Colours of the Earth

Video | Updated 8 months ago

Dr Peter Downes, Curator of Minerals and Meteorites, Earth & Planetary Sciences
Presented as part of the In the Wild West Lecture Series in 2012.

The interaction of the atmosphere and ground waters with mineral deposits over time has produced an exotic array of colourful minerals. Come on a tour through some hotspots for mineral diversity that have been revealed through mining in Western Australia.

Transcription

Today I’ll be taking you on a tour around some Western Australian mineral deposits that are remarkable for the diverse range of mineral species that they contain. To use an analogy with the biological world, they could be called mineralogical rainforests. The stops on our journey are the Telfer gold mine in the Great Sandy Desert, the Whim Creek copper mine and the Ashburton deposits in the Pilbara, and the 132 North nickel mine near Widgiemooltha in the Goldfields.

Rocks exposed at the surface of the Earth are subject to the processes of weathering and erosion, operating through the action of wind and water. In addition, exposure to our oxygen-rich atmosphere produces some interesting chemical reactions between the rocks at or near the surface and oxygen-rich groundwater, a process called chemical weathering. Some minerals break down and others grow, often completely transforming the exposed rocks in a blanket that extends from the surface to depths of up to around 150 metres. When this process of chemical weathering takes place in mineral deposits, for example deposits of copper, lead, zinc, nickel or gold, it can produce an enormous range of products known as secondary minerals that have diverse chemical compositions and are often of great beauty.

Before we go too much further, I’ll just explain a few useful concepts and terms. All matter is made up of atoms, extraordinarily small particles that we can think of in this fashion; a cloud of electrons surrounding a nucleus containing protons and neutrons. To get an idea of the scale involved, take a millimetre length and divide it into 1,000 and this is a micron. To get to the scale of atoms, take a micron and divide it into 10,000 finer widths. So the scale of an atom is one ten millionth of a millimetre. In the words of Bill Bryson, “The nucleus of an atom is tiny. Only one millionth of a billionth of the full volume of the atom, but fantastically dense since it contains virtually all the atom’s mass. If an atom were expanded to the size of a cathedral, the nucleus would be only the size of a fly, but a fly many thousands of times heavier than the cathedral.”

Now if an atom loses an electron to a chemical bond for example, it becomes positively charged and we call it a cation. Iron for example, in forming stable compounds with other elements, may take one of two oxidation states. Ferrous iron has donated two of its electrons in forming bonds with other atoms and is in oxidation state two, or Fe2+. An iron atom may donate three electrons to bonding and is then in oxidation state three, known as ferric iron, or Fe3+. A reaction that causes iron to increase the number of its electrons involved in bonds with other atoms is called an oxidation reaction, as it increases the oxidation state of iron. We will see an example of this in a moment. If an atom gains an electron, it becomes negatively charged and we call it an anion. A reduction reaction reduces the oxidation state of an atom as it receives electrons through bonding.

So all rocks are made up of minerals. Generally, these are inorganic compounds that have a distinct chemical composition and a distinct crystal structure. We see the example here of the iron sulphide pyrite, that has a cubic crystal structure. The iron atoms are in red and the sulphur atoms are in yellow on the right there. Elements such as gold and copper also are minerals that crystallise and crystallise in the cubic crystal system. Carbon also forms minerals. Graphite and diamond are the best known of these. Both are made up of carbon, but they have different crystal structures. Here’s some sulphide minerals from the Kimberley in Western Australia – galena and marcasite.

So what constitutes a mineral deposit? Metallic ores as they’re often called are anomalous concentrations of elements, usually as compounds, which are economically favourable for exploitation. The base metals, copper, lead and zinc, have a similar chemical behaviour and thus form similar minerals. Generally they are recovered from sulphide ore deposits where they occur as sulphides or sulphosalts, minerals in which the metals are bonded with sulphur. These minerals are formed in the primary environment, that is at elevated temperature and pressure with respect to conditions at or close to the surface of the Earth, and is oxygen poor. Minerals formed in the primary environment, often at great depths in the Earth’s crust, are unstable in other environments, notably at the surface of the Earth.

