Zn-isotopic evidence for fluid-assisted ore remobilization at the Balmat Zinc Mine, NY

Journal Pre-proofs Zn-Isotopic Evidence for Fluid-Assisted Ore Remobilization at the Balmat Zinc Mine, NY Peter Matt, Wayne Powell, Ryan Mathur, William F. deLorraine PII: DOI: Reference:

S0169-1368(19)30457-3 https://doi.org/10.1016/j.oregeorev.2019.103227 OREGEO 103227

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Ore Geology Reviews

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15 May 2019 15 October 2019 9 November 2019

Please cite this article as: P. Matt, W. Powell, R. Mathur, W.F. deLorraine, Zn-Isotopic Evidence for Fluid-Assisted Ore Remobilization at the Balmat Zinc Mine, NY, Ore Geology Reviews (2019), doi: https://doi.org/10.1016/ j.oregeorev.2019.103227

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Zn-Isotopic Evidence for Fluid-Assisted Ore Remobilization at the Balmat Zinc Mine, NY Peter Matt (1), Wayne Powell (1,2), Ryan Mathur (3) and William F. deLorraine (4) 1) Department of Earth and Environmental Science, The Graduate Center at CUNY, 365 Fifth Ave., New York, NY 10016, USA; [email protected] 2) Department of Earth and Environmental Sciences, Brooklyn College, 2900 Bedford Ave., Brooklyn, NY 11210, USA 3) Department of Geology, Juniata College, 1700 Moore St., Huntingdon, PA 16652, USA 4) 1 Indian Head Trail, Gouverneur, NY 13642, USA

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Abstract Shear zone-hosted ore bodies at the Balmat, NY, zinc deposit were remobilized at the kilometer scale during amphibolite facies metamorphism ca. 1180 Ma. Despite there being little evidence for interaction of hydrous fluids with ores during deformation, such translocation distances are considered unlikely without the assistance of fluids. Measurements of Zn isotopic composition of sphalerite from six Balmat ore bodies that originated from the same stratigraphic level (Upper Marble unit 6) reveal variation between ore bodies, as well as intraorebody trends of isotopic lightening down plunge. remobilization correlates with decreasing ẟ66Zn.

In general, increasing distance of

The syn-tectonic isotopic fractionation

recorded in Balmat sphalerite is interpreted to have resulted from the interaction between the ore and sulfide melts that were fluxed by H2S localized primarily in Upper Marble unit 7 (fetid dolomite). Lighter isotopes of Zn were enriched in the melt, leaving the residual ore enriched in heavier Zn isotopes. These observations support previous petrographic evidence for the presence of anatectic melts at Balmat and help to explain the unusual scale of translocation at Balmat by means of fluid-assisted remobilization during which intergranular melts decreased rock competency.

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1. Introduction The carbonate- and evaporite-hosted Balmat zinc deposit, with past production in excess of 45M metric tons of zinc, is located near Gouverneur in the Adirondack Lowlands of New York, a part of the global Grenville Orogen (Fig. 1). The deposit formed in the TransAdirondack back-arc basin ca. 1250-1225 Ma through seafloor exhalative processes (deLorraine, 2001; Chiarenzelli et al., 2010), and subsequently underwent upper amphibolite-facies regional metamorphism (~640° C at 6.5 kb) (Kitchen and Valley, 1995) during the Shawinigan Orogeny (1180-1140 Ma) (Chiarenzelli et al., 2010; Baird and Shrady, 2011). Deformation during this time resulted in unusually large distances of ore remobilization. Mineralization in some shearzone hosted ore bodies is known to have moved over 500 m laterally across stratigraphy, and in others has moved over 2000m along plunge (deLorraine, 2001). These distances are considered conservative given that the original position of some ore bodies remains unknown. The mechanisms associated with the large-scale remobilization at Balmat are not fully understood. Historically, solid-state ductile flow was thought to be the predominant means of ore translocation, based in part on a lack of evidence of fluid involvement: 1) the generally anhydrous nature of the host mineral assemblages; 2) the lack of hydrous alteration selvages around ore bodies (deLorraine, 2001); 3) mineralogical differentiation along flow paths corresponding to relative ductilities; and 4) peak metamorphic conditions were well below the melting point of the predominant sphalerite-pyrite assemblage (Barton and Toulmin, 1966). However, external remobilization at the scale observed at Balmat is considered unlikely, if not impossible, without the involvement of fluids, either operating alone (e.g., dissolution and

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reprecipitation) or together with solid-state processes (e.g., decreased competency due to intergrain fluid films) (Marshall et al., 2000). Recently, mineral assemblages were documented in the Cross-Cutting zone of the Fowler orebody which indicate that small volumes of low-melting-temperature sulfide melts were present at Balmat (Matt et al., 2019). For example arsenopyrite plus pyrite, the most abundant low-temperature assemblage they report, react to produce As-S melt plus pyrrhotite above 560⁰C at 5 kb (Tomkins, 2006). Bournonite, another sulfosalt found by Matt et al. (2019), melts at 522⁰C at 1 bar (Barton and Skinner, 1979). Other low melting temperature minerals documented at Balmat include jordanite, sartorite, boulangerite, orpiment and realgar (Matt et al., 2019; Chamberlain et al., 2018, deLorraine 2001). These minerals occur along grain boundaries and within fractures, as well as being components of polysulfide grains in textural equilibrium with silicate host assemblages (Matt et al., 2019). These silicate assemblages were themselves deemed to have been molten, or at least mobile with respect to granoblastic domains with which they are in contact. On the basis of these observations, Matt et al. (2019) concluded that the low-temperature phases must have been molten under peak metamorphic conditions and migrated to structural traps and/or mobile domains before crystallizing during regional cooling. Based on the estimate of 640⁰C at 6.5 kb at the peak of metamorphism (Kitchen and Valley, 1995) and a regional cooling rate of 1.5⁰C/Ma (Mezger et al. 1991; Dahl et al. 2004), some melts must have been present at Balmat as late as 80Ma or more after metamorphic peak. Although it is unlikely that the volume of former melt now preserved in the Fowler ore body would have significantly affected the rheological character of the ore, the actual volume 4|Page

