q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.Geology; September 2003; v. 31; no. 9; p. 765–768; 3 figures. 765

Generation of metal-rich felsic magmas during crustal anatexisAndrew G. Tomkins Department of Geology, Australian National University, Canberra, ACT 0200, AustraliaJohn A. Mavrogenes Department of Geology and Research School of Earth Sciences, Australian National University,

Canberra, ACT 0200 Australia

ABSTRACTWe show that partial melting of a preexisting mineral deposit during regional anatexis

is likely to lead to incorporation of sulfide melt into silicate melt. Simple dissolution ofmetals in silicate melt during anatexis leads to minor metal addition to escaping magma,but physical entrainment of sulfide melt can lead to extreme enrichment and higher totalmetal incorporation. Metal enrichment is enhanced if melt segregation is structurally driv-en; only dissolved metal is incorporated if initial segregation is driven by buoyancy alone.However, once encapsulated within a larger volume of silicate melt, immiscible globulesof sulfide melt are physically carried by buoyantly rising magma. Gradual dissolution ofentrained sulfide melt globules is expected as the silicate magma migrates from the lowercrust. This process may lead to significant variations in metal contents of silicate magmasabove regions that underwent anatexis.

Keywords: ore genesis, granite metallogeny, source, sulfide melt, anatexis.

Figure 1. Au-Bi binary phase diagram (afterOkamoto and Massalski, 1983). L 5 liquid,Ma 5 maldonite (Au2Bi).

INTRODUCTIONFelsic magma–related mineral deposits,

such as porphyry Cu-Au-(Mo) and epithermalAu-Ag-(Cu) deposits, are among the most im-portant metal producers. Because they are wellexposed and easily accessed, near-surface pro-cesses that control the localization of miner-alization are well understood. Nevertheless,deep-crustal processes controlling the uptakeof (chalcophile) metals by granitic melts re-main unclear.

Many have observed that within individualmetallogenic provinces, few intrusions pro-duced mineralization whereas many are barren(e.g., Baldwin and Pearce, 1982). In specificdeposits, there may be only one intrusion ofseveral that is related to mineralization (e.g.,Ulrich and Heinrich, 2001). Similarly, someregions produce many metal-rich magmas,whereas others are apparently barren. Theseobservations may be largely a consequence ofvariability in the source. The source problemis important for two reasons: first, it raisesquestions of geochemical processes in crustalevolution, and second, it addresses whethermetal source terranes, characterized by preen-richment of metals, should be considered byexplorationists (Mathur et al., 2000). Varioussuggestions have been offered for the sourceof metals in felsic magma–related ore depos-its. Some suggest that metal is derived fromsubducted oceanic slab in arc regions (Hed-enquist and Lowenstern, 1994); sedimentaryand volcanic rocks in the crust (Krauskopf,1967; Noble, 1970); remnants of metallogenicheterogeneity in the mantle (Sillitoe, 1972); orthe lower crust during anatexis (Hedenquistand Lowenstern, 1994). Although the sourceis likely to vary from region to region (Hed-enquist and Lowenstern, 1994), conventional

wisdom views metals as indigenous to the re-gion of magma generation (e.g., Mathur et al.,2000). The lack of access to magmatic sourceregions unequivocally linked to mineralizedplutons has forced geologists to speculate onprocesses that enrich magmas.

Tomkins and Mavrogenes (2002) showedthat partial melting of the host rock at theChallenger Au deposit coincided with meltingof Au 1 Bi 1 sulfides during high-temperaturemetamorphism. Segregation of these two meltsproduced silicate-melt leucosomes containing, insome cases, percent levels of silicate-immiscibleAu-sulfide melt. This effect was so compre-hensive that .95% of visible Au in the de-posit is found within leucosomes. If melt ex-traction had been more efficient at thislocality, all of the silicate melt (and Au) wouldbe gone. Examples of efficient silicate-meltextraction were thoroughly described by Saw-yer (2001). However, would inefficientsilicate-melt extraction lead to any enrich-ment? To address this question, we first re-view the physical properties of metal-sulfide(polymetallic) melts and the current under-standing of how felsic silicate melt escapesfrom migmatites, before investigating metalenrichment of felsic magmas.

PHYSICAL PROPERTIES OFSILICATE VS. METAL-SULFIDEMELTS

Frost et al. (2002) introduced the acronymLMCE (low-melting-point chalcophile ele-ments) to group elements that form low-temperature sulfide melts. LMCE flux metalsand sulfides to lower melting temperatures(e.g., Fig. 1). In this way, higher-melting-pointmetals and sulfides are able to contribute tolow-temperature polymetallic melts. Native

metals such as Au and Ag occur in nature insuch low concentrations that they should com-pletely dissolve in metallic melts. This ap-pears to have occurred at the Challenger de-posit (Tomkins and Mavrogenes, 2002), wherenearly all Au is contained in globules thatwere once polymetallic melt (Fig. 2).

