Imagine a column of magma forcing its way upward through the crust, cooling slowly, and leaking hot metal-bearing fluids into the surrounding rock for tens of thousands of years. That single process has produced more copper, molybdenum, and gold than almost any other geological event on Earth. It is called a porphyry system, and it is a useful place to start, because understanding how one deposit forms makes the logic of all the others easier to follow. Ore deposits are not random. Each type is the predictable result of a particular set of geological conditions, and knowing which conditions produced which deposit tells you a great deal about what you are likely to find there.
Porphyry deposits
Porphyry deposits form where magma intrudes into the upper crust, typically above a subduction zone. As the magma cools, water and other fluids are expelled outward. Those fluids carry dissolved metals — copper most commonly, often with molybdenum, and sometimes gold — and deposit them as fine disseminations and veinlets through a large volume of fractured rock. The ore grades are low compared with some other deposit types, but the sheer size of the mineralised zone compensates. To illustrate how this arithmetic works: suppose a mine processes rock at a grade of 0.4% copper. That sounds modest, but if the orebody contains several hundred million tonnes of rock at that grade, the total copper contained is very large indeed. The economics depend on scale and low operating cost per tonne rather than on rich pockets of metal.
Sedimentary deposits
Some ore deposits have nothing to do with magma. Sediment-hosted deposits form when metals are carried in solution through basin waters and then precipitated within sedimentary layers — sometimes on the seafloor, sometimes in continental basins. The result can be remarkably consistent sheets of mineralisation, which makes them attractive to mine because the geometry is predictable. Copper, lead, zinc, silver, and iron are all found in sedimentary settings. The Zambian and Congolese copperbelt is a well-known example of a sediment-hosted copper province, where mineralisation follows ancient basin stratigraphy. Banded iron formations — thick alternating layers of iron oxide and chert laid down in Precambrian seas — are the source of most of the world's iron ore.
Magmatic sulfide deposits
When certain magmas, particularly those rich in iron and magnesium, become saturated in sulfur, a separate sulfide liquid can separate out, much as oil separates from water. That sulfide liquid is an efficient collector of metals, particularly nickel, copper, cobalt, and the platinum-group elements. It can pool at the base of a magma chamber or along channels in the rock, solidifying into bodies of dense sulfide ore. Deposits of this kind tend to be smaller in volume than porphyries but considerably richer in metals per tonne. Nickel mining in regions such as the Sudbury basin in Canada and Norilsk in Russia is largely built on magmatic sulfide mineralisation.
Laterite deposits
Laterites are the product of deep, prolonged weathering in warm, wet climates. When certain types of rock — particularly those rich in olivine and pyroxene — are exposed to intense tropical weathering over millions of years, soluble elements are washed away and less soluble ones, including nickel, cobalt, and aluminium, become concentrated in the residual soil profile. Lateritic nickel and cobalt deposits are typically shallow and of large areal extent. Bauxite, the main ore of aluminium, is a laterite. Because no deep mining or blasting is initially required, extraction can be physically straightforward, though processing the ore into metal is energy-intensive.
Pegmatite deposits
Pegmatites are extremely coarse-grained igneous rocks that crystallise from the last, water-rich fraction of a cooling granitic magma. Because many elements do not fit easily into the crystal structures of common minerals such as quartz or feldspar, they become progressively concentrated in this residual fluid. The result is that pegmatites can carry unusually high concentrations of lithium, beryllium, tantalum, niobium, tin, and rare-earth elements, along with gemstones such as tourmaline and beryl. Individual crystals can be enormous — metres long in some cases — but the deposits themselves are often small and geometrically irregular, which can make mining and resource estimation more complex than for a large disseminated deposit.
Placer deposits
Running water sorts sediment by density. Minerals that are dense and chemically resistant — gold, cassiterite (tin oxide), ilmenite, zircon, and others — tend to settle out in river bends, behind boulders, and at the base of gravel sequences while lighter material is carried further downstream. Concentrations of these heavy minerals in river or beach sediments are called placers. Placer gold drove many of the great nineteenth-century gold rushes, including those in California and Australia. The gold is not created by the river; it is eroded from a primary hard-rock source somewhere upstream and reconcentrated mechanically. Beach placers, where wave action does the sorting, are a significant source of titanium and zircon minerals in several countries.
Evaporite deposits
When a body of water evaporates — whether a shallow inland sea, a coastal lagoon, or a desert lake — dissolved salts crystallise out in a predictable sequence. The most soluble compounds precipitate last and therefore tend to occur in the upper parts of the deposit. Evaporites are the primary source of potash (used in fertilisers), lithium (from certain continental brine systems), boron, and common salt, as well as gypsum and anhydrite. The thick salt beds beneath parts of Europe and North America are ancient evaporite sequences. Lithium-bearing brines in the high-altitude basins of the central Andes represent a related environment, where the evaporating fluid is groundwater rather than a surface sea.
The bigger picture
These seven types account for the great majority of the world's commercial mineral production, but they are not isolated categories. Porphyry systems sometimes have an overlying epithermal gold deposit formed by the same fluids at shallower levels. Laterites develop over magmatic sulfide rocks. Placers concentrate minerals eroded from pegmatites. Understanding the connections between deposit types — and the tectonic settings that produce them — is the territory of economic geology at level three and beyond. Readers wanting to go further might look at the relationship between plate tectonic setting and deposit type, or at the methods geologists use to distinguish one deposit model from another during early-stage exploration.