Page 135 - Handbook of Gold Exploration and Evaluation

114 Handbook of gold exploration and evaluation

High temperature (350±360 ëC) black-smokers occur at the top of a hydro-
thermal mound; lower temperature (260±300 ëC) white-smokers vent from areas
of hydrothermal activity around the apron of the mound. Other important
sources of heat result from chemical reactions between the seawater and certain
constituents of the volcanic rocks, and from radioactive decay. These hydro-
thermal systems play important roles in global heat budgets and geochemical
balances. The smokers themselves are transient in nature; the crustal residence
time of convecting seawater may be less than three years (Cann and Strens,
1982). Smokers grow continually by circulation of the hydrothermal fluids
through the sulphide pile causing abundant re-crystallisation of sulphide
minerals. Most occurrences of metallic sulphides amount to no more than a
few hundred to a thousand tonnes; only where large numbers of black smokers
are active simultaneously over longer periods of time are larger deposits formed.
Recent advances in deep-sea exploration technology have led to the direct
observation of gold-bearing black-smokers at hydrothermal vents and fissures
along active spreading ridges. The ore zones are located within the igneous and
metamorphic rocks of these settings at various depths ranging from near surface
(epithermal) vein systems, to deep-seated (mesothermal) deposits at depths of
10 km and more. Epithermal-like seafloor hydrothermal systems have been
discovered in shallow marine island arc environments of the west and southwest
Pacific. A conceptual cross-section of a seafloor hydrothermal system (Fig.
2.21) is given by Herzig and Hannington (1995) (cf., Ocean Drilling Program
Leg 158: Trans Oceanic Geotraverse Survey `TAG' Hydrothermal Field).
The depth and nature of sedimentation has important implications that reflect
differences in fluid-sediment reaction on deposit styles. Sediment-hosted sul-
phide deposits tend to be larger than sediment-starved deposits and have lower
concentrations of base metals. The larger cover of sediment close to the land
produces longer-term heat retention and entrapment and insulation of vent
fluids, which may account for the large size of the deposits (Kappe and Franklin,
1989). The sediment cover may also help to protect the sulphides from the
effects of submarine weathering and oxidation.
The potential of ancient oceanic crust to survive is low because of the
development of subduction systems and associated hydrothermal processes.
Only remnants may still occur as accretionary prisms obducted over the edge of
the overriding plate and its volcanic arc, whether oceanic or continental
(Coleman, 1971; Moores, 1982). Rona (1984) demonstrated the significance of
the impact of seafloor mineralisation on the development of land-based ore
bodies by locating ancient sources of ore originally formed by seafloor hot
springs and now on land. Gold occurring in trace proportions, but in fact in
massive overall quantities, in volcanically hosted metalliferous sulphides may
be chemically dissolved and re-precipitated or otherwise modified in a near-
surface oxidising environment. The oldest base metal ores in the world are Zn-
Pb-Cu deposits from the eastern Pilbara Craton.