Page 107 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 87

Oceanic rifting

Magma is introduced into the crust at an oceanic divergent plate boundary (rift
zone) or at any other divergent plate boundary generating ophiolite suite.
Ophiolites are tectonic slices that have been tectonically emplaced during
orogenesis. These slices comprise crystalline basement onto which the oceanic
slice of sheeted dykes, lavas and an abyssal sediment sequence have been
emplaced and post-emplacement deposits of laterite, reef limestone, or shallow
marine or subaerial sediments have been developed. Seismic, gravity and
magnetic studies suggest that the full stratigraphy of modern oceanic crust
closely resemble ophiolite stratigraphy (Moores, 1982).
Creation of new oceanic crust begins along a string of hot spots in the Earth's
crust. As a young ocean widens and matures mafic magmas rise repeatedly from
the chambers, filling the rift between spreading plates and developing a layer of
volcanic mountains as they spill out into the cold bottom waters at the seafloor.
Hot spot volcanism at a submarine mid-ocean ridge is characterised by an axial
rift (graben) in the centre of the ridge at its highest point, and is interrupted in
many places by feeder dykes that cut through the crust. Enormous quantities of
molten basaltic lavas are erupted from volcanoes at the edge of the rift and
sometimes hundreds of kilometres to the sides of the axial rift. The process
continues as the lavas move away from the spreading centre and fresh magma
from the mantle is injected into the chambers
Divergent plate margins in back-arc basins are formed by small convection
cells above subduction zones, fissure (flood) volcanism making the ocean basin
wider. As the plate grows, the leading edge is destroyed as it descends back into
the mantle. The locus of volcanism is typically confined to a line of volcanoes
(some submerged and some emergent above sea level) lying parallel to a
similarly linear deep-sea trench. Most `young' island arc rocks are basalts or
basaltic andesites of island arc tholeiitic type, derived either from the subducting
lithosphere and/or the overlying mantle and crust.

Continental rifting
Continental divergent plate settings first appear as a terrestrial rift valley
involved in the break up of some ancient landmass bordered by ocean basins.
Prior to the onset of rifting the continent floats on the asthenosphere in a state of
isostatic equilibrium. This passive condition is disturbed when a plume of hot
mafic or ultramafic magma from deep in the mantle rises towards the surface
creating a hot spot at the base of the continent. Heat from magma accumulating
at the base of the continent causes lifting, stretching and pulling apart of the
continental lithosphere as the result of extentional tectonics causing the crust to
thin upward, crack and fracture on each side of the stressed area. A narrow basin
is formed, floored by new oceanic crust (refer to Fig. 2.11). Cracks appear along