Gravity – a new trend in exploration

  • By Andrew Mortimer MAusIMM

Mineral exploration, especially greenfields exploration, is a precarious venture for a company director to explain (let alone promote) to investors.

Then there is the small matter of undertaking the exploration itself and signing off as a company director on a use of funds that, of itself, pre-assumes an exploration strategy. However, if there is little or no data on the mineral project in question, then what is the initial exploration strategy? What is the first step in mineral exploration, especially in a greenfields area? Obviously this question is most often answered by the reason that the tenement or project was pegged or acquired in the first place. For example, there may be highly anomalous soil samples or gossanous material or preferably an outcrop of some sort. Geophysical anomalies can also provide the hook for tenement pegging or acquisition. Further, the earth geology doesn’t stop at tenement boundaries; hence the much maligned ‘nearology’ makes actual perfect sense, which is why mineral exploration (and especially mineral discoveries) create more exploration by others in the region where the mineral exploration and/or discovery/discoveries are taking place.

Government geological surveys are also involved in mineral exploration for the purpose of geological data mapping and the encouragement of private mineral exploration. Geological surveys often do early preliminary work, mainly regional mapping, and this is where private mineral explorers should start their investigation. Often geological surveys will fly regional geophysics, such as aeromagnetic, and conduct large seismic surveys. However, Government-funded gravity surveys starting in South Australia in the late 80s and early 90s, and much more recently in Western Australia, have yielded very positive exploration results and mineral discoveries in the case of South Australia. More importantly though is the exploration strategy that they have demonstrated, which is a useful lesson for all explorers.

Government geological surveys rarely do drilling as part of their mineral exploration, as they prefer to do broad areas of ground mapping or airborne surveys as first pass exploration. Also, geological surveys can rely on drill data collected from some water bore holes to provide initial drilling data. The very important question then becomes: where does the private explorer start? This is especially the case since investors want to see decent drilling in preference to geophysical surveys. However, it is now clear from the experience of some Australian companies that if there is government gravity data available, it is best to infill this gravity data with ground gravity to inform the later drill testing program. High powered EM is a good new solution for sourcing legitimate bedrock conductors, but a new clear path in exploration has emerged that has shown that in many cases it is best to test the ambient geophysical fields first, like magnetics, gravity and Z-TEM, before pursuing electro magnetics and/or induced polarisation.

Gravity data is especially useful as it is comparatively inexpensive to collect and not as susceptible to false positives and negatives as other geophysical methods, where electricity is imported to create an artificial field that hopefully causes subsurface mineralisation to be highlighted. Mineral exploration is hard enough without having costly false positives or negatives causing an extra layer of difficulty. Furthermore, the market does not have a full understanding, let alone the patience, for the added time and expense that misinformation can cause.

It is now very clear that whatever the outcome of drilling (if indeed any drilling takes place) the first port of call for a private explorer is the government-generated data and an infill program in these ambient fields of gravity and magnetics in particular. These surveys do provide the perfect background picture prior to other methods such as high voltage EM and induced polarisation.

By far gravity and magnetics are the best foundation for ongoing mineral exploration and if Government-run gravity or magnetic data is available then, subject to anomalies, this Government data should be in filled as an absolute first priority by an explorer.

Feature image by Simon Yeo. Used under CC BY 2.0.

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