9.1 Mineral deposits & ore-forming processes

Economic Geology: Ore, Gangue, and Grade

Economic geology is the study of Earth materials that can be extracted and used at a profit, and it underpins the supply of nearly every metal and mineral society uses. An ore is a rock or mineral aggregate from which one or more valuable commodities—usually metals—can be recovered economically. The valuable minerals are the ore minerals (for example, chalcopyrite for copper, galena for lead, sphalerite for zinc, and hematite for iron), whereas the surrounding worthless minerals are gangue (commonly quartz, calcite, and pyrite). Whether a body of rock qualifies as ore is an economic judgment as much as a geologic one: it depends on commodity price, mining and processing costs, metallurgy, depth, and environmental constraints. A deposit that is subeconomic today can become ore if prices rise or technology improves, and a mined-out deposit can revert to worthless rock.

Grade expresses the concentration of the valuable commodity. Base metals (Cu, Pb, Zn, Ni) are reported in weight percent, while precious metals (Au, Ag, and the platinum-group elements) are reported in grams per tonne (g/t) or parts per million. The cutoff grade is the lowest grade that can be mined profitably; material below it is treated as waste. The concentration factor is the ratio of a metal's grade in ore to its average crustal abundance, and ore-forming processes must enrich a metal many times above background. Copper averages roughly 0.006% in the crust but requires about 0.4 to 1% to be mined, a concentration factor near 100; gold requires a factor in the thousands. Understanding these numbers explains why economic deposits are geologic rarities produced only by unusually efficient concentrating processes.

Magmatic Deposits

Magmatic deposits form as metals concentrate within cooling magma. Fractional crystallization and crystal settling cause dense early-formed minerals to accumulate near the floor of an intrusion; layered mafic bodies such as the Bushveld Complex host chromite, platinum-group elements (PGE), and vanadium-rich magnetite. Sulfide liquid immiscibility separates a dense metal-rich sulfide melt from the silicate magma, concentrating Ni-Cu-PGE ores (Sudbury, Norilsk). Diamonds are carried from the mantle in kimberlite pipes. Late-stage, water-rich residual melts crystallize as pegmatites enriched in lithium, beryllium, tantalum, rare earth elements, and gemstones.

Hydrothermal Deposits

Hot, metal-bearing aqueous fluids are the most important ore-forming agents on Earth. These fluids leach metals from source rocks, migrate along fractures and permeable beds, and precipitate ore minerals where temperature, pressure, pH, or fluid chemistry changes—often when hot fluids cool, mix with cooler groundwater, or react with wall rock.

• Vein / lode deposits — metals precipitate in open fractures, classically gold in quartz veins.
• Porphyry deposits — large-tonnage, low-grade disseminated and stockwork Cu (with Mo and Au) surrounding shallow felsic-to-intermediate intrusions; they supply most of the world's copper and molybdenum.
• Volcanogenic massive sulfide (VMS) — Cu-Zn-Pb sulfides deposited on or near the seafloor at submarine hydrothermal vents (black smokers).
• Sedimentary exhalative (SEDEX) — stratiform Pb-Zn (with Ag) hosted in shale, formed where hydrothermal brines vent into reduced marine basins.
• Mississippi Valley-type (MVT) — low-temperature Pb-Zn hosted in carbonate rocks, deposited from migrating basinal brines.
• Skarn deposits — metasomatic replacement at igneous-carbonate contacts, yielding W, Cu, Fe, or Au within calc-silicate gangue.

Secondary enrichment can upgrade hydrothermal ore: weathering of a porphyry cap dissolves copper, which reprecipitates below the water table as a high-grade supergene blanket of chalcocite.

Surface and Sedimentary Processes

• Placer deposits concentrate dense, chemically resistant minerals—gold, platinum, cassiterite (tin), diamonds, and monazite—by moving water; gold's high specific gravity causes it to settle and lag in stream gravels and bedrock traps.
• Residual / laterite deposits form under intense tropical weathering that leaches soluble elements and leaves an insoluble residue: bauxite (the principal ore of aluminum) and nickel laterites are the classic examples.
• Evaporite deposits precipitate from evaporating seawater or saline lakes in a predictable, solubility-controlled sequence—carbonate, then gypsum/anhydrite, then halite, then potash (sylvite)—supplying salt, gypsum, and potassium.
• Banded iron formations (BIF) are Precambrian chemical sediments of alternating iron-oxide and chert layers; they are the world's dominant iron source and record the oxygenation of early oceans.

Deposit Summary Table

Recognizing a deposit's family lets a geologist predict its geometry, likely metals, and exploration signatures—the core reasoning tested on the ASBOG exams. Deposits also cluster in space and time within metallogenic provinces, regions where a particular tectonic setting favored one style of mineralization; for instance, convergent-margin arcs host porphyry copper belts, rifted continental margins host SEDEX and MVT lead-zinc districts, and ancient cratons host the deepest kimberlites and layered intrusions. Many deposits also yield economically important by-products—molybdenum, gold, and silver from porphyry copper, or cadmium and germanium from zinc ores—so grade must be evaluated for the whole suite of recoverable metals, not the primary commodity alone. This systems-level view, tying process to setting to product, is what separates rote memorization from the geologic reasoning the exam rewards.