Did the giant Broken Hill (Australia) Zn–Pb–Ag deposit melt?

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Ore Geology Reviews 34 (2008) 223 – 241 www.elsevier.com/locate/oregeorev

Did the giant Broken Hill (Australia) Zn–Pb–Ag deposit melt? Paul G. Spry a,⁎, Ian R. Plimer b , Graham S. Teale c a

Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, USA b School of Earth and Environmental Sciences, University of Adelaide, South Australia 5005, Australia c Teale & Associates, PO Box 740, North Adelaide, South Australia 5006, Australia Received 9 May 2007; accepted 11 November 2007 Available online 4 December 2007

Abstract Recent published research has proposed that some metamorphosed massive sulfide deposits, including the Broken Hill deposit, Australia, have partially melted. Here we discuss the evidence for and against this process. The Broken Hill deposit is located in the Curnamona Craton within a rift in an apparent upward-coarsening sequence of clastic metasediments into which Paleoproterozoic mafic and felsic melts intruded. The deposit was metamorphosed to granulite facies conditions (750–800 °C and 5–6 kbar) and was subjected to at least five periods of deformation. At peak P–T conditions, a melt phase was produced in silicate rocks, especially granitoids and psammopelites, and in ore characterized by the systems Cu– As–S, Ag–Pb–S, Ag–Sb–As–S, Cu–Sb–Pb–S, Sb–As–S, Cu–Sb–S, and Fe–As–S. However, the proportion of minerals in these S-bearing systems is insignificant (b 1 wt.%) when compared to the volume of sulfides in the system SiO2–FeO–MnO–CaO–Al2O3–P2O5–CO2–ZnS– PbS–FeS–(FeAsS/FeAs2), which is the most relevant system to Broken Hill ores. P–T conditions were never high enough to produce a melt phase in this system. Authors arguing for sulfide rock melting suggest that Mn-rich lithologies (garnetite and quartz garnetite), which are intimately associated with the Pb-rich ores at the Broken Hill deposit, were produced by a reaction between Mn-rich sphalerite and aluminous wall rocks. However, to produce such rocks would require Mn contents of sphalerite compositions that are unrealistically high and yet to be found in nature. Moreover, the implication of the melt model is that wherever Mn-rich quartz garnetite and garnetite are found sphalerite should be located next to them. This is clearly not the case. The presence of polyphase sulfide inclusions within garnet in garnetite has also been considered by some in the literature to be sulfide melt inclusions with the implication that the enclosing garnet was a product of melting. However, we consider these sulfides to be products of hydrothermal processes during retrograde metamorphism. It is impossible to form garnetite and quartz–garnetite in rocks within the Curnamona Craton that formed at upper greenschist–lower amphibolite facies conditions by partial melting. P–T conditions were too low. We consider that the Fe and Mn component of garnetite and quartz garnetite are products of exhalation and inhalation at or near the sea floor, and that these rocks are meta-exhalites or meta-inhalites. Some garnet-rich rocks also formed by metasomatic processes throughout the protracted metamorphic history that affected the Curnamona Craton. © 2008 Elsevier B.V. All rights reserved. Keywords: Partial melting; Sulfides; Broken Hill; Australia

1. Introduction Based in large part on the results of experimental studies derived from the systems Fe–Pb–Zn–S (Avetisyan and Gratyshenko, 1956) and Pb–Fe–S (Brett and Kullerud, 1967), Lawrence (1967) suggested that sulfides in the Broken Hill Zn–Pb–Ag deposit, Australia, partially melted during peak high-grade regional metamorphism and was responsible for the formation of “sulfide metapegmatites”. In following up on the ⁎ Corresponding author. Tel.: +1 515 294 9637; fax: +1 515 294 6049. E-mail address: [email protected] (P.G. Spry). 0169-1368/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.oregeorev.2007.11.001

concept of partial melting of sulfides put forward by Lawrence (1967), Vokes (1971) considered the results of further experiments in the systems Fe–Pb–S (Brett and Kullerud, 1967), Cu– Fe–Pb–S (Craig and Kullerud, 1968), and Ag–As–S (Roland, 1965; 1968) and pointed out that eutectic melts form in the systems Fe–Pb–S, Cu–Pb–S, and Ag–As–S at b 700 °C, 508 °C, and b500 °C, respectively. From this information, Vokes (1971) proposed that sulfide melts may be a common product of regional metamorphism. Based on experimental considerations and ore textures, Hofmann (1994) subsequently showed that a metamorphogenic sulfide melt formed in the Lengenbach Pb– Zn–As–Tl–Ba deposit. The concept of partial melting in ore

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formation has been extended by Frost et al. (2002a), Tomkins (2006) and Tomkins et al. (2007) to include a variety of ore deposits including massive sulfide deposits, disseminated gold deposits, and various types of copper deposits. Frost et al. (2002a), along with a plethora of recent publications and conference presentations (MacIntosh and Mavrogenes, 1998; Mavrogenes et al., 2000, 2001, 2004; Frost et al., 2000a,b, 2001, 2005; Frost and Swapp, 2001, 2003, 2004; Gregory et al., 2003, 2004; Sparks, 2003; Wykes, 2003; Wykes and Mavrogenes, 2003, 2005; Sparks and Mavrogenes, 2003a,b, 2004a,b, 2005; Sparks et al., 2006, 2007; Tomkins et al., 2007), reinforced Lawrence's (1967) partial melt hypothesis for the Broken Hill deposit. The aim of the present study is to evaluate the concept of partial melting for sulfur-bearing minerals in the Broken Hill deposit on the basis of field work conducted almost continuously since 1968 by the authors at Broken Hill and available experimental data, and laboratory studies. The current paper will also evaluate the genetic relationship of manganiferous garnet-rich rocks to high-grade sulfides, particularly garnetite, as it was considered by Frost et al. (2002a), Mavrogenes et al. (2004), and Sparks et al. (2006) to be a product of the melting of sulfides. Although this contribution will primarily center on the Broken Hill deposit, Australia, we will also discuss the recent work of Bailie and Reed (2005) who proposed the presence of partially melted sulfides in the Broken Hill deposit, Aggeneys, South Africa. 2. Geologic setting 2.1. Regional geological setting The Broken Hill mine occurs in the south-eastern part of the Curnamona Craton (Fig. 1), a large domain of high to low

metamorphic grade rock-types ranging in age from ∼1710 Ma to ∼ 1570 Ma and generally overlain by younger cover. In the Broken Hill region, geological mapping at 1:12,500 scale by the New South Wales Geological Survey has compiled a ∼ 7 km thick stratigraphy of the Willyama Supergroup comprising a wide range of metamorphosed deformed lithologies comprising pelite, psammopelite, psammite, quartzofeldspathic and mafic rocks and hydrothermal sediments that formed in an intracontinental rift (Willis et al., 1983). Younging, vergence, fold morphology and overprinting of schistosities were used to compile the regional structure (Laing et al., 1978) and structure associated with the Broken Hill orebodies (Laing, 1996). However, there is still unresolved controversy regarding the detailed structural controls of the Broken Hill ore deposit (White et al., 1994; Laing, 1996; Stevens, 1996; Rothery, 2001; Roache, 2004; Webster, 2006), which, in part, derives from the scales of observation and the availability of stope faces at the time of observation. The age of the Willyama Supergroup is tightly constrained from ≤ 1710 Ma and ≥ 1704 ± 3 Ma in the middle of the Willyama Supergroup (Thackaringa Group; Love 1992; Donaghy et al., 1998; Page et al., 2005b) to ≤ 1642 ± 5 Ma at the top of the Willyama Supergroup (Nutman and Gibson, 1998; Stevens, 2000; Page et al., 2005b). Felsic gneiss intimately associated with the Broken Hill sulfide rocks was dated at 1686 ± 3 and 1689 ± 5 Ma (Stevens, 2000; Page et al., 2005b) and lead isotope studies (Parr et al., 2004) suggest that the Pinnacles deposit (Cues Formation, Thackaringa Group) is some 10 million years older than the Broken Hill deposit (Hores Gneiss, Broken Hill Group). This is in accord with the stratigraphy (Willis et al., 1983) and SHRIMP geochronology (Page and Laing, 1992; Page et al., 2005a,b). Phillips et al. (1985), Plimer (1986) and Parr and

Fig. 1. Geological map of the southern Curnamona province (modified after Laing et al., 2002; Page et al., 2005a).

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Fig. 2. Map of the Broken Hill and Euriowie Domains showing the metamorphic zones for the prograde domains (And + Ms; Sil + Ms; Sil + Ksp; Opx + Cpx) and the staurolite (St) and kyanite (Ky) isograds for the retrograde metamorphism. Drawn after Phillips (1980), Stevens et al. (1988), and Frost et al. (2005). Abbreviations: And = andalusite, Cpx = clinopyroxene, Ksp = K-feldspar, Ms = muscovite, Opx = Orthopyroxene, Sil = sillimanite.

Plimer (1993) argued that deposition of the Broken Hill ore deposit was coeval with bimodal felsic–mafic volcanism and premetamorphic alteration.