of melt that was present during the peak of metamorphism is unknown. This hypothesis is supported by the observations of Matt et al. (2019) who noted that the melt-derived sulfosalt assemblages were preserved in gangue rather than in the massive sphalerite ore itself. Further support comes from experiments by Pruseth et al. (2016) who documented that sphalerite plus pyrite is the first assemblage to crystallize from sulfosalt-bearing Zn-Fe-Cu-Pb melts, with metal-rich and sulfosalt-rich immiscible melts persisting to lower temperatures. This result is consistent with the simple sphalerite + pyrite ore mineral assemblage present at Balmat today, and permissive of the idea that some of the crystalline products of low-temperature sulfide melts are no longer present in the ore, as the viscosity contrast between liquids and solids would cause their eventual segregation in response to stress (Tomkins et al., 2007). In this study, we measured the zinc isotopic character of six individual ore bodies at the Balmat mine in an effort to provide additional evidence for the involvement of sulfide melts during remobilization coeval with metamorphism and deformation. Among the known causes for isotopic fractionation, only two are theoretically applicable to Balmat: 1) dissolution of sphalerite into, or precipitation of sphalerite from, a hydrothermal fluid; and 2) melting of sphalerite as a eutectic component. The largely anhydrous mineral assemblage of the Balmat host rock and the general lack of alteration zones around the ore bodies suggest that a hydrothermal fluid is unlikely to be the fractionating agent. Thus we hypothesized that any systematic variation in the zinc isotopic character of the ore would support the conclusion that the fractionating agent must have been a sulfide melt. 2. Geology of the Balmat Deposit 2.1 Metallogeny and tectonic setting 5|Page

The carbonate- and evaporite-hosted Balmat zinc deposit, with past production in excess of 45M metric tons, is located near Gouverneur in the Adirondack Lowlands of New York, a part of the global Grenville Orogen (Fig. 1). The deposit formed in the Trans-Adirondack back-arc basin ca. 1250-1225 Ma through seafloor exhalative processes (deLorraine, 2001; Chiarenzelli et al., 2010), and subsequently underwent upper amphibolite-facies regional metamorphism (~640° C at 6.5 kb) (Kitchen and Valley, 1995) during the Shawinigan Orogeny (1180-1140 Ma) (Chiarenzelli et al., 2010; Baird and Shrady, 2011). The Balmat zinc deposit has been categorized as part of a Grenville Province metallogenic belt (Gauthier and Chartrand, 2005) which includes a number of smaller carbonate-hosted, stratiform Pb-Zn deposits in southern Ontario and Quebec. The very large Sterling Hill and Franklin, NJ, zinc oxide deposits (combined production of 33 Mt), hosted in rocks of the Grenville inlier known as the New Jersey Highlands, are also included in this metallogenic belt (Gauthier and Chartrand, 2005). Though all these deposits differ in size, mineral abundances, and presence or absence of clastic host rocks, they share a genesis by seafloor exhalative processes, as well as an upper amphibolite to granulite facies metamorphic overprint and associated deformation during the Grenville Orogeny (Gauthier and Brown, 1986; Gauthier and Chartrand, 2005; Johnson and Skinner, 2003). The deposits also share a back-arc basin tectonic setting (Johnson and Skinner, 2003; Peck et al., 2009; Dickin and McNutt, 2007; Volkert et al., 2010; Chiarenzelli et al., 2012). At the scale of the Adirondack Lowlands, Balmat is the southernmost member of a group of historic zinc deposits that extends 28 miles to the northeast and includes the Hyatt, Edwards and Pierrepont deposits. The same mineralized band also contains economic talc. In contrast to the sedimentary exhalative origin for zinc, talc originated from the metamorphism 6|Page

of a silicate plus anhydrite layer, a mappable unit within the largely carbonate-dominated host sequence for zinc mineralization (see 2.2-Stratigraphy) (Brown and Engel, 1956; Chamberlain et al., 2018). The northeast-southwest trend of the Balmat district is paralleled to the west in the Lower Marble by a belt of pyrite, some of which was mined in the past for sulfur (Prucha, 1957) and even further west by a belt of graphite (Johnson and Skinner, 2003, their Fig. 8). This distribution provides evidence for southeast to northwest deepening of the back-arc basin (Johnson and Skinner, 2003), as does thickening of the Lower Marble from the Carthage-Colton line northwesterly towards Black Lake. 2.2 Stratigraphy The Balmat massive sulfide deposit is hosted by the 1100 m thick Upper Marble, (Brown and Engel, 1956; deLorraine, 2001). Surface and underground diamond drilling, detailed surface and subsurface mapping, and structural analysis has allowed definition of 16 units within the formation. These comprise primarily alternating layers of dolomitic and quartz-diopside rock (deLorraine, 2001; Whelan, et al., 1990). Although most units are comprised of anhydrous mineral assemblages, unit 13 is a talc-tremolite-anthophyllite schist (deLorraine, 2001; Lupulescu and Rowe, 2011). Minor talc and serpentine occur elsewhere in the Upper Marble but are retrograde replacements of diopside and forsteritic olivine (Brown and Engel, 1956; deLorraine 2001). A marine depositional setting with significant organic activity is indicated by the presence of stromatolites in units 4 and 11, by the presence of natural gas that was released during initial exploration, by graphitic horizons in numerous dolomitic marble units, and by several fetid (H2S-bearing) dolomitic units within the sequence (Chiarenzelli, 2012; deLorraine, 2001; Brown and Engel, 1956). The basin was likely floored by oceanic crust, at least 7|Page

