Categories:
Base Metals
/
General Market Commentary
Topics:
General Base Metals
/
General Market Commentary
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.
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% zinc and 1 gram gold per tonne within 4 million to 25 million tonnes, but some examples far exceed those amounts. Supergiant examples in Canada include the Kidd mine in Ontario, Windy Craggy in British Columbia, the Flin Flon/Snow Lake district in Manitoba, and Bathurst in New Brunswick.
Porphyry
Porphyry copper-gold plus or minus molybdenum deposits are found along convergent plate boundaries, where slabs of crust are driven downwards and ultimately consumed by the mantle. Magma generated from the sub-ducting slab travels upwards, exploiting pre-existing structures and rupturing the crust as it ascends. Occasionally, the system blows its top and erupts as a volcano, otherwise, it sits well below the surface stewing in its own hydrothermal fluids. The deposits tend to have a high-grade core, noted as significant concentrations of potassium feldspar, magnetite and copper-rich minerals such as bornite.