2.6 Sub-aqueous volcanic-related mineral system

In 1965, the existence of a major hydrothermal vent was confirmed in the Atlantis II Deeps in the Red Sea (Miller et al., 1966). This kicked off a period of deep sea exploration the resulted in discovery of the first black smoker on the East Pacific Rise in 1977. A black smoker is a hydrothermal vent on the seafloor. The name refers to the venting fluids, which appear as black smoke due to the precipitation and entrainment of fine sulfide particulates in buoyant, venting hydrothermal plumes. Hundreds of black smoker deposits have been discovered over the last four decades, initially in the axial valleys of mid-oceanic ridges, but more recently in a large range of tectonic environments, including seamounts, back-arc basins and rifted arcs (Hannington et al., 2005). The discovery of these deposits has validated the volcanic-hosted massive sulfide (VHMS) ore deposit model that has been developing since the 1840s (Stanton, 1990). Analyses of black smoker material indicate that in addition to the base metals, Cu, Zn and Pb, and precious metals, Ag and Ag, these modern VHMS deposits contain a large variety of commodities, including many critical commodities. Analyses of ancient VHMS deposits indicate a similar range in commodities, and some critical commodities (e.g., Sn and In) have been recovered from these deposits.

The original deposits discovered in the Atlantis II Deeps, however, differ in many ways from the more common black smokers. The Atlantis II deposits are presently forming in an ensialic rift, the Red Sea, and involve dense, bottom-hugging brines (Zierenberg and Shanks, 1983), not the chimneys and black smoke plumes characteristic of black smoker deposits. The Atlantis II Deeps deposit is most likely an analogue of Broken Hill-type deposits. Like black smoker and VHMS deposits, Broken Hill-type deposits also contain significant concentrations of critical commodities.

2.6.1 Geological setting

Direct observation of modern systems and interpretations of ancient systems suggest that, by and large, deposits of the sub-aqueous, volcanic-related mineral system occur in extensional and, to a much lesser extent, transtensional tectonic settings (Figure 2.6.1). These include both divergent and convergent geodynamic environments. Although most modern black smokers occur in divergent mid-oceanic ridges, these deposits are unlikely to be preserved owing to seafloor weathering and subduction. Rather, most ancient VHMS deposits are hosted by extensional back-arc basins and rifted arcs in an overall convergent geodynamic system (Franklin et al., 2005). Broken Hill-type deposits, by contrast, form in divergent ensialic rifts and in back-arc basins (Leach et al., 2010), both of which have a higher chance of preservation than mid-oceanic ridges.

There is some evidence that the style of deposit can vary depending upon the geographic position within the mineral system. Mercier-Langevin et al. (2007) and Huston et al. (2011a) suggested that Zn-rich VHMS deposits associated with chlorite±sericite-dominant alteration assemblages occur within back-arc basins, whereas Cu-rich VHMS deposits associated with pyrophyllite±kaolinite-bearing alteration assemblages occur closer to the (rifted) magmatic arc.

As implied by their name, volcanic-hosted massive sulfide deposits are hosted by or closely associated with volcanic rocks. In contrast, an association with volcanic rocks is not as clear cut with Broken Hill-type deposits. Although some of these deposits are associated with (felsic) volcanic rocks (e.g., association of Broken Hill with the metavolcanic Hores Gneiss: Page and Laing, 1992), in most cases the succession in the footwall to these deposits contains abundant mafic volcanic and high level intrusive rocks (Huston et al., 2006).

2.6.2 Sources of fluid, metals and energy

In the sub-aqueous, volcanic-related mineral system, the dominant ore fluid is evolved seawater and the main source of metals is rock leaching. Stable isotope data, particularly oxygen isotope data (Huston et al., 2011a), point to the dominance of evolved seawater in VHMS deposits, and the presence of extensive zones leached in Zn, Pb, Cu and related elements in both VHMS (Brauhart et al., 2001) and BHT districts (Huston et al., 2006) can account for the metals present in the deposits. The main role that the coeval magmatism plays is as a heat source to drive circulation of evolved seawater, although magmatic-hydrothermal fluids may be involved in Cu-rich deposits with pyrophyllite-kaolinite alteration assemblages or in very uncommon, extremely Sn-rich (e.g., Neves Corvo, Portugal) deposits (e.g., Huston et al., 2011a). An important difference between VHMS and BHT systems is the potential roll of evaporites as a source of chloride in the latter.

