By George W. Moore
Explanatory notes for the Mineral-Resources Map of the Circum-Pacific Region, Arctic Sheet: U.S. Geological Survey Map CP-53, p. 4-8 (2000).
INTRODUCTION
The acceptance of the theory of plate tectonics in the 1960s revolutionized our understanding of the origin of mineral deposits. Newly understood processes at plate boundaries gave us new ways to bring up heat from the Earth's interior. Near the surface, the heat mobilized metals to make ores. The same processes also brought saline water such as seawater into contact with the hot rocks. The salt acted as a fluxing agent to further mobilize the metals.
The heat from plate-tectonic processes also converted fossil organic matter into petroleum compounds. In places, microorganisms took energy from those compounds. Because oxygen is lacking at depth, they used sulfate (SO4) from the minerals anhydrite and gypsum as a substitute for oxygen. Burning the organic compounds let them sequester electrons in the sulfate. That process converted sulfate into the sulfide that formed many of the valuable metallic ores such as chalcopyrite (CuFeS2).
The new insights from plate tectonics led us to reappreciate the ore-genesis ideas written by the great German mineralogist Georg Bauer in 1556. Bauer, a physician living in the Erzgebirge Mining District, published in Latin under the name of Agricola, a translation of his family name that means farmer. He visited the mines as a careful observer and rejected fanciful ideas such as that one may find ore by dowsing with a forked stick and that gold comes from the Sun's rays penetrating the Earth. He said that ores are produced largely by surface water that becomes heated in the Earth, dissolves minerals from the rocks, and redeposits ore-grade concentrations.
Bauer's ideas dominated thought on ore genesis and were strongly boosted in 1791 by Abraham Werner, professor at the Freiberg Mining Academy. Werner advocated that ores are deposited by circulating seawater, and his followers became known as the Neptunists. In its most extreme form, the Neptunists said that all rocks--even granites--are produced from the sea.
Werner's nemesis was the Scott, James Hutton, who in 1788 convinced most geologists that granite is not deposited from seawater but forms by the cooling of magma, molten rock at depth. As leader of the group that became known as the Plutonists, he noted the apparent insolubility of ores and stated that they too are injections of molten material.
The general acceptance of Hutton's ideas about the origin of granite and other plutonic rocks led to an eclipse of all of Werner's ideas, including those about ore minerals. By the time of the plate-tectonic revolution, most students of mineral resources had a strong magmatic bias.
Under the theory of plate tectonics, about 20 large shells or plates divide the surface of the Earth at any time in geologic history. They move with respect to each other at about the rate that our fingernails grow. As Tuzo Wilson explained in 1965, the plates interact at three kinds of boundaries now called the following: (1) spreading axes, where two plates pull apart; (2) subduction zones, where two plates collide; and (3) transform faults, where two side-by-side plates slide against each other.
The three kinds of plate boundaries work in separate ways to mobilize metals. Some of the boundaries are within oceans, and others are within continents. The plates, both continental and oceanic, are about 100 km thick. Continental crust is 35 km thick and rich in silica and potassium, whereas oceanic crust is only 6 km thick and rich in iron and magnesium. The three plate boundaries in combination with the two different hosts, continents and oceans, form a spectrum of ore-deposit environments.
To that spectrum, we add related places where mineral deposits form--hotspots. When two plates collide at a subduction zone, one of them, an oceanic plate, descends into the lower mantle. The sea had chilled its shallow mantle material under the thin oceanic crust and hence made it heavier. It is heavier than the hot deep mantle material of an adjacent plate that is capped by thick continental crust.
Such a subducting plate eventually descends all the way to the Earth's core. Once there, the former oceanic plate gradually warms up. With the added heat, it becomes lighter again until it is less dense than the nearby mantle material. It then begins to rise buoyantly back as a plume toward the Earth's surface. Once there, it creates a hotspot such as at Hawaii or Iceland. It carries heat with it from the Earth's interior, and it produces its own family of mineral deposits.
The information that follows leans heavily on explanatory pamphlets on mineral resources prepared for previous maps of the Circum-Pacific Map Project by Philip W. Guild in 1985 and Masaharu Kamitani in 1999. I am also indebted to Cyrus W. Field for many suggestions and for reviewing this chapter and to John H. Dilles for helpful discussions.
RIFTING IN THE OPEN OCEAN
Seafloor spreading produces two main types of mineral deposits, those in slowly chilled basaltic magma within the oceanic crust, and those of the black-smoker type that form on the seafloor.
