Geostatistical modelling of lateritic bauxite orebodies in Surinam: effect of the vertical dimension

Lateritic bauxite deposits in Surinam have until now been modelled in two dimensions. The samples collected by auger drill are composited into 1 m (3 ft) intervals and the accumulated grade per hole used as the input for statistical and geostatistical studies. The aim of the study presented in this paper is to quantify the information lost by working with accumulated grades. In other words. the third dimension is taken into account, by considering successive slices with a thickness of 1 m. The composition of the bauxite orebody is characterised by the loss on ignition (LOI), silica, titanium oxide, iron oxide and aluminium oxide contents. The average grade of silica increases significantly with depth, whereas the decrease in the average LOI is only minor. The statistical study indicates two populations for silica and LOI, at least for the fourth slice (at 3-4 m from the top of the orebody) and deeper. For iron oxide a significant decrease with depth is observed, but for each slice only one population could be distinguished. For titanium and aluminium oxide no significant variation with depth and no multi-populations are observed. All these observations are linked to the weathering process, which is the process of genesis of bauxite. The higher the samples are in the profile, the more intense the weathering has been, as indicated by the leaching of silica. These observations confirm that a horizontal zoning is present. For all five parameters studied, a spatial correlation in the horizontal plane is noted for the successive slices. The range of this correlation is on average 200-250 m, and a relative nugget of 35-80% is calculated depending on the parameter and the depth position. The study presented shows clearly that there is a variation with depth. By considering accumulated values, some of this information is lost. For example, for the accumulations of silica and iron oxide no spatial correlation is observed, whereas for the individual slices a distinct spatial correlation is present. (C) 2001 Elsevier Science B.V. All rights reserved.