Mineral deposits are produced at the earth's active centers. Will plate tectonics become a divining rod? •

by Edward Edelson

"7""or the earth sciences, the theory of . **̂ plate tectonics may be the most dra-' • matic event of the last two decades.

In that time, the theory has developed, ma­tured, and won acceptance. Scientists have begun to deal with the import of this dy­namic picture of geological processes—a picture of continents breaking apart, drift­ing across the earth, and fusing to form new land masses and features. The collisions of migrant continents have thrown up moun­tain ranges and rifted and wracked the earth with volcanoes and earthquakes.

The process is responsible for the shape of the earth as we know it. Additionally, almost from the start, it was clear that the theory of plate tectonics would produce enormous benefits. For one thing, it ex­plained a mass of empirical data, accumu­lated for centuries, on ore prospecting and mining. The early 1960s marked the seminal

discoveries that became evidence of seaf loor spreading; the 1970s saw the first experi­ments to target ore deposits with tectonic data; the early 1980s are becoming the time to apply knowledge of tectonic phenomena.

Some scientists are convinced that plate tectonics can now be a useful tool in the search for mineral deposits. One of them is Robert D. Ballard of the Woods Hole Oceanographic Institution, a member of the East Pacific Rise Study Group. The group has been making a close, extended study in the Pacific of the worldwide ridge, rift, and

MOSAIC November/December 1981 9

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and other minerals glitter in a piece of rock (top) retrieved from the floor of the Pacific. The black smoker at the East Pacific Rise (left) marks a mineral-rich site. The research submersible4/t//r? (right) is used to explore the sea floor.

Emory Kristof and Alvin Chandler/National Geographic Society; Dudley Foster/Woods Hole Oceanographic Institution.

rise system that marks where new ocean crust is formed.

As hot magma wells u p from the mantle below, new crust is created at the lips of the ridge. This crust moves away from the ridge in both directions, carrying with it concentrat ions of elements brought up by the melt, including copper, lead, and zinc. As seawater penetrates fissures in the crack­ing, spreading rock, these and other elements

Edelson writes on many scientific subjects.

are dissolved and a hydrothermal circulat­ing system is formed.

The key to unders tanding how these min­eral deposits form, says Ballard, is that the driving force of the hydrothermal system is the magma chamber, an underground bub­ble of hot rock found at the center of the rift.

As the seawater comes down through the cracks and travels toward the magma cham­ber, Ballard explains, it becomes heated. O n contact with the hot rock, minerals in the rock dissolve and there is a chemical altera­tion in both water and rock. " W h e n the water leaves to go up, it and the rock it is leaving have both been changed," Ballard observes.

Studies of an especially active area of the East Pacific Rise at 21 degrees north latitude, off the west coast of Mexico, revealed hydro-thermal springs rich in suspended minerals. Because the flows contain sulfur, the metals

they carry—copper, zinc, and others—tend to be precipitated as sulfides. By looking at the 21-north location with Alvin, a research submersible, and Angus, a towed, unmanned underwater vehicle carrying high-resolution cameras, researchers in the East Pacific Rise Study Group have been able to get a more precise understanding of where some po­tential ore deposits begin. (See "Hot Springs and Marine Chemistry," Mosaic, Volume 11, Number 4.)

Tha t understanding starts with a general­ized picture of the topography of the Mid-Ocean Ridge. Some 70,000 kilometers long, the ridge is a world-girdling belt of emerging plate boundaries; the Mid-Atlantic Ridge is its bes t -known part. Seen schematically from above, the ridge looks like a series of H-shaped segments, stacked side by side in a staggered array. The cross-bar of each H is a spreading zone where the two plates are moving apart. Each spreading zone lies be­tween two transform faults—the uprights of the H. One way to picture it is to say that the transform faults define ribbons of new crust that are moving away in both directions from the center of the ridge.

"We know that mineral deposits are directly related to the hydrothermal circulation," Ballard says. That is in turn related to the temperature gradient, which is greatest where the magma chamber is closest to the surface.

" O n e game," Ballard continues, "is to search within a segment bounded by two transform faults. If you go from one fault to another, you find that the terrain slopes u p ­hill to a h u m p about halfway between the two faults, then down to the deepest point, at the next transform fault. It's at the h u m p , we believe, where the magma chamber is the shallowest or the lid over it is the thinnest. Tha t ' s where you find these high-tempera­ture vents. So if you want to search for old deposits, you need to understand the setting of present active deposits, the black smokers we saw at 21-north and the high-temperature smokers we saw this year off Easter Is land."

