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History

Silver-lead ore was discovered at Mount Isa in 1923, and lead mining began in 1924, but due to the major problems of isolation, mine flooding and escalating costs, production mining was not achieved until 1931.

Even though the existence of copper ore in quantity was confirmed in the 1930's, it was not mined until a wartime scarcity of copper brought a request from the Australian Government. In 1943, the mine ceased silver-lead-zinc production and switched to copper to help the war effort. In 1946, copper mining stopped, and the mine reverted to silver-lead-zinc. Parallel production of silver-lead-zinc and copper ores commenced in 1953.

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How it All Began

In February 1923, a lone prospector, John Campbell Miles, discovered silver-lead carbonate ore in a low range, about one kilometre across the valley from the sandy bed of the Leichhardt River, in north-west Queensland.

He named his prospecting leases Mount Isa, a corruption of Mount Ida, the West Australian goldfield, which was the subject of incessant talk by Moses Rowlands when he accompanied Miles on a 2,400 kilometre bicycle ride across the Australian outback, in 1908.

Miles discovered Mount Isa while on his way to the Northern Territory in search of gold.

In his first efforts to mine the ore in bulk, he was assisted by four former cattlemen working a near-by copper show, Bill Simpson, Con Davidson, Jim Mullavey and Jim Roberts.

The five men could have divided the entire field between them, if they had the money to pay for the leases. Miles and Simpson pegged 40 acres (16ha) of mining leases that included the Mount Isa, Racecourse and Black Star leases. Davidson, Mullavey and Roberts were so poor that they were restricted to the five acre (2ha) lease of the Rio Grande.

The news of their venture soon spread, attracting men from copper mines and cattle stations to peg out leases, at 10 shillings ($1) an acre (0.4ha), and to gouge the richest surface lead from countless narrow veins.

In shelters of boughs and hessian, beaten out petrol tins, drums and packing cases, they set up home. Soon the valley and spinifex-covered hills echoed to the sound of picks and shovels as they toiled to fill bags with ore for loading onto camel and horse wagons and lorries. They eased their thirst with water bought for sixpence a gallon (about 1c a litre).

Despite the expectations of the miners, Australian business was so pessimistic, few promoters bothered to visit the new field.

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Developing the Field

A Cloncurry publican, storekeeper and stock and station agent, Douglas MacGillivray, became interested in developing the field. He secured options on the most promising leases, but neither he nor his friends had the money to buy them.

The only answer was to float a company that could pay for the leases and develop the field. At this stage, MacGillivray urged William H. Corbould, an engineer who was involved in the development of many new mining ventures, to inspect Mount Isa.

Corbould arrived in late 1923 and wasted little time in urging MacGillivray to secure options over more leases. Shortly after, Corbould left for Sydney to try to raise capital for floating a company. In early January 1924, with $100,000 in cash, Mount Isa Mines Limited was launched.

With the floating of Mount Isa Mines Limited, Miles and the other pioneers, Simpson, Davidson, Mullavey and Roberts, sold their leases and drifted from the field.

Two other companies were floated on the field, but soon sold out to Corbould. As the shafts went down, it became increasingly obvious the ore was poor.

There were a few who persisted in comparing the field to Broken Hill. Both contained silver-lead, but there the likeness ceased. Mount Isa was twice as far from a deepwater port, and its ore was only half as rich as that at Broken Hill.

To attract men to the field, a town with amenities had to be built. There were problems too, in treating the ore. Indeed, if all Mount Isa's disadvantages had been appreciated during those early years of 1925-26, the field would almost certainly have been abandoned.

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A True Believer

Only Corbould believed ardently in the mine which, now, so many began to doubt. More finance was desperately needed, and Corbould set out to hawk his mine overseas. After a number of disappointments, in London in 1927, he succeeded in interesting Leslie John Urquhart, an exuberant, international mining financier, who flung his company's dwindling assets into Mount Isa.

