Skarn Deposits
Skarn Deposits
Adapted to HTML from notes of Larry Meinert and Dr. Kenneth Dawson, Ph.D., P.Geo Tera Geological Consultants from The Prospecting School on the Websource http://www.science.smith.edu/geosciences/skarn/aboutskarn.html
Definitions
Skarn Mineralogy
Evolution of skarns in time and space
Fe,Au, Cu, Fe, Mo, Sn, W, and Zn-Pb skarn deposits
Fe skarn
Au skarn
Cu skarn
Mo skarn
Sn skarn
W skarn
and Zn-Pb skarn
Zonation of skarn deposits
Geochemistry of skarn deposits
Geophysics
Uranium-Thorium as indicators
Petrogenesis and tectonic settings of skarn deposits
Definitions:
There are many definitions and usages of the word "skarn".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. Exoskarn and endoskarn are common terms used to indicate a sedimentary or igneous protolith, respectively.
Magnesian and calcic skarn can be used to describe the dominant composition of the protolith 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.Calc-silicate hornfels is a descriptive term often used for the relatively fine-grained calc-silicate rocks that result from metamorphism of impure carbonate units such as silty limestone or calcareous shale.
Reaction skarns can form from isochemical metamorphism of thinly interlayered shale and carbonate units where metasomatic transfer of components between adjacent lithologies may occur on a small scale (perhaps centimetres) (e.g. Vidale, 1969; Zarayskiy et al., 1987). Skarnoid is a descriptive term for calc-silicate rocks which are relatively fine-grained, iron-poor, and which reflect, at least in part, the compositional control of the protolith (Korzkinskii, 1948; Zharikov, 1970). Genetically, skarnoid is intermediate between a purely metamorphic hornfels and a purely metasomatic, coarse-grained skarn.
For all of the preceding terms, the composition and texture of the protolith tend to control the composition and texture of the resulting skarn. In contrast, most economically important skarn deposits result from large scale metasomatic transfer, where fluid composition controls the resulting skarn and ore mineralogy.
This is the mental image that most people share of a "classic" skarn deposit. Ironically, in the "classic" skarn locality described by Tornebohm at Persberg, skarn has developed during regional metamorphism of a mostly calcareous Proterozoic iron formation. This reinforces the importance of Einaudi et al.'s (1981) warning that the words "skarn" and "skarn deposits" be used strictly in a descriptive sense, based upon documented mineralogy, and free of genetic interpretations.
Not all skarns have economic mineralisation; skarns which contain ore are called skarn deposits. In most large skarn deposits, skarn and ore minerals result from the same hydrothermal system even though there may be significant differences in the time/space distribution of these minerals on a local scale. Although rare, it is also possible to form skarn by metamorphism of pre-existing ore deposits as has been suggested for Aguilar, Argentina (Gemmell et al., 1992), Franklin Furnace, USA (Johnson et al., 1990), and Broken Hill, Australia (Hodgson, 1975).
Skarn Mineralogy
Just as mineralogy is the key to recognizing and defining skarns, it is also critical in understanding their origin and in distinguishing economically important deposits from interesting but uneconomic mineral localities.Skarn mineralogy is mappable in the field and serves as the broader "alteration envelope" around a potential ore body.
Because most skarn deposits are zoned, recognition of distal alteration features can be critically important in the early exploration stages.
Details of skarn mineralogy and zonation can be used to construct deposit-specific exploration models as well as more general models useful in developing grass roots exploration programs or regional syntheses. Although many skarn minerals are typical rock-forming minerals, some are less abundant and most have compositional variations which can yield significant information about the environment of formation.
Some minerals, such as quartz and calcite, are present in almost all skarns. Other minerals, such as humite, periclase, phlogopite, talc, serpentine, and brucite are typical of magnesian skarns but are absent from most other skarn types. Additionally, there are many tin, boron, beryllium, and fluorine-bearing minerals which have very restricted, but locally important, parageneses.
The advent of modern analytical techniques, particularly the electron microprobe, makes it relatively easy to determine accurate mineral compositions and consequently, to use precise mineralogical names.
However, mineralogical names should be used correctly so as not to imply more than is known about the mineral composition. For example, the sequence pyroxene, clinopyroxene, calcic clinopyroxene, diopsidic pyroxene, and diopside, are increasingly more specific terms.
Unfortunately, it is all too common in the geologic literature for specific end member terms, such as diopside, to be used when all that is known about the mineral in question is that it might be pyroxene.
Zharikov (1970) was perhaps the first to describe systematic variations in skarn mineralogy among the major skarn classes. He used phase equilibria, mineral compatibilities, and compositional variations in solid solution series to describe and predict characteristic mineral assemblages for different skarn types. His observations have been extended by Burt (1972) and Einaudi et al. (1981) to include a wide variety of deposit types and the mineralogical variations between types. The minerals which are most useful for both classification and exploration are those, such as garnet, pyroxene, and amphibole, which are present in all skarn types and which show marked compositional variability. For example, the manganiferous pyroxene, johannsenite, is found almost exclusively in zinc skarns. Its presence, without much further supporting information, is definitive of this skarn type.
When compositional information is available, it is possible to denote a mineral's composition in terms of mole percent of the end members. For example, a pyroxene which contains 70 mole percent hedenbergite, 28 mole percent diopside, and 2 mole percent johannsenite could be referred to as Hd70Di28Jo2. In many skarn systems, variation in iron content is the most important parameter and thus, many minerals are described simply by their iron end member, e.g. Hd10 or Ad90. Large amounts of compositional information can be summarized graphically. Triangular plots commonly are used to express variations in compositionally complex minerals such as garnet and pyroxene.
Amphiboles are more difficult to portray graphically because they have structural as well as compositional variations. The main differences between amphiboles in different skarn types are variations in the amount of Fe, Mg, Mn, Ca, Al, Na, and K. Amphiboles from Au, W, and Sn skarns are progressively more aluminous (actinolite-hastingsite-hornblende), amphiboles from Cu, Mo, and Fe skarns are progressively more iron-rich in the tremolite-actinolite series, and amphiboles from zinc skarns are both Mn-rich and Ca-deficient, ranging from actinolite to dannemorite. For a specific skarn deposit or group of skarns, compositional variations in less common mineral phases, such as idocrase, bustamite, or olivine, may provide insight into zonation patterns or regional petrogenesis (e.g. Giere, 1986; Agrell and Charnely, 1987; Silva and Siriwardena, 1988; Benkerrou and Fonteilles, 1989).
