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CONCENTRATION REQUIRED TO OPERATE

ENRICHMENT ELEMENT

NATURAL CONCENTRATION IN CRUST

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FACTOR Aluminum

(% BY WEIGHT)

MAGMATIC PROCESSES ferent minerals may crystallize at different times to create layering in the pluton.

Magmatic processes form mineral deposits as liquid magma solidifies to form an igneous rock. These

The largest ore deposits found in mafic layered plu- processes create metal ores as well as some gems and

tons are the rich chromium and platinum reserves of valuable sulfur deposits.

South Africa’s Bushveld intrusion. The pluton is about 375 by 300 kilometers in area—roughly the size of the

Layered Plutons state of Maine—and about 7 kilometers thick. The Bushveld deposits contain more than 20 billion tons of

Some large bodies of igneous rock, particularly those of chromium and more than 10 billion grams of platinum, mafic (basaltic) composition, solidify in layers. Each

the greatest reserves in any known deposit on Earth. The layer contains different minerals and is of a different

chemical composition from adjacent layers. Some of the layers may contain rich ore deposits.

The layering can develop by at least three processes:

1000°C Magma

1. Recall from Chapter 4 that cooling magma does not

still liquid

solidify all at once. Instead, higher-temperature

Crust

minerals crystallize first, and lower-temperature minerals form later as the temperature drops. Most minerals are denser than magma. Consequently, early-formed crystals may sink to the bottom of a magma chamber in a process called crystal settling (Fig. 19–2). In some instances, ore minerals crystallize with other early-formed minerals and consequently accumulate in layers near the bottom of the pluton.

2. Some large bodies of mafic magma crystallize from the bottom upward. Thus, early-formed ore minerals become concentrated near the base of the pluton by this process.

3. In some cases, a large body of magma may begin to develop layering by either of the two processes just

Crystals that form

described. Then, additional magma of a different

at 1000°C settle

composition or temperature may flow into the Figure 19–2 Early-formed crystals settle and concentrate magma chamber. As a result of these changes, dif-

near the bottom of a magma chamber.

338 CHAPTER 19 GEOLOGICRESOURCES

world’s largest known nickel deposit occurs in a layered Such deposits are sometimes mined even as the sulfur- mafic pluton at Sudbury, Ontario, and rich platinum ores

rich fumes continue to escape from the volcano. are mined from layered plutons in southern Montana and Norilsk, Russia.

HYDROTHERMAL PROCESSES Kimberlites Hydrothermal processes are probably responsible for the

formation of more ore deposits, and a larger total quan- In Chapter 5, we described rare igneous rocks called

tity of ore, than all other processes combined. To form a kimberlites that originate in the mantle and are the

hydrothermal ore deposit, hot water (hence the roots hy- world’s main source of diamonds. Diamonds are used

dro for water and thermal for hot) dissolves metals from both for jewelry and as industrial abrasives because of

rock or magma. The metal-bearing solutions then seep their great hardness. Now, however, most industrial dia-

through cracks or through permeable rock until they pre- monds are produced synthetically.

cipitate to form an ore deposit.

Three main sources provide water for hydrothermal Volcanic Vent Deposits

activity.

Sulfur, used primarily for sulfuric acid in industrial ap-

1. Many magmas, particularly those of granitic compo- plications, precipitates as a pure yellow deposit from

sition, leave behind a water-rich residual fluid after gases escaping from some volcanic vents (Fig. 19–3).

most of the magma has solidified. Under certain conditions, that fluid crystallizes to form pegmatite, as described in Chapter 4. Under other conditions, the water and dissolved ions escape from the magma chamber to form hydrothermal solutions. For this reason, hydrothermal ore deposits are com- monly associated with granite and similar igneous rocks.

2. Ground water can seep into the crust where it is heated and forms a hydrothermal solution. This is particularly true in areas of active volcanism where hot rock or magma heats ground water at shallow depths. For this reason, hydrothermal ore deposits are also common in volcanic regions.

3. In the oceans, seawater is heated as it seeps into cracks along the mid-oceanic ridge and near subma- rine volcanoes.

As you learned in Chapter 6, water by itself is ca- pable of dissolving some minerals. The dissolved salts and high temperature of hydrothermal solutions greatly increase their ability to dissolve minerals. Thus, hot, salty hydrothermal water is a very powerful solvent, ca- pable of dissolving and transporting metals.

Table 19–1 shows that tiny amounts of all metals are found in the average rocks of the Earth’s crust. For ex- ample, gold makes up 0.0000002 percent of the crust, while copper makes up 0.0058 percent and lead 0.0001 percent. As hydrothermal solutions migrate through rock, they dissolve these metals. Although the metals are pres- ent in very low concentrations in country rock, hydrothermal solutions percolate through vast volumes of rock, dissolving and accumulating the metals. The solutions then deposit the metals when they encounter

Figure 19–3 Yellow sulfur coats the vent of Ollagüe changes in temperature, pressure, or chemical environ- volcano, southern Bolivia.

ment (Fig. 19–4). In this way, hydrothermal solutions

How Ore Forms 339

Hydrothermal vein deposits

Granite Figure 19–4 Hydrothermal ore

Disseminated

deposits form when hot water de-

ore deposit

posits metals in bedrock.

scavenge metals from large volumes of average crustal rocks and then deposit them locally to form ore. In ad- dition, some magmas also contain metals, which concentrate with the hydrothermal solutions that form as the magma solidifies.

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