Page 317 - Handbook of Gold Exploration and Evaluation

278 Handbook of gold exploration and evaluation

Age dating

Improved age dating techniques which have been developed for placing the
geological events of Earth history in sequence according to age and denudation
chronology has become an important aspect of placer exploration. Most known
gold resources were already in the continental crust at the end of the Pre-
cambrian; and Phanerozoic ores appear to derive largely from the recycling of
Precambrian gold and its weathered auriferous material. On a local scale, age
dating is used to differentiate surviving remnants of palaeo-erosion surfaces
from those of similar type material of a different age. Using this technique,
marker beds and other features of palaeo-erosion surfaces can be correlated
across the whole of an alluvial goldfield by plotting them to a common datum.
Measurements are derived from the decay of radioactive substances in the rocks,
fossil magnetism and orogenic evolution.

Radiometric dating
The age of a substance can be measured radiometrically by comparing the ratios
of certain radioactive isotopes contained in the specimen. The function of
radioactivity results from the instability of the atomic nucleus in certain atoms
whereby the nucleus experiences a measurably delayed nuclear transition or
transformation with the resulting emission of radiation. The rate of decay is
usually stated in terms of half-life, i.e., the time required for the radioactivity of
an isotope of radioactive material to decay to half of its original value. Isotopes
used for dating the older rocks are listed in Table 5.3.

Radio-carbon dating
14
The carbon C analysis is the best-known technique for dating rocks up to about
70,000 years old, although its accuracy falls away beyond 25,000 years. Its short
half-life (5,730 years) makes it particularly applicable to events of the latter part
of the Pleistocene. The method can be used on a range of materials including
wood, charcoal, shell, bone and various carbonate deposits.

Table 5.3 Radioactive isotopes for dating the older rocks
Isotope Half-life (billion years)

Rubidium (87) 48.6
Thorium (232) 14.0
Potassium (40) 8.4
Uranium (238) 4.5
Uranium (235) 0.7