Compositionally compliant contact analysis

Abstract

Contact analysis assesses 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. This is obtained with contact 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. These concepts are illustrated with data from Murrin Murrin, WA, a Ni-Co laterite deposit where intensive remobilization of both value and deleterious components is known to have occurred.