Page 188 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 163

source rocks cropping out on hillsides eventually gravitates down slope to be
carried away in flowing streams of water. Throughout all of these actions,
gravity is the dominant force, with water as the main processing medium and air
as an essential adjunct. Neither fluid acts entirely alone and water, as a solvent
for the principal agents of chemical weathering (oxygen, carbon dioxide and
other dissolved gases and impurities), is the means by which they are brought
into contact with subsurface materials.

3.3.1 Gravity

Based upon Newton's law of gravitation every body in the universe attracts
every other body with a force, the intensity of which varies directly as the
product of their masses and inversely as the square of their distances apart. Thus,
for two bodies of mass m 1 and m 2 separated by a distance d the gravitational
force:

F ˆ Gm 1 m 2 d ÿ2 3.1
where G is the gravitational constant.
Application of the universal law of gravitation in Newton's second law of
motion relates force F, mass m and acceleration due to gravity g, as expressed in
the formula:

F ˆ mg 3.2
Substituting for F in eqn 3.2 the mass cancels out and
g ˆ Gmd ÿ2 3.3

This equation states, in effect, that all objects in free fall in a vacuum continue to
fall to the Earth with constant acceleration regardless of mass, place and time. In
the context of Newtonian physics, the mass of a body is an intrinsic property that
does not depend upon its chemical composition. Its weight, however, is affected
by various forces such as the gravitational attraction of the Earth on the body
and centrifugal forces due to the rotation of the Earth. The presently accepted
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value of G is 6:67 10 ÿ11 mkg units and the mass of the Earth 5:98 10 g.
The uncertainty of these values appears to be that of the order of one-half unit of
the last place given.
Thus, while mass `M' as the quantity of matter in a body does not change, the
acceleration due to gravity `g' varies with distance from the Earth's centre of
mass hence bodies of equal mass vary in weight from place to place on the
Earth's surface. The variations are small, but have important implications for
geophysicists who are concerned with very slight differences in weights of
bodies of equal mass at various locations and elevations. The value of g is
greatest at sea level at the poles and least on high mountains in higher latitudes.
Reasonable approximations can be made from the calculated values listed in