Page 134 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 113

abundance of mineralised reefs and veinlets widely dispersed throughout the
country rock is usually more important than the size and intrinsic richness of
individual veinlets. The downgrading effects of dilution from all other rocks in
the catchment area inhibit the formation of any but small placer concentrations
from single lode systems, however rich.

2.4.1 Volcanic-hosted metalliferous sulphides

Volcanic-hosted massive sulphide deposits form mostly in back-arc basins and
mid-ocean ridges. Gold is first taken into solution by hot brine solutions at active
seafloor spreading centres when seawater penetrates through deep sections of
the shattered oceanic crust to the underlying mantle. The seawater becomes
heated by flowing near to localised heat sources (e.g. magma chambers) and at
`hot' spots in plate interiors (e.g. the Hawaiian Islands within the Pacific Plate).
The surface manifestation of such dynamic hydrologic systems is manifested in
seafloor hydrothermal vents located along the Earth's mid-ocean ridges at water
depths of about 1,500 to 3,600 m within the upper few kilometres of very young
oceanic crust.
Chemical reactions between the circulating hot seawater and the crustal rocks
result in a two-way exchange of elements between the hydrothermal fluid and
oceanic crust. Elements and compounds such as magnesium and sulphate are
transferred from the water into the crust. Trace (ppb) concentrations of gold and
other metals are dissolved from the mantle and crustal rocks by acidic solutions.
Such solutions evolve from the release of hydrogen ions in the seawater and by
the formation of hydrogen sulphide (H 2 S) from the reaction of seawater sulphate
(SO 4 ) with ferrous iron in the volcanic rock. The hot metal-rich hydrothermal
solutions, having a low pH and redox potential, are transported away at
enhanced metal concentrations in the hydrothermal solutions. Based upon data
from the Broadlands geothermal fluid, the gold solubility window in
mineralising solutions appears to be around a low of 1.5 ppb Au. By analogy
with base metals, this suggests an upper limit of the window at about 100 ppb Au
(Huston, 1997). Precipitation occurs during mixing and cooling in contact with
the cold (around 2 ëC) bottom seawater at the sea-rock interface.
The mode of deposition is strongly influenced by the permeability of the rocks
through which the solutions pass. Within a permeable crust, large massive sulphide
deposits form by extensive alteration and replacement of volcanic rocks at or below
the seawater-volcanic rock interface. An impermeable crust on the other hand
causes the metal-rich hydrothermal fluids to emerge through narrowly confined
channels and discharge directly onto the seafloor. Relative to seawater, the vent
fluids are acidic, metal- and sulphide-rich and magnesium- and sulphate-poor.
Mixing processes between the vent fluids and seawater result in the formation of
massive sulfide deposits including black smoker chimneys and exhalation of
plumes of particle-laden water that rise 200±300 m above the seafloor.