Geology Corner: Deposits that host the world's base metals

The earth has many unique ways of stripping base metals from its crust and concentrating them into deposits. Over the course of civilization, humans have uncovered seven main types: sediment-hosted copper-cobalt; magmatic nickel-copper-platinum group metal; volcanogenic massive sulphide; porphyry; mississippi valley type; sediment exhalative; and laterites.

Sediment hosted copper-cobalt deposits

The majority of world-class sediment hosted copper-cobalt deposits formed shortly after a unique period of time often referred to as the "Boring Billion," the "Earth's Middle Ages", or "the Dullest Time on Earth". The earth's major landmasses combined to form one large supercontinent, and remained relatively stable, positioned along warmer latitudes, from 1.7 billion to 750 million years. The continent began to break apart in the neo-Proterozoic, but many of the rifts failed, becoming significant basins of evaporitic, lacustrine and red-bed sediments mixed with saline and oxidized fluids called brines. The brines circulated through the basins for tens, if not hundreds of millions of years, stripping metals from the rocks only to redeposit them onto chemically reductive horizons.

The sheer size and scale of the mineralizing process created blankets of high-grade copper that extend for tens of kilometres in scale, and account for 20% of the world's copper production today. Supergiant examples are found within the Katangan basin of south-central Africa, whereas other world-class deposits of varying ages are found in the Kodaro-Udokan basin of Siberia, and the Zechstein basin of northern Europe. As an example of a world-class deposit, Ivanhoe Mines' (TSX: IVN; US-OTC: IVPAF) Kamoa and Kakula deposits in the Katangan belt hosts a combined 1.4 billion indicated tonnes of 2.64% copper, and 316 million inferred tonnes of 1.76% copper over a 4.5 to 6.9-metre thickness.

Magmatic nickel-copper-PGM deposits

Magmatic nickel-copper-platinum group metals (PGM) deposits occur when magnesium-rich magma (called ultramafics) sourced from the mantle, break through the crust in a dense network of dykes, sills and magma chambers. The fast-flowing, metal-rich intrusions devour sulfur-bearing crust during its ascent. Once the sulfur is saturated in the melt, it separates out of the magma - much like oil does in water - and begins to absorb the surrounding heavy metals, such as nickel, copper and PGMs.

Underground at Codelco's El Teniente copper porphyry mine in Chile. Credit: Codelco.

In traditional models, the magma becomes trapped in quiescent magma chambers - much like an eddy along a fast-flowing river - allowing the sulfur time to scavenge an abundance of metals from the melt. Within time, the sulfur droplets laden in metal sink to the base of the chamber, whereas lighter concentrations crystallize in-situ as the magma cools. As a result, magmatic-style deposits often have horizons of massive sulphides that grade upwards into less dense, net-textured sulphides, before terminating into a horizon of disseminated sulphides.

Less traditional models take into account the sheer complexity of magma flow. There are often multiple chambers each connected by complex highways of hot magma. The magma bodies are convecting, mixing, changing composition, de-pressurizing or re-pressurizing as new magma is injected into the system, cools then re-melts and cools again.

A perfect example of a non-traditional deposit model can be found at Vale's (NYSE: VALE) Voisey's Bay, where the ore is found in feeder dykes connecting the two chambers (eastern and western deeps), and there is evidence that metal-rich sulphide liquid was injected into the eastern deeps ore body, rather than settling along its base. Similar features are also seen in other world-class camps such as Norilsk or Jinchuan.

There are seven major camps in the world - the largest being the Sudbury complex in Ontario and the Norilsk deposits of Russia, followed by Kambalda in Western Australia, Voisey's Bay in Labrador, the Thompson Nickel belt in Manitoba, Raglen in northern Quebec, and Jinchuan in China.

Volcanogenic massive sulphide copper-lead-zinc deposits (VMS)

VMS deposits are found in areas where the earth's crust is being torn apart, such as along mid-ocean ridges, volcanic islands or back arc spreading centres. Some of the metals are supplied by hydrothermal fluids off-gassing from an underlying intrusive, whereas the rest is sucked in from heated groundwater convecting through basaltic crust for tens of kilometres. The hydrothermal fluids combine to vent upwards along extensional structures, only to spew outwards onto the sea floor as lenses of massive sulphide, and stockworks of mineralization underlying the mound.

