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Carbonatites are rare igneous rocks which are composed of more than 50% primary carbonate minerals, generally mixtures of calcite, dolomite and siderite. About 500 carbonatite intrusions and a small number of localities of extrusive carbonatites are known worldwide.

Many carbonatite intrusions host one or more separate ore bodies, containing different combinations of incompatible trace elements, and in some cases also of some major elements. Important examples of carbonatite-hosted deposits include: ores of LREEs, Nb and Fe at Bayan Obo, Inner Mongolia, China (Yanget al., 2011); resources of Ti and Th at Powderhorn (Iron Hill), Colorado, USA (Van Gosen, 2009); ores of LREEs at Moun-tain Pass, California, USA (Haxel, 2005; Longet al., 2010); ores of Cu, Co, Zr, Hf, Fe, apatite, and vermiculite, and also by-products Au, Ag, Ni and Pt at Palabora, South Africa (Palabora Mining Company staff, 1976); and of LREEs and Nb at Mt Weld, Western Australia (Lottermoser, 1995). Tantalum is present at ore grade in some carbonatites.

Carbonatites and genetically related rocks are the primary global source of the LREEs.

Geochemical nature of LREEs

Many of the trace elements which are extracted from carbonatites and from strongly alkaline silicate igneous rocks are HFSEs. These are trace metals which have a large ionic charge (þ3 to þ5) and relatively small ionic radius, and are consequently incom-patible in common silicate minerals in the mantle (Figure 2.2). The REEs are the row of

elements of atomic number Z ¼ 57 (La) to Z ¼ 71 (Lu), the LREEs are the lower-atomic-number elements in this row (La, Ce, Pr, Nd, Sm and Eu) and the heavy rare-earth elements (HREEs) are those from Gd to Lu. Because they have the same outer-shell electron configuration, the REEs exhibit very similar chemical behaviour in natural environments and readily substitute for each other in minerals. All form ions withþ3 charge, although two, Eu and Ce, can also be present in natural environments with þ2 and þ4 charge, respectively. The increasing atomic number is taken up by addition of electrons to the inner 4f electron shell; this causes progressive reduction in ion size across the row, and hence differences in partitioning constants which allow partitioning of the LREEs from the HREEs. The HREEs are less abundant than LREEs in the crust, and are most strongly concentrated in different types of ores than the LREEs (see page 337 in Chapter 6).

The nature of typical carbonatites and hosted LREE ores

Intrusive carbonatites occur most typically as roughly cylindrical, composite, approxi-mately pipe-shaped, steeply plunging stocks up to about 3 km in diameter, although they can also occur as irregular lenticular bodies (Figure 2.3). Swarms of carbonatite dykes are often present within a few kilometres of larger intrusions. Some carbonatites are isolated intrusions, but most are adjacent to or within complex zoned intrusions of mafic and ultramafic alkaline intrusive rocks. The rocks located within a few hundred metres of the contacts of carbonatite intrusions and of zoned intrusions of carbonatite and alkaline silicate igneous rocks are in almost all cases converted to fenite. This is ametasomatic rock type characterised by high potassium content, such that one or more of K-feldspar, riebeckite, and biotite are important minerals.

50 m

Gneiss Palaeoproterozoic

Mesoproterozoic Mountain Pass Carbonatite Bastnaesite calcite carbonatite Bastnaesite dolomite calcite carbonatite Bastnaesite dolomite carbonatite Monazite carbonatite

Breccia - fenite altered gneiss clasts in carbonatite matrix

W E

Figure 2.3 Cross section through the open pit of the Mesoproterozoic Sulfide Queen carbonatite intrusion at Mountain Pass, California, USA (after Castor, 2008) showing characteristic heterogeneity of these intrusions. The ore zones are essentially the three bastnaesite-bearing units within the intrusion.

