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Fluvial Rutile Deposit Mining Project

Paul Pettigrew (Paul Pettigrew and Associates) writes:

The map was generated for a fluvial rutile deposit being mined by a bucket ladder dredge. Rather than purchase costly optimisation software, it was decided that the mining plan (referred to as a “dredge path”) could be designed on the basis of a map showing profit vs loss areas of the deposit. The writer had successfully used a similar approach in fluvial cassiterite deposits.

A series of gridded seam models were generated to represent (a) value-in-ground of ore based on current recovered sale price and (b) mining and processing costs. These are calculated on a per grid cell basis. As no overburden was involved, a third seam model, cost to strip overburden, was not required. This can easily be added if overburden thickness has been defined (as it was in the tin deposits).

Value-in-ground is based on grid cell area × cell seam thickness × recovered value where value is dependent on mining/processing recovery and current product price per unit volume. Likewise, cost is calculated per grid cell on the basis of cell area × cell seam thickness × cost per unit volume to mine and process. The Surfer Grid Math command [VALUE – COST= PROFIT/LOSS] was used to generate a profit/loss grid model and a contour map generated to display those values.

In the profit/loss map, several other important features which need to be considered are displayed:

  • digging accuracy of the bucket ladder is considered to be limited to recovery of seams with a thickness >2m (6.6ft) – hence a 2m ore isopach is overlaid on the map as limit to mining

  • borehole logs also recorded percentage of coarse chips in the core – defined as >1mm (0.04 in); a plot of areas reported as 60-100% coarse appear to coincide with indurated capping material on topographic highs; such material is deemed not recoverable by dredge mining

  • in order to assist in orienting the user, a major pond level control dam and the dredge construction dock have also been superimposed.

For scheduling purposes, the writer has broken the path up into 12× digitised polygonal outlines. These are used to provide a sequential mining path and each of the polygons are subset to generate reserves as well as an estimate of the timing (in months) to mine each outline. The subsets are generated by defining each as blanking polygons.

Finally, based on maximum dredge digging depth and basement contours, the sequence is subdivided into 3× sub-sequences in which pond level is lowered as the dredge moves from bottom right (SE) downstream to top left (NW). Thus, polygons 1-4 are mined with a pond level of 20m (65ft) above sea level while polygons 5-10 utilise a level of 14m (45ft) and the remaining 2× polygons are mined with the pond level at 11m (35ft). In this phase, the dam is no longer required – replaced by yet another dam further downstream (not shown).

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