Skarn Deposits

Skarn Deposits

Table of Contents

1. Historical usage

2. Definitions

3. Skarn Mineralogy

4. Evolution of skarnsin time and space

5. Au, Cu, Fe, Mo, Sn,W, and Zn-Pb skarn deposits

6. Zonation of skarndeposits

7. Geochemistry of skarndeposits

8. Petrogenesis andtectonic settings of skarn deposits

Historical usage:

Althoughthere are earlier descriptions of deposits now known to contain skarn, thefirst published use of the term "skarn" is by Tornebohm (Tornebohm,A.E., 1875, Geognostisk beskrifning ofver Persbergets Grufvefalt: SverigesGeologiska Undersokning, P.A. Norstedt and Sons, Stockholm, 21 p.). Amongseveral excellent descriptions is the following: (p. 4) "Sasomunderordnade lager i euriten, foretradesvis i dess faltspatsfattigarevarieteter, upptrada vissa egendomliga morka bergarter, som utgora malmaernas egentligaklyftsten. Dessa benamnas i Persbergstrakten skarn ett uttryck, som lampligenskulle kunna anvandas sasom en kollektivbenamning for alla sadana egendomligaoch fran den omgifvande bergartsmassan afvikande bergarter, som upptradanarmast kring malmfyndigheterna."

Thiswas translated thusly by Torbjorn Bergman of the University of Stockholm (1992,written communication), "As subordinate layers in the feldspar-poor felsicvolcanic rocks, there appear peculiar dark rocks which also are the ore's hostrock. These rocks are in the Persberg area denoted 'skarn', a word which likelycan be used as a collective term for all such odd rocks occurring alongside theores." Tornebohm goes on to describe garnet-rich "brunskarn"(brown skarn) and pyroxene-rich "gronskarn" (green skarn).

Forother pre-1970 publications concerning skarn deposits, Burt(1982)provides a very useful annotated historical bibliography. Einaudi etal. (1981),a fairly exhaustive review paper on skarn deposits, is a good source of otherreferences and definitions, some of which are summarized below. Another sourceof historical information is the recently published field trip guide to theclassic Banatregion of Romania.Additional references on skarn deposits are available here.

Definitions:

Thereare many definitions and usages of the word "skarn". Skarns can formduring regional or contact metamorphism and from a variety of metasomaticprocesses involving fluids of magmatic, metamorphic, meteoric, and/or marineorigin. They are found adjacent to plutons, along faults and major shear zones,in shallow geothermal systems, on the bottom of the seafloor, and at lowercrustal depths in deeply buried metamorphic terrains. What links these diverseenvironments, and what defines a rock as skarn, is the mineralogy. This mineralogy includes a widevariety of calc-silicate and associated minerals but usually is dominated by garnet and pyroxene.

Skarnscan be subdivided according to several criteria. Exoskarn and endoskarn arecommon terms used to indicate a sedimentary or igneous protolith, respectively.Magnesian and calcic skarn can be used to describe the dominant composition ofthe protolith and resulting skarn minerals. Such terms can be combined, as inthe case of a magnesian exoskarn which contains forsterite-diopside skarnformed from dolostone.

Calc-silicatehornfelsis a descriptive term often used for the relatively fine-grained calc-silicaterocks that result from metamorphism of impure carbonate units such as siltylimestone or calcareous shale. Reaction skarns can form fromisochemical metamorphism of thinly interlayered shale and carbonate units wheremetasomatic transfer of components between adjacent lithologies may occur on asmall scale (perhaps centimetres) (e.g. Vidale, 1969; Zarayskiy et al., 1987). Skarnoid is a descriptiveterm for calc-silicate rocks which are relatively fine-grained, iron-poor, andwhich reflect, at least in part, the compositional control of the protolith(Korzkinskii, 1948; Zharikov, 1970). Genetically, skarnoid is intermediatebetween a purely metamorphic hornfels and a purely metasomatic, coarse-grainedskarn.

