Technical investigations were conducted for extraction of a high-grade crown pillar in the northern 3500 orebody (N3500) at Glencore's Mount Isa Mines using cemented paste backfill (CPB) underhand stoping. Several stopes have now been successfully extracted, achieving the planned recoveries with minimal dilution. In addition to standard CPB lab testing, in situ testing was conducted in the area planned for underhand exposure. The objective was to determine the variance between actual and design CPB strengths, to ensure the strengths were suitable for the planned underhand exposure dimensions. The testing results indicated the majority of in situ CPB strengths were higher than the lab cured and design strengths, due to the arching of stresses within the fill mass and curing processes. The strength parameters obtained from the testwork were incorporated in numerical modelling assessments using FLAC3D. Model calibrations were conducted using historical vertical CPB exposures to ensure the adopted methodology and material parameters were suitable. This paper discusses the methodology and results of the technical investigations, and how the data fed into analyses to assist with safe and efficient extraction of the crown pillar.

The Black Rock orebody is located at Mount Isa Mines, in northwest Queensland, Australia. Owned and operated by Glencore, the mine has been in operation for almost 100 years. The orebody was last mined as an open cut from 1957 to 1965. It was closed due to two major wall failures and ongoing stability issues one bench short of completion, leaving behind a large quantity of high-grade ore. The remaining orebody is being extracted using the sublevel cave (SLC) mining method. The orebody is located in extensively altered and weak kaolinic shales, with zones of hot and reactive ground. Other challenges include naturally occurring and historic voids, and mining proximity to fixed surface infrastructure. The small SLC has a planned draw of approximately 1.5 million tonnes at 4.5% copper over four years, which commenced in September 2020. This paper discusses the work completed during the project phase and the key learnings during the initial execution phase. This includes geotechnical characterisation, caveability and subsidence assessments, fragmentation testing, cave flow and rock cutter trials, and development of a real-time instrumentation system to monitor surface infrastructure.