Volcanism often provides the heat source for mineralisation in primary environments, driving systems of circulating mineralising fluids. Other sulphide deposits such as the nickel deposits around Kalgoorlie and Kambalda precipitated directly from molten magmas.

An example of a volcanic mineralising system are these ocean floor black smokers where sulphide minerals are deposited on the sea floor from heated brines at temperatures of around 200 to 300 degrees celsius in what is obviously an oxygen-deficient environment. The primary ore in the Whim Creek deposit probably formed in a similar sea floor setting.

What’s called the secondary environment refers to the top few 100 metres of the earth’s crust, or commonly less, far removed from the primary environment conditions. The temperature and pressure in the secondary environment deviate little from atmospheric conditions; oxygen is abundant, water is freely available and sulphur is stable principally as the sulphate iron, SO42-. In the majority of cases the elemental constituents of primary minerals are present in unstable oxidation states relative to their surroundings. With passage of time, they react to form other more stable phases. So the stabilities of chemical compounds or minerals are directly linked to the environment in which they formed, and on this slide we’ve got an example of the oxidation of pyrite to form the iron oxides hematite and goethite. The change in oxidation state from Fe2+ to Fe3+ and you can see the cubes on the bottom right there are iron oxides replacing the original cubes of pyrite.

This next diagram shows a schematic section through an ore body exposed at the surface, and it shows the oxidized zone that contains residual resistant minerals and the products of the decomposition of sulphides and associated primary minerals. The groundwaters in this region are charged with a range of metallic cations and various anions, as shown there, and these groundwaters percolate through this zone. New minerals may be precipitated from these groundwaters, the secondary minerals of the oxide ore bodies, and they’re bewildering in their complexity.

So there are about 5,000 mineral species known at the present time and around a third of these are found in the oxide zone of sulphide ore bodies as a result of these reaction conditions. About 40% of the new minerals that have been described from Western Australia have come from the oxidized zone. Also shown there is the zone of secondary enrichment, just below the water table, and it’s also very important. Secondary sulphides such as chalcocite may be formed here, and we’ll see some beautiful examples of chalcocite from the Telfer gold mine in a few moments.

So the first stop on our tour is the Whim Creek copper mine in the Pilbara and it’s situated just off the highway south of Port Hedland. About 42 minerals from the oxidized zone have been described from Whim Creek, including the only Australian occurrence of murdochite. So the Whim Creek mine was discovered in 1887 by the prospector Phillip Saunders and first mining occurred in 1889–90 and there’s been sporadic production since that time, mostly in the years 1906 to 1924 and more recently from 2005 to 2009. The primary ore body at Whim Creek occurs in the ancient rocks of the Pilbara Craton. It’s a stratabound deposit, but formed in a sequence of sedimentary and volcanic rocks on the sea floor around 3,000 million years ago.

Now there’s a view out across the Whim Creek mine in 1911, and again in 1985. So that’s the Whim Creek Hotel and the mine is situated just behind it, just off the highway south of Port Hedland. Most of the WA Museum collection of Whim Creek minerals were collected in 1985, and here are some photos from that collecting trip.

So that’s the entrance to the underground workings.

I think that’s an area underground called The Ballroom, and here they are collecting a few of the specimens. So the people involved – there’s a couple of them there – David Vaughan who’s one of our museum honorary associates and I think Sheena Elliott as well.

So here’s a list of some of the secondary minerals from the Whim Creek mine. Just to note that you have a whole range of cations and also a whole range of anions as well – sulphate, arsenate, iodide and vanadates and molybdates. So in particular, the lead molybdate wulfenite for which the mine is famous in mineral collecting circles.

So here’s a picture of some wulfenite in the workings underground, little orange crystals studding the wall, and here we have some malachite pseudomorphs after azurite. That’s in situ in the area called The Ballroom.

That’s another nice example of malachite after azurite. There’s some balls of malachite and zooming closer in, you’ve got these lovely little bowties of pyromorphite and pseudomalachite. So the pyromorphite which is a lead mineral, has been capped by little pseudomalachites at the end. So the pseudomalachite is a copper phosphate and the pyromorphite is a lead phosphate.

