Intrusion-centred mineral districts host a diversity of ore deposits of variable metal associations, alteration assemblages and genesis. Porphyry systems represent particularly important exploration targets but the prioritization of conventional geochemical or geophysical anomalies that might represent a deposit, particularly when systems are buried under cover, is extremely difficult. Three key questions arise: (1) is the alteration (particularly when only a propylitic type is observed) related to a porphyry system? (2) how can the fertility of a system be assessed at an early stage of exploration in order to reduce exploration risk? and (3) how can the centre of the system (in 3 dimensions) be predicted ahead of extensive, potentially deep, drilling? These fertility and vectoring challenges have been the subject of recent work, primarily based on mineral chemistry, in a series of AMIRA projects based out of the University of Tasmania, now also being continued at the Natural History Museum in London.

The approach to assessing the presence of a possible porphyry system has been to establish mineral chemical criteria that discriminate between porphyry and non-porphyry environments based on: (1) the composition of igneous minerals (e.g. plagioclase, zircon, apatite, magnetite); and (2) the composition of hydrothermal alteration phases, particularly those developed in the propylitic domain (epidote, chlorite, magnetite, calcite, quartz). Many of these phases may be reworked via erosion into paleo or modern sediment transport systems and are thus available for assessment of catchment area fertility. Some of the characteristics of these minerals may allow the distinction between extensively mineralized and ostensibly barren environments (the system “fertility”); clearly these features are of significant exploration utility.

The vectoring challenge has been addressed by the completion of numerous orientation studies on known porphyry systems to establish any systematic spatial variations in mineral chemistry that may exist, primarily within the propylitic environment. These studies have shown that characteristic and, to variable degrees, reproducible patterns of major and trace element variation exist that allow effective vectoring towards the center of a hydrothermal system, as well as discrimination between porphyry-related and non-porphyry mineral assemblages1,2. In particular, chlorite has proven to be particularly effective for prediction of absolute distances to the system center, even allowing estimation of the depth of a buried system. Both epidote and chlorite appear to contain signals that reflect the potential metal endowment of a system. The ability to define these characteristics of a system from a limited number of samples of distal “green rocks” marks a major step-change in the way that exploration for porphyry systems can be done.

In this presentation, the approach and methodology are summarised and some of the major findings from this work are illustrated with examples from a number of porphyry systems worldwide.