Page 160 - Handbook of Gold Exploration and Evaluation

138 Handbook of gold exploration and evaluation

Table 3.1 Typical mafic minerals and colour indices of common igneous rocks (after
Ernst, 1969)

Rock type Mafic minerals Colour index

Less than 10% quartz
BASALT, gabbro olivine, augite hypersthene 35±65
ANDESITE, diorite hypersthene augite hornblende 20±45
LATITE, monzonite hornblende biotite 10±30
TRACHYTE, syenite biotite hornblende 0±20

More than 10% quartz
DACITE, granodiorite, hornblende augite biotite 20±50
quartz-diorite
QUARTZ-LATITE, hornblende biotite 10±25
quartz-monzonite
RHYOLITE, granite biotite hornblende 0±15

lower situation but not directly underneath overlying strata. They are termed
2
batholiths if their present outcrop areas exceed about 100 km , or stocks or
pendants if they are less extensive. The Sierra Nevada batholith of California, the
Boulder batholith of Montana and Idaho, and the Coast Range batholith of British
Columbia are all associated with clusters of stocks and other inclusive bodies that
are directly connected with extensive and massive gold mineralisation.
Tabular intrusions comprise discordant dykes and sills. Dykes occur along
fissures that cut obliquely across pre-existing country rocks. The largest dykes
are major features; the Great Dyke of Rhodesia stretches for 500 km in length
and ranges from 3 to 12 km in width. As with dykes, all principal magma types
occur in sills. Laccoliths are small intrusions of the more felsic rock types that
have a planar bottom and are domed due to arching of the overlying country
rocks. Lopoliths are very large floored intrusions, concordant with the upper
contacts. They result typically from the intrusion of predominantly mafic
magma.
A third type of intrusive body includes relatively small, discordant plutons as
represented by the filled conduits of eroded volcanoes. They consist of any rock
type that occurs as lava and are appropriately referred to as volcanic necks. Each
of the intrusive rocks represents the emplacement of more than one body;
volcanic necks are associated with dyke swarms.
The weathering characteristics of individual crystals differ largely due their
variable grain size. Coarse-grained feldspathic rocks of the more basic varieties
are particularly susceptible to chemical decay either due to the alteration of
individual crystals or of the matrix binding them together. Of the felsic varieties,
granitic rocks are either very resistant to chemical or physical change or weather
rapidly, depending on the coarseness of their grain size and the relative
abundance of felspar and ferro magnesium minerals. The crystals of fine-grained