Evolution of skarns in time and space
As was recognised by early skarn researchers (e.g. Lindgren 1902; Barrell, 1907; Goldschmidt, 1911; Umpleby, 1913; Knopf, 1918), formation of a skarn deposit is a dynamic process.
In most large skarn deposits there is a transition from early/distal metamorphism resulting in hornfels, reaction skarn, and skarnoid, to later/proximal metasomatism resulting in relatively coarse-grained ore-bearing skarn. Due to the strong temperature gradients and large fluid circulation cells caused by intrusion of a magma (Norton, 1982; Salemink and Schuiling, 1987; Bowers et al., 1990), contact metamorphism can be considerably more complex than the simple model of isochemical recrystallisation typically invoked for regional metamorphism. For example, circulating diverse fluids through a fracture in a relatively simple carbonate protolith can result in several different reactions. Thus, the steep thermal gradients common in most plutonic environments, result in complex metamorphic aureoles complete with small-scale metasomatic transfer as evidenced by reaction skarns and skarnoid.
More complex metasomatic fluids, with the possible addition of magmatic components such as Fe, Si, Cu, etc. , produce a continuum between purely metamorphic and purely metasomatic processes.
This early metamorphism and continued metasomatism at relatively high temperature (Wallmach and Hatton, 1989, describe temperatures > 1200C) are followed by retrograde alteration as temperatures decline. A link between space and time is a common theme in ore deposits and requires careful interpretation of features which may appear to occur only in a particular place (e.g. Barton et al., 1991).
One of the more fundamental controls on skarn size, geometry, and style of alteration is the depth of formation. Quantitative geobarometric studies typically use mineral equilibria (Anovitz and Essene, 1990), fluid inclusions (Guy et al., 1989) or a combination of such methods (Hames et al., 1989) to estimate the depth of metamorphism. Qualitative methods include stratigraphic or other geologic reconstructions and interpretation of igneous textures. Simple observations of chilled margins, porphyry groundmass grain size, pluton morphology, and presence of brecciation and brittle fracture allow field distinctions between relatively shallow and deep environments.
The effect of depth on metamorphism is largely a function of the ambient wall rock temperature prior to, during, and post intrusion. Assuming an average geothermal gradient for an orogenic zone of about 35C per kilometre (Blackwell et al., 1990), the ambient wall rock temperature prior to intrusion at 2 km would be 70C, whereas at 12 km it would be 420C. Thus, with the added heat flux provided by local igneous activity, the volume of rock affected by temperatures in the 400-700C range would be considerably larger and longer lived surrounding a deeper skarn than a shallower one. In addition, higher ambient temperatures could affect the crystallisation history of a pluton as well as minimise the amount of retrograde alteration of skarn minerals.
At a depth of 12 km with ambient temperatures around 400C, skarn may not cool below garnet and pyroxene stability without subsequent uplift or other tectonic changes. The greater extent and intensity of metamorphism at depth can affect the permeability of host rocks and reduce the amount of carbonate available for reaction with metasomatic fluids. An extreme case is described by Dick and Hodgson (1982) at Cantung, Canada, where the "Swiss cheese limestone" was almost entirely converted to a heterogeneous calc-silicate hornfels during metamorphism prior to skarn formation. The skarn formed from the few remaining patches of limestone has some of the highest known grades of tungsten skarn ore in the world (Mathiason and Clark, 1982).
The depth of skarn formation also will affect the mechanical properties of the host rocks. In a deep skarn environment, rocks will tend to deform in a ductile manner rather than fracture. Intrusive contacts with sedimentary rocks at depth tend to be sub-parallel to bedding; either the pluton intrudes along bedding planes or the sedimentary rocks fold or flow until they are aligned with the intrusive contact. Examples of skarns for which depth estimates exceed 5-10 km include Pine Creek, California (Brown et al., 1985) and Osgood Mountains, Nevada (Taylor, 1976).
In deposits such as these, where intrusive contacts are sub-parallel to bedding planes, skarn is usually confined to a narrow, but vertically extensive, zone. At Pine Creek skarn is typically less than 10 m wide but locally exceeds one kilometre in length and vertical extent (Newberry, 1982).
Thus, skarn formed at greater depths can be seen as a narrow rind of small size relative to the associated pluton and its metamorphic aureole. In contrast, host rocks at shallow depths will tend to deform by fracturing and faulting rather than folding. In most of the 13 relatively shallow skarn deposits reviewed by Einaudi (1982a), intrusive contacts are sharply discordant to bedding and skarn cuts across bedding and massively replaces favourable beds, equalling or exceeding the (exposed) size of the associated pluton.
The strong hydrofracturing associated with shallow level intrusions greatly increases the permeability of the host rocks, not only for igneous-related metasomatic fluids, but also for later, possibly cooler, meteoric fluids (Shelton, 1983). The influx of meteoric water and the consequent destruction of skarn minerals during retrograde alteration is one of the distinctive features of skarn formation in a shallow environment.
The shallowest (and youngest) known skarns are presently forming in active geothermal systems (McDowell and Elders, 1980; Cavarretta et al., 1982; Cavarretta and Puxeddu, 1990) and hot spring vents on the seafloor (Zierenberg and Shanks, 1983). These skarns represent the distal expression of magmatic activity and exposed igneous rocks (in drill core) are dominantly thin dikes and sills with chilled margins and a very fine grained to aphanitic groundmass.
The degree to which a particular alteration stage is developed in a specific skarn will depend on the local geologic environment of formation. For example, metamorphism will likely be more extensive and higher grade around a skarn formed at relatively great crustal depths than one formed under shallower conditions.