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The multi-phase (D1–D3) Olarian Orogeny occurred at 1600–1590 Ma according to Page and Laing (1992), Stevens (1999) and Page et al. (2005a,b). During D1, there was a synchronous granulite facies-folding event wherein axial planar pegmatite and granite were generated (Fig. 2). Although outcrops of F1 fold closures are rare, the parallelism of S1 with S0 in both the Broken Hill and adjacent Olary Domains suggests large, recumbent, nappe-like isoclinal folds (Laing et al., 1978; Stevens, 1986). F1 has inverted large areas of the Willyama Supergroup stratigraphy, including the sequence that hosts the Broken Hill orebody. The second ductile deformation event of the Olarian Orogeny (D2) produced widespread tight, upright macroscopic and mesoscopic folds (F2) throughout the Broken Hill Domain (Laing et al., 1978). Three major F2 folds are present within the mines area: the Hanging Wall Synform, the Broken Hill Antiform and the Broken Hill Synform (Laing et al., 1978). Although the presence of the Broken Hill Antiform has been challenged by Webster (2006), recent stratigraphic and structural studies by one of us (IRP) show the presence of an F2 antiform in the center part of the Broken Hill deposit (CML7). F2 folding was coeval with M2 granulite facies metamorphism (Phillips, 1980; Stevens, 1986). The D3 event produced large to small F3 folds with nearvertical axial planes (Laing et al., 1978; Stevens, 1986). Stevens (1986) suggested that F3 was coeval with retrograde metamorphism in the Olarian Orogeny calculated by Phillips (1980), Corbett and Phillips (1981) and Stevens (1986) to be at T = 550 to 600 °C and P = 5.0 to 5.5 kbar. F3 folds are closely associated

Fig. 3. Geological map of the Broken Hill deposit. Abbreviations: N.B.H.C = New Broken Hill Consolidated mine (currently part of Southern Operations operated by Perilya Broken Hill Limited), Z.C. = Zinc Corporation mine (currently part of Southern Operations operated by Perilya Broken Hill Limited), B.H.S. = Broken Hill South mine (currently CML7 operated by New Broken Hill Consolidated Limited), and N.B.H. = North Broken Hill mine (currently North mine operated by Perilya Broken Hill Limited).

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2.2. The sulfide ore lenses and lodes at Broken Hill

Fig. 4. Section 30 cross section though the Broken Hill deposit, Australia, showing the stratigraphically lower Zinc lodes (C lode, B lode, A lode, 1 lens) and the stratigraphically higher Lead lodes (2 and 3 lenses) that have projected up the axes of F2 structures, have thickened at the F2 hinges and have been injected into the Main Lode Shear Zone (after Haydon and McConanchy, 1987; Plimer, 2007).

with retrograde metamorphism and with the initiation of retrograde shear zones (Laing et al., 1978). Evidence for a further deformation event (D4) that is associated with amphibolite facies metamorphism occurs in the floor of Block 14 pit where cross folding produced a D3–D4 dome and basin structure (Plimer, 2006b).

The Broken Hill Zn–Pb–Ag deposit has produced some 200-Mt of high-grade ore since discovery in September 1883 (Fig. 3) and is composed of a number of discrete lenses of high to low metal content sulfide rocks of different chemistry and mineralogy within three distinct stratigraphic horizons. These lenses and lodes have been termed the 3, 2 and 1 lenses and the A, B and C lodes (Fig. 4). The grade and tonnages of these orebodies is given in Table 1. Redevelopment of the southern portion of the Broken Hill field by Perilya Mines Ltd and evaluation by CBH Resources Ltd of the unmined Zinc lodes (the C, B and A lodes) and their down dip facies equivalent (Western Mineralization) in the central 3.8 km of the orebody are adding tonnages of zinc-rich ore. The Potosi mineralization, which occurs in the northern end of the Main Line of Lode, is currently undergoing development drilling by Perilya Mines Ltd. (Teale et al., 2006). The Broken Hill Zn–Pb–Ag deposit occurs towards the top of the Broken Hill Group in a sequence of coarsening upward psammopelitic metasediments laterally equivalent to the Hores Gneiss. The massive sulfides occur as six discrete orebodies in the Main Line of Lode over a strike length of ∼ 8.5 km and minor discontinuous sulfide masses occur over a strike length of 25 km. The orebodies have a characteristic gangue mineralization, ore grade, and trace element signature (Burrell, 1942; King and Thomson, 1953; Johnson and Klingner, 1975, Plimer, 1984; Parr and Plimer, 1993), which lead Johnson and Klingner (1975), for example, to support a syngenetic model of ore formation. Although each orebody contains a multitude of gangue minerals the dominant gangue minerals in each orebody are: rhodonite, fluorite, quartz (3 lens), calcite, rhodonite, wollastonite (2 lens), quartz, calcite, wollastonite (1 lens), rhodonite, manganoan hedenbergite (A lode), quartz (B lode), and quartz (C lode). The relative proportion of one gangue mineral to another varies along the extensive strike length of an orebody and even within a stratigraphic sense. For example, blocks and layers of rhodonite, garnetite, and quartz garnetite are unevenly distributed within the lead-rich orebodies (2 and 3 lenses) with large masses of rhodonite occurring in the basal parts of 2 and 3 lenses (e.g., Johnson and Klingner, 1975). However, more important is that the stratigraphic lowermost orebodies in the deposit are richer in Zn and Cu and poorer in

Table 1 Composition of the principal orebodies at Broken Hill Lode

C B A 1 2 3 (south) 3 (north)

Size

Pb

Zn

F

P

Ag

Cu

As

Sb

Bi

Co

Hg

(Mt)

(%)

(%)

(%)

(%)

(ppm)

(ppm)

(ppm)

(ppm)

(ppm)

(ppm)

(ppm)

11 46 53 10 85 79

3 5 4 8 14 11 15

5 17 10 20 11 15 13

na 0.11 0.06 0.34 1.35 1.1 na

na 0.18 0.11 0.08 0.04 0.09 na

20 40 40 50 100 200 300

na 2000 1200 900 1400 1400 na

na 450 1440 210 560 3985 na

na 113 67 372 413 418 na

na 37 48 28 10 2 na

na 79 120 140 83 68 na

na 12 1 3 2 37 na

Tonnage includes 3 south and north. Data derived from Johnson and Klingner (1975), Plimer (1979), Burton (1994), and Stevens and Burton (1998).

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Pb and Ag. This same metal variation is also apparent along strike within some orebodies, particularly 3 lens (Plimer, 1984). Furthermore, trace element data of Johnson and Klingner (1975) for the southern end of the deposit show an increase in Sb, As, and F as well as a decrease in Cu, Bi, P, and Ni from the stratigraphically lowermost to the uppermost orebodies. Small-, b 10 m long (Fig. 5B), and large-scale (N 10 m long) sulfide projection occur into the wall rocks around orebodies. Garnet is an accessory phase in each orebody and is particularly abundant in garnetite and quartz garnetite that are spatially associated with most of the orebodies (Fig. 5C, D). As was shown by Spry and Wonder (1989), Spry et al. (2003), and Plimer (2006b), the composition of garnet in these rocks is a reflection of the bulk composition of the orebodies. The results of these studies and the characteristic major and trace element composition of each orebody show that sulfides from one orebody has not physically mixed with adjacent orebodies during the intense deformation that has affected the Broken Hill deposit. The origin of the mineralization at Broken Hill has been a source of controversy for most of its mining life. Surface outcrops of the deeply oxidized deposit were hosted by a shear zone and it was concluded initially that the Broken Hill ore deposit was deposited in brittle deformation structures (Wilkinson 1884; Jacquet, 1894). Since then, there have been a multitude of genetic theories but the two most popular models are: 1. Sulfide formation by subaqueous hydrothermal processes that has undergone multi-phase high-grade metamorphism and deformation (e.g., King and Thompson, 1953; Stanton and Russell, 1959; Johnson and Klingner, 1975; Both and Rutland, 1976; Laing et al., 1978; Barnes et al., 1983; Parr and Plimer, 1993; Marshall and Spry, 2000; Spry et al., 2000, 2007; Plimer,

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1979, 1984, 2007); and 2. Syntectonic introduction of metals during peak metamorphism or post-tectonic replacement (The Geological Sub-Committee, 1911; Mawson, 1912; Moore, 1916; Andrews, 1922; Kenny, 1932; Burrell, 1942; Gustafson et al., 1950; Stillwell and Edwards, 1956; Stillwell, 1959; Lewis et al., 1965; Ehlers et al., 1976; Nutman and Ehlers, 1998; Rothery, 2001; Gibson and Nutman, 2004). A third genetic model stems from the work of Lawrence (1967) who suggested that the sulfide rocks were originally conformable to bedding, had undergone multi-phase high-grade metamorphism and may have melted during metamorphism. Mavrogenes et al. (2001) supported the views of Lawrence and considered that the inverted metal zoning pattern of the orebodies could have resulted from partial melting with the Zn lodes being the residual of the Pb-rich melt, which formed the Pb-rich orebodies. Recent synopses of the various proposed genetic models are given in Greenfield et al. (2003) and Webster (2006). 2.3. The location of Ag, Sb, Cu, W, As, Hg and Au in the Broken Hill deposit The mineralogy of the Broken Hill orebodies has been studied by many workers including Gustafson et al. (1950), Johnson and Klingner (1975), Both and Rutland (1976), Plimer (1984), Burton (1994), and Birch (1999). The primary ore is dominated by galena and sphalerite with minor amounts of pyrrhotite, chalcopyrite, tetrahedrite, löllingite, arsenopyrite and various sulfosalts. Löllingite is often rimmed by arsenopyrite and attests to the sulfur-poor nature of the Broken Hill orebodies. The distribution of the most abundant sulfides and sulfosalts varies markedly between and within orebodies.

Fig. 5. A. Vein of massive sulfide injected into the wall rocks (20 level, Zinc Corporation mine, 3 lens. B. Vein of massive sulfide (Sulf vein) injected into the wall rock. Note the end of then is of quartz (Qtz vein) suggesting that the sulfide vein was fluid facilitated (20 level, Zinc Corporation mine, 3 lens). C. Photo of laminated garnetite from A lode (drill hole 6844 47.8m) showing cross-cutting veinlet containing quartz, calcite, tetrahedrite, and galena. The scale rule is in mm. D. Photo of garnetite from A lode (drill hole 6844 50.2 m) containing cross-cutting recrystallized scheelite (Sch). The scale rule is in mm.

