In 1947, MIM geologists discovered a lead-zinc-silver deposit 20km north of Mount Isa, named Hilton after the then Mount Isa mine manager. In 1981, similar mineralisation was located 2km further north. MIM first decided to develop Hilton in 1969 but market factors delayed start-up of the 1Mt/y mine and concentrator until 1990. George Fisher is now owned and operated by Xstrata plc, following its acquisition of MIM Holdings.
Studies in 1994–95 originated a plan with new and existing mining facilities but with all the ore processing at Mount Isa. The development of this expanded George Fisher mine, named after MIM’s chairman in the 1950s and 1960s, was approved in 1998 and production started in mid-2000. By the second-quarter of 2001, George Fisher had become the main ore supplier to the Isa concentrator. Capital expenditures included A$230m on mine development and A$40m on upgrading the No.2 concentrator and lead smelter at Mount Isa.
Despite geotechnical problems encountered at both George Fisher South and George Fisher North in the first quarter of 2007, production steadily increased to reach record production levels in the final quarter with total ore production of 2.8 million tons up by 8% compared to 2006. Following improvements to hoisting capacity made in the second half of 2007, production at George Fisher increased by 17% to 1.46 million tons of ore in the first half.
Geology and reserves
The ore at George Fisher lies in the central part of the Lower Proterozoic Urquhart Shale. However, the mineralised section is only one-third the thickness of the Mount Isa ore zone, extending from 300m to 1,000m depth for 6.5km. It is more folded and more interrupted by faults and dykes. Ore zones are from 3m to 25m thick. 11 orebodies have been assessed, the two to be mined containing 24Mt and providing a ten-year mine life at a production rate of 2.5Mt/y.
As of mid-2005, Xstrata stated the reserves as: George Fisher South – 10Mt grading 7.8% zinc, 5.6% lead and 130g/t silver. George Fisher North – 11.4Mt grading 8.8% zinc, 4.7% lead and 92g/t silver. Resources at the South orebody were 22Mt at 8.9% zinc, 6.5% lead and 150g/t silver, while those at George Fisher North stood at 14Mt at 10% zinc, 5.15% lead and 100g/t silver.
MIM started cut-and-fill mining at Hilton but later switched to bench mining as well as adding sub-level open stoping in three areas and remote control of load-haul-dump machines. The rail haulage was supplied with rolling stock from Mount Isa. The northern ore is accessed by a decline from the base of the Hilton ramp. Below this, a truck haulage runs to the Hilton hoisting shaft. A second ventilation shaft and a fill pass were raise-bored for the new mining area. The high-grade C and D orebodies are open-stoped. The mobile equipment fleet includes drilling, LHD and trucking equipment from competing suppliers. In 2002, the George Fisher mine added a new decline and further changed the mine design during 2004 to improve the mining sequence. A hoist upgrade deployed in 2006 saw a 17% increase in ore mined in the first half of 2007 to 1.46 million tons.
The KG53 decline at George Fisher mine was approved in the last quarter of 2007 and is two-thirds complete. The project is expected to commence production in the final quarter of 2008, producing 500,000 tons pe annum and supplementing production at George Fisher North underground mine. A new tailings filter plant and a paste-fill plant are under construction which will allow for increased backfill to match increased ore production. Both facilities are expected to be completed in the fourth quarter of 2008.
The Hilton concentrator was designed to recover three separate products for shipment to Mount Isa as a slurry. It incorporated techniques new to Mount Isa, including SAG milling, tower-mill regrinding and flotation with MIM-designed column cells and Jameson cells. The plant reached rated output shortly after commissioning but switched to treating copper converter slag from Mount Isa in the mid 1990s when the mining rate was cut and ore was trucked to the Mount Isa lead-zinc concentrator.
Long-run processing of 2.5Mt/y ore from George Fisher required modifications and general upgrading at the Mount Isa lead-zinc concentrator and lead smelter. Engineering contractors Bateman Brown & Root carried out this work. In particular, the flotation circuit was reconfigured to incorporate MIM’s IsaMill technology for very fine grinding and froth pumping. This yielded a near 10% performance improvement in recoveries to 78.8% Pb and 73.4% Zn, with the zinc recovery rising to 80.7% in FY2002. In the lead plant the sintering machine was rebuilt, the blast furnace crucible replaced and a new control system installed.
Planned full rate George Fisher mine output is 2.5Mt/y yielding 170,000t/y of zinc, 100,000t/y of lead and 5Moz of silver-in-concentrate. Ore production in FY2002 was 0.5Mt (6.2% Pb, 8.3% Zn and 150g/t Ag) at Hilton and 1.4Mt (5.6% Pb, 8.2% Zn and 140g/t Ag) at George Fisher. Xstrata Zinc now consolidates the ore production from its George Fisher and Mount Isa operations (see Mount Isa Lead profile).
Total production at George Fisher for 2007 increased 8% to a record 2.8 mt. These and other results helped minimise the damage of lost production following the fire in the zinc lead concentrator in September 2007.
Recent drilling, metallurgical testing and design work has confirmed an open pit reserve of 4.3 million tons in an area south of the George Fisher underground mine. Ore will be mined at a rate of up to 1.75 million tons per annum, trucked to the Mount Isa zinc-lead concentrator from mid 2008.