Conversely, retrograde alteration during cooling, and possible interaction with meteroric water, will be more intense in a skarn formed at relatively shallow depths in the earth's crust compared with one formed at greater depths. In the deeper skarns carbonate rocks may deform in a ductile manner rather than through brittle fracture, with bedding parallel to the intrusive contact; in shallower systems the reverse may be true.
These differences in structural style will in turn affect the size and morphology of skarn. Thus, host rock composition, depth of formation, and structural setting will all cause variations from the idealised "classic" skarn model.
Au, Cu, Fe, Mo, Sn, W, and Zn-Pb skarn deposits
Groupings of skarn deposits can be based on descriptive features such as:- protolith composition,
- rock type,
- and dominant economic metal(s)
- mechanism of fluid movement,
- temperature of formation,
- and extent of magmatic involvement.
This is similar to the classification of porphyry deposits into porphyry copper, porphyry molybdenum, and porphyry tin types; deposits which share many alteration and geochemical features but are, nevertheless, easily distinguishable. Seven major skarn types (Au, Cu, Fe, Mo, Sn, W, and Zn-Pb) have received significant modern study and several others (including F, C, Ba, Pt, U, REE) are locally important. In addition, skarns can be mined for industrial minerals such as garnet and wollastonite
.
Major reviews of this deposit type include
Sangster
(1969), Sokolov and Grigorev (1977), and Einaudi et al. (1981).
Iron Skarns
The largest skarn deposits are the iron skarns. Iron skarns are mined for their magnetite content and although minor amounts of Cu, Co, Ni, and Au may be present, iron is typically the only commodity recovered (Grigoryev et al., 1990). Many deposits are very large (>500 million tons, >300 million tons contained Fe) and consist dominantly of magnetite with only minor silicate gangue. Some deposits contain significant amounts of copper and are transitional to more typical copper skarns (e.g. Kesler, 1968; Vidal et al., 1990).Calcic iron skarns in oceanic island arcs are associated with iron-rich plutons intruded into limestone and volcanic wall rocks.
In some deposits, the amount of endoskarn may exceed exoskarn. Skarn minerals consist dominantly of garnet and pyroxene with lesser epidote, ilvaite, and actinolite; all are iron-rich (Purtov et al., 1989). Alteration of igneous rocks is common with widespread albite, orthoclase, and scapolite veins and replacements, in addition to endoskarn.
In contrast, magnesian iron skarns are associated with diverse plutons in a variety of tectonic settings; the unifying feature is that they all form from dolomitic wall rocks. In magnesian skarns, the main skarn minerals, such as forsterite, diopside, periclase, talc, and serpentine, do not contain much iron; thus, the available iron in solution tends to form magnetite rather than andradite or hedenbergite (e.g. Hall et al., 1989).
Overprinting of calcic skarn upon magnesian skarn is reported from many Russian deposits (Sokolov and Grigorev, 1977; Aksyuk and Zharikov, 1988). In addition, many other skarn types contain pockets of massive magnetite which may be mined for iron on a local scale (e.g. Fierro area, New Mexico, Hernon and Jones, 1968). Most of these occurrences form from dolomitic strata or from zones that have experienced prior magnesian metasomatism (e.g. Imai and Yamazaki, 1967).
Gold Skarns
Although gold skarns had been mined since the late 1800s (Hedley district, British Columbia,Billingsley and Hume, 1941), there was so little published about them until recently that they were not included in the major world review of skarn deposits by Einaudi et al. (1981). In the past decade, multiple gold skarn discoveries have prompted new scientific studies and several overview papers, (Meinert, 1989; Ray et al., 1989; Theodore et al.,1991). A particularly useful WWW site on gold skarns has been created by Gerry Ray (1996). The highest grade (5-15 g/t Au) gold skarn deposits (e.g. Hedley district, Ettlinger, 1990; Ettlinger et al., 1992; Fortitude, Nevada, Myers and Meinert, 1991) are relatively reduced, are mined solely for their precious metal content, and lack economic concentrations of base metals.
Other gold skarns (e.g. McCoy, Nevada, Brooks et al., 1991) are more oxidized, have lower gold grades (1-5 g/t Au), and contain subeconomic amounts of other metals such as Cu, Pb, and Zn. Several other skarn types, particularly Cu skarns, contain enough gold (0.01->1 g/t Au) for it to be a byproduct. A few skarn deposits, although having economic base metal grades, are being mined solely for their gold content (e.g. Veselyi mine, USSR, Ettlinger and Meinert, 1991).
Most high grade gold skarns are associated with reduced (ilmenite-bearing, Fe+3/Fe+2 < 0.75) diorite-granodiorite plutons and dike/sill complexes. Such skarns are dominated by iron-rich pyroxene (typically > Hd50); proximal zones can contain abundant intermediate grandite garnet.
Other common minerals include potassium feldspar, scapolite, idocrase, apatite, and high-chlorine aluminous amphibole. Distal/early zones contain biotite+potassium feldspar hornfels which can extend for 100s of metres beyond massive skarn. Due to the clastic-rich, carbonaceous nature of the sedimentary rocks in these deposits, most skarn is relatively fine-grained.
Some gold skarns contain unusual late prehnite or wollastonite retrograde alteration (Ettlinger, 1990). Arsenopyrite and pyrrhotite are the dominant sulphide minerals at Hedley and Fortitude, respectively. Most gold is present as electrum and is strongly associated with various bismuth and telluride minerals including native bismuth, hedleyite, wittichenite, and maldonite.
The Fortitude deposit is part of a large zoned skarn system in which the proximal garnet-rich part was mined for copper (Theodore and Blake, 1978). Similarly, the Crown Jewel gold skarn in Washington is the pyroxene-rich distal portion of a large skarn system in which the proximal part is garnet-rich and was mined on a small scale for iron and copper (Hickey, 1990). Such zoned skarn systems suggest that other skarn types may have undiscovered precious metal potential if the entire skarn system has not been explored (e.g. Solar et al., 1990).
