Page 121 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 101

The technical feasibility of sea floor mining at depths up to several thousands
of metres has been shown by a number of multi-national companies. Com-
petitive proposals for dredging polymetallic nodules from the deep ocean floor
were reviewed by Macdonald (1987) in the McGraw-Hill Yearbook of Science
and Technology. Possible economic exploitation of the Red Sea muds has also
been studied in small, pilot-scale testing programmes. However, problems of
marketing, materials handling and economic feasibility have yet to be overcome
and none of these proposals has yet been tested at a commercial scale. Not the
least of the problems is the requirement of a very large, long-term and stable
market for all of the contained metals. Studies of the feasibility of dredging
gold-rich polymetallic deposits in back-arc basins are faced also with much
more difficult sampling problems than those encountered in sampling
polymetallic nodule and mud deposits (see Chapter 7).

2.3 Hydrothermal gold systems
Until the 1960s, and the advent of plate tectonic theories, the empirical approach
to hydrothermal ore formation was based generally upon experiments that
simulated natural conditions in trying to deal with the complexities of geo-
chemical processes. Lindgren (1933) classified mineral deposits according to
depth and ore deposits were classified in accordance with temperature±pressure
relationships within the crust as `epithermal', `mesothermal' and `hypothermal'
depending upon the crustal level of ore formation. Since that time, the develop-
ment of new analytical techniques such as fluid inclusion and stable isotype
analysis has allowed much greater insight into the understanding of hydro-
thermal fluid evolution within the ore-forming environment. Sophisticated
computer codes have been developed to simulate depositional processes such as
cooling, boiling, fluid mixing and water-rock interaction, and to couple these
with simulations of fluid flow in porous media.
Significantly large gold deposits require the coincidence of particularly
favourable processes and source parameters. A critical factor is the degree of
element concentration during ore formation. It is now recognised that hydro-
thermal mineral deposits are ultimately the result of chemical reactions and that
they are localised by zones of higher permeability (e.g. faults and aquifers).
Huston (1997) classifies these processes and the regions in which they occur as
`mineral systems'. The characteristics (e.g., T, P, pH, salinity, redox, sulphur
content) of hydrothermal fluids defined by these processes within these systems
determine the metal-carrying capacity of the fluid. Three general groupings of
mineral deposits that contain Cu, Zn, Pb, Ag, and/or Au are:

1. Zn-Pb-Ag ‡ or ÿ Au,
2. Cu ‡ or ÿ Au,
3. Au ‡ or ÿ Ag.