2.8 Basin-hosted mineral system
Sediment-dominated basins (Figure 2.8.1) contain deposits of many different commodities, including hydrocarbons and coal as well as a range of base, precious and strategic commodities. Many of these deposits form as a response to out-of-basin events, which cause within-basin fluid flow and mineralisation or hydrocarbon accumulation. The lithological characteristics of the basin determine to a large extent the geochemistry of these fluids and the geometry and chemical characteristics of the resulting commodity deposits.
2.8.1 Geological setting
Sedimentary basins form when geographic regions subside and are infilled by sediments eroded from adjacent highs or by volcanic rocks produced during basin formation. Four tectonic settings of basins are generally recognised: divergent, convergent, transform and intraplate. In many of these settings, initiation of the basin involves (commonly bimodal) magmatism as the lithosphere thins and is heated from below—the rift phase of basin formation. When extension stops, the crust cools and increases in density, causing further subsidence—the sag phase of basin formation. If extension continues such that oceanic crust develops, passive margins form along the margins of a widening oceanic basin.
Flexure of the crust in response to tectonic loading also can create basins, in this case lacking early phase volcanism. Perhaps the best example of this basin type are foreland basins, which are common inboard responses to collisional orogenesis along convergent margins.
Basin formation is commonly terminated or interrupted by basin inversion, which is generally triggered when out-of-basin tectonic events place the basin in contraction. In some cases, inversion terminates deposition, but in other cases, sedimentation is interrupted for a short period or even continues in parts of the basin. During basin inversion, extensional faults associated with basin formation commonly invert and become contractional faults and the sedimentary succession is deformed. Basin inversion can provide the impetus and focus for fluid flow.
Hydrothermal mineral systems and petroleum systems can be active through all phases of basin development, although many form during the later phases of basin formation, commonly just before or during basin inversion, in response to out-of-basin tectonic triggers. Some basin-hosted mineral systems, such as phosphate deposits, do not form hydrothermally but involve chemical sedimentation in very special environmental conditions.
Finally, other mineral deposits are the result of post-depositional hydrothermal (e.g., iron ore) or metamorphic (e.g., graphite) upgrading of rocks originally deposited during basin formation (e.g., banded iron formation and carbonaceous shale, respectively).
2.8.2 Sources of fluid, metals and energy
By their nature, basins contain a large variety of rocks—mafic volcanic rocks, siliciclastic rocks and carbonate rocks. Because these rocks are deposited in environments dominated by seawater or meteoric water, most late Paleoproterozoic and younger basins are dominated by oxidised rocks, with localised reduced rocks (e.g., carbonaceous shales). Older basins can contain more reduced rocks such as banded iron formation. Basin fill and fluids trapped within the basin determine the chemical characteristics of basin-hosted hydrothermal fluids and can be sources of metals and chlorine.
Studies of modern basinal fluids indicate that generally they are relatively oxidised, H2S-poor and reasonably saline, and can be metal-rich (Hanor, 1996). Because fluid flow commonly occurs late during basin evolution or during basin inversion, periods that lack magmatism, the ore fluids are generally low temperature (<250°C, and commonly <200°C: Huston et al., 2006).
When they interact with oxidised basinal fluids, volcanic rocks at depth within the basin can be excellent sources of metals such as Cu, Zn and Co, whereas siliciclastic rocks (e.g., arkoses) within the basin succession or the basement can be sources of metals such as U and Pb. This potential is illustrated by leaching of mafic volcanic rocks near the base of the McArthur Basin in the Northern Territory by low temperature, oxidised fluids (Cooke et al., 1998).
If basinal fluids are indeed the most important ore fluid in the basin-hosted mineral system (e.g., Sawkins, 1984 and references therein), a potential constraint on the amount of metal transported is the amount of basinal fluid available. Moreover, Sawkins (1984) suggests that fluid flow must have been pulsed or episodic.
