Introduction
The discovery of Hg-bearing Au‒Ag alloys in an historic ‘osmiridium’ concentrate from beach sand at Waratah Bay, in southern Victoria, Australia, has prompted a comparison with numerous world-wide occurrences of these enigmatic phases. Many are recorded as overgrowths on gold grains from placer deposits derived from varied geological sources. For example, the approved mineral aurihydrargyrumite (Au6Hg5) was found in the Oda River draining through crystalline schists, gabbro and serpentinite in Ehime Prefecture, Shikoku Island, Japan (Nishio-Hamane et al., Reference Nishio-Hamane, Tanaka and Minakawa2018). In southern New Zealand, alloys with a wide range in composition occur in Quaternary placers derived from sources within the Mesozoic Otago Schist (Youngson et al., Reference Youngson, Woperis, Kerr and Craw2002). Huang (Reference Huang2011) described Au–Hg phases from placers in the Davao district, Mindanao, Philippines, probably derived from ultramafic–mafic complexes with associated polymetallic deposits. Two hexagonal Au–Hg phases (Au2Hg and (Au,Ag)3Hg2) were detected in river placers in western Switzerland (Meisser and Brugger, Reference Meisser and Brugger2000). Other localities for these alloys in placers include Witwatersrand, South Africa (Oberthür and Saager, Reference Oberthür and Saager1986); the Snake River, Idaho, USA (Desborough and Foord, Reference Desborough and Foord1992); the Tulameen–Similkameen river system in British Columbia, Canada (Barkov et al., Reference Barkov, Nixon, Levson and Martin2009); the Inagli deposit in the Aldan Shield, Russia (Svetlitskaya et al., Reference Svetlitskaya, Nevolko, Kolpakov and Tolstykh2018) and in the Palakharya River, Bulgaria (Atanasov and Iordanov, Reference Atanasov and Iordanov1983).
Other reports describe Au‒Ag‒Hg phases from a range of metallogenic deposits. For example, the approved mineral weishanite (Au,Ag,Hg) was discovered in a gold–silver orebody within granulite-facies metamorphic rocks in Henan Province, China, and in the silver–lead orebody in the Keystone Mine, Colorado, USA (Li et al., Reference Li, Ouyang and Tian1984; Bindi et al., Reference Bindi, Keutsch and Lepore2018). Other occurrences include the Tsugu Au–Sb vein deposit, Japan (Shikazono and Shimizu, Reference Shikazono and Shimizu1988); volcanogenic sulfide deposits at Laangsele, Sweden (Nysten, Reference Nysten1986); at Trout Lake, Manitoba, USA (Healy and Petruk, Reference Healy and Petruk1990); and in the Otago Schist quartz-vein deposits of New Zealand (Mackenzie and Craw, Reference Mackenzie and Craw2005). These occurrences indicate these phases can form over a wide range in temperature and sulfur fugacity.
Studies in the synthetic Au–Hg system have characterised a number of stoichiometric phases, such as Au2Hg, Au3Hg, Au6Hg5 and Au5Hg6, known to be stable under ‘geologically reasonable’ conditions, i.e. below ~300–400°C (Rolfe and Hume-Rothery, Reference Rolfe and Hume-Rothery1967; Okamoto and Massalski, Reference Okamoto and Massalski1989). Of these only Au6Hg5 has been formally defined as an approved mineral, aurihydrargyrumite (Nishio-Hamane et al., Reference Nishio-Hamane, Tanaka and Minakawa2018). Weishanite was originally described as (Au,Ag)3Hg2 by Li et al. (Reference Li, Ouyang and Tian1984), however it was redefined as (Au,Ag,Hg) by Bindi et al. (Reference Bindi, Keutsch and Lepore2018). Hexagonal prismatic crystals of Au3Hg described from Minas Gerais, Brazil by Baptista and Baptista (Reference Baptista and Baptista1987) are likely to be weishanite. The hexagonal Au3Hg phase from Hunan, China, for which the name ‘yiyangite’ was proposed by Enkui (Reference Cao1991), has never been approved as a mineral species.
