Minor and Trace Elements in Natural Tetrahedrite-tennantite: Effects on Element Partitioning among Base Metal Sulphides

Minerals of the tetrahedrite isotypic series are widespread components of base metal ores, where they co-exist with common base metal sulphides (BMS) such as sphalerite, galena, and chalcopyrite. We used electron probe microanalysis and laser-ablation inductively-coupled plasma mass spectrometry to obtain quantitative multi-trace element data on tetrahedrite-tennantite in a suite of 37 samples from different deposits with the objective of understanding which trace elements can be incorporated, at what levels of concentration, and how the presence of tetrahedrite-tennantite influences patterns of trace element partitioning in base metal ores. Apart from Fe and Zn, Hg and Pb are the two most abundant divalent cations present in the analysed tetrahedrite-tennantite (up to 10.6 wt % Hg and 4 wt % Pb). Cadmium, Co and Mn are also often present at concentrations exceeding 1000 ppm. Apart from one particularly Te-rich tetrahedrite, most contained very little Te (around 1 ppm), irrespective of prevailing assemblage. Bismuth is a common minor component of tetrahedrite-tennantite (commonly > 1000 ppm). Tetrahedrite-tennantite typically hosts between 0.1 and 1000 ppm Se, while Sn concentrations are typically between 0.01 and 100 ppm. Concentrations of Ni, Ga, Mo, In, Au, and Tl are rarely, if ever, greater than 10 ppm in tetrahedrite-tennantite and measured W concentrations are consistently <1 ppm. Taking into account the trace element concentrations in co-crystallizing BMS, the results presented allow the partitioning trends between co-crystallized sphalerite, galena, chalcopyrite, and tetrahedrite-tennantite to be defined. In co-crystallizing BMS assemblages, tetrahedrite-tennantite will always be the primary host of Ag, Fe, Cu, Zn, As, and Sb, and will be the secondary host of Cd, Hg, and Bi. In contrast, tetrahedrite-tennantite is a poor host for the critical metals Ga, In, and Sn, all of which prefer to partition to co-crystallizing BMS. This study shows that tetrahedrite-tennantite is a significant carrier of a range of trace elements at concentrations measurable using contemporary instrumentation. This should be recognized when establishing protocols for trace element analysis of tetrahedrite-tennantite, and when assessing the main hosts of trace elements in any given assemblage, e.g., for geometallurgical purposes.

The tetrahedrite isotypic series currently comprises eight named minerals: tetrahedrite (Cu 6 [ 1).The series is notable among sulphides for extensive fields of solid solution and broad ranges of metals that can be incorporated within the structure (e.g., [8][9][10]).This diversity is borne out by the range of minor and trace elements measured in natural specimens.Perhaps most noteworthy, the tetrahedrite isotypic series is an important concentrator of silver, and may also be the main host for antimony and arsenic in many ore deposits.Research interest in tetrahedrite-tennantite has also been generated by the observation of compositional variations across an orefield that may be tied to ore-forming processes (e.g., [11][12][13][14]), and which offer insights into ore genesis as well as potential as exploration vectors in, for example, porphyry-epithermal environments.[24] * Argentotetrahedrite differs from freibergite in that Ag is contained within both the A and B sites as opposed to the A site only.This endmember has only been described on the basis of electron probe microanalysis (EPMA) data.
A proper definition through the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) based on X-ray powder or reflectance data is required.
The present contribution seeks to build on these extensive foundations, using electron probe microanalysis (EPMA) and laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) on a suite of 37 samples from different locations, to address two interdependent goals.Firstly, we seek to understand the range of minor and trace elements which can be incorporated into natural tetrahedrite-tennantite, including those seldom reported in the literature for either natural or synthetic specimens.Secondly, we investigate how the preferred trace element partitioning trends among the common base metal sulphides (BMS) sphalerite, galena, and chalcopyrite outlined in George et al. (2016) [30], are modified if tetrahedrite-tennantite is present in a mineral assemblage and co-crystallizes with BMS.

