Page 195 - Handbook of Gold Exploration and Evaluation

170 Handbook of gold exploration and evaluation

country rock. This causes the roof zone to yield and fractures rock formations
over a large area. New textures and structures are developed within the rock.
Being relatively plastic under conditions of high temperature and pressure,
deeply buried sedimentary rock formations may respond to stress by extensive
bending and folding over a wide area. Shale, which tends to deform rather than
fracture under stress, may be metamorphosed to schist. Planes of parting in the
schist provide two most important factors affecting the flow of hydrothermal
fluids through the rocks: competence and structure.
Following these disturbances each intrusive body exists within its own
uniform stress field and remains in equilibrium with its confining rocks until
again disturbed, either by renewed tectonic movement, which may create further
stresses, or by removal of the overlying rocks which gradually dissipates the
internal stresses. As contraction gives way to expansion the rock develops
patterns of expansion joints along which the rock may split. Massive rocks, such
as granite, are more likely to be affected by this form of pressure release
(dilatation) than rocks, which are initially close jointed.
Additionally, since each type of rock-forming mineral has a different modulus
of elasticity, pressure release also leads to the development of myriad micro-
fractures and loosening of individual grains. Differences in the physical parameters
of the various minerals cause fracturing along mineral-mineral interfaces and rock
disintegration may then be initiated on a grain to grain scale. Thus, when the rock
crumbles, breakage along the plane between a flake of gold and the enclosing
quartz may lead to release of the gold (Fig. 3.17). Some gold grains are plate-like,
some are jagged and irregular in shape, others are rounded, but most have
recognisable characteristics that will affect their subsequent hydraulic behaviour.

3.17 Illustrating how differences in modulus of elasticity and other physical
characteristics can provide potential fracture directions loosening included
gold grains.