There are a number of difficulties associated with characterising the natural occurrences and correlating them with the synthetic system. These include textural complexity, very small sizes and subtle changes in composition. The prime difficulty is distinguishing between textures that have a purely supergene origin, or those that have involved the influence of mercury for amalgamation during gold mining. Such anthropogenically assisted phases cannot be approved as a mineral species.
Regardless of these difficulties, this paper aims to fully characterise the Waratah Bay Au–Hg–Ag species using a combination of electron back scattered diffraction (EBSD) and electron microprobe analysis (EPMA). Using data from other studies, the Waratah Bay phases are shown to have compositions that differ from other localities, nor do they correlate directly with known synthetic phases, thereby limiting any application to nomenclature in the Au–Ag–Hg system. The reasoning behind preferring an origin by supergene alteration of Au–Ag alloys containing small amounts of Hg, without involvement of anthropogenic Hg is also presented.
The Waratah Bay occurrence
Local prospectors were obtaining detrital gold by traditional panning methods in the beach sands at Waratah Bay, in southern Victoria, Australia, from as early as 1871, although not in sufficient amounts to cause a rush, as had occurred throughout central Victoria from the 1850s. It wasn't until 1915 that the occurrence first came to the attention of mining investors in Melbourne, however, regardless of leases being approved and several companies being registered, there are no records of any large-scale mining taking place. ‘Osmiridium’ (a general term for Ir–Os alloys) occurring with the gold had only been mentioned in passing in several of the early newspaper reports, as attention focussed instead on the gold. Both the gold and osmiridium are thought to have been derived from a complex belt of faulted and serpentinised Cambrian ultramafic rocks (Maitland Beach Volcanics; Cayley et al., Reference Cayley, Taylor, VandenBerg and Moore2002) which crop out for ~5 km along the far southwestern margin of the bay (Fig. 1). However, unlike the better known and researched occurrences of platinum-group element (PGE) minerals associated with ultramafic complexes in western Tasmania, there has been no detailed work on the Waratah Bay serpentinites to determine their Au and PGE element contents.
Figure 1. (a,b) Simplified geological and location maps for the Waratah Bay Au–Hg–Ag phases, in Victoria, Australia. The preserved Cambrian source rocks crop out along a narrow coastal strip.
Only one sample of osmiridium has been preserved from the locality, a 35-gram concentrate probably sent to the Victorian Mines Department for assay by an anonymous miner in the late 19th century. It consists almost entirely of osmiridium grains between 1 and 2 mm across, with a few gold grains of similar size. Other minor minerals in the concentrate include quartz, chromite, cassiterite and small red zircons. The presence of these minerals is due to longshore drift in an anti-clockwise direction, bringing the cassiterite from the tin-bearing granite forming Wilsons Promontory to the southeast.
Features of the gold
Historical records described the gold from Waratah Bay as free, coarse, and not water-worn, although in the sample investigated the grains exhibit a range of features, including rounding by abrasion. Generally grains are irregular and up to 2 mm across with dull knobbly surfaces, and partial patinas of iron oxide–hydroxide (Figs 2, 3 and 4). A selection of osmiridium and gold grains were examined by a scanning electron microscope equipped with energy dispersive spectrometry (EDS), in preparation for electron microprobe analysis. Most gold grains were either close to end-member Au or have minor contents of Ag with several grains having detectable Hg. Scanning electron microscopy imagery shows the surfaces of Hg-bearing grains to have a partially etched appearance, with suggestions of crudely hexagonal crystal outlines (Fig. 3b). This texture is remarkably similar to that shown by Hg-bearing gold grains designated as ‘Type 2 Au–Ag–Hg alloy’ from New Zealand (Youngson et al., Reference Youngson, Woperis, Kerr and Craw2002, figure 7b) and by aurihydrargyrumite (Nishio-Hamane et al., Reference Nishio-Hamane, Tanaka and Minakawa2018, Fig. 2c). The appearance suggests progressive removal of Hg along grain boundaries between roughly hexagonal crystals in the surface Au–Hg phase, possibly followed by rounding due to abrasion in the beach sand environment. Two of these Waratah Bay Hg-bearing grains, here designated #2 (Fig. 2) and #20 (Fig. 4), were chosen for further investigation.