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Silver is largely incorporated in the structure of galena (Both, 1973) but the ratio of Pb:Ag varies from within and between orebodies, with the 2 and 3 lenses containing the highest ratios. As was documented by Lawrence (1968) and Plimer (1984), Ag is also incorporated in a variety of other minerals including tetrahedrite, pyrargyrite, polybasite, stephanite, argentite, antimonial silver, allargentum, dyscrasite, and native silver. Tetrahedrite, of differing compositions, also hosts much of the Hg, Sb, Bi, Te and some As and Cu in the primary ore. The ores contain up to 300 ppm Ag, 0.2% Cu, 0.4% As, 418 ppm Sb, 48 ppm Bi and 37 ppm Hg (Johnson and Klingner, 1975; Plimer, 1979) but most of the orebodies contain considerably lower concentrations of these elements (see Table 1). Retrogression, remobilization and general cooling of the ores lead to the development of a myriad of new sulfides and sulfosalts (e.g., gudmundite, bournonite, stephanite, stibnite, argentite, polybasite, pyrargyrite) most of which are dominated by the elements located in the primary tetrahedrite (Lawrence, 1968). They occur as felted aggregates, reaction selvages on tetrahedrite, exsolution bodies within tetrahedrite (see Worner and Birch, 1983) and complete pseudomorphs after tetrahedrite and other minerals (Fig. 6A, B). Faults, fractures, supergene enrichment and tongues of high-grade ore projecting downwards (so-called “droppers”) or upwards from the margins of orebodies, particularly 2 and 3 lenses, tend to concentrate these elements. Lawrence (1968), Boots (1972) and Plimer (1984) describe veinlets containing quartz, calcite, bornite, tetrahedrite, berthierite, galena, scheelite, chalcopyrite, dyscrasite, stephanite, and arsenopyrite, which transgress the high-grade foliation of the enclosing rocks. These veinlets are particularly common in garnetite (Figs. 5C, D and 6C). Furthermore, the presence of galena enriched in silver and the presence of ruby silver minerals in shear and fault zones attests to the formation of these minerals during the retrograde period of metamorphism (Lawrence, 1968; Plimer, 1984). 2.4. Garnet-rich rocks at Broken Hill and their genetic relationship to sulfides Garnet-rich rocks have been the subject of numerous petrographic and geochemical studies and include those of Henderson (1953), O'Driscoll (1953), Stillwell (1959), Segnit (1961), Richards (1966), Jones (1968), Hodgson (1975a,b), Stanton (1976), Billington (1979), Haydon and McConachy (1987), Lottermoser (1988), Spry and Wonder (1989), Schwandt et al. (1992), Spry et al. (2000, 2007), and Plimer (2006b). Details of the mineralogy, descriptions of the three major sub-types of garnet-rich rock, and their geochemistry are given in Spry and Wonder (1989), Plimer (2006b), and Spry et al. (2007) and will not be repeated here. Garnet at Broken Hill occurs as crystals within ore, in the stratigraphic footwall of C lode as part of a pre-metamorphic hydrothermal alteration, on the margins of epidotized dolerite dikes, and as a dominant mineral in lode horizon rocks (Plimer, 2006a,b). Where present in a high-grade ore lens, garnet occurs as isolated fine to coarse grained crystals in sulfides, as trails from garnetite clasts, and as garnetite clasts suggesting that

some grains grew as porphyroblasts whereas others possess a xenoclastic origin. In the lode horizon, three types of garnet-rich rock were recognized by Spry and Wonder (1989) and Plimer (2006b). They are in order of abundance: quartz garnetite, garnetite, and garnet envelope. However, it should be noted that the Zinc lodes (1 lens, A, B, and C lodes) have a quartz–garnet halo (with or without gahnite, plumbian orthoclase, pyrrhotite) whereas the Lead lodes (2 and 3 lenses) are, in general, spatially associated with garnetite (Plimer, 2006b). Quartz garnetite generally occurs as laminated horizons or massive units in and adjacent to all sulfide orebodies. However, it should be stressed here that there are places (particularly adjacent to Lead lode orebodies) where there are contacts between sulfides and seemingly unaltered metapelites. Where quartz garnetite is layered, laminations vary from 2 mm to 10 cm thick and are defined by alternations of garnet and quartz, garnet and sulfide, different colored garnets, and garnet with other silicates. Layering is coplanar with S0 and S1 and could have resulted from shearing coplanar with S1. However, the stratigraphic continuity and position of these rocks, the presence of bedding, cross bedding and bifurcation, the absence of a mylonitic fabric, the grading of quartz garnetite into garnetite and garnet-bearing pelite, the presence of stacked horizons of quartz garnetite, and the absence of laminations parallel to deformation surfaces after S1 (e.g., S2, S3, etc.), strongly suggests that this rock-type was originally a sediment (Plimer, 2006b). Garnetite is a highly friable rock which contains N 90% garnet. The same textural features that produced laminations in quartz garnetite define laminations in garnetite. It is most common along the margins of 2 and 3 lenses and A lode but is stratigraphically equivalent to ore and can occur as boudinaged blocks within ore. One boudinaged layer of garnetite on the footwall of 3 lens in CML7 is 200 m in strike, 50 m in width and 1 to 2 m thick (Plimer, 2006b). In places, garnetite also occurs in metasedimentary rocks adjacent to sulfide ores, as bodies that form coplanar to a retrograde schistosity, and on the margins of droppers that form in the axial plane of F2 and F3 folds (Jones, 1968; Maiden, 1972). Where present, laminations in garnetite parallel S0 and S1 in adjacent rock-types but they can also be chaotic and unrelated to S0 and S1, especially where garnetite is spatially related to retrograde schist zones. Garnetite is also folded, disarticulated, intercalated with sulfide rocks, occurs as isolated clasts in sulfide ore, or is present as rare inclusions in planar unfolded veins. Veinlets of blue-quartz, gahnite, and minor amounts of S-bearing minerals locally crosscut garnetite (Figs. 5C, D and 6D). Garnetite that is coplanar with the enclosing metasediments and shows laminations parallel to S0 and S1 suggests it is a metamorphosed metasediment that has an exhalative or inhalative origin (Spry and Wonder, 1989; Plimer, 2006b; Spry et al., 2000, 2007) whereas garnetite that is spatially associated with retrograde schist zones and droppers that formed along the axial plane of F2 and F3 folds likely formed due to a reaction between manganese in the ore and aluminous wall rocks (Jones, 1968; Maiden, 1972, Hodgson, 1975a,b; Plimer, 2006b).

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Fig. 6. A. Reflected light photo of coarse grain of tetrahedrite (Td) containing rods of berthierite (Brt), and an intergrowth bornite (Bn), galena (Gn), and chalcopyrite (Ccp). This texture is considered to have formed by unmixing during retrograde cooling. Note that the minerals observed here are commonly found as sulfide inclusions in garnet within garnetite and in veinlets cross-cutting garnetite. B. Reflected light photo of a grain of tetrahedrite (Td) in a polygonal positional between three grains of galena (Gn). The tetrahedrite has converted, in places, to chalcopyrite (Ccp) and berthierite (Brt) (sample DDH 3219-44.3 m). Although not shown here, pyrrhotite and dyscrasite are also locally found as a breakdown products of pyrrhotite in the same section. C. Löllingite (Lol) core overgrown by arsenopyrite (Apy) rim. D. Reflected light photo of a veinlet of bornite (Bn), chalcopyrite (Ccp) and galena (Gn) cross-cutting garnetite (sample 2901). Note also the presence of these same minerals along the grain boundaries of garnet. E. Back-scattered electron image of a polyphase sulfide-rich inclusion in garnet (Grt) in garnetite (drill hole 6844 47.8 m). The phases are arsenopyrite (Apy), bornite (Bn), chalcopyrite (Ccp), galena (Gn), and tetrahedrite (Td). F. Back-scattered electron image of a polyphase sulfide inclusion consisting of bornite (Bn), chalcopyrite (Ccp), and galena (Gn), adjacent to a monomineralic galena inclusion in garnet (Grt) in garnetite (drill hole 6844 47.8 m). Note that the melting of galena at 1 bar is 1115 °C (higher at 5–7 kbar) showing that the galena inclusion cannot be a sulfide melt inclusion. G. Backscattered electron image of a multiple polyphase sulfide inclusions consisting of various combinations of bornite (Bn), chalcopyrite (Ccp), galena (Gn), and tetrahedrite (Td) and adjacent to a monomineralic galena inclusion in garnet (Grt) in garnetite (drill hole 6844 47.8 m). H. Back-scattered electron image of an intergrowth of bornite (Bn), chalcopyrite (Ccp), and galena (Gn) that formed along the margins of garnet (Grt) and quartz (Qtz) in garnetite (drill hole 6844 47.8 m). These late-stage minerals are among those that form sulfide inclusions in garnet within the same sample (see E, F, and G).

