Category: Economic and applied geology

Australia is rightly famous for its gold nuggets and some, such as the ‘Golden Eagle’ found at Coolgardie, were as big as a gap-year’s rucksack. The curious thing about them is that they are generally found in the most featureless parts of the continent, Western Australia being a case in point. What sharpens the paradox is that these flat areas have been peneplains for up to a billion years. A nugget found in a Yukon or Californian stream is easily attributed to high-energy transport in water, and indeed most of those show signs of long transport in water: they are rounded and pitted. The one kilogram and weightier nuggets from Australia could never have been physically moved across the featureless plains, and most of them come from the alluvium deposited by sluggish Cenozoic drainages, now as dry as a bone — the ‘deep leads’ famous for their gold rushes in the past. They are also oddly shaped, the ‘Golden Eagle’ having wing-like flanges, which any physical transport would bend into conformity, for gold is of course very malleable. One long-held hypothesis is that they formed by precipitation from the extremely noxious groundwater that still persists tens of metres beneath the surface, gold being water-transportable in the form of complex ions such as those involving Au and Cl­. But it now seems that the mediator is bacterial in origin (Reith, F. et al. 2006. Biomineralization of gold: biofilms on bacterioform gold. Science, v. 313, p. 233-236).

Frank Reith and his Australian colleagues collected soils that contain small gold grains from goldfields across the continent. A great many have strangely knobbly surfaces and branching structure when scanned under an electron microscope, whereas fine gold grains from primary deposits in hard rock often shows signs of gold’s crystal symmetry, or at least highly angular surfaces. The soil-gold particles do look as though they formed in association with living processes. Using stains that fluoresce when bonded to organic matter the researchers found numerous associations between gold and organisms of some kind. When organic material was leached from separated gold grains it revealed DNA closely similar to a bacterium that is known experimentally to precipitate gold from dissolved Au-Cl  complexes. Ordinary soil grains showed no such genetic tracers. It looks as if Reith et al. have discovered living biofilms coating the gold grains that the constituent bacteria are in the process of growing. Amazingly, they also found gold-plated living bacterial cells. The probable explanation is that the bacteria live in water so rich in gold (by no means a great deal of it, however) that they are defending themselves from gold’s known toxicity — Ralstonia metallidurans, as its Latin name suggests, is a highly metal-tolerant organism. Nuggets may well form as a result of bacterial defence mechanisms.

There is a grim determination at NASA, and in the current US presidential administration that funds it, to get back to and stay on the Moon. Of course, it would be absurdly costly to ship out all the necessities for survival beyond a few days, the weightiest item being water. Protected by the frigid permanent shadows inside craters near the lunar poles, there may be some very old ice there (see Puffing up the Moon in April 2006 issue of EPN). NASA intends to crash a two-tonne spent rocket stage from a planned pre-landing mission into Shackleton crater, hoping to detect water vapour in the debris plume thrown up by the impact. Once the surveying satellite carried by the mission has done its job, that too is going to be crashed in the hunt for what is clearly more precious than gold for would be lunar colonisers.

Source: News in Brief. Nature, v. 440, p. 858.

