hydrothermal minerals 2

8/11/2019 Hydrothermal Minerals 2

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Hydrothermal mineralsWhen hot magmaresides in the crust,water in the surroundingrock is heated andbegins to convect. As ittravels, the hot waterdissolves materials(ions) from thesurrounding rock andcarries them to new

locations. If theconditions of the waterchange (temperature,pressure, pH, oxygencontent), new mineralswill precipitate.


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Hydrothermalcirculation in acontinentalsetting

Minerals form in

hydrothermalveins,disseminateddeposits andgrade intopegmatites.


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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 hotwater exits into the cold deep ocean water. Sulfide minerals form

blacksmokers

, sulfates form

white smokers

. Minerals are typically volcanogenic

massive sulfides.

Hydrothermalcirculation 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 mineralsThe white layers are quartz a useless

gangue

mineral which must be removed


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Porphyry copper replacement deposit copper minerals are deposited infractured, altered igneous rock


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See Table 19.2, page 417

The diagram shows some of the maincategories of sulfide mineralassociations in continental settings:

A. Porphyry copper chalcopyrite,other copper sulfides andmolybdenite, 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 415


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Fluid inclusions record mineral and fluid temperature. Minerals entrap fluid asthey grow. When the minerals cool, the fluid contracts, forming a bubble ofgas. By heating the mineral until the bubble disappears (until the fluid reachesits 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 withhydrothermal solutions to formcharacteristic alteration minerals:

Propylite (chlorite and epidoteform)

Argillite (clay minerals form)Sericite (mica forms from clays)

Fig. 19.2, page 416.

Secondary or Supergene HydrothermalMinerals


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Figure 19.3, page 418

Sulfide minerals are unstable inthe presence of oxygenatedgroundwater. Primary (hypogene)sulfides react to form secondary

(supergene) sulfides, and thensupergene oxygen-bearingminerals such as oxides,carbonates, sulfates, andphosphates, depending on theanions that are available in the

groundwater. At the surface,red/orange colored iron oxides(gossan) are left behind andbecome a marker for sulfidemineral prospecting.

See Table 19.3, page 382 fornames of some minerals in theoxidized part of the supergenezone.


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The stability of supergenehydrothermal minerals istypically shown on a plot of Eh(a measure of the availability of

oxygen) versus pH(concentration of hydrogenions, or acidity). The concept ofthis phase diagram is the sameas that of a plot of pressureversus temperature.

Minerals shown are:ChalcociteNative copperCovelliteCupriteMalachite

Which would you expectto form in alkaline, highlyoxidized waters?

Oxygen richenvironments

Oxygen poorenvironments

hydrothermal minerals 2

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