Crystal Structure
Members of the tetrahedrite isotypic series are classified as sulfosalts; a group of sulphides containing As 3+ , Sb 3+ , Bi 3+ or Te 4+ , where one or more of these metalloid cations is associated with one or more metallic cations [4].The metalloids are not bonded to the metals; both exclusively bond to the anion S 2− (or more rarely Se 2− or Te 2− ).They are thus distinct from sulphide minerals such as arsenopyrite (FeAsS), löllingite (FeAs 2 ), gudmundite (FeSbS) or enargite/luzonite (Cu 3 AsS 4 ) where trivalent metalloids bond directly to Fe or Cu.Moëlo et al. (2008) [4] classify the tetrahedrite isotypic series as sulfosalts with an excess of small (univalent) cations (Ag,Cu) relative to (As,Sb,Bi), and further sub-group them as Cu(Ag)-rich sulfosalts.The tetrahedrite isotypic series is the most complex isostructural series among sulfosalts due to the extensive iso-and heterovalent substitutions that are possible.
In Cu pure tetrahedrite (i.e., Cu 12 Sb 4 S 13 ), the structure is most simply understood as linked polyhedra of metal atoms about sulphur atoms [1].The S 2− anion is bonded to six Cu 2+ cations in octahedral coordination, while each S − anion is coordinated tetrahedrally to two Cu + cations, one Cu 2+ cation and one Sb 3+ cation.The angles between bonds in the tetrahedron are significantly distorted from the ideal 109.28 • .Each tetrahedron shares a corner with the octahedron, while the remaining three corners of each tetrahedron share corners with two adjacent tetrahedrons (Figure 1).Antimony cations are positioned at the tetrahedron corners closest to the octahedron.The crystal structure of tennantite is analogous to that of tetrahedrite (with As substituted for Sb), although the degree of tetrahedron distortion is generally less in tennantite [2].
Additional investigations on how the tetrahedrite-tennantite structure is changed through incorporation of major elements such as Ag, Fe, Zn or Hg have been carried out (e.g., [31][32][33][34]), as has research into the possible oxidation and coordination states of such elements (e.g., [35][36][37]).[4] classify the tetrahedrite isotypic series as sulfosalts with an excess of small (univalent) cations (Ag,Cu) relative to (As,Sb,Bi), and further sub-group them as Cu(Ag)-rich sulfosalts.The tetrahedrite isotypic series is the most complex isostructural series among sulfosalts due to the extensive iso-and heterovalent substitutions that are possible.
In Cu pure tetrahedrite (i.e., Cu12Sb4S13), the structure is most simply understood as linked polyhedra of metal atoms about sulphur atoms [1].The S 2− anion is bonded to six Cu 2+ cations in octahedral coordination, while each S − anion is coordinated tetrahedrally to two Cu + cations, one Cu 2+ cation and one Sb 3+ cation.The angles between bonds in the tetrahedron are significantly distorted from the ideal 109.28°.Each tetrahedron shares a corner with the octahedron, while the remaining three corners of each tetrahedron share corners with two adjacent tetrahedrons (Figure 1).Antimony cations are positioned at the tetrahedron corners closest to the octahedron.The crystal structure of tennantite is analogous to that of tetrahedrite (with As substituted for Sb), although the degree of tetrahedron distortion is generally less in tennantite [2].

Documented Substitutions
Major metal/metalloid components of the tetrahedrite-tennantite series are copper, iron, zinc, antimony, and arsenic.Silver readily enters the tetrahedrite-tennantite structure, with argentotetrahedrite and argentotennantite the Ag end-member analogues of tetrahedrite and tennantite, respectively.

Documented Substitutions
Major metal/metalloid components of the tetrahedrite-tennantite series are copper, iron, zinc, antimony, and arsenic.Silver readily enters the tetrahedrite-tennantite structure, with argentotetrahedrite and argentotennantite the Ag end-member analogues of tetrahedrite and tennantite, respectively.
Tin, Co and Ni have been measured in tetrahedrite-tennantite up to a few thousand ppm.Gaspar (2002) [69] report tetrahedrite and tennantite from Neves-Corvo, Portugal, carrying up to 4784 and 1300 ppm Sn, respectively.Tetrahedrite from the same deposit contains up to 4300 ppm Co. Serranti et al. (2002) [66] also measure 4784 ppm Sn in tetrahedrite from the Corvo orebody, Portugal, as well as up to 1369 ppm Ni in tennantite.Up to 3800 ppm Co was measured in tetrahedrite-tennantite in a fluorite vein from Oberwolfach, Clara, SW Germany, and up to 6900 ppm Ni was measured from nearby Urberg, Gottesehre [13].
Gold is occasionally present in tetrahedrite-tennantite, usually only at a few ppm at most.Knittel (1989) [62], however, measured up to 0.02 wt % Au in tellurian tennantite from the Marian gold mine, northern Luzon, Philippines, while Jurković et al. (2011b) [70] record up to 39 ppm Au in mercurian tetrahedrite from the Duboki Vagan deposit, Bosnia.Wohlgemuth-Ueberwasser et al. (2015) [71] also measured up to 19.4 ppm Au in tetrahedrite-tennantite from black smokers in the Manus Basin, Papua New Guinea, and infer that tetrahedrite-tennantite may control Au distributions in a sulphide assemblage.Unique tetrahedrite from Saski Rad, Bosnia, has also been documented containing 92 ppm Tl, 20 ppm W, 7.3 ppm Mo, and 0.5 ppm Ga, elements that are rarely if ever reported as a trace component in tetrahedrite-tennantite [70].

