Page 310 - Handbook of Gold Exploration and Evaluation

272 Handbook of gold exploration and evaluation

5.1.1 Ancient vs. modern geologic settings

Processes associated with the aggregation and break up of continental masses
favour specific variations in metallogeny, which were not recognised until the
advent of plate tectonic theory in the late 1960s. This new and revolutionary
approach to mineral exploration was ushered in with the identification of palaeo-
magnetic reversals across the seafloor and discoveries of hot metal-rich springs
and poly-metallic sulphide deposits containing gold at seafloor spreading
centres. Discovery of visible primary gold grains in white smoker chimneys at
active vents in the Lau back arc suggested a close similarity between seafloor
hydrothermal processes and processes associated with the development of some
epithermal gold ores in volcanic terrains. Many potential metal deposits con-
taining gold, silver copper, lead and zinc have also been identified in volcanic
front and back-arc rifts in sea areas outside Japan (e.g. Izu and Ogasawara Arcs).
Within these rifts, hydrothermally active submarine calderas exist, which have
typically concave features. Seawater sinking through cracks in the seafloor is
heated in zones around the magma chamber, rising again to the seafloor as a hot
fluid solution containing heavy metals. This phenomenon results in the produc-
tion of a kuroko-type sulphide deposit abundant in the above heavy metals.
It is thus essential to understand the age relations between mineral deposits
and their host rocks to determine if the ores comprise some parts of orogenic
belts that may have been transported for hundreds or thousands of kilometres
from their place of origin. Key factors in establishing the criteria for finding
such deposits are regional exploration programmes adapted to all different
geological conditions pertaining to Precambrian Shield versus Phanerozoic
orogenic belt, and to the various climatomorphogenic zones; jungle, desert,
boreal and glacial (Goossens, 1983). A convenient division may be made on the
basis of age and recognition of similarities and differences.

Points of similarity

Points of similarity between Precambrian and Phanerozoic geological conditions
include:

· host rocks that provide a zoned pattern of hydrothermal gold-ore formation
· alteration assemblages and mineral suites of the same general type in
epithermal deposits of both ancient and modern systems
· mesothermal orebodies of both age groups that are sufficiently alike to be
considered as part of a single age-independent class
· features of both Phanerozoic and Precambrian geological environments that
associate gold deposits with massive base metal sulphide
· the continental margins of all previous super-continents appear to have been
subject to similar type morphogenic controls as are apparent around Pacific
Ocean margins today.