Nevada’s gold deposits formed, which may help in exploration efforts for new gold deposits.

The deposits, known as Carlin-type gold deposits, are characterized by extremely fine-grained nanometer-sized particles of gold adhered to pyrite over large areas that can extend to great depths. More gold has been mined from Carlin-type deposits in Nevada in the last 50 years – more than $200 billion worth at today’s gold prices – than was ever mined from during the California gold rush of the 1800s.

This current Nevada gold boom started in 1961 with the discovery of the Carlin gold mine, near the town of Carlin, at a spot where the early westward-moving prospectors missed the gold because it was too fine-grained to be readily seen. Since the 1960s, geologists have found clusters of these “Carlin-type” deposits throughout northern Nevada. They constitute, after South Africa, the second largest concentration of gold on Earth. Despite their importance, geologists have argued for decades about how they formed.

“Carlin-type deposits are unique to Nevada in that they represent a perfect storm of Nevada’s ideal geology – a tectonic trigger and magmatic processes, resulting in extremely efficient transport and deposition of gold,” said John Muntean, a research economic geologist with the Nevada Bureau of Mines and Geology at the University of Nevada, Reno and previously an industry geologist who explored for gold in Nevada for many years.

“Understanding how these deposits formed is important because most of the deposits that cropped out at the surface have likely been found. Exploration is increasingly targeting deeper deposits. Such risky deep exploration requires expensive drilling.

“Our model for the formation of Carlin-type deposits may not directly result in new discoveries, but models for gold deposit formation play an important role in how companies explore by mitigating risk. Knowing how certain types of gold deposits form allows one to be more predictive by evaluating whether ore-forming processes operated in the right geologic settings. This could lead to identification of potential new areas of discovery.”

Muntean collaborated with researchers from the University of Nevada, Las Vegas: Jean Cline, a facultyprofessor of geology at UNLV and a leading authority on Carlin-type gold deposits; Adam Simon, an assistant professor of geoscience who provided new experimental data and his expertise on the interplay between magmas and ore deposits; and Tony Longo, a post-doctoral fellow who carried out detailed microanalyses of the ore minerals.

The team combined decades of previous studies by research and industry geologists with new data of their own to reach their conclusions, which were written about in the Jan. 23 early online issue of Nature Geoscience magazine and will appear in the February printed edition. The team relates formation of the gold deposits to a change in plate tectonics and a major magma event about 40 million years ago. It is the most complete explanation for Carlin-type gold deposits to date.

“Our model won’t be the final word on Carlin-type deposits,” Muntean said. “We hope it spurs new research in Nevada, especially by people who may not necessarily be ore deposit geologists.”

Sulphur proves important in the formation of gold mines

Collaborating with an international research team, an economic geologist from The University of Western Ontario has discovered how gold-rich magma is produced, unveiling an all-important step in the formation of gold mines.

The findings were published in the December issue of Nature Geoscience.

Robert Linnen, the Robert Hodder Chair in Economic Geology in Western’s Department of Earth Sciences conducts research near Kirkland Lake, Ontario and says the results of the study could lead to a breakthrough in choosing geographic targets for gold exploration and making exploration more successful.

Noble metals, like gold, are transported by magma from deep within the mantle (below the surface) of the Earth to the shallow crust (the surface), where they form deposits. Through a series of experiments, Linnen and his colleagues from the University of Hannover (Germany), the University of Potsdam (Germany) and Laurentian University found that gold-rich magma can be generated in mantle also containing high amounts of sulphur.

“Sulphur wasn’t recognized as being that important, but we found it actually enhances gold solubility and solubility is a very important step in forming a gold deposit,” explains Linnen. “In some cases, we were detecting eight times the amount of gold if sulphur was also present.”

Citing the World Gold Council, Linnen says the best estimates available suggest the total volume of gold mined up to the end of 2009 was approximately 165,600 tonnes. Approximately 65 per cent of that total has been mined since 1950.

“All the easy stuff has been found,” offers Linnen. “So when you project to the future, we’re going to have to come up with different ways, different technologies and different philosophies for finding more resources because the demand for resources is ever-increasing.”

New translation reveals ancient metals and minerals

New GSA Special Paper 467, 'Mining and Metallurgy in Ancient Peru,' is a translation of a 1970 publication by Georg Petersen. Translator William E. Brooks notes that many of the ancient Andean mining and metallurgical techniques described in this book precede those known in Europe. -  The Geological Society of America
New GSA Special Paper 467, ‘Mining and Metallurgy in Ancient Peru,’ is a translation of a 1970 publication by Georg Petersen. Translator William E. Brooks notes that many of the ancient Andean mining and metallurgical techniques described in this book precede those known in Europe. – The Geological Society of America

In 2009, Perú was the world’s leading producer of silver, the second leading producer of copper, and the leading producer of gold in Latin America. But this isn’t something new. Perú’s leadership in mining and metallurgy extends for centuries into the past. This Special Paper from The Geological Society of America documents the use in ancient Perú of minerals, metals, and mineral resources for pigments, industrial stone, aesthetics, and art.

The GSA volume is a translation of a 1970 publication by the Instituto de Investigaciones Antropológicas, in Lima, Perú, written by Georg Petersen. Translator William E. Brooks notes that many of the ancient Andean mining and metallurgical techniques described in this book precede those known in Europe.

The volume also provides forward-thinking analytical data on metals, artifacts, and alloys. A detailed pyrite mirror, featured on book’s cover, symbolizes the spectacular workmanship and blending of utilitarian craft and mineral resources in ancient Perú.

Chapters cover minerals, gems, and pigments; ornamental and industrial stone; specific occurrences of gold, silver, copper, iron, mercury, tin, lead, and platinum in Perú, Bolivia, and Colombia; gold, silver, copper, and mercury metallurgy; Inca mining from 1533 to 1534 in the Altiplano, as documented by the Spanish explorers; and even a forensic description of the Chuquicamata Mummy, the remains of an ancient copper miner killed during an earthquake.