Management of a manganese resource, Postmasburg manganese field

Abstract

The primary objective of this research report is to qualify and quantify risks and benefits that may derive from the use of resistivity data when managing PMF manganese resources. To achieve this goal a 3D geological resource model of the project area were constructed using available geological and geochemical data. A second geoelectric resource model were constructed using the same data applied in the construction of the geological model and available resistivity data over the study area.

Errors made (Mc Carthy, 2003) were identified by comparing differences in geological interpretation, resultant estimated stripping ratios and mineral resource estimates. For comparative purposes both models were adjusted by limiting their extent to an equivalent 3D modelling space.

Vast differences resulted from the two modelling regimes employed. These differences relate to:  The shape, position and orientation of the dolomite floor and associated pinnacles;  Areas of mineralization and differentiation of areas with ferruginous and manganiferous alteration for the geoelectric model;  Inability to delineate the ferruginous zone of alteration as a separate domain in the geological model, due to insufficient geological and chemical data;  The shape, position and orientation of overlying quartzite; and  The shape, position and orientation of the unaltered shale.

The financial risks of not conducting resistivity surveys, when modelling manganese resources associated with the PMF, impact both exploration expenditure and profitability.

The study estimated that, as the geological environment is challenging, exploration expenditure will increase by approximately 46.8% if resistivity surveys are not conducted prior to drilling. The increased expenditure is a direct consequence of inaccurate placing of boreholes, the large number of boreholes required to be drilled and the intensive sampling and analytical regimes needed.

Over estimation of manganese resource reduce stripping ratios and resulted in the over estimation of gross profit by as much as 1.55 times for the geological model. This is clearly demonstrated by estimated differences in gross profit for lumpy ore of ZARm 408.640 for the geological and ZARm 263.571 for the geoelectric model resources.

It is clear that, due to the complex depositional environment of the manganese deposits of the Western Belt of the Postmasburg Manganese Field, exploration regimes based on traditional drilling and sampling are limited, costly and produce results that are potentially very misleading. The addition of resistivity data clearly demonstrated vast changes in geological interpretation, resulting in substantial differences in mineral resource estimation and subsequent profitability.