Felsic silicate melts are viscous (Scaillet etal., 1998), although dissolved water lowerstheir viscosity. However, even wet silicatemelts are far more viscous than liquid metalsand sulfides, which have viscosities similar towater (Richardson, 1974; Dobson et al.,2000).

Polymetallic melts, which may contain sev-eral heavy elements, are much denser than sil-icate melts. Silicate melt is more buoyant thanits restite (e.g., Brown, 1994), whereas poly-metallic melt is typically denser.

Silicate melts crystallize over a range oftemperatures, from ;1200 8C for a basalt to;650 8C for a water-saturated granite (e.g.,McBirney, 1993). Polymetallic melts, how-ever, have a wider range of crystallizationtemperatures, encompassing temperaturesdown to 239 8C, the freezing point of mer-cury (e.g., Frost et al., 2002).

It is well documented that once a silicatemelt is saturated in sulfur, an immiscible sul-fide melt exsolves (e.g., Naldrett, 1989). Themetals under consideration have low solubilityin felsic magmas. Au saturates at ;1 ppm(Frank et al., 2002), and Cu concentration ofmuch as 470 ppm in rhyolitic melt (in exper-iments at fluid saturation, 800–850 8C and50–100 GPa) was reported by Williams et al.

766 GEOLOGY, September 2003

Figure 2. Examples from Challenger Au deposit of Au-sulfide grains that were once polymetallic melt globulestrapped within migmatitic leucosomes (Tomkins and Mavrogenes, 2002). Asp—arsenopyrite with some Ni andCo solid solution, Po—pyrrhotite, Cpy—chalcopyrite, Sph—sphalerite, Pnt—pentlandite, Mal—maldonite. Allcompositions have been confirmed by scanning electron microscopy.

(1995). Other metals show similarly low sol-ubility (e.g., Candela, 1989; Jugo et al., 1999).Some of these metals (particularly Cu) aremore soluble in mafic magmas (e.g., Ripley etal., 2002). We suggest that metal solubility infelsic silicate melts may correlate with meltH2O content (i.e., higher H2O content 5 high-er solubility).

SOURCES OF METALS IN THELOWER CRUST

Several authors have noted that the lowercrust is depleted in metals (e.g., Cameron,1989). These observations seem reasonable,because trace metals are stripped by fluids lib-erated during prograde metamorphism. How-ever, many mineral deposits have survivedgranulite facies metamorphism (e.g., BrokenHill, Mavrogenes et al., 2001; Challenger,Tomkins and Mavrogenes, 2002).

Because hydrothermal mineral deposits typ-ically form from vast volumes of fluid focusedthrough small volumes of rock, a similarlylarge amount of fluid would be required to re-move metals during metamorphism. Dewater-ing during prograde metamorphism does notproduce adequate water to redissolve a largeorebody. It is clear that the best sources ofmetals in the lower crust are preexisting min-eral deposits, because the remainder of thecrust is depleted.

ESCAPE MECHANISMS FOR FELSICMELTS

It is now accepted that migmatites representthe source regions for felsic melts (e.g., Saw-yer, 2001). Clemens and Watkins (2001) sug-gested that the most granitoid magmas are theproduct of vapor-absent melting, and it is gen-erally recognized that melting of the lowercrust is often a regional occurrence (e.g., Saw-yer, 2001). Tomkins and Mavrogenes (2002)showed that coeval silicate and polymetallicmelts segregated into leucosomes at the Chal-lenger deposit during partial melting. Thus,processes that control silicate-melt extractionaffect metal incorporation in escaping magma.

Brown (1994) suggested that the physicalprocesses that control melt distribution duringanatexis are either buoyancy related or struc-turally driven. Buoyancy driven segregation isessentially a compaction process that takesplace in the absence of deformation, where thedenser solid matrix settles, expelling the light-er intergranular melt (e.g., Petford et al.,2000). The rate of this process varies withmelt viscosity and density differences betweensolid and melt. However, given the almost uni-versal structural complexity of migmatites(Brown, 1994), this process is probably rare.

During deformation, melt flow is assuredbecause strain is partitioned into the more eas-ily deformable melt (Vigneresse and Tikoff,

1999). Rocks react to deformation by chang-ing grain shape, causing grain boundaries tomigrate, thus allowing melt present at thesesites to move down hydraulic gradients (Vig-neresse and Tikoff, 1999). High-strain areastherefore offer the greatest permeability to themelt because grain boundaries are changingmore rapidly. Thus, most melt movement iskinematically controlled. Deformation effec-tively squeezes melt from the matrix into di-latant zones. This is the principal method bywhich melt leaves migmatitic source rock(Vigneresse and Tikoff, 1999).

