Page 312 - Handbook of Gold Exploration and Evaluation

274 Handbook of gold exploration and evaluation

Palaeoerosion surfaces resulting from peneplanation are represented by
strongly persistent unconformity in the geologic column. The peneplane, which
represents an erosional surface at the end of an earlier depositional phase,
characterises what has occurred across the landscape before the beginning of a
new major depositional phase. In favourable circumstances any such well-
sculptured surface might be a logical setting for all heavy mineral types,
particularly gold. Evans (1981) notes the apparent relevance of peneplanation to
large-scale placer formation in the California Mother Lode country and west slope
of Sierra Nevada. He suggests that the Klamath Mountains/Coast Ranges and
Sierra Nevada foothills both have future potential but only where, as with known
deposits, they are coincident with peneplanation (Fig. 5.2).
Elucidating the size and nature of Cainozoic palaeo-channel systems requires
a good understanding of climatic as well as tectonic variations at various stages
of geomorphic evolution. Douglas (1977) cites the Murray-Darling basin of
southeastern Australia as a `natural laboratory for testing the principles of
hydrologic geometry'. Generally, the ancestral streams were much larger than
are those of the present; they were also of steeper gradient. Adjustments of many
Quaternary river systems resulted from changes in annual precipitation and
episodes of glaciation and deglaciation. The Murrumbidgee River, which now
transports very little sand was preceded by ancestral channels that were much
larger and straighter than the modern channels, and of greater annual run-off and
higher flood flows.
Solving such geologic and geographical problems in the field relies for
success upon the skills and experience of the exploration team, and its ability to
gather and interpret data from both exotic and normal field conditions. The
importance of adopting optimum procedures in all phases of an exploration
programme cannot be overestimated. Eliminating large areas quickly on the
basis of results from indirect methods such as remote sensing and geophysics, in
order to concentrate on the efforts of small but promising target areas, may
ignore the need to field-check non-anomalous and rejected areas at a later date.
Goossens (1983) warns of `the unfortunate but progressive abandonment of
recording direct observations by field geologists ± perspiring in the jungle or
dehydrating in desert areas', and its replacement by the modern use of
sophisticated but more indirect methods such as geophysics, remote sensing and
geochemistry. It is true, nevertheless, that while the more exotic methods of
exploration cannot take the place of fundamental ground studies in the field,
they do provide important additional data for consideration by the field geolo-
gist. The efforts of integrated exploration techniques (geological, geochemical
and geophysical) offer the best chance of rapidly making new discoveries in
areas selected by geological reasoning.
All available literature on the region to be prospected is reviewed, including
studies of any previous mining activities and collection of all relevant
topographic maps and remote sensing data. At this stage, the geologist will