Urquhart, chairman of Russo-Asiatic Consolidated Limited, saw in Mount Isa, the opportunity to recoup the losses incurred by the confiscation of his Russian mining empire to the Bolsheviks, during the 1917 revolution.

Urquhart's enthusiastic support of the Mount Isa mine brought an injection of capital that ensured its survival.

Almost overnight, the town grew. Many of its buildings were carted from the deserted copper towns of Kuridala and Selwyn.

A 2,000 tonnes/day capacity mill, to treat the carbonate ore, rose from the dusty plains. Steel girders began to change the skyline as construction of shaft headframes, the smelter, mill and powerhouse, became more advanced.

In April 1929, the first train pulled into Mount Isa and the construction of Rifle Creek Dam was completed.

Early in 1929, Urquhart split Russo-Asiatic into two companies - Russo-Asiatic, which retained his Russian claims and interests, and Mining Trust Limited. Through this move, Mining Trust Limited acquired two-thirds of the shares in Mount Isa Mines Limited, without reducing its cash reserve.

Urquhart visited the mine and vowed it was larger and richer than any of his Russian mines and, in his enthusiasm, began to commit his company beyond its resources. Mount Isa was greedy for money and Urquhart spent his original budget three times over.

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Injection of Capital

By 1930, the plant was still not completed and new problems arose. The world Depression was beginning, the coffers of Urquhart's company were bare and the mine was still crying out for money. In London, Urquhart desperately sought new capital.

In mid-1930 he called on Senator Simon Guggenheim, president of the American Smelting and Refining Company (now ASARCO), the world's largest smelter and refiner of metals. Urquhart persuaded Senator Guggenheim to provide half a million dollars, sufficient finance, he believed, to complete the treatment plant.

The mine soon swallowed much of the money. Twice during the year, the man and supply shaft was flooded, and the men had to abandon work until stronger engines were installed to drive the pumps.

Once again more finance was needed before operations could begin. It was becoming obvious to the American company that the mine was a burden more than a prize and, to save the money it had already invested six months before, it lent another $1.2 million.

With large sums of money going into the mine at Mount Isa, ASARCO agreed to accept technical control of the operation. Consequently, in November 1930, Julius Kruttschnitt left the United States to take up his new appointment as General Manager of Mount Isa Mines Limited.

With large sums of money going into the mine at Mount Isa, ASARCO agreed to accept technical control of the operation. Consequently, in November 1930, Julius Kruttschnitt left the United States to take up his new appointment as General Manager of Mount Isa Mines Limited.

Shortly before Kruttschnitt arrived in Australia, William McRae, chairman of the Sydney Board (the company was re-formed in Brisbane in 1933), reported to the sixth annual meeting of shareholders, on the location of copper deposits at Mount Isa.

It was probably not realised just how valuable the deposits would ultimately prove to be to the mine, but at the time - 1930 - the management was concerned with bringing the mine into operation as a lead producer. At the time of Kruttschnitt's arrival, everything was being geared to lead and it would be more than 12 years before copper production became a reality.

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Further Challenges

On his arrival in Mount Isa, Kruttschnitt faced a number of technical challenges, the major one being the difficulty in smelting the ore.

In the mine, much more water was being encountered than had been anticipated. Kruttschnitt also found there was a shortage of accommodation, which was generally of poor quality. There was also a shortage of fresh food such as vegetables, milk and fruit.

For the employees at work, there were times of discouragement, particularly when things that should have been done, could not be done because of the shortage of money.

Delays in bringing the mine into production necessitated the raising of more money and, early in 1931, the company's authorised capital was raised to $7.3 million with the issue of second debentures of $1.3 million. ASARCO came to the rescue with a major portion, $1,150,000, since the Mining Trust Limited in London had no further resources due to the generally depressed state of the London money market.

In May of 1931, despite all the problems which had been faced, milling operations started at the new mine, and, within a few months, the plant was producing 3,350 tonnes of crude lead each month.