Wulfenite is one of the most spectacular minerals occurring in the mine. Wulfenite crystals may be up to 9 mm across, but it more commonly occurs as 1-2 mm crystals scattered on and contrasting effectively with dark brown to black goethite lining solution cavities. So it’s generally pale yellow to orange and crystal habits vary from tabular crystals up to about 9 mm wide to tetragonal bipyramids up to about 7 mm long.

Some lovely orange bipyramids. There’s a tabular crystal of wulfenite up to about 4mm across. Some green pyromorphite with tabular wulfenite.

Just as a bit of an interlude, this is a scanning electron microscope which is very important to this sort of work, looking at really small crystals. This is one at the CSIRO here in Perth and basically what happens is a beam of electrons is scanned across the surface of your sample and you get electrons scattered back, and these are used to form images of the crystals that you can see at very small scales. To get some lovely pictures like this, this is bladed fornacite with tabular wulfenite crystals and tiny acicular mottramite. Again, the fornacite and mottramite – this is all from Whim Creek – and also there is some pyromorphite.

Some other SEM images. This is a wulfenite crystal that’s grown around a ball of malachite from Whim Creek, again showing some of the bipyramidal wulfenite crystals, more of a tabular wulfenite crystal there, and just to finish off, a wulfenite studded with tiny crystals of murdochite, just showing the chemical micro-environments involved here that murdochite is very rare and generally only occurs on the surface of wulfenite.

So moving along to the Telfer gold mine in the Great Sandy Desert. The Telfer gold-copper mine is about 485 kilometres southeast of Port Hedland. There was some speculation at the time of its discovery that it was the lost reef of Harold Lasseter. It’s proved to be one of Australia’s largest gold deposits producing almost 6 million ounces of gold from the start of mining operations in 1977 until 2000. It’s best known among collectors for its world class chalcocite specimens.

The oxidized zone of the Telfer deposit also contains a diverse range of secondary minerals. Some of the oxidized zone material was collected by the museum associate David Vaughan in the early 1980s and preserved in the mineral collection of the late Blair Gartrell. A large proportion of Gartrell’s collection was donated to the museum by Mark Creasy in 2003.

So the Telfer gold deposits occur within a sequence of Proterozoic marine sedimentary rocks within the Paterson provenance adjacent to the eastern edge of the Pilbara Craton.

This next slide shows the geology of the Telfer mine, and so in the mine area the sedimentary sequence is composed of interbedded quartz sandstone, siltstone and mudstone, and folding of the sedimentary rocks has produced the Telfer Dome, a north west trending doubly-plunging anticline. Mineralisation occurs in a series of gold-copper bearing horizons or reefs. The Middle Vale and E Reefs have been the most important historically. They’re now mined out, but they’re the source of specimens in the museum collections. The reefs consisted of thin sheets of quartz-pyrite-chalcopyrite mineralisation or their oxidized equivalents. Oxidation and supergene enrichment of the primary sulphide mineralisation in which the pyrite was completely replaced by chalcocite, increased ore grades up to 90 grams per tonne gold and up to 20% copper.

Here’s a list of sulphide minerals from the Telfer mine. Most notable here of course is the chalcocite.

Here’s a lovely group of chalcocite crystals. This specimen is about 1.5 cms across. So at least several hundred specimens of single Chalcocite crystals and crystal groups were collected during the late 1990s from the Telfer mine. The specimens rival those from classic localities such as Redruth in Cornwall, England, and the Bristol copper mine in Connecticut in the US. So the specimens I’ll show you over the next little bit, they range from single or composite crystals up to 6 cms long, to clusters of crystals up to about 6 cms across. So cruciform twins and interpenetrating crystals are common, and some prismatic crystals have arrowhead terminations.

That’s a lovely group of chalcocite crystals. That’s about 4.5 cms long.

That’s a group of interpenetrating crystals of chalcocite with cruciform twinning.

That’s a group of chalcocite crystals and some have this cruciform twinning associated with calcite and the largest crystal there is about 1.5 cms long.

Another group of chalcocite crystals about 2.5 cms across.

This specimen consists of numerous thin, tabular crystals of chalcocite encrusted by colourless calcite crystals on matrix, and the field of view there is about 5 cms.

