Page 161 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 139

granitoids, being closely interlocked, are less affected by chemical attack than
the coarser-grained varieties and hence less prone to mechanical disintegration.

Extrusive varieties
The physical properties of extrusive igneous rocks depend largely upon the
viscosity of the liquid phase of erupting magma and the nature and abundance of
its gas content. Viscosity is a function of the chemical composition and
temperature of the magmatic fluid, its content of rock fragments broken off as it
rises to the surface and its gas content, either dissolved or as bubbles. In order of
increasing viscosities, the magmas are mafic (basaltic), rhyolitic and felsic.
Sudden cooling and solidification of lava at the surface is accompanied by an
equally sudden and sometimes violent release of pressure. Major eruptions are
explosive in action, forming typically steep-sided cone-shaped volcanoes. Minor
eruptions occur as bubbles combine in the magmatic fluid and burst their way
out at the surface. Basaltic lava, being of low viscosity, releases its gas content
easily and tends to flow rapidly and evenly away from the volcanic centre. The
greatest volumes of lavas are issued from fissures in the ocean floor. On the
continent, lavas accumulate to form broad plateaux, which approach a kilometre
or more in thickness and tens of thousands of square kilometres in area. In some
major goldfields (e.g., California, USA; Victoria and NSW, Australia) basaltic
lava flows cover large areas of valley floors, thereby protecting any underlying
Tertiary gold placer deposits from erosion. These buried placers described in
Chapter 4, are usually referred to as `Tertiary' deep leads.
Evidence for mineralisation is small within presently active arc regions, where
large andesitic volcanoes dominate the landscape, although some exhalative
deposits can be important. Being more viscous than basaltic magmas, andesitic
lavas retain a higher gas content than basaltic lavas because the gas does not
escape so readily. However, in terrains where superficial deposits have been cut
through by erosion, evidence of mineralisation becomes more common. Slightly
altered lavas, which are intermediate in composition between basalt and rhyolite
are the principal rock types associated with primary gold-silver mineralisation of
epithermal type, while porphyry copper deposits are found associated with
intrusive diorites and granodiorites in the sub-volcanic environment.
Although rhyolitic magmas may originate from the simple differentiation of
basaltic magmas, processes of melting of crustal rocks and magma at colliding
oceanic-continental or continental-continental plate boundaries also contain the
essential ingredients for producing rhyolitic type magmas. Completely crystal-
lised rhyolites consist largely of alkali felspar and quartz or other polymorph of
SiO 2 , minor mafic minerals and subordinate oligoclase. Porphyritic varieties
contain numerous large crystals (phenocrysts) disseminated through the matrix.
Lesser amounts of glassy rhyolite include such rocks as obsidium, pumice and
perlite. The large variations in composition, which occur between rhyolites and