When Julius Kruttschnitt had visited London on the way out to Mount Isa, the price of lead was $36 a tonne. Within a year of the new plant coming into operation, the price was down to $19. The company was not always able to meet interest payments and, with continuing capital investments required for plant improvements and extensions, its debts grew to almost $6 million by the middle of 1933. During this period it consistently sustained losses.

Yet significantly, Mount Isa Mines Limited had weathered the Great Depression, providing work for about 1,000 men at a time when one in four of all male adults in the country was unemployed.

Australian Government policy of encouraging overseas investment had paid off. Without English and American funds, the new mine could not have been brought into production, let alone survive the troubled days of the early 1930's.

With an improvement in the price of lead, 1934-35 saw the company's loss reduced to $16,390, after paying over $458,000 in interest.

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First Operating Profit

The year 1937 was a momentous one in the history of Mount Isa Mines Limited - for the first time, the company operated at a profit.

The balance sheet for the year showed that after paying all interest and writing off certain items of abandoned and obsolete plant, there was an operating profit of $140,618. It was also the year in which Kruttschnitt, a director since 1931, became Chairman of the company.

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Change to Copper Production

During World War 2, the Australian Government's 1943 request led to a complete production change from lead to copper.

Milling of the copper ore was achieved through the existing mills and flotation cells with only minor changes, but for the smelting, it was necessary to provide entirely different equipment from that required for lead.

Company metallurgists scoured the smelters of the old Cloncurry field, most of which had now been abandoned for 20 years. From Mount Elliot they brought an electric crane for the smelter aisle; from Mount Cuthbert and Kuridala came converter vessels. Through Cloncurry, almost daily, passed train loads of second hand steel destined to be part of the "new" copper smelter at Mount Isa.

Sintering of copper concentrates started in the smelter on April 9, 1943. Three days later the furnace was blown in and the initial pour of blister copper was made on the same day. In the first full year of production - July 1, 1943 to June 30, 1944 - the "new" converters of Mount Isa produced 10,865 tonnes of blister copper.

As the war continued, the labour position became increasingly acute with the total payroll falling from 1,200 in 1940 to 750 in the early days of 1945. Production of blister copper increased to a peak of 12,436 tonnes in 1945, largely through improved efficiency and, in some areas, through higher grade.

The company ceased copper production and returned to lead in 1946. The soaring post-war demand for lead meant a corresponding leap in prices. But, as announced more than a year earlier, preparations were being pushed ahead for the parallel production of both ores, which began in January 1953.

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First Shareholder Dividend

A milestone in the development of Mount Isa Mines Limited was reached on July 17, 1947, when a dividend of 20 cents per $2 share was paid, the first in the history of the company which, by then, had been in existence for 23 years. An economist, who much later made a study of the affairs of Mount Isa Mines, commented: "The shareholders had been very patient indeed."

The growth of Mount Isa Mines Limited, from a single company to a group with diversified interests, began in 1951. Buying out the Mining Trust, the company also acquired the Britannia lead refinery in London, which had been built in the early thirties to refine the Mount Isa lead.

In the same year, a majority interest was acquired in a coal mining operation near Collinsville, south of Townsville. This ensured continuity of coal supplies for the company-owned power station at Mount Isa.

Copper Refineries Pty Ltd was established at Stuart, in Townsville, in 1959. Four years later, Mount Isa Mines also moved into stevedoring and transport to service company shipments moving through the port of Townsville.

A major change in company structure came in 1970 with the establishment of M.I.M. Holdings Limited, incorporated in Queensland, which was to take over all the issued capital of Mount Isa Mines Limited and certain assets outside Mount Isa.

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History of the Hilton Mine

The Hilton orebodies, located 20 kilometres north of Mount Isa, were discovered in 1947. The mine was named in honour of Charlie Hilton, the general manager of Isa Mine at the time of the discovery.

Following several periods of intense exploration, the decision was made to proceed with development of the zinc-lead-silver deposit in May 1969.