Forall of the preceding terms, the composition and texture of the protolith tendto control the composition and texture of the resulting skarn. In contrast,most economically important skarn deposits result from large scale metasomatictransfer, where fluid composition controls the resulting skarn and oremineralogy. This is the mental image that most people share of a "classic" skarndeposit.Ironically, in the "classic" skarn locality described by Tornebohm atPersberg, skarn has developed during regional metamorphism of a mostlycalcareous Proterozoic iron formation. This reinforces the importance ofEinaudi et al.'s (1981) warning that the words "skarn" and"skarn deposits" be used strictly in a descriptive sense, based upondocumented mineralogy, and free of genetic interpretations.

Notall skarns have economic mineralization; skarns which contain ore are calledskarn deposits. In most large skarn deposits, skarn and ore minerals resultfrom the same hydrothermal system even though there may be significantdifferences in the time/space distribution of these minerals on a local scale.Although rare, it is also possible to form skarn by metamorphism ofpre-existing ore deposits as has been suggested for Aguilar, Argentina (Gemmellet al., 1992), Franklin Furnace, USA (Johnson et al., 1990), and Broken Hill, Australia(Hodgson, 1975).

Skarn Mineralogy

Justas mineralogy is the key to recognizing and defining skarns, it is alsocritical in understanding their origin and in distinguishing economicallyimportant deposits from interesting but uneconomic mineral localities. Skarnmineralogy is mappable in the field and serves as the broader "alterationenvelope" around a potential ore body. Because most skarn deposits arezoned, recognition of distal alteration features can be critically important inthe early exploration stages. Details of skarn mineralogy and zonation can beused to construct deposit-specific exploration models as well as more generalmodels useful in developing grass roots exploration programs or regionalsyntheses.

Althoughmany skarn minerals are typical rock-forming minerals, some are less abundantand most have compositional variations which can yield significant informationabout the environment of formation (e.g. pyroxene - Takano, 1998; scapolite -Pan, 1998). Table 1 lists many of thecommon skarn minerals and their end member compositions. Some minerals, such asquartz and calcite, are present in almost all skarns. Other minerals, such ashumite, periclase, phlogopite, talc, serpentine, and brucite are typical of magnesianskarns but are absent from most other skarn types. Additionally, there are manytin, boron, beryllium, and fluorine-bearing minerals which have veryrestricted, but locally important, parageneses.

Theadvent of modern analytical techniques, particularly the electron microprobe,makes it relatively easy to determine accurate mineral compositions andconsequently, to use precise mineralogical names. However, mineralogical namesshould be used correctly so as not to imply more than is known about the mineralcomposition. For example, the sequence pyroxene, clinopyroxene, calcicclinopyroxene, diopsidic pyroxene, and diopside, are increasingly more specificterms. Unfortunately, it is all too common in the geologic literature forspecific end member terms, such as diopside, to be used when all that is knownabout the mineral in question is that it might be pyroxene.

Zharikov(1970)was perhaps the first to describe systematic variations in skarn mineralogyamong the major skarn classes. He used phase equilibria, mineralcompatibilities, and compositional variations in solid solution series todescribe and predict characteristic mineral assemblages for different skarntypes. His observations have been extended by Burt (1972) and Einaudi et al.(1981) to include a wide variety of deposit types and the mineralogicalvariations between types. The minerals which are most useful for bothclassification and exploration are those, such as garnet, pyroxene, andamphibole, which are present in all skarn types and which show markedcompositional variability. For example, the manganiferous pyroxene,johannsenite, is found almost exclusively in zinc skarns. Its presence, withoutmuch further supporting information, is definitive of this skarn type.

Whencompositional information is available, it is possible to denote a mineral'scomposition in terms of mole percent of the end members. For example, apyroxene which contains 70 mole percent hedenbergite, 28 mole percent diopside,and 2 mole percent johannsenite could be referred to as Hd70Di28Jo2. In manyskarn systems, variation in iron content is the most important parameter andthus, many minerals are described simply by their iron end member, e.g. Hd10 orAd90. Large amounts of compositional information can be summarized graphically.Triangularplotscommonly are used to express variations in compositionally complex mineralssuch as garnet and pyroxene.

