Page 82 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 63

systems, source rocks, provenance and the time rate of unroofing of
orebodies.

2.1 Crustal evolution
No direct evidence exists of crustal evolution earlier than about 4.57 billion
years ago but it is generally believed that while the Earth was still in a molten
state gravitational forces acted to concentrate the densest material towards the
centre and lighter material closer to the surface. The geological timescale (see
Table 5.2) describes the timing and relationship between events that have
occurred during Precambrian times in accordance with the dates and nomen-
clature proposed by the International Commission on Stratigraphy. Highly
siderophile elements such as the platinum-group elements and Au were thus
effectively concentrated into the Fe-Ni core during Earth's accretionary stage
(Solomon and Shen, 1997). At this time the core was probably enclosed in a
partly molten `magma ocean' over the surface of which thin platelets of simatic
and lesser sialic material were moved about and subducted under the influence
of the hot convecting mantle fluids (Lowe and Ernst, 1992). When the surface
layer cooled and solidified, a thin film of crustal rock formed around the Earth.
Large cracks developed because of thermal stress breaking crustal material into
individual rigid plates of `lithosphere' of continental size, which moved as
independent units across the hot plastic part of the upper mantle (asthenosphere).
Depressions in the crust formed as natural basins filled with water rising up
through fissures in the crust and from the Earth's early atmosphere to
accumulate as oceans.
Various explanations are given for the observed spatial variations of the ore-
forming elements in the mantle and crust and particularly of mantle hetero-
geneity generated during the Earth's accretionary stage of enriched mantle
resources. One possibility is the addition of a small amount (<1 mass%) of
oxidised accretional veneer from the core after its formation. The introduction of
highly siderophile elements into the Earth's mantle might then account for the
heterogeneous nature of distributions of Au and platinum group elements in the
mantle (Kimura et al., 1974; McDonough and Sun, 1995). Another speculation
is that core-mantle interaction and rising of the boundary layer might have
introduced important amounts of Au and Pt into the mantle through underplating
of mantle plume and subducted lithosphere. Solomon and Shen (1997) suggest
that while the evolution and final solidification of a circum-global magma ocean
in the upper mantle might have resulted in large-scale mantle heterogeneity,
these effects could have been counterbalanced by vigorous mantle convection.
Available geophysical and geochemical observations point to a pyrolitic type
mantle without major differences between the upper and lower mantle
(McDonough and Sun, 1995).