C u in Cobar EarthCache

Because copper reacts readily with other substances it can be found in a variety of ways in the Earth's crust. It is often found in deposits with other metals such as lead, zinc, gold and silver. Copper is sometimes found in its pure form, which is called "native copper". In its natural state, mineral compounds such as copper are found surrounded by rock or matrix.

By far the largest amounts of copper are found in bodies known as porphyry copper deposits. These deposits were once large masses of molten rock that cooled and solidified deep in the Earth's crust. As they cooled, some large crystals grew, which were then surrounded by smaller crystals - geologists call these rocks porphyries.

At first, the molten rock contained a small amount of copper. As it cooled and crystals began to form the amount of fluid became smaller. The copper remained in the fluid, becoming more and more concentrated. When the rock was almost completely solid, it contracted and cracked and the remaining copper-rich fluid was squeezed into the cracks where it too finally solidified.

The land around Cobar is flat and there are very few outcrops of rock. Geologists have drilled the region and mapped the rocks making up the area. It is one of complex geology with a number of different rock types present.

The oldest recognised rock strata in the Cobar region are the Girilambone Group which is made up of Early Ordovician turbidite interspersed with thin chert and volcanic layers. Turbidite is a type of sedimentary rock composed of layered particles that grade upward from coarser to finer sizes. They are thought to have originated from ancient currents in the oceans while the basaltic volcanic rocks erupted probably as oceanic islands.

During the Late Ordovician to Early Silurian periods the rocks were heated and it was at this time that the Cobar porphyry copper deposits were developed. Granites also began to intrude the deposit layers. Layers alternately consist of conglomerates fining up into siltstones. The siltstone is characterised by wide layers interspersed with fine sandstone bands and volcanic rocks.

In Australia, copper is mainly found in rocks as the mineral chalcopyrite. These rocks are more than 250 million years old.

Other less commonly occurring minerals that contain copper are azurite and malachite which are both composed of copper carbonate. Malachite is green and is a weathering product of azurite, which is a blue copper carbonate mineral.

Over millions of years weathering of the Cobar Basin rock walls created steeply plunging pipe-like clusters which reach deep down into the earth. Intense chemical alteration related to this long weathering history of the Cobar terrain resulted in silica changing the siltstone. The overall geology in Cobar is one of silica altered sandstone and quartz sulphide veins grading to massive chalcopyrite. It is in these rocks deep underground that the Cobar Basin copper has been found.

However, in a few areas where the rock has been greatly eroded it has thrusted up through the Earth’s crust. There it has been largely destroyed or dramatically reduced by further chemical alteration and samples of azurite and malachite can be found on the surface.

The main ore mineral of copper here in Cobar (and worldwide) is chalcopyrite (CuFeS2) which has a golden yellow appearance. As a processed metal, copper is red, with a bright metallic lustre. The unusually textured and interestingly coloured rocks found at S31 30.026 E145 50.691 are slag - they are what is left after the copper ore has been extracted during the smelting process.

Australia has about 6% of the world's economic copper resources and is ranked third after Chile (25%) and the USA (16%). Here at CSA mine the ore grade averages about 6% but in some places is as high as 12% of the rock mined. This places it as one of the richest copper deposits in the world where some copper is mined in mineral averages as low as 0.2%.

Top ten company holdings of economic demonstrated resources of copper in Australia.

In 1870 three tank sinkers, a Cornish, a Scottish and an Australian were camped at 'Kubbur' waterhole in an area that is behind the heritage centre. They noticed the unusual colour of the water and took samples of the rock. Those samples returned assays up to 39% copper content. This led to the creation of the CSA mine.

The sulphides, which yield most of the copper produced throughout the world, generally occupy the deeper parts of lodes which have not been exposed to weathering. Near the surface they are altered by oxidation and other chemical actions to the native metal, oxides and carbonates. These secondary copper minerals may form rich ore in the upper parts of many deposits, and, owing to their characteristic green or blue colour, even small amounts are easily seen in the rocks in which they occur.

Although large copper deposits are mined by open-cut methods in many of the major producing countries, most of the copper ore produced in Australia comes from underground mines.

You may log this Earthcache straight away but then please email your answers to the questions to the CO. We will contact you ONLY if your answers need extra work. Logs with no answers sent will be deleted. Please do not include your answers in your log.

1. What is the chemical symbol for copper?
a. Cu
b. Pb
c. Ag

2. Most of the copper deposits in Australia were formed
a. Thousands of years ago
b. More than 250 million years ago
c. 250 years ago

3. Bronze is an alloy of
a. gold and copper
b. copper and lead
c. copper and tin
d. copper and zinc

4. In Australia, copper is most commonly found as the mineral
a. malachite
b. chalcopyrite
c. azurite

5. What are the three common copper ore minerals found in Australia and throughout the world?

6. If wandering throughout the bush and you were to stumble across a copper mineral deposit, what would you expect to see that may identify it as copper?

7. When standing at S31 30.026 E145 50.691, what are the unusual looking rocks you are standing on?

You can obtain most of your answers from within the above information or a bit of research. Further information is available at the old mine office, now the tourist information and mining museum nearby.