Tungsten Skarns
Tungsten skarns are found on most continents in association with calc-alkaline plutons in major orogenic belts. Major reviews of tungsten skarns include Newberry and Einaudi (1981), Newberry and Swanson (1986), and Kwak (1987). As a group, tungsten skarns are associated with coarse-grained, equigranular batholiths (with pegmatite and aplite dikes) surrounded by large, high-temperature, metamorphic aureoles. These features are collectively indicative of a deep environment. Plutons are typically fresh with only minor myrmekite and plagioclase-pyroxene endoskarn zones near contacts.The high-temperature metamorphic aureoles common in the tungsten skarn environment contain abundant calc-silicate hornfels, reaction skarns, and skarnoid formed from mixed carbonate-pelite sequences. Such metamorphic calc-silicate minerals reflect the composition and texture of the protolith and can be distinguished from ore-grade metasomatic skarn in the field and in the laboratory.
Newberry and Einaudi (1981) divided tungsten skarns into two groups : reduced and oxidized types, based on host rock composition (carbonaceous versus hematitic), skarn mineralogy (ferrous versus ferric iron), and relative depth (metamorphic temperature and involvement of oxygenated groundwater). Early skarn assemblages in reduced tungsten skarns are dominated by hedenbergitic pyroxene and lesser grandite garnet with associated disseminated fine-grained, molybdenum-rich scheelite (powellite). Later garnets are subcalcic (Newberry, 1983) with significant amounts (up to 80 mole %) of spessartine and almandine. This subcalcic garnet is associated with leaching of early disseminated scheelite and redeposition as coarse-grained, often vein-controlled, low-molybdenum scheelite. It is also associated with the introduction of sulphides, such as pyrrhotite, molybdenite, chalcopyrite, sphalerite, and arsenopyrite, and hydrous minerals such as biotite, hornblende, and epidote.
In oxidized tungsten skarns, andraditic garnet is more abundant than pyroxene, scheelite is molybdenum-poor, and ferric iron phases are more common than ferrous phases. For example at the Springer deposit in Nevada, garnet is abundant and has andraditic rims, pyroxene is diopsidic (Hd0-40), epidote is the dominant hydrous mineral, pyrite is more common than pyrrhotite, and subcalcic garnet is rare to absent (Johnson and Keith, 1991). In general, oxidized tungsten skarns tend to be smaller than reduced tungsten skarns, although the highest grades in both systems typically are associated with hydrous minerals and retrograde alteration.
Copper Skarns
Copper skarns are perhaps the worlds most abundant skarn type. They are
particularly common in orogenic zones related to subduction, both in
oceanic and continental settings. Major reviews of copper skarns include
Einaudi et al. (1981) and Einaudi (1982a,b). Most copper skarns are
associated with I-type, magnetite series, calc-alkaline, porphyritic
plutons, many of which have co-genetic volcanic rocks, stockwork veining,
brittle fracturing and brecciation, and intense hydrothermal alteration.
These are all features indicative of a relatively shallow environment of
formation. Most copper skarns form in close proximity to stock contacts
with a relatively oxidized skarn mineralogy dominated by andraditic
garnet. Other phases include diopsidic pyroxene, idocrase, wollastonite,
actinolite, and epidote.Hematite and magnetite are common in most deposits and the presence of dolomitic wall rocks is coincident with massive magnetite lodes which may be mined on a local scale for iron. As noted by Einaudi et al. (1981), copper skarns commonly are zoned with massive garnetite near the pluton and increasing pyroxene and finally idocrase and/or wollastonite near the marble contact. In addition, garnet may be colour zoned from proximal dark reddish-brown to distal green and yellow varieties. sulfide mineralogy and metal ratios may also be systematically zoned relative to the causative pluton. In general, pyrite and chalcopyrite are most abundant near the pluton with increasing chalcopyrite and finally bornite in wollastonite zones near the marble contact. In copper skarns containing monticellite (e.g. Ertsberg, Irian Jaya, Indonesia, Kyle et al., 1991; Maid of Erin, British Columbia, Meinert unpub. data) bornite-chalcocite are the dominant Cu-Fe sulfides rather than pyrite-chalcopyrite.
The largest copper skarns are associated with mineralized porphyry copper plutons. These deposits can exceed 1 billion tons of combined porphyry and skarn ore with more than 5 million tons of copper recoverable from skarn. The mineralized plutons exhibit characteristic potassium silicate and sericitic alteration which can be correlated with prograde garnet-pyroxene and retrograde epidote-actinolite, respectively, in the skarn. Intense retrograde alteration is common in copper skarns and in some porphyry-related deposits may destroy most of the prograde garnet and pyroxene (e.g. Ely, Nevada; James, 1976).
Endoskarn alteration of mineralized plutons is rare. In contrast, barren stocks associated with copper skarns contain abundant epidote-actinolite- chlorite endoskarn and less intense retrograde alteration of skarn. Some copper deposits have coarse-grained actinolite-chalcopyrite-pyrite-magnetite ores but contain only sparse prograde garnet-pyroxene skarn (e.g. Monterrosas and Ral-Condestable deposits, Peru: Ripley and Ohmoto, 1977; Sidder, 1984; Vidal et al., 1990; Record mine, Oregon, Caffrey, 1982; Cerro de Mercado, Mexico, Lyons, 1988). These deposits provide a link between some copper and iron skarns and deposits with volcanogenic and orthomagmatic affinities.
Zinc skarns
Most zinc skarns occur in continental settings associated with either subduction or rifting. They are mined for ores of zinc, lead, and silver although zinc is usually dominant. They are also high grade (10-20% Zn+ Pb, 30-300 g/t Ag). Related igneous rocks span a wide range of compositions from diorite through high-silica granite. They also span diverse geological environments from deep-seated batholiths to shallow dike-sill complexes to surface volcanic extrusions. The common thread linking most zinc skarn ores is their occurrence distal to associated igneous rocks. Major reviews of zinc skarn deposits include Einaudi et al. (1981) and Megaw et al. (1988).Zinc skarns can be subdivided according to several criteria including distance from magmatic source, temperature of formation, relative proportion of skarn and sulfide minerals, and geometric shape of the ore body. None of these criteria are entirely satisfactory because a magmatic source cannot be identified for some deposits, because most skarns develop over a range of temperatures, and because most large skarn deposits contain both skarn-rich ores and skarn-poor ores in a variety of geometric settings including mantos and chimneys.
