Broken Hill: one of the world's largest lead-zinc-silver deposits

Geoscience Australia

The lead-zinc-silver ore deposit of Broken Hill, in far‑western New South Wales, is among the great ore deposits of the world because of its size, richness, and continuity.

The Broken Hill ore body formed as a result of volcanic activity about 1685 million years ago. Seawater heated by magmatism associated with this volcanic activity flowed up through the underlying rocks to the seafloor where it mixed with cold ocean water, forming clouds of black sulphide precipitates. These then settled on the seafloor to form layers of sediment very rich in zinc, lead and silver sulphide minerals. Erosion over the past 500 million years has exposed these minerals to weathering. Exposure to air and groundwater has triggered a series of chemical reactions generating a suite of new minerals such as smithsonite, azurite and malachite.

Discovery
The Broken Hill ore deposit was discovered by boundary rider, Charles Rasp in 1883. Charles Rasp worked on Mount Gipps Station and was attracted to a hill on the station after purchasing a prospectors guide while on holiday in Adelaide. He thought that the dark surface of the hill could be a sign of tin oxide.

Accompanied by two others, James Poole and David James, Rasp set out for the hill and examined and pegged out the blackest section. His claim became the first mining lease on Broken Hill.

The owner of Mount Gipps Station, George McCulloch, decided he would support the exploration, resulting in the beginnings of Broken Hill mining.

In 1885 rich silver ore was found. Knox’s shaft was sunk on this spot, and assays of 2000 to 3000 ounces of silver per ton (6-9%) were found, guaranteeing the future of Broken Hill. Ore was carted to South Australia by camel trains, wagons and pack mules. On 3 June, the shareholders met and registered the Broken Hill Proprietary Co., which has grown over the last 130 years to become BHP-Billiton, one of the largest global mining houses and one of Australia's largest companies. BHP left Broken Hill in the 1940s, but still maintains active mining in Australia, particularly in the Hamersley iron ore province in Western Australia.

The Broken Hill ore body
The deepest point of the Broken Hill ore body descends over 1.5 km underground. Broken Hill had a massive sulphide lode of over 200 million tonnes with 50 million tonnes of lead and zinc and 20 000 tonnes of silver. Broken Hill has been mined continuously for almost 120 years, however, there are now only two operators in this area, both utilising highly efficient bulk underground mechanised mining.

Geoscience Australia at Broken Hill
Geoscience Australia has worked in the Broken Hill region over many decades with the New South Wales and South Australian governments to better understand the mineral and water resources of the region.


In the 1990s Geoscience Australia together with the New South Wales and South Australian governments began a study to address the long term economic prospects of the city of Broken Hill and the surrounding area. The city was facing an uncertain future following predictions that the ore body would be exhausted within 10–12 years. This project sought to better understand the regional geology and mineral potential of the Broken Hill region in New South Wales and the Olary region in South Australia.

Broken Hill Managed Aquifer Recharge project
Following a prolonged drought, the Broken Hill Managed Aquifer Recharge project was undertaken from 2008-2012. The project was undertaken by the Australian Government to investigate groundwater‑related options to help secure Broken Hill’s water supply. The project investigated ways to reduce evaporation and improve water efficiency at the Menindee Lakes Storages, protect the local environment and heritage, and return up to 200 gigalitres to the Murray-Darling Basin.

You can access maps with water observations from: http://www.ga.gov.au/interactive-maps/#/theme/earthobservation/map/wofs

Minerals of Broken Hill

As you go through the following minerals, click on the photograph title to see them in more detail.

Raspite is a rare and valuable mineral which is found in abundance at Broken Hill. The mineral is named after German-Australian prospector Charles Rasp, the man who discovered the Broken Hill lode in 1883.

The Broken Hill raspite is one of the rarest species recorded. Raspite is actually a rare dimorph of the mineral stolzite. Raspite is more deeply coloured and elongated than stolzite. Heating raspite to 395°C leads to a transformation into stolzite.

Raspite is found in small amounts in the upper levels of the Proprietary Mine. Only a small number of specimens were actually collected when it was first discovered. It was recognised as a new form of ‘stolzite’ in 1895. In 1898, raspite was established as a new mineral, following an optical, crystallographic and chemical study conducted by Karl Hlawatsch, of the Natural History Museum, Vienna. The specimen was provided by Baron H Foulon-Norbeeck, who had collected it on a visit to Broken Hill and whom, after discussion with George Smith, proposed the name raspite.

