Some improved approaches to mineral inventory estimation of porphyry-type mineral deposits
Creators: name: Postolski, Tomasz Apolinary
Publisher: name: University of British Columbia
Year: 2009

description: Geological information utilized at early stages of resource/reserve calculation highly improves mineral inventory estimation in porphyry-type deposits. It is useful to review this topic in the context of modern approaches to geological data accumulation and interpretation as well as methodologies of mineral inventory estimation. Detailed geology provides information for a geometric model of a deposit. Substantial effort is required to characterize the geometric margins of a deposit and the relation of these margins to simplistic geometric forms that normally emerge as an interpretation. Several widely accepted models are discussed to illustrate the range of geological features that require special attention in establishing mineral inventory in porphyry-type deposits. Recognition of the different styles of mineralization allow division of the deposit into different mineralization domains having different continuities, which has a profound impact on the development of semivariogram models that are used for geostatistical resource/reserve estimation. Mineralogical studies are also emphasized, because they relate to many aspects of deposit evaluation including abundances of ore, spatial distribution of ore and finally liberation properties of ores. Three separate mineralized zones (Main and East zones of Huckleberry deposit and the Virginia zone of the Copper Mountain porphyry system) are used to illustrate the impact of close geological control on semivariogram modeling and, consequently, the economic impact on geostatistical resource/reserve estimation. Analyses are done using both geostatistical and metal accounting procedures. In the Main zone of the Huckleberry deposit copper grade contour maps show significant variations in trend directions, that coincide with dominant direction of stockwork development. These directions of dominant mineralization control, effectively separate the Main zone into three domains, each domain having a substantially different semivariogram model. Metal accounting calculations showed that the application of deposit-general (less accurate) semivariogram model produces an annual loss of 300 tonnes of metal in operating profit. Similar procedures applied to the East zone of Huckleberry deposit reveal the possibility of an annual loss of 700 tonnes of metal in operating profit. In the case of Virginia zone the principal control on mineralization is a set of easterly striking, vertically dipping veins. Contour maps of Cu and Au grades for all levels showed remarkable similarity and reflected the direction of strongest geological continuity (east striking vertical plane). The widely spaced exploration data are barely adequate to demonstrate the existing anisotropy. The geology thus provided insight into principal directions controlling the semivariogram model for the deposit. The effect of average errors of block grade estimates can be evaluated quantitatively using computer program GATNLOSS that has been developed for this purpose by the author. For a given estimation error and cutoff grade, GATNLOSS calculates both the quantity of metal that is lost as a result of misclassifying ore blocks as waste and the dilution that results from misclassifying waste blocks as ore. Calculations are presented using realistic block grade distribution parameters for both porphyry-type and gold deposits. In addition, the effect of dilution and ore loss on grade of production is calculated. As a result the following fundamental relations were revealed: