Compositionally compliant contact analysis

Contact analysis provides an assessment of the evolution of the average value along boreholes of a given variable at increasing distances from the contact between two facies or domains. The concept is long established in the geostatistical literature and software, albeit for studying the behavior of a single variable. This contribution explores practical ways for studying this transient behavior of a set of variables forming a composition, in such a way that spurious correlation effects are avoided. The main idea is to complement a classical contact analysis of raw percentages with similar diagrams for each possible pairwise logratio of two components, as well as with a contact analysis of the centered-logratio transformed components. This approach is particularly promising when the set of components considered account for a considerable amount of the total mass, or dilution effects are suspected to have affected only a subset of the components. In these scenarios, the classical approach would show an apparent parallel absolute enrichment of the unaffected components, although in reality, the relations between them would have not changed. Thus, a classical assessment would lead the analyst to consider all contacts as “hard”, while in nature a subset of parts would actually be virtually unchanged near the contact. This might be unimportant for simple tonnage or grade calculations (where the raw percentage of value metal present is relevant), but it becomes critical where the relations between several value and/or deleterious elements are necessary, like in complex polymetallic deposits and in geometallurgical studies. These concepts are illustrated with toy examples and data from Murrin Murrin, WA, a Ni-Co laterite deposit where intensive remobilitzation of both value and deleterious components is known to have occurred.