Page 318 - Handbook of Gold Exploration and Evaluation

Gold exploration 279

Radiochemical dating

Certain aspects of geochemistry involve radiochemical-dating studies, which
exploit the high sensitivity of isotope tracer techniques. In this field, the tech-
niques measure minute quantities of matter such as fluid inclusions in rocks and
isotope abundance and decay. Spontaneous decay of radioactive atoms in the
rocks is a dependable means of timing Earth processes; the rate of disintegration
is fixed and does not change in other chemical or physical conditions.

Fossil magnetism

Age dating of rock formations across the seafloor is vividly portrayed by the
alternation of fossil magnetism (normal and reversed) in strips of the spreading
seafloor. Each strip (or stripe) dates a particular segment of floor and is dupli-
cated on both sides of the mid-ocean rift from which the oceanic plates evolve.
Plates spread at an average rate of about 5 cm/y. The magnetism of alternate
strips reverses at widely different intervals which may be as short as 50,000
years, but are sometimes as long as a million years. Occasionally, as in Permian
times, the magnetic reversal may not take place for as long as 20 million years.
Observation of rock magnetism on land has produced evidence of recent
continental drift from remnant magnetism in some of the younger rocks.
However, the method has been less effective for dating the older rocks because
there is usually some blurring of the geological record by later events. On the
seabed itself, the stripes tend to be less magnetic over the older ocean areas,
possibly because of chemical action or decay with time.

Organic fossil dating
This method of age determination is applicable to both old and recent rock
formations. In certain conditions, the method may be more precise than the
radiometric method. The life cycles of the various fossil types and their environ-
ments are fairly well known and the relationship between organic evolution and
geologic time can be used to correlate widely separated beds of sedimentary rock.
Pollen analysis is particularly useful in helping to decipher palaeo-environments
affected by frequent climatic change. Being very resistant to destruction, pollen
can be identified and counted in such weathered rocks as peat, organic soils and
muds.

5.1.3 Techniques and targets for field investigation

Following an overview of geologic, geographic and geochemical data, hard facts
are needed to support and where necessary supplement the preliminary
geomorphic findings. Techniques include geological mapping supported by