Page 149 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 127

The gold grains themselves are liberated sequentially by chemical, physical and
biological weathering effects in the physical environment at different depths of the
weathering zone and on slopes and along drainage lines. Following stage one
deposition, further chemical and mechanical decomposition ensues and one of the
most important features of provenance is that of being able to account for the
various non-economic minerals and rock particles found with the gold. Provenances
have generally predictable gold-rock paragenesis and regardless of age certain
indicator rock-forming minerals act as pointers to particular rock types that may
represent the source or origin of the gold. Quartz is dominant in most mineralised
zones. Other important minerals are Ca, Fe, Mg carbonates, and sulphides (pyrite,
arsenopyrite and chalcopyrite) less commonly galena and sphalerite. Such minerals
as graphite, scheelite, pyrrhotite, tourmaline and tetrahedrite usually appear in trace
quantities only. Gold in mesothermal ore bodies is paragenetically late (Nesbitt and
Muchlenbachs, 1989) and is commonly associated with quartz, carbonate, galena,
and sphalerite plus or minus tellurides in fractures in early sulphide ore bodies.
In this book, placer gold provenances are divided between Phanerozoic and
Precambrian rock types on the basis of age and recognition of similarities and
limitations. Points of similarity include:
· host rocks that provide a zoned pattern of hydrothermal gold-ore formation
· gold-bearing, base metal sulphides in both Phanerozoic and Precambrian
environments
· mesothermal deposits of both age groups that may be considered as part of
one metamorphic change in an environment of tectonic uplift age-
independent class
· that, while settings and conditions of Phanerozoic volcanic processes may
have been different from those of the Precambrian, the same basic principles
should be applicable to both ancient and modern volcanic successions (Cas
and Wright, 1995)
· some evidence to suggest that gold-rich deposits, possibly of porphyry style,
are widespread in Archaean greenstone belts (Symonds et al., 1987) and that
they, like mesothermal deposits which form much deeper in the Earth's crust,
also had a long preservation potential.

2.5.2 Limitations of provenance

Important limitations of the provenance principle are:
· Not all rocks of apparent paragenetic relationship carry the expected gold
mineralisation.
· The primary gold deposit must be sufficiently large for a viable secondary
gold deposit to form.
· The incidence of indicator minerals (rock-forming silicates) in a regolith is no
guarantee that an economic gold deposit will be present.