Between 1970 and 1973, a 4.3 metre diameter exploration shaft (J53) was built, giving access to orebodies, provided geological and rock mechanics information, bulk samples for metallurgical testing and access for diamond drilling. From 1971 to 1975 an 8 metre diameter service and hoisting shaft (P49), was sunk to a depth of 1,040 metres.

Development was suspended for several years as mining focussed on Mount Isa. It was resumed in the 1980's and Hilton was officially opened in 1990.

Hilton North, renamed George Fisher, is now being developed using much of the existing Hilton infrastructure.

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Geology

Silver-lead-zinc and copper from the Mount Isa orebodies are produced from four mines - the X41 copper mine, the lead mine, the Enterprise copper mine at Mount Isa, and the George Fisher zinc-lead-silver mine located 22 kilometres to the north of Mount Isa. The zinc-lead-silver and copper orebodies occur adjacent to each other but are mined as separate entities.

The zinc-lead-silver orebodies extend from the surface to about one kilometre below the surface. Individual orebodies range in widths up to 35 metres and may persist for hundreds of metres in length.

The large 1100 copper orebody extends from 400 metres to 1000 metres below the surface. It is three kilometres in length and is one of the largest and richest copper orebodies in the world.

The newer Enterprise mine has copper in two orebodies (the 3000 Orebody and 3500 Orebody) which occur 1200 metres to 1800 metres below the surface. It is economical to mine these orebodies even though they are very deep because they contain very high-grade copper.

The original discovery in 1923 by John Campbell Miles was of high-grade lead carbonate ore at the surface. It was not until 1927 that the rich copper orebodies below the surface were discovered. The Hilton orebodies were discovered in 1947.

All of the orebodies occur within the Urquhart Shale Formation, which consists of a sequence of alternating pyrite-rich dolomitic siltstone and shale beds up to 1000 metres thick.

These sediments were deposited in a shallow sea or lake about 1600 million years ago. Most geologists believe that the zinc-lead-silver orebodies were formed by hot metal-rich water that circulated through the sediments whilst they were being compacted into rock.

They formed as a set of horizontal, parallel sulphide-rich bands comprised of galena (lead), sphalerite (zinc) and pyrite (iron).

About 1500 million years ago, tremendous mountain building movements in the earth's crust folded and tilted the Urquhart Shale into its present attitude. It is believed that the copper orebodies were formed during this time.

The main copper mineral is chalcopyrite. Closer to the surface, weathering and ground water leaching has changed the chalcopyrite to secondary copper minerals such as chalcocite. This weathering has also affected some of the near-surface lead orebodies, forming cerussite, a secondary lead carbonate mineral.

At Hilton and George Fisher, the orebodies are very similar to the Mount Isa zinc-lead-silver orebodies. They are narrower and disrupted by more folding and faulting. There is a small amount of copper as well, but at this time no major copper orebodies have been discovered below Hilton or George Fisher.

Refer to Ore Reserves & Mineral Resources for more detailed information about the Mount Isa orebodies.

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Mining processes

The zinc-lead-silver orebodies and copper orebodies of Mount Isa are mined and treated separately. The position, extent and metal content of the orebodies have been established by exploration drilling from the surface and underground. Explosives are used to break the hard ores before they are hoisted to the surface.

The two most common mining methods at Mount Isa are open stoping (employed in wide orebodies) and bench stoping.

Open stoping is used to win the copper ore and, now, much of the silver-lead-zinc ore.

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Bench stoping

Before bench stoping was introduced, the cut and fill mining method was used to extract the high grade zinc-lead-silver orebodies. It is a very selective method, but also expensive, so to reduce costs and improve productivity, the benching technique was developed.

In the cut and fill method, a horizontal slice of ore, up to four metres high, is extracted along the length of the orebody. Horizontal tunnels, called sill drives, are driven the length of the orebody at regular intervals rising vertically through the orebody.

