For more than two decades, geologists have scoured the ores and rocks surrounding the world's largest zinc deposit at Red Dog, Alaska, for clues as to how this giant ore body formed. In some zinc deposits elsewhere, volcanic activity was responsible for bringing metals to the upper levels of the Earth's crust. But at Red Dog, which lies about 85 miles north of the Arctic Circle, there are no obvious signs of volcanic activity to explain the presence of so much zinc. In fact, the zinc was deposited in black shale -- a sedimentary rock -- that accumulated slowly in an ancient sea about 330 million years ago. Although it is not a glitzy metal that sparks dreams of riches, zinc is essential for its role in the manufacture of hundreds of consumer goods, from automobiles and batteries to galvanized nails and sunblock.
Now, a Johns Hopkins University hydrogeologist may have discovered the source and mechanism by which all of that zinc was deposited at Red Dog. According to Grant Garven, professor in the Morton K. Blaustein Department of Earth and Planetary Sciences, the zinc (dissolved within hot, salty fluids below the sea floor of Red Dog Basin) flowed along vertical fault conduits and roiled upward, much the way liquids in a lava lamp do. The zinc-rich fluids then moved up toward the sea floor, saturating the black muds within nearby faults. As the zinc-bearing fluids cooled near the sea floor and mixed with sulfur-bearing fluids, chemical reactions occurred, causing tiny crystals to precipitate in the porous black mud.
"A lava lamp is the best analogy I could think of to explain the way we believe this basically worked, " said Garven, who will present the geological fluid mechanics studies he did on Red Dog at the 101st annual meeting of the Cordillera Section of the Geological Society of America and at the 80th annual meeting of the Pacific Section of the American Association of Petroleum Geologists, both to be held from April 29 to May 1 in San Jose, California. "Geothermal heating of fluids at depth really created an ideal environment where trace metals like zinc and other elements were leached out from older sandstone and basement rocks, and transported to the sea floor to form submarine hot springs."
Garven and his team modeled this process mathematically, and their conceptual model promises to enhance understanding of the physical and chemical processes controlling ore formation worldwide.
"As more than one billion tons of zinc are used for manufacturing in the USA alone each year -- three-quarters of which is imported -- understanding the geologic origin of the zinc deposits at Red Dog is crucial for planning where to explore for the discovery of the next super giant deposit," he said.
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