2.6.3 Fluid pathways

In many well studied VHMS systems, district-scale alteration zones include semi-conformable alteration zones 1–3 km below the seafloor from which transgressive zone extend upwards to the seafloor and VHMS deposits (e.g., Franklin et al., 1981; Brauhart et al., 1998). The semi-conformable zones involve high temperature chlorite-rich, albite-rich and/or epidote-rich alteration assemblages that commonly have been leached of metals (Galley, 1993; Huston et al., 2006). These zones are most likely zones of lateral fluid flow at the base of convective cells driven by subvolcanic intrusions. In addition, these zones have acted as chemical reactors where the convecting fluids are heated, reduced (so that ∑H₂S > ∑SO₄) and charged with metals, including some strategic commodities (Brauhart et al., 2001).

The crosscutting zones are interpreted as upwelling zones, commonly controlled by synvolcanic faults, that focus fluid flow into the depositional environment (Franklin et al., 1981; Brauhart et al., 1998). Semi-conformable albitic alteration zones present in BHT districts (Huston et al., 2006), by analogy, may have also been fluid pathways.

2.6.4 Depositional processes

The main depositional process in the sub-aqueous volcanic-related mineral system is mixing between upwelling ore fluids and ambient seawater at or near the seafloor. This results in cooling, which leads to rapid precipitation of Cu, Zn, Pb and Ag as metal sulfides (Ohmoto et al., 1983). This mechanism also results in deposition of a large range of strategic commodities, including Bi, As, Ga, In and Cd. Sulfide precipitation and dilution by seawater also rapidly decreases the amount of H₂S in the fluid, which can cause deposition of Au, and possibly Sb. If the ambient seawater is sulfate-rich, mixing can also cause deposition of barium in the ore fluid as barite (Ohmoto et al., 1983). Both VHMS and BHT deposits are commonly well zoned (Cu → Zn-Pb-Ag → barite, from base: Eldridge et al., 1983), and contain a large range of strategic commodities (Sb, Te, Hg, As, Ga, In, Ge, Mn, Cd, Sn, Mo, Bi and Se) in addition to the main commodities (Zn, Pb, Cu, Au and Ag), attesting to the efficiency of seawater mixing as a depositional mechanism.

2.6.5 Australian examples

After the basin-hosted mineral system, the sub-aqueous volcanic-related mineral systems are the largest source of Zn, Pb and Ag in Australia as well as a major source of Cu and Au. The ˜1690 Ma (Page and Laing, 1992) Broken Hill (New South Wales) deposit was, before mining, the world's largest single Zn-Pb-Ag accumulation, and the similarly-aged Cannington deposit in Queensland is currently the world's largest Ag producer. Minor BHT deposits include ˜1700 Ma (Kositcin et al., 2010) sub-economic deposits (e.g., Chloe) near Einasleigh in Queensland. Although BHT deposits are known as major Zn, Pb and Ag producers, they have also produced significant Au.

Major Australian VHMS districts include the ˜500 Ma (Mortensen et al., in press) Mt Read province in western Tasmania (Rosebery, Hellyer, Mount Lyell), the ˜2950 Ma (Barley, 1992) Golden Grove district in Western Australia (Scuddles, Golden Grove), and the ˜420 Ma (Champion et al., 2009) Woodlawn province of eastern New South Wales (e.g., Woodlawn, Captains Flat, etc) and northeastern Victoria (e.g., Currawong and Wilga). There are a large number of smaller VHMS deposits and prospects throughout Australia that range in age from ˜3480 Ma (North Pole barite: Van Kranendonk et al., 2008) to ˜277 Ma (Mount Chalmers: Crouch, 1999), with significant production from the North Pilbara Granite Greenstone Terrane in Western Australia (e.g., Whim Creek, Mons Cupri, Wundo), the Eastern Goldfields Superterrane in Western Australia (e.g., Teutonic Bore, Jaguar, Bentley, Nimbus; ˜2695 Ma: Pidgeon and Wilde, 1990), the ˜480 Ma (M Fanning in Rae, 2000; Hutton et al., 1997) Balcooma and Mt Windsor (e.g., Thalanga) districts in Queensland, the ˜480 Ma (D Huston, unpub. data) Girilambone district (e.g., Tritton and Avoca Tank) in New South Wales and the New England Province in eastern New South Wales and Queensland (e.g., Mount Chalmers). In Australia, VHMS deposits are not only major producers of Zn, Pb and Ag, but also produce significant Cu and Au.

