2.9 Alkaline intrusion-related mineral system
In comparison with calc-alkaline or tholeiitic magmatism, magmatic rocks associated with alkaline magmatism are generally small in volume, but can have a large range in composition. For example, alkaline rocks that form the ˜1132 Ma (Hoatson and Claoué-Long, 2002) Mordor Complex in the Northern Territory range in composition from pyroxenite to syenite (Hoatson and Claoué-Long, 2002), and alkaline magmatic rocks include kimberlites and related rocks, and carbonatites (igneous rocks with more than 50% modal carbonate). Mineral deposits that form in the alkaline intrusion-related mineral system (Figure 2.9.1) are also quite diverse, ranging from diamond, through REE-P-U, to Ni-Cu-PGE and vermiculite deposits. Moreover, these deposits contain a large number of critical commodities, including REE, PGE, Ni, Th and Zr.
2.9.1 Geological setting
Although small in volume, alkaline igneous rocks are found in a large range of geological settings, including continental rift valleys (the main setting), intraplate magmatic provinces with uncertain tectonic settings, and destructive plate boundaries (Fitton and Upton, 1987). In the latter case, calc-alkaline magmas tend to become more potassic as subduction proceeds and may give way to volcanic rocks of the shoshonitic association (Fitton and Upton, 1987). For this discussion, the shoshonitic association is considered in the porphyry-epithermal mineral system (Section 2.2).
Carbonatites occur in continental shields and are commonly related spatially to fault lineaments such as in rift systems. Locally, they are related to alkaline volcanism. Almost all known carbonatite complexes are intrusive into Precambrian shields, however, the carbonatites themselves may be much younger (Woolley and Kjarsgaard, 2008).
2.9.2 Sources of fluid, metals and energy
Radiogenic and stable isotopic data and geochemical data all suggest that alkaline magmas and carbonatites originated as generally low degree partial melts of metasomatised or crustally contaminated mantle, with the depth of melting and the degree and age of contamination determining the type of geochemical and isotopic characteristics of the magma (Fitton and Upton, 1987; Menzies and Murthy, 1980). Ultimately, this metasomatism and contamination is thought to be a consequence of the subduction of crust into the mantle (Fitton and Upton, 1987).
The parental carbonatite magmas originate by the separation of an immiscible carbonate liquid phase from a CO2-saturated nephelinite or phonolite magma. The separation of the carbonate liquid from its source leads to strong fractionation between the carbonate liquid and silicate and oxide solid phases. Strontium and Nd isotope ratios indicate that the sources of carbonatites are geologically old, inhomogeneous, and variably depleted in the radioactive parent elements Rb and Sm. This fractional process typically results in strong enrichment in light rare-earth elements, U, Th, and Pb, but much less enrichment in T, Zr, Nb, and Sr.
2.9.3 Fluid pathways
The alkaline intrusion-related mineral system differs from most other mineral systems in that many deposits are orthomagmatic, that is the ore fluid was the magma. Hence, fluid pathways in this mineral system are largely the magmatic pathways, which are generally controlled by larger-scale architecture. For example, young examples of larger volume alkaline magmatic provinces are most closely associated with continental rifts such as the East Africa Rift (Kampunzu and Mohr, 1991), suggesting the pathways for the emplacement of these magmas are provided by the rift architecture and the impetus for magmatism was provided by rifting. Ernst and Bell (2010) suggest that possible drivers for some alkaline magmatic events through time were mantle plume events, which also can initiate rifting.
Kimberlite and related melts are generally emplaced along the margins of old and cold cratons as indicated by lithospheric tomographic data (Begg et al., 2009). An important aspect of the emplacement of many alkaline magmas is a rapid ascent from source (Eggler, 1989), which must occur along weak zones such as the margins of old-cold cratonic blocks.
2.9.4 Depositional processes
Typically, mineralisation is the product of the emplacement of volatile-enriched (F and water) alkaline magmas as hypabyssal intrusions, (pyroclastic) volcanic rocks, or deeper level intrusions. During this process, the volatiles and incompatible elements (e.g., Th, Zr, and REE) are concentrated in the upper parts of the magma chamber. Late-stage hydrothermal fluids, containing F and Cl, are important for enhancing the grades in these deposits. If a magmatic-hydrothermal fluid evolves from the alkaline magma, depositional mechanisms can include depressurisation, fluid-rock interaction or mixing with ambient fluids.
Carbonatite magma, ascending through lithospheric mantle, commonly is trapped before it can invade the crust. In addition to the factors that can stop the rise of any magma (heat loss, increase of solidus temperature with decrease in pressure, decrease in density and increase in strength of wall rock), carbonatite magma can be halted by reaction with wall rock to form Ca and Ma silicates plus CO2. In more reducing conditions carbonate can be reduced to elemental carbon (graphite or diamond) or to methane. Both of these changes remove dissolved CO2 from the magma, causing crystallisation.