Where to look

In a few cases, Ballard says, a piece of ocean floor carrying sulfide deposits has been shoved upward more or less intact. The literally classic example is on Cyprus , whose Troodos massif has been mined for copper and other minerals for several millenia. A more recently discovered example is in O m a n , where exploration geologists have discovered an exploitable mineral deposit that, like Cyprus , is within an ophiolite-^a piece of the old ocean floor pushed up onto a continent.

Geologists have a pretty good perspective on the location of ophiolites on the globe,

10 MOSAIC November/December 1981

notes Ballard, along with a reasonably good idea of where to search within them for ore deposits. "The geologists in Oman more or less stumbled on that deposit ," Ballard ob­serves, "but now they know where else to look." The key, he says, is to "look into the past record to see where a condition pre­viously existed. Then you have a handle on where to search."

But there seems to be a more direct method than "waiting for these formations to travel for 90 million years and come up on land," Ballard adds. While newly created undersea mineral deposits once seemed beyond com­mercial reach, advances in technology are bringing them closer. Toward the end of this year, the East Pacific Rise Study Group plans to be back at the 21-north location, this time looking for deposits that might be commercially exploitable.

Black smokers are not likely sources of such deposits because they are ores in the process of formation. The geologists will be looking for deposits already formed and being moved laterally across the ocean basin by seafloor spreading.

The United States Geological Survey has plans, as resources become available, for a similar look at another section of the Pacific system: the Juan de Fuca Ridge about 400 kilometers off the coast of Washington and Oregon. The area has two major disadvan­tages: high winds and seas that limit explo­ration to the summer months. Its advantages, says William Normark of the geological survey office in Menlo Park, California, in­clude the fact that "'it is the only spreading area that might lie in the economic zone of the United States." And it is on the path that the survey's vessels follow each year on their way to work on the continental shelf off Alaska.

The survey has proposed a seven-year program of drilling and sampling at the Juan de Fuca Ridge, first identifying areas of hydrothermal activity and then examin­ing those areas in great detail. " W h a t we learn about the modern era on the ocean floor would help our prospecting for minerals on the continents," Normark says. " W e might learn something about these processes that tells us to look at the rocks on land in a different way."

One reason the focus is on the Pacific is that the most intense activity is where the plates are moving fastest. At the Atlantic section of the Mid-Ocean Ridge, the plates are moving apart at a rate of about 2 centi­meters a year, slowed because they are carry­ing continents with them. By comparison, the annual rate of separation at the 21-north site in the Pacific is 6 centimeters and, off

Easter Island, 18 centimeters. Such rapid separation is possible because the Pacific plate, the earth's largest, is descending into a huge, arc-shaped trench system that runs from the Aleutian Islands down past Japan and then farther south. In addition, it is not burdened with a continent.

That arc is a textbook example of a con­verging plate margin. Where two plates meet head-on, one can dive under the other, pro­ducing a deep trench, upthrusted islands, or mountains, and it can stimulate hydrother­mal activity capable of forming concentrated mineral deposits. Among the most impor­tant deposits formed this way are those of the Andes Mounta ins in South America and the Kuroko deposits, whose most valuable portions are in a relatively small area in the northeastern part of Honshu Island, Japan. In both cases, there are exploration geol­ogists who believe that the application of plate tectonics offers a guide to those min­eral deposits.

In the Andes, says Frederick J. Sawkins of the University of Minnesota, ore deposits seem to have formed in well-defined zones, with iron closest to the trench, copper farther inland, lead-zinc deposits farther still, then tin. Moreover, Sawkins says, it is possible to describe the spacing of ore deposits ("some­thing of critical importance to exploration geologists") by whether they are formed through what he calls single-stage or two-stage mechanisms.

One stage or two Single-stage deposits are formed by water

coming out of solution in the rising magma; Sawkins includes among them the porphyry copper bodies—like those in the Andes Mountains—that are among the world's most important metallic ore deposits. Por­phyry is igneous rock with certain crystalli­zation characteristics that tends to be associ­ated with areas of volcanic activity.

Typically, single-stage ore deposits are tens of kilometers apart, Sawkins says. The porphyry copper deposits of southern Peru and Chile are about 70 kilometers away from each other; the copper-iron deposits of Chile, also of single-stage origin, are about 80 kilometers apart.