Fluid flow and mineralisation in basin-hosted mineral systems is commonly triggered by out-of-basin tectonic events, which can be manifested directly as basin inversion, or can have more subtle expression. For example, Cu mineralisation in the Neoproterozoic Yeneena Basin in Western Australia is closely associated with early structures (e.g., Anderson et al., 2001) probably related to basin inversion. Paleomagnetic studies in the North Australian Craton suggest a close relationship between bends in apparent polar wander paths and Zn-Pb-Ag and U depositional events (Idnurm, 2001; Huston et al., 2012). This relationship suggest tectonic events linked to changes in plate motion, which causes these bends, trigger mineralisation. Finally, Mississippi Valley-type Zn-Pb-Ag deposits around the world are related to regional orogenic events, with topographic head generated during mountain building driving fluid flow and mineralisation (Leach et al., 2005).
A feature of basin-hosted mineral systems not present in other mineral systems is the presence of hydrocarbon fluids—oil and gas—that can contain gaseous critical commodities such as helium or can act as a reductant, causing deposition of ore minerals.
2.8.3 Fluid pathways
The main fluid pathways in the basin-hosted mineral system are faults, stratigraphic aquifers within the basin and permeable zones within the basement or along unconformities. These fluid pathways, however, are not open to fluid movement at all times. In particular, diagenetic processes, which depend on a number geological parameters, including rock composition and temperature, can occlude or create porosity and permeability in aquifers (c.f., Polito and Kyser, 2006), and fault zones can open or close depending upon stress conditions. Hence, basin-hosted mineral systems can only be active during restricted periods when the fluid pathways are permeable. Out-of-basin tectonic events that trigger fluid flow may also open fluid pathways.
2.8.4 Depositional processes
The basin-hosted mineral system differs from most other hydrothermal mineral systems in that the main ore fluid is H2S-poor and generally oxidised. These characteristics, particularly the low H2S concentrations, allow the fluid to transport high concentrations of Cu, Zn, Pb, Ag and U at low temperature (<250°C: Cooke et al., 2001). The main mechanism for ore deposition is reduction of the ore fluid accompanied by provision of H2S, which causes deposition of sulfide minerals. H2S can be produced by inorganic or organic reduction of sulfate either in the ore fluid or in the wall rock, with inorganic reduction more likely at higher temperatures (>120°C) and organic reduction more likely at lower temperature (<120°C: Goldhaber and Orr, 1994; Machel, 2001). Alternatively, H2S can be provided if the ore fluid mixes with a second, ambient fluid enriched in H2S, for example sour gas. These depositional mechanisms are enhanced in reduced rock packages, for instance carbonaceous sediments, or in hydrocarbon-rich environments.
Uranium deposition, on the other hand, requires only reduction of U6+ in the ore fluid to insoluble U4+, which deposits as uraninite or pitchblende (UO2). Although H2S is not needed to deposit U, reduced rocks such as carbonaceous sediments or Fe2+-rich rocks, or fluids in equilibrium with such rocks, can cause U deposition (Bastrakov et al., 2009). It must be stressed however, that under the low temperature conditions of most basin-hosted mineral systems, redox reactions are kinetically slow (Ohmoto and Lasaga, 1982), hence, these reactions must be facilitated organically or by the presence of a catalyst such as H2S or Mn2+ (Goldhaber and Orr, 1994; Machel, 2001). Redox gradients produced by the presence of organic-rich sediments or Fe2+-rich rocks (mafic volcanics or iron formation) in an oxidised sedimentary succession are required for metal deposition.
In addition to metallic commodities, sedimentary basins are also the main source of phosphate rock, coal and hydrocarbons. However, formation of these resources does not involve hydrothermal fluids, but occurs in special sedimentary environments.
2.8.5 Australian examples
In Australia, major basin-hosted Zn-Pb-Ag and Cu-Co deposits are hosted by late Paleoproterozoic to early Mesoproterozoic North Australian Basin System (Queensland and Northern Territory), the Neoproterozoic to Paleozoic Central Australian Basin System (Western Australia, South Australia, Northern Territory and Queensland) and the Canning Basin (Western Australia) (Huston et. al, 2012). Uranium deposits are also hosted in Paleoproterozoic basins in the Northern Territory and Cenozoic basins in South Australia (Skirrow, 2011).
