Page 311 - Handbook of Gold Exploration and Evaluation

Gold exploration 273

Important differences

Important differences between epithermal and deeper-seated source rocks
include:

· epithermal style orebodies found mainly in igneous and volcanic-hosted
rocks of Palaeozoic and Mesozoic age but appear to be largely missing from
rocks older than about 200 my
· generally low preservation potential of epithermal orebodies due to greater
exposure to erosion at shallow depths of emplacement in the Earth's crust
· much greater heterogeneity of deposit types within the Phanerozoic (Nesbitt
and Muchlenbachs, 1989)
· greater density of lode gold deposits per unit area in the Archaean relative to
the Phanerozoic
· less extensive structural pattern of orogenic activity in Archaean times i.e.,
crustal development on a thinner continental crust without major drift and
collision and from the recycling of sediments at continental margins.
Clearly, while many of the basic geological principles apply to both ancient and
modern volcanic activity, the larger scale tectonic controls, processes and
settings may have been to some extent different. Large areas of the Earth's
surface have not yet been properly explored and at the present state of
knowledge, most interpretation is necessarily based upon the geological history
of the Cainozoic Era and more particularly, on the erosion and recycling of
Tertiary placers during rapid climatic changes in the Pleistocene.

5.1.2 Geomorphic reconstruction

The study of geomorphic reconstruction involves tracing the denudation history
of an area or region back to its beginnings. Its purpose is to investigate the nature
of past events and thus throw light upon the possible prior existence of geologic
conditions in which significant ore deposition may have taken place. Evidence
to support the reconstruction will come partly from surface mapping but must
finally be supported by drilling and sub-surface mapping. Diagnostic features
include evidence of old erosion surfaces that were adjusted to the base levels of
their time and aspects of lithology that could relate to periods of active down-
cutting or conversely to channel fill. Present channel sediments usually contain
reworked gravels in which gold, derived from downcutting of the river into its
bedrock is redistributed in a diluted form. Most of the older deposits will have
undergone sequences of pulsatory tectonic uplift, superposition of drainage and
climatic extremes; often, the paystreak will have been partly or wholly
remobilised and redeposited in other forms and in other settings. For many
Cainozoic gold settings only remnants of palaeo-drainage systems may still
remain as terraces around valley walls.