Various definitions of geochronology can be found, including those which incorporate some aspects of relative time. However, for our purposes, it is useful to consider geochronology as the discipline of the geosciences which quantitatively measures the age of earth materials and provides the temporal framework in which other geological phenomena can be investigated. For industry geoscientists this is usually focused around determining the age of the mineralisation in a given system, or, if not the age of a specific ore body itself, the age of a mineralising system or event, in order to facilitate exploration for other, potentially mineralised portions of the same system.

That said, mineralised systems are themselves notoriously difficult to date. This is due to several reasons, the most common being the composition of the mineralised assemblages themselves. Often there can be a perception that an age determination is sacrosanct, and therefore it or its interpretation cannot be changed. More commonly, reported ages record something other than what the user thinks it is. Therefore it is critical to understand how various ages are obtained and hence gain an appreciation of the approaches and limitations of geochronology.

Some techniques are based around a single principle which can be applied to many different isotopic systems (e.g. isochrons, Chapter 7). Other techniques, such as U–Pb zircon (Chapter 8) and Ar–Ar (Chapter 9) geochronology and thermochronology (Section 8.4) are based upon individual technological approaches specific to a single decay system and select minerals. Still other approaches exploit unusual parent–daughter behaviour or chemical affinities (e.g. Re–Os in molybdenite and petroleum systems; Section 8.5) or use the measured isotopic ratios to calculate theoretical ages for materials based upon certain assumptions (e.g. Nd model ages, Chapter 10).

Accuracy and precision in geochronology

All of these approaches rely on inherent assumptions regarding closed system behaviour (see Section 5.4 for a summary of what this means) in order to produce accurate ages. Accompanying accurate determinations should be some estimate of the precision of the analysis. Accuracy and precision are two very different but equally important parameters. Precision is a measure of how well you can make a measurement, whereas accuracy is a measure of whether the answer you get is the ‘right’ answer. There is no point making highly precise measurements if they are not returning the right answer.