Amphibolesare more difficult to portray graphically because they have structural as wellas compositional variations. The main differences between amphiboles indifferent 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 areprogressively more iron-rich in the tremolite-actinolite series, and amphibolesfrom zinc skarns are both Mn-rich and Ca-deficient, ranging from actinolite todannemorite. For a specific skarn deposit or group of skarns, compositionalvariations in less common mineral phases, such as idocrase, bustamite, orolivine, 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

Aswas recognized by early skarn researchers (e.g. Lindgren 1902; Barrell, 1907;Goldschmidt, 1911; Umpleby, 1913; Knopf, 1918), formation of a skarn deposit isa dynamic process. In most large skarn deposits there is a transition fromearly/distal metamorphism resulting in hornfels, reaction skarn, and skarnoid,to later/proximal metasomatism resulting in relatively coarse-grainedore-bearing skarn. Due to the strong temperature gradients and large fluidcirculation cells caused by intrusion of a magma (Norton, 1982; Salemink andSchuiling, 1987; Bowers et al., 1990), contact metamorphism can be considerablymore complex than the simple model of isochemical recrystallization typicallyinvoked for regional metamorphism.

Forexample, circulating diverse fluids through a fracture in a relatively simplecarbonate protolith can result in several different reactions. Thus, the steepthermal gradients common in most plutonic environments, result in complexmetamorphic aureoles complete with small-scale metasomatic transfer asevidenced by reactionskarnsand skarnoid.

Morecomplex metasomatic fluids, with the possible addition of magmatic componentssuch as Fe, Si, Cu, etc. , produce a continuum between purely metamorphic andpurely metasomatic processes. This early metamorphism and continuedmetasomatism at relatively high temperature (Wallmach and Hatton, 1989,describe temperatures > 1200C) are followed by retrograde alteration astemperatures decline. A link between space and time is a common theme in oredeposits and requires careful interpretation of features which may appear tooccur only in a particular place (e.g. Barton et al., 1991).

Oneof the more fundamental controls on skarn size, geometry, and style ofalteration is the depth of formation. Quantitative geobarometric studiestypically use mineral equilibria (Anovitz and Essene, 1990), fluid inclusions(Guy et al., 1989) or a combination of such methods (Hames et al., 1989) toestimate the depth of metamorphism. Qualitative methods include stratigraphicor other geologic reconstructions and interpretation of igneous textures.Simple observations of chilled margins, porphyry groundmass grain size, plutonmorphology, and presence of brecciation and brittle fracture allow fielddistinctions between relatively shallow and deep environments.

Theeffect of depth on metamorphism is largely a function of the ambient wall rocktemperature prior to, during, and post intrusion. Assuming an averagegeothermal gradient for an orogenic zone of about 35C per kilometre (Blackwellet al., 1990), the ambient wall rock temperature prior to intrusion at 2 kmwould be 70C, whereas at 12 km it would be 420C. Thus, with the added heat fluxprovided by local igneous activity, the volume of rock affected by temperaturesin the 400-700C range would be considerably larger and longer lived surroundinga deeper skarn than a shallower one. In addition, higher ambient temperaturescould affect the crystallization history of a pluton as well as minimize theamount of retrograde alteration of skarn minerals.

Ata depth of 12 km with ambient temperatures around 400C, skarn may not coolbelow garnet and pyroxene stability without subsequent uplift or other tectonicchanges. The greater extent and intensity of metamorphism at depth can affectthe permeability of host rocks and reduce the amount of carbonate available forreaction with metasomatic fluids. An extreme case is described by Dick andHodgson (1982) at Cantung, Canada, where the "Swiss cheese limestone"was almost entirely converted to a heterogeneous calc-silicate hornfels duringmetamorphism prior to skarn formation. The skarn formed from the few remainingpatches of limestone has some of the highest known grades of tungsten skarn orein the world (Mathiason and Clark, 1982).