in part (Chiarenzelli, et al., 2011) and underwent repeated episodes of shallowing, evidenced by thick layers of anhydrite in units 6, 10, 11 and 13 as well as the presence of stromatolites in units 4 and 11. Within the Upper Marble, sixteen distinct ore bodies have been recognized. Eleven are the remobilized and disaggregated products of stratiform massive sulfides originally located at three different stratigraphic horizons in the Balmat section; five are conformable massive sulfide orebodies that have remained in their original stratigraphic horizons. Orebodies deposited at or remobilized from each horizon have unique trace element signatures (deLorraine, 2001; Swanson, 1979). The Upper Gleason, Lower Gleason, Loomis, Mahler, Upper Fowler, and Mud Pond ore bodies (Fig. 2), on which this study focused, are all members of the lowest syngenetic ore horizon in unit 6 and are characterized by high Hg. 2.3 Structure The Adirondack Lowlands exhibit a marked northeast-southwest structural grain defined by the trends of major folds, lithological contacts, gneissic layering and schistosities, major shear zones and major faults. Schistosities, gneissic layering and migmatitic veining developed during an early, F1, phase of deformation at high metamorphic grade. Underground mapping at the Balmat Zinc mine shows ore remobilization into large scale F2 fractures referred to as macrofractures (deLorraine, 1979), followed by F3 shear folds of pervasive “top side east” asymmetric sense, and subsequently by large scale NE-SW trending, overturned, doubly plunging, F4 isoclinal folds primarily responsible for the prominent regional grain. Of these, the Sylvia Lake syncline, host to the zinc ore bodies of the Balmat-Edwards mining district and the Great Somerville anticline, (deLorraine and Sangster, 1997 and references therein) are 8|Page

most prominent.

F4 folds are recognized as generally coaxial with F3 folds but are variable in

plunge orientation and produce hook and crescent type interference patterns (deLorraine, 2001; Baird and Schrady, 2011). F5 folds are upright and open with steeply dipping axial planes that strike northwest. Although it is clear that F5 folds refold earlier structures (deLorraine, 1979), their exact timing or influence on regional map patterns is not well understood (Baird and Schrady, 2011).

2.4 Ore Bodies The ore bodies analyzed in this study occur in the upper overturned limb of the Sylvia Lake syncline (Fig. 2) and thus have been inverted after remobilization, as have the host rock units 6-10. Structural inversion occurred later in the deformational history of the region, and after the large-scale translocation of ore bodies, as is indicated by the refolding of the transgressive, durchbeweget ore bodies and their connecting durchbewegung sheets by minor folds related to the formation of the F4 Sylvia Lake syncline (e.g., Figs. 3 and 6). The terms parent and daughter are used to describe the source beds and final positions, respectively, of externally remobilized ore bodies (Marshall et al., 2000). At Balmat, parent and daughter ore bodies may be distinguished on the basis of compositional and textural characteristics. Parent ore bodies are massive, coarse-grained mixtures of sphalerite, pyrite, gray quartz, and very minor galena, and are conformable with relict bedding in the host rocks. Daughter ore bodies are finer-grained, generally display durchbewegung texture, cut relict bedding in host rocks and are composed of monomineralic sphalerite with traces of pyrite

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(deLorraine, 2001). Photographs of representative hand samples (Fig. 3) show these contrasting textures. The Upper Gleason, Lower Gleason and Loomis ore bodies are directly linked to each other along the Gleason fault and the Gleason-Loomis Slide, a thin sheet of milled (durchbewegt) ore (Fig. 4). On this basis, as well as on the textural differences between them, they have been recognized as parent (Upper Gleason) and cogenetic metamorphic daughter (Lower Gleason and Loomis) ore bodies (deLorraine, 2001). They have been described by deLorraine (2001) as follows. The North-trending Upper Gleason is massive and relatively compact, measuring some 470 m long by 200 m wide with a maximum thickness of 26 m. It is composed of sphalerite and variable amounts of coarse-grained pyrite and gray quartz. The NESW trending Lower Gleason is recognized at its intersection with the Upper Gleason by a ~37° change in trend, from which it extends 900 m both up and down-plunge along the GleasonLoomis brittle-ductile fault, cutting the upper subunits of unit 6 up to the contact with unit 7. The Loomis ore body is parallel to, and approximately the same size as the Lower Gleason but is localized further east along the Gleason Loomis fault where it cross-cuts Upper Marble units 7, 8, and 9, ending at the contact with unit 10. Much of the ore exhibits durchbewegung texture, although it is coarse-grained in some places and is described as banded marble-sphalerite tectonite where it occurs in unit 9 (deLorraine, 2001). The Mahler-Upper Fowler trend (Fig. 5) has a mineable length of ~4600 m within a cross-stratigraphic shear zone, displays the fine-grained durchbewegung texture characteristic of daughter ore bodies, and is composed of nearly monomineralic sphalerite. The Upper Fowler is connected to the Mahler by a folded durchbewegt sheet which indicates a cogenetic 10 | P a g e

daughter relationship between the Mahler and Upper Fowler. Since no parent ore body for the Mahler-Upper Fowler trend has been discovered, its translocation distance is unknown. The Mud Pond (Fig. 6) is compositionally similar to other daughter ore bodies, being composed predominantly of fine-grained sphalerite with minor pyrite. It crosses the same stratigraphic units, 6 through 10, as the Gleason-Loomis and Mahler-Upper Fowler groups and extends more than 2100 m along plunge. Its parent remains undiscovered. The Mud Pond has a single cogenetic daughter orebody, the Davis (not sampled in this study.) 2.5 Mining history From 1903 until 2005, the mines of the Balmat district were very important sources of zinc. For example, in 1968 the mines were estimated to account for 10% of all U.S. zinc production (Lea and Dill, 1968). In 1997, the district was credited with 45 million tons of past production (deLorraine and Sangster, 1997). However exhaustion of reserves in the smaller mines together with falling prices of zinc caused the successive closure of the mines. Recently, all of the historic zinc mines of the Balmat district were purchased by the Titan Mining Corp. which has renamed them collectively as The Empire State Mine. Production was resumed at the former Balmat #4 mine in January, 2018. 3. Methods Samples of drill core from the Balmat mine core library were selected on the basis of location along the trend of each of six ore bodies (Figs. 7-9) that originated from unit 6 of the Upper Marble. Samples were spaced as evenly as possible between end points. Individual sphalerite grains (1-2 mm) were hand-picked from crushed drill core, and further crushed to 100 mesh in an alumina mortar. Approximately 30-50 mg of powdered sphalerite were 11 | P a g e