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A third, minor-type, of garnet-rich rock that is spatially related to the margins of the Lead lode orebodies is referred to as garnet envelope. In this setting, garnet, 1 mm to 1 cm in diameter, occurs in trains parallel to the schistosity in retrograde shear zones and along the margins of D3 quartz veins. Like garnetite, with which they are locally associated, they formed as a result of reaction between manganese in the orebody, or from manganese carried in quartz veins, and aluminous wall rocks. 3. Metamorphism, sulfide melts, sulfide textures, hydrothermal fluids and the Broken Hill orebody: A discussion 3.1. Peak metamorphic conditions affecting the Broken Hill deposit Binns (1964) first established that regional metamorphic grade increased southward in the Broken Hill Domain. Phillips (1980) extended the work of Binns and identified that this increase in grade from an andalusite + muscovite zone (approximately 500 °C), through sillimanite + muscovite (580 to 680 °C) and sillimanite + K-feldspar zones (680 to 760 °C) to the twopyroxene zone (760 to 800 °C), corresponded to a maximum pressure of 5 to 6 kbar in the sillimanite + K-feldspar and twopyroxene zones (Fig. 2). According to Phillips (1980) and Phillips and Wall (1981), the Broken Hill deposit was subjected to peak metamorphic conditions of 780 °C and 5.2 kbar. Application of the sphalerite–hexagonal pyrrhotite geobarometer of Bryndzia et al. (1990) to sulfides from Broken Hill ore yielded a pressure of 5.8 ± 0.7 kbar, consistent with the findings of Phillips (1980) and Phillips and Wall (1981). These peak conditions coincide with D1 and D2. Based on oxygen isotope geothermometry, Cartwright (1999) showed peak metamorphic temperatures in the two pyroxene zone of 753 ± 76 °C (1 sigma) for quartz–biotite pairs and 789 ± 43 °C for garnet–quartz pairs. No samples were taken directly from the Broken Hill deposit by Cartwright (1999) and individual samples that showed temperatures N 800 °C were collected from locations N 10 km from the mine. Two samples analyzed from the Alma Gneiss, approximately 3 km from the mine (the closest samples to the mine), yielded temperatures based on quartz–biotite pairs of 726 °C and 706 °C. Recent studies by Frost and Swapp (2003), Swapp and Frost (2003), and Frost et al. (2005) applied two pyroxene geothermometry to three samples, one from Round Hill, 2 km NW of the Broken Hill deposit, and two from Black Bluff, 4 km SE of the deposit. The two Black Bluff samples yielded temperatures of 827 ± 37 °C and 840 ± 17 °C, whereas that from Round Hill gave a temperature of 764 ± 27 °C. Two pyroxene-bearing assemblages in the vicinity of the Broken Hill mine, for example Round Hill, are rare and are possibly a function of bulk rock composition and oxygen fugacity. The assemblage K-feldspar–quartz–garnet– cordierite–sillimanite–biotite gave temperatures and pressures from an area 4 km SE of the Broken Hill deposit, and Round Hill of 750 °C–6 kbar and 750 °C–5.5 kbar, respectively. Pressures of N 9kbar and N 8 kbar were derived by Swapp and Frost (2003)

using the GRAIL assemblage (garnet–rutile–aluminosilicate– ilmenite) from samples collected at Round Hill and the airport locality, respectively. These pressures are unrealistically high when compared to all other geobarometers that have been applied to rocks in the Broken Hill Domain. Stevens (2006) noted that the high pressures reported by Swapp and Frost (2003) were incorrect (B.R. Frost, pers. comm.., 2004 to B.P.J. Stevens) and that they have reinterpreted the rutile–ilmenite–garnet textures as having formed during lower P–T conditions. Regardless of the validity of the temperatures obtained by two pyroxene geothermometry from the two samples collected by Frost and Swapp (2003) and Swapp and Frost (2003), they were not derived from the orebody. Given the fact that the orebodies and immediately adjacent rocks were affected by isoclinal folding, it has not been demonstrated that the temperatures derived from samples at Black Bluff represent conditions that affected the orebody. It should be noted here that application of the garnet–biotite geothermometer to pelitic rocks close to the ore by Gregory et al. (2003) produced temperatures of only 580 oC to 630 °C. These lower temperatures for garnet– biotite pairs are understandable considering that temperatures in the Broken Hill area probably decreased slowly from approximately 780 °C at 1600 Ma to 550 to 600 °C at 1545 Ma (G.S. Teale and C.M. Fanning, unpubl. data). The conclusion that can be reached from geothermometric and geobarometric studies is that there is no evidence for the orebodies to have been metamorphosed to pressure and temperature conditions greater than the 780 °C and 5.2 kbar originally suggested by Phillips (1980) and Phillips and Wall (1981). However, the orebodies were subjected to high temperatures over a prolonged period causing major metasomatism, generation of high to low temperature hydrothermal fluids, retrogression and major movement of metals. 3.2. Experimental studies relevant to possible partial melting of sulfides 3.2.1. Melting in the systems Pb–Zn–Fe–S and Pb–Zn–Fe–Ag–S In an attempt to determine whether or not melting of sulfides took place during peak metamorphism, various types of hydrothermal experiments have been conducted at pressures and temperatures that were believed to have affected the Broken Hill ores. Among the first of these experiments were those conducted by Mavrogenes et al. (2001) who showed that the eutectic for the system Pb–Zn–Fe–S is 830 °C at 5 kbar and 850 to 870 °C at 10 kbar (average of 860 °C). Based on these data, the first sulfide melt should form at about 838 °C assuming a peak metamorphic pressure of 6 kbar. However, we consider this temperature to be higher than the peak metamorphic temperature that affected the Broken Hill ores. In order to lower the eutectic temperature and to better simulate the ore system Mavrogenes et al. (2001) also added 1 wt.% AgS to saturate their experiments, but they recognized that this silver content is considerably higher than the average ore content of 0.05 wt.% Ag2 S, determined by Plimer (1982) for the Lead lodes; it is less than this for the Zinc lodes. Mavrogenes et al. (2001) suggested that the addition of 1 wt.% AgS will lower the eutectic temperature to “b 810 °C” assuming a pressure of 5 kbar.

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However, this temperature will be higher if 0.05 wt.% Ag2S is used. Mavrogenes et al. assumed that the primary Fe sulfide at Broken Hill was pyrrhotite rather than pyrite. In their experiments, they use a starting pyrrhotite composition of NFeS = 0.96, which they based on the work of Bryndzia et al. (1990). Mavrogenes et al. pointed out that this S-rich pyrrhotite, rather than troilite, will lower the solidus in the system Pb–Zn–Fe–Ag–S to 795 ± 5 °C at 5 kbar. This temperature will be about 803 °C if a pressure of 6 kbar is assumed. However, the average pyrrhotite composition of Bryndzia et al. (1990) is NFeS = 0.953 not 0.96, which will raise the eutectic temperature. There is pyrrhotite at Broken Hill that exhibits NFeS N 0.953, but this pyrrhotite (most of which is monoclinic pyrrhotite) has mostly formed along with secondary pyrite upon retrograde cooling of the ores. 3.2.2. Melting in the systems Ag–Pb–S, Ag–Sb–As–S, Cu–Pb– Sb–S, Cu–As–S, Sb–As–S, Cu–Sb–S, Fe–As–S and ZnS–FeS2– PbS–CuFeS2 Frost et al. (2002a) evaluated phase diagrams in the systems Ag–Pb–S, Ag–Sb–As–S, Cu–Pb–Sb–S, Cu–As–S, Sb–As– S, Cu–Sb–S, and Fe–As–S and concluded that a melt phase existed in all of these systems at temperatures between 280 °C and 496 °C. Of these systems, only the system Fe–As–S has been investigated at P N 1 bar. However, the results of studies involving this system is conjectural since the stabilities of members of the system Fe–As–S from experiments do not match those derived by thermodynamic calculations (Clark, 1960; Kretschmar and Scott, 1976; Sharp et al., 1985). Notwithstanding this debate, there should be a melt phase in the system Fe–As–S and the other systems discussed by Frost et al. (2002a) at peak metamorphic conditions that have affected the Broken Hill deposit. However, the proportion of sulfides in these systems is relatively insignificant (b 1%) when compared to the system Pb–Zn–Fe–Mn–S, which is more relevant to Broken Hill ores. Moreover, it should also be noted here that some of the As at Broken Hill was introduced during the Olarian Orogeny (i.e., after peak metamorphic conditions were reached). Mavrogenes et al. (2001) suggested that impurities such as As, Sb, and Bi, which now appear in the form of various sulfides and sulfosalts in the ore, would have lowered the solidus temperatures as peak metamorphic conditions were approached. Johnson and Klingner (1975) and Plimer (1979) showed that the concentrations of major and trace elements varies markedly between orebodies as shown by the minor element content of composite bulk samples of ore (Table 1). If one sums the maximum concentration of each minor element in a given orebody, the total range of trace element concentrations ranges from 2473 to 3391 ppm (or 0.25 to 0.34 wt.%) for four of the six orebodies (B and A lodes, 1 and 2 lenses), and 6622 ppm (or 0.66 wt.%) for 3 lens. More than 60% of the trace element content of 3 lens is As (3985 ppm). Despite the occurrence of a wide variety of rare sulfides and sulfosalts in pockets in the deposit, the concentration of these elements is small (0.25 to 0.66 wt.%) by comparison with the two major ore elements, Pb and Zn. The solidus should be lowered but it is unclear by how much (if at all). Part of the uncertainty relates to the results of recent experiments conducted by Stevens et al. (2005) in the

