Conference Agenda

Carbonatites are currently enjoying something of a renaissance due to increasing awareness that these rocks are among our best sources of critical metals. The dominant petrogenetic paradigm suggests that economic rare-earth element mineralization in carbonatites forms only after protracted magmatic differentiation and post-magmatic overprinting by carbo(hydro)thermal fluids in the crust. There are indications, however, that some mantle sources – especially in collision zone settings – are more conducive to the formation of economic carbonatites than others, meaning that not all carbonatites were created equal with respect to metal budgets.

In this contribution, I provide an overview of currently debated models for the origin of carbonatites including various types of economic mineralization. New data and ideas are discussed for some of world's oldest carbonatites to better understand the processes that led to inception of the carbonatite mineral system.

Pyroxenitic veins dominantly composed of aegirine augite are ubiquitous in aureole rocks and ijolites of the Iivaara alkaline complex. Titanite is second most abundant followed by apatite. Both are heterogeneously distributed and occur in clusters locally aligned along vein orientation. Clustered appearance of texturally distinct titanite indicates vein mineralization in successive steps. Veins in ijolitic rocks with elongate aegirine augite oriented perpendicular to vein margins into the host formed in a sequence of metasomatic decarbonation reactions of a carbonatite with nepheline. The carbonatite incorporated Ca, Mg, Fe, Ti, PO₄, S, CO₂ and H₂O into the ijolite while removing substantial Al, Na and K. In the host of the veins the reactions result in a mineral association of idiomorphic concentrically zoned aegirine augite next to cancrisilite, titanite, apatite, minor cancrinite, sodalite as well as late natrolite and analcite replacing nepheline. While rock domains with relict nepheline reflect the incomplete alteration process, rocks altered at an advanced stage obtained a texture that can hardly be distinguished from a magmatic rock. The observations suggest that ijolite at the type locality Iivaara was significantly affected by injection of carbonatite leading to metasomatic “melteigitization” of former ijolite or urtite compositions. Textural and mineral chemical evidence shows that Ti was transported in the carbonatite and not leached from the vein host. Titanium either was a primary constituent of the carbonatite or was taken up in metasomatic reactions at depth.

Molybdenite in ferrocarbonatites and associated fenites from NW Namibia provides an example of Mo precipitation during the transition from magmatic crystallization to metasomatic fluid overprint in a carbonatite system. Textural relations, sulfur isotopes, trace-element data, fluid inclusions, Raman spectroscopy, and Re–Os geochronology indicate that Mo enrichment was controlled by both carbonatite fractionation and subsequent high-salinity Na–Cl-rich brines. Sulfur isotope compositions of molybdenite near -1‰ δ³⁴S V-CDT, together with the limited Mo potential of the exposed country rocks, point to a dominantly mantle-derived Mo source. Molybdenite contains generally low trace-element concentrations, with enrichments mainly in Se, Te, Pb, and minor Re, W, Nb, and Co. Early fluid inclusions in metasomatic sodalite and magmatic ankerite have salinities of 25–30 wt.% NaCl eq. and contain molybdenite and magnetite daughter minerals, showing that the metasomatic brines were metal-bearing. Fluid inclusions in ankerite with lower salinities around 4 to 6 wt.% NaCl eq. are barren of metals. These inclusions indicate molybdenite precipitation at temperatures of at least ca. 500 °C, probably after transport of Mo in Na–Mo–O–Cl complexes and subsequent cooling and local sulfur saturation. Textures around molybdenite can be interpreted in line with dissolution reprecipitation processes affecting ankerite, and apatite. Later albite, chlorite, quartz, and Mo oxides reflect a lower-temperature overprint at ca. 250–150 °C. Re–Os molybdenite ages of 1183.4 ± 7.5 Ma and 1174.2 ± 6.4 Ma constrain the timing of mineralization and link the Swartbooisdrift ferrocarbonatites to nearby Mesoproterozoic carbonatites of the Epembe area.