Thedepth of skarn formation also will affect the mechanical properties of the hostrocks. In a deep skarn environment, rocks will tend to deform in a ductilemanner rather than fracture. Intrusive contacts with sedimentary rocks at depthtend to be sub-parallel to bedding; either the pluton intrudes along beddingplanes or the sedimentary rocks fold or flow until they are aligned with theintrusive contact. Examples of skarns for which depth estimates exceed 5-10 kminclude Pine Creek, California (Brown et al., 1985) and Osgood Mountains,Nevada (Taylor, 1976). In deposits such as these, where intrusive contacts aresub-parallel to bedding planes, skarn is usually confined to a narrow, butvertically extensive, zone. At Pine Creek skarn is typically less than 10 mwide 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 sizerelative to the associated pluton and its metamorphic aureole. In contrast,host rocks at shallow depths will tend to deform by fracturing and faultingrather than folding. In most of the 13 relatively shallow skarn depositsreviewed by Einaudi (1982a), intrusive contacts are sharply discordant tobedding and skarn cuts across bedding and massively replaces favorable beds,equalling or exceeding the (exposed) size of the associated pluton. The stronghydrofracturing associated with shallow level intrusions greatly increases thepermeability of the host rocks, not only for igneous-related metasomaticfluids, but also for later, possibly cooler, meteoric fluids (Shelton, 1983).The influx of meteoric water and the consequent destruction of skarn mineralsduring retrogradealterationis one of the distinctive features of skarn formation in a shallow environment.

Theshallowest (and youngest) known skarns are presently forming in activegeothermal systems (McDowell and Elders, 1980; Cavarretta et al., 1982;Cavarretta and Puxeddu, 1990) and hot spring vents on the seafloor (Zierenbergand Shanks, 1983). These skarns represent the distal expression of magmaticactivity and exposed igneous rocks (in drill core) are dominantly thin dikesand sills with chilled margins and a very fine grained to aphanitic groundmass.

Thedegree to which a particular alteration stage is developed in aspecific skarn will depend on the local geologic environment of formation. Forexample, metamorphism will likely be more extensive and higher grade around askarn formed at relatively great crustal depths than one formed under shallowerconditions. Conversely, retrograde alteration during cooling, and possibleinteraction with meteroric water, will be more intense in a skarn formed atrelatively shallow depths in the earth's crust compared with one formed atgreater depths. In the deeper skarns carbonate rocks may deform in a ductilemanner rather than through brittle fracture, with bedding parallel to theintrusive contact; in shallower systems the reverse may be true. Thesedifferences in structural style will in turn affect the size and morphology ofskarn. Thus, host rock composition, depth of formation, and structural settingwill all cause variations from the idealized "classic" skarnmodel.

Au, Cu, Fe, Mo, Sn, W, and Zn-Pb skarndeposits

Groupingsof skarn deposits can be based on descriptive features such as protolithcomposition, rock type, and dominant economic metal(s) as well as geneticfeatures such as mechanism of fluid movement, temperature of formation, andextent of magmatic involvement. The general trend of modern authors is to adopta descriptive skarn classification based upon the dominant economic metals andthen to modify individual categories based upon compositional, tectonic, orgenetic variations. This is similar to the classification of porphyry depositsinto porphyry copper, porphyry molybdenum, and porphyry tin types; depositswhich share many alteration and geochemical features but are, nevertheless,easily distinguishable. Seven major skarn types (Au, Cu, Fe, Mo, Sn, W, andZn-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 forindustrial minerals such as garnet and wollastonite.

Major skarn types:

Other skarn types

Thereare many other types of skarn which historically have been mined or exploredfor a variety of metals and industrialminerals.Some of the more interesting include rare metal and rare earth element enrichedskarns (e.g. Kato, 1989; Birkett and SInclair, 1998). REEs tend to be enrichedin specific mineral phases such as garnet, idocrase, epidote, and allanite.Vesuvianite and epidote with up to 20% REE (Ce>La>Pr>Nd) have beenfound in some gold skarns and zinc skarns (Gemmel et al., 1992; Meinert,unpublished data).