VMS deposits are often found in clusters or districts about 40 km in diameter, yet only one or more of the deposits contain more than half of the district's resources. Average VMS grades run 5% copper, 4% zinc, 1% copper. Although low grade, the sheer size of porphyry deposits allow for bulk mining for multiple decades, and the mines are therefore not as vulnerable to challenging market conditions compared to those shorter lived. World class deposits include Codelco's Chuquicamata and El Teniente, BHP Billiton's (LON: BHP) and Rio Tinto's (NYSE: RIO) Escondida, Rio Tinto's Bingham Canyon, and Freeport McMoRan's (NYSE: FCX) Grasberg in Papua New Guinea.

Mississippi Valley Type lead-zinc deposits (MVT)

Mississippi Valley Type lead-zinc deposits account for 24% of the global resources of lead and zinc and are known for their high quality ore. The deposits tend to have low iron in the system, which means the ore will be less penalized at the smelter, and the minerals are very coarse grained, making the zinc easier to recover. MVT's have a median size of 7 million tonnes at grades of 7.9 % lead and zinc, but they occur in districts where there can be hundreds of them in one place.

The driving factors behind MVT deposits are rock type-they form in coarse-grained carbonate rocks, often enriched with hydrocarbons-and how permeable the rocks are to mineralizing fluids. The deposits are found in extensional fault zones, and associated with fractures and dilatant zones. Brines flush along basin-bounding faults into structurally-prepared cavities created by the dissolution of carbonates and/or along permeable horizons. The fluids react with the carbonates and the metals are deposited.

Major districts include the area surrounding the drained basin of the Mississippi River in central U.S., Pine Point in Yellowknife, Teck Resources' (TSX: TECK.A and TECK.B; NYSE: TECK) former Polaris mine in Nunavut, ScoZinc Mining's (TSXV: SZM; USOTC: SWNLF) former Gays River mine in Nova Scotia, or Lennards Shelf in Western Australia.

Sediment Exhalative lead-zinc deposits (Sedex)

Unlike MVT's, sediment exhalative lead-zinc deposits are hosted in shales. The basinal brines carrying the metals travel upwards along structures only to exhale onto the seafloor, or permeate outwards into underlying sediments adjacent to the vent. Some sedex deposits are found really close to the vent, whereas others are more distal-either a plume fallout that's been dispersed by bottom currents or reworked sulfide mounts.Giant and supergiant deposits range from 100 million to 400 million tonnes, with grades from 5% to 30% lead and zinc.

World class deposits include Teck Resources Sullivan in southern B.C., Glencore's (LON: GLEN) McArthur River in Australia, and Teck's Red Dog in Alaska, whereas notable deposits are found at Faro, Mac Pass and Howard's Pass in the Yukon, where Selwyn Chihong and Fireweed Zinc (TSXV: FWZ; US-OTC: FWEDF) are currently exploring.

Nickel-Cobalt Laterite Deposits

The earth's crust tends to rot at low latitudes. The intense chemical and mechanical weathering at surface makes for thick soil profiles, with varying compositions depending on the original rock type. Nickle-cobalt laterites occur when ultramafic rocks, which carry an elevated concentration of metals, weather into a soil.

Valueless minerals are flushed away, and metals such as nickel and cobalt find their way into multiple minerals throughout the weathered profile. Over 70% of the world's nickel resources come from laterites, however only 50% account for global production. Deposits range in size from 2.5 million to 400 million tonnes, grading 0.66% to 2.4% nickel and 0.01% to 0.15% cobalt. New Caledonia contains 21% of the world's nickel laterites, followed by Australia (20%), the Philippines (17%) and Indonesia (12%).

Lesley Stokes is a geologist and a former staff writer for The Northern Miner. She currently works for Freeport McMoRan in Vancouver.