Approach and Methodology
The published data above show that alongside the essential constituents Cu, Fe, Zn, Sb, As, and S, the tetrahedrite isotypic series may incorporate up to wt % levels of Ag, Mn, Cd, Bi, Pb, In, Hg, Se, and Te, thousands of ppm Ni, Co, and Sn, at least ppm-level concentrations of Au, Tl, W, and Mo, and possibly also Ga.We have sought to analyse the concentrations of these 23 elements in a suite of 37 samples (Table 2).Sample material mostly derives from collections of the South Australian Museum and the Tate Museum (The University of Adelaide; as indicated in Table 2), while some material originates from the author's personal collections.Although some samples comprise solely tetrahedrite-tennantite (i.e., without other sulphides present), most were selected because they consist of tetrahedrite-tennantite co-existing with other base metal sulphides.Textural evidence suggests that these assemblages co-crystallized at equilibrium (~120 • triple-junctions between sulphides; see Figure 2).The trace element partitioning rules for BMS outlined in George et al. (2016) [30] were employed as an additional check for assessing BMS co-crystallization.Thus through trace element analysis of the sulphides in each co-crystallized assemblage, the preferred partitioning of trace elements can be determined for a BMS assemblage comprising tetrahedrite-tennantite.
Quantitative analysis on tetrahedrite-tennantite was carried out using a Cameca SX-Five EPMA (Cameca, Gennevilliers, France), which utilizes five wavelength dispersive spectrometers (WDS).The elements analysed were S, Pb, Cd, As, Se, Fe, Cu, Mn, Ag, Sn In, Hg, Zn, Ni, Co, Sb, Te, Bi, Tl, and Ga.Beam operating conditions were maintained at 20 kV and 20 A with a beam size of 5 µm.Further details on EPMA methodology, including standards, count times and typical minimum limits of detection, are given as Electronic Appendix A. EPMA element maps were generated using the same instrument.An operating voltage of 20 kV and 224 nA were applied for EPMA mapping of selected grains, with a step size of 4 µm.
LA-ICP-MS instrumentation and analytical procedures followed those given in George et al. (2016) [30] and George et al. (in press) [72].A Resonetics M-50-LR 193 nm Excimer laser (Resonetics, Nashua, NH, USA) and an Agilent 7700cx Quadrupole ICP mass spectrometer (Agilent, Santa Clara, CA, USA) were used for the LA-ICP-MS analysis of tetrahedrite-tennantite (Adelaide Microscopy, The University of Adelaide).An atmosphere of ultra-high purity (UHP) He (0.7 L/min) was created inside the ablation cell, and Ar (0.93 L/min) was mixed with the ablated material upon exiting the cell.The mixture then passed through a pulse-homogenizing device (squid) before being directed to the torch.Regular calibration of the ICP-MS was performed so that the sensitivity on the isotopes of interest was maximized, and formation of unwanted molecular oxide species was kept at a minimum.
Laser beam energy was kept at 100 mJ with a 26 µm spot size, while the repetition rate was maintained at 10 Hz.Each analysis was a total of 60 s, consisting of a 30 s background measurement and 30 s of sample ablation.A delay of 40 s was allowed between each analysis for cell washout and gas stabilization.The following isotopes were analysed: 34 S, 55 Mn, 57 Fe, 59 Co, 60 Ni, 65 Cu, 66 Zn, 69 Ga, 75 As, 82 Se, 95 Mo, 107 Ag, 111 Cd, 115 In, 118 Sn, 121 Sb, 125 Te, 182 W, 197 Au, 202 Hg, 205 Tl, 206 Pb, 207 Pb, 208 Pb, and 209 Bi.The dwell time for each element was 0.01 s, while In, Au, and Tl were set to 0.05 s.Unless tetrahedrite-tennantite was fine-grained and sparse within any given sample, we aimed to make 10 spot analyses on each sample.Electronic Appendix B shows the range of minimum detection limits and mean errors for the trace elements analysed.Electronic Appendix C shows the full tetrahedrite-tennantite LA-ICP-MS dataset, as well as the data for co-existing sphalerite, galena and chalcopyrite also analysed in each sample according to methodology outlined in George et al., (2016) [30].The external standard used was MASS-1 (previously PS-1; [73]), utilizing the latest certificate of analysis [74].Up to 10 unknown analyses were bracketed by multiple analyses of this reference material.Thus instrument drift was monitored, and a linear correction was applied to the unknown analyses.All data reduction was carried out using GLITTER [75].Internal standardization was conducted using Cu concentration values obtained by EPMA on each individual sample.While certain interferences may have affected the reliability of some elements analysed by LA-ICP-MS (e.g., sulphur interference on 66 Zn; [76], or 59 Co 16 O interference on 75 As; [77]), corrections were not applied as reliable measurements on these typically major elements were made by EPMA.

