22.2: Metal Deposits

Metal Deposits

A ore deposit is a body of rock in which one or more metals are sufficiently concentrated to be economically viable for recovery. Some background levels of important metals in average rocks are shown on Table 18.1, along with the typical grades necessary to make a viable deposit, and the corresponding concentration factors. In the case of copper, average rock has around 40 ppm (parts per million) of copper, but a grade of around 10,000 ppm or 1% is necessary to make a viable copper deposit. In other words, copper ore has about 250 times as much copper as typical rock. For the other elements in the list, the concentration factors are much higher. For gold, it’s 2,000 times and for silver it’s around 10,000 times.

Metal	Typical Background Level	Typical Economic Grade*	Concentration Factor
Copper	40 ppm	10,000 ppm (1%)	250 times
Gold	0.003 ppm	6 ppm (0.006%)	2,000 times
Lead	10 ppm	50,000 ppm (5%	5,000 times
Molybdenum	1 ppm	1,000 ppm (0.1%)	1,000 times
Nickel	25 ppm	20,000 ppm (2%)	800 times
Silver	0.1 ppm	1,000 ppm (0.1%)	10,000 times
Uranium	2 ppm	10,000 ppm (1%)	5,000 times
Zinc	50 ppm	50,000 ppm (5%)	1,000 times
*It’s important to note that the economic viability of any deposit depends on a wide range of factors including its grade, size, shape, depth below the surface, and proximity to infrastructure, the current price of the metal, the labour and environmental regulations in the area, and many other factors.

Table 18.1 Typical background and ore levels of some important metals Source: Steven Earle (2015), CC BY 4.0. View source.

It’s clear that some very significant concentration must take place to form a mineable deposit. This concentration could happen during the formation of the host rock, or after the rock forms, through a number of different types of processes. There’s a wide variety of ore-forming processes, and hundreds of types of mineral deposits. The origins of a few of them are described below.

Types of Metal Deposits

This section will describe the following types of deposits:

• Placer deposits
• Lode (hydrothermal vein) gold deposits
• Porphyry deposits
• Carlin-type gold deposits
• Banded iron formation
• Sandstone-hosted uranium deposits

Placer Deposits

A placer deposit is a concentration of valuable minerals by gravity separation during sedimentary processes. When heavy, stable minerals are freed from their matrix by weathering, they are washed down slope into streams that quickly winnow away the lighter material, concentrating the heavy minerals in stream gravels and gravel beds. For effective concentration, placer minerals must have a high density (greater than about 3.3 g/cm³), a high degree of resistance to dissolution, and mechanical durability. Gold satisfies all three conditions: it is dense, chemically inert, and extremely resistant to weathering.

Stream placers depend on swiftly flowing water for their concentration. Because the ability to transport solid material varies approximately with the square of the water velocity, where velocity decreases, heavy minerals are deposited much more quickly than light ones. As a result, gold tends to settle out in predictable locations: on the inside bends of rivers, behind boulders and bedrock irregularities, at the base of waterfalls, and in cracks and potholes in the streambed where water slows. These are the spots that experienced Gold Rush miners learned to target with their pans and sluice boxes.

Placer deposits are secondary deposits, meaning the gold they contain did not originate where it is found. The source is always a primary deposit — typically a hydrothermal gold-bearing quartz vein in bedrock upstream. In California, the placer gold that triggered the 1848 Gold Rush was ultimately derived from the lode gold deposits of the Mother Lode, a belt of gold-bearing quartz veins extending roughly 200 kilometers along the western foothills of the Sierra Nevada. As erosion stripped away overlying rock over millions of years, gold was liberated from those veins and carried downstream by rivers, eventually accumulating in the gravel beds where James Marshall made his discovery at Sutter's Mill. This relationship between placer and lode deposits is explored further in the next section.

Lode (Hydrothermal Vein) Deposits

A lode deposit is a primary ore deposit in which valuable minerals occur in veins or networks of veins within the host rock. Lode deposits form through hydrothermal mineralization: hot, mineral-laden water circulates through fractures and fault zones deep in the crust, dissolving metals from the surrounding rock. As this fluid migrates upward and cools, it deposits its dissolved metals as veins, typically composed of quartz studded with ore minerals. Vein deposits formed this way account for most of the world's gold and silver mines, as well as many copper and lead-zinc mines.

Figure \(\PageIndex{1}\): Native gold deposited in a quartz hydrothermal vein. This sample is from the California Mother Lode. Source: James St. John (2014), CC BY 4.0. View source.

