The Mount Isa zinc, lead and silver mine located in Queensland, Australia is one of the biggest zinc mining operations in the world. Owned and operated by Glencore through its subsidiary Mount Isa Mines (MIM), it comprises two underground mines namely, George Fisher and Lady Loretta.

George Fisher and the Lady Loretta mines respectively produced 2.9 million tonnes (Mt) and 1.5Mt of ore in 2019. The total concentrates production from the Mount Isa operations during the year included 326,373 tonnes (t) of zinc, 157,984t of lead, and approximately 5.5 million ounces (Moz) of silver.

The Mount Isa multi-metal ore body that also hosts significant copper resources were discovered by John Campbell in 1923 which was followed by the establishment of Mount Isa Mines in January 1924.

The George Fisher mine, previously known as the Hilton mine, began operations in 1990, while the Lady Loretta mine began commercial operations in 2013 and was placed on care and maintenance in 2015 until it was restarted in March 2018.

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The Mount Isa mines were previously owned and operated by Xstrata, which merged with Glencore in 2013.

Location, geology, and mineralisation

The George Fisher and the Lady Loretta underground mines are located approximately 20km and 110km north of the city of Mount Isa respectively in Queensland, Australia. The George Fisher mine is located on mining lease ML8058 which expires in November 2036 while the Lady Loretta is located on mining lease ML5568 that expires in January 2026.

The George Fisher deposit is part of the Mount Isa Inlier and lies within the Leichhardt River Fault Trough of the Western Fold Belt. The Lady Loretta deposit lies within the Proterozoic Paradise Graben, a north-trending formation encompassing the folded and faulted anticlines.

The George Fisher North deposit comprises 11 mineralised stratigraphic intervals while the South deposit has up to 10. The mineralisation occurs as stacked stratabound lenses, hosted by the pyritic siltstone, as well as the interbedded pyritic siltstone including the banded mudstone units of the Urquhart Shale.

The mineralisation at Lady Loretta occurs over a strike length of 300m and it is composed of finely banded sphalerite, pyrite, and galena. The ore zone is underlain by a thick massive to banded pyrite-black shale unit and sideritic sediments.

Zinc, lead, and silver reserves at Mount Isa

The Mount Isa mines were estimated to hold a total of 79Mt of proven and probable reserves grading 7.4% zinc, 3.6% lead, and 66g/t silver as of December 2019.

It comprised 56Mt of proven and probable mineral reserves at the George Fisher North deposit and 17.5Mt at the George Fisher South deposit, while the proven and probable mineral reserves at Lady Loretta were estimated to be 5.5Mt grading 13.4% zinc, 4.9% lead, and 83g/t silver.

Mining and mineral processing

The Mount Isa zinc mining operation comprises the George Fisher and Lady Loretta underground mines with annual ore production capacities of 4.6Mt and 1.6Mt respectively.

The run-of-the-mine (ROM) ore undergoes crushing in the mines before further processing at the mill. At the processing mill, the ore undergoes grinding in a series of ball mills, and the finely ground ore is mixed with water before passing through a flotation circuit for the separation of minerals from waste rock.

Air is pumped through the flotation slurry to create bubbles that bind to the zinc and lead particles and produce a mineral loaded froth. The froth material is to obtain the zinc and lead concentrates.

The zinc concentrates are directly sold domestically or exported overseas, while lead undergoes further processing.

A combination of high-grade silica, limestone, and copper slag is mixed with the lead concentrate and fed into the sinter plant where it is heated by a blast furnace to 1,450 degrees Celsius to produce lead bullion.

The lead bullion is transported by rail to Townsville from where it is shipped to Britannia Refined Metals in England for further refining where silver is extracted as a by-product.

Contractors involved

Redpath Australia was engaged by Glencore as the operator of the Lady Loretta mine in December 2017. The contract encompasses the entire underground and surface operations and associated facilities management.

Byrnecut was contracted for cable bolt installation services at the George Fisher mine from August 2012 through December 2014.

Modular Mining was contracted to implement their DISPATCH Underground 2015 Fleet Management System (FMS) at the George Fisher mine in June 2016.