dissolved in 4 ml of heated ultra-pure aquaregia (3:1 HCl to HNO3), and complete dissolution was confirmed visually. These solutions were dried and the redigested salts were purified with ion exchange chromatography using MP-1 BioRad resin following the technique described by Marechal et al. (1999). The solutions were measured in two locations, on the Isoprobe MC-ICPMS at the University of Arizona and the Neptune at Rutgers University. Solutions and standards were diluted to 150 ppb Zn, and sphalerite samples were doped with 976 NIST Cu isotope standard. Standards and ore samples were measured within 0.5V. Mass bias was corrected using the exponential correction using Cu, and sample bracketing. Thirty ratios were measured for each sample, and each sample was measured in duplicate. Values were measured relative to the IRMM 3702 Zn standard (equivalent composition to AA-ETH standard) which can be correlated with the JMC-Lyon standard using the equation ẟ66ZnJMC-Lyon = ẟ66ZnIRMM 3702 + 0.28‰ (Archer et al., 2017).

ẟ66Zn values are reported relative to the JMC-Lyon standard to facilitate

comparisons with prior studies. Experimental error (2σ) was 0.04‰.

4. Results Zinc isotopic compositions were determined for 47 samples of sphalerite from six ore bodies that are geochemically related (Swanson, 1979; deLorraine, 2001) and constitute a set of translocated ore mobilizates that originated from a single stratigraphic horizon (Upper Marble unit 6) in the pre-metamorphic deposit (Table 1). Our results are compared to those obtained from analyses of other massive sulfide deposits globally (Fig. 10), as well as from analyses of hydrothermal fluids from active seafloor 12 | P a g e

vents. Although these studies vary in both the average and range of measured ẟ66Zn values, they describe a mean value of 0.15‰ with a standard deviation of 0.18. This makes them closer to the isotopic character of the mantle (~0.18‰) than that of bulk silicate Earth (~0.28‰) (Wang et al., 2017). The mean ẟ66Zn value for Balmat ore bodies is 0.18‰, and thus within the range of other zinc ores globally. However Balmat ores are the only ones among those reported to date that have undergone high grade metamorphism. Therefore, the distinct variations in the Zn isotopic composition of Balmat sphalerite, both between ore bodies and along the length of single ore bodies (Fig. 11) must be explained in the context of deformation and metamorphism, rather than in the context of precipitation and sedimentary deposition. For the related Gleason-Loomis ore bodies, there is an apparent pattern of decreasing ẟ66Zn from the parent Upper Gleason, to the daughter Lower Gleason, to the cogenetic daughter Loomis, although within the typical range of analytical error (Table 2). The Upper Gleason, the parent orebody that remained within its original stratigraphic unit (Upper Marble unit 6), does not exhibit systematic isotopic variation along plunge. Nor does the daughter Lower Gleason which, despite crossing several subunits, remains hosted within the strata of unit 6. In contrast, three of the four other ore bodies that occur in cross-stratigraphic shear zones (Upper Fowler, Mahler, Mud Pond, and Loomis), and which were structurally emplaced in stratigraphically higher units (units 7 through 9), display decreasing ẟ66Zn along the current down-plunge trend (Fig. 12). In all four ore bodies, the lowest ẟ66Zn values occur toward the current down-plunge end. The sample with the highest ẟ66Zn value (0.56‰) occurs at the current up-plunge end of the Mahler orebody, and the lowest value (-0.02‰) occurs at the current down-plunge ends of the Loomis and Mud Pond ore bodies (Fig. 12). 13 | P a g e

5. Discussion 5.1 Epigenetic versus Metamorphic Isotopic Patterns - Undeformed massive sulfide deposits exhibit a spatial pattern of increasing ẟ66Zn in sphalerite with distance from the hydrothermal vent. This is attributed to rapid kinetic fractionation of light zinc into the solid phase during precipitation and concurrent progressive increase in ẟ66Zn of the fluid as it moves away from its source (Mason et al., 2005; Wilkinson et al., 2005; Kelley et al., 2009; Zhou et al., 2014a, 2014b). Thus, it would be expected that prior to metamorphism, disaggregation, and remobilization, the more distal ores at Balmat would have exhibited the highest ẟ66Zn values. The observed pattern of decreasing ẟ66Zn from parent to daughter (i.e., further from the source) is counter to the expected syngenetic signature, and so must be post-depositional in its origin. Remobilization is thus associated with an isotopic partitioning process. 5.2 Current positions of the ore bodies Overturning of the Sylvia Lake syncline with the adjacent Fowler syncline occurred late during a major phase (D2) of regional deformation (deLorraine, 2001; Sangster and deLorraine, 1997) (herein designated D4). Associated shearing refolded previously remobilized ore bodies, preexisting shear zones and thin sheets of durchbewegt ore such as the one connecting the Lower Gleason and Loomis ore bodies (Fig. 4). Since overturning produced a recumbent fold (Fig. 2), complete stratigraphic inversion resulted in the upper limb of the Fowler syncline. For this reason, the current plunge directions of ore bodies do not indicate the actual plunge directions at the time of remobilization.

