Hydrothermal and Skarn
Hydrothermal mineral deposits are those in which hot, mineral laden water (hydrothermal solution) serves as a concentrating, transporting, and depositing agent. They are the most numerous of all classes of deposit. The solutions are thought to arise in most cases from the action of deeply circulating water heated by magma. Other sources of heating that may be involved include energy released by radioactive decay or by faulting of the Earth's crust.
The mineral deposit may be precipitated from the solution with or without demonstrable association with igneous processes. These waters may deposit their dissolved minerals in openings in the rock, thus filling the cavities, or they may replace the rocks themselves to form so-called replacement deposits. The two processes may occur simultaneously, the filling of an opening by precipitation accompanying the replacement of the walls of the opening.
Conditions necessary for the formation of hydrothermal ore deposits include:
Deposition is most affected by changes in the temperature and pressure: the solubility decreases, hence precipitation occurs as the temperature and pressure decrease. Although hydrothermal ore deposits may form in any host rock, deposition is influenced or localized by certain kinds of rock. For example, lead-zinc-silver ores in some parts of Mexico occur in dolomitic rather than pure limestone; the reverse is true at Santa Eulalia, where massive sulfide deposits end abruptly at the limestone-dolomite contact.
Origins of the Solutions
Hydrothermal deposits are never formed from pure water, because pure water is a poor solvent of most ore minerals. Rather, they are formed by hot brines, making it more appropriate to refer to them as products of hydrothermal solutions. Brines, and especially sodium-calcium chloride brines, are effective solvents of many sulfide and oxide ore minerals, and they are even capable of dissolving and transporting native metals such as gold and silver.
The water in a hydrothermal solution can come from any of several sources. It may be released by a crystallizing magma; it can be expelled from a mass of rock undergoing metamorphism; or it may originate at the Earth's surface as rainwater or seawater and then trickle down to great depths through fractures and porous rocks, where it will be heated, react with adjacent rocks, and become a hydrothermal solution. Connate waters, when set into motion by tectonic activity, may also constitute hydrothermal fluids.
During wet partial melting, the water that causes the melting is released when the magma solidifies. This water carries with it soluble constituents such as NaCl, as well as elements such as Au, Ag, Cu, Pb, Zn, Hg, and Mo that do not easily enter into the common minerals (e.g. quartz, feldspar) by ionic substitution.
Meteoric and seawater can also form hydrothermal solutions if they are heated sufficiently and a convection system is generated. The source of this heat is magmatic intrusions, so magma is a key ingredient in the generation of hydrothermal mineral deposits. Hydrothermal mineral deposits are thus associated with convergent and divergent plate boundaries.
Regardless of the origin and initial composition of the water, the final compositions of all hydrothermal solutions tend to converge, owing to reactions between solutions and the rocks they encounter.
Composition of the Solutions
The principle ingredient of hydrothermal solutions is water. Pure water, however, can not dissolve metals. Hydrothermal solutions are always brines, containing dissolved salts such as NaCl, KCl, CaSO4 and CaCl2. The range in salinity varies from that of seawater (around 3.5 wt %) to about ten times the salinity of seawater. Such brines are capable of dissolving small amounts of elements such as Au, Ag, Cu, Pb and Zn. High temperatures increase the effectiveness of the brines to dissolve metals.
Hydrothermal solutions are sodium-calcium chloride brines with additions of magnesium and potassium salts, plus small amounts of many other chemical elements. The solutions range in concentration from a few percent to as much as 50 percent dissolved solids by weight. Existing hydrothermal solutions can be studied at hot springs, in subsurface brine reservoirs such as those in the Imperial Valley of California or the Cheleken Peninsula on the eastern edge of the Caspian Sea in Turkmenistan, and in oil-field brines. Fossil hydrothermal solutions can be studied in fluid inclusions, which are tiny samples of solution trapped in crystal imperfections by a growing mineral.
Causes of Precipitation
Because hydrothermal solutions form as a result of many processes, they are quite common within the Earth's crust. Hydrothermal mineral deposits, on the other hand, are neither common nor very large compared to other geologic features. It is apparent from this that most solutions eventually mix in with the rest of the hydrosphere and leave few obvious traces of their former presence. Those solutions that do form mineral deposits (and thereby leave obvious evidence of their former presence) do so because some process causes them to deposit their dissolved loads in a restricted space or small volume of porous rock. It is most convenient, therefore, to discuss hydrothermal mineral deposits in the context of their settings.
Hot brines can hold in solution greater concentrations of metals than cold brines. As a hydrothermal solution moves upwards, it cools and the dissolved minerals precipitate out of solution. To be effective in generating sufficient mineralisation to form ore bodies, the process must be continuous over a large period of time, so a convection cell is required to maintain a constant precipitation.
If the upward movement is slow, the precipitation of the minerals would be spread over a wide area and may not be sufficiently concentrated to form an ore body. Sudden cooling, caused by rapid movement of the fluid into porous layers such as volcanic tephra or into open fractures such as veins and brecciated rocks, leads to rapid cooling and the rapid precipitation of minerals over a limited region.