On paper, metal resources lying on the deep ocean floor look like an economic panacea. Large areas are covered with either a crust or scattered, potato-sized nodules rich in manganese, copper, cobalt, nickel and several other metals. In some ocean basins, one scoop might provide ore grades for all of them, as in the best onshore multi-metal deposits. ‘Black smokers’ and the metal-rich pillars and muds that develop from them seem just as promising for lead, zinc, copper and even gold: such submarine hydrothermal exhalations probably formed many of the rich massive sulphide deposits sought on land. The 1960s and early 70s seemed likely to foster a fundamental shift in metal extraction, but despite rises in metal prices after the 1973 Yom Kippur war and Iranian revolution of 1978, the excitement faded to insignificance.  There were a few ironies too. A ship was designed and almost completed by one of Howard Hughes’ many companies, Global Marine, supposedly to harvest ocean-floor manganese nodules. In fact, the venture was to be secretly directed at salvaging a sunken Soviet nuclear submarine, and the code books that it carried, from the floor of the Pacific Ocean. It now seems that ocean-floor mining might be resurrected – assuming that all does not descend into further wrangling over the Law of the Sea and who should benefit from profits (Thwaites, T. 2005.  Treasure Ocean. New Scientist, 17 December 2005, p. 40-53). An Australian company called Seacore is soon to drill around New Guinea and New Zealand to evaluate the potential of exhalative deposits.  They claim that if thicknesses greater than 15 m, at decent grades for gold, copper, zinc, silver and lead, are found dredging up the ores would be commercially possible.  Essentially it would be literally a smash and grab job, unlike the massive logistics of on-shore open-pit and subsurface mining, albeit tempered by problems connected with depths of several kilometres. Understandably, there are environmental concerns about exposing highly anomalous concentrations of metals and associated sulfide minerals, probably in a fine-grained soft state. Ocean ecosystems are fundamentally based on clear water, and mud plumes could wreak havoc far afield.  The deposits would have to be sucked to the surface using the air-lift dredge technique pioneered by marine archaeologists, but on a much larger scale.  Yet this appears to be more than a means of attracting and siphoning off venture capital, for the groundwork of identifying targets has already been done by Placer Dome, a well-heeled Canadian mining company.  Also, the thorny issue of the legality of harvesting the global oceanic ‘commons’ in international waters is being avoided by drilling within national offshore limits, as has long happened with offshore oil development.

Banded iron formations (BIFs) are by far the largest repositories of economic iron ore on Earth, and mines in them dwarf all but the largest surface coal mines. They also present one of the most enduring paradoxes in geochemistry. BIFs consist of oxidised iron in the form of iron(III) oxide (mainly hematite, Fe₂O₃), yet formed before about 2 billion years ago, when the Earth’s atmosphere and oceans were devoid of free oxygen. In fact the very formation of BIFs presupposes that iron must have been freely available in seawater as dissolved ions of its reduced form, iron(II). Their formation has been linked to the excretion of oxygen by photosynthesising cyanobacteria in the photoc zone of Archaean and Palaeoproterozoic seas, which would immediately combine with iron(II), thereby buffering environmental oxygen at very low levels. The problem with that hypothesis is BIFs show every sign of having accumulated in extremely quiet conditions: they contain the most exquisitely fine banding that in some cases has been linked to a diurnal cycle. The photic zone would have been one of high wave energy. A more environmentally viable hypothesis has to take account of that and place the environment of BIF deposition in deeper water. Biogeochemists of the California Institute of Technology and the University of Alberta have perhaps helped to resolve all the paradoxes surrounding BIFs (Kappler, A. et al. 2005. Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology, v.  33, p. 865-868). The bacteria that they cite as agents for iron(III) precipitation use the photon energy of ultraviolet radiation to oxidise iron(II) to iron(III), and in doing so use the freed electrons to reduce CO₂ and water to carbohydrate – this is not photosynthesis that uses light energy to increase the energy of electrons so that they perform the life-giving reduction. Solar ultraviolet radiation penetrates to much greater depths than the red light exploited by photosynthesisers, and could therefore fuel BIF formation below storm wave base at depths greater than 200m.

One of the most revealing field trips that I ever made was to the now-closed Pine Point lead-zinc deposit in Canada’s Northwest Territories. Being in the company of the late Doug Shearman (dcd. 2003) of Imperial College London helped a great deal, but the evidence exposed in and around the mine reawoke my interest in sedimentary processes that lead to economic mineralisation.  To cut a long story short, Pine Point developed by the passage of Devonian seawater from a vast evaporating basin through a barrier-reef complex, in which a variety of chemical and biological environments, and products of karst formation encouraged the fluid to deposit the metals that it contained on an awesome scale.  Limestone-hosted Pb-Zn ores occur widely in Britain and Ireland in rocks of Carboniferous age, the most familiar to me in the English Pennines being in narrow veins.  The biggest in Ireland, and they are world-class, are more pervasive of the carbonate host.  How they formed has been conjectural and based on geological relationships in what is a small area by comparison with the vast Late Palaeozoic sedimentary systems of the Canadian Shield.  Crucial large-scale evidence is meagre.  Studying the chemistry of the ore-bearing solution trapped in Irish fluid inclusions reveals a familiar picture (Wilkinson, J.J. et al. 2005.  Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield.  Geology, v. 33, p. 805-808).