Results
As discussed in Cook et al. (2016) [78] and George et al. (in press) [72], mineral microanalysts are continually faced with the challenge of determining whether measurable concentrations of an element in any given mineral are present in solid solution, within microscale mineral inclusions hosted within the analysed mineral, or a mixture of both.Every effort has been made in the present study to report data that reflects elements in solid solution within the tetrahedrite-tennantite structure.This was done by examining all samples in back-scatter electron mode with a scanning electron microscope (SEM) prior to microanalysis so that only clean areas free of any noticeable inclusions were analysed.In addition, all downhole spectra from the LA-ICP-MS datasets were additionally checked for any peaks that suggest the presence of micro-inclusions beneath the sample surface (e.g., [72,79]).If present, such analyses were discarded.It is evident that some elements, notably Pb, are more commonly present as micro-inclusions in tetrahedrite-tennantite than others, perhaps due to the common presence of co-existing galena.
Table 3 summarizes the concentrations of 16 elements in tetrahedrite-tennantite as determined by EPMA.Nickel, Ga, In, and Tl were also measured, but analyses were consistently below the minimum limits of detection (average values of 0.022, 0.041, 0.031, and 0.126 wt %, respectively).Figure 3 shows the major element variation in the tetrahedrite-tennantite sample suite.Specimens spanning the entire tetrahedrite-tennantite solid solution range are represented (Figure 3a), as are samples ranging in composition from essentially Fe to Zn pure end-members (Figure 3b).Most tetrahedrite-tennantites here contain more Cu than Ag; only in sample EV8 (Evelyn Mine, NT, Australia) could freibergite be classified (Figure 3c; Table 3).
Concentrations of 20 trace and minor elements measured in tetrahedrite-tennantite by LA-ICP-MS are summarized in Table 4. Individual spot analyses are plotted as cumulative plots (Figures 4 and 5) that are sorted by the co-crystallized BMS assemblage.This allows a visualization of element concentrations and variation in different assemblages.In one sample (G871; Pulganbar, Grafton, NSW, Australia), clear oscillatory zoning was observed and mapped by EPMA (Figures 6 and 7).The zoning shows the inverse correlation between the trivalent cations Sb and As, such that Sb enriched zones are As depleted, and vice versa.Zones of high Cu, Fe, and Zn correlate with As enriched, Sb depleted zones, while high Hg zones correlate to Sb-enriched, As depleted zones.While such strong zoning appeared to be uncommon in the sample suite, other analysed tetrahedrite-tennantites may be considered as zoned to some degree based on variation of concentration values between spots.In general, grain-scale compositional zoning is common in tetrahedrite-tennantite, although this is more commonly observed as patchy heterogeneity (e.g., [80]).Micron-scale oscillatory zoning of the kind illustrated here is, however, well known from epithermal systems (e.g., [81]).Apart from Fe and Zn, Hg and Pb are the two most common divalent cations determined as present in tetrahedrite-tennantite. 10.6 wt % Hg and 4 wt % Pb are the highest measured levels of the two elements in tetrahedrite-tennantite, measured in samples G871 (Pulganbar, Grafton, NSW, Australia) and Mo17A (Mofjell, Norway), respectively.Tetrahedrite-tennantite (TetTen) crystallizing in isolation, or with chalcopyrite (Cp; i.e., TetTen-Cp assemblages), hosts the most Hg (see Hg plot in Apart from Fe and Zn, Hg and Pb are the two most common divalent cations determined as present in tetrahedrite-tennantite. 10.6 wt % Hg and 4 wt % Pb are the highest measured levels of the two elements in tetrahedrite-tennantite, measured in samples G871 (Pulganbar, Grafton, NSW, Australia) and Mo17A (Mofjell, Norway), respectively.Tetrahedrite-tennantite (TetTen) crystallizing in isolation, or with chalcopyrite (Cp; i.e., TetTen-Cp assemblages), hosts the most Hg (see Hg plot in Figure 5).While those tetrahedrite-tennantites crystallizing with sphalerite (Sp) and galena (Gn; TetTen-Sp-Gn assemblages) also host high levels of Hg, this trend is swayed by one population of Hg-rich tetrahedrites from sample G10847 (Mt.Camel, Heathcote, Australia; see Hg plot in Figure 5).The tetrahedrite in this sample co-crystallized with the most Hg-rich sphalerite of any sample here, implying a particularly Hg-rich crystallization environment.Thus, excluding this one sample, the highest Hg concentrations are present in tetrahedrite-tennantite, crystallizing without sphalerite or galena, which is in close agreement with Hg partitioning trends outlined in George et al. (2016) [30], where sphalerite is the preferred Hg host in BMS assemblages.
Cadmium, cobalt, and manganese are the other common divalent cations in tetrahedrite-tennantite.All may be present at concentrations greater than 1000 ppm.Whereas the EPMA data shows 0.674 wt % Cd in sample G6951 (Yerranderie, NSW, Australia), the LA-ICP-MS data shows almost 30,000 ppm in the same sample, hinting at heterogeneity from area to area of the sample.The highest Co and Mn in tetrahedrite-tennantite here is 0.362 wt % and 0.107 wt %, in samples G16152 (Siegen, Westphalia, Germany) and VF1031 (Emperor Gold Mine, Fiji), respectively.The LA-ICP-MS data for these samples is in close agreement.Cadmium concentrations are highest in tetrahedrite-tennantite that has not crystallized with sphalerite or chalcopyrite (see Cd plot in Figure 4).This is anticipated since Cd displays a strong preference for the sphalerite structure in BMS systems [30].The only exception is the tetrahedrite from sample G14549b (Consols Mine, Broken Hill, NSW, Australia), which co-crystalized with sphalerite hosting up to 10,000 ppm Cd (see Cd plot in Figure 4).The exceptionally Cd-rich environment this assemblage formed in explains the high Cd concentrations in tetrahedrite, even though sphalerite co-crystallized.In a similar way, Co concentrations are also highest in those tetrahedrite-tennantites that have not crystallized with sphalerite (see Co plot in Figure 4), in line with findings reported by George et al. (2016) [30].
Tetrahedrite in sample VFI031 (Emperor Gold Mine, Fiji) contains as much as 3648 ppm Te, the highest concentration measured in this study.Apart from this sample, and two samples from the Kalgoorlie ore district, Western Australia (G11579 and G6940 with tetrahedrites containing 50 and 38 ppm Te, respectively), the mean Te concentrations in all tetrahedrite-tennantites from all assemblages are remarkably constant around the 1 ppm level (see Te plot in Figure 5).
Bismuth concentrations exceed 1000 ppm in tetrahedrite-tennantites from all assemblages except those co-crystallizing with galena and chalcopyrite, where concentrations only exceed 10 ppm in two spots.Bismuth concentrations in tetrahedrite-tennantite from TetTen-Sp-Gn and TetTen-Gn assemblages are also very low if samples EV8 (Evelyn Mine, NT, Australia) and G14549a (Consols Mine, Broken Hill, NSW, Australia) are excluded, both of which contain galena with up to 10,000 ppm Bi (see Bi plot in Figure 5).The Bi-rich galena co-crystallizing with high Bi tetrahedrite points at particularly Bi-rich crystallization environments for these samples.Such samples are anomalous for the otherwise Bi-depleted tetrahedrite-tennantites in TetTen-Sp-Gn and TetTen-Gn assemblages (see Bi plot in Figure 5).As shown in George et al. (2016) [30], the presence of galena can be considered the cause of this Bi depletion in tetrahedrite-tennantite since it is the preferred host of Bi in BMS assemblages.
Tetrahedrite-tennantite typically hosts between 0.1 and 1000 ppm Se.The most Se-rich tetrahedrite is recorded in sample VFI031 (Emperor Gold Mine, Fiji; 624 ppm).In general, the highest levels of Se in tetrahedrite-tennantite are reached when galena does not co-crystallize; i.e., from TetTen, TetTen-Sp-Cp, and TetTen-Cp assemblages (see Se plot in Figure 4).This is to be expected since galena is always the preferred host of Se in BMS assemblages [30].
Tin concentrations in tetrahedrite-tennantite are usually an order of magnitude lower than those of Se (typically between 0.01 and 100 ppm); they are the highest in EV8 (Evelyn Mine, NT, Australia; 404 ppm).Interestingly, the EPMA data records up to 0.264 wt % Sn in the tetrahedrite from EV8, though this may be due to inclusions of a Sn-rich phase (possibly stannite), since some were recognized on LA-ICP-MS downhole spectra from this sample.