There’s a list of the oxides from Telfer. You get some quite nice examples of cuprite and here it’s associated with chrysocolla.

There’s some of the carbonates and sulphates.

Here we have some globular masses of malachite on pale blue chrysocolla and intergrown with dark-grey cuprite in parts, and that field of view is about 20 cms across.

Again we have malachite associated with chrysocolla.

Here are some quite nice orange barite crystals on matrix. The crystals are up to about 6 mm wide here.

But the Telfer mine also contained quite a wide range of what are called arsenates, and you see a list of them here. I’ll just go through and show you a few of those.

This is some olivenite associated with chrysocolla. So they’re the olive-green acicular crystals up to about 2mm long. That’s the olivenite and here’s an SEM image showing the overgrowth by chrysocolla.

Another Arsenate is cornwallite. It’s a monoclinic mineral and it’s dimorphous with cornubite. Dimorphous means that it’s got the same chemical composition as cornubite, but has a different crystal structure. So generally the cornwallite forms dark-green hemispherical to barrel-shaped crystal clusters, generally less than 1 mm diameter on matrix, and these crystal groups are composed of elongate bladed crystals that appear to form length-parallel twins.

That’s another mat of dark green cornwallite coating the cavity surface and here we have an SEM image showing cornwallite associated with cornubite in this case. Also there’s a large cylindrical group of cornwallite crystals at the centre of the view associated with fibrous bundles of cornubite crystals, example to the top left of the cornwallite group, and also associated with minor fibres of agardite.

Here we have some intergrown bundles of cornubite crystals on cornwallite and associated with some fibres of agardite as well. So the cornubite is triclinic.

More cornwallite and cornubite. Here we have some cornwallite on matrix. This is a hemispherical group of twin cornwallite crystals surrounded by tufts and fibres of agardite. So the agardite is that pale green colour. They’re generally quite fibrous and sometimes form mats on cavity surfaces.

Here’s an SEM image showing some agardite, sort of radiating hemispheres of this fibrous mineral.

Telfer also contains copper and gold, and here we have a branching copper nugget that’s about 10–11 cms long.

These are some native copper pseudomorphs after a cubic mineral. The field of view is about 4.3 cms there.

Here we have black tarnished dendritic native copper on quartz and the field of view is about 2.5 cms wide there.

And under the SEM you can see the branching structure of one of the copper specimens.

So we’ve also found some lovely examples of gold crystals at the microscopic scale in the Telfer specimens. So ideally formed gold crystals are rare. Natural gold crystals tend to be distorted. Their cubic symmetry is often indiscernible. Typically gold forms skeletal crystals, sheets, wires and dendritic or fern-like aggregates, and twinning plays an important role in the origin of gold’s diversity of habits. This is a flat group of gold crystals here, shaped like a Christmas tree, and the field of view is about 440 microns wide, so less than half a millimetre.

Here we have a complex network of gold rods, wires and tabular dioctrahedral crystals.

Another group of bladed gold crystals on iron oxides on quartz.

Here we have a lovely complex network of gold rods and tabular crystals on quartz, and this is slightly larger. The field of view is about 1.25 mm wide.

Here are some more tabular gold crystals encrusted by very fine-grained gold balls and crystals on quartz, and you’ve got gold encrusting quartz in the background there as well.

Another nice group of gold crystals, and here they are situated next to a tuft of agardite, the arsenate mineral.

Here is a group of very small gold crystals and rods.

A nice group of tabular gold crystals and then just to finish off - I think this might be the last couple – we’ve got this lovely group, it looks a bit like cheese.

So that’s the Telfer gold mine, and so we’ll just move along to the Ashburton Downs deposits in the Capricorn Range in the Pilbara.

So over 80 different species of secondary minerals have been discovered in the Ashburton deposits and mineralisation has been known there since the early 1960s. There’s been minor work, but no large scale mining there.