Gold is particularly well-suited to this type of deposit because it readily dissolves in hot hydrothermal fluids and precipitates cleanly as native metal when those fluids cool. California's Mother Lode is one of the most significant lode gold districts in North America. The California Mother Lode is a 190-kilometer-long, 1.5- to 6-kilometer-wide alignment of hard-rock gold deposits in the Sierra Nevada foothills, bounded on the east by the Melones Fault Zone. The deposits formed along the suture where the Smartville Block, a Jurassic oceanic terrane, accreted onto North America. Hydrothermal fluids emplaced gold-bearing quartz veins during the Early Cretaceous, approximately 127 to 108 million years ago. Heat from the intruding Sierra Nevada Batholith drove these fluids through the surrounding rocks, remobilizing and concentrating gold from the accreted oceanic terrane rocks into the veins we see today. In the western Sierra Nevada foothills, gold is predominantly found in these quartz veins rather than alloyed with other elements to form gold-bearing minerals. The veins can range from 30 centimeters to 30 meters in thickness and often include other minerals such as calcite and pyrite.

Recent research has added an intriguing layer to our understanding of how gold concentrates within these veins. Quartz is the only abundant piezoelectric mineral on Earth, meaning it generates an electric charge when deformed by stress. The cyclical nature of earthquake activity that drives orogenic gold deposit formation means that quartz crystals in veins experience thousands of episodes of stress over geological time. When earthquake stress deforms quartz crystals, the resulting electrical field draws gold nanoparticles out of the surrounding hydrothermal solution and deposits them as grains along the vein. Once gold particles are present, they become the focus for ongoing electron donation because gold is a conductor and quartz is an insulator, small grains can grow into larger nuggets over repeated seismic events. This mechanism may help explain why large gold nuggets are so commonly found in quartz veins, a pattern that the hydrothermal precipitation model alone could not fully account for. In California, where the Mother Lode formed along an active fault zone and seismicity remains a defining feature of the landscape, this connection between earthquakes and gold concentration is particularly fitting.

The connection between lode and placer deposits is direct. Lode deposits are the upstream source of placer gold; erosion liberates gold from the veins and rivers carry it downstream, concentrating it in gravel bars and stream channels as described in the previous section. Early forty-niners were keenly aware that the gold had to be coming from somewhere in the bedrock, and they found quickly that it was associated with quartz veins that ran the length of the Mother Lode district. Once placer gold in the rivers and streams was depleted, miners shifted to hardrock mining of the lode deposits themselves, a more technically demanding and expensive undertaking that required tunneling directly into the quartz veins.

Porphyry Deposits

Porphyry deposits are the most important source of copper and molybdenum in the western United States and Central and South America. Most porphyry deposits also host some gold, and in rare cases it can be the primary commodity. In the United States, major porphyry copper deposits are concentrated in a northwest-trending belt across Arizona, New Mexico, Nevada, and Utah. The Bingham Canyon Mine in Utah, located about 30 kilometers southwest of Salt Lake City, is one of the largest open-pit mines in the world and exploits a porphyry copper deposit formed by crystal-rich magma moving upward through pre-existing rock layers. The copper mines of southern Arizona, which account for approximately 70% of domestic U.S. copper production, are also among the most significant examples of this deposit type.

Porphyry deposits form around a cooling felsic stock in the upper part of the crust. They are called “porphyry” because upper crustal stocks are typically porphyritic in texture, the result of a two-stage cooling process. Metal enrichment results in part from convection of groundwater related to the heat of the stock, and also from metal-rich hot water expelled by the cooling magma (Figure 18.6).

Figure 18.6 A model for the formation of a porphyry deposit around an upper-crustal porphyritic stock and associated vein deposits. Source: Steven Earle (2015), CC BY 4.0. View source.

The host rocks, which commonly include the stock itself and the surrounding country rocks, are normally highly fractured. During the ore-forming process, some of the original minerals in these rocks are altered to potassium feldspar, biotite, epidote, and various clay minerals. The important ore minerals include chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄), and pyrite in copper porphyry deposits, or molybdenite (MoS₂) and pyrite in molybdenum porphyry deposits. Native gold is present as minute flakes.

Banded Iron Formation

Most of the world’s major iron deposits are of the banded iron formation type (classified as a type of chemical sedimentary rock), and most of these formed during the initial oxygenation of Earth’s atmosphere between 2,400 and 1,800 Ma. At that time, iron that was present in dissolved form in the ocean as Fe²⁺ became oxidized to its insoluble form, Fe³⁺, and accumulated on the sea floor, mostly as hematite interbedded with chert (Figure 18.7). Unlike many other metals, which are economically viable at grades of around 1% or even much less, iron deposits are only viable if the grades are in the order of 50% iron and if they are very large.

