The Earth’s mineral deposits, as for much of Nature, are a continuum. Geologists attempt to better understand these deposits through classification and interpretation of the variables they can understand, while being aware that there are many more they may never understand. The rationale for classification is to facilitate communication, which then facilitates the development of exploration models and methodology.

Exploration success is often attributed to a rigorous application of scientific methodology, but in truth, the scientific method can only go so far. Thus, the term “technical success” is used  to describe an exploration program that did not find an economic deposit, but did get some results that appeared to fit the geological model.

The Earth and its processes are exceedingly complex. In addition to a sound model and methodology, exploration success requires good fortune.

Gold deposits can be, but are not always, classified into three systems, along with an “other” classification. These classes are:

  1. Orogenic

Typically greenstone-hosted deposits (for example Kalgoorlie in Western Australia), but also hosted by banded iron formation Hill 50 in Western Australia) and turbidite hosted (Bendigo in Victoria).

  1. Intrusion Related

Also known as reduced intrusion related deposits. They are typically hosted by intrusive bodies themselves or by surrounding sediments. Examples include Kidston in Queensland and Fort Knox in Alaska.

  1. Porphyry Related

Also known as oxidised intrusion related deposits. They are typically in, or near porphyry intrusive bodies and include skarn and epithermal mineralisation. Examples include Grasberg in Indonesia and Porgera in PNG.

  1. Other

Here I include deposit types that have few peers ( for example Carlin in Nevada and Witwatersrand in South Africa), or that don’t easily fit the above classifications (such as the gold-rich VMS deposit of Henty in Tasmania).

Now, let’s have a look at the currently popular Intrusion Related Gold Systems (“IRGS”).