Lava Lamp-Like Process Caused World's Largest Zinc
Deposit
Grant Garven
PHOTO BY HIPS / WILL KIRK
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By Lisa De Nike Homewood
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 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 was scheduled to 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, Calif. "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 1 billion tons of zinc are used for
manufacturing in the U.S.A. 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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2005
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