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The Lost Riches of the Himalaya

Aerial view of several Himalayan glaciers.

Aerial view of several Himalayan glaciers.

By Lloyd White

Most of the world’s gold and copper deposits are formed at tectonic plate boundaries. It’s a pity, then, that geologists find it difficult to locate the ancient plate boundaries in the Himalayan mountains.

The Earth’s surface is divided into a number of tectonic plates that are constantly moving. They grind against one another, tear apart and crash into each other, triggering earthquakes and volcanic eruptions. Over time this movement also results in the formation and destruction of vast oceans such as the Indian and the Pacific, and great mountain chains such as the Himalaya and the Andes.

The world’s oceans and mountain ranges take many millions of years to form because the tectonic plates move at about the rate at which your fingernails grow. Geologists are able to understand more about the rates of these processes by determining the age of rocks, which can be dated if they contain fossils of a known age or by measuring the relative amounts of certain isotopes produced by the radioactive decay of minerals trapped in the rock.

Fossils can provide very useful information about how old a rock is, but they are only found in some sedimentary and metamorphic rocks. For the other sedimentary, igneous and metamorphic rocks we must use isotopic dating to get a precise measurement of a rock’s age.

Over the course of my PhD project I have dated the age of rocks and minerals from the Himalayan mountain chain to determine how they were emplaced, deposited or deformed. It is important to understand how the plate boundaries have evolved over time as this is precisely where we believe most of the world’s major gold and copper deposits form.

I have travelled with my PhD supervisors to north-western India to collect rock samples from the remote and mountainous regions of the Himalaya and Karakorum ranges, which are at the northern and southern sides of the Eurasian/Indian plate boundary, in order to confirm far more precisely its location and how it evolved.

Samples were taken from strategic locations to either side of where most geologists believe the ancient plate boundary lies. The samples were shipped back to the ANU’s Research School of Earth Sciences in Canberra for testing.

In the 1970s the boffins at ANU designed and created a powerful instrument for this purpose called the “SHRIMP”, which is used to measure the amount of uranium and lead in zircon minerals. Zircon crystals are present in many sedimentary, igneous and metamorphic rocks, and the date when the zircon actually crystallised can be calculated based upon the known “half-life” of each isotope and the amounts measured by SHRIMP.

By combining the results obtained from SHRIMP with other data that tells us where the Earth’s tectonic plates existed in the past, we can construct a picture of what the Earth looked like many millions of years ago. We were hoping this data would tell us more about how the Himalayan mountains evolved.

The Himalayan mountain chain is considered to have formed when the Indian plate ploughed into the Eurasian continent, causing the rocks to buckle and thicken and to be uplifted to their present elevation. However, there is considerable debate within the scientific community about when India and Eurasia collided. One of the aims of my PhD thesis was to collect new data with SHRIMP to understand the age of the Himalayan rocks and help to resolve the disputes about when India collided with Eurasia.

Many geological researchers had previously envisaged the India/Eurasia collision to be a relatively simple system where two tectonic plates rammed into each other. However, our research indicated that it was far more complicated than this.

As a result of our samples, and additional scientific data, we now believe that the formation of the Himalaya was much more like a multi-vehicle freeway pile-up rather than a sudden crash between two vehicles. That is, a number of micro-continents or island chains got in India’s way before it rammed into Eurasia. Our role was to establish which continents hit the other continents, and how many millions of years passed between each of these smaller collisions!

Some of the rocks we collected from north-western India were expected to be part of the Eurasian plate, but surprisingly the rocks had an age “fingerprint” similar to the rocks of the Indian plate further to the south.

Our isotopic results were further supported by older palaeontological studies which showed that fossils collected in this area were from the ancient supercontinent Gondwana, of which India was a part along with Australia, Antarctica, South America and Africa. For some reason these earlier fossil findings had largely gone unnoticed by the geological research community.

These findings could mean several things. One explanation is that the northern margin of India extended much further north than previously thought. This would mean that a great deal of scientific work devoted to understanding the history of the Himalayan mountain range, and where India and Eurasia collided, have actually been focusing on the wrong location.

Another explanation is that the Indian/Gondwanan rocks had been eroded and redeposited as a new rock on the Eurasian side of the plate boundary, or that parts of the Indian continent were pulled away from, and possibly torn off, the northern margin of India and were later sandwiched between India and Eurasia after the major collision.

The uncertainty means that we are unable draw a line on the world map to define which rocks belonged to the Indian plate and which rocks belonged to Eurasia.

This gets even more complicated when we start using advanced software tools to consider the shape of the continents in three-dimensions, as the shape of the continents changes at deeper levels in the Earth’s crust.

We also need to consider that the shape and composition of the edge of the continents can change over time – the immense forces generated by the movement of tectonic plates cause the edges of the continents to deform. For example, about 100 million years ago the shape of Australia’s coastline would have looked much different, with New Zealand joined onto the east coast of Australia and Australia still joined to Antarctica.

While it is a challenge to understand the location and geometry of the Earth’s ancient plate boundaries, it is not all for academic purposes. It is important that we understand how the plate boundaries evolved over time as this is where we think most of the world’s major gold and copper deposits form.

Our isotopic dating of minerals collected from the north-west side of the Himalayan mountain range reveals that it is much more difficult to identify the location of the ancient plate boundary between India and Asia than previously thought. As a result, the hunt for these elusive new reserves of gold and copper continues.

Lloyd White is now a postdoctoral research fellow at the Australian National University’s Research School of Earth Sciences.