IMPLICATIONS FOR METALCONTENT OF ESCAPED MAGMAS

Metals can be extracted from a migmatiticsource area by dissolution in silicate melt(e.g., Frank et al., 2002) and possibly by beingcarried as physically entrained polymetallicmelt. In the rare case where compaction-relatedbuoyancy-driven processes alone are respon-sible for the initial segregation of silicate meltfrom restite (before sulfide melt globules areencapsulated within accumulations of silicatemelt), it is likely that polymetallic melt (beingvery dense) would remain in the restite, andthe only metal incorporated in the escapingmagma is the dissolved component.

However, polymetallic melts, because oftheir low viscosity, are even more readily mo-

GEOLOGY, September 2003 767

Figure 3. Conceptual cross-sectional model of Earth’s crust showing link between anatexisof disseminated sulfide deposit in lower crust and formation of felsic magma–related min-eral deposits in upper crust. Arrows in circled enlargement show migration of polymetallicmelt (solid dots) with silicate melt, initially into leucosomes and eventually into progres-sively larger magmatic lenses.

bilized during deformation than silicate melts.Therefore, any silicate-melt segregation pro-cess that involves deformation will undoubt-edly mobilize polymetallic melt to the samedilational sites, producing accumulations ofsilicate melt containing immiscible polyme-tallic melt.

After accumulation into larger bodies, up-ward movement of silicate melt through thecrust may be largely governed by buoyancy.Immiscible globules of metal-sulfide melthave been shown to be carried along withinsilicate melts in experiments designed to testsulfide melt 1 mafic magma systems (de Bre-mond d’Ars et al., 2001). However, felsicmagmas are known to migrate more slowlyand to be more viscous than mafic magmas,and thus should be considered separately.Stokes’ law of settling applies:

2W 5 {[(r 2 r)g]/18m}d ,s (1)

where W is the settling rate of a sulfide meltglobule, rs is the density of the sulfide meltglobule, r is the density of silicate melt, g isthe acceleration due to gravity, m is the vis-cosity of granitoid magma, and d is the di-ameter of the sulfide melt globule. With theuse of appropriate values,

28 21W 5 1.059 3 10 m·s . (2)

This estimate is based on the viscosity ofgranitic melt (105 Pa·s; from Scaillet et al.,1998), the density of rhyolite melt at 800 8C(2220 kg·m23; from McBirney, 1993), and thedensity of the sulfide melt globule at 10,000kg·m23. The size of the sulfide melt globuleis approximately equal to the largest found atthe Challenger deposit (500 mm; Tomkins andMavrogenes, 2002). These factors are conser-vative in that (1) rhyolite is less dense thansilicic melts derived during anatexis, (2) thedensity of the sulfide melt globule at 10kg·m23 is probably denser than most possiblesulfide melts, and (3) larger globules settlefaster. Of these factors, the most critical is theglobule diameter, e.g., a globule with a di-ameter of 5 mm would fall two orders of mag-nitude faster, at ;1 3 1026 m·s21. When com-pared to granitic melt velocities of 1 3 1027

to 1028 m·s21 (Watt et al., 1996), we find thatthe rate of settling of globules ,500 mm isnot fast enough to allow differentiation. Thus,small globules of encapsulated sulfide meltshould be physically carried by accumulationsof buoyantly rising silicate melt.

TRANSPORT OF MAGMA TO THENEAR SURFACE

Lower-crustal melts (6 entrained polyme-tallic melt) from a large basement region

eventually coalesce into granitic plutons (Fig.3). However, immiscible polymetallic meltglobules are only rarely observed in granites.Instead, metals are generally considered to bedissolved at trace concentrations in the mag-ma. Yet some leucosomes at the Challengerdeposit contain as much as 1%–2% Au (or;4% polymetallic melt). If high levels of po-lymetallic melt are to be gradually dissolvedin the host silicate melt during transport of themagma to the near surface, there needs to beextensive dilution of the metal concentration.For example, a volume of silicate melt con-taining 2% Au as polymetallic melt wouldhave to be diluted by a factor of 20,000 todissolve all of the Au (based on Au solubilityin magma of 1 ppm). However, this estimateis based on solubility experiments conductedat water saturation and at low pressure, so theactual dilution factor may differ. Nevertheless,it is clear that significant dilution is required.