In the United States, the most significant banded iron formations are found in the Lake Superior region, particularly in the Mesabi Range of northeastern Minnesota. The Mesabi Range has been the foundation of American steel production for over a century and today supplies the majority of domestically mined iron ore. The iron ore extracted there is processed into steel which is then used in construction, vehicles, appliances, and infrastructure. Iron ore ranked eighth among U.S. nonfuel mineral commodities by production value in 2025, with an estimated value of $3.38 billion.

Figure 18.7 ~2.69 Ga Banded Iron Formation from Soudan Underground State Park, Soudan, Minnesota. Source: James St. John (2015), CC BY 2.0. View source.

Carlin-Type Gold Deposits

Carlin-type gold deposits are found almost exclusively in Nevada and are the primary reason the state accounts for approximately 80 percent of U.S. gold production. Unlike the visible gold associated with placer and lode deposits, Carlin-type deposits contain what geologists call "invisible gold": particles so microscopically small they can only be detected through chemical analysis. This gold occurs in disseminated form within arsenic-rich pyrite and arsenopyrite hosted in carbonate-bearing sedimentary rocks.

The deposits form when low-temperature, low-salinity hydrothermal fluids rich in hydrogen sulfide circulate through permeable carbonate rocks such as limestone. As these fluids interact with the host rock, carbonate minerals are dissolved or replaced by silicates, and gold is precipitated within the crystal structure of pyrite. The deposit type was not recognized until 1961, when Newmont Mining Corporation discovered the original Carlin deposit. This finding demonstrated that enormous quantities of gold could exist in rocks with no visible metallic minerals.

The Carlin Trend, a five-mile-wide, 40-mile-long strip of land in north-central Nevada, is the world's third-richest gold-mining district of all time. Large-scale open-pit mining combined with cyanide heap leach processing makes these low-grade but volumetrically enormous deposits economically viable, and new deposits along the trend continue to be discovered.

Sandstone-Hosted Uranium Deposits

There are several different types of uranium deposits, but some of the largest and richest are those within the Athabasca Basin of northern Saskatchewan. These are called unconformity-typeuranium deposits because they are all situated very close to the unconformity between the Proterozoic Athabasca Group sandstone and the much older Archean sedimentary, volcanic, and intrusive igneous rock (Figure 18.8).

Figure 18.8 Model of the formation of unconformity-type uranium deposits of the Athabasca Basin, Saskatchewan Source: Steven Earle (2015), CC BY 4.0. View source.

The origin of unconformity-type U deposits is not perfectly understood, but it’s thought that two features are particularly important: (1) the relative permeability of the Athabasca Group sandstone, and (2) the presence of graphitic schist within the underlying Archean rocks. The permeability of the sandstone allowed groundwater to flow through it and leach out small amounts of U, which stayed in solution in the oxidized form U⁶⁺. The graphite (C) created a reducing environment (non-oxidizing) that converted the U from U⁶⁺ to insoluble U⁴⁺, at which point it was precipitated as the mineral uraninite (UO₂).

Practice with Types of Metal Deposits

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Mining and Mineral Processing

Metal deposits are mined in a variety of different ways depending on their depth, shape, size, and grade. Relatively large deposits that are close to the surface and somewhat regular in shape are mined using open-pit mine methods (Figure 18.1). Creating a giant hole in the ground is generally cheaper than making an underground mine, but it’s also less precise, so it’s necessary to mine a lot of waste rock along with the ore. Relatively deep deposits or those with elongated or irregular shapes are typically mined from underground with deep vertical shafts, declines (sloped tunnels), and levels (horizontal tunnels) (Figures 18.9 and 18.10). In this way, it’s possible to focus the mining on the ore body itself. With relatively large ore bodies, it may be necessary to leave some pillars to hold up the roof.

Figure 18.9 Underground at the Myra Falls Mine, Vancouver Island. Source: Steven Earle (2015), CC BY 4.0. View source.

Figure 18.10 Schematic cross-section of a typical underground mine. Source: Steven Earle (2015), CC BY 4.0. View source.

In many cases, the near-surface part of an ore body is mined with an open pit, while the deeper parts are mined underground (Figures 18.10 and 18.11).

Figure 18.11 Entrance to an exploratory decline (arrow) for the New Afton Mine situated in the side of the open pit of the old Afton Mine, near Kamloops, B.C. Source: Steven Earle (2015), CC BY 4.0. View source.

A typical metal deposit might contain a few percent of ore minerals (e.g., chalcopyrite or sphalerite), mixed with the minerals of the original rock (e.g., quartz or feldspar). Other sulphide minerals are commonly present within the ore, especially pyrite.

