Category: Economic and applied geology

With the price of gold having climbed to above $1400 oz^-1 while social revolutions develop in North Africa and the Middle East, articles on how gold deposits form will get a wider readership than they would have during its doldrum years in the late 20^th century – the price has increased 7-fold since 2000. The bulk of gold nowadays can be mined profitably from ores in which it cannot be seen, except using a microscope, at grades well below 1 g t^-1 (1 part per million) thanks to cheap heap-leaching with sodium cyanide of lightly milled ore. The epitome of low-grade gold is that produced at huge open-pit mines in Nevada from sedimentary host rocks. The gold is far too fine grained to have been deposited as placers, and also occurs dissolved in pyrite (Fe₂S), so most experts regard it as having been introduced by hydrothermal fluids. Yet that covers two possibilities: by deep penetration of groundwater or from magmatic waters, and it is hard to decide which, again because the mineralisation is too fine grained to allow conclusive studies of fluid inclusions and stable isotopes. Also, such evidence as there is suggests low temperature fluids (~200° C) with low salinity; ambiguous data.

By using a synergy of ore-mineral chemistry, experimental data and ages of magmatism and mineralisation, Nevadan geologists have developed a convincing model for these ‘Carlin-type’ deposits (Muntean, J.L. et al. 2011. Magmatic-hydrothermal origin of Nevada’s Carlin-type gold deposits. Nature Geoscience, v. 4, p. 122-127). First, the mineralisation is of Eocene age and was introduced in Lower Palaeozoic sediments. The Eocene in the western USA saw the end of a period of compressional tectonics related to subduction since the Jurassic, fluids from which gave rise to partial melting of the overlying mantle wedge. This was succeeded by extensional tectonics and further intrusive magmatism dated between 40 to 36 Ma. This provided thermal energy and passageways for fluid migration. The second line of evidence is that hydrogen- and oxygen isotopes from fluid inclusions in hydrothermal gangue minerals show evidence that both mantle-derived and meteoric water mixed in the ore-forming fluids, and sulfur isotopes are similarly evidence of dual origin. Thirdly, the authors reasonably postulate from experimental data that basaltic back-arc magmas of Jurassic to Eocene age may have repeatedly added metals, including gold, to the mantle wedge that underpinned Nevada during subduction over a 175 Ma period. Thus later extension-related magmatism sourced in the wedge would itself have become metal enriched from this ‘fertile’ source. Moreover, conditions would have been ripe for highly oxidised conditions in the magmas and high concentrations of water in their fractionated descendants. Under such conditions gold and other metals favour entry into hydrothermal fluids. Given the extensional tectonic conditions such fluids could rise efficiently. Initially highly saline and very hot, rapid rise of the fluids would eventually result in them cooling adiabatically and separating into a dense salty liquid or brine and remaining vapour. That would force down the chlorine content in the vapour, favour some metals (Fe, Ag, Pb, Zn and Mn) ending up in the brine, while others (Au, Cu, As and Sb together with S) would remain in the vapour phase together with dissolved CO₂ in large amounts, making the vapour acidic. Able to pass into the fractured Palaeozoic cover, the fluids widened fractures in the carbonate sediments and facilitated their own precipitation of minerals, the foremost being gold-bearing pyrite. Nevada is probably unique, but my goodness it is a big gold province; >6000 t of gold in tham thar hills.

Of the fossil fuels coal has long been assumed to be the most plentiful, even the most pessimistic forecasters having acknowledged a global lifetime of centuries for known reserves. The determination of the emerging giant economies of China and India and of the USA to fuel themselves through coal-burning seems inevitable if highly risky for the climate. But that depends on coal remaining the cheapest fuel, largely because of the sheer abundance of supplies. A recent commentary on coal (Heinberg, R. & Fridley, D. 2010. The end of cheap coal. Nature, v. 268, p. 367-369) suggests that there is a growing tendency for reserve estimates to decrease as geologists factor in practical restrictions – place, depth, seam thickness and quality – on feasibility under current mining conditions, instead of just looking at known masses of coal. Astonishingly, the end-19^th century estimate of five thousand years of US coal supplies dropped to about 400 years by 1974 and is currently judged to be 240 years. China and India look likely to have less than 60 years-worth left. On top of that, the widely publicized turn to carbon capture and storage (CCS)for ‘clean-coal’ future supplies will inevitably drive-up prices of coal-fired energy. The two main factors in this remarkable transformation of ‘King Coal’ are fundamental economic forces in capitalism and the increasing refusal of miners to accept dangerous working conditions. The second is especially the case for China, where most coal is deep-mined; in the late 1990s it saw a drive to close down unsafe mines that caused production to fall, although it has greatly accelerated this century – further driving down coal’s lifetime there. It seems from this analysis that any realistic hope for a CCS-based coal economy, especially in China and India, depends on declining safety and environmental standards in their largely underground mines, which in turn depends on the highly unlikely willingness of their workforces to accept worse conditions.

