Mount Isa Mines Limited ("MIM") operates the Mount Isa copper, lead, zinc and silver mines near Mount Isa, Queensland, Australia as part of the Glencore group of companies. For a brief period in 1980, MIM was Australia's largest company. It has pioneered several significant mining industry innovations, including the Isa Process copper refining technology, the Isasmelt smelting technology, and the IsaMill fine grinding technology, and it also commercialized the Jameson Cell column flotation technology.
History In 1923 the orebody containing lead, zinc and silver was discovered by the miner John Campbell Miles.N Kirkman, Hilton Mine, (Mount Isa Mines Limited: Mount Isa, Queensland, 1990), 2–3. Prominent mining engineer William Henry Corbould was invited to the field by Douglas MacGilvray, who held options over several tenements, and immediately noted similarities between the Mount Isa orebodies and those of Broken Hill, New South Wales. Corbould secured an option over and in January 1924 floated Mount Isa Mines Ltd (MIM) in Sydney with himself as director and general manager.
MIM was one of three companies founded in 1924 to develop the minerals discovered by Miles, but production did not begin until May 1931. The other two companies were Mount Isa Silver Lead Proprietary and Mount Isa South. These were both acquired by MIM by late 1925. Corbould played a key role in consolidating the ground under MIM, investing his own funds and persuading the state government to extend the railway from Duchess. He resigned as general manager in 1927 but remained a major shareholder.
The lean years (1924–1945) The early years were characterized by the struggle to develop the lead–zinc ore bodies, including the need to finance drilling, metallurgical test work and shaft sinking, and there was significant doubt that Miles' discovery would ever amount to much.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960). However, by the end of 1928, the drilling had allowed an estimate of reserves of 21.2 million tons, which were at the time the largest in Australian history, and rose to an estimate of 32 million tons in 1930.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 131–132.
Crucial to the success of any mining venture was a rail line connecting the area to the coast. However, the Queensland Government was reluctant to invest in a railway to what might be a mine of limited life. After the mining company guaranteed to cover any losses, construction of the railway from Cloncurry to Mount Isa began in 1926, and the line opened on 27 May 1929, giving Mount Isa an essential link to Queensland's eastern seaboard.
The cost of developing the Mount Isa ore body was so high that the owners had to turn to ASARCO to obtain sufficient finance to bring the operation into production.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 139–148. The project was running behind schedule and over budget, which ultimately resulted in ASARCO sending its own man, Julius Kruttschnitt II, to take charge. Kruttschnitt arrived in 1930 to find that bills were going unpaid because there was no money to pay them, the shafts were flooding, and the construction of the surface plants was months behind schedule.
When mining commenced in 1931, the mine was mechanized to an extent not previously seen in Australia, with mechanized drilling and mechanical shovels rather than "hammer and tap" hand drilling and hand shovels.R J Lloyd, "Equipment - the Mount Isa experience," in: Design and Operation of Caving and Sublevel Stoping Mines, Ed. D R Stewart (Society of Mining Engineers, 1981), 653–660. The initial mine production was 660,000 tonnes per year ("t/y") of ore and stayed at about this level until 1953.
Even after the first ore had been mined and processed, the Mount Isa operations struggled. The smelter proved inadequate and required a third blast furnace and additional sintering machines.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 160–166. The recovery of valuable minerals in the concentrator was less than expected, and the metal prices were depressed by the Great Depression of the 1930s. The poor recoveries were found to be caused by the unusually (for the time) fine nature of the mineral grains in the Mount Isa ore. While the metal prices eventually recovered as the Depression passed, the fine mineral grains would never be resolved by the Mount Isa lead–zinc operations.N W Johnson, M Gao, M F Young and B Cronin, "Application of the ISAMILL (a horizontal stirred mill) to the Lead–Zinc Concentrator (Mount Isa Mines Limited) and the mining cycle," in: AusIMM '98 - The Mining Cycle: Proceedings of the Annual Conference, Mount Isa, Queensland, 19–23 April 1998 (Australasian Institute of Mining and Metallurgy: Melbourne, 1998), 291–297.
By June 1933, the debt owed by MIM to creditors around the world, £2.88 million, was equal to 15% of all income tax paid in Australia in 1932. It was not until the 1936–1937 financial year that MIM made its first profitG Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 179–181. and the company could begin to pay down its burden of debt. However, the outbreak of The Second World War was not kind to MIM, because it could no longer find markets for all its production, and the price of lead did not increase as it had during the First World War.
While some copper mineralization had been discovered during drilling in the late 1920s, the major find did not come until 1930, when drilling to explore the lead–zinc ore body passed through almost 38 meters of copper mineralization with an average grade of 4.3% copper.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 182–191. While this was a very good grade, MIM did not have the financial resources to develop the copper, and it was not until global copper prices increased in 1937 that there was an incentive for further copper exploration. These efforts were initially unsuccessful, but yielded fruit in 1940 and 1941. However, it was not until 1941–1942 that mining of the No. 7 level of the Black Star lead-zinc ore body allowed the existence of an economic copper deposit to be established.The smelting staff at Mount Isa, "Notes on copper smelting at Mount Isa, 1943–1946," Proceedings of the Australasian Institute of Mining and Metallurgy, No. 171, 1953, 43–54.
