Page 315 - Handbook of Gold Exploration and Evaluation

276 Handbook of gold exploration and evaluation

familiarise himself with the geology and geography of the environment within
which he will be working. Operational sequences can then be defined and linked
together in a logical time frame to ensure that priorities are identified and that
efficient use is made of the available resources of manpower, money and
equipment.
With the indication of a favourable source rock environment the next stage is
to identify and map the geological features of deposit size targets by their
geophysical and geochemical expression and/or by remote sensing. The
individual ore zones will then be examined in a ground geological survey
including geological mapping, geochemical sampling, pitting and trenching. The
boundaries of potential areas, where a reasonable probability exists of economic
development, can then be defined for more detailed exploration. Hodgson
(1986) proposes geological criteria for selecting areas of gold mineralisation in
belts dominated by volcanic rocks in Table 5.1.
Account must also be taken of the variable nature of past climates and the
wide range of timescales within which individual changes may have occurred.
The problem is to recognise those elements of the landscape that were adjusted
to the base level at the time of their formation and to fix their elevation relative
to the present base level. In a valley-fill sequence, separate deposits may exist at
different levels and any apparently significant features should always be
observed at a number of different points to help avoid mistaking local for
regional features. The separate mapping of a sequence of erosion surfaces may
be prone to error if based upon stratigraphic succession alone. It is not
uncommon for older deposits to undergo a wide variety of pulsatory tectonic
uplift, superposition of drainage and climatic extremes, and sometimes the
paystreak will be reworked and relocated several times. In a pulsatory but net
aggradational succession the lower erosion surface is least likely to have been
disturbed by subsequent events and should be mapped first.

Denudation chronology

Investigating the denudation chronology of a residual gold setting entails
differentiating between individual elements of palaeo-drainage systems and
plotting the sequence of events from the first stages of weathering to the present.
The process of fitting individual features into the geological framework requires
sub-dividing the rock record into intervals representing small units of time.
Historical records are sparse and generally, the older the geological setting the
greater the gaps in knowledge and the less easily is its geomorphic record read.
Earth probably formed about 4.5 billion years ago and its geology has been
changing ever since. It is only possible to relate geological time units in the
oldest, Precambrian settings to major episodes of igneous intrusion, metamor-
phism and mountain building with possible errors as large as 60 million years,
close to the entire age of the Cainozoic Era. For the younger Mesozoic orogenic