When ore is processed (typically very close to the mine), it is ground to a fine powder and the ore minerals are physically separated from the rest of the rock to make a concentrate. At a molybdenum mine, for example, this concentrate may be almost pure molybdenite (MoS₂). The rest of the rock is known as tailings. It comes out of the concentrator as a wet slurry and must be stored near the mine, in most cases, in a tailings pond.

The tailings pond at the Myra Falls Mine on Vancouver Island is shown in Figure 18.12, and the settling ponds for waste water from the concentrator are shown in Figure 18.13. The tailings are contained by an embankment. Also visible in the foreground of Figure 18.12 is a pile of waste rock, which is non-ore rock that was mined in order to access the ore. Although this waste rock contains little or no ore minerals, at many mines it contains up to a few percent pyrite. The tailings and the waste rock at most mines are an environmental liability because they contain pyrite plus small amounts of ore minerals. When pyrite is exposed to oxygen and water, it generates sulphuric acid—also known as acid rock drainage (ARD). Acidity itself is a problem to the environment, but because the ore elements such as copper or lead are more soluble in acidic water than neutral water, ARD is also typically quite rich in metals, many of which are toxic.

Figure 18.12 The tailings pond at the Myra Falls Mine on Vancouver Island. The dry rock in the middle of the image is waste rock. The structure on the right is the headframe for the mine shaft. Myra Creek flows between the tailings pond and the headframe. Source: Steven Earle (2015), CC BY 4.0. View source.

Figure 18.13 The tailings pond (lower left) at Myra Falls Mine with settling ponds (right) for processing water from the concentrator. Source: Steven Earle (2015), CC BY 4.0. View source.

Tailings ponds and waste-rock storage piles must be carefully maintained to ensure their integrity, and monitored to ensure that acidic and metal-rich water is not leaking out. In August 2014, the tailings pond at the Mt. Polley Mine in central B.C. failed and 10 million cubic metres of waste water along with 4.5 million cubic metres of tailings slurry was released into Polley Lake, Hazeltine Creek, and Quesnel Lake (Figure 18.14, a and b). As of July 2015, the environmental implications of this event are still not fully understood.

Figure 18.14a The Mt. Polley Mine area prior to the dam breach of August 2014. The tailings were stored in the area labelled “retention basin.” Source: Source: NASA (2014), Public Domain. View source.

Figure 18.14b The Mt. Polley Mine area after the tailings dam breach of August 2014. The water and tailings released flowed into Hazeltine Creek, and Polley and Quesnel Lakes. Source: NASA (2014,) Public Domain. View source.

Most mines have concentrators on site because it’s relatively simple to separate ore minerals from non-ore minerals and thus significantly reduce the costs and other implications of transportation. But separation of ore minerals is only the preliminary stage of metal refinement. For most metals the second stage involves separating the actual elements within the ore minerals. For example, the most common ore of copper is chalcopyrite (CuFeS₂). The copper needs to be separated from the iron and sulphur to make copper metal, and that involves complicated and very energy-intensive processes that are done at smelters or other types of refineries. Because of their cost and the economies of scale, there are far fewer refineries than there are mines.

There are several metal refineries (including smelters) in Canada; some examples are the aluminum refinery in Kitimat, B.C. (which uses ore from overseas); the lead-zinc smelter in Trail, B.C.; the nickel smelter at Thompson, Manitoba; numerous steel smelters in Ontario, along with several other refining operations for nickel, copper, zinc, and uranium; aluminum refineries in Quebec; and a lead smelter in New Brunswick.

Practice with Mineral Processing

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References

Britannica. (1998). Placer deposit. Encyclopaedia Britannica. https://www.britannica.com/science/placer-deposit

Cline, J. S., Hofstra, A. H., Muntean, J. L., Tosdal, R. M., and Hickey, K. A. (2005). Carlin-type gold deposits in Nevada: Critical geologic characteristics and viable models. Economic Geology, 100th Anniversary Volume, 451–484.

Perkins, D., Peck, T., Edition, J. N., & Donovan, J. (n.d.). 9.3.3: Sedimentary ore deposits. In Mineralogy. Geosciences LibreTexts. https://geo.libretexts.org/Bookshelves/Geology/Mineralogy_(Perkins_et_al.)/09:_Ore_Deposits_and_Economic_Minerals/9.03:_Types_of_Ore_Deposits/9.3.03:_Sedimentary_Ore_Deposits

Voisey, C. R., Hunter, N. J. R., Tomkins, A. G., Brugger, J., Liu, W., Liu, Y., and Luzin, V. (2024). Gold nugget formation from earthquake-induced piezoelectricity in quartz. Nature Geoscience, 17(9), 920–925. https://doi.org/10.1038/s41561-024-01514-1