A comparative study of geometric and geostatistical methods for qualitative reserve estimation of limestone deposit

Mining projects especially relating to limestone deposits require an accurate knowledge of tonnage and grade, for both short and long-term planning. This is often difficult to establish as detailed exploration operations, which are required to get the accurate description of the deposit, are costly and time consuming. Geologists and mining engineers usually make use of geometric and geostatistical methods, for estimating the tonnage and grade of ore reserves. However, explicit assessments into the differences between these methods have not been reported in literature. To bridge this research gap, a comparative study is carried out to compare the qualitative reserve of Oyo-Iwa limestone deposit located in Nigeria, using geometric and geostatistical methods. The geometric method computes the reserve of the limestone deposit as 74,536,820 ​t (mean calcite, CaO grade ​= ​52.15) and 99,674,793 ​t (mean calcite, CaO grade ​= ​52.32), for the Northern and Southern zones of the deposit, respectively. On the other hand, the geostatistical method calculates the reserve as 81,626,729.65 ​t (mean calcite, CaO grade ​= ​53.36) and 100,098,697.46 ​t (mean calcite, CaO grade ​= ​52.96), for the two zones, respectively. The small relative difference in tonnage estimation between the two methods (i.e., 9.51% and 0.43%), proves that the geometric method is effective for tonnage estimation. In contrast, the relative difference in grade estimation between the two methods (i.e., 2.32% and 1.26%) is not negligible, and could be crucial in maintaining the profitability of the project. The geostatistical method is, therefore, more suitable, reliable and preferable for grade estimation, since it involves the use of spatial modelling and cross-validated interpolation. In addition, the geostatistical method is used to produce quality maps and three-dimensional (3-D) perspective view of the limestone deposit. The quality maps and 3-D view of the limestone deposit reveal the variability of the limestone grade within the deposit, and it is useful for operational management of the limestone raw materials. The qualitative mapping of the limestone deposit is key to effective production scheduling and accurate projection of raw materials for cement production.