Metal and Ores
Metal and Its Discovery
Metals are opaque, shiny, smooth solids that can conduct electricity and can be bent, drawn into wire, or hammered into thin sheets. In this regard, they look and behave quite differently from wood, plastic, meat, or rock. This is because, unlike in other substances, the atoms that make up metals are held together by metallic bonds, so electrons can ﬂow from atom to atom fairly easily and atoms can, in effect, slide past each other without breaking apart. The ﬁrst metals that people used copper, silver, and gold can occur in rock as native metals. Native metals consist only of metal atoms, and thus look and behave like metal.
|Gold occurs as native metal within quartz veins. The quartz breaks up to form sand, leaving nuggets of gold.|
Gold nuggets, for example, are chunks of native metal that have eroded free of bedrock (figure above). Over the ages, people have collected nuggets of native metal from stream beds and pounded them together with stone hammers to make arrowheads, scrapers, and later, coins and jewellery. But if we had to rely solely on native metals as our source of metal, we would have access to only a tiny fraction of our current metal supply. Most of the metal atoms we use today originated as ions bonded to non-metallic elements in a great variety of minerals that themselves look nothing like metal. Only because of the chance discovery by some prehistoric genius that certain rocks, when heated to high temperatures in ﬁre (a process called smelting), decompose to yield metal plus a non-metallic residue called slag, do we now have the ability to produce sufﬁcient metal for the needs of industrialized society.
What Is an Ore?
|Examples of ore minerals.|
The minerals from which metals can be extracted are called ore minerals, or economic minerals. These minerals contain metal in high concentrations and in a form that can be easily extracted. Galena (PbS), for example, is about 50% lead, so we consider it to be an ore mineral of lead (a in figure above). We obtain most of our iron from haematite and magnetite. Copper comes from a variety of minerals, none of which look like copper (b in figure above). Geologists have identiﬁed a great variety of ore minerals. Many ore minerals are sulphides, in which the metal occurs in combination with sulphur (S), or oxides, in which the metal occurs in combination with oxygen (O).
To obtain the metals needed for industrialized society, we mine ore, rock containing native metals or a concentrated accumulation of ore minerals. To be an ore, rock must not only contain ore minerals, it must also contain a sufﬁcient amount to make the rock worth mining. Iron constitutes only about 6.2% of the continental crust's weight but makes up about 30% to 60% of iron ore. The concentration of a useful metal in an ore determines the grade of the ore the higher the concentration, the higher the grade. Whether or not an ore of a given grade is worth mining depends on the price of metal in the market.
How Do Ore Deposits Form?
Ore minerals do not occur uniformly through rocks of the crust. If they did, we would not be able to extract them economically. Fortunately for humanity, geologic processes concentrate these minerals into accumulations called ore deposits. Simply put, an ore deposit is an economically signiﬁcant occurrence of ore. The various kinds of ore deposits differ from each other in terms of which ore minerals they contain and which geologic conditions led to their formation. Below, we introduce a few examples.
|Various processes that form ore deposits.|
When a magma cools, sulphide ore minerals crystallize early, then, because sulphides tend to be dense, hey sink to the bottom of the magma chamber, where they accumulate; this accumulation is a magmatic deposit. When the magma freezes solid, the resulting igneous body may contain a concentration of sulphide minerals at its base. Because of their composition, such concentrations are known as “massive- sulphide deposits” (a in figure above).
Hydrothermal activity involves the circulation of hot-water solutions through a magma or through the rocks surrounding an igneous intrusion. These ﬂuids dissolve metal ions. When a solution enters a region of lower pressure, lower temperature, different acidity, and/ or different availability of oxygen, the metals come out of solution and form ore minerals that precipitate in fractures and pores, creating a hydrothermal deposit (figure above b). Such deposits may form within an igneous intrusion or in surrounding country rock. If the resulting ore minerals disperse through the intrusion, we can also call the deposit a disseminated deposit, but if they precipitate to ﬁll cracks in pre-existing rock, we can call the deposit a vein deposit; veins are mineral-ﬁlled cracks.
In recent decades, geologists have discovered that hydrothermal activity at the submarine volcanoes along mid-ocean ridges leads to the eruption of hot water, containing high concentrations of dissolved metal and sulphur, from a vent. When this hot water comes in contact with cold seawater, the dissolved components instantly precipitate as tiny crystals of metal-sulphide minerals (c in figure above). The erupting water, therefore, looks like a black cloud, so the vents are called “black smokers”. The minerals in the cloud eventually sink and form a pile of ore minerals around the vent. Since the ore minerals typically are sulphides, the resulting hydrothermal deposits constitute another type of massive-sulphide deposit.
