Page 150 - Handbook of Gold Exploration and Evaluation

128 Handbook of gold exploration and evaluation

· The source of minerals such as gold cannot always be predicted from
available evidence, where that evidence is geographic and there is no
predictable mineral-rock paragenesis.
· The application of provenance in placer exploration is governed by the
drainage pattern but it may also be influenced by the existence of a secondary
source for the gold-bearing minerals.

For example, one tributary of a river may rise in platinum-bearing ultramafics,
whilst another may cut through granite or sandstone terrain. Ultimately this
would bring together unrelated rock-forming minerals in which quartz would
predominate. The author's experience of such an example includes the Uraido
River gold/platinum placer, Colombia, South America.
Because of these limitations, any model for reconstruction of the geomorphic
history of a residual placer gold environment must take account of the variable
nature of past climates and the wide range of timescales within which individual
changes may have occurred. All past and present processes that relate to the
modification and release of gold grains in the weathering zones of orebodies will
be critical to investigations of both primary and secondary gold deposits. Neither
primary nor alluvial gold deposition can be studied in isolation without
neglecting possibly vital evidence from the other. A detailed knowledge of a
source area (petrology, structure and geological history) is thus of inestimable
value to both primary and placer gold explorationists.

2.6 Time rate of unroofing ore bodies
During the active life of a volcano, the aggregation of magma exceeds the
enormous amounts removed, the principal effect being to continuously raise the
surface of the land against the base level of erosion to which the erosional
processes operate. Uplift occurs in stages and is rapid during the most active
stages of mountain building. Only during extended periods of tectonic calm, e.g.
in later stages of orogeny will erosion rates exceed those of upward movement
and thus eliminate the majority of the surface relief overlying the mineralised
zones. On a global scale, orogenic belts exposed at various levels of erosion
make up much of the world's land area, and differences in the vertical continuity
of gold-bearing ores reflect differences in the geology of their formation, as well
as their levels of emplacement and regional associations. Interpretation in terms
of active arc systems and collision belt processes requires detailed geological
investigation and careful comparison with presently active systems.

2.6.1 Volcanic uplift
Volcanic eruptions are short-lived and occur only intermittently during the total
history of a volcano. Regionally extensive crustal structures include volcanic