The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

Around 1970 the production of natural gas in the US reached its peak and has been slowly declining since then. The degree to which the US economy has grown to depend on natural gas and growing fears of becoming dependent on insecure supplies on the international LPG market has seen a stealthy growth in unconventional technologies to maintain indigenous supplies. The greatest growth has been in winning the useful fuel from ‘tight’ organic-rich shales that are usually regarded as source rocks for conventional petroleum rather than resources in their own right (Kerr, R.A.. 2010. Natural gas from shale bursts onto the scene. Science, 328, p. 1624-1626). The technology relies on drilling methods developed in the oil industry that allow several holes from a single platform to bend to pass at low angles through thin, gently dipping strata. That allows far larger volumes to be tapped than through a single, vertical well. Oil shales are not yet targeted for liquid petroleum because of the cost, but as Richard Kerr, a news writer for Science, reveals they are supplying an increasing proportion of US gas demand: from 1% to 20% since 2000. Being less of a source of carbon dioxide than coal or oil that might seem to be a ‘good thing’ all round, but there are worrying and little known problems with the technology.

To get the gas out demands that the permeability of shale is artificially increased by jacking open joints and fractures using very high-pressure fluids that carry sand to wedge them open when production begins open: this is ‘fracking’ in driller-speak. Not only gas starts to move, but also water locked into the shale for millions of years and often highly toxic. Drillers hope that all the fluids will follow the holes, but that is by no means guaranteed and some may make their way into aquifers and up to the surface. The fluids used in fracking are deliberately full of chemicals that help open up cracks and even biocides that keep them from being clogged by bacterial films: around 15 million litres used per well. Although aimed to be recycled these noxious fluids can escape, sometimes in massive blowouts. Uncontrolled gas and formation water escapes can cause explosions and kill of forested areas by disrupting tree-root biota.

The NASA and German Aerospace Centre Gravity Recovery and Climate Experiment (GRACE) launched in 2002 aims to measure variations over time in the gravity field by gauging tiny changes in distance between two satellites using radar. The only significant changes in the short term are due to movements of water in one form or another. The best-known result from GRACE is its assessment of shrinking ice caps, and it can also detect shifting ocean currents and the drainage of lakes. The GRACE science team has noted a major change in gravity since launch over a nearly 3 million km² area of NW India centred on Delhi. The only conceivable mechanism is gradual loss of groundwater though irrigation of the Gangetic plains (Rodell, M. et al. 2009. Satellite-based estimates of groundwater depletion in India. Nature, v. 460, p.999-1002). The authors estimate a decline of around 109 km³ of groundwater since 2002 – more than twice the storage capacity of India’s largest reservoir. In places local farmers are reportedly having to sink deeper and deeper wells as the water table sinks by over 6 m each year. The area is one of Asia’s largest producer of food grains and occupied by 600 million people. Most likely there has been a surge in withdrawal for irrigation during poor monsoons in the early 21^st century, for pumping rates seem to be 70% greater than they were in the 1990s.

 

Gas hydrates soon to come on stream?

 The looming prospects of petroleum production outside of Arabia passing its peak and flexing of Russian economic power that stems from its control of the largest  untapped natural gas reserves are spurring evaluation of methane production from gas hydrates in onshore frozen peat mires and marine sediments. Gas hydrates are more equitably distributed than are much older petroleum reservoirs: even Japan, which is currently entirely dependent on foreign supplies, has what appear to be huge offshore reserves of gas hydrates. Estimates of the world‘s potential resources are enormous, at around 2 x 10¹⁶ m³ (annual US natural gas consumption is ~ 6 x 10¹¹ m³) but in a variety of sands and muds at different concentrations (Boswell, R. 2009. Is gas hydrate energy within reach? Science, v. 325, p. 957-958). Experiments in northern Canada (see Onshore gas hydrate reserves close to recovery in March 2004 issue of EPN) indicates that drilling to induce lower pressure in gas hydrate bearing sediments induces dissociation of the hydrate crystals to release methane while retaining also present within their structure. Injection of CO₂ into deposits should displace methane while CO₂ enters the crystalline structure: killing two birds, including carbon sequestration, with one stone. The main technical stumbling block is that gas hydrates occur in unconsolidated sediments that may be destabilised during production, resulting in uncontrollable release of the powerful greenhouse gases as well as collapse of surface structures. From an environmental standpoint, all gas hydrates do is sustain reliance on carbon-based fossil fuels and continue emissions of greenhouse gases, though burning methane is a good deal ‘cleaner’ than coal or oil.