Secondary-enrichment depositsSometimes groundwater passes through ore-bearing rock long after the rock ﬁrst formed. This groundwater dissolves some of the ore minerals and carries the dissolved ions away. When the water eventually ﬂows into a different chemical environment (for instance, one with a different amount of oxygen or acid), it precipitates new ore minerals, commonly in concentrations exceeding that of the original deposit. A new ore deposit formed from metals that were dissolved and carried away from a pre-existing ore deposit is called a secondary-enrichment deposit. Some of these deposits contain spectacularly beautiful copper-bearing carbonate minerals, such as azurite and malachite.
Rain falling along one margin of a large sedimentary basin may sink into the subsurface and then ﬂow as groundwater along a curving path that takes it ﬁrst down to the bottom of the basin, and then eventually back up to the opposite margin of the basin, hundreds of kilometres away. At the bottom of the basin, temperatures become high enough that the water dissolves metals. As the water returns to the surface and enters cooler rock, these metals precipitate in ore minerals. Ore deposits formed in this way, containing lead- and zinc-bearing minerals, appear in dolomite beds of the Mississippi Valley region of the United States, and thus have come to be known as Mississippi Valley–type (MVT) ores.
Sedimentary deposits of metals
|A Precambrian banded iron formation from northern Michigan.|
Some ore minerals accumulate in sedimentary environments under special circumstances. For example, between 2.5 and 1.8 billion years ago, the atmosphere, which previously had contained very little oxygen, gained oxygen because of the evolution of abundant photo synthetic organisms. This change affected the chemistry of seawater so that large quantities of dissolved iron precipitated as iron oxide minerals that settled as sediment on the sea ﬂoor. The resulting iron-rich sedimentary deposits are known as a banded iron formation (BIF) (figure above), because after lithiﬁcation they consist of alternating beds of Gray iron oxide (magnetite or haematite) and red beds of jasper (iron-rich chert).
The chemistry of seawater in some parts of the ocean today leads to the deposition of manganese-oxide minerals on the sea ﬂoor. These minerals grow into lumpy accumulations known as manganese nodules. Mining companies have begun to explore technologies for vacuuming up these nodules; geoscientists estimate that the worldwide supply of nodules contains 720 years’ worth of copper and 60,000 years’ worth of manganese, at current rates of consumption.
Residual mineral deposits
Recall from Interlude B that as rainwater sinks into the Earth, it leaches (dissolves) certain elements and leaves behind others, as part of the process of forming soil. In rainy, tropical environments, the residue left behind in soils after leaching includes concentrations of iron or aluminium. Locally, these metals become so concentrated that the soil itself becomes an ore deposit. We refer to such deposits as residual mineral deposits. Most of the aluminium ore mined today comes from bauxite, a residual mineral deposit created by the extreme leaching of rocks (such as granite) containing aluminium-bearing minerals. The figure below shows the residual mineral deposits as well as placer deposits.
|Placer deposits form where erosion produces clasts of native metals. Sorting by the stream concentrates the metals.|
Ore deposits may develop when rocks containing native metals erode, producing a mixture of sand grains and metal ﬂakes or nuggets (pebble-sized fragments). For example, gold accumulates in sand or gravel bars along the course of rivers, for the moving water carries away lighter mineral grains (quartz and feldspar) but can’t move the heavy metal grains (gold) so easily. Concentrations of metal grains in stream sediments are a type of placer deposit (figure above). Panning further concentrates gold ﬂakes or nuggets swirling water in a pan causes the lighter sand grains to wash away, leaving the gold behind.
Where Are Ore Deposits Found?
The Inca Empire of ﬁfteenth-century Peru boasted elaborate cities and temples, decorated with fantastic masks, jewellery, and sculptures made of gold. Then, around 1532, Spanish ships arrived, led by conquistadors who quipped, “We Spaniards suffer from a disease that only gold can cure.” The Incas, already weakened by civil war, were no match for the armorclad Spaniards with their guns and horses. Within six years, the Inca Empire had vanished, and Spanish ships were transporting Inca treasure back to Spain. Why did the Incas possess so much gold? Or to ask the broader question, what geologic factors control the distribution of ore? Once again, we can ﬁnd the answer by considering the consequences of plate tectonics. Several of the ore-deposit types mentioned above occur in association with igneous rocks. Igneous activity does not happen randomly around the Earth, but rather concentrates along convergent plate boundaries (speciﬁcally, in the overriding plate of a subduction zone), along divergent plate boundaries (along mid-ocean ridges), continental rifts, or hot spots. Thus, magmatic and hydrothermal deposits (and secondary-enrichment deposits derived from these) occur in these geologic settings. Placer deposits are typically found in the sediments eroded from such magmatic or hydrothermal deposits. The Inca gold formed in the Andes along a convergent plate boundary.
Credits: Stephen Marshak (Essentials of Geology)