Page 81 - Handbook of Gold Exploration and Evaluation

2

Geology of gold ore deposits

Gold, because of its high density and attraction to iron, is believed to have been
mainly concentrated in the Earth's solid and compacted Fe-Ni core during the
Earth's accretionary stage along with Sn, Mo, and other highly siderophile
minerals. These minerals are also present in Fe-sulphides in the surrounding
mafic-ultramafic rock, the uppermost part of which is in a semi-molten state.
During partial melting of the mantle, gold derived from the sulphides is
contributed to the magmatic fluids and vapours that circulate to the surface
through rifts in the crustal rocks in either extentional or compressional tectonic
regimes. Tectonic regimes (cratons, ocean basins, divergent margins, convergent
basins and transform boundaries) are related to one another through those earth
processes encompassed by plate tectonic theory.
In the plate tectonic model, intensive research on the nature of continental
and oceanic plate interactions and the associated geological processes that
control the genesis of ores has made possible an understanding of many
unresolved geological features and processes. The theory provides logical
explanations for many aspects of Earth's geology and history including the
opening and closing of ocean basins, origins of mountain ranges, geological
structures, distribution of mineral resources, and palaeoclimates. Important
deposit types are distinguished according to geological setting, host rock type,
associated minerals and depths of emplacement. They comprise volcanic hosted
massive sulphides (VHMS), mesothermal ore bodies, intrusion related porphyry
and non-porphyry deposits, and epithermal deposits of both low and high
sulphidation styles. Residual and detrital deposits are developed wherever the
unroofing of a sufficiently large primary gold orebody contributes gold to the
regolith under stable conditions of weathering erosion and deposition. This may
be done directly, e.g. Palaeozoic to present or in stages, e.g. Palaeozoic to
Mesozoic to present. Some authors also apply the same tectonic principles to
explanations of certain aspects of Precambrian geology, particularly the genesis
of greenstone belts, but this is a contentious issue.
Key geological features of gold ore geology discussed in this chapter are
crustal evolution, tectonic elements of plate boundaries, hydrothermal gold