Miles had found the Black Star, Racecourse, and Rio Grande lodes, the central ore bodies of a field stacked with lead, zinc, silver, and copper. The mining engineer William Henry Corbould recognized the prize at once, comparing it to the legendary silver-lead field of Broken Hill, and in January 1924 floated Mount Isa Mines Limited in Sydney. But riches in the ground meant nothing without a way out. The Queensland Government hesitated to build a railway to what might be a short-lived mine, agreeing only after the company guaranteed any losses. The line from Cloncurry reached Mount Isa on 27 May 1929, finally linking the field to the coast. Even then the costs were staggering. By June 1933, MIM's debt to creditors around the world equaled fifteen percent of all the income tax paid in Australia the previous year.

Production crept along through the Depression and the war, hobbled by markets that vanished and stockpiles that could not be sold. Copper, which would become the operation's signature, did not flow in earnest until January 1953. From there the field exploded outward. Between 1952 and 1960 alone, proven reserves multiplied many times over, and Mount Isa transformed from a struggling venture into an industrial colossus. The most visible monument to that scale rose in 1978: a lead smelter stack soaring 270 meters into the desert sky, at the time the tallest free-standing structure in Australia. It is visible from every corner of the city and from forty kilometers out across the plains, a steel exclamation mark announcing the mine that made the place.

Mount Isa did more than dig metal out of the ground. As ore grades fell and costs climbed through the 1970s and beyond, its engineers turned the operation into a global laboratory. They developed the Isa Process for copper refining, now used under license at more than a hundred operations worldwide, replacing back-breaking manual labor with stainless-steel plates. With Australia's CSIRO they built the energy-efficient ISASMELT furnace; they pioneered the IsaMill for grinding ore impossibly fine, and commercialized the Jameson Cell for separating valuable minerals from waste. A remote outback mine, half a world from anywhere, quietly rewrote the textbooks of how metal is won from rock.

No story of this scale comes without cost. For decades, lead and the dust it rode on drifted over the township, and the health of children growing up in its shadow became a matter of public concern and litigation. The mine changed hands too, sold to Xstrata in 2003 for nearly three billion US dollars and folded into the Glencore empire by 2013. Then, in October 2023, came the announcement many had long feared: copper mining would cease, with low-grade ore no longer worth the digging. The underground copper mines closed in July 2025, ending more than seventy years of copper production and costing some five hundred direct jobs. A century after Miles posted his rocks to Cloncurry, the mine that built a city now faces the question every great mining town eventually must: what remains when the ore runs out.