Discussion
5.1.Element Partitioning Between Tetrahedrite-Tennantite, Sphalerite, Galena, and Chalcopyrite Figure 8 is a series of tri-plots that show where various elements are hosted in co-crystallized three-component assemblages in which one component is tetrahedrite-tennantite.Each individual point is a sample average, and where it plots shows how much of a given element is contained within the sulphides of that assemblage as a fraction of the overall amount of that element in the assemblage (using molar percentages for comparisons between sulphides).For example, in TetTen-Sp-Gn assemblages, sphalerite always concentrates more than 60% of the Mn budget, no more than 40% is ever contained in tetrahedrite-tennantite, and co-crystallizing galena essentially contains none of the available Mn.In the TetTen-Sp-Cp assemblage, sphalerite again always concentrates >50% of the Mn budget.We can thus say that whenever sphalerite is present, it is always the primary Mn host.
Apart from cases where chalcopyrite (or other Fe-sulphides such as pyrite and pyrrhotite) co-crystallizes, Fe is always hosted in either tetrahedrite-tennantite or sphalerite.In most cases tetrahedrite-tennantite is the primary host, but in four TetTen-Sp-Gn assemblages, sphalerite hosts > 50% of the Fe budget.Cobalt does not have a strong affiliation with any of the four sulphides, although it is never primarily hosted in chalcopyrite.In the absence of sphalerite, Zn always partitions to tetrahedrite-tennantite. Gallium seems to be most affiliated with sphalerite, although in TetTen-Sp-Cp assemblages, chalcopyrite hosts the larger share of Ga in four samples.Two of these samples (V446 and V538) come from the Bleikvassli deposit, Norway, a sedimentary-exhalative deposit which is interpreted to have recrystallized at conditions of granulite facies [82,83].The other two (G6940 and G11579) come from Kalgoorlie, Western Australia, where Boulter et al. (1987) [84] describe recrystallization of pyrite during deformation associated with regional metamorphism.Thus, those assemblages in which chalcopyrite hosts more Ga than sphalerite, both come from environments where sulphides have recrystallized under regional metamorphism.Such conditions result in chalcopyrite becoming the primary Ga host, distinct from lower temperature/pressure environments where sphalerite is the preferred host [30,72].
Selenium is always concentrated in galena whenever that mineral is present, and tetrahedrite-tennantite seems to be the secondary host in most cases.Tetrahedrite-tennantite typically hosts > 90% of the Ag budget whenever it co-crystallizes with any combination of BMS.In all but one sample, Cd is principally hosted in sphalerite when present.In the single exception (sample G10847; Mt.Camel, Heathcote, Australia), tetrahedrite-tennantite is the preferred host.The same is true in assemblages in which sphalerite is absent.
As is the case in the three-component Sp-Gn-Cp assemblage, Sn does not have a strong affiliation with any sulphide [30].It is concentrated in different phases in different samples.In most cases, however, Sn seems to prefer the BMS over tetrahedrite-tennantite. Tellurium has a strong affiliation with galena, while Hg appears to be preferentially incorporated into either sphalerite or tetrahedrite-tennantite in different samples.Galena will concentrate Bi whenever it is present, and in its absence tetrahedrite-tennantite always becomes the primary host.Each individual point is a sample, and where it plots shows how much of a given element is contained within the sulphides of that assemblage as a fraction of the overall amount of that element in the assemblage (using molar percentages for comparisons between sulphides).TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite.