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system ZnS–FeS2–PbS–CuFeS2. In an attempt to evaluate whether melting occurred in Broken Hill-type deposits with more Cu (in the form of chalcopyrite) than observed at Broken Hill, Australia, Stevens et al. (2005) conducted experiments on the assemblage ZnS–FeS2–PbS–CuFeS2 between 750 °C and 1000 °C at 2 kbar. While confirming that the addition of S, in the form of pyrite (rather than pyrrhotite), lowers the eutectic in the system Zn–Fe–Pb–Cu–S, they also concluded that “the presence of N 3 wt.% Cu in the starting compositions did not lower the solidus”. 3.2.3. Melting in the systems Pb–Zn–Fe–Mn–Ag–S and PbS– FeS–ZnS–MnS–PbCl2–H2O In a further attempt to evaluate whether or not other components reduce the eutectic in the system Pb–Zn–Fe–Ag–S, and to explain the presence of garnetites, quartz garnetites, and Mn–bearing pyroxenoids adjacent to sulfides, Mavrogenes et al. (2004) argued that the ore system was better represented by the system Pb–Zn–Fe–Mn–Ag–S than the Mn-free system. Their experiments showed that when Mn-bearing sphalerite (in the presence of pyrrhotite and galena) was sandwiched between layers of pelite at 5 kbar for one week at various temperatures, a Mn-silicate halo formed along the contact between the sulfides and melt and that experiments without a so-called “sulfide melt” did not form a Mn-silicate halo. According to recent experiments by Sparks et al. (2006), when a powdered pellet of composition PbS–FeS–ZnS–MnS–PbCl2–H2O at 800 °C and 5 kbar was mixed with natural Broken Hill pelite manganoan garnet formed. Mavrogenes et al. (2004) and Sparks et al. (2006) interpreted the results of these experiments to conclude that partial melting of Broken Hill sulfides produced the garnetite spatially related to the various orebodies. However, there are problems with this interpretation for several reasons that include mass-balance considerations, textural relationships and the distribution of garnetite in the Curnamona Complex. Johnson and Klingner (1975) showed that ore composites analyzed from 1961 to 1970 contained 3.8 wt.% MnO (or 2.9 wt.% Mn) and 9.3 wt.% Zn. If one assumes that all the Zn in the original ore-forming solution was in the form of sphalerite or wurtzite (ZnS) then approximately 24% of the tetrahedral site of the Zn sulfide was filled with Mn. Note that this figure is only a minimum value since much of the wall rock, where most of the Mn-rich rocks occur, was not mined and was not incorporated in the metal budget of Johnson and Klingner (1975). The use of a Mn-rich sphalerite in the experiments of Mavrogenes et al. (2004) is inappropriate since sphalerite in hydrothermal systems contains low amounts of Mn (b 1 wt.% Mn). Mn-rich sphalerite, with compositions required to generate the volume of garnetite and other Mn-rich rocks in and adjacent to the orebodies, has yet to be reported in nature. Note that the Mn content of sphalerite at Broken Hill is b1wt.% Mn (Edwards, 1956). Manganese is likely to have been present in the ores prior to metamorphism as Mn carbonates, oxides, and oxyhydroxides, which are generally precipitated from hydrothermal solution rather than being deposited as Mn-rich sphalerite/wurtzite or alabandite (e.g., Spry and Wonder, 1989). Upon metamorphism, Mn went into the structure of various Mnsilicates eventually resulting in the assemblage of Mn-bearing

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silicates (e.g., garnet, rhodonite, bustamite, manganoan hedenbergite) that formed during granulite facies metamorphism. The precursor Mn carbonates, oxides, and oxyhydroxides are also known to occur adjacent to sulfides in unmetamorphosed and weakly metamorphosed terrains. For example, these minerals coexist with Mn-free sphalerite near active hydrothermal vents sites on the sea floor and in rift-zones in fresh-water lakes (e.g., Dymond et al., 1973; Hackett and Bischoff, 1973). 3.2.4. The effects of water on sulfide melting Mavrogenes et al. (2001) also considered the effects of water on sulfide melting and cited the work of Naldrett and Richardson (1968) who concluded that “water does not have any influence on the melting temperatures of pyrrhotite– magnetite mixtures and would almost certainly be the same for oxide-free iron-bearing sulfide systems”. A similar conclusion was reached by Craig and Kullerud (1968). Despite these two studies, Wykes and Mavrogenes (2003) proposed that the addition of water depresses the solidus in the system FeS–PbS– ZnS by 35 oC ± 5 °C relative to the dry eutectic of 900 °C and 1.5 GPa. It is unlikely that the conclusions derived from these experiments are appropriate since the P–T conditions used in their experiments do not match P–T conditions at Broken Hill, and the system FeS–PbS–ZnS does not accurately duplicate that at Broken Hill. Furthermore, the aH2O at Broken Hill during peak metamorphic conditions was ∼0.5 (Phillips, 1980), which is considerably lower than that used in the experiments of Wykes and Mavrogenes (2003). 3.2.5. The effect of excess of sulfur on sulfide melting and whether the Broken Hill deposit was sulfur-rich or sulfur-poor Experiments by Stevens et al. (2005) in the system FeS– PbS–CuFeS2–ZnS–S at 700 °C and 2 kbar show that an excess of S will lower the eutectic of the sulfide melt relative to a more S-poor system. As has been proposed by many workers, including for example Vokes (1971), an increase in metamorphic grade may convert pyrite to pyrrhotite and release sulfur, via the following reaction: 2FeS2 ¼ 2FeS þ S2

ð1Þ

This reaction, if it proceeds from left to right, results in an increase in the sulfur fugacity. However, the assumption that pyrite must ultimately react to form pyrrhotite during prograde metamorphism is incorrect. The fluid composition may dictate that the reaction goes in the opposite direction. The studies of Nesbitt (1982) and Spry (2000) show that if the host rocks to an orebody contain Mg-rich silicates they can convert to more Fe-rich silicates by consuming pyrrhotite and forming pyrite. Such reactions take place in conjunction with a decrease in sulfur or oxygen fugacities or both and have been documented at for example, the Geco (Petersen, 1984) and Ducktown massive sulfide deposits (Nesbitt, 1982) both of which were metamorphosed to amphibolite facies. The Geco deposit was identified by Frost et al. (2002a) as being a sulfide deposit that may have melted. Therefore, pyrite can be stable at granulite facies conditions in the absence of a reducing agent. Given that pyrrhotite and troilite are the only primary members of the system Fe–S in the Broken Hill

deposit, Australia, the possibility that the primary Fe sulfide was pyrite, as was proposed by Spry (1987) on the basis of sulfur isotope data, must be considered. However, the presence of primary pyrite in pelitic and psammitic metasedimentary rocks metamorphosed to granulite facies in several locations elsewhere in the Broken Hill Domain (e.g., Stirling Vale, Pyrite Hill, Thackaringa) suggests that pyrite is stable at granulite facies conditions (Plimer, 1977) and that it did not convert to pyrrhotite. This suggests that pyrrhotite and troilite at Broken Hill are likely to be primary in origin and simply reflect the sulfur-poor character of the Broken Hill deposit. The presence of other sulfurpoor or sulfur-free minerals such as gahnite, löllingite, safflorite, plumbian orthoclase, zincian manganese olivine, native lead, zincian staurolite, zincian muscovite, zincian biotite, and zincian ilmenite in the ore is in accord with this idea (Plimer, 1977). The experiments of Stevens et al. (2005) showed that the melt composition obtained by them in their experiments, using pyrite in the starting composition, resulted in a melt being enriched in S (i.e., 52 wt.% S). Approximately 65% of the S in the melt was added by sulfur being released via reaction (1). If such a melt was extracted and crystallized, pyrite and native sulfur should appear in the melt. The extracted melt would be 25% Pb and 20% Fe. No such rock of this composition is, to the best of our knowledge, present at Broken Hill and so it is unclear whether the geochemical conditions of their experiments are appropriate to Broken Hill-type deposits. Of particular relevance to the discussion of whether or not there has been an excess of sulfur during metamorphism at Broken Hill involves minerals in the system Fe–As–S. Recent studies by Tomkins (2006), and Tomkins et al. (2006, 2007) suggest that excess sulfur may be generated by several reactions involving löllingite (FeAs2), pyrite (FeS2), pyrrhotite (Fe1 − xS), and a melt (As–S). Tomkins et al. (2006, 2007) used nine bulk compositions ore in Ryall (1979) to argue that the maximum amount of melt in the system As–Pb–S in 3 lens at Broken Hill could be as high as ∼3 wt.% (this included one outlier sample with an extremely high As content of 9300 ppm As). However, this assumes that all of the As in the rock forms a melt and that all the As–S melt can communicate with excess galena. As Tomkins et al. (2007, p. 523) noted “in many rocks these are not valid assumptions”. This is, in part, due to the lack of equilibrium between the melt and galena and the likelihood that Pb–As sulfosalts would form. This would result in a reduction of galena in the melt. But what is even more important is that the bulk of As in the 3 lens actually occurs at the margin of the orebody and in the garnetite (as arsenopyrite and löllingite). This means that the amount of As available to melt in the sulfide systems As–Pb–S would be reduced. At Broken Hill, one of the most common textures involving minerals in the system Fe–As–S is for löllingite to form cores to arsenopyrite rims (Fig. 6D). It is unclear how this texture formed but one possibility is via the reaction: FeAs2 þ 2FeS þ S2 ¼ 4FeAsS

ð2Þ

The production of arsenopyrite, in this case, would be via a sulfur consuming reaction rather than a sulfur producing reaction. However, it should be stressed here that the compositions of and textures involving members of the system Fe–As–S are