Figure 2. Typical grains of ‘osmiridium’ and gold (to 2 mm across) in the Waratah Bay concentrate. Gold grains #2 (the larger) and #3 are in contact on the left. Brown areas on these grains are patinas of iron oxides.
Figure 3. (a) SEM image of gold grains #2 and #3, prior to preparation of a polished section. The dark patches are iron oxide patina. The green line shows the edge of grain #2 overlapping grain #3, and the blue outline shows the area in (b). (b) SEM image of surface texture of Au–Hg phase on grain #3.
Figure 4. SEM image of grain #20. The dark areas are iron oxide patina.
Methods
Electron back-scattered diffraction and energy dispersion spectroscopy
Grain #2 was mounted and polished for examination at Caltech (Caltech GPS Division Analytical Facility, California, USA). A ZEISS 1550VP Field-Emission scanning electron microscope (SEM) with an Oxford X-Max EDS was used for back-scatter electron (BSE) imaging and elemental analysis. Electron back-scattered diffraction (EBSD) analyses at a submicrometre scale were performed using methods described in Ma and Rossman (Reference Ma and Rossman2008, Reference Ma and Rossman2009). An HKL EBSD system on the ZEISS 1550VP was operated at 20 kV and 6 nA in focused beam mode with a 70° tilted stage and in a variable pressure mode (25 Pa). The EBSD system was calibrated using a single-crystal silicon standard. Structural information was obtained by matching the experimental EBSD patterns with structures of Au, Au–Ag, Au–Hg and Au–Ag–Hg phases from the Inorganic Crystal Structure Database (ICSD, https://icsd.products.fiz-karlsruhe.de/). Back-scattered electron (BSE) imagery revealed a thin prominent marginal alteration zone around the entire grain (Fig. 5). High magnification revealed at least four phases in this marginal zone (Fig. 6a,b). EDS showed that the apparent relict patches in the structure are Au–Ag–Hg, probably weishanite (Fig. 6a, 7a,b), being replaced by a phase with compositions in the range Au2Hg–Au3Hg. EBSD (Fig. 8a,b) showed that this phase is hexagonal P63/mcm, the same as the structure of aurihydrargyrumite. EBSD on another phase appearing to be gradational to the marginal zone showed it to be face-centred cubic, with a composition determined by EDS to be close to Au83Hg17 (see Fig. 6b).
Figure 6. (a,b) Parts of marginal alteration on grain #2 showing texture and formulae obtained by EDS analyses at Caltech. (a) The red and yellow lines mark possible reaction fronts (see text for discussion). The area outlined in blue shows the area and orientation (blue arrow indicates top) of the sample used for Figs 9 and 10. The red arrows in (a) and (b) mark the location of the samples used for EBSD in Figs 7 and 8, respectively.
Figure 7. (a) EBSD pattern for weishanite (Au,Ag)3Hg as shown on Fig. 6a; (b) the pattern indexed with the P63/mmc Au3Hg structure.
Figure 8. (a) EBSD pattern of Au2Hg~Au3Hg2 as shown on Fig. 6b; (b) the pattern indexed with the P63/mcm Au6Hg5 structure.
Figure 9. BSE image of area marked on Fig. 6a (blue arrow shows orientation) showing weishanite grains (black) with a gold-rich marginal phase (red arrows). The hosting phase is a Au–Hg alloy.
Back-scattered electron imagery of one small area (15 × 11 μm) in this marginal zone (see area outlined on Fig. 6a) showed the irregular patches of weishanite (with a hexagonal P63/mmc structure) surrounded by thin mantles of another phase (Fig. 9) which is close to pure gold in composition.
Element distribution images
The area shown in Fig. 9 was selected for X-ray mapping to reveal more detail of the compositional relationship between the weishanite relicts and the surrounding Au–Hg phase. The image for the Ag distribution best shows the weishanite relicts (red) with gold-rich (and Hg-depleted) mantles (yellow to green) grading into the Au–Hg phase (Fig. 10).