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5.3 Parent and Daughter Ore bodies -The known relationship between the Upper Gleason, Lower Gleason and Loomis ore bodies allows for correlation of isotopic trends with distance of ore remobilization. This is the only “Parent—Daughter” ore system at Balmat exhibiting direct linkage of stand-alone daughter ore bodies to parent that has remained intact following syn-F2 deformation, and therefore stands as a vitally important empirical model. The relatively uniform, slightly positive ẟ66Zn of both the Upper Gleason and Lower Gleason suggest that little or no isotopic fractionation of Zn occurred during metamorphism and deformation of these ores (Fig. 3). In contrast the Loomis displays a trend of decreasing ẟ66Zn in the current down-plunge direction, from 0.23 to -0.02‰. The lower mean ẟ66Zn of the Loomis (0.12‰) relative to the Upper and Lower Gleason (0.19‰ and 0.16‰, respectively) suggests that the fractionating mechanism enriches the daughter ore bodies in lighter Zn isotopes, and that longer distances of remobilization are associated with greater enrichment in light Zn. The Loomis, Mahler and Upper Fowler ore bodies exhibit a clear trend of isotopic lightening in the current down plunge direction. Although lacking such a clear trend, the lowest ẟ66Zn values obtained from the Mud Pond were found at its current down plunge end. However it is much more challenging to interpret this data, since the Mud Pond/Davis complex is by far the most structurally intricate ore body at Balmat. deLorraine (2001) suggests that strain from three folding events (F2a, F2b and F3) was experienced by this ore body. Furthermore, strong variability in the thicknesses and lithologies of the rocks in contact with the Mud Pond along its shear zone host (Mud Pond slide) affect the geometry of the ore body at any given point (deLorraine, 2001). For this reason, we limit our interpretation of Mud Pond

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data to the observation that it neither conflicts with nor precludes the more general interpretations made above, and later in this section. Because the Mahler, Upper Fowler and Mud Pond ore bodies are daughters with undiscovered parents, it is difficult to interpret their average isotopic character in terms of remobilization distance. For example, although the low mean ẟ66Zn (0.07‰) of the Mud Pond could be interpreted as an indication that this ore body traveled farther from a parent with a mean ẟ66Zn similar to that of the Upper Gleason (0.19‰) than did the Loomis (mean ẟ66Zn of 0.12), there is no way of knowing the isotopic character of the undiscovered parent. The high mean ẟ66Zn (0.33‰ and 0.27‰, respectively) of the Mahler and Upper Fowler could be interpreted as indicating their origin from a source of even heavier character, or as indicating very thorough removal of light zinc by the partitioning process. 5.4 Mechanisms of Isotopic Fractionation A number of physical and chemical processes are known to cause isotopic fractionation in zinc and other transition metals. Many of these processes may be excluded as explanations for the fractionation observed in Balmat sphalerite. Evaporation and condensation produce large isotopic fractionation in meteorites (e.g. Luck et al., 2005) and in some industrial processes (e.g. smelting, Sonke et al., 2008) but occur at temperatures far in excess of peak metamorphic conditions at Balmat. Equilibrium partitioning between coexisting phases, the phenomenon on which the use of isotopic geothermometers depends, may be excluded since sphalerite is the only significant Zn-bearing phase at Balmat. Because zinc only occurs in chemical compounds with a single valence state (Zn2+), its isotopes are not fractionated by redox-related reactions, unlike many other metals including Cu (Mathur et al., 2005), Fe (Lee et 16 | P a g e

al., 2010), Ag (Mathur et al., 2018), and Sn (Wang et al., 2019). Dissolution of primary zinc sulfides followed by precipitation of secondary oxide and silicate phases produces significant isotopic fractionation (Mondillo et al., 2018) but the Balmat ore assemblage is characterized by sulfide minerals only. Dissolution and precipitation associated with hydrothermal processes have been shown to act as fractionation mechanisms for Zn isotopes. Rapidly precipitating sphalerite at seafloor vents preferentially incorporates lighter isotopes of zinc, resulting in increasing ẟ66Zn in the residual fluids (e.g. Wilkinson et al., 2005; John et al., 2008). In addition, Weiss et al. (2014) demonstrated that dissolution of Zn-bearing biotite granite preferentially incorporates lighter isotopes of zinc into the fluid. Although such near-surface processes are inapplicable to fractionation under upper amphibolite facies metamorphic conditions at Balmat, synmetamorphic interactions with hydrothermal fluids are known from the region. For example, low-Ti, Kiruna-type magnetite deposits in the Adirondack Highlands have been attributed to the interaction of host rocks with evolved magmatic hydrothermal fluids from late-orogenic granites and pegmatites (Valley et al., 2011), based in part on extensive zones of chemical alteration between the ores and host rocks. However the general lack of host rock alteration at the contacts with Balmat ore bodies, and the absence of granitic plutons in close proximity to the mine precludes an analogous process at Balmat. Thus the only hydrous fluid type that could have possibly affected the remobilizing Balmat ores would have been metamorphic. Metamorphic fluids are in equilibrium with their host (e.g. Rauchenstein-Martinek et al., 2016) and therefore would not be expected to induce a chemical gradient, absent evidence of metasomatism. The delineation of metamorphic isograds in the Adirondacks based on the 17 | P a g e

partitioning of oxygen between graphite and calcite (e.g. Kitchen and Valley, 1995) demonstrates

the

non-reactive

nature

of

regional

metamorphic

fluids.

Thus

hydrothermal/metamorphic fluids cannot have been responsible for the isotopic variation in syn-metamorphically remobilized ore bodies. The exclusion of hydrous fluid-driven fractionation mechanisms discussed in the previous paragraphs leave partial melting and fractional crystallization as the one (combined) mechanism remaining for consideration. Partial melting and fractional crystallization are known to produce isotopic fractionation in zinc (Telus et al., 2012; Chen et al., 2013; Wang et al., 2017; Sossi et al., 2018; McCoy-West et al., 2018; Xu et al., 2019). Micro-petrographic examination of ore and gangue assemblages from the Fowler ore body show the presence of Zn-bearing sulfosalts such as tennantite and tetrahedrite deemed to have crystallized from melts, based on thermodynamic and textural evidence (Matt et al., 2019). Thus anatectic sulfide melts at Balmat included some zinc. Tomkins et al. (2004) suggested that compounds forming solid solutions with FeS such as ZnS, MnS and HgS would expand the field of polysulfide melts in multi-component systems at 600⁰C and above. Significant trace amounts of Mn and Hg were reported in analyses of sphalerite from some of the ore bodies analyzed in this study (Doe, 1962; Swanson, 1979; Whelan et al., 1984). Pruseth et al. (2014) demonstrated experimentally that a Zn-bearing (4.61 mole %) melt could be produced at 600⁰ C in evacuated silica tubes from the components FeS, PbS, Cu2S, ZnS and S. Free sulfur has been shown to lower the melting temperature of common sulfide systems (Stevens et al., 2005; Tomkins et al., 2007; Pruseth et al., 2016). At Balmat, pyrite, barite (Tomkins et al., 2004; Pruseth et al., 2014, 2016), and several H2S-bearing dolomitic units in the host rock are possible sources for free sulfur released 18 | P a g e