Multi-element geochemistry plus strontium and sulfur isotope composition of the included fluids in Irish deposits reveals the signature of considerable concentration of the brines by evaporation, together with their having scavenged metals from crustal rocks as they circulated at depth.  Returning to the surface along fault-controlled conduits, the metal-rich brine seems to have mixed with another. As at Pine Point, the sulfur needed to precipitate metal ions as insoluble sulfide ore minerals was probably supplied as hydrogen sulfide excreted by anaerobic bacteria that reduce sulfate ions in seawater. Doug demonstrated this phenomenon in 1981 with a linen handkerchief soaked in lead acetate solution, which he dipped into a foetid swamp seething with such ‘bugs’ on the Pine Point muskeg. ‘Instant orebody’, he cried, as the hanky turned black from fine galena particles.

Although the Irish Zn-Pb ores are more related to faults than to limestone reefs, nonetheless local geology demonstrates considerable relief on the floor of the shallow Carboniferous sea.  Fully understanding the ‘plumbing’ and the geochemistry requires, as Wilkinson and colleagues suggest, a regional view of Carboniferous tectonics just before Africa collided with Laurussia. Just before that amalgamation, restricted, evaporation-prone basins would have formed.  On a continental scale the circulation of their concentrated brines would have followed active faults systems that reached the shallow sea bed: a great deal more complicated than what is plain to see at Pine Point, given the eye of one of post-WW2 Britain’s lions of geology.

Since the origin of life it is certain that a proportion of biological materials would have been preserved in sediments after organisms died. As today, such material would have evolved or matured as the host sediments were buried and heated. There is plenty of evidence that such maturation did occur as far back as 3250 Ma ago, but signs that oil-fields formed by migration and trapping have proved elusive. Several lines of evidence, such as carbon-isotope anomalies in Precambrian limestones, point to periods when enormous amounts of organic material were buried, much as happens in the formation of Phanerozoic petroleum source rocks during periods of ocean anoxia. Before about 2400 Ma, when evidence for an oxidising surface environment first appears in the rock record, such conditions would have been pervasive. The first hints of large-scale petroleum formation and migration have been found in the low-grade Pilbara craton (3500-2850 Ma) of Western Australia and 2770-2450 Ma sediments that overlie the older Archaean complex (Rasmussen, B. 2005. Evidence for pervasive petroleum generation and migration in 3.2 and 2.63 Ga shales. Geology, v. 33, p. 497-500). Black shales in the Pilbara contain not only lots of fine-grained carbonaceous matter, but some in forms that clearly suggest that they had been thermally matured (‘cracked’) to low-viscosity fluids that could migrate. There are blobs of bitumen contained within iron sulfide layers that seem to have formed later, to engulf petroleum liquids. Molecules within the bitumens resemble those formed by photosynthesising blue-green bacteria, methanogen and sulfate-reducing bacteria and arguably perhaps primitive eukaryotes. It appears that the bitumens probably formed as residues as lighter and more fluid hydrocarbons migrated out of these substantial source rocks. What has yet to be demonstrated are Archaean and Palaeoproterozoic reservoir rocks where such migrating petroleum accumulated. Another question is whether or not the source rocks, which are extremely widespread and thick, might have retained some potential for sourcing petroleum much later in the geological history of Western Australia and similar cratons elsewhere.

In their slack moments, petroleum geologists ponder on when oil and gas got into a particular reservoir and became trapped.  One aspect of the conundrum is easy to answer: after the reservoir rock and trap formed.  But timing is not so trivial, for an important consideration in exploration for new oilfields concerns the actual rock that sourced hydrocarbons in known fields, almost always a highly reduced, black mudrock in which unoxidised dead organic matter accumulated and matured. Repeated anoxic events, both regional and global, provide several alternatives in many petroleum provinces.