Boiling, rapid pressure decrease, reactions with adjacent rock types, and mixing with seawater can also cause rapid precipitation and the concentration of mineral deposits.
Types of Deposits:
deposits are formed within a temperature range of 500 to 50°C. The modes of
formation are replacement and cavity filling. Lindgren divided this class into
three subclasses viz. (a) hypothermal, (b) mesothermal and (c) epithermal,
according to the temperature of formation of the minerals of each sub-class.
replacement deposits are formed at the higher end of the temperature range and
close to the intrusive. Most deposits of gold, silver, copper, lead and zinc,
mercury, antimony and molybdenum come under this class. Most deposits of minor
metals and many non-metallic minerals are formed by this process. Cr, Ti, V, Zr,
U, Ce, Ta and Pt are absent in deposits of this class.
replacement deposits have always an alteration zone surrounding the ore-bodies.
The nature of the alteration varies with the kind of enclosing rocks. The
different types of wall rock alteration characteristic of different sub-classes
of hydrothermal deposits may be summarised as follows :
Sericitization, silioification. and argillic alteration
Silicification, argillic alteration & alunitization.
The wall rock alterations have
often been used as a guide to ore-finding for where weathering has removed the
top of the ore-body, these alteration haloes serve as indicator of hidden
Hydrothermal Deposits Forming Today
In 1962, oil/gas drilling struck a 350°C brine at 1.5 km depth. As the brine flowed upwards and cooled, it deposited a siliceous scale. Over a period of 3 months, some 8 tons were precipitated, containing 20 wt % Cu and 8 wt % Ag. This was the first unambiguous evidence that mineral deposits can be formed from hydrothermal fluids.
In 1964, oceanographers discovered a sries of hot, dense brines at the bottom of the Red Sea. The higher density of the brines (i.e. increased sanility) means that they remain at the bottom of the sea, despite being hot. The sediments at the bottom of these pools contain ore minerals such as chalcopyrite, sphalerite and galena. The Red Sea is a stratabound mineral deposit in the making.
In 1978, deep-sea submarines on the East Pacific Rise, at 21°N, found 300°C hot springs emerging in plumes along the oceanic ridge, 2500 m below sea level. Minerals precipitated out of the solution as soon as it emerged, and around the vents was a blanket of sulphide minerals. This is the modern analogue of volcanogenic massive sulphide (VMS) deposits.
Skarns are generally thought of as being the result of contact metamorphism of impure limestone. Although the majority are found in lithologies containing at least some limestone, skarns can form during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. They are found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafloor, and at lower crustal depths in deeply buried metamorphic terrains. What links these diverse environments, and what defines a rock as skarn, is the mineralogy. This mineralogy includes a wide variety of calc-silicate and associated minerals but usually is dominated by garnet and pyroxene.
Skarns can be subdivided according to several criteria, the most common being their mineralogy and their enclosing rock types. Exoskarns are skarns developed in the sedimentary rocks surrounding the themal source (pluton). Endoskarns are those developed within the igneous intrusion. Magnesian and calcic skarn can be used to describe the dominant composition of the original rock and resulting skarn minerals. Such terms can be combined, as in the case of a magnesian exoskarn which contains forsterite-diopside skarn formed from dolostone.
vast majority of skarn deposits are associated with magmatic arcs related to
subduction beneath continental crust.
Types of Skarn Deposits
A descriptive skarn classification can be based on the dominant economic minerals.
The largest skarn deposits, with many over 500 milliion tonnes. They are mined for their magnetite. Minor amounts of Ni, Cu, Co and Au may be present, but typically only Fe is recovered. They are dominantly magnetite, with only minor silicate gangue.
Most gold skarns are associated with relatively mafic diorite - granodiorite plutons and dyke/sill complexes. Some large Fe or Cu skarns have Au in the distal zones. There is the potential that other skarn types have undiscovered precious metals if the entire system has not been explored.
These are found in association with calc-alkaline plutons in major orogenic belts. They are associated with coarse grained, equigranular batholiths (with pegmatite and aplite dykes), surrounded by high temperature metamorphic aureoles. This is indicative of a deep environment.
These are the world's most abundant type and are particularly common in orogenic zones related to subduction both in continental and oceanic settings. Most are associated with porphyritic plutons with co-genetic volcanic rocks, stockwork veining, brittle fracturing, brecciation and intense hydrothermal aleteration. These features are all indicative of a relatively shallow environment. The largest copper skarns can exceed 1 billion tonnes and are associated with porphyry copper deposits.
Most occur in continental settings associated either with subduction or rifting. They are also mined for lead and silver, and are high grade. They form in the distal zone to associated igneous rocks.
Most are associated with leucocratic (lacking ferromagnesian minerals) granites and form high graade, small deposits. other metals are also commonly associated, the most common being Mo-W-Cu skarns.
These are almost exclusively associated with high silica granites generated by partial melting of continental crust. Greisen alteration by fluorine produces a characteristic yellowish mica.
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