Hydrothermal minerals

Hydrothermal minerals
When hot magma
resides in the crust,
water in the surrounding
rock is heated and
begins to convect. As it
travels, the hot water
dissolves materials
(ions) from the
surrounding rock and
carries them to new
locations. If the
conditions of the water
change (temperature,
pressure, pH, oxygen
content), new minerals
will precipitate.


Hydrothermal
circulation in a
continental
setting –
Minerals form in
hydrothermal
veins,
disseminated
deposits and
grade into
pegmatites.

A hot spring on the surface is a sign of deep hydrothermal circulation.


Hydrothermal
circulation on the
ocean floor.
Seawater circulates through the ridge basalt. Most minerals form when the
hot water exits into the cold deep ocean water. Sulfide minerals form “black
smokers”, sulfates form “white smokers”. Minerals are typically volcanogenic
massive sulfides.


Hydrothermal veins


The dark colored layers are chalcopyrite, sphalerite and galena – valuable ore minerals
The white layers are quartz – a useless “gangue” mineral which must be removed


Porphyry copper replacement deposit – copper minerals are deposited in
fractured, altered igneous rock


See Table 19.2, page 417
The diagram shows some of the main
categories of sulfide mineral
associations in continental settings:
A. Porphyry copper – chalcopyrite,
other copper sulfides and

molybdenite, near the top of a felsic
igneous intrusion
B. Hydrothermal vein with
chalcopyrite, galena and sphalerite
C. Galena and sphalerite in limestones,
typically with dolomite
D. Low temperature (epithermal) gold,
silver, cinnabar vein
E. Low temperature (epithermal gold,
cinnabar deposit)

Fig. 19.1, page 415


Fluid inclusions record mineral and fluid temperature. Minerals entrap fluid as
they grow. When the minerals cool, the fluid contracts, forming a bubble of
gas. By heating the mineral until the bubble disappears (until the fluid reaches
its original volume) you can estimate the temperature of entrapment.


Secondary or Supergene Hydrothermal
Minerals
The silicates in igneous rocks
(feldspar, hornblende and micas)
are altered by reaction with
hydrothermal solutions to form
characteristic alteration minerals:
Propylite (chlorite and epidote
form)
Argillite (clay minerals form)

Sericite (mica forms from clays)

Fig. 19.2, page 416.


Sulfide minerals are unstable in
the presence of oxygenated
groundwater. Primary
(hypogene) sulfides react to form
secondary (supergene) sulfides,
and then supergene oxygenbearing minerals such as oxides,
carbonates, sulfates, and
phosphates, depending on the
anions that are available in the
groundwater. At the surface,
red/orange colored iron oxides
(gossan) are left behind and
become a marker for sulfide
mineral prospecting.
See Table 19.3, page 382 for
names of some minerals in the
oxidized part of the supergene
zone.
Figure 19.3, page 418


Oxygen rich
environments

Oxygen poor

environments

The stability of supergene
hydrothermal minerals is
typically shown on a plot of Eh
(a measure of the availability of
oxygen) versus pH
(concentration of hydrogen
ions, or acidity). The concept of
this phase diagram is the same
as that of a plot of pressure
versus temperature.
Minerals shown are:
Chalcocite
Native copper
Covellite
Cuprite
Malachite
Which would you expect
to form in alkaline, highly
oxidized waters?