Organized by InfoMine and SRK Consulting

Open Pit To Underground Transition


Simon Hanrahan

First Presented:

SRK News - Issue 56
SRK carried out a scoping study on an existing open pit operation (12 Mtpa) that is soon to be reaching a depth where the mine will need to transition to a large-scale underground mining method. The goal was to provide production continuity at a rate that will still be suitable to maximise use of the existing surface processing facilities. The ore body dips below the bottom of the planned open pit and is still open at depth. Around the periphery of the main ore body is a halo of disseminated lens-shaped types of mineralisation.

Due to this mineralisation, the ore halo requires smaller scale or selective mining methods, while the main orebody lends itself to mass mining where block caving or inclined caving are viable options.

SRK conducted a series of mining method evaluations to identify what the most suitable mining method options would be to maximise recovery, while also maintaining production continuity as the mine transitions from surface to underground. This resulted in a sublevel open stoping (with backfill) mine design for the peripheral halo ore zone and two side-by-side block caves for the more massive ore area. Given the geometry of the ore body, an inclined cave could become more of a real option, but additional resource drilling is required and a firmer definition of the final open-pit bottom.

A critical aspect that is often underestimated is the transition time frame, particularly when additional ore body knowledge (including resource, geotechnical, hydrogeology, metallurgy and rock temperature) is still required to support detailed designs and approvals for a major underground mining complex. It is not uncommon for these transitions to take more than 10 years to complete the required sequences of work from orebody knowledge sourcing to feasibility studies, construction, development, and production ramp up. If the transition timeframe is underestimated, shortcuts could be required to achieve production continuity. However, there is significant risk that if these plans are not realistic, the successful operation of a cave mine for the longer term can be comprised.

The results of the scoping study provided the client with a solid understanding of a conceptual transition plan that is now being used to optimise production continuity while making plans to continue life beyond an existing open pit operation. A key outcome of the study was that the client now has a vastly improved understanding of what a transition will require, and plans are now progressing for a further open pit pushback to create adequate time to transition to an underground mine.
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