Someskarns contain economic concentrations of REEs and uranium (Kwak andAbeysinghe, 1987; Lentz, 1991, 1998). The Mary Kathleen skarn deposit inQueensland, Australia is unusual in that REEs and uranium daughter minerals influid inclusions suggest that these elements can be strongly concentrated inhigh-temperature hydrothermal fluids (Kwak and Abeysinghe, 1987). This suggeststhat other metasomatic environments should be examined for possibleconcentrations of REEs and uranium.

Theoccurrence of platinum group elements is reported in some skarns (e.g. Knopf,1942; Korobeynikov et al., 1998). These deposits have not been well documentedin 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 differentskarn types because PGEs have not been routinely analyzed until recently.Geochemical considerations suggest that PGEs could be transported under veryacidic, oxidized conditions (Wood, 1989). In the skarn environment suchconditions might be reached in the greisen alteration stage of tin skarns. Thismight be a direction for future research and exploration.

Zonation of skarndeposits

In most skarns thereis a general zonation pattern of proximal garnet, distal pyroxene, and idocrase(or a pyroxenoid such as wollastonite, bustamite, or rhodonite) at the contactbetween skarn and marble. In addition, individual skarn minerals may displaysystematic color or compositional variations within the larger zonationpattern. For example, proximal garnet is commonly dark red-brown, becominglighter brown and finally pale green near the marble front (e.g. Atkinson andEinaudi, 1978). The change in pyroxene color is less pronounced but typicallyreflects a progressive increase in iron and/or manganese towards the marblefront (e.g. Harris and Einaudi, 1982). For some skarn systems, these zonationpatterns can be "stretched out" over a distance of several kilometresand can provide a significant exploration guide (e.g. Meinert, 1987). Detailsof skarn mineralogy and zonation can be used to construct deposit-specificexploration models as well as more general models useful in developing grassroots exploration programs or regional syntheses. Reasonably detailed zonationmodels are available for copper,gold, and zinc skarns (Meinert,1997). Other models can be constructed from individual deposits which have beenwell studied such as the Hedley Au skarn (Ettlinger, 1992;Ray et al., 1993) or the GroundhogZn skarn(Meinert, 1982).

Geochemistry of skarndeposits

Skarnformation spans almost the complete range of potential ore-formingenvironments. Most geochemical studies of skarn deposits have focused onmineral phase equilibria, fluid inclusions, isotopic investigations of fluidsources and pathways, and determination of exploration anomaly and backgroundlevels. Experimental phase equilibria studies are essential for understanding individualmineralreactions.Such studies can be extended using thermodynamic data to include variablecompositions). Another approach is to use a self-consistent thermodynamicdatabase to model potential skarn-forming solutions (e.g. Flowers and Helgeson,1983; Johnson and Norton, 1985; Ferry and Baumgartner, 1987). Fractionation ofelements between minerals (e.g. Ca:Mg in carbonate, Bowman et al., 1982; Bowmanand Essene, 1984) also can be used to estimate conditions of skarn formation. Ageneral review of phase equilibria applicable to skarn systems is presented byBowman (1998). A more specialized treatment of the vector representation ofskarn mineral stabilities is presented by Burt (1998). Recent work hasincorporated standard phase equilbria treatment of skarn mineralogy along withfluid dynamics to model the metasomatic evolution of skarn systems (Dipple andGerdes, 1998).

Fluidinclusion studies of many ore deposit types focus on minerals such as quartz,carbonate, and fluorite which contain numerous fluid inclusions, are relativelytransparent, and are stable over a broad T-P-X range. However, this broad T-P-Xrange can cause problems in interpretation of fluid inclusion data, becausethese minerals may grow and continue to trap fluids from early high temperatureevents through late low temperature events (Roedder, 1984). In contrast, hightemperature skarn minerals such as forsterite, diopside, etc. are unlikely totrap later low temperature fluids (beyond the host mineral's stability range)without visible evidence of alteration. Thus, fluid inclusions in skarnminerals provide a relatively unambiguous opportunity to measure temperature,pressure, and composition of skarn-forming fluids.

