Gold
171 Gold The gold nugget rests in your hand, still wet. Its lustrous surface reflects a world unique among the stars—your world, a place that has the power to produce this glittering gem from base rock. That alchemy requires enormous pressures and temperatures, millions of years to carry out, and a special setting as well—as far as we know, it could only happen here. Your nugget was once gold dust scattered at infinitesimally sparse concentrations across the void of outer space. Gold constituted less than 0.0000005 percent of the matter that consolidated from star dust into the primordial Earth—a negligible trace constituent of our growing sphere. But Earth was destined to be a water world, and that would bring sweeping change. Oceans now rest uneasily on the thin rock firewall covering Earth’s core, where the incandescent heat of its creation is retained. The interplay of the fires below and waters within the crust has energized a dynamic geology that drives the tectonic motions of Earth’s surface. That motion has created folded ridges tens of thousands of feet high, still showing the sedimentary strata on which they were built. Those strata were assembled at the bottom of the sea, and as they grew, they came to be shot through with veins of crystalline ore and sprinkled here and there with pieces of solid gold. The fires beneath the mountains show through the Earth’s crust where it is thinnest—at the bottoms of canyons deep beneath the seas. There, newly created seafloor spreads away from its origins—a broad conveyor belt following a path that will lead to its eventual destruction. Seafloor is destroyed at the end of a multimillion-year migration, when it slides into canyons steeper still, meeting the fires of its birth again deep beneath the beaches of the continental margins. At those regions where the crust is created and destroyed—beneath the centers and the edges of the oceans—groundwater exists in a form incomprehensible to those who know only the surface of the land. Deep underground , water has been forced into contact with the Earth’s fiery magma, where it grows so hot and pressurized that it dissolves quartz. It also dissolves gold. Gold is not soluble in water as we know it. Where we live, a gold ring 172 will stay in intimate contact with the skin moisture of a finger for a lifetime and never tarnish. But at depth, gold readily dissolves and leaches from the rocks. It is scoured one atom at a time from cubic miles of the Earth’s crust by the superheated solution percolating through the cracks. This is the first step in its concentration. The plates of the Earth’s crust carry their mineral-laden groundwater with them as they move away from the volcanic boilers where seafloor is created. The temperature of the hot groundwater declines as the volcanic fires are left behind. And as the solution cools, it undergoes a series of transformations. Each of the minerals dissolved in the buried brine cools to its own critical temperatures, and then each materializes in solid form. They appear as crystalline deposits on the walls of flooded fissures. These crystals grow pure—self-selecting their own atoms one at a time from the solution. Their formation concentrates dissolved manganese into pink seams in the rocks. As the temperature continues to decline, brown bands of iron oxide are deposited. The process continues, and veins of solid gold grow on surfaces previously varnished in translucent quartz. Over the millennia, all the dissolved atoms in cubic miles of groundwater are purified from each other and concentrated into clefts in the rock. This creates lodes of mineral deposits that remain in place when the water finally runs away. Some continental margins are particularly well suited to the accumulation of gold. Among them are the mountain ranges behind the shores of the eastern Pacific—the Andes, the Sierra Nevada—terrains shaped both by rocks derived from seafloor sediment and by active volcanism. The sedimentary strata there are the remnants of generations of mountain ranges long since reduced to sand, washed down on each other and compacted into rock, then uplifted tens of thousands of feet above the level of their creation. Harder mineral-bearing blocks shaped by volcanic action have been sutured to those sedimentary strata during their tectonic uplift. The thundering energy of the water planet we live on is readily apparent to anyone hiking past the...
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