Here’s a map showing the location of the Ashburton Downs and a simplified geology there as well. So, basically it consists of sedimentary rocks, the Capricorn and Ashburton formations of Lower Proterozoic age. Mineralisation occurs in a west–northwest trending anticlinal structure and the prospects shown there are situated along a sheared fault zone. So at the Anticline prospect shown there, the average width of the shear zone on the surface is about a metre, and secondary ore minerals occur along the shear zone, frequently in cavities and generally the secondary  minerals form very small crystals.

This is a view of the Bali Lo deposit in 2005. The primary sulphide minerals in the Ashburton deposits consist of arsenopyrite, pyrite, marcasite, galena and tetrahedrite as well as tennantite and chalcopyrite. So their primary mineralogy is dominated by sulphides and arsenides of lead, iron, copper and zinc.

This is more of a close-up view of that hill at Bali Lo and looking in on the right hand side you might be able to see a greenish patch of rock there. That’s a boxwork after tennantite which is a copper-arsenic sulphide and it’s been replaced by secondary minerals including olivenite, chenevixite, mawbyite, gartrellite, mimetite, arsentsumebite and some alunite minerals.

Here we are doing a bit of mineral collecting in 2005 at Bali Lo, and this is a view of the Bali East deposit. So in underneath that ledge there is where all the secondary minerals have been uncovered.

This list just shows some of the diversity of those secondary minerals, from the Ashburton deposits and the large variety of secondary minerals indicates a complex weathering history for these deposits with many changes in equilibrium conditions. The fine grain size probably indicates a rapid dropping of the water table, allowing insufficient time for large crystals to grow from solution. So the Ashburton deposits are the type locality for ashburtonite and gartrellite, and you can see them listed there as well.

So obviously some of these minerals are quite colourful. Here we have images of ashburtonite, the lovely blue. Metazeunerite – that’s a copper uranium arsenate, the yellow on the right there, tsumcorite on the bottom left, and libethenite is the copper phosphate on the bottom right.

This is another group of cornwallite crystals and this time from Bali East. They’ve got that very interesting sort of stacked crystal morphology.

Again, cornwallite from Bali East in the Ashburton deposits.

I’ll now move along and look at some nickel deposits, well in particular the 132 North nickel mine near Widgiemooltha, but I’ve just listed there some of the other nickel mines in Western Australia that have produced a lot of rare and unusual nickel minerals.

So this map shows the location of the 132 North mine near Widgiemooltha.

So the 132 North mine has produced the greatest variety of secondary nickel minerals found anywhere in the world, some being the best examples of their species. It’s the type locality for widgiemoolthalite and hosts second known occurrences for kambaldaite and nullaginite. So about 20 secondary nickel mineral species have been discovered there and some 11 of the 16 new minerals described from the oxidized zone in Western Australia come from nickel deposits.

The 132 North deposit, the primary ore consists of a small komatiite-hosted nickel sulphide deposit that’s around 2,700 million years old. The primary sulphides are pyrrhotite, pentlandite, chalcopyrite and pyrite and the supergene profile or the weathering profile at the deposit is the product of weathering from at least the Permian to the present. 132 North is also the type locality of gillardite and more recently another newly-discovered copper nickel chloride.

Here we have some examples of kambaldaite from the 132 North deposit. So the little bright green kambaldaite crystals on the top left are about 0.8mm in length.

Here are some examples of the other nickel minerals from the deposit – annabergite at the top left, nullaginite, nepouite and widgiemoolthalite.

Here we have examples of pecoraite and gillardite on hydrohonessite.

This another photograph of gillardite. The crystals there are up to about 0.3 mm in length and gillardite was described in 2006. It’s actually named in honour of Professor Robert D. Gillard in recognition of his contributions to the field of inorganic chemistry. It’s not named after our Prime Minister.

Most recently another new mineral has been found by Professor Pete Williams and his colleagues in Sydney, amongst the copper-nickel chlorides from the 132 North mine specimens in the museum collection, and a description of this should be published soon.

So this is where we end our tour. Now because mining is a destructive process, if deposits such as these are economic, then material from the oxidized zone will find its way to the crusher and all traces of these mineralogical rainforests will be lost. That is unless some finds its way into collections like the museums where all sorts of startling discoveries can then be made.

Thank you.

This video recording was made possible with the support of Chevron Australia.