Two-stage deposits are formed by hydro-thermal activity Involving water that orig­inates outside the magma—either from the sea or the upper crust. Massive sulfide de­posits and veins of precious metals are formed by two-stage processes, Sawkins says. They are called two-stage deposits because " the metal In such deposits must have undergone a staging process prior to final t ransporta­tion and deposit ion."

Two-stage deposits tend to be only kilo­

meters apart, Sawkins points out. Their spacing is related to the size of the hydro-thermal convection cells formed when water encounters the magma. The clusters of gold-silver veins in Durango, Mexico, are about five kilometers apart, as are the massive sulfide deposits in Tasmania. Similar spac-ings are present in the Kuroko sulfides of northern Japan. The two kinds of deposits also differ in their customary depth: Single-stage deposits, usually more than 800 meters from top to bottom, are at least twice the vertical extent of a typical two-stage deposit.

Nature introduces complexities into this simple picture, Sawkins adds. Sometimes magmatic water can surge into a two-stage deposit, carrying metals with it; sometimes single-stage and two-stage deposits are in­termingled. But, Sawkins notes, knowing that there are two ways deposits can form at converging plate boundaries can help the exploration geologist plan a strategy.

Exploration strategies

Lawrence Cathles of Pennsylvania State University has concentrated on the Green Tuff region of Honshu, the linear belt 100 kilometers wide and 1,000 kilometers long in which the Kuroko sulfide deposits lie. This leads him to exploration strategies of a specific kind.

One important fact about those deposits, Cathles says, is that they were all formed in a relatively brief period about 13 million years ago, even though subduction—the descent of one plate beneath the other—has been going on in the region for more than 25 million years. " W h a t ' s so special about 13 million years ago?" Cathles asks. It was a time, he answers, when a rift did not quite form. The kind of rift he envisages is a by­product of subduction. As a plate descends, he says, it produces circulation currents in the mantle. The crust that forms mountainous Islands is then in tension and can break so that a rift forms. The rifting, if unchecked, can create a small ocean or marginal sea. (See "Ocean Margins, the Scars of Creation," Mosaic, Volume 12, N u m b e r 2.)

That sequence has happened several times in the geological history of Japan, Cathles points out. But 13 million years ago, the crust did not break completely. As it thinned under tension, the place that would have been the rift subsided rapidly. Magma wound up close to the surface, causing Intense hy­drothermal activity and also causing mineral deposits to form. When the rift did not occur, the f ailed-rif t area rebounded upward. Weath­ering eventually exposed the deposits.

While this scenario must still be regarded as a hypothesis, Cathles says it outlines a clear strategy for geologists (a strategy iden-

MOSA1C November/December 1981 11

Possible sources. Candidate regions for ore deposits include plate boundaries and related tectonic settings. Mineral deposits can form in areas of continental collision (top), or at island arcs and inter-arc basins such as Honshu, Japan (below). Salty Bensusen.

12 MOSAIC November/December 1981

tical with that used in exploring the Mid-Ocean Ridge): Look for rift areas between transform faults. "I can draw a very nice set of transform offsets and spreading centers in Japan and account for all the mining d is t r ic t s / ' Cathles says.

A fourth dimension There are implications for exploration in

other areas, he adds. "If you want to find Kuroko- type sulfide deposits, look for rift­ing—but rifts that have failed. Once you find them, concentrate at [zones] of the age when rifting occurred and look for spread­ing segments between transform offsets."

Hiroshi Ohmoto , Cathles 's colleague at Penn State, has a more specific suggestion: Look for a caldera, the crater left after the center of a volcano explodes or collapses. "A caldera can cause fracture systems that allow seawater in, bringing metals with it," Ohmoto says. "The caldera model doesn't contradict the failed-rift model. It's on a much smaller scale. Calderas can form in the rift sys tem." Ohmoto says there are indica­tions that calderas produced by submarine volcanic activity played a role in forming mineral deposits found in Tasmania, eastern Canada, and elsewhere.

A major problem in trying to apply this sort of plate tectonics model is that geology must deal with the fourth dimension, time, and all the changes it brings. Plates have subducted and separated for billions of years; ocean basins have opened and closed again; continents have collided; mineral deposits have been formed and eroded, leaving no trace behind. (See "Before Pangea," Mosaic, Volume 12, Number 2.)

"The geological record is highly selective," says Kevin Burke of the State University of New York at Albany. " In one area you might have 100 percent of the record of what happened for a hundred million years. In another area there's no record of what happened for a thousand million years."

