Ore deposits and economic minerals: the processes that concentrate metals into economic ore deposits (magmatic segregation, hydrothermal and vein deposits, placer deposits, secondary enrichment and sedimentary deposits); the concepts of ore grade, cut-off grade and reserves; and the calculation of the tonnage of metal from grade and tonnage data.

What this dot point is asking

Part of the Earth materials and natural resources concept. Eduqas wants you to explain the processes that concentrate metals into economic ore deposits (magmatic segregation, hydrothermal and vein, placer, secondary enrichment and sedimentary), to use the concepts of ore grade, cut-off grade and reserves, and to calculate the contained metal from grade and tonnage. An "ore" is only an ore if the metal is concentrated enough to mine at a profit, so the geology and the economics are linked.

The answer

What makes a deposit an ore

The metals are present in ordinary rock only in tiny crustal abundances, far too dilute to mine. An ore deposit forms where a natural process has concentrated a metal to many times its crustal abundance. The required concentration (the enrichment factor) is large for rare, valuable metals and smaller for common ones.

The ore-forming processes

Several processes concentrate metals:

• Magmatic segregation. Dense, early-crystallising minerals (for example chromite, or sulphides carrying nickel and platinum) settle and accumulate at the base of a cooling magma chamber.
• Hydrothermal and vein deposits. Hot, watery, mineral-rich fluids (from a cooling intrusion or heated circulating groundwater) carry dissolved metals through fractures; as they cool, depressurise or react with the wall rock, the metals precipitate as sulphides in veins (galena, sphalerite, chalcopyrite, with quartz). This is the most important source of many base and precious metals.
• Placer deposits. Weathering releases dense, resistant minerals (native gold, cassiterite for tin), which rivers then sort: the heavy grains drop where the current slows, concentrating them while lighter grains are carried on.
• Secondary enrichment. Weathering of a low-grade sulphide deposit dissolves metals near the surface and reprecipitates them lower down, enriching a zone above the water table.
• Sedimentary deposits. Chemical precipitation from water (for example banded iron formations, and evaporite salts) and the residual concentration of insoluble metals (for example bauxite, aluminium ore, from intense tropical weathering).

Grade, cut-off grade and reserves

The economics use three precise terms:

Because the cut-off grade depends on the metal price and costs, the same rock can be ore when prices are high and waste when they are low; reserves therefore change with economics, not just geology.

Calculating contained metal

The mass of metal in a deposit is simply:

so a 0.8 percent grade means $0.008$ of the rock mass is the metal. This calculation, and judging it against the cut-off grade, is the standard quantitative task.

Examples in context

Example 1. Vein sulphides. Classic lead-zinc-copper veins form where hydrothermal fluids precipitated galena, sphalerite and chalcopyrite along fractures, the deposits worked in many historic mining districts.

Example 2. Gold placers. Dense, inert gold released by weathering accumulates in river gravels where the current slows, which is why panning concentrates gold from the heavy fraction of the sediment.

Try this

Q1. An orebody is 10 million tonnes at 2 percent zinc. Calculate the mass of zinc metal. [2 marks]

• Cue. $2\% = 0.02$; zinc $= 10{,}000{,}000 \times 0.02 = 200{,}000$ tonnes.

Q2. Define cut-off grade. [2 marks]

• Cue. The lowest grade that can be mined and processed at a profit at current prices and costs; rock above it is ore, rock below it is waste.

Q3. Explain how a placer deposit concentrates gold. [2 marks]

• Cue. Weathering releases dense, resistant gold grains, which rivers sort: the heavy grains are deposited where the current slows while lighter grains are carried on, concentrating the gold.