Geological continuity in the context of resource modeling and estimation usually refers to the lithological (sometimes structural) features that define the ore zone with a defined consistency. This might be defined, for example, by a particular sandstone unit which consistently hosts uranium mineralization over a certain area, a vein (ore shoot) hosting gold mineralization or even a contact area between to rock types (skarn deposit). Value continuity however, is defined as the degree of consistency with which the value of the mineralization itself is consistent within a particular deposit. This value speaks of grade, thickness and could even be extended to geo-metallurgical consistency (the continuity of similar mineralogy). But why do we need to differentiate between these two concepts as opposed to just pure continuity?
In some cases, geological continuity and value continuity seem to be mutually inclusive and not deserving of separate attention. The platinum rich Merensky and UG2 reefs of the Bushveld Complex are an example. Lithology and the associated PGE grade may remain constant within 5 – 10% for up to 100 km of strike length. This of course is the exception. Take for example a hydrothermal copper deposit in the Andes (El Soldado perhaps). Sulphide-rich fluid (and the corresponding Cu mineralization) has not been shown to have a particularly strong association with lithological controls. There is, to a certain degree some association between volcanic host rock porosity and mineralization but there often lacks even a internal porosity consistency within the same rock types in the area. Despite the stronger association with apparent structural controls its application has often been surprisingly inconsistent in guiding exploration.
Sinclair and Vallée (1993) were some of the first guys to stress the importance of distinguishing between the two types of continuity. In many deposit types there is both a degree of variability in the geology as well as variability in mineralization. These two variabilities (or continuities) may be dependent or independent. The effects that erroneous assumptions around any of these two can have on a resource estimation are detrimental. The underlying fundamental in this discussion is the geology and mineralization model. Geological features that affect physical continuity can pre- or post date the mineralization process, or be synchronous.
Before resource modelling takes place it is essential that a robust holistic deposit formation model including all lithological and structural aspects is in place and well understood by the modeller. The same holds true for value continuity. The context of a resource model, geological continuity needs to be understood on at least a mining unit scale, and value continuity an a substantially smaller scale. Drill hole spacing becomes important in this area but the possibly the most important factor in accurate value continuity delineation is sample composite widths during the modelling process. Composites that are large relative to sample width result in too much smoothing and therefore the false impression of the degree of continuity within deposits.
Whether you are a geologist working in early stage exploration, pre-feasibility or on mine, a resource modeller or a mining engineer working on feasibility, we need to continually ask ourselves questions regarding the underlying geology. Do I understand the local geology and mineralization model? How much confidence do I have in this model? Where are the areas of doubt, concern or unexplained areas? Do the constraints put on the modelling process (eg. Composites) do justice to the true nature of the deposit? Are my assumptions around the geology and mineralizations well founded? The geology, modelling and feasibility (or mining planning) spheres should be well integrated in terms of location, conversation and understanding.
Keep asking the hard questions and keep moving toward the answers by doing good fundamental geology!
Sinclair, A. J., and M. Vall ́ee, 1994, Reviewing continuity: an essential element of quality control for deposit and reserve estimation; Expl. Min. Geol., v. 3, no. 2, pp. 95–108.
Figures “3.1” and “3.4” are from Sinclair and Blackwell’s Applied Mineral Inventory Estimation