Raspite at Broken Hill ranges in colour from pale yellow to reddish-orange and reddish brown. The crystals are lustrous and striped in appearance and are found in monoclinic prisms, with edges that are either blunt or chisel shaped. The fine-grained rock surrounding them is usually composed of siliceous ‘limonite’, or manganese-stained botryoidal goethite.

Smithsonite is a zinc carbonate mineral and was the second most abundant secondary carbonate found at Broken Hill after cerussite. Smithsonite has some of the most interesting and beautiful structures of all the Broken Hill minerals. The Broken Hill smithsonite varies in colour from nearly colourless to white, pale yellow, smoke grey, pale green, bluish green and olive green. A variety of crystal habits are seen at Broken Hill, including mammillary, botryoidal, stalactitic and columnar masses. Also seen are crusts of tiny smithsonite crystals, sharp pointed ‘dog tooth’ crystal groups (which often looked like tiny haystacks), hemispherical and teardrop shaped globules and, at times, ‘bowtie’ like crystal clusters. Single crystals ranged from 1 mm up to 2 cm long, and a single hemisphere may be up to 6 cm across.

The finest specimens of smithsonite at Broken Hill were found in the Proprietary Mine, with examples of the entire range of habits and colours. The most distinctive specimens were the teardrop structures. These crystals are white or pale grey, translucent, and pearly looking, up to 6 or 7 mm across. They resemble solidified droplets and are found coating coronadite stalactites.

The mineral that is now smithsonite was re-named in 1832 by François Sulpice Beudant in honour of James Smithson, British chemist, mineralogist and benefactor of the Smithsonian Institution. Smithson discovered that what was known as ‘calamine’ was actually three different minerals: hemimorphite, hydrozincite, and zinc carbonate (now smithsonite). Smithsonite has been and continues to be used as an important, although minor, ore of zinc.

Cerussite was one of the most common and widely distributed secondary minerals in the Broken Hill lode. It was most commonly found in reticulated arrangements. The term ‘reticulated’ means that the crystals are arranged in a network structure, giving the reticulated cerussite of Broken Hill some of the most spectacular crystal structures known.

Cerussite is a lead mineral which is often found in the oxidised parts of lead ore deposits. Its name stems from the Latin word cerussa, which means ‘white lead’, which it is often called in English. It has also been known as horn silver and lead spar. Cerussite’s bizarre twinning habits are one the reasons for its fame. Twinning is the intergrowth of two or more crystal grains so that each grain is a reflected image of its neighbour or is rotated with respect to it. Other grains added to the twin form crystals that often appear symmetrically joined, sometimes in a star-like or cross like shape. Cerussite has one of the highest densities for a transparent mineral; it is over six and half times as dense as water, whereas most rocks and minerals are only around three times the density of water! This density is due to its high lead content, which is also the reason for its brilliant sparkle.

Most cerussite crystals found at Broken Hill were opaque white, sometimes cream or grey. Crystals of other colours were also observed, both transparent and translucent. It was found in large crystal lined cavities, known as vugs, often in delicate structures of lattice works or tabular and prismatic crystals, some up to 30 cm long. The largest mass of cerussite crystals at Broken Hill was found in the Central Mine and measured many metres across.

Cerussite is an important lead ore and was once a main ingredient of lead paint and some cosmetics, before being discontinued because of the lead poisoning it caused. Its attractive appearance causes it to be a sought after collector’s item.

The Broken Hill lode is the richest source of rhodonite crystals in the world and some magnificent specimens have been preserved from it. Rhodonite is a manganese silicate and is known for its beautiful pink and red colour. When rhodonite is red, its colour can be so deep that it rivals that of ruby. An uncommon mineral, its name is derived from the term rhodos, Greek for ‘rose coloured’.

Broken Hill rhodonite was recognised around 1889–90, its general occurrence was first outlined in 1900 and the first descriptions of fine crystals of rhodonite were made in 1908.