Where plates move apart, the separation creates so-called new ground. Basalt boils out of the underlying mantle to produce new oceanic crust, and a crystal mush of mantle material moves up from under the plate to fill the gap beneath the new crust. In the lower part of the crust, the basalt crystallizes as gabbro. Magma chambers within the gabbro often persist long enough for valuable metals such as chromium to seggregate out. For such deposits to become ore deposits on the land, the oceanic crust must be dismembered and transported from the seafloor to a continental margin. This process happens when subduction zones accrete oceanic crust. Whereas oceanic crust normally descends into the mantle at subduction zones, reverse faulting may transport fragments of oceanic crust onto the land surface. Montana's Stillwater Complex may be an example of chromium-bearing oceanic gabbro accreted to an Archean continent at a subduction zone. It is a major producer of platinum and palladium, used in jewelry and increasingly in automobile catalytic converters.
Accreted plate fragments also include parts of the mantle below the oceanic crust. These ultramafic rocks contain podiform bodies of chromite. When altered to serpentine, they also are a source of asbestos, for example, at Cassiar, Canada.
The metal deposits that occur at and near the seafloor along spreading axes form spectacular chimneys of copper, zinc, and iron minerals. Seawater moves into the crust at the flank of a spreading axis, and it then picks up metals where it circulates past hot basalt. Finally, the hot water issues at the seafloor, where it meets with cold seawater that chills it and precipitates the metals. The uppermost chimneys are rarely preserved in ore deposits. What we usually find is a stockwork of ores that formed a little below the surface.
Copper and zinc are abundant at modern spreading axes far from land, whereas lead becomes abundant where terrestrial sediment floods the orifices of seafloor hot springs. An example of this is the present-day Guaymas polymetallic sulfide deposit in the Gulf of California. Guaymas also contains small amounts of petroleum that were generated by the magmatic heat. Such dispersed organic compounds are especially important in mobilizing metals at areas of continental rifting.
For open-ocean rift deposits to appear as ore deposits on land, accretion at a subduction zone is necessary. Such open-ocean and then accreted deposits are difficult to distinguish, however, from those formed later in the accreted terrane by continental rifting. Among the likely accreted open-ocean Cu(AuAg) deposits is the basalt-hosted deposit at Besshi, Japan.
Banded iron formation formed in the Archean sea during the interval from 2.6 to 2.7 billion years ago. Geologists generally assume that this period represents a time when organically produced oxygen reached a concentration where former dissolved ferrous iron in seawater became increasingly oxidized and precipitated in part as ferric iron. Large economic deposits are at Atlantic City, Wyoming, Anshan, China, and Isua, Greenland.
RIFTING WITHIN CONTINENTS
Processes of continental rifting cause several dissimilar types of deposits as follows: (1) Rift and exhalation deposits in sedimentary basins that are similar to those of black smokers in the open ocean; (2) diamond-bearing kimberlite pipes from the incipient rifting of very thick crust; and (3) salt and sulfur deposits at continental rifts that initially produce small ocean basins.
Gently deformed volcanogenic massive sulfide deposits usually have the silicic volcanic rock dacite as an associate. They indicate rifting within or at the margin of a continent. Japan's Kuroko PbZn(AgCuAu) deposits are a classic example. Typical large deposits of this type are Hanoaka, Kosaka, and Shakanai, Japan; Granduc, Sullivan, and Anvil, Canada; and Red Dog, United Verde, and Coeur d'Alene, United States.
Incipient rifting at relatively cool and thick cratons--ancient stable areas--has produced all of the world's diamonds. At the beginning of the Carboniferous, kimberlite pipes penetrated Archean crust at Mir, Russia, and similar pipes were emplaced at the beginning of the Paleogene at Ekati, Canada.
The new diamond fields in Canada are a good illustration of the balance between basic research and traditional prospecting methods. John Gurney identified microscopic inclusions in South African diamonds. He found that they are made of chrome diopside and a rare pyrope garnet. Those distinctive mineral varieties also occur as free crystals in the kimberlite pipes that contain the diamonds. The diamonds themselves are too rare to pan for in prospecting, but Gurney's indicator minerals are abundant enough to use.
In Canada, Charles Fipke learned of Gurney's research, and he laboriously panned his way for hundreds of kilometers across the Northwest Territories. He tracked the drift left 9,000 years ago by Ice Age glaciers that streamed out from central Canada. Eventually the trail became hot downdrift from Ekali Lake. In 1991, Fipke and his son dug under the frozen lake and made the diamond discovery. In 1998, the Ekali Mine went into production, and today industry experts predict that Canada will produce as much as 12% of the world's diamonds.
Continental rifting initially produces small ocean basins such as the Red Sea. Seawater pours into these basins, and where the climate is dry, they soon fill with salt. The salt itself is an important mineral resource. Because of its low density, salt commonly rises buoyantly through overlying sediment to produce salt domes. Petroleum at the salt domes chemically reduces gypsum in the salt, and the product is native sulfur. At the edge of the Gulf of Mexico, the United States produces much of the world's sulfur from salt domes such as Grand Isle, Boiling Dome, and Gulf Hill. One of the last products to from from seawater desiccation is potassium. Major producers of potassium from evaporite deposits are Berezniki-Solikamsk, Russia, Saskatoon, Canada, and Carlsbad, New Mexico.
