Hydrothermal and Skarn Deposits

 Hydrothermal Deposits

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:

a)      Presence of hot water to dissolve and transport minerals

b)      Presence of interconnected openings in the rock to allow the solutions to move

c)      Availability of sites for the deposits, and

d)      Chemical reaction that will result in deposition

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:

The hydrothermal 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.  Mutation

The 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.

The 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 : ­

Hypothermal: Greisenization, Serpentinization

Mesothermal : Sericitization, silioification. and argillic alteration

Epithermal : 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 ore-bodies.

Hydrothermal Deposits Forming Today

1.      Imperial Valley, southern California

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.

  1. Red Sea

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.

  1. East Pacific Rise

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 Deposits

Skarn is an old Swedish mining term used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide.  In modern usage the term "skarn" has been expanded to refer to rocks containing  calcium-bearing silicate minerals. In America the term "tactite" is often used synonymously with skarn.

Skarns and tactites are most often formed at the contact zone between intrusions of granitic magma bodies into contact with carbonate sedimentary rocks such as limestone and rdolostone. Hot waters derived from the granitic magma are rich in silica, iron, aluminium, and magnesium. These fluids mix in the contact zone, dissolve calcium-rich carbonate rocks, and convert the host carbonate rock to skarn deposits in a metamorphic process known as "metasomatism". The resulting metamorphic rock may consist of a very wide variety of mineral assemblages dependent largely on the original composition of the magmatic fluids and the purity of the carbonate sedimentary rocks.

Typical skarn minerals include pyroxene, garnet, idocrase, wollastonite, actinolite, magnetite or hematite, and epidote. Because skarns are formed from incompatible-element rich, siliceous aqueous fluids a variety of uncommon mineral types are found in the skarn environment, such as: tourmaline, topaz, beryl,corundum, fluorite, apatite, barite, strontianite, tantalite, anglesite, and others. Often, feldspathoids and rare calc-silicates such as scapolite are found in more marginal areas.

Skarns are sometimes associated with mineable accumulations of metallic ores of iron, copper, iron, zinc, lead, gold, and several others. In such cases these deposits are called "skarn deposits".

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.

The 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.

1.   Iron Skarns

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.

2.   Gold Skarns

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.

3.   Tungsten Skarns

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.

4.   Copper Skarns

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.

5.   Zinc Skarns

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.

6.   Molybdenum Skarns

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.

7.   Tin 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.

 

Role of Replacement in the formation of Hydrothermal and Skarn Deposits

Replacement is the process of almost simultaneous solution and deposition whereby one mineral replaces another. It is an important process in the formation of epigenetic (those formed after the formation of the host rock) mineral deposits, in the formation of high- and intermediate-temperature hydrothermal ore deposits, and in supergene sulfide enrichment (enriched by generally downward moving fluids). Replacement is the method whereby wood petrifies (silica replaces the wood fibres), one mineral forms a pseudomorph of another, or an ore body takes the place of an equal volume of rock.  Hence replacement deposits are those in which solutions have reacted with host rocks, leading to replacement of silicates and carbonates by new gangue and ore minerals.

Replacement occurs when a mineralizing solution encounters minerals unstable in its presence. The original mineral is dissolved and almost simultaneously exchanged for another. The exchange does not occur molecule for molecule, but volume for volume.  This means that  fewer molecules of a less dense mineral will be replaced by more molecules of a denser mineral.  Replacement takes place first along major channels in a host rock through which the hydrothermal fluids enter it.  Replacement along smaller openings follows later.  Eventually, rocks or at least some of their constituent minerals are replaced even along capillary sized openings.  Replacement occurs even in those parts of the rock where fluids cannot flow.  This happens by diffusion of ions at the replacement front.  

Early-formed replacement minerals are themselves replaced by later minerals, and definite mineral successions have been established. The usual sequence among the common hypogene (deposited by generally ascending solutions) metallic sulfide minerals is pyrite, enargite, tetrahedrite, sphalerite, chalcopyrite, bornite, galena, and pyrargyrite.

Although replacement can occur at any temperature or pressure, it is most effective at elevated temperatures, at which chemical activity is enhanced. Replacement by cold circulating waters is mostly confined to soluble rocks, such as limestone. These may be replaced by iron oxides, manganese oxides, or calcium phosphates. 

Many ore deposits have been enriched in terms of their metal content where supergene (downward moving) fluids have dissolved metallic minerals from the near-surface parts and redeposited these in the lower parts by replacement of other (gangue) minerals.  At higher temperatures the degree of replacement of preexisting minerals increases, till in extreme cases, there is complete replacement.  Mineralizing solutions at intermediate temperatures form simple sulfides and sulfosalts while those at higher temperatures form sulfides and oxides. Replacement deposits are the largest and most valuable of all metallic ore deposits except those of iron.  Large deposits of lead-zinc deposits have formed where carbonates rocks (limestone, marl, dolomite) have been replaced.  The orebodies range in size from 0.5 million tonnes to 20 million tonnes with over 14 % combined metal content.  Replacement deposits commonly show an envelope of weak mineralization around the ore, giving a larger dispersion pattern and therefore a larger exploration target.  The dispersion is controlled by degree of disequilibrium between fluids and host rocks and level of diffusion/interaction.  This also gives rise to zoning of replacement deposits.