Broken Hill rhodonite ranges in size from microscopic inclusions in sulphides and other silicates that surround the primary ore, to angular crystals and aggregates up to a metre across. The best specimens found at Broken Hill were found in the North Mine. The rhodonite crystals that are considered the finest ever found at Broken Hill formed later than those found in the primary ore. They were discovered in 1966 in two cavities in the Zinc Corporation Mine. The first cavity contained perfect transparent, rich red colour rhodonite crystals, several centimetres long, in a mass of partially altered feldspar. The second cavity was much smaller. It had bigger crystals, up to 6 cm long, but they were less gem-like.

Rhodonite was once used as an ore of manganese in India. Because of its pleasing appearance, it is used to make beads and other ornamental objects. Rhodonite crystals that are well formed and transparent are highly sought after by mineral collectors.

One of the most beautiful minerals found at Broken Hill is azurite, a well-known mineral named for its colour. The word ‘azure’ is derived from the Arabic word for blue. The azurite crystals found at Broken Hill are medium to dark blue in colour and are up to 15 cm in length.

The most notable azurite deposit of Broken Hill spanned between the Proprietary and Block 14 mines. Azurite is unstable in open air, so is often pseudomorphically replaced by malachite. The weathering process involves the replacement of some of the carbon dioxide (CO2) units with water (H2O).

The most striking occurrences of malachite at Broken Hill are the pseudomorphs from azurite crystals. These were usually partial replacements with contrasting deep blue and green. Azurite’s blue colour is caused by the presence of copper, which is a strong colouring agent. At times, the colour is so deep that larger crystals can appear black.

In the past, azurite was crushed and used as a blue pigment for dyes and paints. This pigment was used extensively during the Middle Ages and Renaissance, but there are records that suggest that it was used even earlier, during the fourth dynasty in Egypt (around 2575 BC).

However, due to the continuous oxidation process of azurite, skies that were once a beautiful blue colour in old paintings coloured using azurite based paint, have over the centuries changed to a green colour! Azurite is also used in jewellery and other ornamental pieces.

Azurite can be used as an ore of copper. There are over 45 well-known forms of azurite, and over 100 forms that have been described. It is often associated with its close cousin, the green coloured malachite, but is also associated at Broken Hill with cuprite, Iodargyrite, embolite, cerussite, and less commonly with mimetite, smithsonite, native silver and anglesite.

Australia possesses the largest share of the world’s economic silver resources. Most silver is extracted from zinc and lead rich sulphide ores, but significant amounts are also mined from native silver and other non-sulphide silver minerals. About a quarter of Australia’s silver output is refined to silver metal and sent to Japan, and most of the remainder is exported in lead bullion to the United Kingdom, where it is extracted and refined.

Native silver is commonly found in the oxidised zones of Broken Hill. The silver at Broken Hill is found in a variety of forms, including films, plates, wires, curled ‘ram horn’ shapes and arborescent (tree-like) masses. Silver has been found in most of the mines at Broken Hill.

The relative scarcity of silver, along with its attractive appearance and ability to be easily shaped and moulded has caused it to be used in jewellery, ornaments and silverware since before ancient Roman times. Throughout history, silver has been used in currency to make coins, but in recent years this has become less common. In 1966, a 50 cent piece was the last coin in general use to contain silver in Australia. In more recent times, the main use of silver was in photographic paper and film, followed by the electronics, jewellery and tableware industries. But with the advent of digital cameras, the use of silver in photography has declined, replaced by a newly discovered use as in medicine as an anti-bacterial agent. Other uses include brazing alloys, solder, mirrors, medicines, tooth fillings, medallions and in scientific equipment.

Goethite is abundant in the upper levels of the Broken Hill lode, in the oxidised zone. Goethite was named in 1806 by Johann George Lenz in honour of the German poet, novelist, playwright, philosopher, politician and geoscientist Johann Wolfgang von Goethe.

The name ‘goethite’ was originally applied to lepidocrocite, a more uncommon mineral that has the same chemical composition as goethite, but a different crystal structure. In goethite, the oxygen and hydroxyl (oxygen and hydrogen) atoms are in hexagonal arrangements, while in lepidocrocite they are in cubic arrangements.

