Hydrothermal minerals

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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 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? [...]... 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. ..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
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