Page 120 - Handbook of Gold Exploration and Evaluation

100 Handbook of gold exploration and evaluation

massive sulphides in the Okinawa Trough. Preliminary analyses of sulphides in
the Eastern Manus Basin indicate an average gold content of more than 30 ppm
Au with maximum 54 ppm Au. In contrast, sulphide deposits related to mature
back-arc spreading centres associated with mid-ocean ridge basaltic type
volcanics (e.g. North Fiji Basin, Mariana Trough) have gold contents of only
0.1±4.3 ppm Au, similar to deposits on mid-ocean ridges.
Volcanic dominated sediment-free deposits, e.g. high-temperature (350 ëC)
black smoker chimneys composed of Cu-Fe sulphides typically contain less than
0.2 ppm Au whereas the 350 ëC member fluids contain about 100 to 200 ppm Au
in solution (Hannington et al., 1991). Hydrothermal reworking of gold during
sustained venting of the mineralising fluids through the sulphide mounds may
provide local enrichment comparable to that found on land, but sampling is at a
very early stage. Because of sampling difficulties during submerged operations,
most values reported from mid-ocean ridge deposits have been obtained from
sulphide chimneys at the surface of the seabed. Such values are unlikely to
represent the underlying interiors of the massive sulphide bodies from which
only a few samples have been taken, none from any systematic deep drilling
programme. More information may be obtained from a current feasibility study,
which is investigating the possibility of mining Pacmanus deposits at a depth of
some 1,600 m in the Eastern Manus Basin.
The likely preservation of Phanerozoic gold deposits is influenced by whether
they become parts of peripheral or interior orogens. Peripheral orogens form
adjacent to an ocean external to the continent (Murphy and Nance, 1991). As
with the Pacific during the Phanerozoic, they do not undergo major continental
collisions and are favourable for the preservation of richly mineralised, low
metamorphic-grade island arc and marginal terraines. Deposits such as granite-
associated and mesothermal Au-Cu mineralisation that form in the upper crust of
continental magmatic arcs are locally preserved. Interior orogens (e.g. the
Himalayas) on the other hand undergo intensive crustal thickening as the result
of continental collision during ocean closure. Uplift and exposure of medium to
high-grade rocks to rapid weathering and erosion gives them a much lower
preservation potential.
With the general acceptance of seafloor tectonic and hydrothermal processes
as being either analogous to or integral parts of geological processes operating in
the continental crust, proposals postulating a seafloor origin for many land-based
deposits now seem more plausible. For example, an overall picture of Mesozoic
orogenic mountain building episodes may be developed from the identification
of assemblages such as opiolites, blue schist series and melange characteristic of
former plate margins or suture zones between major plates. Discovery of visible
primary gold in white smoker chimneys at active vents in the Lau back arc has
shown that seafloor hydrothermal processes are in many ways similar to
hydrothermal processes involved with the development of some epithermal
gold-only ores in volcanic arcs.