Megaw et al. (1988) make the important point that many zinc skarn districts:
"grade outward from intrusion-associated mineralisation to intrusion-free ores, which suggests that those districts lacking known intrusive relationships may not have been traced to their ends".
Similarly, most zinc skarn districts grade outward from skarn-rich mineralisation to skarn-poor ores, veins, and massive sulfide bodies which may contain few if any skarn minerals. Incompletely explored districts may only have some of these zones exposed. But as previously noted, the presence of skarn minerals, such as garnet and pyroxene within the system, is important because it indicates a restricted geochemical environment which is entirely distinct from ore types, such as Mississippi Valley-type deposits, which also contain Zn-Pb-Ag ores but which absolutely lack skarn minerals.
Besides their Zn-Pb-Ag metal content, zinc skarns can be distinguished from other skarn types by their distinctive manganese- and iron-rich mineralogy, by their occurrence along structural and lithologic contacts, and by the absence of significant metamorphic aureoles centered on the skarn. Almost all skarn minerals in these deposits can be enriched in manganese including garnet, pyroxene, olivine, ilvaite, pyroxenoid, amphibole, chlorite, and serpentine.
In some deposits, the pyroxene:garnet ratio and the manganese content of pyroxene increase systematically along the fluid flow path (e.g. Groundhog, New Mexico, Meinert, 1987). This feature has been used to identify proximal and distal skarns and proximal and distal zones within individual skarn deposits. A typical zonation sequence from proximal to distal is: altered/endoskarned pluton, garnet, pyroxene, pyroxenoid, and sulfide/oxide replacement bodies (sometimes called mantos and chimneys based upon geometry and local custom).
The occurrence of zinc skarns in distal portions of major magmatic/hydrothermal systems may make even small deposits potentially useful as exploration guides in poorly exposed districts. Thus, reports of manganese-rich mineral occurrences may provide clues to districts that have not yet received significant exploration activity.
Molybdenum skarns
Most molybdenum skarns are associated with leucocratic granites and range from high grade, relatively small deposits (Azegour, Morocco, Permingeat, 1957) to low grade, bulk tonnage deposits (Little Boulder Creek, Idaho, Cavanaugh, 1978). Numerous small occurrences are also found in Precambrian stable cratons associated with pegmatite, aplite, and other leucocratic rocks (Vokes, 1963). Most molybdenum skarns contain a variety of metals including W, Cu, Zn, Pb, Bi, Sn, and U and some are truly polymetallic in that several metals need to be recovered together in order for the deposits to be mined economically. Mo-W-Cu is the most common association and some tungsten skarns and copper skarns contain zones of recoverable molybdenum.Most molybdenum skarns occur in silty carbonate or calcareous clastic rocks; Cannivan Gulch,Montana (Darling, 1990) is a notable exception in that it occurs in dolomite. Hedenbergitic pyroxene is the most common calc-silicate mineral reported from molybdenum skarns with lesser grandite garnet (with minor pyralspite component), wollastonite, amphibole, and fluorite. This skarn mineralogy indicates a reducing environment with high fluorine activities. These deposits have not received significant study outside of the Soviet Union and there has not been a modern review since the brief summary by Einaudi et al. (1981).
Tin skarns
Tin skarns are almost exclusively associated with high-silica granites generated by partial melting of continental crust. Major reviews of tin skarn deposits include Einaudi et al. (1981) and Kwak (1987). Tin skarns can be subdivided according to several criteria including proximal versus distal, calcic versus magnesian, skarn-rich versus skarn-poor, oxide-rich versus sulfide-rich, and greisen versus skarn. Unfortunately, few of these categories are mutually exclusive.Many large tin skarn systems are zoned spatially from skarn-rich to skarn-poor (or absent). For example, in the Renison Bell area of Tasmania, Australia there is a single large magmatic/hydrothermal system zoned from a proximal calcic tin skarn with minor cassiterite disseminated in a sulfide-poor garnet-pyroxene gangue to a distal magnesian massive sulfide replacement body containing abundant cassiterite and a complete absence of calc-silicate minerals. The distal massive sulfide ore body (Renison Bell) is a major ore deposit and the proximal skarn body (Pine Hill) has not and probably never will be mined.
Einaudi et al. (1981) emphasized that there is a common thread linking the several types of tin skarn deposits and that is the characteristic suite of trace elements (Sn, F, B, Be, Li, W, Mo, and Rb) in the ore and in associated igneous rocks. This suite distinguishes tin skarns from all other skarn types. Kwak (1987) makes a further distinction in that many tin skarn deposits develop a greisen alteration stage which is superimposed upon the intrusion, early skarn, and unaltered carbonate.
Greisen alteration is characterized by high fluorine activities and the presence of minerals like fluorite, topaz, tourmaline, muscovite, grunerite, ilmenite, and abundant quartz. In many cases this greisen-stage alteration completely destroys earlier alteration stages. Of particular importance, greisen-style alteration is absent from all other skarn types.
There are several mineralogical features of tin skarns that should be highlighted. From a mining standpoint, the most important is that tin can be incorporated into silicate minerals, such as garnet, sphene, and idocrase, where it is economically unrecoverable. Dobson (1982) reports garnet containing up to 6% Sn in skarn at Lost River, Alaska. Thus, large deposits such as Moina in Tasmania (Kwak and Askins, 1981), can contain substantial amounts of tin that cannot be recovered with present or foreseeable technology.
Extensive retrograde or greisen alteration of early tin-bearing skarn minerals can liberate this tin and cause it to precipitate in oxide or sulfide ore. Thus, the skarn destructive stages of alteration are particularly important in tin skarn deposits. As noted by Kwak (1987), the most attractive ore bodies occur in the distal portions of large skarn districts where massive sulfide or oxide replacements occur without significant loss of tin in calc-silicate minerals like garnet.