The space between these sill drives varies depending on local ground conditions. However, typical spacing is 15 metres. Benching employs blast holes drilled vertically down from one sill drive to the lower drive.

Starting at one end of the bench, a row of holes are filled with explosives and blasted to remove the rock between the two drives. The broken ore drops to the ore pass at the bottom of the orebody and is removed by a load-haul-dump (LHD) unit to the ore pass.

To remove the ore, it is necessary for the LHD to go inside the stope. To ensure operator safety, at times a remote control device is used. The operator remains in a secured area and manoeuvres the LHD via a control panel, while it is in the stope.

Remote LHD's are limited by the line of sight and tele-remote units have been developed using television cameras mounted at both ends of the LHD. Sitting in front of two television screens, the tele-remote operator does not have to leave the comfort of the air-conditioned cabin, often over 100 metres away, to load, haul and dump the ore.

As benching progresses, fill is often progressively introduced to the cavity to add stability to the hanging wall. The fill could be underground development mullock, heavy media reject or hydraulic sand fill from the surface.

Benching has increased productivity, improved safety, reduced costs and provides better utilisation of equipment.

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Mining processes - Copper

Open stoping

Sub Level Open Stoping (SLOS) is the term used to describe the mining method in the copper mine.

What is an open stope? Imagine a mass of copper ore 40 metres by 40 metres and 200 metres high. This represents only a part of the orebody - something like a single block in a structure made from children's building blocks. The object is to remove that single block of copper ore and, ultimately, all the blocks.

Drilling sub-levels (tunnels 4.2 metres by 4.3 metres) are developed at 40 metre intervals. At the bottom of the stope a number of drawpoints are mined to extract the ore.

Blasthole drilling is carried out using mechanised drill rigs. On the extraction level, upholes in a "V-shape" are used to shape the trough. On the drilling sub-level, fans of radial holes are drilled. A slice of ore the height of the stope is extracted first (cut-off slot). This means there is now an open area along one side of the stope into which ore is progressively blasted.

Ammonium Nitrate Fuel Oil (ANFO) is the main explosive used and it is not uncommon to blast 100,000 tonnes in one firing.

The broken ore falls to the bottom of the stope and is extracted at the drawpoints by diesel-powered load-haul-dump (LHD) units. These big loaders have a 6.1 cubic metre bucket capacity.

Ore is either tipped directly into passes leading to the crusher, or tipped into trucks - for stopes a long distance from the crusher - which run back to tip to the crusher.

The crushed ore is then conveyed, by a 1.6 kilometre cable belt, to the U62 hoisting system where 36 tonne skips are used to take the ore to the surface.

With a philosophy of 100% extraction, pillars also need to be recovered. The open stopes are filled with a cement based slurry and/or rock mixture. This allows adjacent ore blocks to be extracted without dilution from fill material.

The slurry is a mixture of Portland cement and tailings (waste from the concentrator), while the rock is sourced from surface stockpiles or heavy medium aggregate rejects from the lead concentrator or slag waste from the copper smelter. This mixture sets into a hard, rocklike formation providing a stable face for extraction of the adjacent ore pillar.

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Enterprise ore handling system

The new ore handling system for the Enterprise Mine consists of a jaw crusher, the V63 conveyor, the M62 internal shaft and the M62 conveyor.

These allow ore to be crushed and transported to MIM's existing U62 copper ore handling system.

The 84" x 60" jaw crusher crushes ore down to less than 400 mm at a rate of up to 1,000 tonnes per hour. The 378 metre long V63 conveyor transports this crushed ore to the M62 shaft.

Once at the shaft, more than 1.6 km below the surface, ore is hoisted 700 metres to 20 Level where it is tipped onto the M62 conveyor which delivers the ore to the existing U62 system via a short ore pass.

The internal M62 shaft is 5.3 metres in diameter and 713 metres long. Its base lies approximately 1,660 metres below the surface. The shaft's hoisting system is controlled from a surface control room and hoists skips at speeds of up to 16.8 metres per second.