Hydrocarbons, having formed under highly reducing conditions, contain several metals and other elements well above normal crustal concentrations.  Among these are rhenium and osmium, which allow radiometric dating through the decay of ¹⁸⁷Re to ¹⁸⁷Os.  In principle, therefore, it is possible to date oil and relate it to a particular source rock. Interestingly, it is easier to date the actual time at which oil has accumulated in a trap.  In an analogous way to the equilibration of parent and daughter isotopes in magmas, which is halted by crystallization so that the system evolves and dating can be done, once oil settles in a trap after migration the timing can be dated sing the Re-Os method.  David Selby and Robert Creaser of the University of Alberta, Canada applied this approach for the first time, using the vast reservoirs of oil sand in Alberta as a test (Selby, D. & Creaser, R.A. 2005.  Direct radiometric dating of hydrocarbon deposits using rhenium-osmium isotopes.  Science, v. 308, p. 1293-1295). The oil in the sands were emplaced around 112 ±5 Ma ago, during the Early Cretaceous, not long after the host sandstones had been deposited.  Previous work using ideas on oil maturation suggested that migration had taken place during the Early Palaeocene, around 60 Ma ago, when potential source rocks were heated by tectonic burial during the Laramide orogeny.  The Re-Os results point to migration from the west while the Cretaceous sedimentary basin was filling.  This may explain the high viscosity of the oils as a result of near-surface biodegradation.

Another product of isotopic dating is establishing the initial ¹⁸⁷Os/¹⁸⁸Os ratio of the petroleum system, which relates to that of the original source and its isotopic evolution.  In the case of the oil sands this value points to source rocks of earlier Mesozoic and even Palaeozoic age, rather than a Cretaceous source that had been suggested previously.

On May 24 the government of Tanzania cancelled a contract with the commercial water giant Biwater, which was supposed to bring clean water to the country’s largest city Dar es Salaam, and establish a privatised water supply.  The UK-based company had won a £76.5 million contract from the World Bank, with the support of the British government’s Department for International Development (DfID).  DiFID had paid the free-market thinktank £0.5 million in fees to advise the Tanzanian government and promote privatisation, out of a total expenditure of more that £36 million since 1998 for similar consultancies.  In two years Biwater has failed to install a single pipe (Vidal, J. 2005.  Flagship water privatisation fails in Tanzania.  The Guardian 25 May 2005, p. 4).

In her statement to the International Conference on Water and Sustainable Development in Paris (March 1998) Clare Short (British minister then heading DfID) outlined the New Labour government’s “vision” on water resources in the Third World, “Partnerships among governments, the private sector and civil society are critical to sustainable development [of water resources]”.  Policy of the International Monetary Fund is to enforce “structural adjustment programmes” on poorer countries as a condition for rescheduling debt repayments. Into these are written the privatisation of formerly public assets, such as water utilities. The first targets for this in Africa were the townships of South Africa, following the removal of apartheid.  Although very poor by western standards, and with unemployment running at up to 50%, people in South African townships are better off than the majority of sub-Saharan Africans.  Potential profits from water metering seemed attractive.  However, a great many people found themselves cut off from this most basic necessity in 2000, being unable to pay the increased water rates.  This led to nationwide protests, the most violent being in the arid Transvaal.  The company involved in that region was also Biwater, with bids for contracts worth 12 billion rand.  The company has an interesting history, having been an early beneficiary of the Conservative government’s “aid for trade” programme in the 1980s, including dam and water distribution contracts in Malaysia and Thailand that were linked to British arms supplies to the governments involved.

Water privatisation is a target outside Africa, perhaps the most notorious case being in South America. Bolivian trades unionists demonstrated on 6 April 2000 against a 35% rise in water prices imposed on the city of Cochabamba.  Military forces opened fire, killing 6 demonstrators, and a state of siege was declared by the authorities. The price hike stemmed from the new owner of the region’s water system – International Waters Ltd (IWL) of London, a subsidiary of Bechtel, based in San Francisco.  IWL’s Bolivian operation centres on the Misicuni dam project.  Water from the dam will cost 6 times more than it would from alternative sources.  The increased water charges were to recover the cost of the dam, with one problem: the dam had not been built, and IWL/Bechtel had put no funds into the construction project.  Subsequently, public pressure forced the ending of the contract.  Similar upheavals have been seen in Ghana, Trinidad, Argentina and the Phillipines.