The North Australian Basin System contains three of the ten largest Zn-Pb-Ag deposits in the world, Mount Isa, Hilton-George Fisher and McArthur River (HYC), making it the largest global zinc province (Huston et al., 2012). Moreover, unconformity uranium deposits, such as Jabiluka, Ranger, Westmoreland and Coronation Hill, are present in basement near (mostly below) the basal unconformity of this basin system in the Northern Territory (Mernagh at al., 1998). This uranium province is the second largest in Australia (after the Olympic IOCG Province; see Section 2.4) and one of the largest in the world.
Although not as well mineralised as the North Australian Basin System, the geographically extensive Central Australian Basin System contains a number of significant Cu, and U deposits, particularly in the Yeneena Basin in Western Australia and the Adelaide Rift Basin in South Australia. Shale-hosted Cu±Co±Ag deposits (e.g., Nifty and Maroochydore) are hosted by carbonaceous shale (Broadhurst Formation) near the base of the Yeneena Basin, and are present at several stratigraphic levels in the Adelaide Rift Basin (e.g., Mount Gunson, Burra and Kapunda).
During the latter half of the 19th century, the Adelaide Rift Basin was one of the largest copper producers in the world. Unconformity-related uranium deposits (e.g., Kintyre) and Mississippi Valley-type (e.g., Warrabarty) are also present within the basin or immediately below the basal unconformity of the Yeneena Basin. The Paleozoic section of the Amadeus and Ngalia basins contains several small sandstone-hosted uranium deposits in the Northern Territory (e.g., Bigrlyi and Pamela-Angela). The Amadeus Basin also contains producing oil and gas field at Mereenie and Palm Valley, respectively. These hydrocarbon fields are part of the Larapintine petroleum system, which was active during the Paleozoic (Bradshaw, 1993).
The lower Paleozoic section of the Georgina Basin, the northeastern constituent basin of the Central Australian Basin System, contains a number of phosphate deposits in Queensland and the Northern Territory. The most significant of these are Phosphate Hill, which produced 2.49 Mt of phosphate rock in 2011 (Geoscience Australia, 2013), and Paradise South in Queensland, and Wonarah in the Northern Territory. Although large deposits have not yet been discovered, sedimentary phosphate prospects are present in other constituent basins of the Central Australian Basin System, including the Amadeus Basin and the Adelaide Rift Basin. Economically demonstrated phosphate rock resources, mostly within the Georgina Basin, amounted to 945 Mt at 31 December 2011 (Geoscience Australia, 2013).
The Paleozoic to Mesozoic Canning Basin in Western Australia is host to early Carboniferous and possibly older Mississippi Valley type Zn-Pb-Ag deposits, including those on the Lennard Shelf (˜354 Ma: Christensen et al., 1995) and at Admiral Bay (possibly 425-410 Ma: McCracken et al., 1996). The timing of mineralisation on the Lennard Shelf corresponds to the Brewer movement of the Alice Springs Orogeny and the formation of foreland basins in central Australia (Huston et al., 2012). The Canning Basin also hosts producing oil fields on the Lennard Shelf (Blina, Sundown and smaller fields), which form part of the Gondwanan petroleum system (Bradshaw, 1993).
The Eromanga-Carpentaria basin system, which covers much of eastern and south central Australia, and the overlying Lake Eyre basin in South Australia, hosts a number of small U (Beverley (including Beverley North), Four Mile and Honeymoon in South Australia) and V-Mo (Julia Creek in Queensland: Table 2.2.1) deposits associated with paleochannels or stratigraphic units rich in carbonaceous material. Currently, U is being extracted at the Beverley deposit using in situ leaching.
Historically, production of hydrocarbons in Australia has been from conventional oil and gas fields, including the Bass-Otway-Gippsland basin system in Bass Strait, the Perth, Carnarvon, Brouse and Bonaparte basins off the coast of Western Australia, and the Cooper, Surat, Canning and Amadeus basins in central and eastern Australia. Following developments elsewhere in the world, non-conventional hydrocarbon resources—coal seam gas, shale gas and shale oil—are currently being explored and developed through large parts of Australia, including the Sydney-Gunnedah-Bowen basin system.