during prograde heating. To the extent that pyrite was a sulfur source, the lack of pyrrhotite in ore bodies derived from parents in stratigraphic units 6 or 11 at Balmat implies that there was a complete reversion of pyrrhotite to pyrite during cooling (Hall, 1986; Hall et al., 1987). Since barite is rare in the current assemblage at Balmat, nearly quantitative breakdown would have been required for it to have made a significant contribution to free sulfur. However organicrich fetid dolomites such as unit 7 in the host rock have retained free sulfur up to the present time, suggesting that even more abundant sulfur was available from these rocks during metamorphism. In view of the foregoing information, we attribute the observed fractionation of zinc isotopes to partial melting of sulfide phases including sphalerite. Because these melts were likely to have been initially of very low volume and to have grown slowly through a combination of increased temperature and mobilization-assisted melting (Tomkins et al., 2007) melts would initially have remained in close contact with the ore-rich domains where they originated. Highly ductile sphalerite would have aided mixing between ore and melt, creating a viscous crystal mush (deWaal et al., 2004) which acted as a weak rheological unit.

Later fractional

crystallization would have facilitated segregation of evolved melts from the solids. 5.5 Fractionation of Fe and Zn in Magmatic Systems-Melting of silicate assemblages induces isotopic fractionation of zinc, with the heavy isotopes being enriched in the melt. This has been documented in peridotites and basalts (Chen et al., 2013; Doucet et al., 2016; Wang et al., 2017; McCoy-West et al., 2018) as well as granitoids (Telus et al., 2012; Xu et al., 2019). In each case, the fractionation results from Zn having a lower coordination number (tetrahedral) in silicate melts than residual silicate minerals (olivine, pyroxene, biotite) where Zn is 19 | P a g e

octahedrally coordinated (Sossi et al., 2018; Williams et al., 2018). Given that heavier isotopes are favored by stronger bonding environments, δ66Zn is greater in the melt fraction. For the same reason spinels, in which Zn is tetrahedrally coordinated, exhibit higher δ66Zn than coexisting olivine and pyroxene (Wang et al., 2017). Given their similar chemical behavior, Zn2+ and Fe2+ exhibit similar fractionation patterns with respect to partial melting and equilibrium partitioning between silicate phases (Wang et al., 2017; Sossi et al., 2018). Although no experimental results have been published regarding the isotope partitioning of zinc in sulfide melts, the more extensively studied behavior of Fe2+ may provide insight into the behavior of Zn in sulfide systems. Within silicate melts, immiscible sulfide melts have been shown to be enriched in light isotopes of iron due to the Fe-S bond being weaker than the Fe-O bond (Williams et al., 2018). Iron in molten FeS is held in 6-fold coordination in sulfide melts (Urakawa et al., 1998), whereas it is tetrahedrally coordinated in silicate magmas. Given the geochemical similarity of ferrous iron and Zn2+, it is expected that the melting or diffusion of Zn from tetrahedrally coordinated sphalerite into a sulfide melt in 6fold coordination would result in enrichment of the melt in light isotopes of Zn, leaving the residual sphalerite with higher δ66Zn. The pattern in Zn isotopic data (decreasing ẟ66Zn along the current trend of shear zonehosted ore bodies and from parent to daughter) is consistent with syn-deformational interaction between sphalerite ores and migrating sulfide melts. Lighter isotopes of Zn were partitioned from the ore during melt migration along grain boundaries, leaving the residual solid sphalerite enriched in heavier Zn isotopes. During subsequent cooling, light isotope-

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enriched melt migrated up plunge, opposite the direction of ore flow.

Overturning of

stratigraphy produced the pattern observed today of isotopic lightening down plunge. 5.6 Initiation and evolution of anatectic sulfide melts Experiments have demonstrated that polymetallic sulfide melts evolve during cooling. Early crystallization products include sphalerite and pyrite (Pruseth et al., 2016) galena and pyrrhotite (Mavrogenes et al., 2013). Remaining liquids are enriched in metals and sulfosalts, with these components becoming immiscible on further cooling (Mavrogenes et al., 2013; Pruseth et al., 2016). Mavrogenes et al. (2013) produced parent melts from the components galena, pyrrhotite, and sphalerite, with added Sb, Ag and As, replicating the actual assemblage at Broken Hill. Pruseth et al. (2016) used synthesized Cu2S, PbS, FeS and ZnS, replicating the major ore phases at Rajpura-Dariba. Broken Hill and Rajpura-Dariba have significantly different abundances of sulfide minerals compared to Balmat. Broken Hill is enriched in Pb and Ag. Remobilized vein ore at Rajpura-Dariba is enriched in Pb, Cu, As, Sb and Fe. These enrichments explain in part why higher volumes of melt were likely at these deposits relative to Balmat. Higher temperature was another important factor at Broken Hill. The Broken Hill and Rajpur-Darida deposits and experiments designed to illuminate metamorphic processes affecting them are nevertheless relevant to Balmat, for four reasons. First, minor to very rare minerals at Balmat could not have been solid under peak metamorphic conditions.