The goethite at Broken Hill is found in three different shapes; botryoidal (an arrangement that resembles a cluster of grapes), mammillary (having several smoothly rounded convex surfaces) and stalactitic. The arrangements are concretionary masses, meaning that they are held together by a concrete like substance and are either smooth or glossy, or pale brown and velvet-like. Goethite is distinguished by its dark brown colour, fibrous appearance and its yellow brown streak (powder colour). Stalactites of goethite up to 30 cm long were recovered from the old open cut mine at Broken Hill.

Goethite is a widespread iron oxide mineral and is the most common ingredient in rust. It is normally formed under oxidising conditions, as a weathering product of iron minerals, such as pyrite and magnetite. Goethite is an important component of high-grade iron ore and has industry uses in this area. Consumer uses include the making of construction materials, floor coverings, paints and coatings, plastic and rubber products and water treatment products. It is also the source for the pigment known as yellow ochre.

Manganoan calcite, also known as manganocalcite, is a variety of calcite rich in manganese. Its name stems from the Greek chalx, meaning ‘lime’ or any other white, calcareous mineral. The term ‘manganoan’ indicates the presence of the accessory element manganese. Manganese is a powerful chromophoric (colour causing) agent and gives manganoan calcite its pink colour.

The first description of a manganese-bearing calcite from Broken Hill was written in 1926. A large amount of the calcite found at Broken Hill contains at least trace amounts of manganese and some of the veins found in the deeper levels of the Zinc Corporation and New Broken Hill Consolidated Mines are composed of calcite rich enough in manganese to be considered manganoan calcite.

The manganoan calcite found at Broken Hill varies in colour from pale to bright pink, sometimes making it difficult to distinguish it from rhodochrosite and kutnohorite in a hand specimen. It is found in a variety of crystal arrangements, including botryoidal and mammillary masses, twisted ropy forms, stalactitic, sheaf-like structures, as well as pale pink translucent ‘dog tooth’ forms. The most beautiful variety is the clusters of crystals which look like bunches of pink tulips or cauliflowers; they range in size from 2 cm up to 18 cm.

An interesting feature of manganoan calcite is its deep pink to pinkish orange fluorescence under UV light. The easiest way to identify calcite in the field is to drop a small amount of hydrochloric acid on it. If its calcite, the rock will effervesce (bubble) upon contact with the acid. The bubbles released are carbon dioxide gas, produced by the reaction between the calcite and the acid.

Pyrite is found sporadically throughout the main ore body of Broken Hill. It is more abundant in cavities within fractures and fault zones at the deeper levels of the Zinc Corporation and New Broken Hill Consolidated Mines. The crystals of pyrite are usually cube shaped, with an average edge length of 1 mm and are frequently found speckled across white calcite and pink manganoan calcite. Pyrite can form as a cluster of crystals or as thin coatings of tiny crystals on other rocks; these coatings are brassy looking and shimmery in appearance.

In the old mining days, pyrite was sometimes mistaken for gold, earning its nickname ‘fool’s gold’. Pyrite and gold frequently occur together and the metallic lustre and pale brass yellow colour of pyrite means it resembles gold. However, it is easy to tell the difference between the two with some simple tests; pyrite is much lighter in weight than gold, but also much harder. It is not possible to scratch pyrite with a fingernail or pocket knife, unlike gold. Pyrite has a black streak (powder colour), while gold has a yellow one.

Pyrite consists of both iron and sulphur, but does not serve as an important ore of either of these elements. Pyrite was once important in the production of sulphur and sulphuric acid, but now most sulphur is obtained as a by-product of natural gas and crude oil processing. The most important use of pyrite is as an ore of gold, as they form under the same conditions, so in some rocks small amounts of gold are found in pyrite. Pyrite is also occasionally used as a gemstone in jewellery; this was popular in the United States and Europe in the mid-to-late 1800s.

The name Pyrite comes from the Greek ‘pyr’ meaning fire. It earned this name because when iron is struck with a lump of pyrite, a spark is produced. Pyrite has been used as a spark producing material in flintlock firearms.

Credits: Story

The National Mineral and Commonwealth Paleontological Collection, Geoscience Australia and The National Museum of Australia Mineral Collections (specimens).

Genevieve Crutchley and Kate Lehane (writing and editing).

Peter Butler and Dave Huston (scientific review).

Steve Keough (photography).

Marie Lake (graphic design).

Copyright for content: http://www.ga.gov.au/copyright

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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