On average, carbonatites have the highest concentrations of rare-earth element oxides (REOs) of all crustal rock types. Summed REO concentration is about 0.5%, which is around 30 times that of average continental crust. In unweathered REE-rich igneous carbonatite, the REEs are hosted in different minerals in which they are essential components, especially the REE fluorocarbonates bastnaesite [(La,Ce)CO3(F,OH)] or more rarely parisite [Ca(Ce,La)2(CO3)2F2], or the phosphate monazite [(Ce,La,Th)PO4].

The ores have igneous textures with the ore minerals either disseminated relatively uniformly or as concentrations in clusters with other minerals through the carbonatite.

Economic ores of LREEs are hosted in the small number of carbonatites that have significantly higher concentrations in part or all of the intrusion, with grades of up to a few weight per cent (wt %) combined LREEs. At Mt Weld, ore is in weathered carbonatite and the enrichment to ore grade is a result of lateritic weathering (see Section 6.1). In contrast, the high concentrations of LREEs at Bayan Obo and Mountain Pass occur in rocks with essentially fresh igneous minerals and textures and the ores are thus interpreted as ‘primary’ magmatic ores. These two LREE ore-bearing carbonatites are unusual compared to other carbonatites. At Mountain Pass (Figure 2.3), ore is in calcitic and dolomitic carbonatite with barite as an important gangue mineral. The carbonatite is unusually associated with an ultrapotassic alkaline intrusion. At Bayan Obo, ore is present in calcite carbonatite, but the highest grades are in composite lenses of unique iron oxide–

fluorite–aegerine-augite rock which is hosted within a large (10 by 2 km outcrop area) carbonatite intrusion. Other carbonatites with high primary REE concentrations are known but many have not been fully evaluated as possible deposits of REEs, for instance, along the East African Rift and at Kanneshin (Afghanistan).

Genesis of LREE ores in carbonatites

The environments of melting to form carbonatite magmas are inferred using geochem-ical reasoning and through comparison of rock compositions with results of petrologgeochem-ical experiments of melting at high pressures. At pressures greater than about 2.5 GPa, hence depths greater than about 90 km, dolomite can be a stable mineral in mantle peridotite. Up to a few per cent of carbonate is present in some mantle-derived xenoliths, hence suggesting that it is a minor component of some mantle peridotite.

Carbonatite melts are produced in experiments by low percentages of partial melting of carbonate-bearing peridotite at between about 2.5 GPa and 6 GPa. The carbonate minerals are fully melted after a few per cent partial melting, and the maximum percentage of melting that will produce a carbonatite melt from dolomite-bearing mantle peridotite is thus estimated to be less than about 3%. With slightly higher temperatures and degrees of melting, the same source rock will produce carbonate-bearing alkaline magmas (Figure 2.4). As the solidus is at a higher temperature at lower pressures (Figure 2.4) carbonatite melts may in most cases freeze on rising through the upper mantle and may only reach the surface in unusual circumstances of rapid rise from the depths of melting.

The geochemical behaviour and mineralogical setting of REEs in mantle environments is not fully known; however, their likely behaviour on melting can be interpreted. All REEs have ionic radii too large for easy substitution in crystal lattice sites in major mantle minerals, and unless there is a minor mineral in which they are compatible they will be incompatible and will be concentrated in a melt. Partition coefficients are lower for

LREEs than for HREEs, hence the former will be more strongly concentrated in melts.

The concentrations of LREEs in typical carbonatites and the relative concentration of LREEs over HREEs are of the order that is expected in low-percentage batch partial melts of carbonate-bearing mantle peridotite (Figure 2.5).

Small degrees of partial melting of mantle with average REE concentrations can produce melts with approximately 0.5% REOs. Additional processes are, however, required to produce enrichment from about 0.5% to per cent levels of LREEs in ores.