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complicated at Broken Hill since arsenopyrite generally contains between 1 and 3 wt.% Co, As/S ratios can either increase or decrease from core to rim, and composite arsenopyrite–löllingite crystals can contain no Fe-sulfide (Spry, 1978). Since there is no textural evidence to show that pyrite reacted with arsenopyrite at Broken Hill then the likelihood that excess sulfur was generated via an arsenopyrite consuming reaction remains unlikely. This suggests that given the absence of primary pyrite at Broken Hill there was little, if any, excess S available to produce a sulfide melt. Therefore, the idea of lowering the eutectic of the system Pb–Zn– Fe–Mn–Ag–S by having an excess of sulfur cannot be entertained for the Broken Hill deposit. 3.2.6. Which geochemical systems are most appropriate for the Broken Hill orebodies? Despite considerable attempts to mimic the ore system and the peak metamorphic conditions at Broken Hill, Mavrogenes and coworkers have been unable to demonstrate that the presence of water, Ag, Mn, and other trace elements in the ore prior to metamorphism were sufficient to lower the solidus of the system Pb–Zn–Fe–Mn–Ag–S to the peak metamorphic temperature determined by Phillips (1980) and Phillips and Wall (1981). The question arises as to whether the experimental studies simulate the ore system and the geochemical conditions that are associated with the prograde and metamorphic history of the Broken Hill deposit. Lawrence's (1967) initial assumption that high-grade ore at Broken Hill had partially melted was based on the experiments of Avetisyan and Gratyshenko (1956) and Brett and Kullerud (1967) for the systems Fe–Pb–Zn–S and Pb–Fe–S, respectively. However, for these and some other experiments (e.g., Mavrogenes et al., 2001) involving systems that may be relevant to Broken Hill ores they have been conducted under sulfur saturated or high fS2 conditions (see Plimer, 1984). Such conditions are inappropriate given the presence of S-poor phases in the deposit and the previous discussion concerning the lack of evidence for excess S in the ore system. Moreover, the experimental charges may contain too much Ag and Fe, and ignore other major components. Experiments conducted by Mavrogenes et al. (2004) on the system PbS–FeS–ZnS–MnS–PbCl2–H2O were considered appropriate for simulating the ore system at Broken Hill. However, it must be stressed that there are gross mineralogical variations within an orebody and that the appropriate system for one orebody does not match that for another orebody. The geochemical system that is relevant to the Broken Hill deposit if all orebodies are considered is probably SiO2–FeO–MnO–CaO– Al2O3–P2O5–CO2–ZnS–PbS–FeS–(FeAsS/FeAs2). However, this system is not relevant to each orebody. For example, based on ore and gangue mineralogy, the system for B-lode is better described as SiO2–Al2O3–FeO–ZnS–PbS–FeS whereas those for 3 and 2 lenses are SiO2–Al2O3–FeO–MnO–F2–PbS–ZnS– FeS–(FeAsS/FeAs2) and SiO2–Al2O3–FeO–MnO–CaO–PbS– ZnS–FeS–CO2, respectively. 3.3. The origin of sulfide inclusions in garnet within garnetite If sulfides at Broken Hill partially melted, melt textures would not be preserved in sulfide masses because of subsequent

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deformation and recrystallization during the long protracted metamorphic history that affected the Broken Hill deposit. Consequently, Sparks and Mavrogenes (2003a,b, 2004a,b) considered that the only direct evidence of sulfide melt is socalled sulfide melt inclusions (SMINCs), which they proposed formed in garnet within garnetite and as planar features in quartz veins. They argued that these inclusions, which consist of various combinations of galena, sphalerite, arsenopyrite, chalcopyrite, tetrahedrite–tennantite, argentite, dyscrasite, gudmundite, fluorite, calcite, chlorite, and quartz, formed directly from a homogeneous sulfide melt and crystallized upon cooling (Fig. 6E). Such inclusions were also discussed by Frost et al. (2002a). Sparks and Mavrogenes (2003b) recognized as many as eight phases in inclusions within garnet. However, an important feature of some of the sulfide inclusions at Broken Hill is that they formed in open systems since the inclusions commonly occur in fractures in quartz and garnet or along grain boundaries. This can be seen particularly well in Fig. 2 of Frost et al. (2002a) where the sulfide inclusion occurs in contact with garnet and another silicate (possibly quartz), and a fracture. Note that sulfide inclusions also occur in contact with minerals, such as chlorite (Frost et al., 2002a, Fig. 2) that formed during a retrograde event. Melt experiments of sulfide inclusions at temperatures and pressures as low as 720 °C and 5 kbar by Sparks and Mavrogenes (2003b) reinforced the concept to them that the orebody melted. However, if sulfide inclusions are evidence of sulfide melting, this is incongruous with the presence of negative crystal-shaped monomineralic inclusions in sphalerite and galena adjacent to multi-phase low temperature sulfide inclusions because the melting points of galena and sphalerite at 1 bar are 1115 °C and 1850 °C, respectively (Fig. 6F, G). The melting point of galena increases to 1191 °C at 5.9 kbar (Wheeler et al., 2007). Moreover, there is no textural evidence to prove that sulfide inclusions did not form during retrograde metamorphism and that the concentration of the sulfide assemblage is related to differential sulfide mobility rather than melting. The same minerals that occur in so-called SMINCs also occur in massive ore along the grain boundaries of the most common sulfides at Broken Hill, sphalerite, galena, and pyrrhotite, and in fractures. However, what is more pertinent to this discussion is that samples of garnetite, which contain sulfide inclusions in garnet with multiple phases, are generally the same samples that contain the same minerals in veinlets cross-cutting garnetite or as intergrowths along the grain boundaries of garnet and quartz (Figs. 5C, D and 6B, H). We suggest that many sulfide inclusions are part of an open system with sulfides being introduced into open spaces along grain boundaries and cracks rather than having formed as a trapped liquid in a closed system. This is clearly the case for quartz as shown by Sparks and Mavrogenes (2003b), since the sulfide inclusions form along healed fractures. If our contention is correct that most sulfide inclusions formed from a retrograde metamorphic fluid and were trapped in garnet, then this suggests that garnet formed prior to the sulfide inclusion. Other sulfide inclusions, particularly those containing one or more of the following sulfides, sphalerite, galena, or pyrrhotite indicate that mineralization was present during sedimentation or diagenesis. The composition and textural relationships of sulfide inclusions in

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garnet within garnetite suggest that garnetite formed as a metamorphosed chemical sediment rather than as a reaction product between a partially molten Mn-bearing melt and the aluminous wall rocks. However, if our contention is incorrect and that the multi-phase inclusions garnet are indeed melt inclusions their compositions are quite different from what is seen in any orebody at Broken Hill. Sparks and Mavrogenes (2005) suggested that the average composition for the sulfide inclusions in garnet is 51.1 wt.% Pb, 11.3 wt.% Cu, 0.8 wt.% Zn, 0.5 wt.% As, 0.6 wt.% Ag and 0.7 wt.% Sb. However, no orebody of this composition, or even parts of orebodies, are known (including droppers). Table 1 shows that the average composition of sulfide inclusions in garnet are enriched in Ag (20 times), Sb (15 times), and Cu (100 times), which shows that, if partial melting has occurred, they are derived from very minor amounts of sulfide melting. If the Pb-rich orebodies (2 and 3 lenses) represent melt enriched portions of the Broken Hill deposit they should also be strongly enriched in Ag, As, Sb, and Cu by these levels relative to the Zn-rich orebodies. Table 1 clearly shows that this is not the case. 3.4. Fluid inclusion studies Several fluid inclusion studies have been conducted on various minerals in the Broken Hill deposit. Wilkins (1977), Wilkins and Sverjensky (1977) and Wilkins and Dubessy (1984) described fluid inclusion assemblages in gangue silicates (quartz, clinopyroxene, bustamite), carbonates and fluorite in high-grade ore and suggested that these assemblages provide a record of metamorphic fluids during the period of retrograde metamorphism. Subsequent studies by Spry (1978) confirmed the fluid assemblages recognized by Wilkins (1977) in quartz and showed that similar assemblages were found in garnet in quartz–garnetite, garnetite, and garnet envelope. Spry (1978) proposed that particular fluid assemblages characterized each of the D2–D4 deformation events. By combining the observations of Wilkins (1977), Spry (1978), and Prendergast et al. (1998), these studies show that the metamorphic fluids were complex and contained H2O, CO2, CH4, NaCl, KCl, MgCl2, CaCl2, unknown chlorides of Na–Ca–Fe, Pb–K and Mn–Ca–Fe, and possibly N2, anhydrite, and dawsonite. Raman spectroscopy studies of fluid inclusions in rhodonite by Millsteed et al. (2005) also confirmed the presence of CH4 and N2. Williams et al. (2005) analyzed fluid inclusions in blue and white quartz in A lode using PIXE, LA-ICP-MS, and laser Raman techniques and revealed the existence of methane, a solid Pb–K–Cl phase and brines with N 1% Pb and N 1000 ppm Zn. Due to the presence of Pb/Zn ratios similar to eutectic melts in the PbS–ZnS–FeS system but Pb/Fe ratios lower than such melts, Williams et al. (2005) raised the possibility that the brines could have been synmetamorphic sulfide-rich melts that were modified after they had been trapped or that there was post-metamorphic entrapment of brines with pre-existing sulfides. However, subsequent studies by Williams et al. (2006) using LA-ICP-MS techniques on mixed fluid-sulfide inclusions in quartz showed that the fluids are markedly depleted in Fe and Zn relative to eutectic compositions. Williams et al. (2006) argued that for the fluid to

have been ultimately related to partial melting, the fluid would have had to be retained in the ore system “until it were able to interact with cool external fluids at temperatures well below those of the metamorphic peak”. Such a scenario seems unlikely. 3.5. Sulfide textures in high-grade ore Stanton (1965) proposed that interfacial angles ranging from 100° to 140° between sulfides in stratiform massive sulfide deposits must have equilibrated in the solid state. However, Frost et al. (2002a) used interfacial angles of galena against sphalerite–sphalerite pairs in the Broken Hill ore, which range from 5° to 115°, to argue that they reflect sulfide melting rather than solid-state equilibration. The likelihood that original melt textures have been retained throughout the complex deformation and metamorphic history is tenuous at best. Not only will the sulfides have recrystallized but they will have also been subjected to directed stress associated with retrograde metamorphism. Based on the results of experiments carried out between 280 °C and 980 °C, Lusk et al. (2002) derived a sulfide geothermometer based on the dihedral angle in sphalerite– galena–sphalerite triple junctions. They applied the results of these experiments to four massive sulfide deposits, including Broken Hill. Lusk et al. (2002) obtained a peak metamorphic temperature of ∼700 °C, which is well below the peak derived by Philips et al. (1980). Lusk et al. argued that the range of dihedral angles present in sulfides from Broken Hill recorded declining temperatures as the ore cooled from 700 °C down to 540 °C. Their data reinforces the concept that the massive sulfides at Broken Hill continuously re-equilibrated texturally during a retrograde cooling path and show that interfacial angles in sulfides cannot be used as evidence for sulfide melting at Broken Hill as proposed by Frost et al. (2002a). Moreover, the presence of curved cleavage planes in galena, subgrain textures on the cleavage faces of galena, lamellar twins and inclusions of chalcopyrite along its cleavage and twinplanes, as well as the presence of sulfide schists and breccias along the margins of galena-rich sulfide masses are examples of the effects of post-peak modification to assemblages involving galena and sphalerite. 3.6. Sulfide minerals filling fractures Sulfide-filled projections into the country rocks occur at a range of scales from mm-sized veinlets to sulfide dikes or socalled “droppers”, which extend into the country rock for up to 160 m. Droppers project upward and downward from the main lodes and are enriched in silver and lead relative to them. Maiden (1975, 1976) considered that they formed in shear zones by plastic deformation of sulfides. However, Sparks and Mavrogenes (2003b) argued that it was “unlikely that deformation can account for the fractionation of trace metals”. Sparks and Mavrogenes recognized two different types of droppers, a tectonized variety (type D) and a partial melt variety (type P). They suggested that type P droppers were injected into the wall rocks during peak metamorphism when the orebody was molten. However, there are features about their model that are difficult to explain. For example, Spry (1987) described a

