Page 165 - Handbook of Gold Exploration and Evaluation

Gold deposition in the weathering environment 143

Table 3.2 Metamorphic rocks (from Whittow, 1984)

Metamorphic Sandstone Limestone Mudstone Clay Basic
grade lava

Low quartzite marble slate slate greenstone
Medium quartzite marble mica schist amphibole amphibole
High quartzite marble gneiss amphibole amphibole

temperatures and pressures, tend to become plastic and warp and fold when
subjected to additional prolonged stress. Because there is little recrystallisation
and chemical action, any structures present will have been formed principally
through granulation and shearing and the moving out of fragments of pre-
existing material. Friction breccias, which may result from crustal deformation,
comprise abundant fractured and angular rock fragments with dimensions
ranging from about a millimetre to a metre in length. Gneiss forms as the result
of more intense shearing and recrystallisation and the development of almond-
shaped grains or grain clusters set in finely grained matrix.
Contact metamorphic rocks form as the result of pronounced temperature
increases in the virtual absence of increased stress. Physical change occurs when
magmatic fluids, which often bring about recrystallisation, permeate pre-existing
minerals. These rocks are characterised by alternating bands of darker minerals
such as chlorite, biotite mica and graphite. The areas of contact called aureoles or
concentric shells surround hot igneous bodies emplaced at upper levels.
The most durable metamorphosed sediments (metasediments) are silica-
cemented quartzites. Resistant quartzitic types comprise an almost pure quartz
sandstone with quartz cement, other quartz sandstones (e.g. carbonated sand-
stone) and a chemical variety of chert (monomineralic quartz rock composed of
interlocking cryptocrystalline quartz veins). Of other common rocks in a gold
placer environment, limestone is mechanically resistant but is easily broken
down chemically, slate is chemically durable but splits easily along well-defined
cleavage planes, and coarse grained metamorphic rock such as gneiss exhibits a
weathering pattern similar to the coarser grained granites. Schistose rocks,
particularly those containing large plates of mica, are more susceptible to
weathering than most other schists because of the easy penetration of aqueous
solutions along the micaceous layers. The above rocks are described in terms of
metamorphic grade in Table 3.2.

3.2 Earth's atmosphere and climate

The Earth's surface comprises atmosphere, land and sea each one exerting an
influence upon the properties of the others. Atmosphere is a shallow mixture of
gases surrounding the Earth held by gravity. The atmosphere is subdivided into