Hydrothermal Deposits - Metals and Society: An Introduction to Economic Geology - page 74

Fig. 4.1 Key factors that
control the formation of
hydrothermal ore deposits

Key factors in the formation of a hydrothermal ore deposit

Magmas ; volcanic, sedimentary
or metamorphic rocks.

Source of metals

Magmatic water; meteoric water;
connate water ; sea water ;
metamorphic water

Source of fluids

Heat source; compaction;
tectonic deformation;
metamorphic dehydration

Trigger of fluid
circulation

Fractures; cavities; porous rock
Temperature decrease;
pH, Eh, or composition change

Site and a mechanism
of precipitation

deposits”, a class that yields about 50% of the world's copper. For most other types
of hydrothermal deposit, however, a link to magmas cannot be demonstrated and in
these deposits the ore metals come from diverse rock types. In most cases they are
leached from these rocks by the circulating hydrothermal fluids. Perhaps the best-
known example are the volcanogenic massive sulfide or VMS deposits which form,
and indeed are continuing to form at the present time, on the ocean floor. Seawater
circulates through the oceanic crust, leaches out metals present at trace levels in the
volcanic and sedimentary rocks of the crust, and reprecipitates them at the ocean
floor to form the ore deposit. In other types of hydrothermal deposit, sedimentary or
metamorphic rocks provide the ore metals.
The types of metals in the ore deposit are directly related to their source. The
granitic source of porphyry copper deposits produces deposits that are rich not
only in copper but also molybdenum, tungsten and in lesser quantities gold and
silver. VMS deposits on basaltic crust are also rich in copper, in this case
associated with zinc; when the substrate consists of felsic volcanic or sedimentary
rocks, lead is present in addition to the two other metals. In deposits in a wholly
sedimentary setting, copper is less present and the ores are dominated by lead and
zinc. Finally hydrothermal gold and uranium deposits are hosted in a wide variety
of crustal rocks and their origin depends crucially on the type of fluid, the manner
in which the fluid circulates and the process that causes the metals to become
concentrated.

4.2.2 Source and Nature of Fluids

The hydrothermal fluids that produce ore deposits are brines or more dilute aqueous
fluids of diverse origins. Some also contain large CO 2 concentrations.