Electron microprobe analysis
Grain #20 was mounted and polished for microprobe analysis, using a Joel JXA-830 field emission microprobe in the School of Geography, Earth and Atmospheric Sciences (SGEAS) at the University of Melbourne. Operating conditions in WDS mode were 15 kV, 20 nA, beam diameter 1 μm and with standards of pure Au and Ag, and HgS (cinnabar) for Hg. Overlap of the AuMβ and HgMα lines on PET was compensated for by subtracting a proportional amount determined by obtaining the ratio of AuLα counts on the LIF crystal to the Au interference at the Hg position on the PET crystal. This value, 0.007, was then subtracted from the actual Hg peak count before doing the ZAF calculation. This is especially significant for analyses with small amounts of Hg. Detection limits are 90 ppm for Ag and 440 ppm for both Au and Hg. Examination of part of an outer edge of grain #20 using back-scattered electron imaging showed three phases are present, distinguished by differing Au, Ag and Hg contents (Fig. 11). The innermost phase is Au–Ag alloy containing a very small amount of Hg, partly fringed by irregular areas of Au–Hg–Ag, probably weishanite. The outermost phase showing an open framework of roughly hexagonal crystals to 30–40 μm is a Au–Hg alloy with an average composition close to Au2Hg. (Table 1). Electron microprobe analysis was also undertaken on the phases in part of grain #2 identified at Caltech (Table 1) (Figs 5 and 12). The innermost phase is Au–Ag alloy with minor Hg, surrounded by Au with minor Ag and Hg, with an outer-edge phase of Au–Hg alloy with compositions mostly between Au3Hg and Au2Hg (see Fig. 13). The difference between the two is that the middle phase in Fig. 11 is weishanite (Au–Ag–Hg) whereas it's Au with minor Ag and Hg in Fig. 12.
Figure 11. Part of grain #20 showing positions of analysed points on three phases: blue crosses are on Au–Ag alloy with minor Hg; red crosses are on weishanite; and green crosses are on Au–Hg alloy. Numbers refer to analyses in Table 1.
Table 1. Electron microprobe determined compositions of Waratah Bay Au–Hg–Ag phases.
Figure 13. Ternary plot for Waratah Bay mineral data given in Table 1. Red circles are points along the Au–Hg join (Au–Hg alloy in Table 1); blue triangles are interpreted as weishanite; green diamonds are electrum; and black crosses are near-pure gold. Open circles show data for weishanite and aurihydrargyrumite (as defined formally), and for synthetic phases along the Au–Hg join. See text.
Results and discussion
Compositions
The Waratah Bay electron microprobe data (Table 1) plotted in the Hg–Au–Ag ternary diagram (Fig. 13) have compositions spread along the binary join between Au2Hg and Au3Hg, though not extending to Au6Hg5. Data points for aurihydrargyrumite and weishanite are also plotted, as well as known synthetic phases along the Au–Hg join. The field boundaries are taken from Youngson et al. (Reference Youngson, Woperis, Kerr and Craw2002) who extrapolated them from the Au–Hg and Ag–Hg experimental binary systems, and assuming 25°C. Nearly all published compositions with known crystal structures conform to this scheme, which also includes ‘γ-goldamalgam’ being cubic (Chen et al., Reference Chen, Yang, Ma and Peng1981). The exception is the monoclinic symmetry assigned to euhedral crystalline grains with compositions in the range Au94Hg6–Au88Hg12 found in placers in Snake River, Idaho, USA (Desborough and Foord, Reference Desborough and Foord1992). There is no clear explanation for this anomalous result other than that the grains were heated to 500°C prior to examination by X-ray diffraction. Data for Waratah Bay weishanite cluster in a different position to those of other published descriptions (Bindi et al., Reference Bindi, Keutsch and Lepore2018; Li et al., Reference Li, Ouyang and Tian1984) suggesting a wide compositional field. Compositions along the Au–Ag join have Hg contents too low to display on Fig. 13.
Comparison with other localities
Compositional data for Au–Ag–Hg phases from placer deposits in the Philippines, British Columbia (Canada) and South Island (New Zealand) are shown in Fig. 14.
