The Need for a Connectivity Index in Mining Geostatistics

When orebodies are mined at high cutoff grades (relative to the mean grade), only a fraction of the selective mining units is above cutoff, and worse these blocks are split up into disjoint patches that cannot always be accessed for mining economically. This paper addresses the question of how to characterize the breakup of the reserves above cutoff for the case of 2D orebodies of the Witwatersrand type, firstly by studying a mined out deposit where the true grades are known and then via simulations. Three criteria were found to be important for describing the breakup of the reserves: the proportion of ore above cutoff, the number of connected components (i.e. of clumps of blocks above cutoff) and their size distribution. Looking at images of the reserves above cutoff, we felt that the images needed to be ”tidied up” by removing small clumps of ore in the waste or of waste in the ore. Two morphological transformations were found that do this, and more interestingly seem to provide upper and lower bounds for the effective mineable reserves. As this work is just starting, no definitive conclusions can yet be proposed. However, simulation combined with suitably chosen morphological operators seems to be a promising approach for studying the breakup of orebodies with increasing cutoff grades. Having said that, we feel that the work has raised far more questions than it has resolved.