Muchof the skarn fluid inclusion literature prior to the mid-1980's has beensummarized by Kwak (1986), especially studies of Sn and W skarn deposits. Suchstudies 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¡Cexcept for copper and zinc skarns, deposits in which most fluid inclusions arein the 300-550¡C range. This is consistent with the relatively shallow anddistal geologic settings inferred respectively for these two skarn types.

Salinitiesin most skarn fluid inclusions are high; documented daughter minerals in skarnminerals include NaCl, KCl, CaCl2, FeCl2, CaCO3, CaF2, C, NaAlCO3(OH)2, Fe2O3,Fe3O4, AsFeS, CuFeS2, and ZnS (Table 2). Haynes and Kesler (1988) describesystematic variations in NaCl:KCl:CaCl2 ratios in fluid inclusions fromdifferent skarns reflecting differences in the fluid source and the degree ofmixing of magmatic, connate, and meteoric fluids. In general, magmatic fluidshave KCl>CaCl2 whereas high-CaCl2 fluids appear to have interacted more withsedimentary wall rocks.

Fluidinclusions can provide direct evidence for the content of CO2 (both liquid andgas), CH4, N2, H2S and other gases in hydrothermal fluids. Studies of gasphases and immiscible liquids in fluid inclusions typically show a dominance ofCO2, a critical variable in skarn mineral stability. Although no comparativestudies have been done, it appears that CH4 is slightly more abundant than CO2in reduced systems like tungsten skarns (Fonteilles et al., 1989; Gerstner etal., 1989) whereas CO2 is more abundant than CH4 in more oxidized systems likecopper and zinc skarns (Megaw et al., 1988).

Studiesof fluid inclusions in specific skarn mineral phases are particularly useful indocumenting the temporal and spatial evolution of skarn-forming fluids and howthose changes correlate with compositional, experimental, and thermodynamicdata (e.g. Kwak and Tan, 1981; Meinert, 1987). Fluid inclusions also providedirect evidence for the temperature and salinity shift in most skarn systemsbetween prograde and retrograde skarn events. For example, most garnet andpyroxene fluid inclusions in iron skarns have homogenization temperatures of370->700¡C and 300-690¡C, respectively, with salinities up to 50 wt. % NaClequivalent, whereas retrograde epidote and crosscutting quartz veins havehomogenization temperatures of 245-250¡C and 100-250¡C, respectively, withsalinities of less than 25 wt. % NaCl equivalent.

Ingold skarns, prograde garnet and pyroxene homogenization temperatures are up to730¡C and 695¡C, respectively, with salinities up to 33 wt. % NaCl equivalent.In contrast, scapolite, epidote, and actinolite from these skarns havehomogenization temperatures of 320-400¡C, 255-320¡C, and 320-350¡C,respectively. In tungsten skarns, prograde garnet and pyroxene homogenizationtemperatures are up to 800¡C and 600¡C, respectively, with salinities up to 52wt. % NaCl equivalent. In contrast, amphibole and quartz from these skarns havehomogenization temperatures of 250-380¡C and 290-380¡C, respectively withsalinities of 12-28 and 2.5-10.5 wt. % NaCl equivalent (data summarized inMeinert, 1992).

Isotopicinvestigations, particularly the stable isotopes of C, O, H, and S, have beencritically important in documenting the multiple fluids present in most largeskarn systems (Shimazaki, 1988; Bowman, 1998). The pioneering study of Taylorand O'Neill (1977) demonstrated the importance of both magmatic and meteoricwaters in the evolution of the Osgood Mountain W skarns. Bowman et al. (1985)demonstrated that in high temperature W skarns, even some of the hydrousminerals such as biotite and amphibole can form at relatively high temperaturesfrom water with a significant magmatic component (see also Marcke de Lummen,1988).

