Ring of Fire
British Columbia sits on the Pacific Ring of Fire, a geological feature that is associated with many of the largest and richest mineral deposits in the world, including:
- Grasberg in Indonesia (largest gold mine and third largest copper mine in the world);
- Nevada’s gold deposits;
- Chile’s massive copper deposits;
- The incredible silver riches of Mexico and:
- The rich mineral deposits of British Columbia.
Without getting too technical, it’s worth a moment to understand the geological processes associated with the Ring of Fire and how those processes create mineral deposits.
The earth’s upper layer is called the lithosphere and consists of the crust (on which we live) and the uppermost mantle immediately beneath. The lithosphere is divided into sections, or plates, some larger, some smaller.
Lithosphere comes in two main types. Continental lithosphere is typically about 40-280 km thick and in places as old as 4,200 million years (Ma). Oceanic lithosphere is created by upwelling of basaltic lava along oceanic plate margins called spreading ridges (an example is the Juan de Fuca Ridge off the coast of British Columbia). The oceanic lithosphere is much younger than continental lithosphere (rarely older than 180 Ma). It also varies in thickness from only a few kilometres at its recently-created margins to as much as 140 km in the oldest sections.
Between the lithosphere and the earth’s core is the huge thickness of the rest of the mantle. The layer immediately beneath the rigid lithosphere is called the asthenosphere. The asthenosphere is a "isco-elastic solid", meaning that it flows, but so slowly as to make a glacier look like a Lotus. Very slow convection in the asthenosphere carries the overlying plates at speeds generally only a few centimetres per year.
Where plates move apart, molten rock (called magma) moves up into the rift, creating new oceanic lithosphere. This is presently happening along the Juan de Fuca spreading centre, just off the coast of British Columbia. One of the largest spreading rifts on the planet is the Mid-Atlantic Ridge which, as its name suggests, runs roughly north to south along the centre of the Atlantic Ocean. The volcanic island of Iceland is one of the few areas where this vast volcanic rift is visible above sea level.
Where lithospheric plates converge, the denser plate, usually oceanic, sinks or is
pushed beneath the more buoyant plate, in a process known as subduction. The
subducted rock is mostly the volcanic (and related) rock formed at the spreading
centres, but includes seawater and ocean sediment deposited on the seabed. The
older the lithosphere, the more sediment present.
Heating and devolatilisation of the subducted lithosphere generates magma which can ascend to form a chain of volcanoes on the overriding plate. Where the overriding plate is oceanic, a volcanic island arc forms (Japan is an example). On an overriding continental plate, a continental volcanic arc such as the Cascades arc in the western U.S.A. is formed. These present-day volcanic arcs define the Ring of Fire.
It’s not all volcanoes. Plates slide laterally as well, creating fault zones that can extend for hundreds of kilometres, such as the San Andreas Fault in southern California. However, it’s safe to say that plate boundaries are where volcanoes happen, fluids flow and rock gets buckled and broken. All this is excellent news for forming mineral deposits.
That was now, this is then (no, it’s not a misprint)
You will read in many places, including here, about “Early Jurassic” and the mineral wealth of the Golden Triangle. The Early Jurassic was a time period 200 Ma to 176 Ma ago. To give you an image of that “length” of time, at a scale where 1 millimetre represents a year, 200 Ma is 200 km. What has the present Ring of Fire to do with anything that formed that far in the past?
In fact, the Pacific Ocean, or its ancestors, has been around for considerably longer than that and the western edge of ancestral North America has been oceanfront property for at least the past 500 Ma. In the time before the Early Jurassic, several Madagascar-sized microcontinents separated from ancestral North America, only to collide with the big continent later. One of these, which we now call Stikinia, contained the area of the future Golden Triangle. In the Early Jurassic, Stikinia was a volcanic arc separate from North America, but within the ancestral Ring of Fire. The best bit is that the Early Jurassic volcanic arc was built on several previous volcanic arcs, which had already concentrated metals in the continental fragment.
How to build a mineral deposit
Some like it hot – a heat source
Mineral deposits are often formed in conjunction with the magmatic activity related to convergent plate boundaries. Where the magma comes to surface, it creates volcanoes. However, much of the magma solidifies (crystallizes) below the surface in masses that are kilometres across. The resulting intrusive rock masses are called stocks, or plutons if they are larger (several kilometres). That magma is very hot, starting with temperatures above 650°C. When successive bodies of magma rise from a source region, creating a polyphase intrusion, the duration of heating is prolonged, allowing solutions to build up more metal.
If it’s not there, it can’t be concentrated – a metal source
The molten rock generated from subduction zones commonly contains higher concentrations of both precious and base metals. In the case of the Early Jurassic Texas Creek suite of intrusions in the Golden Triangle, the magmas contained a lot of metal. When multiple intrusions occur over a geologically short period of time, the intrusion becomes polyphase – and the metals just keep on coming.
Follow the yellow brick road – make a pathway
As the magma cools, some minerals form crystals. Gold, silver and the base metals do not bond into the rock-forming minerals and so are concentrated into the residual fluid. The water in that fluid, at a temperature of several hundred degrees and under intense pressure, mixed with chlorine, fluorine and other elements is extremely corrosive. The metals and even silicon dioxide (quartz) are dissolved and carried by that fluid. Water from rain and other sources contribute to its volume and, with every grain of pyrite dissolved, the fluid becomes more corrosive and, as it forms a convection cell, more laden with base and precious metals.
However, without a pathway, that convecting fluid – and the metal in it – isn’t going anywhere. The popular statement is that it takes ten times the volume of water in Sydney harbour to carry enough metal to make an ore deposit. Trying to force that much hot water through solid rock, before the molten rock cools (and the “kettle goes off the boil”) is a non-starter. A pathway is crucial.
Sometime the fluids will make their own pathway, by fracturing rock with steam explosions. However, it’s always preferable to have the previously broken rock of faults and shear zones act as pathways from the intrusive sources to the traps and reservoirs where the metals finish their journey.
Sometimes the metals travel far from their source, to form epithermal deposits on the fringes of the intrusion-related systems, where boiling of the fluids changes their chemistry and deposits precious and base metals a few hundred metres beneath the surface of the volcano. Often the fluids precipitate at deeper levels, forming porphyry deposits, in excess of a kilometre beneath the ancient land surface, whose main metal component is iron, with economically significant copper, molybdenum and gold. Occasionally the fluids reach the ancient surface or seafloor, as happened at Eskay Creek, depositing metals and metal sulphides on the seafloor as exhalative deposits.
There are thousands of porphyry systems, some of which contain meaningful amounts of copper and other metals. However, only a limited number of those deposits have the right combination of grade and tonnage to be economically viable. A high concentration of metals in a porphyry requires a number of factors to come together, including the nature of the source magma and the length of time that the system remains active.
Let nature do most of the digging
Every one of the mineral deposits formed in the Golden Triangle has one thing in common. Since their formation, they have been covered (in the case of deposits formed near surface) or remained covered (such as the porphyry systems formed at depths in excess of 2 km below an ancient surface). The magic of the Golden Triangle is that the topographic variation, in excess of 2,500 metres, means that Nature has already done a lot of the digging.