In an attempt to reduce the inherent risk in mineral exploration, SRK, with their clients, continues to use innovative and new technology, approaches and concepts that assess the increasing amounts of geological data in the most time efficient manner. One approach that has become increasingly accepted is the Mineral Systems Approach to assessing prospectivity.
The Mineral Systems Approach considers the origin of deposits in the framework of lithospheric-scale processes from the time-honoured perspective of source, fluids, transport and traps. Applied to exploration strategy, this approach allows for more predictive models. Rather than matching patterns, knowledge of the underlying geological processes and tectonic setting can be used to identify prospectivity.
Furthermore, a Mineral Systems Approach can broaden the scope of prospectivity indicators and therefore allows for earlier, more efficient fertility assessments.
The five questions of mineral system theory are geodynamic history and setting, architecture, fluid reservoirs, fluid pathways, and driving forces for transport and deposition. The questions were formulated for hydrothermal mineral systems but are equally applicable to magmatic ores. Translating this theory into a useful tool for exploration involves understanding how critical processes of the mineral system are reflected in the geology, and using them to define targeting criteria to detect elements directly or by proxy.
A good example is that of porphyry Cu-Au-Mo deposits. Both the well-known tectonic environment proximal to subduction zones within magmatic arcs, and the classic Lowell-Gilbert model of propylitic-phyllic-potassic alteration are widely used to identify areas of mineralisation. Thus, exploration focuses on identifying areas indicating alteration at the surface, on a scale of hundreds to thousands of metres. However, using a Mineral Systems Approach considers the range of geological processes that occur to produce a porphyry deposit. The source-fluid-transport-sink pathway in this case includes mantle melting, magma transport, lower and upper crustal magma chambers, sulfide saturation and volatile exsolution. By understanding the signatures that these processes leave in the common and widespread magmatic and volcanic rocks in arcs can mean a much broader range of rocks can indicate a fertile system. These processes leave signatures in minerals like zircons and apatites, which are common in most magmatic rocks, mineralised or not. Thus, any magmatic rock in a district can help identify fertile or barren systems as a proxy to the essential elements of the mineral system – and not just direct evidence of mineralisation or alteration.
At SRK, we are active in the continued research of mineral systems and are currently involved in a large research project focused on these deposits (FAMOS: From Arc Magmas to Ore Systems); the project aims to make exploration for porphyry deposits cheaper, quicker and more efficient. Applying the Mineral Systems Approach is now becoming an increasingly advantageous tool for exploration across all deposit types.
David Holwell: dholwell@srkexploration.com
