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Mobile Metal Ions is a term used to describe ions which have moved in the weathering zone and that are only weakly or loosely attached to surface soil particles. It has now been proven in a CAMIRO study using Pb isotopes that these Mobile Metal Ions are transported from deeply-buried ore bodies to the surface. Scientists from around the world have been studying this phenomenon for many years.

Convection, electrochemistry, diffusion, capillary rise and seismic pumping are some of the theories which have been put forward. However, research and case studies over known ore-bodies have shown that mobile metal ions accumulate in surface soils above mineralization, indicating that the metals are derived from oxidation of the mineralization source. Capillary rise is thought to be a very important process in the near surface environment which is responsible for maintenance of anomalies and dictates depth for sampling. The diagram below demonstrates a hypothetical model by which mobile ions are released from ore bodies, migrate vertically and accumulate in surface soils.

 

As the ions reach the surface, they attach themselves weakly to the soil particles. These are the ions that are measured by the MMI Technique to find mineralization at depths. The weakly attached ions are at very low concentrations. Because the ions have recently arrived to the surface they provide a precise ‘signal’ on where the ore-bodies are.

When the mobile metal ions have arrived at the surface they have a limited lifetime as ‘mobile’ ions. At the surface the ions are subject to weathering and are bound up by soil forming processes (i.e. they become part of the soil). The diagram below demonstrates this process. Note that bound ions (yellow) are subject to lateral movement away from the mineralization. The mobile ions (blue), however, do not move away from the source (mineralization) because they have a limited lifetime before they are converted to a bound form.

 

By only measuring the mobile metal ions in the surface soils, MMI Geochemistry will produce very sharp responses (anomalies) directly over the source of mobile ions. This source is ore-bodies at depth, which emit metal ions, which make up that ore-body. For example a Cu, Pb, Zn base metal deposit will emit (release) Cu, Pb and Zn ions.

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