The development of the liquefied natural gas industry in Australia has also allowed the first production of helium in the southern hemisphere. In March 2010, BOC Limited opened a helium production facility in Darwin. This plant produces approximately 4.25 million standard cubic metres of helium a year by refining a waste stream (containing up to 3 mole % helium) released during liquefied natural gas (LNG) processing in the nearby Conoco Phillips plant (Department of Resources and Energy, Minister for Tourism, 2010).
2.8.6 Associated critical commodities
Like the subaqueous volcanic-related mineral system, the basin-hosted mineral system has high potential to contain a number of critical commodities associated with base metals. For example, Cd, Ga, Ge, In and Hg are likely to be hosted by sphalerite in Zn-rich deposits. These Zn-rich deposits may also be sources of semi-metals such as Bi, Sb and As. As an example, using data from McGoldrick (1982) for 12 orebody at Mount Isa, Cd is weakly correlated (correlation statistically significant at 95% confidence) with Zn (r2 = 0.212, n = 25, Cd/Zn (by mass) = 1.7 × 10-3; excludes two Cd-rich outliers) suggesting an overall concentration of 65 ppm, although with high variability (Table 2.6.1). Other sediment-hosted deposits, such as Hilton-George Fisher, Century, HYC (McArthur River) and Lady Loretta also probably contain significant concentrations of Cd. Carr et al. (1986) indicated that the Hg content of sediment-hosted Zn-Pb-Ag deposits ranged up to 33 ppm, with geometric means of 1.3 ppm, 8.3 ppm and 22 ppm reported for the HYC, Mount Isa and Lady Loretta deposits, respectively.
As discussed in Section 2.6, base metal deposits also can contain significant levels of the semi-metals, As, Sb and Bi. Table 2.6.1 estimates the concentration of Sb at Mount Isa based upon a strong correlation between Sb and Ag (r2 = 0.776, n = 25; data from McGoldrick, 1986) which indicated Sb/Ag (by mass) ˜1.03. These results indicate that the Mt Isa resources as of 31 December 2011 contained ˜24 kt Sb, although this Sb is not recovered presently (and may not be recoverable). Arsenic concentrations in the ore samples analysed by McGoldrick (1982) were typically between 100 ppm and 450 ppm; a weak but significant correlation (r2 = 0.264, n = 25; excluded two outliers) was noted, suggesting and overall As/Zn of ˜ 1.5 × 10-3. In contrast to volcanic-hosted massive sulfide deposits (Section 2.2.5), very limited data from high-grade ore samples suggest low Bi levels (<1 ppm, n = 2: McGoldrick, 1986) in Zn-Pb ores from Mount Isa.
In addition to the major deposits of the Australian Zinc Belt, Zn and Pb are also present at sub-economic grades in sediment-hosted Cu-Co-Ag deposits (e.g., Anderson et al., 2001; Tonkin and Creelman, 1990). Consequently, elements such as Cd, Ga, Ge, In, Hg, Bi, Sb and As may also be anomalous in these systems.
Platinum-group elements such as Pt and Pd are present in a number of basin-hosted deposits, including unconformity-related U and sediment-hosted Cu-Co. For example, the Coronation Hill unconformity-related deposit has a pre-JORC resource of 4.84 Mt grading 4.31 g/t Au, 0.19 g/t/ Pt and 0.65 g/t Pd, making it one of Australia’s largest known PGE deposits1. Importantly, the Au-PGE resource is spatially separate from the nearby U resource. A number of other unconformity-related deposits also have anomalous PGE, Au and Cu (Jabiluka, NT, and Cluff Lake, Saskatchewan, Canada: Peterson, 1994). Platinum-group elements are also present in a sediment-hosted Cu-Co-Ag deposits including the Polish Kupferschiefer and some deposits in Zambia. Kucha (1982, 1983) reported Pt and Pd contents of up to 370 g/t and 120 g/t, respectively (and unquantified though anomalous Ir), in the Kupferschiefer, and Peterson (1994) indicated that ore grade rocks at the Musonoi deposit in Zambia contain 0.75 g/t Pt and 6 g/t Pd. No PGE data are available for sediment-hosted Cu-Co-Ag deposits in Australia.