One small example of the influence of time is the scarcity or nonexistence of por­phyry copper deposits in Vermont and Mas­sachusetts , Burke adds. Vermont does have large talc deposits, the mainstay of a major manufacturer of baby powder; they were created when an ocean basin closed 450 mil­lion years ago. Most , if not all, of the por­phyry deposits that presumably came into existence in that collision of ancient plates, Burke explains, have long since eroded.

Suture zones The New England talc deposits are in what

geologists now call suture zones—the sites of ancient ocean basins where mountains have formed because of plate collisions and

where mineralization presumably occurred. Geologists have just begun to apply plate

tectonic reasoning to mineral exploration in suture zones, says Andrew H. G. Mitchell, an exploration geologist with the United Nations Development Program. For example, the large porphyry copper deposits of the Philippines, mined for 40 years, are now known to lie close to a suture zone, Mitchell notes. "Abou t 20 years ago, rocks similar to those of the Philippines were found in the continents—the Canadian shield, and in Australia," he adds. "People didn't know about plate tectonics then. Now thafc plate tectonics gives an explanation of how you get an ancient plate margin in the midst of what is now a continent, you can accept that where you find Philippine-like rocks you might find porphyry copper deposits."

More detailed information about suture zones is needed by exploration geologists trying to find exploitable mineral deposits, Mitchell says. He has a model, which he describes as " ra ther speculative," that could be a step in that direction.

There seems to be a basic difference in the kind and location of mineral deposits found in suture zones: one kind in remnants of east-facing convergence margins (that is, those whose subduct ing plate was on the east), and another in sutures formed from west-facing margins. East-facing suture zones are similar to the active convergence margin in Japan, where the Kuroko sulfide deposits are found. West-facing zones are similar to the Andes mineral belt, with its porphyry copper deposits. If the model is correct, an exploration geologist can use plate tectonics data to determine which way a suture zone faces and thus get a reasonable idea about the sort of mineral deposits that may be present.

Over time Reasoning from basic knowledge about

plate tectonics is often not easy because of the confounding effects of time. Strik­ing examples of the problem's complexity are the mineral deposits of the American West. This area is the worst place to start applying plate tectonics, Frederick Sawkins asserts, because its geological history is so complicated. "Seven thousand kilometers of ocean floor have disappeared under the continental United States," he says. "There has also been a lot of extension. The area has spread out several hundred kilometers. The Andes are still very tightly compressed, by comparison."

"There are a number of major porphyry copper and molybdenum deposits in Nor th America in the center of the continent"—

this from J. David Lowell, a consulting geol­ogist in Tucson. " T h e theory says that the plate couldn ' t penetrate to this area. To rationalize those deposits, you must first cause the plate to head down at a 45-degree angle, then level off so that it stays above the zone of melting. Since the deposits are in layers, this would have had to happen re­peatedly, so it doesn' t seem that this mech­anism is correct."

The case is certainly not open and shut. Some earth scientists are less than enthusi­astic about the ability of plate tectonics to replace the prospector 's divining rod. An exploration geologist well along in his career, for instance, may have a considerable per­sonal stake in the old way of doing things. For some others, plate tectonics paints with too broad a brush. It "explains the broad features of mountain belts, trenches, ridges, continents, and oceans, but not the details," says Ulrich Petersen of Harvard University. " O r e deposits are part of the details. An ore district might be as small as five kilometers by five, or one by one. The ore is commonly found in veins that are much smaller. A vein might be less than ten meters wide and only a kilometer long."

"We ' re still at a very early stage in under­standing what's going on," says Penn State's Lawrence Cathles. Or, as H. D. Holland of Harvard sums it up : "Plate tectonics is a superb intellectual framework that has managed to put all sorts of geological, geo-chemical, and geophysical problems in a new perspective. But its use in the search for new ore deposits has not been spectacularly successful."

The revolution is certainly not complete. A basic method of mineral exploration still is what Sawkins calls "lithological analogy." Geologists look for ore deposits in forma­tions similar to those where such deposits have already been found. An impressively large portion of the earth has not been mapped well enough to make such analogies possi­ble, he says. But "plate tectonic theory allows us to sharpen our perspectives regarding geologic environments. This in turn can help the explorer focus on potentially favor­able areas ."

Geologists will continue to search for ore by drilling a lot of holes, notes SUNY's Kevin Burke. "Bu t , " he adds, "if you have a system that is more comprehensive than the old system, as plate tectonics is, it gives you a better idea of where to drill your holes." •

The National Science Foundation contrib­utes to the support of research discussed in this article through its Divisions of Earth and Ocean Sciences.

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