Mineralisation is commonly restricted to carbonatite dykes, sills, breccias, sheets, veins, and large masses, but may occur in other rocks associated with the complex rocks. Alteration known as fenitisation (widespread alkali metasomatism of quartzo-feldspathic rock; mostly alkalic feldspar with some aegerine and subordinate alkali-hornblende and accessory sphene and apatite) typically occurs near the contact of carbonatite intrusions (Brøgger, 1921). The fluid-rock interaction recorded by fenitisation may be a prime mechanism for carbonatite crystallisation, as described above. Trace elements typically associated with carbonatites include Th, U, Cu, V, P, Mn, S, La, Sm, Pb, Zr, Ba, and Eu; enrichment of Be, B, Li, Sn, Ta, Hf, and W, though present in some carbonatites, is less common.
2.9.5 Australian examples
Mineral deposits related to alkaline magmatism in Australia are relatively uncommon, but include important current producers, such as the Argyle diamond mine, deposits from which production has just commenced, such as the Mount Weld carbonatite, and deposits in the advanced stages of feasibility, such as the Toongi and Nolans Bore deposits. Much of the information presented herein is based on Hoatson et al., (2011).
The Argyle diamond mine in the Halls Creek Orogen in Western Australia is the world’s largest diamond producer. The only other current producer is the Ellendale field, also in Western Australia, although the Merlin field in the Northern Territory may be reopened in the next few years. Total Australian diamond production in 2011 was 7.5 million carats, with Argyle producing over 98% (Geoscience Australia, 2013).
The Toongi Zr-Nb-REE deposit in New South Wales (Table 2.2.1) occurs within an alkaline trachyte plug about 30 km south of Dubbo in NSW. In 2008 a demonstration plant was constructed in collaboration with ANSTO in Sydney, which demonstrated the technical viability for of Zr, Nb, LREE and HREE recovery. The project is currently the subject of a defensible feasibility study scheduled for release in Quarter 1 2013 (alkane.com.au). The peralkaline granitic intrusions of the Narraburra Complex, also in New South Wales, contain anomalous amounts of Zr, REE and low concentrations of Th (Table 2.2.1).
The Hastings-Brockman deposit in Western Australia is a large low-grade Zr-Nb-REE deposit (Table 2.2.1) hosted in altered trachytic tuff of Paleoproterozoic age. It occurs in a Paleoproterozoic sequence (>1000 m thick) of alkaline lavas, volcaniclastic sedimentary rocks, and felsic volcanics that is intruded by subvolcanic sheets and interlayered with greywacke, siltstone and mudstone.
The Mount Weld deposit in Western Australia is a large, high-grade Y, Nb, Ta, P, Zr, Ti and REE resource in the lateritic profile of a large circular Proterozoic carbonatite, which intrudes Archean greenstones of the Yilgarn Craton. The deposit was enriched as a result of supergene enrichment processes in the deep weathering profile. The weathered zone is rich in REE, Nb, and Ta, whereas the unweathered carbonatite at depth contains these elements in lower amounts more typical of carbonatites. Mining commenced in 2008, and in May 2011 the concentrating plant was opened. Currently, a total of 14.365 kt of REO concentrate has been processed and is ready for export to a processing plant in Malaysia. In January 2012 Lynas Corporation announced total resources at the Mount Weld deposit (including the Central Lanthanide and Duncan lenses) of 23.9 Mt grading 7.9% REO (rare-earth oxides).
The Cummins Range carbonatite in Western Australia is a roughly circular body and is about 1400 m in diameter. The complex is deeply weathered and covered by a thin layer of aeolian soil and silicified limonitic collapse breccia mounds. Navigator Resources reported inferred resources for the Cummins Range deposit of 4.17 Mt at 1.72% REO, 11.0% P2O5, 187 ppm U3O8 and 41 ppm Th (Geoscience Australia, 2012).
Other Australian carbonatites include Yangibana, Ponton Creek, and the Yungul dykes in Western Australia and Mud Tank in the Northern Territory. Carbonatites and/or related veins are also reported at the Mordor Igneous Complex in the Northern Territory and Walloway in South Australia (Hoatson et al., 2011). The Mordor Complex also contains low-grade Au-PGE mineralisation associated with a layered mafic intrusion of lamprophyric affinity (Barnes et al., 2008).
Another Australian deposit that may be associated with alkaline magmatism is the Nolans Bore REE-P-U deposit (Table 2.2.1), which consists of a series of massive apatite veins. This deposit is thought to be a hydrothermal deposit, and a direct relationship to alkaline magmatism has not been established, although it is suspected. This interpretation is supported by radiogenic and stable isotope data, which suggest a metasomatised or contaminated mantle source, possibly like EM1 (Huston et al., 2011b).
2.9.6 Associated critical commodities
As described above deposits associated with alkaline magmatism are Australia’s (and the world’s) dominant resource for a number of critical commodities, including REE, Nb, Zr and Th. Other critical commodities contained within some of these deposits include Ga (Table 2.2.1) and PGE.