Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 110

EPITHERMAL DEPOSITS

high sulfidation

hot spring

sea level

0 km

rhyolite dome

paleoplacer low sulfidation

1 km

Au-rich massive sulfide

sericite

Breccia-pipe Au

Stockwork
disseminated Au

Carbonate rocks

INTRUSION RELATED
DEPOSITS 5 km Carlin type (USA)

Au Manto type (Chili)

permeable unit

porphyry Au

Au skarn

dyke vein

stock

Turbidite-hosted vein

Wacked-shale

10 km

Volcanic

BIF hosted vein

GREENSTONE VEINS AND
SLATE BELT DEPOSITS

granitoid

shear zone

Banded Iron Formation

Fig. 4.13 Geological settings of the various types of gold deposits (Modified from Dub ´ and
Gosselin, 2008 )

convergent margin settings of all ages. The metamorphic grade is greenschist facies
or more rarely amphibolite to granulite. The gold-bearing veins are spatially
associated with crust-scale deformation zones and usually show strong local struc-
tural control. Hydrothermal alteration surrounding the veins contains the same
minerals as in the gangue - quartz, carbonates and sulfides with additional low-
temperature silicate minerals such as sericite, albite, biotite, and chlorite. One group
of well-known deposits includes examples in Archean greenstone belts such as
Kalgoorlie in Australia and Timmins-Kirkland Lake-Val d'Or in Canada. These are
the source of about 15% of global gold production. Another group includes the
deposits in California and the Klondike in North America and Victoria in Australia.
Weathering of these deposits produced the placers that fuelled the gold rushes in
these regions, as described in the following chapter.
Orogenic gold deposits have no direct association with magmas. Opinions differ
as to the source of the ore fluids: possible origins include (1) metamorphic fluids
released by the dehydration that accompanies the breakdown of hydrous minerals
during amphibolite to granulite facies metamorphism in the middle to lower
crust; (2) magmatic fluids from deep granitic intrusions; (3) CO 2 -rich fluids from
a subcrustal source; (4) deeply circulating meteoric water. As these fluids are driven
through the crustal sequence of metavolcanic and metasedimentary rocks, they
leach out gold which is transported, once again, as chloride or sulfide complexes.
The fluids are channelled along major structural discontinuities and react with the