Maybe it was a coincidence, but the April issue of Geology contain a paper whose title looked suspiciously unreal (White, K. et al. 2009. Hydrologic evolution of the Edwards Aquifer recharge zone (Balcones fault zone) as recorded in the DNA of eyeless Cicurina cave spiders, south-central Texas. Geology, v. 37, p. 339-342). Seemingly, the Cretaceous Edwards Aquifer now flows through cavern systems at the base of a fault-controlled escarpment. At higher levels in the unit are air-filled caves, that are relics of previous karstic events. It is in these dark, dry caves that the arachnid troglobites dwell. Troglobitic animals (those that inhabit totally dark caves and have no eyes) originate as normal surface dwellers, which through successive generations lose functioning eyes and coloration. Conversely, they evolve improved senses of smell, taste and vibration detection. The species that emerge are among the rarest of creatures, for they often occur in only a single cave: a special case of allopatric speciation that may happen when small populations are cut off from one another. Technically, then, this study is no joke, for analysis of mtDNA from the spiders in different caves ought to show evidence of microcosmic evolution, and possible provide a molecular ‘clock’ to chart the times of cave colonisation. And this is what the authors from the University of Mississippi and the endangered invertebrate group of a Texan consulting company have tried to do. The spiders in the higher caves are more evolved than those at progressively lower levels. Moreover, since the karst evolution has developed in a structurally active setting, the spider data correlates with tectonic history…

Metal thefts in the UK have increased to such an extent in 2008 that police are marking lead on church roofs with the same identification tags as televisions and DVD players. Similarly there has been an outbreak of filching heating oil and diesel from isolated farmsteads. This follows the surge in commodity prices during the first two quarters of 2008. On a more legal note, oil and mining companies have found that their assets have soared, and unsurprisingly they want more of the same, while the prices hold or rise even further. Exploration managers with increased budgets are set to thrust out to the frontiers, and consultants are rubbing their hands with glee. On the surface, these developments might seem to foretell a welcome rise in the employability of people with a geoscience degree; or so think three contributors to the 8 August 2008 issue of Science (Gramling, C. 2008. In the geosciences, business is booming. Science v. 321, p. 856-7. Laursen, L. 2008. Geoscientists in high demand in the oil industry Science v. 321, p. 857-9. Coontz, R. 2008. Hydrogeologists tap into demand for an irreplaceable resource. Science v. 321, p. 858-9).  It is claimed that geoscience jobs in the US will rise by 22% in the next decade, compared with an overall jobs forecast around 10%. Low place-value physical resources being, by definition, potentially profitable world-wide, prospects ought to be good for ‘geos’ globally.  Salaries also seem to be set to rise, along with employability for individuals with first degrees, as opposed to master’s qualifications. The ruthless downsizing, outsourcing and  lay-offs of the 80s and 90s have also placed greater value on Earth science qualifications, simply because there has been a decline in students opting for seemingly moribund career prospects; a matter of increased demand facing diminished supply, as any trader at the London metal exchange or the world’s petroleum spot markets would verify. At the same time, shifts in research funding from rock-oriented geosciences to Earth system science have created a bear market for geological academic posts. High-flying geologists in universities and surveys may well be polishing up their CVs in anticipation of a growing wage differential between the public and private sectors.

Set against such rosy prospects are the inherent economic risks that are bound up with inflation in commodity prices. Historically, there has been a tendency for boom then bust in mining and the oil industry. The contrast between the surge in petroleum and metals prices following the Yon Kippur War and the Iranian Revolution and recession in the 80s and 90s being too recent to ignore, as many ‘geos’ who found themselves ‘over the hill’ in its aftermath will admit. It would be wise to look on prospects with caution. One area that is likely to rise in prominence is ‘environmental’ geology: the likes of hydrogeology; geotechnics; coastal and flood defence. The problems that global warming may bring, an increased focus on leisure learning and heritage, and the fact that around 20% of all living people have little if any access to clean drinking water and adequate standards of public hygiene compete in many ways for young geoscientists’ aspirations. On a mercenary yet acutely practical note, growing environmental legislation and provision of development funds by non-governmental agencies that range in scale from the UN ‘family’ to small charitable bodies suggest that these fields are likely to provide satisfyingly useful employment with longer-term stability than the uncontrollable vagaries of the commodity markets, albeit at somewhat more modest salaries.