Figure 14. Ternary diagram for analysed Hg–Au–Ag phases from the Philippines (Huang, Reference Huang2011) (red triangles), British Columbia, Canada (Barkhov et al., Reference Barkov, Nixon, Levson and Martin2009) (green triangles) and New Zealand (Youngson et al., Reference Youngson, Woperis, Kerr and Craw2002) (blue symbols). The blue diamonds are type 2 secondary Au–Ag–Hg alloy and the blue triangles are type 1 (α-phase). Synthetic phases are indicated by open circles.
If it is assumed that the weishanite composition can vary, as expressed by the formula (Au,Ag,Hg), these data indicate that aurihydrargyrumite and weishanite occur in the Philippines placers. The British Columbia placers contain weishanite close to the type composition of Bindi et al., (Reference Bindi, Keutsch and Lepore2018), and the New Zealand occurrences (Youngson et al., Reference Youngson, Woperis, Kerr and Craw2002) are a mix of Au–Ag alloys containing up to ~10 at.% of Hg (Type 1 α-phase, Au–Ag–Hg alloy), possible weishanite and points along the Au–Hg join between Au2Hg and Au6Hg5 (Type 2 Au–Ag–Hg alloy). The Type 1 phases are ultimately of hydrothermal origin, whereas Type 2 are entirely secondary in origin (Youngson et al., Reference Youngson, Woperis, Kerr and Craw2002).
Comparison with synthetic phases
Synthetic Au–Hg phases which have close to stoichiometric formulae and for which crystal structure data are known include Au2Hg, Au3Hg, Au4Hg and Au6Hg5 (Lindahl, Reference Lindahl1970; Rolfe and Hume-Rothery, Reference Rolfe and Hume-Rothery1967; Berndt and Cummins, Reference Berndt and Cummins1970). These, and their mineralogical equivalents, are given in Table 2, adapted from Nishio-Hamane (Reference Nishio-Hamane, Tanaka and Minakawa2018). Attempts to align naturally occurring Au–Hg alloys with synthetic equivalents have proved problematic, mainly because of the wide compositional ranges shown by the natural phases, and the difficulty of obtaining structural data for them. To date the only approved Au–Hg mineral (i.e without Ag) which has a synthetic equivalent is aurihydrargyrumite.
Table 2. Synthetic phases and their equivalent minerals in the Au–Hg–Ag system.
Implications for nomenclature
As shown in Fig. 13, along the Au–Hg join there is essentially a continuous composition between Au2Hg and Au3Hg for the Waratah Bay compositions. Au2Hg has been shown to be hexagonal P63/mcm (Berndt and Cummins, Reference Berndt and Cummins1970), whereas Au3Hg is P63/mmc, indicating that somewhere along this compositional interval there is a change in structure from one polytype to another. It is not possible to determine accurately where this occurs without extremely precise EBSD measurements. However, given that the compositional range encompassing Au3Hg might be from 21.3 to 25.8 at.% (at 150°C), compared to <1 at.% for Au2Hg (Rolfe and Hume-Rothery, Reference Rolfe and Hume-Rothery1967), the change is probably closer to Au2Hg.
Although compositional continuity between Au2Hg and Au6Hg5 (aurihydrargyrumite) has not yet been confirmed, both are hexagonal P63/mcm (Lindahl, Reference Lindahl1970), suggesting that the former might have a wider range in composition. At higher proportions of Hg than aurihydrargyrumite, only cubic ‘γ-goldamalgam’ has been found naturally, however it has not been sufficiently characterised to have IMA approved mineral status.
In common with Au3Hg, weishanite is hexagonal P63/mmc, suggesting the possibility that its compositional field might extend closer to the Au–Hg join near Au3Hg. Data for weishanite from British Columbia (Fig. 14) plot close to the presumably limiting composition before Ag becomes dominant over Au. As noted by Bindi et al. (Reference Bindi, Keutsch and Lepore2018), weishanite is isostructural with schachnerite (Ag1.1Hg0.9) (Seeliger and Mücke, Reference Seeliger and Mücke1972), and the two minerals could be considered simple polymorphs of silver and gold, respectively, as the metals are disordered in the same structural position.