These include boulangerite, bournonite, dufreynosite, jordanite, orpiment,

pyrargite, realgar, sartorite, tetrahedrite and the assemblage arsenopyrite plus pyrite (Tomkins et al., 2007; Matt et al., 2019; Chamberlain et al., 2018). The low melting temperature 21 | P a g e

chalcophile elements present in these phases would be expected to lower the eutectic dominated by more abundant phases (Mavrogenes et al., 2001; Frost et al., 2002; Tomkins et al., 2007; Pruseth et al., 2014). Second, both galena and chalcopyrite have been documented at Balmat. During the early years of exploration galena was judged to represent slightly over 1% of the total ore components, by weight (Brown, 1936). Chalcopyrite has been documented as exsolution blebs in sphalerite (deLorraine, 1979). Other Cu-bearing phases were documented among the sulfosalts in polysulfide grains from the Fowler ore body (Matt et al., 2019). Third, sulfur was available as a flux. The likely source is unit 7 of the Upper Marble, a mediumgrained, dark gray, graphitic, dolomitic marble which emits a distinct odor of H2S when crushed, and accordingly is referred to as fetid dolomite or “stinkstone”

(Brown and Engel, 1956).

Barite, now quite rare at Balmat, may have also contributed free sulfur, as postulated for Rajpura-Dariba (Pruseth et al., 2016) and Hemlo (Tomkins et al., 2004). Fourth, the simple assemblage sphalerite plus pyrite that characterizes most Balmat ore bodies is consistent with early crystallization of these phases from a Zn- and Fe-bearing polysulfide melt, while the low volume polysulfide and sulfosalt assemblages documented by Matt et al. (2019) are consistent with later crystallization of evolved melts enriched in Pb, Cu, As and Sb (Pruseth et al., 2016). Given the documented fluxing effect of sulfur (Pruseth et al., 2016; Stevens et al., 2005) increased melting would have occurred when Balmat ores were tectonically emplaced in contact with unit 7 under amphibolite facies conditions. Migration of the relatively low-volume melt would have resulted in the redistribution of lighter isotopes of zinc. This is consistent with our observation that only those ore bodies that are in contact unit 7 exhibit along-trend Zn isotope fractionation. 22 | P a g e

5.7 Fluids and remobilization The important role of fluids in the remobilization of metals in ore deposits is well established, and in the case of large-scale external remobilization is considered essential (Marshall et al., 2000). These authors suggested that fluids may be involved even in cases where solid-state transfer is considered the primary mechanism, such as the healing of fractures in high competence sulfides by minerals with low competence. Fluid pressure is invoked as causing fractures in wall rock, and dissolution and precipitation are at least partly involved in sulfide infilling of those fractures at the Black Angel mine (Greenland), based on the metal content of fluid inclusions (Pedersen, 1980). At temperatures above ~250⁰ C, sphalerite is considerably weaker than metamorphosed silicates and some carbonates, e.g. Solenhofen limestone and Hasmark dolomite (Clark and Kelly, 1973). Sphalerite at Balmat has focused strain, in part because of its evident ductility contrast with the host rock, particularly those units containing abundant quartz and diopside. The presence of melts within ore bodies where sphalerite is the dominant phase would have increased their weakness and their ability to focus strain, leading to increased distances of remobilization. Migration of melts along grain boundaries at the same time as crystalline sphalerite was undergoing ductile deformation created a low viscosity, well-mixed material which facilitated locally homogeneous isotopic exchange between liquid and solid phases. Textural evidence of former liquid-derived sphalerite has been overprinted by annealing recrystallization. Field observations at Balmat, e.g. the sharp contacts between ore bodies and host rock and the lack of hydrous alteration in the host rock at those contact, preclude a hydrous fluid as 23 | P a g e

an agent in remobilization.

However the data presented here, in conjunction with the

petrographic analysis of Matt et al. (2019), indicate that anatectic sulfide melts were present during metamorphism at Balmat and are thus invoked as facilitators in the remobilization of massive sulfide ores. Intergranular sulfide melts would have decreased the competency of the ores and thereby enhanced predominantly solid-state deformational processes. 6. Conclusions Systematic variations in Zn isotopic composition of sphalerite occur in ore bodies spatially associated with unit 6 of the Upper Marble at the Balmat mine: 1) daughter ore bodies display lower average ẟ66Zn than their parents; 2) ore bodies that lie in cross-stratigraphic fault positions in contact with unit 7 (fetid dolomite) or that have traversed unit 7 before final emplacement tend to display decreasing ẟ66Zn along their current down-plunge trend; 3) ore bodies that remained hosted within unit 6 do not display down-plunge Zn isotope fractionation or trends toward low ẟ66Zn compositions. The observed differences and trends are interpreted to have resulted from involvement of low-volume sulfide melts during the remobilization of ore undergoing upper amphibolite facies metamorphism. Sulfide melt generation was augmented when units traverse or were emplaced in the sulfur-rich dolomite of unit 7. The presence of small volumes of magmatic fluid affected the overall rheological character of the ores by wetting grain boundaries, thereby facilitating long-distance remobilization.

Acknowledgements We thank Jeff Chiarenzelli for providing duplicate drill core samples. We thank Mark Baker for his assistance in operation of the mass spectrometer at University of Arizona and Joaquin Ruiz 24 | P a g e

for making the U. of Arizona facility available. We thank Linda Godfrey for use of and assistance in operation of the mass spectrometer at Rutgers University. We thank Paul Spry for a helpful pre-publication review. Partial funding for this research was provided by PSC-CUNY Research Grant #69706-00 47.