2.6.6 Associated critical commodities

As discussed above, the efficiency of seawater mixing as a depositional mechanism means that BHT and VHMS deposits contain anomalous to very high concentrations of a number of critical commodities. Table 2.6.1 provides indicative concentrations ¹ for a number of critical commodities in Australian BHT and VHMS deposits. The most important of these include those that occur in solid solution in sphalerite (Cook et al., 2009) and, hence, have a strong geochemical affiliation with Zn: Cd, Ga, Hg, In, Ge and Mn. Of these, the best established correlation with Zn is Cd, followed by Ga. At the Rosebery (Mount Read province, Tasmania) deposit, a strong correlation (r²= 0.947, n = 75) suggests the Cd/Zn ratio (by mass) of the ores is 2.7 × 10^-3, whereas at the Dry River South (Balcooma Province, Queensland) deposit this ratio is 1.9 × 10^-3 (r² = 0.986, n = 20). Using these ratios and EDR data as of 31 December 2011, Rosebery and Dry River South grade 273 ppm for 6.4 kt and 131 ppm for 0.1 kt Cd, respectively (Table 2.6.1). Zinc smelters (e.g., Nyrstar's Hobart Zinc Works) commonly produce Cd as a by-product although Cd can attract a smelter penalty.

The estimates of concentrations and contained metal for critical elements are based upon reported ore reserves/mineral resources and geochemical relationships obtained from publically available sources (at 31 December 2011). The estimates are indicative only and do not consider the technical viability or economics of extraction. Data sources for the geochemical data are McGoldrick (1986), Carr et al. (1984) and Huston (1988).

Analytical data for Ga are also available for the Dry River South deposit. Although not as strong as that of Cd and Zn, the correlation between Zn and Ga (r² = 0.679; n = 20) is statistically significant at 99% confidence level, and a Ga/Zn ratio (by mass) is 8.7 × 10 -4 , suggests a very approximate grade at Dry River South of 60 ppm for 44 t Ga (Table 2.6.1).

The Hg concentrations of the Broken Hill deposit and a number of VHMS deposits are reasonably well known. Carr et al. (1986) report ranges and geometric means for a large number of Australian Zn-Pb-Ag deposits. The Broken Hill deposit has geometric mean of 8.2 ppm and a range of 1–37 ppm (n = 35). VHMS deposits are even more variable, with Carr et al. (1986) reporting a total range in geometric means of 0.1-18 ppm (seven deposits) and Smith and Huston (1992) reporting an arithmetic mean of 43 ppm for the Rosebery deposit, which suggests an indicative resource of 1.0 kt Hg based on the current EDR (Table 2.6.1), although the economic and technical viability of this resource is uncertain. MacPhersons Resources has published a JORC-compliant in situ total resource at the Nimbus deposit citing a grade of 66 ppm Hg for 267 t Hg (Table 2.6.1: macphersonsresources.com.au).

Although public-domain assay data for In, Ge and Mn in VHMS and BHT deposits are not available for Australian deposits, Schwarz-Schampera and Herzig (2002) indicate that In is a significant minor to trace metal in many VHMS ores, and Ge and Mn is present in VHMS sphalerite (Cook et al., 2009). Although data for BHT deposits are not available, it is likely that Cd, Ga, In, Ge and Mn are present at trace levels. Indium is also produced as a by-product from zinc smelters.

Although not present as solid solution in sphalerite, Sb also has a broad association with Zn, Pb and, particularly, Ag. At Rosebery, Sb is moderately correlated with Ag (r² = 0.651; n = 75), with an average Sb/Ag ratio is 2.3. Using this ratio and reported Ag grade for Rosebery, the indicated Sb grade is approximately 270 ppm for 6.4 kt (Table 2.6.1). In the early 1990s, Sb, along with Ag and Au, mostly reported to the copper-precious metals concentrate at Rosebery, with a grade of ˜0.47% (Huston et al., 1992). In most cases, such a 'dirty' copper concentrate would attract a penalty to offset smelter costs of treatment and disposal of Sb (e.g., Larouche, 2001). Hence, although an Sb resource may exist at Rosebery, it may not be economically or technically viable to recover. Zinc-rich ore samples from the Dry River South deposit typically contain 20–100 ppm Sb (Huston, 1988), and a dump sample from the Mt Chalmers deposit in Queensland returned <50 ppm Sb (Huston et al., 1992). Although generally anomalous, the Sb content of VHMS ores is highly variable.

Broken Hill-type deposits are even more enriched in Sb, with the 1, 2 and 3 lenses at Broken Hill averaging ˜400 ppm (Table 2.6.1: Johnson and Klingner, 1975) and the Broadlands and Nurnham-Nisdale lenses at Cannington averaging 505 ppm and 680 ppm, respectively (Walters and Bailey, 1998). Due to its large size, prior to mining, Broken Hill contained total Sb of similar order to the Hillgrove deposit in New South Wales, currently Australia's largest Sb resource. However, this Sb has not been recovered.