Specifically,garnet, pyroxene, and associated quartz from the skarn deposits summarized inMeinert (1992) all have 18O values in the +4 to +9 range consistent withderivation from magmatic waters. In contrast, 18O values for sedimentarycalcite, quartz, and meteoric waters in these deposits are distinctlydifferent. In most cases, there is a continuous mixing line between originalsedimentary 18O values and calculated 18O values for magmatic hydrothermalfluids at the temperatures of prograde skarn formation. Similar mixing isindicated by 13C values in calcite, ranging from typical sedimentary 13C valuesin limestone away from skarn to typical magmatic values in calcite interstitialto prograde garnet and pyroxene (Brown et al., 1985). Hydrous minerals such asbiotite, amphibole, and epidote from different skarn deposits also display 18Oand D values ranging from magmatic to local sedimentary rocks and meteoricwaters (Layne et al., 1991). Again, mixing of multiple fluid sources isindicated.

Sulfurisotopic studies on a variety of sulfide minerals (including pyrite,pyrrhotite, molybdenite, chalcopyrite, sphalerite, bornite, arsenopyrite, andgalena) from the skarn deposits summarized in Table 2 indicate a very narrowrange of 34 values, consistent with precipitation from magmatic fluids. Forsome of the more distal zinc skarns, sulfur isotopic studies indicate that themineralizing 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 mineralequilibria studies which demonstrate that most large skarn deposits form fromdiverse fluids, including early, high temperature, highly saline brinesdirectly related to crystallizing magma systems (e.g. Auwera and Andre, 1988).In many systems, the highest salinity fluids are coincident with peak sulfidedeposition. In addition, at least partial mixing with exchanged connate ormeteoric fluids is required for most deposits with the latest alteration eventsforming largely from dilute meteoric waters.

Eventhough skarn metal contents are quite variable, anomalous concentrations ofpathfinder elements in distal skarn zones can be an important explorationguide. Geochemical studies of individual deposits have shown that metaldispersion halos can be zoned from proximal base metal assemblages, throughdistal precious metal zones, to fringe Pb-Zn-Ag vein concentrations (e.g.Theodore and Blake, 1975). Anomalies of 10s to 100s of ppm for individualmetals can extend for more than 1000 meters beyond proximal skarn zones.Comparison of geochemical signatures among different skarn classes suggeststhat each has a characteristic suite of anomalous elements and that backgroundlevels for a particular element in one skarn type may be highly anomalous inother skarns. For example, Au, Te, Bi, and As values of 1, 10, 100, and 500ppm, respectively, are not unusual for gold skarns but are rare to absent forother skarn types (e.g. Meinert et al., 1990; Myers and Meinert, 1991).

Someskarns have a strong geophysical response (Chapman and Thompson, 1984; Emerson,1986). Almost all skarns are significantly denser than the surrounding rock andtherefore may form a gravitational anomaly or seismic discontinuity. This isparticularly evident in some of the large iron skarns which may contain morethan a billion tons of magnetite (specific gravity, 5.18). In addition, bothskarns and associated plutons may form magnetic anomalies (Spector, 1972).Relatively oxidized plutons typically contain enough primary magnetite to forma magnetic high whereas reduced plutons typically contain ilmenite rather thanmagnetite and may form a magnetic low (Ishihara, 1977). Skarns may form amagnetic 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 todistinguish original protolith as well as the presence of skarn (Hallof andWinniski, 1971; Chermeninov, 1988).

Electricalsurveys of skarns need to be interpreted carefully. Either disseminated ormassive sulfide minerals may give strong IP, EM, or magnetotelluric responsesin skarn (Emerson and Welsh, 1988). However, metasomatism of carbonate rocknecessarily involves the redistribution of carbon. The presence of carbonaceousmatter, especially if in the form of graphite, can strongly effect electricalsurveys. Such carbon-induced anomalies may be distant from or unrelated toskarn ore bodies.

Afew skarns contain sufficient uranium and thorium to be detectable by airborneor ground radiometric surveys (e.g. Mary Kathleen, Australia, Kwak andAbeysinghe, 1987). Detailed studies of such deposits demonstrate thatrelatively small skarns can be detected and that different types of skarns canbe distinguished (e.g. Lentz, 1991). Although gravity, magnetic, electrical,and radiometric methods have all been applied to skarn deposits, their use hasnot been widespread. Because of the variability of skarn deposits, it probablyis necessary to tailor specific geophysical methods to individual skarndeposits or types.