Most carbonatites in the upper crust are not primitive magmas whose composition is unchanged between formation of melt in the mantle and final crystallisation in the upper crust, but have evolved through fractional crystallisation and other petrogenetic processes, probably at various depths in the upper mantle and in the crust. Processes that could cause enrichment to per cent levels of LREEs include fractionation during crystallisation, immiscible separation of alkaline silicate melts and carbonatite melts, and hydrothermal processes. At both Bayan Obo and Mountain Pass there are multiple phases of carbonatite dykes; later dykes have higher concentrations of REEs, a fact that suggests fractional crystallisation was a contributing process in the formation of these ores (see discussion of this process in Section 2.2.5).

Lherzolite + CO₂

carbonatitic melts alkali basaltic melts

5% 10% 20% 40%

Figure 2.4 Left: Schematic pressure–temperature diagram of the environment of formation of carbonatitic and alkali–basaltic melts from carbonate-bearing mantle peridotite (dolomite and magnesite lherzolite), after Dalton and Presnall (1998). Numerical percentages of partial melts at any pressure and temperature, and the boundary between carbonatitic and basaltic melts are based on experiments with 2.5 wt % CO2of Dasguptaet al. (2007). The exact positions of these isotherms will depend on the modal per cent carbonate in the mantle and are shown to illustrate trends only. Right: Element enrichment in melt compared to source rock as functions of fraction melted (F) and bulk partition coefficient (D_i) for the rare-earth elements La, Eu and Lu, based on the equation for batch (one-step) partial melting, as shown.

The shaded circles correspond to melt extraction at the conditions shown on theP–T diagram.

Partition coefficients are taken from Dasguptaet al. (2009).

Mantle will melt to a low degree where temperatures are not significantly elevated above normal temperatures at that depth in the mantle. Were temperatures more elevated, more voluminous silicate melts would form, in the first instance alkaline mafic to ultramafic rocks. Some of the geologically most recent carbonatites are along the East African Rift, which is an incipient continental rift cutting relatively cold lithosphere of a part of a continent that has otherwise been tectonically stable over time periods of hundreds of millions of years, as is typical of many continental shields. This is thus one possible environment of subtle, minor heating of the mantle to produce low-temperature, low-volume melts. Other carbonatites, for instance those in the Kola magmatic province of the Russian Federation, intruded at the same time as large-volume extensive intraplate mafic magmatism in Eastern Europe, but at the edges of the main area of magmatism and many hundreds of kilometres from its centre. This mafic magmatism is an example of a large igneous province (LIP) in which large volumes of mantle-derived magma formed and intruded into and through continental crust in an intraplate setting over a period of at most a few million years. The carbonatites may thus have formed at the cooler edges of a volume of mantle that was undergoing large-scale melting.

A critical factor for the genesis of carbonatites is the presence of a small amount of carbonate in mantle peridotite. We do not know how widespread carbonate is in the

La (ppm)

1% 10%

1000 10

La content of mantle melt based on bulk partition coefficient D_La = 0.002

carbonatites

carbonatite-hosted LREE ores primitive

mantle

average crust

0 0.5 1 2

carbonate-bearing peridotite xenoliths

granites

1 100

enrichment

on melting second phase

of enrichment

Figure 2.5 Lanthanum concentrations (ppm) in rocks and reservoirs in the mantle and in carbonatites and ores in carbonatites: La is shown here as proxy for LREEs. Partial batch melts of primitive mantle of percentages appropriate to form carbonatites will have La concentrations shown by the circles, which correspond well with concentrations in

carbonatites. A second phase of enrichment is required to produce concentrations of per cent level of ores. Bulk peridotite–carbonatite melt partition coefficients are those of Dasgupta et al. (2009). Lanthanum concentrations in carbonate-bearing peridotite xenoliths were measured by Ionovet al. (1993) and indicate that carbonate-bearing peridotite is not unusually rich in REEs compared to normal mantle.

mantle. One possible origin of carbonate in the mantle is that it is added metasomatically from fluids that are released in metamorphic mineral reactions in sediments or carbonate-bearing ocean crust that are being subducted and which percolate through the mantle wedge of peridotite that overlies subducting crust.