Page 127 - Handbook of Gold Exploration and Evaluation

106 Handbook of gold exploration and evaluation

Table 2.5 Average values for gold in country rocks (from various sources)

Rock type Number of Weighted
analyses average,
all analyses

Plutonic rocks
Salic
Granite 310 1.7
Granodiorite 380 3.0
Aplite (including dykes and veins of granite and greisen) 23 4.2
Intermediate
Diorite, quartz diorite, monozonite, quartz monzonite,
tonalite 261 3.2
Mafic
Gabbros (including gabbro-diorite) 580 4.8
Ultramafic
Mainly dunite and peridotite 149 6.6

Volcanic and hypabyssal rocks
Salic
Mainly rhyolite (including rhyoandacite, dacite, felsite,
latite and obsidian) 188 1.5
Mafic and intermediate
Mainly basalt, andesitic basalt, andesite (51)
trachyandesite and trachyte (2) 696 3.6

Sedimentary and metasediments
Rock type Au (ppb) Au (ppb)
average high
Detrital
Sandstone 7.5
Shale 3.9 67.7
Metasediments
Gneiss 1.8
Schist 5.0

Although some authors, e.g. Symonds et al. (1987), doubt the importance of
the metal content of gases in relation to ore formation, Meeker (1998) has since
estimated from snow analyses that about 1 kg of gold per day was emitted as
volcanic gas from Mt Erebus (Antarctica) in 1986. At this rate, the emission of
gold would amount to a million oz. in only 83 years. Estimates of the amounts of
copper and gold discharged over short intervals of time from a number of
hydrothermal systems including the White Island volcanic-hydrothermal system
are listed in Table 2.6.
Modern analytical techniques now permit examination of ancient hydrothermal
solutions trapped as fluid inclusions in such hydrothermal minerals as quartz,
sphalerite and calcite. Detailed fluid inclusion studies have been applied to
epithermal ore formation in both modern and ancient volcanic settings. Exploration