Although phases close to Au2Hg have been reported from a number of localities including in this investigation, it has not been possible to obtain sufficient data to establish it as an IMA approved mineral name, regardless that the crystal structure is known.
Explanation of textures
Based solely on textures in these marginal Au–Hg–Ag phases at Waratah Bay, it is difficult to distinguish between supergene reactions and anthropogenic amalgamation as being responsible for their formation. These overgrowths can be observed in placer deposits where mercury was used to recover gold, but also from sites where there is no evidence for its use. For the latter case, the source of the mercury is the primary gold itself, which might also contain variable Ag contents, i.e. it is a three-phase alloy.
Several factors need to be considered when deciding on an origin for the Waratah Bay alloys. First, the grains have not been transported a long distance from their possible source i.e. the suite of ultramafic rocks that crop out on the beach where the grains were found. Though some rounding has occurred, this is probably due to wave action affecting the beach sands. Second, there is no strong historical evidence for the use of mercury in the recovery of gold from the beach sands. Third, there are no recorded sources of natural liquid mercury in the region (see below). In combination these three factors suggest an entirely supergene origin for the phases.
Though several features of the grains suggest amalgamation might have occurred, there are other explanations. The morphology of grains #2 and #3 (Fig. 3a) suggests that each might consist of several separate rounded grains which have been amalgamated with Hg. However, another interpretation is that these are rounded, deeply recessed, knobbly grains whose entire margins have been altered with the texture giving the impression there were once separate grains.
Another feature requiring an explanation is the small irregular outgrowths seen, for example in Figs 5, 6a,b. Is it expected that these would not have survived any significant transport, in which case they would only form after the grain had come to rest. This could have been due either to supergene reactions or the effect of amalgamation during collection. However, the nature of the concentrate suggests that traditional panning methods were used to obtain the osmiridium, with the gold grains being accidental additions.
The textures shown in Fig. 6a, b appear to indicate replacement reactions, with two different reaction fronts. In Fig. 6a, one is marked in red and a second one marked in yellow. The region between these interfaces shows a number of pores (dark pores), which are characteristic of a fluid-driven replacement reaction. The area to the left of the red line seems to be a two- or three-phase region which is a mixture of Au3Hg2 and (Au,Ag)3Hg. This appears to be an exsolution texture resulting from the solid-state unmixing of a metastable Au–Hg phase. Analogous textures can be seen in the Au–Te system, in which sylvanite is replaced to form calaverite and another phase, which breaks down by exsolution to a mixture of hessite and petzite (Zhao et al., Reference Zhao, Brugger, Xia, Ngothai, Chen and Pring2013). These can result from supergene hydrothermal reactions, which might occur at ambient temperatures, albeit sluggishly (A. Pring, pers. comm.).
Formation of the Au–Ag–Hg phases
The mechanism by which these marginal Au–Ag–Hg phases form in placers is uncertain, with or without, the presence of liquid mercury. Clearly, the phases are secondary and formed under supergene conditions at the expense of primary Au‒Ag alloys, which are now preserved in the core of the grains. However, there is considerable disagreement over the mechanism. Youngson et al. (Reference Youngson, Woperis, Kerr and Craw2002) attributed the New Zealand occurrences to diffusion between detrital gold grains and liquid Hg that was either hydrothermal in origin or derived from the local breakdown of cinnabar. Barkov et al. (Reference Barkov, Nixon, Levson and Martin2009) favoured an origin for zoned Hg-amalgam as being due to a process of electro-refining involving liquid mercury introduced into the placer deposit, i.e. due to mining operations. Nishio-Hamane et al. (Reference Nishio-Hamane, Tanaka and Minakawa2018) explained the formation of aurihydrargyrumite from a Hg component already in the core of the grain and invoked a complex process of ionisation and precipitation, or self-electro-refining, on the surface of gold grains.