Because the viscosity of felsic silicate meltis high, small globules of polymetallic meltare likely to be relatively immobile within it.However, the common view is that most felsicmagma migration takes place along narrowstructurally controlled pathways as dikes anddoes not form batholithic bodies until reach-ing the upper crust (Petford et al., 2000). Con-sidering that partial melting of the lower crusttypically takes place at 5–10 kbar (;18–34km depth), melts must travel many kilometersalong narrow pathways, with major additionof sulfur-undersaturated silicate melt fromother sources before it ponds (Fig. 3). The

deformation-induced nature of this migrationis likely to ensure that considerable physicalmixing occurs, thus allowing each sulfide meltglobule to chemically interact with a largevolume of silicate magma. Therefore, duringascent, this mixed melt, with increasing sili-cate melt relative to polymetallic melt, beginsdissolving sulfides as the magma becomes in-creasingly sulfur-undersaturated. If the addedmagmas are more mafic, the solubility in-crease is enhanced. If H2O is added to the meltduring migration through the middle crust, wespeculate that metal solubility will increasefurther. Despite these factors, complete dis-solution of entrained sulfide melts may not al-ways eventuate. A direct link between sulfidemelt inclusions (preserved in felsic intrusives)and porphyry-type ore deposits was interpret-ed by Halter et al. (2002).

As pressure decreases during upward mi-gration, the melt approaches water saturation.Eventually, water saturation is reached as themagma is emplaced in the upper crust, and thesilicate melt exsolves a metal-rich hydrother-mal fluid. For example, if granitic melt con-taining ;3.5 wt% H2O, derived from vapor-absent melting of the Challenger deposit at;850 8C and 7.5 kbar, followed an isothermaldecompression path, it would become saturat-ed and exsolve fluid at ;1 kbar or ;3 kmdepth, a fairly typical crustal level for the for-mation of porphyry or epithermal deposits. IfH2O is added to the granitic magma during itsupward migration, saturation and exsolutionoccur at greater depths.

768 GEOLOGY, September 2003

RECOGNITION OF INTERACTIONBETWEEN SULFIDE MELT ANDSILICATE MELT IN MIGMATITES

Many migmatites retain some silicate melt.In these cases, some sulfide melt may also bepreserved within leucosomes. In addition, sul-fide melt may be trapped as inclusions in leu-cosome minerals, even where melt removalhas been very efficient.

Migmatitic rocks that show evidence of in-teraction between sulfide melt and silicatemelt may be recognized by depletion in me-sosomes, and enrichment in leucosomes, oftrace metals and minerals that partition strong-ly into sulfide melts (e.g., Au, Ag, Bi, Sb- andTl-bearing sulfides; Frost et al., 2002). Com-mon sulfides are likely to show a similar, butless obvious heterogeneous distribution. Someminerals such as sphalerite and pyrrhotite con-tribute less to sulfide melts than others, suchas galena (Mavrogenes et al., 2001), arseno-pyrite, and stibnite. Determination of sulfidemelting requires an analysis of phase relation-ships between coexisting mineral phases, be-cause some assemblages melt at very hightemperatures.

On a large scale, areas that have lost sulfideand silicate melts are nevertheless likely topreserve elevated sulfide mineral contents be-cause, even at the highest metamorphicgrades, it is unlikely that the entire sulfide as-semblage melted. Although they do contributeto sulfide melts, some minerals are likely tobecome concentrated in the restite if theycompose a large proportion of the sulfide as-semblage (Mavrogenes et al., 2001).

CONCLUSIONSWe have indicated two means by which ore-

forming metals may be incorporated into fel-sic silicate melts during anatexis. Metals maybe simply dissolved within silicate melts, andsulfide melts may be physically entrainedwithin escaping silicate melts. The differentphysical properties of sulfide melts vs. silicatemelts affect how and when sulfide melts areentrained as immiscible inclusions within thesilicate melts. Synchronous melting of a sul-fide assemblage (of any deposit type) and thesilicate host rock during deformation repre-sents the most favorable scenario for incor-porating the largest proportion and overallamount of metal in silicate magma. Thesephysically entrained sulfide melts are likely tobe gradually dissolved during transport of thesilicate magma to the upper crust.

Because mineral deposits are relatively rare,significant heterogeneity in metal content offelsic magmas produced from any one regionis expected. Migmatites likely to have under-gone this process are recognized by elevatedsulfide contents, mesosomes that are depleted

in trace chalcophile metals and minerals, andcorrespondingly enriched leucosomes.

ACKNOWLEDGMENTSWe thank Jon Standing, Dave Francis, and Je-

rome Gillman (geologists at Resolute Ltd.) for lo-gistical and financial support, and Andrew Rankinand Tony Naldrett for helpful reviews.

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Manuscript received 3 February 2003Revised manuscript received 16 May 2003Manuscript accepted 18 May 2003

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