News of Tanzania’s decision to end the ill-fated contract with Biwater followed announcements in the same week that the EU would effectively double its Third World aid.  In early July, Britain will host the 2005 G8 summit, which will be dominated by discussion of ways to increase the flow of finance into Africa in particular.  This follows the publication in early 2005 of the Commission for Africa Report sponsored by the New Labour government. Two thirds of the world’s population lacks sanitation that is adequate for healthy living.  Of them, one billion people, including the majority of Africans, have no access to safe drinking water.  Poor water supplies form the main contributor to the death of children under five years old.  For hundreds of millions of people, getting water for domestic use consumes much of their daily labour, which involves mainly women and children trudging to distant water sources and carrying it home, on average twice each day.  The failure of private enterprise to deliver water to the needy suggests that the small print of any declaration from the G8 summit needs the most careful scrutiny.

Scientists in developed countries are more or less unanimous that climate is warming because of rising CO₂ levels from the burning of fossil fuels.  That spurs calls for less reliance on fossil fuels and more use of renewable energy resources, including biomass.  The situation for the other two-thirds of humanity is much different.  The majority depends on biomass fuels (wood products, agricultural waste or animal dung).  Unprotected burning of biofuels releases such levels of carcinogens that 1.6 million people including 400 thousand in sub-Saharan Africa, mainly women and infants, meet an early death each year.  By 2030 this may rise to over 9 million, if current fuel use continues.  Biofuels also devastate woodland cover, and burning animal dung reduces natural fertiliser used on fields: two contributors to the inexorable decline in conditions of life in the “Two-Thirds World”.

Energy researchers at Harvard and the University of California have examined the options for household fuels in the light of these “counter-environmentalism” facts (Bailis, R. et al. 2005.  Mortality and greenhouse impacts of biomass and petroleum energy futures in Africa.  Science, v. 308, p. 98-103).  A safer alternative to wood and dung burning is the use of charcoal, yet that would increase CO₂ emissions by around 50%, as well as increasing loss of woodland.  The higher energy content of non-coal fossil fuels would actually decrease the “greenhouse” burden, while improving health dramatically.  They estimate that a shift to petroleum-based household fuels would delay between 1.3 to 3.7 million deaths per annum, by 2030

From time to time, truly odd ideas emerge, even from such a conservative bunch as geoscientists.  They are often based on quite mundane science.  If you pour sulphuric acid on limestone, of course it fizzes violently because CO₂ is a product of the simple reaction.  Less noticeable is that the other product, hydrated calcium sulphate or gypsum, is considerably less dense than the calcite in limestone.  The solid residue swells.  “What if….?”, thought Dutch geochemist Roelof Schuiling (Ravilious, K 2004.  The new stone age.  New Scientist, 20 November 2004, p. 38-41).  His idea was to put the simple phenomenon to practical use; infiltrate sulphuric acid into porous limestone and the swelling will bulge up the surface.  This does happen naturally, where sulphide-sulphate oxidising bacteria generate sulphuric acid, which renders limestone to an easily erodable mess, and in some soils generates gypsum lenses that bulge up the ground into surface blisters.  Schuiling reckons that the huge sulphuric acid surplus, created partly by removing sulphur from vehicle fuels, could be used as a kind of geo-engineering tool on a vast scale.  For instance, the coralline shallows beneath the shallow Palk Straits that separate India and Sri Lanka, could be induced to bulge up and create an island ridge, and so complete what is known as Adam’s Bridge that nearly links the two countries.  Closer to home, the Low Countries might become the “Slightly Higher Countries”.  Worryingly, the technology to make the process viable is simple, if a little expensive on the scales envisaged.  The worry, of course, is yet more CO­₂ emission plus the effect on the environment of so much sulphate and a massive fall in pH.