TOWARDS A TECTONIC AND LITHOSPHERE-SCALE APPROACH TO GOLD EXPLORATION:THE KERRICH LEGACY

As discovery rates for mineral deposits are declining, we need to positively influence the nature of greenfields exploration using the research philosophy of Rob Kerrich, one of the influential economic geologists to recognize the critical importance of scale.

Orogenic, IRGD, Carlin-type, and IOCG deposits are classically grouped as such on the basis of their deposit-scale characteristics. At this scale, some deposit types have similar features, but contrasts are more common and different deposit-scale genetic processes are invoked for each group. Each formed at particular times within the supercontinent cycle, suggesting a specific tectonic control. When the driving forces for these deposits are viewed at such a crustal to lithosphere to mantle scale, they have many more common parameters than at the deposit scale. IRGD, Carlin-style and IOCG deposits all formed above metasomatized lithosphere from fluids at least in part exsolved from mixed basic to felsic alkaline or sub-alkaline intrusions. Although the source of orogenic gold deposits is still hotly debated, such magmas are considered highly unlikely as a direct fluid or metal source. However, at least for the Jiaodong deposits, China, the ore fluids must be derived from either the subduction slab, with overlying oceanic sediments, or the lithosphere below. A deep source is also implied by the common spatial association of orogenic gold deposits with lamprophyre dikes. As expected, IRGDs, Carlin-type, and IOCG deposits all show strong spatial associations with craton margins and other lithospheric boundaries where previous subduction events have enriched the lithosphere with incompatible elements and metals.

It follows that the potential of each district to deposit- scale exploration target should be examined both in terms of its timing within the supercontinent cycle for its specific deposit type and its tectonic setting as a generic factor. Specifically, craton margins and other lithosphere boundaries should be buffered to determine the probability that any specific target potentially lies within a world-class gold province. This screening process should reduce the number of targets selected for drilling based on local anomalism, with the legacy of such scale-related, Kerrich-style thinking a new generation of world-class discoveries.