Petrogenesis and tectonic settings ofskarn deposits

Mostmajor skarn deposits are directly related to igneous activity and broadcorrelations between igneous composition and skarn type have been described byseveral 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 besummarized on a variety of compositional diagrams to show distinctions amongskarn classes. Tin and molydenum skarns typically are associated with highsilica, strongly differentiated plutons. At the other end of the spectrum, ironskarns usually are associated with low silica, iron-rich, relatively primitiveplutons. Such diagrams are less useful for detailed studies, however, because ofthe wide range of igneous compositions possible for an individual skarn depositand the difficulty of isolating the effects of metasomatism and latealteration.

Otherimportant characteristics include the oxidation state, size, texture, depth ofemplacement, and tectonic setting of individual plutons. For example, tinskarns are almost exclusively associated with reduced, ilmenite-series plutonswhich can be characterized as S-type or anorogenic. These plutons tend to occurin stable cratons in which partial melting of crustal material may beinstigated by incipient rifting. Many gold skarns are also associated withreduced, ilmenite-series plutons. However, gold skarn plutons typically aremafic, low-silica bodies which could not have formed by melting of sedimentarycrustal 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 tendto be porphyritic and emplaced at shallow levels in the earthÕs crust. Tungstenskarns, on the other hand, are associated with relatively large,coarse-grained, equigranular plutons or batholithic complexes indicative of adeeper environment.

Tectonicsetting,petrogenesis, and skarn deposits are intimately intertwined. Some moderntextbooks use tectonic setting to classify igneous provinces (Wilson, 1989) ordifferent kinds of ore deposits (Sawkins, 1984). This approach has been lesssuccessful in describing ore deposits such as skarns which are the result ofprocesses that can occur in almost any tectonic setting. A useful tectonicclassification of skarn deposits should group skarn types which commonly occurtogether and distinguish those which typically occur in specialized tectonicsettings. For example, calcic Fe-Cu skarn deposits are virtually the only skarntype found in oceanic island-arc terranes. Many of these skarns are alsoenriched in Co, Ni, Cr, and Au. In addition, some economic gold skarns appearto have formed in back arc basins associated with oceanic volcanic arcs (Ray etal., 1988). Some of the key features that set these skarns apart from thoseassociated with more evolved magmas and crust are their association withgabbroic and dioritic plutons, abundant endoskarn, widespread sodiummetasomatism, and the absence of Sn and Pb. Collectively, these featuresreflect the primitive, oceanic nature of the crust, wall rocks, and plutons.

Thevast majority of skarn deposits are associated with magmatic arcs related to subduction beneath continentalcrust. Plutons range in composition from diorite to granite althoughdifferences among the main base metal skarn types appear to reflect the localgeologic environment (depth of formation, structural and fluid pathways) morethan fundamental differences of petrogenesis (Nakano et al., 1990). Incontrast, gold skarns in this environment are associated with particularlyreduced plutons that may represent a restricted petrologic history.

Thetransition from subduction beneath stable continental crust to post-subductiontectonics is not well understood. Magmatism associated with shallowsubduction anglesmay have more crustal interaction (Takahashi et al., 1980) and floundering ofthe downgoing slab may result in local rifting. During this stage the magmaticarc may widen or migrate further inland. Plutons are granitic in compositionand 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 importantAu and As.

Someskarns are not associated with subduction-related magmatism. These skarns maybe associated with S-type magmatism following a major period of subduction orthey may be associated with rifting of previously stablecratons. Plutons are granitic in composition and commonly contain primarymuscovite and biotite, dark gray quartz megacrysts, miarolitic cavities,greisen-type alteration, and anomalous radioactivity. Associated skarns arerich in tin or fluorine although a host of other elements are usually presentand may be of economic importance. This evolved suite includes W, Be, B, Li,Bi, Zn, Pb, U, F, and REE.