Geologists are pretty clear about how Broken Hill formed. About 1700 million years ago, there was some volcanic activity, probably under the sea, and hot volcanic fluids – containing dissolved metal sulphides – were bubbling up, and when these hot solutions encountered the cold ocean water, you got clouds of black precipitates forming. And these settled to form layers of very rich metal sulphides.
The layers were deeply buried, heated up – cooked, if you like – in the process known as metamorphism. During that stage they were also folded and the metal sulphides re-crystallised. And they turned into this rather glittery looking mixture of lead, zinc and silver sulphide minerals. And this is the typical ore that is mined at Broken Hill.
The metal sulphides were not pure in the sense that they also contained other elements. They contained manganese, and silicon, and other things all mixed up with them. So when this metamorphism took place, other crystals were able to form. And so Broken Hill’s famous for big crystals of manganese silicates like this rhodonite crystal. It also formed some wonderful garnet crystals, these big red crystals enclosed in the metal sulphides. And these formed as a result of the metamorphism at the same time.
For the last, say, 400 or 500 million years not much has happened at Broken Hill except the landscape’s been lowered through erosion. And that’s gradually brought the sulphide layers, which now are sort of in a folded aspect, brought them to the surface.
And that means they can be attacked by the atmosphere, by acid ground water, and that immediately sets up a whole series of new chemical reactions, which generate a suite or a set of new minerals.
In particular, zinc carbonates, this mineral here smithsonite, for copper carbonates — azurtite and malachite, for lead phosphates, which is this yellow mineral here — pyromorphite. There are also other unusual lead minerals, a range of copper minerals. And these are the minerals that Broken Hill is famous for.