The Practicalities And Implementation Of A NAF Grade Control Programme For Placement Of A WRD Cover

Mount Isa Mines (MIM) has commenced rehabilitation of the Handlebar Hill Open Cut (HHOC) Waste Rock Dump (WRD). This includes reshaping of the WRD and placement of an interim inert cover over potentially acid forming (PAF) and neutral mine drainage (NMD) waste rock exposed following the reshape. The interim cover material was recovered from the adjacent Magazine WRD.  To ensure that only non-acid forming (NAF) non-NMD material was used, a grade control programme was implemented which ran simultaneously with the active mining of the Magazine WRD (November 2016 – April 2017).  

To ensure that only NAF non-NMD material was used for construction, a sampling and grade control programme was implemented to delineate NAF non-NMD materials contained in the Magazine WRD.  Material was sampled from test-pits, excavated to a depth equal to the mining flitch, on a variable grid in advance of mining.  Samples were then analysed to support material classification.  Several logistical, scheduling and safety challenges were faced, including:

•    Classification sufficiently in advance of active mining.  
•    Safe working procedures without causing delays or interference to mining.  
•    Strategies to reduce mining time and costs associated with unsuitable materials.

Waste rock classification was found to agree well with the NAF/PAF boundary identified from a preliminary site geochemical field investigation.  Whilst “NAF” and “PAF” materials could mostly be identified based on geological characteristics, in the southern end of the Magazine WRD “NAF” lithologies contained higher sulphur and had been impacted by seepage from overlying PAF materials.

Post-placement verification indicated that the resolution of the sampling and testing was sufficient to meet the requirements of the cover specification.