In 1999 the late Thomas Gold, cosmologist and quite a lot more, annoyed the geoscience community with publication of his book The Deep Hot Biosphere: The Myth of Fossil Fuels (Springer-Verlag: New York). In that book Gold reached the acme of his lone campaign for recognition that oil, gas and even coal formed from carbon and hydrogen feedstock that had been residing in the mantle since the Earth’s accretion. He suggested that it was mediated by a hidden yet teeming biosphere at much deeper levels than suspected at the time. I did my share of carpet gnawing, but was sorry to learn of the death in 2004 of such a supreme scientific provocateur. Although without mentioning Gold, a recent paper hints at a possibility that he may have been on to something (Proskurowski, G. et al. 2008. Abiogenic hydrocarbon production at Lost City hydrothermal field. Science, v. 319, p. 604-607).

Hydrocarbons are often found as blobs in fluid inclusions within gangue minerals of a variety of ore bodies. The US-Swiss research team examined hydrocarbons within the fluids that gush from a hydrothermal vent at 30˚N on the Mid-Atlantic Ridge; i.e. where there is no older sediment that might host biologically generated hydrocarbons, but where heat-loving microbial life could play a role. Molecular structure and carbon-isotope composition of the hydrocarbons point strongly to their formation by reduction of CO₂ to methane and low molecular weight hydrocarbons by the catalytic action of mineral surfaces in the presence of a great deal of hydrogen. This is known as a Fischer-Tropsch reaction, the basis for making oil from coal, as in Nazi Germany and South Africa when under economic blockade.

The CO₂ could have come from two possible sources: seawater or the mantle beneath the Lost City vents. Hydrogen can form abundantly when the olivine in peridotite beaks down to serpentinite as seawater is convected through the oceanic mantle. The vents have created towers made partly of carbonates, in whose pores there are microbes whose metabolism is based on use of hydrogen. However, the key finding is that the hydrocarbons contain no radioactive ¹⁴C, which forms by cosmic-ray interaction with nitrogen atoms in the atmosphere and is easily detectable in seawater. This rules out a seawater source for the CO₂, but supports a mantle origin.

As they say, ‘Gold is where you find it’ – gold mineralisation has a great diversity of settings. One of the oddest gold mines is the Ladolam deposit on the island of Lihir off Papua New Guinea — it is also one of the largest, with reserves of around 1300 tons (~41 million troy ounces). There, gold is being extracted from an open pit, cooled by water injection, in the crater of a geothermally active volcano. Aside from that it is one of many different kinds of hydrothermal deposit in which metals are transported and deposited by a plumbing system that delivers hot watery fluids. The hydrothermal system on Lihir is obviously still active, and it is possible to sample the fluid itself by drilling to depths up to a kilometre. Deep sampling is needed to obtain pristine fluids, uncontaminated by mixing with groundwater. Their chemical composition trns out to be surprising (Simmons, S.F. & Brown, K.L. 2006. Gold in magmatic hydrothermal solutions and the rapid formation of a giant ore deposit. Science, v. 314, p. 288-291).

The ground in which the deposit occurs is a breccia produced by explosive decompression when the volcano collapsed in its last magmatic throes, at about 400 ka. It is this brecciation that provided the intricate pathways in which gold was able to precipitate from the hydrothermal fluids. The samples have deuterium and oxygen isotopes that show that it is derived directly from magma. The fluid is extremely saline with very high chloride and sulfate ion concentrations. Around 50 kg of the fluid reaches the surface every second. Because it contains about 15 parts per billion of gold, it is possible to estimate how long it might have taken to produce the gold ore body: a surprisingly rapid 55 thousand years at the current rate of 24 kg of gold per year. Even more surprising is that the Lihir hydrothermal fluid is not particularly rich in gold compared with the fluids emerging from some active volcanoes. For instance Mount Etna is estimated to be delivering up to a tone of gold every year. However, before setting off on a gold rush to extinct volcanoes in the last hydrothermal phase, it is worth bearing in mind that forming a super-rich giant gold deposit requires that both gold transport and deposition are closely synchronised in a small volume of rock, otherwise the gold merely ends up in such a vast volume of rock that its extraction is not economic.