hydrothermal minerals

7/29/2019 Hydrothermal Minerals

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HYDROTHERMAL MINERALS

Dr. Arifudin Idrus

Department of Geological EngineeringGadjah Mada University


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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 theconditions of the water

change (temperature,

pressure, pH, oxygen

content), new minerals

will precipitate.


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Hydrothermalcirculation in a

continental

setting

Minerals form in

hydrothermalveins,

disseminated

deposits and

grade into

pegmatites.


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


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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

circulation on the

ocean floor.


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Hydrothermal veins


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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


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Porphyry copper replacement deposit copper minerals are deposited in

fractured, altered igneous rock


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See Table 19.2, page 381

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 intrusionB. Hydrothermal vein with chalcopyrite,

galena and sphalerite

C. Galena and sphalerite in limestones,

typically with dolomite

D. Low temperature (epithermal) gold,

silver, cinnabar veinE. Low temperature (epithermal gold,

cinnabar deposit)

Fig. 19.1, page 379


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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.


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The silicates in igneous rocks

(feldspar, hornblende and micas)

are altered by reaction with

hydrothermal solutions to form

characteristic alteration minerals:

Propylite (chlorite and epidoteform)

Argillite (clay minerals form)

Sericite (mica forms from clays)

Fig. 19.2, page 380.


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Figure 19.3, page 382

Sulfide minerals are unstable in

the presence of oxygenated

groundwater. Primary (hypogene)

sulfides react to form secondary

(supergene) sulfides, and thensupergene oxygen-bearing

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 fornames of some minerals in the

oxidized part of the supergene

zone.


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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?

Oxygen richenvironments

Oxygen poor

environments

hydrothermal minerals

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