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Gwen Preston – “Exploration Explained: Dating Rocks”

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Knowing the age of the rocks that contain the metals and minerals we explore and mine might sound like an esoteric pursuit for academic geologists. Why should a savvy investor care how old the rocks are? Does it really matter if the gold is hosted in rocks that are 2 billion or 3 billion years old? Isn’t the amount of gold all that’s important?

Wrong. Understanding the ages of the rocks that host economic mineralization is critical to finding more mineralization, from the property scale to a global scale, and it can be a guide to how prospective a patch of ground really is.

You might remember from previous explainer articles that economic mineral deposits often form when magma (molten, or partially melted, rock beneath the Earth’s surface) is pushed up and into other rocks nearer the surface. The magma brings heat and metal-rich fluids that perforate through rocks and into faults and fractures, which then cool and trap metals to form mineral deposits.

Over Earth’s 4.5-billion-year history, there have been periods of intense magmatic activity. Most of the major mining districts around the world formed during these ‘supercontinent cycles’.

One example is the Archean, the period 4 to 2.5 billion years ago. Some of the oldest rocks on Earth are found in the Canadian Shield of central Canada and the Archean Shield in Western Australia. Some of the richest mineral deposits in the world are found in similar rocks of the same age! It is not a coincidence.

What do we mean by ‘magmatic activity’? You can witness present-day magmatic activity around the edges of the Pacific Ocean’s ‘Ring of Fire’. At these tectonic plate boundaries, hot magma is being intruded into the crust, volcanoes are erupting, and new mountain chains are being created all around the planet’s largest ocean.

That magmatic activity is could be forming future ore reserves and is just a small-scale example of the activity that has occurred during a small number of intense cycles in the past, when many of the world’s major mineral deposit districts were formed.

Even ore deposit types that are associated less closely with magmatic intrusive rocks, such as Nevada’s Carlin deposits, have unique age controls. Carlin deposits are hosted in sedimentary rocks that formed at the right time relative to tectonic movements and global sea levels. Those particular rocks were in the right place at the right time to be mineralised. Older and younger rocks are typically barren, and so are the right kind of rocks in the wrong place.

Are these rocks the right age?

By understanding the when, where, and why of these historical episodes of magma activity, rock formation, and mineral accumulation, we should be able to locate new mineral-rich areas and new deposits.

Exploration teams should try to understand the ages of the rocks and processes on their property. A robust exploration program will be designed to find the ideal rock types and structures to host the mineralization and the age relationship between the rock units hosting the deposit and the mineralization itself.

Understanding the processes that emplaced the ore guides explorers to ask the right questions. With the right data they can follow these clues to find the richest portion of a mineral deposit, find the next mineral deposit area, or spend less money sterilising a property and moving on.

Rock dating is not often a standard part of the exploration process. It tends to be something students, researcher and academics do on a regional scale, rather than on an exploration lease. But often, companies conducting exploration development and mining will cooperate with universities to provide samples and access to the property, and possibly funding, to support this research because it helps everyone understand the overarching, big picture idea of the region and the setting of the mineralization. 


Radiometric dating

The methods used to directly date rocks (determine the absolute age) are based on measuring the radiometric decay of certain elements in certain minerals.

If you cast your mind back to high school science class, you might recall that elements contain protons, neutrons, and electrons. Some elements have unstable isotopes, variations that have an extra neutron.The radioactive isotopes decay, discarding those neutrons to become more stable. Laws of physics govern how quickly an isotope decays, making them reliable ‘clocks’. Scientists compare the remaining amount of unstable isotope to the more stable form to estimate the time that has passed.

When certain minerals crystallize from cooling molten rock, the ‘decay clock’ within them starts ticking. The mineral zircon, for example, contains radioactive uranium (U) that decays to lead (Pb). The ratio in the rock today can be used to estimate when it cooled.

Zircons are relatively common in granite rocks. In the laboratory, scientists can isolate a single zircon crystal from a rock, analyze it for U and Pb isotopes, and use a ratio of the two to determine the age of the zircon crystal and the rocks around it.

Different elements and isotope pairs are used to date different materials, including carbon, potassium, thorium, uranium, lead and others. Radiometric dating allows geologists to determine the age of rocks that are millions or even billions of years old. The study of dating rocks is called geochronology.

In short, age matters. Knowing the timing of the magma intrusions, and the relationships between the rocks on an exploration property is vital to understanding the mineralization – and where to find more!

Posted May 19, 2020

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