An introduction to geology
An introduction to geology

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

An introduction to geology

2.13 Bursting balloons of copper mineralisation

Porphyry copper deposits are usually formed by small igneous intrusions – formed by hot, molten rock forcing their way through existing rocks. The ore minerals that contain the copper are mainly copper sulphides like chalcopyrite (CuFeS2) and bornite (Cu5FeS4), and they’re found in networks of veins called stockwork. Each of the veins are only a few mm across, so you can imagine why it isn’t efficient to try to remove just the copper minerals.

Described image
Figure 2.14 Mineral grains called stockwork

The minerals in the veins are deposited by water being forced through the rocks. As the magma rises through the earth, it cools and solidifies first at the edges, where the hot magma is in contact with the cold rock which it is being forced through. It rises up like a balloon because it is hotter and less dense than the rocks that surround it, just like a hot air balloon.

The magmas which form the ores also contain water, not a lot – probably only about 3% – but still enough to be important. Bubbles of hot water dissolve elements (like copper) which don’t go into the minerals forming as the magma solidifies. The super-heated copper-laden water is even less dense than the surrounding magma and so rises to the top of the balloon of magma. The water starts to expand and form steam, and the overpressure cracks the balloon of solidifying magma, allowing the copper-filled water to burst out, forcing open cracks in the rocks as it goes. As the water reaches cooler rock, it cools down and can no longer keep the copper dissolved, so it’s deposited along the cracks that it’s formed.

Described image
Figure 2.15 Copper deposition

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371