Other skarn types
There are many other types of skarn which historically have been mined or explored for a variety of metals and industrial minerals. Some of the more interesting include rare earth element enriched skarns (e.g. Kato, 1989). REEs tend to be enriched in specific mineral phases such as garnet, idocrase, epidote, and allanite. Vesuvianite and epidote with up to 20% REE (Ce>La>Pr>Nd) have been found in some gold skarns and zinc skarns (Gemmel et al., 1992; Meinert, unpublished data).Some skarns contain economic concentrations of REEs and uranium (Kwak and Abeysinghe, 1987; Lentz, 1991). The Mary Kathleen skarn deposit in Queensland, Australia is unusual in that REEs and uranium daughter minerals in fluid inclusions suggest that these elements can be strongly concentrated in high-temperature hydrothermal fluids (Kwak and Abeysinghe, 1987). This suggests that other metasomatic environments should be examined for possible concentrations of REEs and uranium.
The occurrence of platinum group elements is reported in some skarns (e.g. Knopf, 1942). These deposits have not been well documented in the literature and most appear to represent metasomatism of ultramafic rocks (e.g. Yu, 1985). It is difficult to evaluate the abundance of PGEs in different skarn types because PGEs have not been routinely analyzed until recently. Geochemical considerations suggest that PGEs could be transported under very acidic, oxidized conditions (Wood, 1989). In the skarn environment such conditions might be reached in the greisen alteration stage of tin skarns. This might be a direction for future research and exploration.
Another skarn type that has received recent study is related to metasomatism in regional metamorphic environments (Mueller, 1988; Llotka and Nesbitt, 1989; Pan et al., 1991). In the Yilgarn craton of western Australia, Archean volcanic rocks are cut by regional shear zones which host gold-quartz veins with typical carbonate-sericite alteration in most deposits (Groves et al., 1988). In some of the deeper deposits, mineralized gold-quartz veins have zoned alteration envelopes of calcic pyroxene and garnet (Mueller, 1988).
Skarn alteration is locally massive and best developed in iron-rich metabasalt, banded iron formation, and komatiite. Based upon detailed underground mapping, mineral equilibria, and structural fabrics, Mueller (1988) interprets the skarn alteration as post-dating peak metamorphism and related to synkinematic granite domes.
In the Archean Great Slave Province of northern Canada, banded iron formation contains disseminated and vein-controlled gold mineralisation associated with arsenopyrite and pyrite at the Lupin Mine.
Metamorphism of these rocks has formed hedenbergitic pyroxene skarn along with grunerite and garnet. Llotka (1988) notes that hedenbergite skarn is most abundant in the central sheared part of the Lupin mine and concluded that metasomatic fluids circulating along the shear zone were responsible for stabilizing the calcic hedenbergite pyroxene in the iron-rich but calcium-poor host rocks. The arsenopyrite-bearing hedenbergite skarn at Lupin is very similar in composition and texture to some of the reduced Phanerozoic gold skarns such as Hedley and Fortitude.
Zonation of skarn deposits
In most skarns there is a general zonation pattern of- proximal garnet,
- distal pyroxene,
- and idocrase (or a pyroxenoid such as wollastonite, bustamite, or rhodonite)
For example, proximal garnet is commonly dark red-brown, becoming lighter brown and finally pale green near the marble front (e.g. Atkinson and Einaudi, 1978). The change in pyroxene colour is less pronounced but typically reflects a progressive increase in iron and/or manganese towards the marble front (e.g. Harris and Einaudi, 1982). For some skarn systems, these zonation patterns can be "stretched out" over a distance of several kilometres and can provide a significant exploration guide (e.g. Meinert, 1987). Details of skarn mineralogy and zonation can be used to construct deposit-specific exploration models as well as more general models useful in developing grass roots exploration programs or regional syntheses.
Reasonably detailed zonation models are available for copper, gold, and zinc skarns. Other models can be constructed from individual deposits which have been well studied such as the Hedley Au skarn (Ettlinger, 1992; Ray et al., 1993) or the Groundhog Zn skarn (Meinert, 1982).
Geochemistry of skarn deposits
Skarn formation spans almost the complete range of potential ore-forming environments. Most geochemical studies of skarn deposits have focused on mineral phase equilibria, fluid inclusions, isotopic investigations of fluid sources and pathways, and determination of exploration anomaly and background levels. Experimental phase equilibria studies are essential for understanding individual mineral reactions. Such studies can be extended using thermodynamic data to include variable compositions). Another approach is to use a self-consistent thermodynamic database to model potential skarn-forming solutions (e.g. Flowers and Helgeson, 1983; Johnson and Norton, 1985; Ferry and Baumgartner, 1987). Fractionation of elements between minerals (e.g. Ca:Mg in carbonate, Bowman et al., 1982; Bowman and Essene, 1984) also can be used to estimate conditions of skarn formation.Fluid inclusion studies of many ore deposit types focus on minerals such as quartz, carbonate, and fluorite which contain numerous fluid inclusions, are relatively transparent, and are stable over a broad T-P-X range. However, this broad T-P-X range can cause problems in interpretation of fluid inclusion data, because these minerals may grow and continue to trap fluids from early high temperature events through late low temperature events (Roedder, 1984).
In contrast, high temperature skarn minerals such as forsterite, diopside, etc. are unlikely to trap later low temperature fluids (beyond the host mineral's stability range) without visible evidence of alteration. Thus, fluid inclusions in skarn minerals provide a relatively unambiguous opportunity to measure temperature, pressure, and composition of skarn-forming fluids.
Much of the skarn fluid inclusion literature prior to the mid-1980's has been summarized by Kwak (1986), especially studies of Sn and W skarn deposits. Such studies have been very useful in documenting the high temperatures (>700¡C) and high salinities (>50 wt. % NaCl equiv. And multiple daughter minerals) which occur in many skarns. All the skarn types summarized in Meinert (1992) have fluid inclusion homogenization temperatures up to and exceeding 700¡C except for copper and zinc skarns, deposits in which most fluid inclusions are in the 300-550¡C range.
This is consistent with the relatively shallow and distal geologic settings inferred respectively for these two skarn types.