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4 m long vein that extended from 2 lens into a body of wall rock garnetite. Not only did samples near the end of the vein have lighter sulfur isotope values than in high-grade ore at the beginning of the vein but temperatures derived from sulfur isotope geothermometry ranged from 421 °C to 622 °C. If the ore was injected as a molten sulfide liquid there would be no reason to expect any isotopic fractionation along the length of the vein. However, the lighter isotope (32S) would be expected to be enriched relative to the heavier isotope (34S) with increasing distance during plastic deformation. The temperatures derived from the sulfides are consistent with a protracted history of movement and re-equilibration from peak metamorphic conditions through the retrograde metamorphic episode. On a gross scale, droppers primarily cut the fabric in the wall rocks and generally have a retrograde schist zone along their margins. 3.7. Mn- or Ca-rich selvage Frost et al. (2002a), Sparks and Mavrogenes (2003b), and Mavrogenes et al. (2004) suggested that Mn-rich rocks, including garnetite, quartz–garnetite, and pyroxenoid rocks, consisting of rhodonite, bustamite, hedenbergite, and wollastonite, formed as a result of a reaction between a sulfide melt and surrounding silicates. If this was the case then these rocks should envelope each of the orebodies. However, these three rock-types are not always found on the margins (Plimer, 1984; Webster, 2006). Bustamite, hedenbergite, rhodonite are common in 2 lens, but are not found exclusively along the margins of this orebody. Of these minerals, rhodonite is common in 3 lens, in places present as elongate almost monomineralic bodies (bustamite occurs in trace amounts), and in A lode along with hedenbergite. However, rhodonite and hedenbergite occur in only trace amounts in B lode, while bustamite and wollastonite occur in just minor amounts in 1 lens. Even if the Mn–Ca rocks formed around the orebodies as reaction halos due to partial melting of sulfides as proposed by Frost et al. (2002a), Sparks and Mavrogenes (2003b), and Mavrogenes et al. (2004), it is impossible for the melt model to explain the presence of Mnrich rocks (including garnetite) with sulfides in the Curnamona Complex that were metamorphosed to temperatures and pressures well below the 780 °C and 5 to 6 kbar conditions that are required to produce melting in the system Zn–Pb–Mn– Fe–S (Heimann et al., 2006). Although Frost et al. (2002a) note that Mn-rich rocks may form by exhalative processes and by melting, quartz–garnetite and garnetite in and adjacent to the Broken Hill deposit exhibit the same mineralogical, textural, and chemical characteristics as those metamorphosed to upper greenschist–lower amphibolite facies that are found in the Olary Domain. Frost et al. (2002a) suggested that Mn-rich rocks formed by melt processes should lack compositional layering, cut the regional fabric, and be markedly high in variance. However, there are manganiferous garnetites and quartz– garnetites that fulfil these criteria that occur throughout the Broken Hill and Olary Domains in the absence of sulfides. For these situations, the implication of the sulfide melt model is that wherever garnetite and quartz garnetite are present, Mn-bearing partially melted sulfides should also be present. Clearly this is

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not the case as shown by the presence of lode rocks without sulfides throughout the Broken Hill Domain. It is also apparent that the development of manganoan garnet is associated with pre-, syn-, and retrograde-metamorphic alteration, and syngenetic processes (Plimer, 2006a,b). 3.8. Metal zonation patterns Laing et al. (1978) proposed that the mine sequence at Broken Hill, including the six major orebodies, occurs on a single inverted limb of an F1 fold. This folding resulted in an inverted metal zoning pattern typical of syngenetic massive sulfide deposits. However, this view has recently been challenged by Webster (2006) on the basis of the disposition of lithologies in the Broken Hill lode. It has also been questioned by Mavrogenes et al. (2001) who argued that the inverted metal zoning pattern could have resulted from partial melting with the Zn lodes being the residual of the Pb-rich melt. In such a scenario, one would expect to find altered rocks between orebodies and zoned stringers of sulfides between the orebodies, notwithstanding the complex folding that has affected the orebodies. Such features are not observed at Broken Hill. The proposal of Mavrogenes et al. also raises questions whether the fluid pressure associated with partially melted sulfides and density of galena-rich melts is sufficient to percolate downwards through rocks being simultaneously metamorphosed to granulite facies. To generate the two biggest orebodies, the lead-rich 2 and 3 lenses, would require almost complete melting of the smaller Zn-rich orebodies. However, a bigger problem with the Mavrogenes et al. sulfide segregation model is that it cannot explain the orebody zonation at the Pinnacles deposit, which also occurs in the two pyroxene zone in the Willyama Domain, and the formation of the Western Mineralization in CML7. The Western Mineralization consists of the three stratabound units, a quartz–gahnite-bearing unit (C lode equivalent), a hedenbergite-rich unit (B lode equivalent), and a spessartine± rhodonite-rich unit (A lode equivalent). The ore formed as stringers, disseminations, remobilizates, and within syn-metamorphic quartz veins (Plimer et al., 2003). Given the structural and stratigraphic setting of the ore, the melt model would require that it is a residual melt product with melt fractionations preserved at the intersection of F3 and F4 structures. To have Pb–Zn–Ag-rich sulfide melts preserved during the retrograde metamorphic history in these structures seems highly unlikely. The Pinnacles deposit, which is the second largest BHT deposit in the Curnamona Complex, comprises three stratiform zinc-rich lodes and one lead-rich lode within the Cues Formation (Barnes et al., 1983; Parr, 1992b,c, 1994). It is very similar texturally, mineralogically and geochemically to the Broken Hill deposit (Parr 1992a, 1994). Some 200,000 t of ore have been mined from the Pinnacles deposit (Stevens and Burton, 1998), which has an inferred Zn–Pb resource of 2 Mt of 2.4% Pb, 8.0% Zn, and 92 g/t Ag (Williams and Hopwood, 2006). An unknown tonnage of pyrrhotite–gold and pyrrhotite–chalcopyrite–gold developed in extremely retrogressed (D3) gahnite-bearing rocktypes. The Pinnacles deposit consists of a lead-rich orebody with a thinner zinc-rich lode in its hanging wall and two zinc-rich lodes

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in its footwall. The deposit has strike equivalent quartz–gahnite, quartz–garnet, tourmalinite, quartz–magnetite rocks and silicate Fe formations. The edge of the lead-rich lens contains interdigitated laminated quartz–ferroan spessartine and quartz– galena–sphalerite–ferroan spessartine rocks. The major body of garnet-rich rock-type occurs on the hanging wall of the Pb-lode. If the Broken Hill deposit has undergone sulfide migration across the stratigraphy as a result of sulfide partial melting, then it is logical that the Pinnacles deposit, which has been subject to metamorphic P–T conditions similar to or possibly higher than at Broken Hill should have also been affected by partial melting of sulfides. However, at the Pinnacles deposit the possible zincrich “restite” ores occur stratigraphically above and below the lead-rich orebody. Despite considerable drilling of the Pinnacles deposit, a Pb-rich orebody, which would be required by the segregation model of Mavrogenes et al., has not been found stratigraphically below the lowermost Zn-rich lode. 3.9. Evidence of sulfide partial melting in other metamorphosed massive sulfide deposits Hofmann (1994) showed overwhelming evidence in support of partial melting of sulfides and sulfosalts in the system Pb– Tl–As–Sb–Bi–S in the Lengenbach massive sulfide deposit, Switzerland that was metamorphosed to upper greenschist– lower amphibolite facies (∼520 oC). These minerals, some of which include sartorite (PbAs2S4), baumhauerite (Pb3As4S9), orpiment (As2S3), and realgar (AsS) in the sub-system Pb–As–S all have melting points b 500 oC. There was no indication that common sulfides pyrite, galena, and sphalerite were the products of partial melting but this is to be expected given that metamorphic conditions were below the eutectic for the system FeS2–PbS–ZnS. However, Frost et al. (2002a) proposed that Cannington, Aguilar, and Balmat also are examples of metamorphosed massive Pb–Zn deposits that have partially melted. Frost et al. also considered that these deposits have a reaction halo consisting of Ca–Mn pyroxenoid rocks or garnet-rich rocks, similar to that which they considered as having formed by a reaction between a sulfide melt and the surrounding country rocks. Although Cannington shows considerable resemblance to Broken Hill, Australia, the orebody was only metamorphosed to 640 °C to 690 °C and 4 to 6 kbar (Mark et al., 1998). For Balmat, results of geothermometric and geobarometric studies, which are summarized in DeLorraine (2001), indicate conditions of formation of 625 °C to 725 °C and 6.5 kbar, at XCO2 N 0.7. Metamorphic conditions that affected the Aguilar deposit were 350 °C to 650 °C and 1 to 2 kbar (de Brodtkorb et al., 1978). The peak metamorphic conditions that affected these three ore deposit are well below the solidus of the system Zn–Pb–Mn–Fe–Ag–S system. Therefore, it was impossible for the ore to have melted and to have released Mn to react with silicate wall rocks in the manner described by Frost et al. (2002a). It should be noted here that while Brown et al. (1980) reported the presence of pyroxenoids in the Balmat deposit, there is no Ca–Mn pyroxenoid or garnet-rich rock that occur as a selvage to the Balmat deposit (DeLorraine, written comm., 2004).

