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Transition of the Ernest Henry Mine from Open Pit to an Underground Sublevel Cave


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Author A T McGrath, A D Campbell and J B Tucker


Situated in north-west Queensland, Australia, the Ernest Henry Mine (EHM) has been producing copper and gold since 1996. Owned and operated by Glencore Xstrata, ore is currently sourced from a newly developed underground sublevel cave (SLC) mine below the old EHM open pit (the pit). Ore feed to the concentrator is supplemented by satellite open pits comprising the Mount Margaret operation.

The transition from open pit to a producing underground mine involved a complex extraction sequence for the first two production sublevels on RL1650 and RL1625. Underground mining during cave initiation was further complicated when south wall instability prevented completion of the final open pit shell. The transition mining phase included mass firing of an inclined crown pillar between the initial SLC void and the pit followed by a combination of transverse and longitudinal SLC mining.

The inclined crown pillar mass firing was the first significant milestone in the extraction of the transition ore block. Comprising approximately 890 000 t of in situ material; 70 000 m of drilling was charged with 351 t of emulsion and 6026 electronic detonators to initiate the single mass firing. The firing contained five direction changes over 19 seconds making it one of the largest and most complex underground blasts ever conducted in Australia.

Risk management was integrated into
every facet of the technical and operational planning and execution of the
transition mining phase, whilst maintaining daily mine construction and
production commitments. This paper provides a detailed overview of the
transition objectives and constraints, mining method selection, post mass blast
outcomes and the journey ahead.


McGrath, A T, Campbell, A D and Tucker, J B, 2014.
Transition of the Ernest Henry Mine from open pit to an underground sublevel
cave, in Proceedings 12th AusIMM Underground Operators’ Conference
, pp 297–308 (The Australasian Institute of Mining and Metallurgy: Melbourne).