Page 85 - Handbook of Gold Exploration and Evaluation

66 Handbook of gold exploration and evaluation

suspended fine to coarse bedload sediment are discharged by rivers into the
ocean each year; and along the ocean trenches where turbulent currents
distribute great quantities of pebbles, sand, silt and clay. Chemically formed
sediments are precipitated as a result of reactions of the seawater to products of
both on-shore erosion and off-shore volcanic action. In warm tropical waters,
extraction of calcium carbonate from seawater by marine organisms and
biological precipitation as shells may cause extensive carbonate sedimentation.
The mantle of sediments becomes increasingly silicic with distance from the
magmatic source, and its assimilation into the melt during magma-forming
processes in subduction zones strongly influences the composition and
mineralogy of the hydrothermal fluids. Differences in nature and distribution
of the rock types are functions of the average compositions of the continental
and oceanic crusts. In modern settings, involvement of crustal magma is evident
by the high percentage of silicic and intermediate volcanics as in the Andes and
Cascades. In arc and back-arc settings, sediments are largely continental, e.g.
fluvial, alluvial fan and lacustrine; and will range in the fore-arc area from
continental to shallow marine to deep marine (Cas and Wright, 1995). However,
silicic magmas may be generated in areas where the basement is oceanic as well
as where it is continental; significant proportions of mafic magmas in
continental settings are also not uncommon.
Based upon seismic properties, a complete section of oceanic crust has a
depth of about 5±7 km (Anderson et al., 1982). Figure 2.3 is a schematic
representation of the section intercepted in hole 508B of the Deep Sea Drilling
Programs DSDP/IPOD, which was drilled to a depth of 1,350 m on the southern
flanks of the Galapagos spreading centre, south of Costa Rica. This hole is fairly
typical of what might be expected of mid-ocean ridge volcanism where the only

2.3 Schematic representation of section intercepted in hole 508B of the Deep
Sea Drilling Programmes DSOP/1POD.