Using criteria outlined by Frost et al. (2002a) to indicate sulfide-rich partial melts in metamorphosed ore deposits (including Mn and Ca-rich alteration halos on the margins of sulfide ore bodies; low interfacial angles between sulfides; occurrence of ore sulfides in dike-like and fracture fillings; concentrations of low melting point chalcophile elements [LMCE] in ore; the presence of LMCE sulfide inclusions in metamorphic silicates), Bailie and Reed (2005) considered that sulfides in the Broken Hill deposit, South Africa, were products of a sulfide partial melt. The deposit was metamorphosed to peak metamorphic conditions of 660 to 690 °C and 3.5 to 4.5 kbar, which is well below the eutectic temperature in the system Pb–Zn–Mn–Fe–Ag–S. It should be noted that chalcopyrite is a major constituent of the ore (only present in trace amounts in Broken Hill, Australia) and is a sulfur-rich deposit, since primary pyrite is a common sulfide in the ore. The presence of chalcopyrite should lower the eutectic in the system Pb–Mn–Fe–Ag–Cu–S relative to that in the Cu-free system but it seems to have had an insignificant affect on the eutectic (Stevens et al., 2005). If the experimental data of Stevens et al. (2005) for the system FeS–PbS–CuFeS2–ZnS–S, which were conducted at 2 kbar, are extrapolated to 3.5 to 4.5 kbar, the eutectic temperature is ∼ 740 °C. This temperature is higher than the peak metamorphic conditions which have affected the Broken Hill deposit, South Africa. While LMCE may have aggregated to form sulfides and sulfosalts, their volume is insignificant compared to sulfides in the system FeS– PbS–CuFeS2–ZnS–S. Bailie and Reed (2005) contended that quartz garnetite at Broken Hill, South Africa, is a product of the partial melting of sulfides. This suggestion is at odds with the views of Spry et al. (2000) who considered them to be primarily metamorphosed manganese-rich exhalites. The presence of sulfide inclusions in garnet (Bailie and Reed, 2005; Figs. 3a, 4a–f, 5b) and biotite (Bailie and Reed, 2005; Fig. 3b) were considered by Bailie and Reed as evidence for sulfide melting. However, it should be noted that the inclusions shown by them were of sphalerite, galena, chalcopyrite, and pyrrhotite only. All of these minerals melt at temperatures N1000 °C and cannot be considered as melt products. None of the photographs shown by Bailie and Reed (2005) contained sulfides or sulfosalts with LCME as components. Although it should be noted in this context that Sparks and Mavrogenes (2004a) recognized sulfide inclusions containing LCME within garnet spatially associated with rare garnetite. It is unclear whether the sulfide inclusions formed during the retrograde period of metamorphism as we have proposed for similar inclusions at Broken Hill, Australia. 4. Conclusions The main conclusions of this study are: 1. Field, theoretical and experimental studies of Frost et al. (2002a), Tomkins and Mavrogenes (2002), Tomkins et al. (2004, 2006), Ciobanu et al. (2006) and Tomkins (2006) have shown that partial melting of sulfides has taken place in a variety of ore deposits. Where present in Broken Hill-type

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2.

3.

4.

5.

massive sulfide deposits, partial melting of sulfides/sulfosalts involves LCME elements and is volumetrically insignificant (b 1 wt.%) when compared to the volumetrically important system SiO2–FeO–MnO–CaO–Al2O3– P2O5–CO2–ZnS–PbS–FeS–(FeAsS/FeAs2). We believe that sulfides/sulfosalts in low temperature systems such as Ag–Pb–S, Ag–Sb–As–S, Cu–Pb–Sb–S, Cu–As–S, Sb– As–S, Cu–Sb–S, and Fe–As–S may have been scattered in pockets throughout the deposit. However, we do not consider the peak metamorphic conditions at Broken Hill to have been high enough to have partially melted the most common sulfides sphalerite, galena, and pyrrhotite. The low interfacial angles between sulfides in the Broken Hill deposit reflect sulfide re-equilibration during the retrograde period of metamorphism and are not evidence for galena having crystallized from a sulfide melt. Textural relations among garnet, sulfide inclusions and fractures and the presence of minerals in cross-cutting veins that are the same as those in spatially related multi-phase sulfide inclusions in garnet show that these sulfide inclusions in the Broken Hill deposits of Australia and South Africa formed from a hydrothermal fluid during the retrograde period of metamorphism. These inclusions are not considered to be sulfide melt inclusions as it is impossible to form monomineralic or multi-phase mineral inclusions composed of chalcopyrite, sphalerite, galena, or pyrrhotite, since these sulfides only melt at T N 1000 °C, which is hundreds of degrees hotter than peak metamorphic conditions at both of these deposits. Manganoan garnet rocks (quartz garnetite and garnetite) at Broken Hill, Australia, are deformed and metamorphosed hydrothermal precipitates that formed from exhalative or inhalative fluids. However, garnet-rich rocks formed throughout the prograde and metamorphic history that affected the Broken Hill deposit. Some of these rocks formed as a result of a reaction between manganoan sulfide rocks and aluminous wall rocks. Garnetite and quartz garnetite are not the residue of partial melting of sulfides. The precursor phase to garnet in these rocks is not Mn-rich sphalerite because mass-balance calculations indicate that the composition of sphalerite does not incorporate high enough quantities of Mn to produce the volume of garnet-rich rocks associated with the Broken Hill deposit. Furthermore, the presence of garnet-rich rocks up to 300 m from sulfide orebodies in highly refractory rocks that still retain their sedimentary structures at Broken Hill is hard to explain using the partial melt hypothesis. Even more difficult to explain is the presence of quartz garnetite and garnetite in several locations in rocks metamorphosed to the amphibolite facies in the Olary Domain. These rocks are metamorphosed at more than 100 oC below the eutectic temperature in the system Zn–Pb–Mn–Fe–Ag–S. If the partial melt model is correct, then it requires that sulfides should be present wherever garnetite and quartz garnetite are observed. Field observations show this to not be the case. Sulfide bodies that impinge the wall rocks at Broken Hill formed as a result of sulfide mobilization and plastic injection in the wall rocks during the waning stages of high-grade metamorphism and the retrograde period of metamorphism.

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These bodies cross-cut the S1–S3 foliation. The spatial association of sulfide inclusions in garnetite spatially associated with sulfide dikes (“droppers”) does not prove they were products of sulfide melting. The presence of secondary pyrite and sericite along the margins of these dikes suggests that sulfides moved during the retrograde period of metamorphism. 6. Although there is controversy concerning whether the sulfide orebodies at Broken Hill, Australia, were structurally overturned, there is no field evidence to suggest that Zn-rich orebodies formed as a restite to melted Pb-rich ores as a result of gravitational differentiation. On the basis of mass-balance considerations it is also likely that the Zn-rich orebodies (residues) would have almost certainly had to have completely melted in order to form the Pb-rich (2 and 3 lenses), which are the two largest orebodies in the Broken Hill deposit. The partial melt model cannot explain the presence of Zn-rich orebodies structurally above and below the Pb-rich orebodies at the nearby Pinnacles deposit and the presence of sulfide mineralization in the Western Mineralization that is located in F3 and F4 structures. Acknowledgements This study was financially supported by National Science Foundation Grant EAR 03-09627. The authors thank the following for discussions concerning the concept of partial melting of massive sulfides: Ron Frost, Adriana Heimann, John Mavrogenes, Heather Sparks, Barney Stevens and Andy Tomkins. While some of these people may not agree with our ideas, they are gratefully acknowledged for sharing their views, thoughts, and opinions. The comments of reviewers Ron Berry, Nigel Cook (Editor) and Laurence Robb greatly helped to improve the revised version of the paper. Todd Bonsall and Adriana Heimann are also thanked for assistance with drafting. References Andrews, E.C., 1922. The Geology of the Broken Hill District. Memoirs of the Geological Society of New South Wales, 8. 432 pp. Avetisyan, K.K., Gnatyshenko, G.I., 1956. Thermal and Metallographic Study of the Lead–Zinc Sulphide–Iron Sulphide System. Izvestiya Akademi Nauk Kazakhstan Soviet Socialist Republic, Seriya Gornogo Dela Metallov I Stroimaterialov, 6, pp. 11–25 (in Russian). Bailie, R.H., Reid, D.L., 2005. Ore textures and possible sulphide partial melting at Broken Hill, Aggeneys, South Africa I: petrography. South African Journal of Geology 108, 51–70. Barnes, R.G., Stevens, B.P.J., Stroud, W.J., Brown, R.E., Willis, I.L., Bradley, G.M., 1983. Zinc, manganese and iron-rich rocks and various minor rock types. Records of the Geological Survey of New South Wales 21, 289–323. Billington, L.G., 1979. The relationship between of the garnet quartzite rock types to the orebodies in the ZC-NBHC, N.S.W., Australia. Unpublished MSc thesis, University of New South Wales, Sydney, Australia, 178 pp. Binns, R.A., 1964. Progressive regional metamorphism in the Willyama Complex, Broken Hill district, New South Wales. Journal of the Geological Society of Australia 11, 283–330. Birch, W.D., 1999. The minerals. In: Birch, W.D. (Ed.), Minerals of Broken Hill. Broken Hill City Council, Broken Hill, pp. 88–256. Boots, M.K., 1972. The textural, chemical and mineralogical effects of retrograde metamorphism on the Main Lode Horizon, Broken Hill, NSW, Unpublished PhD thesis, University of New South Wales, Sydney, Australia, 271 pp.

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