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Figure CaptionsFigure 1-Balmat location maps. Left, the Canadian Grenville Province with some Appalachian inliers. Adirondack region in dotted box. Right, the Adirondack region comprising Lowlands and Highlands, showing Balmat near the sub-regional boundary. Figure 2-Cross section of the Balmat mining district. Ore bodies in black are MA-Mahler; MPMud Pond; UF-Upper Fowler; F-Fowler; UG-Upper Gleason; LG-Lower Gleason; L-Loomis; SLSylvia Lake; M-Main; HW-Hanging Wall. Recognized sub-units of the Upper Marble: 1-3, two units of dolomitic marble separated by pyritic schist; 4, laminated quartz-diopside rock; 5, dolomitic marble; 6, quartz-diopside rock, dolomitic marble and anhydrite; 7, fetid dolomitic marble; 8, quartz-diopside rock with interlayered dolomitic marble; 9, white dolomitic marble; 10, serpentine-talc rock; 11, quartz-diopside rock with interlayered dolomitic and calcitic marble; 11a, anhydrite; 12, dolomitic marble; 13, talc-tremolite-anthophyllite schist; 14, laminated quartz-diopside rock, calcitic marble, serpentinous dolomitic marble; 15, phlogopitic calcitic marble; 16, quartz-biotite-diopside-scapolite gneiss. Figure 3-Representative photos of massive, coarse-grained, parent-type and fine-grained, milled (durchbewegt), daughter-type ore. Dotted lines on photo of parent-type ore show boundaries of pyrite-rich layer. Anh=anhydrite; Qtz=quartz. Figure 4-Cross section of the Gleason-Loomis family, No. 3 mine. Upper Gleason is the parent, Lower Gleason the daughter, and Loomis the cogenetic daughter. The three ore bodies are structurally connected by faults. Rock unit symbols as in Fig. 2. Figure 5-Mahler-Upper Fowler cross section, enlarged from Fig. 2. Rock unit symbols as in Fig. 2. Figure 6-Mud Pond cross section. Rock unit symbols as in Fig. 2.

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Figure 7-Sample locations in the Upper Gleason, Lower Gleason and Loomis ore bodies. Plunging ore bodies are projected onto a horizontal plane. Plunge directions are north (Upper Gleason) to northeast (Lower Gleason and Loomis). Figure 8-Sample locations in the Upper Fowler and Mahler ore bodies. Northeast plunging ore bodies are projected onto a horizontal plane. Figure 9-Sample locations in the Mud Pond ore body. Northeast plunging ore bodies are projected onto a horizontal plane. Figure 10-World ore bodies whose zinc isotopic character has been analyzed. Circles show mean ẟ66Zn for each locality. Short horizontal lines show upper and lower limits of range. Data sources are: Irish Midlands, sphalerite (Wilkinson, 2005); Red Dog, sphalerite (Kelley et al., 2009); Hydrothermal vent fluids and sulfide minerals from chimneys, (John et al., 2008); Dongshengmiao, sphalerite (Gao et al., 2018); Navan, sphalerite (Gagnevin et al., 2014); Cantabrian, Zn-bearing minerals (predominantly sphalerite) (Pašava et al., 2014); Alexandrinka, whole rock (predominantly sulfides) (Mason et al., 2005); Balmat, this study; Franklin/Sterling Hill, two mineral separates, one of zincite and one of franklinite from the Sterling Hill mine; Tanquiao and Banbanquiao, sphalerite (Zhou et al., 2014) . All values normalized to the Lyon JMC 3702 standard. Figure 4-Graphs of ẟ66Zn vs. down plunge distance. Circles show analytical result, lines above and below show analytical error. On the x axis, zero is fixed at the position of the sample located farthest up plunge. Note lack of spatial trends in Upper and Lower Gleason ore bodies. Loomis, Mahler, and Upper Fowler show clear trends of isotopic lightening down plunge. Mud

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Pond lacks a clear trend but has the lowest mean isotopic values of all ore bodies and lowest internal values at the down plunge end.

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Table captions

Table 1-Analytical results for Balmat samples. UG = Upper Gleason; LG = Lower Gleason; LO = Loomis; MA = Mahler; UF = Upper Fowler; MP = Mud Pond. Values of ẟ66Zn have been normalized to the Lyon JMC 3702 standard.

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Table 2-Analytical results for Balmat ore bodies. Ore bodies are listed from heaviest to lightest mean values of ẟ66Zn relative to the Lyon JMC 3702 standard.

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Figures for Zinc Isotope Manuscript

Fig. 1

Fig. 2

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Fig. 3

Fig. 4

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

Fig. 10

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Fig. 11

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Tables for Zn-isotope manuscript Table 1 Drillhole# ẟ66Zn UG-613 0.16 UG-769 0.31 UG-773 0.21 UG-787 0.20 UG -88 0.22 UG-971 0.06 LG-625a 0.19 LG-625b 0.23 LG-634 0.19 LG-643 0.20 LG-951 0.06 LG-998 0.02 LG-1157 0.25 LG-1446 0.19 LG-1475 0.09 LO-720 0.15 LO-770 0.15 LO-781 0.16 LO-921 -0.02 LO-930 0.04 LO-931 0.11 LO-1315 0.23 LO-1366 0.17

Drillhole# ẟ66Zn MA-1110 0.25 MA-1826 0.56 MA-2200 0.49 MA-1615 0.28 MA-1699 0.44 MA-1585 0.39 MA-1619 0.13 MA-1627 0.19 MA-2366 0.20 UF-604 0.19 UF-661 0.23 UF-1020 0.30 UF-1029 0.30 UF-1027 0.34 MP-1194 -0.01 MP-1136 0.08 MP-1296 0.07 MP-1122 0.12 MP-1234 0.08 MP-577 0.13 MP-1281 0.11 MP-1206 -0.02 MP-1213 0.13 MP-1315 0.04

Table 2 Ore Body Mahler Upper Fowler Upper Gleason Lower Gleason Loomis Mud Pond All

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Avg 0.33 0.27 0.19 0.16 0.12 0.07 0.18

Min 0.13 0.19 0.06 0.02 -0.02 -0.02 -0.02

Max 0.56 0.34 0.31 0.25 0.23 0.13 0.56

Range 0.43 0.15 0.24 0.23 0.25 0.15 0.58

No. 9 5 6 9 8 10 48

With regard to my submission to Ore Geology Reviews of the manuscript entitled “Zn-Isotopic Evidence for Fluid-Assisted Ore Remobilization at the Balmat Zinc Mine, NY,” I have no conflicts of interest.

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Highlights: 

Individual ore bodies are distinguished by zinc isotopic composition



Isotopic trends within ore bodies correspond to downplunge distances



Isotopic lightening corresponds to longer remobilization distances

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Isotopic fractionation is evidence of sulfide anatexis during metamorphism



Zinc isotopic analysis may provide a new exploration tool

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