Controls on Fe and Hg Partitioning
Both tetrahedrite-tennantite and sphalerite may be the primary host phase for Fe and Hg in different samples (excluding, of course, samples containing Fe-sulphides).Which mineral will host a greater molar percentage of Fe or Hg in a given system seems to depend on the amount of that element available in the system.In systems with lesser Fe, tetahedrite-tennantite is the primary Fe host, indicating that Fe prefers to partition into tetrahedrite-tennantite relative to sphalerite (Figure 9a).Sphalerite is the primary host of Fe only in those systems where Fe concentrations in tetrahedrite-tennantite approach 2 apfu, i.e., the maximum concentration of Fe allowed in the tetrahedrite-tennantite structure ( [4]; Figure 9b).Thus, in assemblages free of Fe-sulphides, if there is more Fe than can be incorporated by tetrahedrite-tennantite, then the excess will partition to sphalerite such that sphalerite may host more Fe than tetrahedrite-tennantite.

Controls on Fe and Hg Partitioning
Both tetrahedrite-tennantite and sphalerite may be the primary host phase for Fe and Hg in different samples (excluding, of course, samples containing Fe-sulphides).Which mineral will host a greater molar percentage of Fe or Hg in a given system seems to depend on the amount of that element available in the system.In systems with lesser Fe, tetahedrite-tennantite is the primary Fe host, indicating that Fe prefers to partition into tetrahedrite-tennantite relative to sphalerite (Figure 9a).Sphalerite is the primary host of Fe only in those systems where Fe concentrations in tetrahedrite-tennantite approach 2 apfu, i.e., the maximum concentration of Fe allowed in the tetrahedrite-tennantite structure ( [4]; Figure 9b).Thus, in assemblages free of Fe-sulphides, if there is more Fe than can be incorporated by tetrahedrite-tennantite, then the excess will partition to sphalerite such that sphalerite may host more Fe than tetrahedrite-tennantite.Similar patterns are observed for Hg.In systems with lesser Hg, sphalerite is the primary Hg host, indicating that Hg preferentially partitions into sphalerite (Figure 9c).Tetrahedrite-tennantite is the primary host of Hg only in those systems where Hg concentrations in sphalerite are ~0.001mol% (Figure 9d).The present dataset may thus indicate that this is an approximate upper limit to Hg concentrations in sphalerite, at least when co-crystallizing with tetrahedrite-tennantite.Only in one sample is the concentration of Hg in sphalerite significantly higher than this; sample G10847 (Mt.Camel, Heathcote, Vic., Australia) which concentrates 0.0062 mol% Hg in sphalerite (equivalent to ~250 ppm Hg), and where co-crystallizing tetrahedrite hosts more than 0.07 mol% Hg (>2500 ppm Hg equivalent).This is the most Hg in any tetrahedrite-tennantite co-crystallizing with Similar patterns are observed for Hg.In systems with lesser Hg, sphalerite is the primary Hg host, indicating that Hg preferentially partitions into sphalerite (Figure 9c).Tetrahedrite-tennantite is the primary host of Hg only in those systems where Hg concentrations in sphalerite are ~0.001mol% (Figure 9d).The present dataset may thus indicate that this is an approximate upper limit to Hg concentrations in sphalerite, at least when co-crystallizing with tetrahedrite-tennantite.Only in one sample is the concentration of Hg in sphalerite significantly higher than this; sample G10847 (Mt.Camel, Heathcote, VIC, Australia) which concentrates 0.0062 mol% Hg in sphalerite (equivalent to ~250 ppm Hg), and where co-crystallizing tetrahedrite hosts more than 0.07 mol% Hg (>2500 ppm Hg equivalent).This is the most Hg in any tetrahedrite-tennantite co-crystallizing with sphalerite analysed here.This particularly Hg-rich system seems to allow for exceptional Hg concentrations in sphalerite.In any case, if a given system contains more Hg than can be incorporated by sphalerite, then the excess will partition to tetrahedrite such that tetrahedrite may host more Hg than sphalerite.