SUBDUCTION AT ISLAND ARCS
When oceanic crust descends down into the mantle at a subduction zone, it carries with it seawater in its sediment cover and in underlying hydrated basalt. When the subducting slab reaches a depth of about 100 km near the bottom of the upper tectonic plate, the high local temperature begins to release the water. It moves upward into the mantle material of the overlying plate. The water changes the chemical conditions of the mantle material and causes basalt to separate from it. The basalt then begins to move upward. Some of the basalt rises all the way to the surface and produces lava flows and cinder cones. Most of it, however, rises to a level where its density matches that of the adjacent crust. At that level, a magma chamber forms, and heavy dark-colored minerals crystallize and settle out. The remaining liquid is light in weight and light in color. Depending on the local crustal density, it either erupts to the surface as felsic (light-colored) volcanic rocks or solidifies at depth as a felsic pluton.
Hot, water-rich, and salty plutons of this sort are the major source of the world's mineral deposits. Valuable metals commonly are the last things to crystallize from the residual fluids, and they accumulate around and above the top of a pluton.
Plutons at island arcs--so-called first-cycle plutons--differ in composition from plutons emplaced into thick continental crust. They generally consist of tonolite and granodiorite, which are on the darker side of the felsic rocks. On the other hand, plutons emplaced far into continents consist of quartz monzonite and granite, which are on the lighter side. Mineral deposits follow along with these differences.
SUBDUCTION WITHIN CONTINENTS
The Arctic Sheet contains five cratons as follows: Laurentia, Baltica, Siberia, and North and South China. Originally these Archean continental cores had the rough topography of present-day mountain regions, but repeated uplift and erosion over a period of about 1 billion years led to their present stability.
Multiple cycles of early subduction and pluton intrusion into the cratons have resulted in the concentration there of the so-called incompatible elements--elements that do not enter into the crystals of the major rock-forming minerals but do form their own late-stage minerals. These incompatible elements are available to produce ore deposits when a young pluton remobilizes them. Molybdenum is an example. By contrast, copper is a compatible element that does crystalize as a trace element in the rock-forming minerals, for example, in those of basalt. Hence copper ore deposits tend to be associated with oceanic and island-arc rocks, whereas molybdenum is associated with cratonic rocks.
Repeated accretion of oceanic materials has affected the northern part of the Circum-Pacific region. These processes were particularly active during the Mesozoic Era. Australia and Antarctica had rifted away from North America during the Proterozoic. Over the ensuing geologic periods, continental-margin deposits formed along the edge of the continent, which then passed through western Idaho and Utah. Starting in the Triassic, submarine fans and other oceanic materials came toward North America on subducting plates but were too buoyant to descend. The subduction zone then jumped outboard from the first accreted terrane and reentered the mantle at a new location. This process was repeated three times, ending in the Cretaceous. By that time, all of western North America consisted of accreted material with oceanic affinities.
During a similar chiefly Mesozoic time, island-arc and seafloor-fan deposits accreted to eastern Asia. The accreted rocks include terranes in Japan, Sakhalin, Sikote Alin, and Kolymia. Earlier accretion had incorporated similar seafloor-fan deposits at the margins of the China Cratons.
Regarding ore deposits, the broad tracts of accreted terranes had many of the properties of oceanic crust. At the cratons, porphyry deposits containing molybdenum are especially common. Examples are Dexing, China, and Bingham and Climax, United States.
The sandstone-type uranium deposits, such as those at Gas Hills, Wyoming, and Uravan, Colorado, probably formed from uranium leached by groundwater from volcanic ash. The ore minerals were redeposited at bodies of organic matter that constituted chemical reducing agents.
HOTSPOT EMPLACEMENT
Hotspot plumes are responsible for an important class of Ni(CoCuPt) ore deposits. The preeminent example is Norilsk-Talnakh, Russia. It consists of olivine and augite ultramafic sills containing pyrrhotite, chalcopyrite, pentlandite, and magnetite.
During the latest Permian, a cold former oceanic plate descended to above the Earth's core, warmed up to restore its buoyancy, and then rose as a plume back to the surface. It created the 1-million-cubic-km Siberian Flood Basalt and intruded the bodies that contain the ore deposits.
A curious aspect of this district is that ultramafic rocks do not contain sufficient sulfur to account for the sulfide in the ore minerals. Studies of sulfur isotopes in the minerals discovered heavy sulfur of the kind found in evaporite deposits but not found in mantle rocks. And indeed Devonian evaporites containing gypsum do underlie Norilsk-Talnakh. Apparently iron in the rising ultramafic rocks reduced the sulfate in the gypsum and made sulfide available to the ore minerals.