MIM was still not in a position to mine the copper, because it had stockpiles of lead bullion and zinc concentrate that could not be sold due to the war. However, the Australian government needed copper for its war effort and lent MIM £50,000 to allow the mining to proceed. Further drilling expanded the copper reserves and MIM decided to switch from lead to copper production. The lead–zinc concentrator could treat the copper ore with little modification, but the lead smelter required the addition of second-hand equipment lying idle at the Kuridala, Mount Cuthbert and Mount Elliott mines.
Lead smelting ceased on 9 April 1943 and sintering of copper concentrate commenced on the same day. While the copper had the potential to be more profitable, MIM's run of bad luck did not end then: the Australian Government's Department of Supply and Shipping decided that it no longer needed MIM's copper and recommended returning production to lead and zinc as from January 1944, without compensation for the expense of converting the operations to copper production. After much discussion between MIM and the Australian government, MIM was permitted to continue to produce copper until six months after the end of the Pacific War. The last copper was produced on 2 May 1946 and lead production resumed at a time of rising lead prices.
From survival to prosperity (1946–1973)
In 1947, MIM paid its first dividend, signaling an end to its early troubles, after 16 years of continuous production and 23 years after the company's formation.E M Bennett, "History, geology and planned expansion of the Mount Isa Mines properties," in: World Symposium on [sic] Mining and Metallurgy of Lead and Zinc, Volume 1, (American Institute of Mining, Metallurgical and Petroleum Engineers, 1970), 139–170.
That same year, exploration began north of the Mount Isa ore bodies, in an area that later became the Hilton mineG Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 203. and, following the discovery of an outcrop of rocks similar to the host rocks of the Mount Isa ore bodies, diamond drilling began in August 1948.B N Black and B K Mutton, "The development of the Hilton mine, 1947–1985," in: 13th Congress, The Council of Mining and Metallurgical Institutions, Singapore, 1986, 11–19. That first drill hole intersected a small amount of zinc mineralization. From then until 1957, a significant drilling program was undertaken and by 1950, the Hilton ore reserves stood at 26 million tonnes. The drilling program was curtailed in 1957 due to a fall in metal prices and heavy capital expenditure in the existing operations.
To ensure adequate supplies of coal for its power station, which supplied both MIM's operations and the city of Mount Isa, MIM bought a controlling interest in Bowen Consolidated Coals Mines Limited in 1951.
The profitable years following the war allowed MIM to repay its debts, including those to ASARCO. ASARCO used the money it received from MIM to buy shares, and owned 53% of MIM's shares by 1960.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 194. MIM was also able to construct a copper concentrator and copper smelter, and copper production resumed in January 1953.G Blainey, Mines in the Spinifex, (Angus and Robertson: Sydney, 1960), 199. Later that year, Kruttschnitt resigned as Chairman of MIM's Board of Directors.
With the recommencement of copper mining, total ore production doubled from the 660,000 tonnes per year level that had been maintained since production began in 1931. Both the copper and lead-zinc ores were treated in separate circuits in the same concentrator, which was later referred to at the "No. 1 concentrator".R A Eaton and K H Ehlers, "Lining of ball and rod mills at Mount Isa—installation, design and materials," in: The Aus.I.M.M. North West Queensland Branch, Mill Operators' Conference, June 1978 (The Australasian Institute of Mining and Metallurgy: Melbourne, 1978), 237–245.
Initial copper production used two multiple-hearth roasters, a single coal-fired reverberatory furnace and two to produce a design 1,500 tons of blister copper per month (18,000 tons per year).R V Anderson, "Brief outline of [sic] Mount Isa copper smelter," Proceedings of the Australasian Institute of Mining and Metallurgy, No. 171, 1953, 33–40. The copper smelter produced 15,000 tons of copper during 1953.J Pritchard, "Copper smelting at Mount Isa Mines Ltd, Mount Isa, Qld," in: Mining and Metallurgical Practices in Australasia - The Sir Maurice Mawby Memorial Volume, Ed. J T Woodcock (The Australasian Institute of Mining and Metallurgy: Melbourne, Victoria, 1980), 340–344.
Exploration activities between 1952 and 1960 expanded the Mount Isa ore reserves from 9.9 million tons of lead-zinc-silver ore to 25.6 million tons, and from 3.8 million tons of copper ore to 24.2 million tons.J W Foots, "Review of operations, Mount Isa Mines Limited, 1952–1960," Proceedings of the Australasian Institute of Mining and Metallurgy, No. 197, 1961, 1–15. As a consequence of the expansion of reserves, MIM decided to expand production. The quantity of copper produced rose from zero in 1952 to 60,000 tons in 1960, while the lead bullion output increased from about 36,860 tons in 1952 to 60,000 tons in 1959 and then reduced to 52,000 tons in 1960 as a consequence of a decision to reduce output in the face of a global oversupply of lead metal.