Implications and Conclusions
This study shows that tetrahedrite-tennantite is a significant carrier of a range of trace elements at concentrations measurable using contemporary instrumentation.This should be recognized when establishing protocols for trace element analysis of tetrahedrite-tennantite, and when assessing the primary hosts for trace elements of interest in any given assemblage, e.g., for geometallurgical purposes.Most noteworthy, tetrahedrite-tennantite will always be the primary host of Ag in co-crystallizing BMS assemblages.Iron, Cu, Zn, As, Cd, Sb, Hg, and Bi are the additional elements that are controlled to some degree by the presence of tetrahedrite-tennantite.In contrast, tetrahedrite-tennantite does not appear, from the data here, to be a very good host for the critical metals Ga, In, and Sn, all of which prefer to partition to co-crystallizing BMS.
As an extension of arguments put forward by George et al. (2016) [30], the partitioning trends outlined here may be used as a tool to assess co-crystallization of a given TetTen ± Sp ± Gn ± Cp assemblage.If the hosts of various trace elements in such an assemblage do not match the preferred hosts outlined in Table 5, then it strongly suggests that assemblage did not co-crystallize.On the other hand, if the primary hosts do match those given here, that is typically suggestive of a co-crystallized assemblage.Abbreviations: TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite. 1Trend observed in all examined samples. 2 Recrystallization increases element concentration in Cp and may make Cp primary host. 3Trend generally, yet not always, true.* Recrystallization increases Sn concentration in Cp such that Cp is usually primary host in recrystallized samples.Below recrystallization conditions, no trend is observed.

Figure 1 .
Figure 1.Crystal structure of tetrahedrite drawn from .ciffile in American Mineralogist Crystal Structure Database after data in [31].

Figure 1 .
Figure 1.Crystal structure of tetrahedrite drawn from .ciffile in American Mineralogist Crystal Structure Database after data in [31].

Figure 3 .
Figure 3. Scatter-plots showing the variation in major elements in the tetrahedrite-tennantite sample suite.(a) Sb vs. As; (b) Zn vs. Fe; and (c) Ag vs. Cu.Atoms per formula unit is calculated by normalizing mol % values to 29.

Figure 3 .
Figure 3. Scatter-plots showing the variation in major elements in the tetrahedrite-tennantite sample suite.(a) Sb vs. As; (b) Zn vs. Fe; and (c) Ag vs. Cu.Atoms per formula unit is calculated by normalizing mol % values to 29.

Tet
= tetrahedrite, Ten = tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite, St. Dev.= standard deviation.Minerals in brackets are very minor phases.(X) = number of individual spot analyses in that sample.Dash = insufficient data to perform calculation (all analyses <mdl).Other <mdl values were treated as mdl/2.ME = major/minor element (concentration from EPMA > 1 wt %).* Evidence suggests sulphides in sample did not co-crystallize (based on textures and partitioning trends among base metal sulphides).

Figure 4 .
Figure 4. Cumulative plots showing individual spot concentrations of Mn, Fe, Co, Zn, As, Se, Ag and Cd in tetrahedrite-tennantite from different assemblages.Tetrahedrite-tennantite concentration data for each assemblage is sorted in ascending order and plotted in succession along the X axis.Y axis = concentration (parts per million).The average composition for each assemblage is given as a horizontal coloured line.TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite.

Figure 4 .
Figure 4. Cumulative plots showing individual spot concentrations of Mn, Fe, Co, Zn, As, Se, Ag and Cd in tetrahedrite-tennantite from different assemblages.Tetrahedrite-tennantite concentration data for each assemblage is sorted in ascending order and plotted in succession along the X axis.Y axis = concentration (parts per million).The average composition for each assemblage is given as a horizontal coloured line.TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite.

