Orogenic Gold Deposits

Orogenic Gold Deposits


Behind me is the Eureka Reef, discovered by
gold prospectors 160 years ago. It was a truly spectacular outcrop of white
quartz. Stained orange from the minerals in it, 10
metres wide and rising high above the ground. What led the miners here, was gold-rich gravels,
known as placer deposits, in the surrounding gullies. But it was here, in this beautiful quartz
reef, that they found the source of the placer gold. The gold was embedded in the quartz in small
but visible grains, like in this sample here. This type of gold-bearing quartz is typical of what I’m familiar with, here in southeastern Australia. So, after travelling half way around the world
to California I was surprised to find that the gold deposits
there, looked so familiar. And this is the same for many of the gold
lode districts around the world. The veins look the same and even the chemistry
is similar. To earth scientists this suggests they may
have formed in the same way. By 1998 a lot more was known about these distant
gold deposits. So an international team could now draw some
conclusions, about their likely origins. I met with Dr Rich Goldfarb on his visit to
Victoria, he was a member of the team and they decided to give all these similar
gold deposits a new common name. We call them orogenic gold deposits because they’re related to orogeny or the mountain-building process. Any time we have a mountain building process
along the margin of a continent we’re going to form orogenic gold deposits as part of
that process. Rich Goldfarb became fascinated with gold
working for the United States Geological Survey. In the 1980s he was looking for new deposits
in the Alaskan coastal mountain ranges by using thousands of geochemical soil samples this is a routine job but the results set
him thinking about how Alaskan gold had formed. These deposits at the time I started were
actually were called epithermal gold. Which is gold that forms at very shallow levels,
often from circulation of surface waters but I started looking at the distribution
of the gold and those type of deposits, epithermal deposits, are associated with granites. But the gold wasn’t associated with granites. When I plotted up all the gold and stream
sediments and glacial moraines the gold showed a distribution with metamorphic rocks. It was mainly in rocks that had been metamorphosed at moderate temperatures and pressures. And that suggested to me there might be a different type of model and it might be related to metamorphism. Other researchers also realised that metamorphism, gold deposition and orogeny were often closely related in many quartz lode type deposits. So the name Orogenic Gold has stuck. Metamorphism might be the key but where did
the gold come from in the first place. Where do you get the gold? It has to be in
the rocks to move during metamorphism. Most of us would argue that gold comes into
the rocks during sea-floor processes. If you look at sediments or volcanic rocks
on the ocean floor they often have very disseminated grains of pyrite, or iron sulphide. There’s often a high background perhaps
a hundred plus per billion gold in those pyrite grains. As the ocean floor is moved towards the edge of the continent, subducted, and what’s called, what we call often accreted, or added to the continental margin those oceanic sediments with the pyrite are
then heated up during metamorphism so that gold in that pyrite is mobile along
with any water or CO2, carbon dioxide, that is in other minerals so that fluid and that gold move together
during the metamorphism. Metamorphism occurs when seafloor rocks are
thickened and deeply buried by deformation. With burial, the gold-bearing rocks are subjected to rising pressure and temperature causing a dramatic chemical change. The rocks then release a gold-rich metamorphic
fluid. When we heat a rock to 400 or 450 degrees
along a continental margin we change 5 to 6% of that rock volume to the
fluid phase. That’s a hell of lot of fluid being produced. To create this amount of metamorphic fluid,
you need a big engine. So what is the earth process that can do this? The unifying process is actually plate tectonics
we, up till 25 years ago, we could not understand the distribution of gold, but as our understanding of plate tectonics
grew, we started to being able to understand the spatial and temporal distribution of gold
on the planet. We realise along active continental margins
where we have collision of terranes and then after collision the terranes strike
slip, or tend to move apart and we have earthquakes we form orogenic gold
deposits. The youngest examples of these collisions
are continental margins of the Pacific. Around the Pacific collisions created deposits along the Asian and American margins between 170 to 50 million years ago. In North America, the driving force for collision
lay beneath the continental margin, where subduction pushed oceanic sediments
against the continent. Those reflect young collisions along the margins
of continents, metamorphism and mountain building along the edge of continents and the formation
of young orogenic gold deposits. These relatively young collisions can help
us explain the origin of more ancient gold deposits like in Australia or Asia? Plate tectonics went way back in Earth’s
history, so if we look at other gold belts of the world, we have other collisional margins that are
ancient examples of western North America. Much of central Asia into western China is an analogue to what we see in western North America. So from what we learned in western North America,
from the very young collisions in Alaska. We can now look at central Asia and say “that’s
what happened 250 to 500 million years ago” and that’s why we have all these gold deposits
spread across central Asia. And the orogenic model fits my homeland Australia
like a glove? Like Asia, this place where I’m now standing,
once lay next to a continental margin. And the rocks here were completely smashed
in a monumental collision. As a tectonic plate pushing from the east
deformed and raised these rocks from the seafloor. And just as in Asia and Alaska orogenic gold
was formed during this collision. I’m 300 metres below the peak and the gold-bearing
lodes that we find around here were injected into these rocks during or just
after that major collision. And this timing is one of the defining characteristics
of orogenic gold deposits. It happened here, in southeastern Australia,
more than 400 million years ago. Collision led to metamorphism, which in turn
created these veins. And they’re still being mined here today. Oh, there’s no doubt about it the Victoria
gold deposits are just a classic example of orogenic gold deposits. Their structure, their mineralogy, the fact that they have large placer fields along with the lode fields. They are a world-class orogenic gold deposit
province.

17 Comments

  1. Thanks for fantastic films.   I have just completed my 2nd year geology subjects as a mature aged student at Monash and my passion is gold Geo particularly Victoria, so this film is right up my alley.   Thanks so much, this film has helped me to order the general geo theory I've learned up to date, i.e. plate tectonics, metamorphism and chemical origins and how they interact to make orogenic deposits.   Thanks Heaps and keep up the great work  🙂

  2. I first saw Dr Richard Goldfarb on the excellent "How the Earth was made" series by History channel.  Amusing play on words Rich Gold farb. He was down a gold mine looking at a white quartz seam with visible gold nuggets in it. , is it a pen name ? On another geology video there was a geologist called Prof Underhill.

  3. I am just an average part time prospector…I found this very informative …heading out tomorrow to get rich….with Australian outback culture 🙂

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