Estimating bulk density for mineral resource reporting

Mineral resources are reported in tonnage and grades. While great care is placed on generating a reliable estimate of grade, determining bulk density for resource estimation is often overlooked. Instead, a simple average value of bulk density is used. While this estimate can work for some deposit types, an overly simplistic estimate of bulk density can cause serious errors in tonnage estimates and an over or under estimate of metal content. Constructing a reliable bulk density model requires developing a good understanding of how bulk density varies with grade and geology. A reliable bulk density database is essential in determining the appropriate modelling technique to minimise the errors in mineral resource statements. 

Modelling bulk density for resource estimates can be complex and intricate, and each deposit type needs to be carefully evaluated to determine the most appropriate methodology to best model bulk density. In a survey of 50 publicly filed technical reports on SEDAR, 29 precious metal, 10 base metal, 7 porphyry and 4 rare metal deposits, bulk density is only marginally considered when preparing resource estimates. The reports were selected at random and included 6 pre-feasibility (PFS) reports, 6 preliminary economic assessment (PEA) reports and 38 mineral resource reports. Nine of these reports failed to mention bulk density or to indicate how bulk density was assigned to the resource estimate.  Of the reports that included bulk density sampling, most used the water displacement method for calculating it. On average, bulk density was measured for less than 20% of the assay data and 29 reports used a simple average value. Of the 10 base metal deposits examined, 4 used a simple average, and 6 estimated bulk density geostatistically with 2 reports including bulk density weighting in the estimate. All reports indicated that multiple geological domains were present, yet most used the same bulk density value to estimate the resource tonnage for all domains, ignoring the multiple geological domains that indicate multiple bulk density domains.

Bulk density is a critical component of the resource estimate. For mineral deposits with low metal content and simple mineralogy, calculating an average of all bulk density measurements can be adequate with sufficient data and a meaningful average. However, each geological domain must be examined individually with a separate bulk density value calculated for each.

For deposits with more complex mineralogy, where a relationship exists between density and grade, simply averaging bulk density for each geological domain will produce errors in local estimates and errors in resource tonnage. A better approach is to apply similar interpolation parameters for grade estimates. For deposits with high grade and density variability, weighting grades by density and estimating grade times bulk density can produce a more accurate resource estimate. 

Gilles Arseneau:


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