Salinities in most skarn fluid inclusions are high; documented daughter minerals in skarn minerals include NaCl, KCl, CaCl2, FeCl2, CaCO3, CaF2, C, NaAlCO3(OH)2, Fe2O3, Fe3O4, AsFeS, CuFeS2, and ZnS (Table 2). Haynes and Kesler (1988) describe systematic variations in NaCl:KCl:CaCl2 ratios in fluid inclusions from different skarns reflecting differences in the fluid source and the degree of mixing of magmatic, connate, and meteoric fluids. In general, magmatic fluids have KCl>CaCl2 whereas high-CaCl2 fluids appear to have interacted more with sedimentary wall rocks.
Fluid inclusions can provide direct evidence for the content of CO2 (both liquid and gas), CH4, N2, H2S and other gases in hydrothermal fluids. Studies of gas phases and immiscible liquids in fluid inclusions typically show a dominance of CO2, a critical variable in skarn mineral stability. Although no comparative studies have been done, it appears that CH4 is slightly more abundant than CO2 in reduced systems like tungsten skarns (Fonteilles et al., 1989; Gerstner et al., 1989) whereas CO2 is more abundant than CH4 in more oxidized systems like copper and zinc skarns (Megaw et al., 1988).
Studies of fluid inclusions in specific skarn mineral phases are particularly useful in documenting the temporal and spatial evolution of skarn-forming fluids and how those changes correlate with compositional, experimental, and thermodynamic data (e.g. Kwak and Tan, 1981; Meinert, 1987).
Fluid inclusions also provide direct evidence for the temperature and salinity shift in most skarn systems between prograde and retrograde skarn events. For example, most garnet and pyroxene fluid inclusions in iron skarns have homogenization temperatures of 370->700¡C and 300-690¡C, respectively, with salinities up to 50 wt. % NaCl equivalent, whereas retrograde epidote and crosscutting quartz veins have homogenization temperatures of 245-250¡C and 100-250¡C, respectively, with salinities of less than 25 wt. % NaCl equivalent.
In gold skarns, prograde garnet and pyroxene homogenization temperatures are up to 730¡C and 695¡C, respectively, with salinities up to 33 wt. % NaCl equivalent. In contrast, scapolite, epidote, and actinolite from these skarns have homogenization temperatures of 320-400¡C, 255-320¡C, and 320-350¡C, respectively. In tungsten skarns, prograde garnet and pyroxene homogenization temperatures are up to 800¡C and 600¡C, respectively, with salinities up to 52 wt. % NaCl equivalent.
In contrast, amphibole and quartz from these skarns have homogenization temperatures of 250-380¡C and 290-380¡C, respectively with salinities of 12-28 and 2.5-10.5 wt. % NaCl equivalent (data summarized in Meinert, 1992).
Isotopic investigations, particularly the stable isotopes of C, O, H, and S, have been critically important in documenting the multiple fluids present in most large skarn systems (Shimazaki, 1988).
The pioneering study of Taylor and O'Neill (1977) demonstrated the importance of both magmatic and meteoric waters in the evolution of the Osgood Mountain W skarns. Bowman et al. (1985) demonstrated that in high temperature W skarns, even some of the hydrous minerals such as biotite and amphibole can form at relatively high temperatures from water with a significant magmatic component (see also Marcke de Lummen, 1988).
Specifically, garnet, pyroxene, and associated quartz from the skarn deposits summarized in Meinert (1992) all have ¶18O values in the +4 to +9 range consistent with derivation from magmatic waters.
In contrast, ¶18O values for sedimentary calcite, quartz, and meteoric waters in these deposits are distinctly different. In most cases, there is a continuous mixing line between original sedimentary ¶18O values and calculated ¶18O values for magmatic hydrothermal fluids at the temperatures of prograde skarn formation.
Similar mixing is indicated by ¶13C values in calcite, ranging from typical sedimentary ¶13C values in limestone away from skarn to typical magmatic values in calcite interstitial to prograde garnet and pyroxene (Brown et al., 1985). Hydrous minerals such as biotite, amphibole, and epidote from different skarn deposits also display ¶18O and ¶D values ranging from magmatic to local sedimentary rocks and meteoric waters (Layne et al., 1991). Again, mixing of multiple fluid sources is indicated.
Sulfur isotopic studies on a variety of sulfide minerals (including pyrite, pyrrhotite, molybdenite, chalcopyrite, sphalerite, bornite, arsenopyrite, and galena) from the skarn deposits . indicate a very narrow range of ¶34 values , consistent with precipitation from magmatic fluids. For some of the more distal zinc skarns, sulfur isotopic studies indicate that the mineralizing fluids acquired some of their sulfur from sedimentary rocks (including evaporites) along the fluid flow path (Megaw et al., 1988).
Overall, stable isotopic investigations are consistent with fluid inclusion and mineral equilibria studies which demonstrate that most large skarn deposits form from diverse fluids, including early, high temperature, highly saline brines directly related to crystallizing magma systems (e.g. Auwera and Andre, 1988). In many systems, the highest salinity fluids are coincident with peak sulfide deposition. In addition, at least partial mixing with exchanged connate or meteoric fluids is required for most deposits with the latest alteration events forming largely from dilute meteoric waters.
Even though skarn metal contents are quite variable, anomalous concentrations of pathfinder elements in distal skarn zones can be an important exploration guide. Geochemical studies of individual deposits have shown that metal dispersion halos can be zoned from proximal base metal assemblages, through distal precious metal zones, to fringe Pb-Zn-Ag vein concentrations (e.g. Theodore and Blake, 1975).
Anomalies of 10s to 100s of ppm for individual metals can extend for more than 1000 meters beyond proximal skarn zones. Comparison of geochemical signatures among different skarn classes suggests that each has a characteristic suite of anomalous elements and that background levels for a particular element in one skarn type may be highly anomalous in other skarns. For example, Au, Te, Bi, and As values of 1, 10, 100, and 500 ppm, respectively, are not unusual for gold skarns but are rare to absent for other skarn types (e.g. Meinert et al., 1990; Myers and Meinert, 1991).
Geophysics
Some skarns have a strong geophysical response (Chapman and Thompson, 1984; Emerson, 1986).Almost all skarns are significantly denser than the surrounding rock and therefore may form a gravitational anomaly or seismic discontinuity.