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Production processes - Zinc-lead-silver

Zinc-lead-silver ore is supplied from two separate sources namely, the Mount Isa lead mine and the George Fisher Mine (incorporating the former Hilton mine).

Lead mine ore is hoisted via the R62 shaft system while the George Fisher ore is trucked from the former Hilton mine, 20 kilometres north of Mount Isa. All the ore is handled by the R62 ore system and is stored in six crude ore bins which feed a crushing plant. This plant reduces the average size of the ore from 200mm to 14mm.

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Heavy Medium Plant

Crushed ore is then conveyed to the Heavy Medium Plant where a separation is made on the basis of ore density. Around 30-35% of the ore is rejected as gangue, waste material, containing very little valuable mineral. Around 95-97% of the lead, zinc and silver metal is recovered and fed to the lead zinc concentrator. The pre-concentrated ore is held in bins which feed the concentrator.

The valuable minerals are liberated from the remaining gangue by grinding in rod mills and ball mills, by the action of the tumbling steel rods and balls. Finer liberation is carried out using specially developed IsaMills technology.

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Flotation

Separation of the valuable minerals from the gangue is done in the flotation section. Chemical reagents are added to the ore slurry. These reagents selectively coat the surface of the valuable minerals, allowing them to adhere to air bubbles produced at the bottom of the flotation cells.

The mineral-laden bubbles rise to the top of the cells and are recovered as a concentrate. The flotation section of the concentrator is used to obtain two products, lead concentrate at 50-55% lead and containing one kilogram of silver per tonne and zinc concentrate at 51% zinc.

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Concentrate

The zinc concentrate is thickened, filtered and dried. This is railed to Townsville for delivery to the Sun Metals zinc refinery in Townsville and for shipment to overseas customers.

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Lead Smelter Sinter Plant

The lead concentrate is sent to thickeners in the concentrator and before going to a filter plant in the lead smelter for the removal of water. From here it is fed into a mixer, where fluxes and return material are added and moisture is adjusted in readiness for sintering.

In the sinter plant, sulphur is burned off, producing a sinter containing lead oxide with the sulphur leaving as gaseous sulphur dioxide. The sinter produced in this way is mixed with coke and fed to a blast furnace, where the lead oxide is reduced to lead and a slag is formed containing other impurities.

The lead is then drossed to remove some minor impurities.

Following drossing, the crude lead is cast into four tonne blocks and railed to Townsville for shipment to MIM's lead-silver refinery in England where the lead is refined and silver is recovered and refined.

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Production processes - Copper

Concentrate

The mined copper ore from Mount Isa's X41 and Enterprise mines and as well as ore from Ernest Henry mines goes through the concentrating process, where waste is removed from the ore to produce concentrate containing copper, iron and sulphur.

The concentrate grade produced is between 25% and 33% copper.

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Smelting

The next phase of the process is smelting and it is during this stage that sulphur dioxide is formed.

During smelting, impurities such as sulphur and iron are removed from the copper concentrate to form anode copper.

The sulphur is removed as sulphur dioxide and sulphur trioxide gases, which are sent to the WMC Resources Acid Plant and also vented up the copper smelter stack. Up to 80% of the copper smelter sulphur dioxide gases can be captured by the WMC Resources Acid Plant. The iron is removed as slag which is then used as underground fill.

In the smelter there are three main stages - namely the copper ISASMELT furnace, which produces matte copper (58% copper), the four converters which produce blister copper (97% copper) and the anode furnace producing copper anode (99.7% copper).

The copper anodes are then railed to MIM's Townsville copper refinery for refining to cathode copper (99.999% copper).

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Environment

This year MIM has again provided information on emissions from all its operations to be included in the National Pollutant Inventory (NPI).

Because of the size and nature of Mount Isa's operations, MIM rated as a significant emitter of sulphur dioxide, lead, arsenic, cadmium, zinc and copper.