Page 159 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 137

and recycling of Tertiary placers during rapid climatic changes in the
Pleistocene. Because of the effects of multiple metamorphic phases, repeated
deformation and discrete thermal events (e.g. magmatic intrusion) are not
always observable on Precambrian rocks by petrography. Whether a particular
rock is of Archaean or Proterozoic age may be determined only by its syngenetic
concentrations. `It is not enough for a gold bearing quartz vein to be within
Archaean rocks for the mineralization to be Archaean, it could be anything from
Archaean to Recent' (Goossens, 1983). Anhaeusser (1981) even suggests that
post 3.8 billion-year Archaean ore deposits are essentially secondary in origin.
The principal factors determining the resistance of the rocks to chemical and
mechanical decay, are the chemistry of the rocks and physical properties of
hardness, toughness, cleavage and texture.

3.1.1 Igneous rocks

Igneous rocks constitute about 80% of the Earth's crust. They solidify from
molten or partly molten silicate magma in which the oxide of silicon ranges
from about 45% to 75%. Two main categories are distinguished by the abund-
ance and composition of the major phases particularly quartz, felspars and ferro-
magnesium minerals, and whether the rocks are intrusive or extrusive. Extrusive
varieties include lava flows that reach the surface in either a molten or partly
molten state, and molten ash that has been blown apart by the explosive action of
dissolved gases as pressure is released. Intrusive igneous rocks crystallise from
magmas that do not reach the surface.
Rocks are `felsic' if they are high in silica and `mafic' if they are low in silica
and high in ferro-magnesium minerals (e.g., pyroxenes, amphiboles and
olivines). Rocks in which the silica content exceeds about 60% by weight
contain quartz and alkali feldspars with or without muscovite. They are more
resistant to weathering and hence are better represented in sediments than are the
darker-coloured mafic varieties.
Colour is an important diagnostic tool. The colour index of a rock is defined
as the volume percentage of dark or ferro-magnesium minerals; the lower the
index, the more felsic and silicic the rock. Table 3.1 lists the colour indices of
common igneous rocks having quartz contents of less than 10% and more than
10%.

Intrusive varieties
Because of slower cooling, intrusive igneous rocks are coarser grained and
generally retain a higher proportion of hydrous phases than do their extrusive
counterparts. The rock varieties range from the most felsic (granite) through
granodiorite, diorite, gabbro, and peridotite to the most mafic (dunite).
Massive discordant igneous bodies are called subjacent because they form in a