crust periodically saturated the magma in ore-forming minerals, including chromite, magnetite, and sulfides that settled out of the magma chamber to formed distinct bands. In contrast, hydromagmatic ores such as porphyry copper deposits, which are the primary source of copper ore, form when a saline aqueous fluid exsolves from a magma and intrudes, often with violent force, into surrounding rock. Laboratory experiments together with analysis of fluid inclusions in these ores have revealed that many metals, including Cu, Zn, Pb, Co, Sn, and Au, form highly soluble chloride and sulfide complexes in these fluids at elevated temperatures and partition into the fluid phase from the magma, then precipitate when the solution cools. These form mainly from subduction-related magmas because they are rich in water and oxidizing; the latter prevents premature precipitation from the magma of the ore metals as sulfides. Many tin deposits form in a similar way but the magmas are produced by melting of Sn-rich sediments within the crust and reducing conditions allow Sn concentrations to build up through fractional crystallization and Sn is often complexed by F rather than Cl.
Hydrothermal ores also precipitate from aqueous solution and chloride complexes are also important in transporting metals in these deposits. The fluid, however, is derived from seawater or formation brines within the crust. These types of deposits include volcanogenic massive sulfides (VMS); mid-ocean ridge hydrothermal systems are actively forming examples of this type of deposit. The ore-forming fluids can be directly sampled and their chemistry determined; study of these systems has provided much insight into how VMS deposits form. Seawater is warmed as it penetrates the hot, young ocean crust and a series of reactions result in the solution becoming acidic and reducing. Under these conditions, metals, most notably Cu, Zn, and Pb, are leached from the rock. When temperatures reach 350–400°C, the fluid rises, eventually mixing with seawater whereupon the metals precipitate as sulfides.
We'll examine two examples of sedimentary ore deposits. The first is banded iron formations, which are the principal source of iron ore. Most of these formed around the time the atmosphere first became oxidizing about 2.3−2.4 billion years ago as ferrous iron-rich deep ocean water upwelled to the surface and the iron was oxidized to the insoluble ferric form. Directly or indirectly, the evolution of photosynthetic life appears responsible for them. Brines associated with saline lakes and salars, or salt flats, and their associated brines, particularly from the high plateaus of the Andes and Tibet, are becoming the most important source of lithium, which is needed for high performance batteries in everything from cell phones to electric cars. But not all such brines are Li-rich; we learn the conditions under which Li-rich brines form. Weathering-related ore deposits include bauxite, the ore of Al, and laterites, which are sources of Ni, Fe, and rare earths. These form through extreme weathering of soils such that little remains but these highly insoluble elements; what we have learned about weathering, soil-forming processes, and the geochemistry of these metals will serve us well understand how these deposits form. Because of their importance to everything from flat panel computer displays and televisions to high performance magnets in wind turbines, electric cars, and speakers, we briefly examine rare earth ore deposits, which fall into many of the above categories.
Finally, we put our geochemical toolbox to use to understand how human activities can degrade environmental quality and how this can be addressed. Like ore deposits, this is an enormous topic and we have space to consider only a few examples. We begin with the problem of eutrophication and associated anoxia in fresh water lakes, using Lake Erie, one of the Great Lakes of North America, as an example. Eutrophication refers to situations where nutrient levels in water allow excessive growth of algae, usually cyanobacteria, which produce microcystin toxins. Lakes typically become temperature-stratified in summer such that oxygen in the deep water is not replenished. Bacteria consuming the remains of algae falling into the deep water can consume all available oxygen leading to anoxic conditions in the deep water and consequent fish kills. Persistent eutrophication in Lake Erie was successfully addressed by regulations in the 1970s that severely limited nutrients from sewage, industrial effluents and particulate phosphorus in agricultural runoff and the lake was restored to health. In the late 1990s eutrophication is summer began to occasionally reoccur due to dissolved phosphorus from agricultural runoff. Solving the problem will require further modification of farming practices.
Toxic metals are another important environmental problem. One source is mining of sulfide deposits, such as the several types described above. Sulfides exposed to water and atmospheric oxygen quickly weather to produce sulfuric acid, resulting in a problem known as acid mine drainage. Not only is the acidity a problem with pH values as low as 2, but under these conditions many otherwise insoluble toxic metals become soluble. The solution is certainly not to simply shut down mines as when pumps are shut off, water penetrates in mine shafts, pits, and tailings ponds and the problem worsens. Indeed, the bigger problem is old, abandoned mines as a number of strategies are deployed in modern mining operations to prevent the problem. Lead and mercury are highly toxic metals and anthropogenic release of these elements to the atmosphere has polluted the entire surface of the planet. Lead, however, is an example of an environmental success story largely due to the efforts of one geochemist, Claire Patterson. Regulations that eliminated Pb from gasoline and emissions from smelters have dramatically reduced the amount of Pb in the environment. Regulations have also starkly reduced emissions of Hg, at least in developed countries, and local sources of extreme pollution, such as in Minamata, Japan, where mercury poisoning killed over 1700 people and disabled many more, have been eliminated in most cases. Nevertheless, levels in the atmosphere, soils, plants, the ocean, and many fish species remain high and will decrease only slowly in the future, even if all emissions are eliminated. An understanding of the unique geochemistry of Hg will enable us to understand why.
Finally, we examine the problem of acid rain. This results from burning of fossil fuels, particularly coal, which oxidizes sulfur and nitrogen ultimately to sulfuric and nitric acid, although use of nitrogen fertilizers also contributes. This can lower pH in rain to values as low as 4. Depending on the nature of the soil and bedrock this may or may not be a problem, and the understanding of weathering reactions we gained in earlier chapters will help understand why. In areas where soils have developed through weathering of rocks with low acid neutralizing capacity, the low pH alone can have deleterious effects on trees, fish, and aquatic invertebrates, but that is not the main problem. Instead, the principal problems are loss of cations such as Ca2+ and aluminum toxicity. Aluminum is one of the most abundant elements in the Earth's crust, yet natural Al toxicity is rare. Once we understand the geochemistry of Al, we'll be able to understand why this is usually not an issue but can be when rain is acidic. Acid rain is another environmental success story, although a still unfolding one. Regulations have greatly reduced emissions in the developed world, but it will take decades before soils and stream chemistry returns to natural levels and for damaged ecosystems to heal.
REFERENCES AND SUGGESTIONS FOR FURTHER READING
1 Clarke, F. W. 1908. The Data of Geochemistry. US Geological Survey Bulletin 770. Washington, US Government Printing Office.
2 Vernadsky, V. I. 1926. Biosfera. Leningrad, Scientific Chemico-Technical Publishing.
3 Goldschmidt, V. M. 1937. The principals of distribution of chemical elements in mineral and rocks. Journal of the Chemical Society of London 1937: 655–73. doi: 10.1039/JR9370000655.
4 Gribbin, J. R. 1984. In Search of Schrödinger's Cat: Quantum Physics and Reality. New York, Bantam Books.
5 Lindley, D. 2001. Boltzmann's Atom: The Great Debate that Launched a Revolution in Physics, New York, The Free Press.
6 Morris, R. 2003. The Last Sourcers: The Path from Alchemy to the Periodic Table, Washington, DC, Joseph Henry Press.
7 Strathern, P. 2000. Mendeleyev's Dream: The Quest for the Elements, London, Berkley.
NOTES
1 ‡ Christian Friedrich Schönbein (1799–1868) was born in Metzingen in Swabia, Germany and served as professor at the University of Basel from 1835 until 1868. He is best known for his discovery of ozone.