Figure 4 .
Figure 4. Cumulative plots showing individual spot concentrations of Mn, Fe, Co, Zn, As, Se, Ag and Cd in tetrahedrite-tennantite from different assemblages.Tetrahedrite-tennantite concentration data for each assemblage is sorted in ascending order and plotted in succession along the X axis.Y axis = concentration (parts per million).The average composition for each assemblage is given as a horizontal coloured line.TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite.

Figure 5 .
Figure 5. Cumulative plots showing individual spot concentrations of In, Sn, Sb, Te, Au, Hg, Pb, and Bi in tetrahedrite-tennantite from different assemblages.Tetrahedrite-tennantite concentration data for each assemblage is sorted in ascending order and plotted in succession along the X axis.Y axis = concentration (parts per million).The average composition for each assemblage is given as a horizontal coloured line.

Figure 5 .
Figure 5. Cumulative plots showing individual spot concentrations of In, Sn, Sb, Te, Au, Hg, Pb, and Bi in tetrahedrite-tennantite from different assemblages.Tetrahedrite-tennantite concentration data for each assemblage is sorted in ascending order and plotted in succession along the X axis.Y axis = concentration (parts per million).The average composition for each assemblage is given as a horizontal coloured line.

Figure 5 .
Figure 5. Cumulative plots showing individual spot concentrations of In, Sn, Sb, Te, Au, Hg, Pb, and Bi in tetrahedrite-tennantite from different assemblages.Tetrahedrite-tennantite concentration data for each assemblage is sorted in ascending order and plotted in succession along the X axis.Y axis = concentration (parts per million).The average composition for each assemblage is given as a horizontal coloured line.

Figure 8 .
Figure8.Series of tri-plots that show where the Mn, Fe, Co, Zn, Ga, Se, Ag, Cd, Sn, Te, Hg, and Bi budget is hosted in co-crystallized three-component assemblages comprising tetrahedrite-tennantite.Each individual point is a sample, and where it plots shows how much of a given element is contained within the sulphides of that assemblage as a fraction of the overall amount of that element in the assemblage (using molar percentages for comparisons between sulphides).TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite.

Figure 8 .
Figure8.Series of tri-plots that show where the Mn, Fe, Co, Zn, Ga, Se, Ag, Cd, Sn, Te, Hg, and Bi budget is hosted in co-crystallized three-component assemblages comprising tetrahedrite-tennantite.Each individual point is a sample, and where it plots shows how much of a given element is contained within the sulphides of that assemblage as a fraction of the overall amount of that element in the assemblage (using molar percentages for comparisons between sulphides).TetTen = tetrahedrite-tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite.

Figure 9 .
Figure 9. Scatter-plots that show the primary host of Fe and Hg depends on the concentration of Fe and in tetrahedrite-tennantite and sphalerite.(a) Fe in tetrahedrite-tennantite and sphalerite vs. Fe in tetrahedrite-tennantite as a % of the assemblage Fe budget.(b) Fe in tetrahedrite-tennantite vs. Fe in tetrahedrite-tennantite as a % of the assemblage Fe budget.(c) Hg in tetrahedrite-tennantite and sphalerite vs. Hg in sphalerite as a % of the assemblage Hg budget.(d) Hg in sphalerite vs. Hg in sphalerite as a % of the assemblage Hg budget.TetTen = tetrahedrite-tennantite, Sp = sphalerite.

Figure 9 .
Figure 9. Scatter-plots that show the primary host of Fe and Hg depends on the concentration of Fe and Hg in tetrahedrite-tennantite and sphalerite.(a) Fe in tetrahedrite-tennantite and sphalerite vs. Fe in tetrahedrite-tennantite as a % of the assemblage Fe budget.(b) Fe in tetrahedrite-tennantite vs. Fe in tetrahedrite-tennantite as a % of the assemblage Fe budget.(c) Hg in tetrahedrite-tennantite and sphalerite vs. Hg in sphalerite as a % of the assemblage Hg budget.(d) Hg in sphalerite vs. Hg in sphalerite as a % of the assemblage Hg budget.TetTen = tetrahedrite-tennantite, Sp = sphalerite.

Table 1 .
Named minerals in the tetrahedrite isotypic series.

Table 3 .
Cont.Ten = tennantite, Sp = sphalerite, Gn = galena, Cp = chalcopyrite, apfu = atoms per formula unit.Minerals in brackets are very minor phases.(X) = number of individual spot analyses in that sample.* Evidence suggests sulphides in sample did not co-crystallize (based on textures and partitioning trends among base metal sulphides).
Dash = insufficient data to perform calculation (all analyses <mdl).Other <mdl values were ignored, thus data can be considered maximum concentrations.Totals are calculated by averaging the total for each spot in each sample.Hence each row will not add to the total.

Table 4 .
Summary of trace element concentrations in tetrahedrite-tennantite determined by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) (data in ppm).

Table 5 .
Preferred hosts of various trace elements in a co-crystallizing TetTen-Sp-Gn-Cp assemblage.