FIGURE 4.10 Processes active at the scale of individual vent fields. Hydrothermal venting (plumes) and low-temperature, diffuse venting (at tube worm/clam beds) release significant heat to the deep ocean. Massive sulfide-sulfate precipitation can result in the growth of complex vent structures or simple chimneys when high-temperature fluids mix rapidly with seawater.

supply sulfur that is subsequently incorporated into sulfides. Thus, organic matter has a powerful control over the mobility of metals in the crust, especially under near-surface conditions. One of the most exciting research frontiers in organic geochemistry is the development of engineered bacterial species that leach metals from ores—dubbed biometallurgists—or that precipitate and isolate metals from waste—biotrashmen, of course. The challenges, opportunities, and rewards in this field are great and are being vigorously pursued.

Organic matter also has a passive role in the search for and recovery of minerals, as a guide to conditions instead of as an active agent. For example, the thermal maturation of organic matter can provide a guide to related ore-forming thermal events. In addition, living or fossil species provide clues to the environment prevailing at sites of ore deposition. One of the best guides to active deep-sea smoker sites is the distinctive biota of giant white clams, tube worms, and deep-sea crabs. Fossils of that distinctive biota provide telltale evidence that mineral deposits formed at ancient ocean spreading centers may lie nearby (Figure 4.10).

Finally, geochemists have refined laboratory experiments that attempt to duplicate natural processes. Experiments in genetic studies use two broad but contrasting approaches. The first is to design and reconstruct a natural process and then use the reconstruction as a model. The second is to assemble the assumed necessary thermodynamic and chemical data and then calculate just how nature might perform under those conditions. With the second method, developing a model is a process of trial and error. The first method is most effective when the natural system is well established and reactions are rapid. Unfortunately, nature presents major obstacles to the study of certain natural processes: unrestricted time spans and complex environments. Nature has taken millions of years to weather a rock or to crystallize a glass, while experiments requiring more than a few months are difficult to maintain, and studies lasting years are rarely funded. Natural environments simultaneously involve high-temperatures, high pressures, and corrosive chemistries; only some of these combinations that alter and dissolve rocks can be duplicated in the laboratory.

The solution to this problem has been to combine several experimental approaches in the laboratory. Artificial systems can be simplified to eliminate extraneous factors for short periods. For instance, experiments can be performed at high-temperatures, where the reaction rates provide definitive results in relatively short time frames. Thermodynamic data can also be acquired that permit extrapolation of high-temperature results from experiments performed at lower temperatures or to a range of pressures and chemical reactivities.

Models in the Study of Mineral Deposits

Humankind has searched for minerals throughout recorded history and undoubtedly developed prospecting theories very early. Perhaps the approach was to search for another gold nugget in a stream in which one nugget had already been found. Then came the idea that other streams might have similar nuggets—the first germs of an analog model. Intensive prospecting in and around a stream might have led to an outcrop of gold-bearing quartz vein or perhaps to finding a nugget still attached to a piece of vein quartz, and eventually to the idea that there might be more than one place to look for gold. In this way the idea of alternative descriptive models would develop, one for stream placers and another for veins. Such models continue to develop today, and just as the first gold-bearing vein was a new kind of deposit to ancient miners, modern research-

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