Replacement deposits commonly occur as massive (containing more than 50 % sulfides) lenses, pipes, veins and disseminations often associated with intrusives

In general, it has been observed that certain minerals replace others preferentially. Accordingly, a set of "rules" has been proposed:

a.      Sulfides replace gangue or ore minerals

b.     Gangue minerals replace host rock, but not the ore minerals

c.      Oxides replace host rock and gangue, but rarely replace sulfides.

Evidence of Replacment:

1.      Pseudomorphs.

2.      Widening of fractures

3.      Vermicular unoriented intergrowths

4.      Islands (of the host or replaced mineral) having the same optical orientation and surrounded by the new mineral

5.      Relicts

6.      Cusp and caries texture: (host or replaced mineral). Cusps are relict protuberances of the replaced mineral or host rock between “caries”. The caries are embayed surfaces concave towards the replacing mineral into the replaced one.

7.      Non-matching walls of a fracture. This is a feature common when replacement works outward from a central fissure (compare with open space filling textures) (Fig. 6).

8.      The occurrence of one mineral crosscutting older structures.

9.      Selective association: Since replacement is a chemical process, specific selective associations of pairs or combinations of minerals can be expected. For example, chalcopyrite is more likely to replace bornite by a change in the Cu/Fe ratio or in fS2 than it is to replace quartz. Therefore, the occurrence of minerals with some chemical similarity in some textural relationship is often a good indication of replacement.

 

Role of Colloidal Deposition in Hydrothermal Processes

A colloidal system consists of two phases:

  1. The dispersed phase - is the diffused phase

  2. The dispersion medium - in which the diffused phase is dispersed.

Colloidal particles range in size from those in true solutions to those in coarse suspension.  The limit of size are 10-3 to 10-7 cm (> solution < coarse suspension). The colloidal material may be solid, liquid or gas and may be dispersed in one of these same phases. In the study of ore transport, we are concerned with solids suspended in liquids or a gaseous medium.

A colloidal system consisting of solids dispersed in a liquid is called a Sol. Colloidal particles have large surface areas per unit volume.  Ions absorbed on the surfaces of such particles control their behaviour. If the particles absorb cations they become positively charged, if they absorb anions they become negatively charged. These charges prevent the particles of the sol from coagulating or flocculating, but if an electrolyte is added the particles neutralize and flocculate.

Most sulfides and organic sols are negative, whereas most oxide and hydroxide sols are positive.  There are some exceptions, eg. colloidal silica is negative. Colloids are most stable in cool, dilute solutions and in the presence of a second (protective) colloid. Egcolloidal gold is stable below 150oC and coagulates between 150-250oC.  In the presence of colloidal silica this colloidal gold is stable upto 350oC.

It is difficult to explain the colloidal migration of metals in depth because the rocks at depth are dense and relatively impermeable. Some geologists suggest that ore fluids change from solutions at depth to colloidal sols in the near surface environments.

Precipitation from colloids involves two distinct stages:

1.     Nucleation (the formation of centres of crystallisation), and

2.     Crystal growth

Leaving aside the question of stability, it is the relative rates of these processes, which determine the particle size of the precipitate so formed. A high degree of dispersion and large number of crystals is obtained when the rate of nucleation is high and the rate of crystal growth is low.

The initial rate of nucleation depends on the degree of supersaturation, which can be reached before phase separation occurs, so that colloidal sols are most easily obtained when the substance in question has a very low solubility. With material as soluble as, for example, calcium carbonate, there is a tendency for the smaller particles to dissolve and recrystallize on the larger particles as the precipitate is allowed to age.

The rate of particle growth depends mainly on the following factors:

1.     The amount of material available.

2.     The viscosity of the medium, which controls the rate of diffusion of material to the particle surface.

3.     The ease with which the material is correctly orientated and incorporated into the crystal lattice of the particle.

4.     Adsorption of impurities on the particle surface, which act as growth inhibitors.

5.     Particle-particle aggregation.

Evidence of precipitation from colloids is furnished by certain minerals which form colloform textures that are indicative of flocculation from a sol. Colloform textures have the following characteristics:

1.   Colloform textures occur in a series of concentrically curved or scalloped layers in which the curvature is always convex towards the younger or free surface.

2.     The free surface is botryoidal, reniform or stalactitic.

3.   Colloform textures are typically exhibited by agate, chalcedony, malachite, azurite, collophane, manganese oxides, spodument, wurtzite, lead and zinc ores, pyrite, cassiterite and wolframite.

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