This is particularly evident in some of the large iron skarns which may contain more than a billion tons of magnetite (specific gravity, 5.18). In addition, both skarns and associated plutons may form magnetic anomalies (Spector, 1972).
Relatively oxidized plutons typically contain enough primary magnetite to form a magnetic high whereas reduced plutons typically contain ilmenite rather than magnetite and may form a magnetic low (Ishihara, 1977).
Skarns may form a magnetic high due to large concentrations of magnetite (Chapman et al., 1986) or other magnetic minerals such as high temperature pyrrhotite (Wotruba et al., 1988). Since metasomatism of dolomitic rocks tends to form abundant magnetite, in magnesian skarn deposits a strong magnetic signature may be able to distinguish original protolith as well as the presence of skarn (Hallof and Winniski, 1971; Chermeninov, 1988).
Electrical surveys of skarns need to be interpreted carefully. Either disseminated or massive sulfide minerals may give strong IP, EM, or magnetotelluric responses in skarn (Emerson and Welsh, 1988). However, metasomatism of carbonate rock necessarily involves the redistribution of carbon.
The presence of carbonaceous matter, especially if in the form of graphite, can strongly effect electrical surveys. Such carbon-induced anomalies may be distant from or unrelated to skarn ore bodies.
Uranium-Thorium as indicators
A few skarns contain sufficient uranium and thorium to be detectable by airborne or ground radiometric surveys (e.g. Mary Kathleen, Australia, Kwak and Abeysinghe, 1987). Detailed studies of such deposits demonstrate that relatively small skarns can be detected and that different types of skarns can be distinguished (e.g. Lentz, 1991). Although gravity, magnetic, electrical, and radiometric methods have all been applied to skarn deposits, their use has not been widespread. Because of the variability of skarn deposits, it probably is necessary to tailor specific geophysical methods to individual skarn deposits or types.Petrogenesis and tectonic settings of skarn deposits
Most major skarn deposits are directly related to igneous activity and broad correlations between igneous composition and skarn type have been described by several workers (Zharikov, 1970; Shimazaki, 1975,1980; Einaudi et al., 1981; Kwak and White, 1982; Meinert, 1983; Newberry and Swanson, 1986; Newberry, 1987; 1990).Averages of large amounts of data for each skarn type can be summarized on a variety of compositional diagrams to show distinctions among skarn classes. Tin and molydenum skarns typically are associated with high silica, strongly differentiated plutons. At the other end of the spectrum, iron skarns usually are associated with low silica, iron-rich, relatively primitive plutons. Such diagrams are less useful for detailed studies, however, because of the wide range of igneous compositions possible for an individual skarn deposit and the difficulty of isolating the effects of metasomatism and late alteration.
Other important characteristics include the oxidation state, size, texture, depth of emplacement, and tectonic setting of individual plutons. For example, tin skarns are almost exclusively associated with reduced, ilmenite-series plutons which can be characterized as S-type or anorogenic. These plutons tend to occur in stable cratons in which partial melting of crustal material may be instigated by incipient rifting. Many gold skarns are also associated with reduced, ilmenite-series plutons.
However, gold skarn plutons typically are mafic, low-silica bodies which could not have formed by melting of sedimentary crustal material. In contrast, plutons associated with copper skarns, particularly porphyry copper deposits, are strongly oxidized, magnetite-bearing, I-type and associated with subduction-related magmatic arcs. These plutons tend to be porphyritic and emplaced at shallow levels in the earthÕs crust. Tungsten skarns, on the other hand, are associatedwith relatively large, coarse-grained, equigranular plutons or batholithic complexes indicative of a deeper environment.
Tectonic setting, petrogenesis, and skarn deposits are intimately intertwined.
Some modern textbooks use tectonic setting to classify igneous provinces (Wilson, 1989) or different kinds of ore deposits (Sawkins, 1984). This approach has been less successful in describing ore deposits such as skarns which are the result of processes that can occur in almost any tectonic setting. A useful tectonic classification of skarn deposits should group skarn types which commonly occur together and distinguish those which typically occur in specialized tectonic settings.
For example, calcic Fe-Cu skarn deposits are virtually the only skarn type found in oceanic island-arc terranes.
Many of these skarns are also enriched in Co, Ni, Cr, and Au.
In addition, some economic gold skarns appear to have formed in back arc basins associated with oceanic volcanic arcs (Ray et al., 1988).
Some of the key features that set these skarns apart from those associated with more evolved magmas and crust are their association with gabbroic and dioritic plutons, abundant endoskarn, widespread sodium metasomatism, and the absence of Sn and Pb. Collectively, these features reflect the primitive, oceanic nature of the crust, wall rocks, and plutons.
The vast majority of skarn deposits are associated with magmatic arcs related to subduction beneath continental crust.
Plutons range in composition from diorite to granite although differences among the main base metal skarn types appear to reflect the local geologic environment (depth of formation, structural and fluid pathways) more than fundamental differences of petrogenesis (Nakano et al., 1990). In contrast, gold skarns in this environment are associated with particularly reduced plutons that may represent a restricted petrologic history.
The transition from subduction beneath stable continental crust to post-subduction tectonics is not well understood. Magmatism associated with shallow subduction angles may have more crustal interaction (Takahashi et al., 1980) and floundering of the downgoing slab may result in local rifting.
During this stage the magmatic arc may widen or migrate further inland.
Plutons are granitic in composition and associated skarns are rich in Mo or W-Mo with lesser Zn, Bi, Cu, and F. Many of these skarns are best described as polymetallic with locally important Au and As.
Some skarns are not associated with subduction-related magmatism.
These skarns may be associated with S-type magmatism following a major period of subduction or they may be associated with rifting of previously stable cratons.
Plutons are granitic in composition and commonly contain primary muscovite and biotite, dark gray quartz megacrysts, miarolitic cavities, greisen-type alteration, and anomalous radioactivity.
Associated skarns are rich in tin or fluorine although a host of other elements are usually present and may be of economic importance.
This evolved suite includes W, Be, B, Li, Bi, Zn, Pb, U, F, and REE.