Limited data are also available for elements that geologically are more closely allied with Cu (e.g., Mo, Re, Se and Te), or can be associated with Cu or Zn (e.g., Bi and Sn). As an example, Cu concentrate from the Mount Lyell copper field typically contained 0.1% Mo, 80 ppm Se and 40 ppm Te in the early 1990s (KE Faulkner, written comm., 1991). It is probable that the Se and Te are (have been) recovered during smelting and refining, as discussed in Section 2.2.1, although the Mo is probably not. A single analysis of Mt Lyell molybdenite yielded 147 ppm Re (D Huston, unpub. data), suggesting an indicative Re grade for the Cu concentrate of around 0.2 ppm. Although Se and Te are present in significant levels in most VHMS deposits, molybdenite is not commonly reported in these deposits, suggesting that significant Mo concentrations are not that common. No data are available for these elements in BHT deposits.

Bismuth can be present in both Zn-Pb-rich and Cu-rich ores from VHMS deposits. At Rosebery, Bi is associated with Cu, whereas at Dry River South it is enriched in Zn-Pb-rich ores. In both cases, concentrations run to several hundred ppm, approaching 1000 ppm at Dry River South (Huston, 1988). At the Broken Hill deposit, average Bi contents of individual lodes range up to 50 ppm, and decrease in the stratigraphically upper lodes. Because Bi is commonly a trace to minor solute in galena, it may report to lead concentrates, where is can be recovered. In Cu concentrates, high Bi levels can attract a penalty (Larouche, 2001).

The Kidd Creek deposit in Ontario, Canada has produced significant amounts of Sn through its mining history (Hennigh and Hutchinson, 1999), and the Neves Corvo deposit is a major tin deposit (Relvas et al., 2006). According to Hennigh and Hutchinson (1999), Sn is generally associated with Zn in VHMS deposits, although at Neves Corvo, it occurs towards the stratigraphic base, in association with Cu (Relvas et al., 2006). In Australia, very little information is available about Sn contents of VHMS deposits. At the Dry River South deposit, Sn is associated with Zn, with grades of Zn-rich samples typically between 100 ppm and 500 ppm (Huston, 1988). This Sn is present as cassiterite and Sn sulfide minerals. No data on Sn concentrations in BHT deposits are available.

Other critical commodities that can be anomalous in the sub-aqueous volcanic-related mineral system include As, fluorite and barite. Assays of ore samples from Zn-rich Rosebery and Dry River South deposits indicate As grades of hundreds to thousands of ppm (Huston, 1988), whereas the Cu-rich Mount Lyell and Mount Chalmers deposits have much lower (<50–400 ppm) concentrations (Huston et al., 1992). These data suggest that As concentrations in VHMS deposits are quite variable and relate to the metallogenic characteristics of individual deposits. Although no As data are available for BHT deposits, Johnson and Klingner (1975) report F contents (as fluorite) at Broken Hill up to 1.35% (in the Number 2 lens).

The last critical commodity present in the sub-aqueous volcanic-related mineral system is barite. Barite is widely distributed in Australian VHMS deposits, although relatively uncommon in BHT deposits. Barite is common in VHMS deposits from the North Pilbara Granite Greenstone Terrane, the Mount Read province, and the Woodlawn province. All three of these mineral provinces contain significant barite deposits with minor base metals as well as Zn-Pb-Ag and Cu-Au deposits with barite-rich lenses or zones.

The sub-aqueous volcanic-related mineral system has a diverse suite of trace elements, some of which have been identified as critical. Of these, Cd, Se and Te are probably recovered as by-products from smelting and refining of Zn and Cu concentrates from Australian deposits, and In and Sn have been recovered as by-products from VHMS deposits elsewhere in the world. In addition to these commodities, Ga, Hg and Ge also have potential for recovery from Zn concentrates, whereas Sb, and possibly Mn, although anomalous, can cause metallurgical difficulties and are not recovered. Barite deposits formed in this mineral system have been mined and have potential to be mined, with by- or coproduct precious and base metals. Although these and other commodities (e.g., Bi, Mo and Re) are known to be present in this mineral system (Figure 2.5.1), controls on the distribution, both between and within deposits, are poorly understood.

Endnotes

1 The concentrations presented in Table 2.6.1 are based on limited publicly available data and must be regarded as indicative values only.