Textures revealed by BSE imaging of the Waratah Bay phases provide some evidence for the sequences in which the observed phases have formed. In grain #2, Fig. 6 shows relics of weishanite enclosed in the Au3Hg–Au2Hg phase, whereas in Fig. 12, primary Au–Ag–(Hg) is replaced in succession by Au with minor Ag and Hg, in turn replaced at the margins by Au3Hg–Au2Hg. For grain #20, a different relationship between the three phases is shown (Fig. 11). It seems clear that the first reaction involved Ag and Hg diffusing towards the margins of the grains, initially forming weishanite. The phenomenon of Ag-depleted rims is commonly observed in Au–Ag grains locked in placer deposits and is broadly described as being due to leaching of silver in supergene environments (Butt et al., Reference Butt, Hough and Verrall2020). The narrow gold-rich margins observed around relic weishanite in grain #2 (Fig. 9) have not been observed previously, however they appear to be due to a similar process, in which Ag was removed from weishanite, giving rise to the spongy texture observed in the marginal Au–Hg phase. Understandably, the mechanism(s) controlling this process, as well as its timing and environmental conditions, are not well understood and are likely to remain so without considerably more research.
Source of the mercury
The source of both the gold and osmiridium in the Waratah Bay beach sands is considered to be rocks within the Cambrian mafic and ultramafic sequence that crops out along the southwestern coast (Fig. 1b). Although there is no actual evidence for this conclusion, it is supported by the absence of any streams draining into the bay from known goldfields. As far as is known, this is the only occurrence along the Victorian coast of gold being present in detectable amounts in beach sands.
The Victorian gold province is poor in mercury occurrences, with only one minor deposit known in which liquid mercury and cinnabar occur in quartz veins (Birch, Reference Birch2003). No data for Hg contents of natural gold in Victoria appear to exist. Large amounts of mercury were imported into Victoria during the mid-19th century gold rushes to recover gold from pulverised ore and alluvium (placer deposits) by amalgamation (Davies et al., Reference Davies, Lawrence and Turnbull2015). According to contemporary records published in Mineral Statistics of Victoria, an estimated 665 tons of mercury were imported into Victoria between 1868 and 1888. Over that period, the use of mercury (or quicksilver) was recorded for each goldfield in the state. Because of the inefficiency of the amalgamation process, a minimum estimate of 121 tons of mercury were lost from crushing, either flowing into nearby creeks or retained in tailings, with the loss varying from goldfield to goldfield (Davies et al., Reference Davies, Lawrence and Turnbull2015). Though Waratah Bay was nominally placed in the Russell's Creek mining district in the Gippsland Division, it was not listed as an official goldfield (see Fig. 1a). There are no records of gold production and according to the official records mercury was not used in the Russell's Creek mining district. The possibility of some early prospectors using mercury when panning for gold in the beach sands is remote.
Conclusions
Au–Hg–Ag phases have formed on the margins of several gold grains in beach sands at Waratah Bay, Victoria, Australia. They have similar features and compositions to those present in other placer-gold deposits around the world. Weishanite (Au,Hg,Ag) is the only IMA approved mineral present, however it has a different composition to that from British Columbia, the Philippines and New Zealand. Compositions plotting along the Au–Hg join between Au3Hg and Au2Hg do not extend to aurihydrargyrumite (Au6Hg5) and do not represent accepted species. High-magnification X-ray imagery shows a complex assemblage of phases formed as weishanite is resorbed to form the marginal Au–Hg phase simultaneously with leaching of Ag to the environment. In the absence of any historical evidence for the use of mercury to recover gold, it is probable that the marginal Au–Hg phases at Waratah Bay have formed from primary Au–Ag alloys (in some cases electrum) containing minor amounts of Hg, by a process which is not well understood, but which might involve supergene replacement reactions. These phases are further evidence for the complexity of the natural Au–Hg–Ag system and the difficulty involved in refining their nomenclature. Methods such as detailed EBSD combined with precise EPMA are required to understand completely the Au–Ag–Hg system.
Acknowledgements
Graham Hutchinson (University of Melbourne's School of Geography, Earth and Atmospheric Sciences) is thanked for the electron microprobe data and SEM and BSE imagery. SEM-EBSD analyses were carried out at the Caltech GPS Division Analytical Facility, which is supported, in part, by NSF Grants EAR-0318518 and DMR-0080065. Oskar Lindenmayer (Museums Victoria) carried out photography and assisted with drafting.
Competing interests
The authors declare none.