Because of the large redox gradients involved in ore deposition, basin-hosted mineral deposits commonly contain high concentrations of redox sensitive elements such as Co, V, Se and Mo. Of these, Co is the most significant, being recovered as a by-product in sediment-hosted Cu-Co-Ag deposits overseas (e.g., Zambian copper belt), although not in Australia. Vanadium, Se and Mo are commonly found geochemically dispersed around sandstone-hosted U deposits (Nash et al., 1981; not shown in Figure 2.8.1), although not in sufficient quantities to warrant recovery.
Mernagh et al. (1998) indicate that Th, Zr and REE can be enriched in the basin above unconformity-related U deposits, although it is unlikely that the enrichment would be sufficient to allow recovery. Although not as common as in the sub-aqueous volcanic-hosted mineral system, the basin-hosed mineral system can contain significant barite. The most important example is the Lady Loretta deposit, which contains barite lenses spatially associated with the massive Zn-Pb-rich sulfide lenses.
Sedimentary phosphate deposits contain anomalous concentrations of some critical commodities. These metals include Sr, REEs, B, F, Cr, V, Mn, U, Ni, Cd, Mo, Co, Se, Te, As, Th and Sb (Bech et al., 2010; Simandl et al., 2011). Although these commodities are presently not being recovered, and some (e.g., Sr) can be deleterious, phosphorites are potential sources of REEs (Simandl et al., 2011), and historically uranium has been recovered from phosphate rock in several countries, most prominently the United States (http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/Uranium-from-Phosphates/).
Graphite deposits (not shown in Figure 2.8.1), which form mostly as the consequence of moderate to high grade metamorphism of carbonaceous sedimentary rocks, are hosted by deformed and metamorphosed basins. The best example in Australia is the Mikkira graphite province on the Eyre Peninsula, South Australia, which contains the Uley deposit (Table 2.2.1). This deposit intermittently produced graphite from the late 1920s to 1993. The total resource for the Mikkara province has been estimated at 350 Mt grading 6–7% graphite (http://www.pir.sa.gov.au/minerals/geological_survey_of_sa/commodities/graphite). The only other deposit of significance in Australia is the Munglinup deposit near Ravensthorpe in Western Australia.
As helium, a gas produced by radioactive decay of U and Th, is trapped in the subsurface under conditions that also trap natural gas, many natural gas deposits contain economically significant helium. As described above, helium is now being extracted from the waste stream of a liquefied natural gas plant in Darwin. Australia has two other currently producing liquefied natural gas plants, the North West Shelf Venture (NWSV) at Karratha and the Pluto LNG project, located 190 km north-west of Karratha, but neither plant is presently extracting helium. Apart from these projects, seven large Australian LNG schemes are currently under construction (APPEA, 2012). Four draw from gas fields in northern Western Australia (Gorgon, Prelude, Wheatstone and Ichthys) and three are in Queensland (Queensland Curtis LNG, Gladstone LNG and Australia Pacific LNG).
Australia has vast conventional gas resources. In 2008, Australia’s EDR and subeconomic demonstrated resources (SDR) of conventional gas were estimated at 180,400 PJ (164 tcf) (Geoscience Australia and ABARE, 2010). Based on the above estimate of 3 mole % helium in Australian LNG, this equates to a resource of approximately 4.92 tcf helium. Most (around 92 %) of Australia’s conventional gas resources are located in the Carnarvon, Browse and Bonaparte basins off the north-west coast. There are also resources in south-east and central Australia. As well as the conventional gas resources, Australia also has large supplies of unconventional gas. Large coal seam gas (CSG) resources exist in the coal basins of Queensland and New South Wales. Tight gas accumulations are located in onshore Western Australia and South Australia, while potential shale gas resources are located in the Northern Territory (Geoscience Australia and ABARE, 2010). The potential of unconventional gas resources for helium extraction is unknown.
1 Coronation Hill will not be developed as it is within the confines of Kakadu National Park.