Managing borehole samples of unequal lengths to construct a high-resolution mining model of mineral grades zoned by geological units

The objective of this research is to construct a high-resolution model of the mineral grades of the Farim-Saliquinhe sub-horizontal layer-type phosphate deposit in Guinea-Bissau. A statistical analysis identified two data-driven challenges: (i) grades have been previously measured using borehole samples of unequal lengths; and (ii) grades need to be zoned by geological units. For the first issue, this work simulates grades at borehole locations and high-resolution spacing, approximately the shortest length of the samples, and then enlarges the average of grades to the final resolution. A calibration of the variance following an inverse approach is used to simulate values at the high resolution. For the second issue, an automatic classification procedure is applied to the simulated values at borehole locations, leading to three geological units. These units are extended to the entire deposit in the form of a surface-based morphological model. Finally, P2O5 grades are simulated in a 3D grid of blocks by making use of a modified version of the direct sequential simulation algorithm to constrain the grades according to the morphological model via local histograms. The related results are validated through a comparison of phosphate reserves with those obtained by a weighted average of grades by thicknesses and areas of polygons of influence. A discussion concerning the local and global uncertainties provided by the set of the simulated images is made at the end of the case study. (C) 2013 Elsevier B.V. All rights reserved.